Category: 7. Science

The typical American home requires 15 to 25 solar panels to address 100% of its energy needs, and millions of households are making the switch. But as this solar revolution accelerates, a critical question emerges: What happens to these millions of panels when they reach the end of their 25- to 30-year lifespan?

Solar power has become the driving force of the global energy transition. In 2024 alone, the world installed a record-breaking 597 gigawatts of solar capacity—a 33% increase over 2023, which brought global solar installations to over 1.6 terawatts. To put this growth in perspective, solar panels are now being installed at a pace that would have seemed impossible just a decade ago, with predictions that the world could be installing one terawatt of solar annually by 2030—enough energy to power the state of California for a week-and-a-half.

The Coming Wave of Solar Waste

Unlike the relatively modest recycling needs of today, we’re heading toward a tsunami of solar panel waste. The International Renewable Energy Agency (IRENA) projects that global solar panel waste could reach 78 million tons by 2050, equivalent to disposing of more than 4 billion of today’s panels. In the United States alone, the Environmental Protection Agency estimates recyclers will need to process one million tons of solar panel waste by 2030 and up to 10 million tons by 2050.

Photovoltaic panel recycling isn’t just an environmental challenge; it’s a massive economic opportunity. IRENA estimates that recovered materials from recycled panels could be worth $450 million globally by 2030, growing to $15 billion by 2050. These materials could supply enough resources to manufacture 2 billion new panels without mining new raw materials.

Why Solar Panel Recycling Matters

Solar panels aren’t just glass and metal. While about 75% of a panel’s weight is recyclable glass, panels also contain valuable materials, including silicon, silver, copper, aluminum, and sometimes rare elements such as tellurium and indium. More concerning, some panels contain potentially hazardous materials, such as cadmium and lead, that shouldn’t end up in landfills.

“In a clean energy industry, we can’t advocate for clean energy while choosing to landfill and not properly recycle solar panels,” explained Brad Henderson, CEO of Solar Panel Recycling, a company that has processed hundreds of thousands of panels.

The good news? Modern recycling technologies can now recover up to 95% of materials from silicon-based panels and up to 98% from thin-film panels. It is now the case that yesterday’s solar installations can be recycled to provide the raw materials for tomorrow’s panels.

Recycling Infrastructure Takes Shape

The solar recycling industry is rapidly maturing. The global solar panel recycling market is projected to grow from $384.4 million in 2025 to $548 million by 2030. Early adoptors will eventually begin panel retirements and the implementation of more stringent photovoltaic panel recovery regulations will keep those older systems out of landfills.

The companies scaling up operations across the country include:

• SOLARCYCLE has processed nearly 500,000 panels and is on track to recycle one million panels by the end of 2025. The company has partnerships with over 90 energy companies and operates advanced facilities that can extract high-purity materials.
• Solar Panel Recycling (SPR), which has facilities in North Carolina and Georgia, offers full decommissioning, transportation, and compliance management services.
• First Solar has been operating a comprehensive recycling program for its thin-film panels for over a decade, achieving some of the highest material recovery rates in the industry.

New facilities are opening regularly, with many nations adding end-of-use options for used panels. In October 2024, Australia’s Pan Pacific plant opened with the capacity to process 240,000 panels annually.

Regulations Drive Responsible Disposal

Policy makers are trying to get ahead of the waste curve. Europe leads the way with the first-of-its-kind Waste Electrical and Electronic Equipment (WEEE) directive that requires solar panel manufacturers to finance collection and recycling costs for panels sold in European markets.

In the United States, regulations are emerging state by state:

• California was the first state to establish solar-specific recycling regulations, which require comprehensive reporting by companies that handle more than 200 pounds of used panels.
• North Carolina will require decommissioning plans for solar projects larger than 2 megawatts starting November 1, 2025.
• Twenty-nine states currently have decommissioning and recycling policies for utility-scale solar projects.

These regulations ensure that solar project developers plan for end-of-life management from Day One, which can prevent future environmental problems.

How Solar Panel Recycling Works

Modern solar panel recycling involves a process that separates and purifies the different materials used in a panel:

1. Disassembly: Aluminum frames and junction boxes are removed for standard metal recycling
2. Glass separation: The glass cover, which accounts for 75% of panel weight, is separated and cleaned for reuse
3. Laminate processing: High-temperature or chemical processes separate the polymer layers that encapsulate the solar cells
4. Cell recovery: Silicon solar cells are extracted and can often be reused directly in new panels
5. Metal extraction: Copper wiring, silver contacts, and other valuable metals are recovered through specialized processes

As recovered materials flow back into manufacturing supply chains, there will be a reduced need for virgin materials. Solar energy can be self-sustaining and by mid-century could eliminate the need for new raw materials.

Preparing for Residential Solar Recycling

While most current recycling efforts focus on utility-scale installations, residential solar recycling is on the horizon. SEIA and SPR launched a pilot program on January 1, 2025, in Mecklenburg County, N.C., the first drop-off program for residential solar panels.

For homeowners with aging solar systems, here’s what you should know:

• Don’t throw panels in the trash: Solar panels often contain materials that shouldn’t go to landfills
• Check with your installer: Many solar installers are developing take-back programs
• Look for certified recyclers: Choose recyclers certified under standards like SERI’s R2 Standard or the e-Stewards standard
• Plan ahead: Include end-of-life costs in your solar investment planning

The Economic Promise of Solar Recycling

Solar panel recycling isn’t just about environmental responsibility; it’s also becoming a good business practice. Australia’s government projects that the total material value from end-of-life solar panels could exceed $1 billion by 2033, while global projections suggest the industry could create thousands of green jobs.

As panel prices continue to fall and solar installations continue to grow, the recycling industry expects that recycling will become increasingly profitable. Some companies are already signing long-term contracts to supply recycled materials to solar manufacturers, creating dedicated supply chains for secondary materials.

Recycling Is Ready

Solar energy is critical to addressing climate change, but its environmental benefits depend on responsible end-of-life management. The good news? The infrastructure, technology, and economic incentives for comprehensive solar panel recycling are rapidly falling into place.

As we race toward a clean energy future powered by unprecedented solar growth, building a robust recycling industry today ensures that tomorrow’s clean energy remains truly clean from cradle to grave. For environmentally conscious consumers, solar installations that include recycling commitments make the renewable energy choice even more beneficial for both people and nature.

Want to find recycling options in your area? Use Earth911’s recycling search to find electronics recyclers near you and inquire about their capabilities for recycling solar panels.

Editor’s note: Originally published on April 6, 2017, this article was most recently updated in July 2025.

A shocking new discovery beneath Canada’s remote northern frontier may have just revealed one of the nation’s most powerful and underestimated natural threats. Scientists from the University of Victoria, working in partnership with the Geological Survey of Canada and the University of Alberta, have confirmed that a massive fault line running across the Yukon is still very much active, and dangerously overdue for a major earthquake.

The Tintina fault, which spans over 1,000 kilometers across northwestern Canada, was long believed to have been dormant for millions of years. But new high-resolution imaging from satellites, airplanes, and drones tells a different story. Beneath the forests and permafrost near Dawson City, researchers have uncovered surface scars, physical evidence of massive prehistoric earthquakes that tore through the Earth in the not-so-distant past.

Now, scientists are warning that this “long-forgotten” fault may be capable of unleashing a magnitude 7.5 or greater earthquake, potentially one of the most powerful in Canadian history.
“This fault has been silently accumulating tectonic strain for over 12,000 years,” said Dr. Theron Finley, UVic geologist and lead author of the study published in Geophysical Research Letters. “That strain is going to release at some point and when it does, it could be catastrophic.”

A geological sleeping giant

The Tintina fault is a major lateral strike-slip fault, the kind of fault that moves horizontally, like California’s San Andreas. Throughout its lifetime, it has slipped over 450 kilometers, but researchers believed its activity ceased tens of millions of years ago.
That belief has now been overturned.
Using lidar (light detection and ranging) mounted on drones and aircraft, as well as data from the ArcticDEM satellite elevation project, the research team identified a 130-kilometer-long segment of the fault showing unmistakable signs of repeated seismic ruptures during the Quaternary Period (2.6 million years ago to present).
Some of the fault scarps, narrow ridges that mark the surface rupture of past earthquakes, were found to have offset glacial landforms by up to 1,000 meters. More recent features, about 132,000 years old, were displaced by 75 meters, proving that the fault has remained active into Canada’s recent geologic history.

But the most startling revelation came when the team examined younger landforms, around 12,000 years old, that appeared undisturbed.

“That tells us the last major rupture occurred just before 12,000 years ago,” Finley explained. “And based on the current rate of tectonic strain accumulation, estimated at 0.2 to 0.8 mm per year, we believe the fault may now be nearing the end of a seismic cycle.”

In plain terms, a significant amount of energy has built up underground. If released in a single event, it could trigger an earthquake measuring 7.5 or greater on the Richter scale, capable of causing widespread devastation.

The implications are particularly dire for Dawson City, a historic town located within 20 kilometers of the newly identified fault scarps. Known more for its Gold Rush past than for seismic activity, the region lacks the kind of earthquake-resistant infrastructure common in places like British Columbia or California.

The threat doesn’t end with shaking. The surrounding landscape is highly prone to landslides, many of which are already unstable. Two major slopes, the Moosehide landslide to the north of Dawson City and the Sunnydale landslide across the Yukon River, are showing signs of ongoing motion. A strong quake could send millions of tons of earth cascading into nearby valleys or rivers, potentially blocking waterways, destroying property, and endangering lives.

National seismic blind spot

Perhaps most troubling is the fact that the Tintina fault is not currently recognized as an active seismic source in Canada’s National Seismic Hazard Model (NSHM), the model that underpins building codes and engineering standards across the country.

That is now expected to change.

Officials at Natural Resources Canada have confirmed that the data from the new study will be used to revise the NSHM. These revisions could have far-reaching impacts on everything from construction permits and zoning regulations to emergency planning and national infrastructure projects.

The findings are also being shared with local authorities, First Nations governments, and emergency managers across the Yukon, including the Tr’ondëk Hwëch’in and Na-Cho Nyäk Dun, on whose traditional territories the research took place.

While there’s no way to predict precisely when the next earthquake will strike, the science suggests that the conditions for a major rupture are already in place.

“This fault has been silent for over 12,000 years,” said Finley. “That’s well within the recurrence interval for a fault of this size and behavior. It’s not a question of if, it’s a question of when.”

Without immediate action to update hazard maps, strengthen infrastructure, and prepare northern communities, experts warn that Canada could face a disaster on a scale not seen in modern history.

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Early this morning, the raised, flat lava plain of Wargentin is stands out near the large crater Schickard in the lunar southwest.

Scientists say they have finally discovered the cause behind the death of more than 5 billion sea stars along the Pacific coast of North America, solving a mystery that has lasted over a decade.

As per an AP report, starting in 2013, a mysterious disease called sea star wasting disease led to a massive die-off of sea stars (often called starfish), from Mexico to Alaska. More than 20 species were affected, with the sunflower sea star hit hardest, losing nearly 90% of its population in just five years.

“It’s really quite gruesome,” said Alyssa Gehman, a marine disease expert at the Hakai Institute in British Columbia, who worked on the new research. “Healthy sea stars have puffy arms sticking straight out, but with the disease, they develop lesions and then their arms actually fall off.”
The cause? A bacteria known as Vibrio pectenicida, the same type that also affects shellfish, was identified in a new study published in the journal Nature Ecology and Evolution.

“This solves a long-standing question about a very serious disease in the ocean,” said Rebecca Vega Thurber, a marine microbiologist at the University of California, Santa Barbara, who was not part of the study.

For years, scientists believed the cause might be a virus, particularly a densovirus. But further research revealed that this virus exists in healthy sea stars too, and wasn’t responsible for the illness.Researchers had also missed the real cause earlier because they mainly studied dead sea stars, which no longer had the internal fluid needed for proper analysis. This time, scientists focused on coelomic fluid, the liquid inside living sea stars, and found the harmful bacteria there.“It’s incredibly difficult to trace the source of environmental diseases, especially underwater,” said microbiologist Blake Ushijima from the University of North Carolina, who wasn’t involved in the study. He called the research “really smart and significant.”

Now that the cause is known, scientists believe they can start efforts to protect and restore sea star populations.

According to Melanie Prentice, co-author of the study, researchers can now test which sea stars are still healthy, explore breeding in captivity, and move healthy individuals to areas where the population has collapsed. They may also test whether some sea stars have natural immunity, and whether probiotics could help protect them.

This work is essential not just for the sea stars, but for the entire marine ecosystem. Sunflower sea stars are known for eating sea urchins, which helps keep their numbers in check.

Since the sea stars have vanished, sea urchins have taken over, destroying about 95% of kelp forests in Northern California within ten years. These kelp forests are often called the “rainforests of the ocean,” and they provide food and shelter for fish, sea otters, seals, and many other marine species.

“Sunflower sea stars look sort of innocent when you see them,” said Gehman, “but they eat almost everything that lives on the bottom of the ocean. They’re voracious eaters.”

Now, with this new breakthrough, scientists hope to bring sea star numbers back, and help restore the Pacific’s kelp forests.

Inputs from AP

Structural analysis

The geometrical parameters of the host molecules, [6]CPPAs and [6]CPPDs, and their corresponding host-guest complexes have been meticulously analyzed to elucidate the structural attributes governing their interactions with various guest molecules, carbon nanotubes, and fullerenes (C₆₀ and C₇₀). These analyses are crucial for optimizing the design and functionality of these supramolecular assemblies. As previously designed, the optimized structures of both [6]CPPAs and [6]CPPDs were re-optimized, as shown in Figure. 2. The optimized structures of [6]CPPAs and [6]CPPDs reveal distinct geometrical differences, primarily driven by the substitution of acetylene (C ≡ C) units in [6]CPPAs with azobenzene (N = N) units in [6]CPPDs. This substitution deviates from perfect circularity in the molecular rings, particularly affecting the long-axis measurements. In [6]CPPAs, the long axis is measured at 13.25 Å, whereas in [6]CPPDs, it is slightly reduced to 12.15 Å, reflecting the increased rigidity and reduced conformational flexibility associated with the nitrogen-nitrogen bond in [6]CPPDs. Similarly, the short-axis measurements further corroborate the structural impact of the azobenzene substitution, with [6]CPPAs having a short axis of 12.89 Å and [6]CPPDs slightly lower at 11.81 Å. These dimensions indicate that due to their planar nature and steric bulk, the azobenzene units cause a slight contraction of the ring structure in [6]CPPDs, resulting in a more elliptical and less circular geometry than [6]CPPAs.

A detailed analysis by Ali et al. shows the importance of understanding the strain energy (SE) and enthalpy of the formation of [6]CPPDs¹⁹. Their finding is of utmost importance in explaining the slight structural distortions observed in [6]CPPDs compared to [6]CPPAs. It helps contextualize the geometrical differences between these molecules, underlining the significance of our research in supramolecular chemistry.

To validate the accuracy of these geometrical observations, the findings were rigorously compared with previous computational and experimental data, which consistently aligned with established trends in guest encapsulation efficiencies and intermolecular distances⁴¹. The detailed analysis of critical geometrical parameters, including radius, long axis, and short axis, provides a comprehensive understanding of how the structural modifications between [6]CPPAs and [6]CPPDs influence their ability to interact with and encapsulate guest molecules are tabulated in Supporting Information Table S4. These optimized geometries serve as the basis for designing host-guest complexes, where each guest molecule is individually optimized under the same computational level as the hosts (Fig. 3). The study reports that the cavity size of [6]CPPAs, approximately 13.3 Å in diameter, is nearly ideal for encapsulating C₆₀, while [6]CPPDs, with a slightly smaller cavity, still effectively encapsulates C₆₀ but with a slightly altered binding energy and interaction dynamics.

The geometry optimization of guest systems, including fullerenes (C₆₀ and C₇₀) and CNTs, reveals that the host’s structural parameters largely govern their encapsulation within the host molecules. The experimental diameters of CNT(5,5) and fullerene (C₇₀) are consistent with the optimized values, falling within 6.6-6.9 Å and 7-7.2 Å, respectively (6.80 Å and 7.01 Å)^46,47. The study by Zhao et al. emphasizes the importance of size selectivity in these interactions, where the alignment and overlap of π-systems play a crucial role in the strength of the host-guest binding²⁵. The interfacial length, defined as the distance between the centroids of the phenyl rings of the host and the nearest carbon atoms of the guest, emerges as a critical determinant of interaction strength. For instance, in the C₆₀@[6]CPPAs complex, the interfacial length is measured at 3.1 Å, indicative of π-π interactions. In contrast, the C₆₀[6]@CPPDs complex exhibits a slightly longer interfacial length of 3.2 Å. An important finding from the nano-Saturn study, a seminal work in molecular encapsulation, is the relationship between the interaction angle and binding energy⁴⁸. The study notes that the closer the angle between the plane of the host and the guest molecule is ~ 90°, the stronger the π-π interactions, leading to more incredible binding energy. This is relevant for the [6]CPPAs and [6]CPPDs systems, where the orientation of the guest molecule within the host cavity can significantly influence the stability and strength of the interaction​.

The distance between the centers of the host and guest molecules (d_c-c) is another critical parameter that provides insight into the binding affinity and structural alignment within these complexes. In [6]CPPAs-based complexes, particularly those involving (5,5)CNT and fullerene C₆₀, the d_c-c distances are notably shorter, indicating a robust binding affinity and efficient encapsulation of the guest molecules. Conversely, in [6]CPPDs-based complexes, the d_c-c distances are slightly longer, suggesting a less intimate interaction, which may affect the overall stability and strength of the host-guest complex. These variations in dc-c distances underscore the importance of precise geometrical alignment in determining the effectiveness of molecular encapsulation and the resulting interaction strength.

The analysis of size selectivity within the host molecules further reinforces the differences between [6]CPPAs and [6]CPPDs. For example, among the CNT guests within [6]CPPAs, the CNT(5,5) configuration exhibits the closest binding affinity, followed by CNT(4,4) and CNT(3,3), highlighting the size-selective nature of the [6]CPPAs ring. In contrast, within the [6]CPPDs host, the CNT(4,4) guest is optimally encapsulated, while more prominent guests such as CNT(5,5) and C₆₀ demonstrate significant embedding, as evidenced by minimal d_c-c distances indicative of near-complete encapsulation. Notably, the complexation of C₇₀ within [6]CPPDs adopts a “ball in a bowl” configuration, where the guest molecule is positioned above the host ring, leading to limited interaction compared to the more intimate and pronounced interactions observed in the [6]CPPAs counterpart. This configuration suggests that [6]CPPDs may be better suited for guest molecules not requiring complete encapsulation, potentially due to the ring strain and steric effects introduced by the azobenzene substitution. The broader interfacial distances observed in the C₇₀@[6]CPPDs complex further support this interpretation, indicating that the azobenzene units may disrupt the ideal π-π stacking interactions necessary for strong host-guest binding. Our findings align with established trends in guest encapsulation efficiencies and intermolecular distances, reinforcing the robustness of our computational approach and affirming the reliability of predicted host-guest interactions. The complete encapsulation of the smaller molecule, HMB, in both [6]CPPAs and [6]CPPDs complexes is a concrete demonstration of their effective size discrimination capabilities without inducing structural deformations (see the Supporting Information Table S5 and Table S6).

Thermodynamic and kinetic stability

The thermodynamic data regarding the encapsulation of CNTs, C₆₀, and C₇₀ by the hosts, computed at the M06-2X/6-31G(d) level of theory, are tabulated in Table S7 of Supporting information and shown in Fig. 4. The analysis of ΔG(298 K) and ΔS values alongside ΔE_cp. values reveals a consistent pattern, underscoring the meticulousness and reliability of the thermodynamic parameters analyzed. The results obtained for complexes involving [6]CPPAs are in close agreement with previously reported findings, affirming the robustness of the M06-2X optimization with dispersion correction. In CNT@[6]CPPAs complexes, all CNT species exhibit heightened stability and adequate size selectivity within the host ring. The (3,3)@[6]CPPAs complex exhibits the weakest binding (ΔE = −20.8 kcal/mol) among them. The stability improves significantly in the (4,4)@[6]CPPAs complex (ΔE = −31.17 kcal/mol), indicating that a slight increase in nanotube diameter enhances interaction strength. The (5,5)@[6]CPPAs complex exhibits the strongest interaction (ΔE = −42.39 kcal/mol), confirming that larger CNTs achieve better stabilization within the CPPA ring. The increasing negative entropy values (ΔS) across these complexes further support the idea that host-guest interactions stabilize the system while restricting configurational flexibility. In contrast, CNT encapsulation within [6]CPPDs follows a different binding pattern due to the structural rigidity and electrostatic contributions introduced by nitrogen incorporation. The (3,3)@[6]CPPDs complex exhibits moderate stability (ΔE = −27.37 kcal/mol), indicating that electrostatic effects slightly enhance binding despite steric constraints. Among the [6]CPPDs complexes, the encapsulation of CNT(4,4) stands out with a notable interaction energy of −41.31 kcal·mol⁻¹, comparable to the stability observed in (5,5)@[6]CPPAs complexes. However, the (5,5)@[6]CPPDs complex shows a positive Gibbs free energy (ΔG = + 21.71 kcal/mol), indicating non-spontaneous binding due to severe steric hindrance.

The encapsulation of fullerenes within [6]CPPAs is primarily stabilized by π-π stacking and van der Waals interactions. The C₆₀@[6]CPPAs complex exhibits moderate binding affinity (ΔG = −19.95 kcal/mol), with interaction strength influenced by guest curvature and host adaptability. Despite its partial immersion, the C₇₀@[6]CPPAs complex exhibits more favorable complexation (ΔG = −25.19 kcal/mol), suggesting that a larger guest surface area enhances host-guest interactions. The results indicate that C₇₀ is more effectively stabilized within [6]CPPAs than C₆₀, likely due to enhanced dispersion interactions and a better geometric fit.

Fullerene complexation within [6]CPPDs presents greater steric hindrance, resulting in less favorable thermodynamics. The C₆₀@[6]CPPDs complex exhibits a highly unfavorable Gibbs free energy (ΔG = + 30.42 kcal/mol), confirming that the size mismatch between the host and guest leads to destabilized interactions. Conversely, C₇₀@[6]CPPDs forms a more stable complex (ΔG = −19.20 kcal/mol), indicating that it is better accommodated within the CPPDs cavity than C₆₀. The partial immersion of C₇₀@[6]CPPDs, as indicated by d_i−i distances, results in lower interaction energy. Notably, C₇₀ forms a stable “ball in a bowl” complex more effectively than fully immersed CNT(5,5) and C₆₀, a feature attributed partly to CH.π interactions⁴⁵. The proximity of host and guest centers enhances crucial interactions that stabilize the complex and increase the overall complexation energy in size-selective complexes. The binding becomes more spontaneous as the CNT(5,5) goes further from the ring center. Similarly, the ball-in-a-bowl configuration of C₆₀ has an interaction energy comparable to that of the [6]CPPAs.

The thermodynamic trends observed in these host-guest systems underscore the distinct binding preferences of [6]CPPAs and [6]CPPDs. While [6]CPPAs exhibit a size-dependent increase in stability, favoring larger guests such as CNT(5,5) and fullerenes, [6]CPPDs demonstrate a preference for medium-sized guests like CNT(4,4), with larger guests experiencing steric destabilization. The instability of (5,5)@[6]CPPDs and C₆₀@[6]CPPDs suggests that alternative binding models, such as the hand-lever and ball-in-a-bowl configurations, play a crucial role in stabilizing host-guest interactions when steric limitations are present. Figure 5 depicts geometry-optimized structures of these newly designed complexes, such as the “hand-lever” model of (5,5)@[6]CPPDs and the ball-in-a-bowl structure of C₆₀@[6]CPPDs. These structural models highlight how host-guest interactions manifest, influencing stability and functional properties in supramolecular chemistry.

By analyzing the binding constants (K_a) at various temperatures, we can infer critical thermodynamic properties, shedding light on the factors influencing the stability and behavior of these molecular complexes. The temperature dependence of the equilibrium constants, as seen in Fig. 6, reveals that the host-guest interactions in both systems are exothermic, as indicated by the increasing K_a values at lower temperatures. This trend aligns with the expected behavior of exothermic processes, where lower thermal energy (lower temperatures) favors stronger binding interactions, leading to higher equilibrium constants. This analysis ties into the established Gibbs free energy, showing that the negative free energy change is directly related to a more spontaneous binding process. A more negative Gibbs free energy signifies a higher equilibrium constant, reinforcing that the interactions are thermodynamically favored as temperature decreases.

Interaction between complex

Non-covalent forces govern the molecular mechanisms driving these interactions, particularly van der Waals interactions and π-π stacking. These non-covalent interactions are vital in stabilizing the complexes in host-guest systems such as [6]CPPAs or [6]CPPDs rings interacting with fullerenes or CNTs. To better understand thermodynamics and kinetics stability, we analyzed Intermolecular interaction based on scatter plots of the reduced electron density. The hostile regions in the scatter plot correspond to bonding interactions such as hydrogen bonding. In this case, significant van der Waals interactions or π-π stacking occur near the zero region of the plot. The vicinity with a red color or positive value in the scatter plot indicates steric repulsion. The scatter plots for (5,5)@[6]CPPAs, (5,5)@[6]CPPDs, C₆₀@[6]CPPDs, C₇₀@[6]CPPAs, C₇₀@[6]CPPDs are shown in Fig. 7 and for and for (3,3) and (4,4)CNTs, are shown in Supporting information Figure S1 to S6. For the (3,3)@[6]CPPAs complex, van der Waals attraction forces are prominent on the distorted side. The discontinuation or breakage of density can be attributed to the geometrical distortion from the center. The part of the nanotube far from the host experiences no interactive forces, suggesting that van der Waals forces alone are responsible for the observed distortion. Additionally, repulsion between the nanotube and the benzene rings of the host ring is also apparent. Within the CNT, steric repulsion is evident.

In the scatter plot of the [6]CPPDs counterpart, ring-based steric repulsion is prominent, as indicated by the positive values. The denser gradient of van der Waals interaction is closer to the region where the CNT(4,4) and [6]CPPAs rings are nearby, likely causing the complex’s distortion. Here, repulsion within the tube is also observed. There is a slight density decrease in the (4,4)@[6]CPPDs complex where the guest is far from the host, suggesting that the main interacting forces are van der Waals interactions and steric repulsion. Notably, the [6]CPPDs complex exhibits more van der Waals interaction than its [6]CPPAs counterpart.

The RDG analysis of (5,5)@[6]CPPAs and (5,5)@[6]CPPDs reveals distinct non-covalent interaction patterns governing their stability. In (5,5)@[6]CPPAs, van der Waals interactions dominate, with a balanced charge distribution and minimal steric repulsion, leading to more substantial host-guest accommodation. In contrast, (5,5)@[6]CPPDs exhibit stronger steric repulsion and asymmetric charge localization, primarily due to nitrogen-induced electrostatic effects. The scatter plots confirm that (5,5)@[6]CPPDs have more repulsive interactions, leading to weaker binding (ΔG = 21.71 kcal/mol) compared to (5,5)@[6]CPPAs (−36.68 kcal/mol). The increased rigidity of [6]CPPDs limits its ability to encapsulate larger guests like CNT(5,5), whereas [6]CPPA’s flexible framework allows for better host-guest interactions. C₆₀@nanohoop has a uniform distribution of forces that help the guest hold within the host. Except for the repulsion from the benzene rings in [6]CPPAs and the aversion among the carbon atoms in C₆₀, no other significant repulsive forces are acting on this complex. This could be the reason why it is floating over the ring. A powerful steric repulsion has been found in the case of C₆₀@[6]CPPDs. The almost symmetric immersion of C₆₀ in the host might be due to this repulsion; the strong aversion among themselves and the ring might not have allowed the guest to immerse in complete symmetry. Even though Van der Waals’s force is seen in the plot, the repulsive forces dominate. Notice that the ball in a bowl model of C₆₀@[6]CPPDs has more vital π-π concave-convex interaction than the immersed one. Also, the bonding interaction is sharper. This could be the reason for the floating model’s higher complexation energy and stable kinetics. Furthermore, nitrogen substitution in the [6]CPPDs systems introduces additional thermodynamic and electrostatic considerations. Nitrogen in the diazene moieties of [6]CPPDs nanorings creates regions of partial positive charge, which can engage in electrostatic interactions with the fullerene or CNT guests. The “Hand-lever” configuration for CNT (5,5) also benefits from nitrogen-induced polarizability, resulting in higher K_eq values and more robust binding as temperature decreases. Nitrogen enhances the host’s ability to stabilize larger guest molecules through inductive effects, increasing the binding strength and favorability of the host-guest interactions⁴⁹.

The π-π interactions also contribute significantly to the binding, as these interactions arise from the overlap of delocalized π-electron systems between the [6]CPPAs or [6]CPPDs host and the guest molecule. This overlap is particularly effective when the guest is a fullerene or CNT with large, conjugated π-systems. The thermodynamic stability of these interactions increases as the temperature decreases, allowing for closer proximity and more pungent π-π stacking between the host and guest molecules⁵⁰.

The QTAIM molecular graphs of the CNT host-guest complexes offer crucial insights into the nature and extent of non-covalent interactions governing their supramolecular behavior, as shown in Fig. 8(for 5,5, C₆₀ and C₇₀) and Supporting Information Figures S7 and S8 (for 33, 4,4). For the CNT(3,3) encapsulated in [6]CPPAs, a clear pattern of bond critical points (BCPs) emerges, with orange bond paths connecting the carbon atoms of the cyclic host to those of the guest nanotube. These interactions are symmetrically distributed around the nanotube axis, confirming a coaxial alignment and supporting stable π–π stacking interactions between the curved π-systems. As the CNT diameter increases to (4,4), a corresponding increase in the number and distribution of BCPs is observed, suggesting improved spatial complementarity with the [6]CPPAs ring. The bond paths in this complex are more radially distributed, covering the guest’s outer surface more evenly, which indicates a more extensive and uniform interaction zone. This shift strengthens the host-guest binding and aligns with energetic trends that favor CNT(4,4) over CNT(3,3) encapsulation.

This trend culminates in the (5,5)@[6]CPPAs complex, which displays the most extensive and continuous BCPs network among the [6]CPPAs-hosted systems. The bond paths cover both the equatorial and axial regions of the guest, forming a cylindrical shell of interactions that reflects an almost ideal dimensional match between the host cavity and the nanotube. This configuration represents the most stable [6]CPPAs-based complex, confirming that optimal geometric alignment enhances non-covalent stabilization.

Transitioning to the nitrogen-containing [6]CPPDs systems, a comparable pattern is noted in the (3,3)@[6]CPPDs complex. Here, incorporating diazene (N = N) units introduces electronic asymmetry to the host, leading to a skewed distribution of BCPs. These bond paths still prominently connect the host’s inner wall to the nanotube. However, they are slightly denser near nitrogen-rich zones, suggesting localized electrostatic stabilization in addition to π–π interactions. Notably, compared to (3,3)@[6]CPPAs, the [6]CPPDs host enhances directional interactions due to its polarizable π-surface, possibly improving selectivity and responsiveness.

In the case of the (4,4)@[6]CPPDs complex, the QTAIM graph reveals more peripheral and asymmetric BCP distribution, reflecting the looser fit of the slightly larger CNT within the host cavity. Nonetheless, nitrogen atoms once again appear to guide and enhance localized interactions, especially where the curvature matches most closely. The diazene units seemingly compensate for reduced van der Waals overlap by facilitating polarization-induced stabilization. This behavior reflects the improved thermodynamic favorability of this complex over its CPPA counterpart, as seen in the calculated ΔG and binding energy values.

However, the (5,5)@[6]CPPDs complex presents an intriguing divergence. Although its molecular graph shows numerous BCPs connecting the nanotube and the host, indicating significant non-covalent contact, the overall interaction is thermodynamically unfavorable. Both the counterpoise-corrected interaction energy and ΔG are positive, suggesting that geometric strain, repulsion, or curvature mismatch negates the stabilizing effect of the observed electron density pathways. This highlights a key insight: multiple BCPs do not necessarily imply thermodynamic favorability. In this case, the rigidity and polarity introduced by the N = N linkers in the [6]CPPDs host may hinder its ability to conform to more prominent guests, such as the CNT(5,5), despite an apparent topological connection.

This limitation is resolved in a modified configuration, the “hand-lever” (5,5)@[6]CPPDs complex, where the nanotube sits above the ring rather than fully encapsulated. The QTAIM graph here shows an asymmetric but rich network of BCPs, and more importantly, the interaction is thermodynamically favorable. The negative ΔE_cp. and ΔG values suggest that this partial immersion strikes an optimal balance between dispersive interactions and steric relief. The flexibility imparted by the diazene linkers allows the host to adapt to this non-standard geometry, leading to a stable, albeit unconventional, host-guest interaction. This adaptation reflects the nuanced role of electronic and geometric tuning in supramolecular assembly. It underscores the potential of nitrogen-doped systems for hosting a broader range of molecular guests through flexible or semi-open configurations.

In the case of C₆₀@CPPAs, the visualization exhibits a highly symmetric cage-like interaction, where the fullerene is centrally positioned within the macrocyclic ring. This central alignment results in evenly distributed BCPs radiating from the surface of C₆₀ to the inner wall of [6]CPPAs. These uniformly spaced bond paths suggest isotropic van der Waals interactions and π–π stacking, stabilizing the encapsulated guest via weak but extensive dispersion forces. The radial symmetry of the BCP network confirms strong encapsulation with minimal directional bias, reflecting optimal shape complementarity between the spherical guest and the cyclic host.

By contrast, the encapsulation of C₇₀ within [6]CPPAs yields a distinctly asymmetric QTAIM signature. The ellipsoidal shape of C₇₀ introduces curvature mismatch, resulting in an uneven distribution of BCPs. These interactions are most concentrated along the “waist” of the C₇₀ molecule, its equatorial region where the surface comes closest to the host ring. Fewer or more elongated bond paths are observed at the poles, underscoring reduced interaction strength due to geometric misalignment. This partial asymmetry aligns with the characteristic floating configuration reported for similar systems, where the fullerene guest sits deeper into the ring without complete immersion. Despite the asymmetry, these interactions remain strong, and the thermochemical data corroborate this, showing favorable binding energies and spontaneous encapsulation for both C₆₀ and C₇₀ in [6]CPPAs.

Transitioning to [6]CPPDs-based hosts, the encapsulation dynamics shift significantly. The molecular graph of C₆₀@CPPDs displays a network of bond paths similar in spatial arrangement to that of C₆₀@[6]CPPAs, indicating the geometric feasibility of encapsulation. However, despite the presence of multiple BCPs, the counterpoise-corrected interaction energy and Gibbs-free energy for this complex are both positive, indicating that the encapsulation of C₆₀ in [6]CPPDs is thermodynamically unfavorable. This disconnect between topology and energetics likely arises from the electronic influence of the diazene (N = N) units in the [6]CPPDs framework. While they introduce dipolar and potentially interactive sites, they also disrupt uniform dispersion interactions or induce steric/electrostatic repulsions that negate the stabilizing effects of π–π stacking. Thus, although electron density bridges exist between host and guest atoms, they do not correspond to energetically viable complexation.

This contrast becomes even more striking when comparing the regular C₆₀@[6]CPPDs complex with its “ball-in-a-bowl” variant, where the fullerene is elevated slightly above the central axis of the ring. In this configuration, QTAIM analysis reveals a directional, bowl-like pattern of BCPs focused predominantly on one hemisphere of the C₆₀, specifically the lower portion interfacing closely with the [6]CPPDs ring. This spatial reorganization reduces steric congestion and maximizes interaction efficiency without inducing substantial strain on the host. The thermodynamic data affirm this, with both ΔE_cp. and ΔG being negative, indicating spontaneous and favorable complexation. The elevated position of the fullerene increases the entropy of the system by reducing confinement and facilitates a broader interaction surface. This configuration offers a unique potential for dynamic applications, including stimuli-responsive molecular shuttles or controlled capture–release systems, where partial insertion could aid in reversibility or guest mobility.

The case of C₇₀@[6]CPPDs showcases the most robust and favorable interaction among all fullerene complexes studied. Here, the QTAIM graph exhibits an extensive and dense network of BCPs, mainly clustered along the equatorial region of the C₇₀ guest. This enhanced contact zone matches well with the wider midsection of the ellipsoidal fullerene, reflecting size and shape complementarity. The nitrogen atoms in [6]CPPDs amplify host-guest affinity by increasing local polarizability and fine-tuning the electronic environment to match the fullerene’s π-surface. This synergy results in both a topologically rich interaction profile and highly favorable thermodynamics, as evidenced by the negative ΔEcp and ΔG values for C₇₀@[6]CPPDs. Compared to the C₆₀-based complexes, this system stands out for its superior compatibility and encapsulation efficiency, emphasizing the role of host flexibility and heteroatom doping in optimizing supramolecular architectures.

The influence of guest encapsulation on the aromatic character of [6]CPPAs and [6]CPPDs hosts is clearly delineated through a combined assessment of NICS(0), isotropic, and anisotropic values (see Supporting Information Table S8). In [6]CPPAs-based complexes with carbon nanotubes, the aromatic system demonstrates notable resilience. The (3,3)@[6]CPPAs complex shows an isotropic NICS value of ~ 0.27 ppm and high anisotropy of ~ 37.39 ppm, indicating intact π-delocalization. As guest diameter increases, a progressive rise in isotropic values is observed, 7.55 ppm for CNT(4,4) and 9.78 ppm for CNT(5,5), accompanied by decreasing anisotropy, reflecting growing disruption to the aromatic ring currents. These results highlight moderate electronic perturbation while preserving the characteristic aromaticity of CPPAs. In contrast, the CPPDS host exhibits a stronger response to CNT encapsulation. The NICS(0) values increase sequentially from 6.80 ppm for (3,3)@CPPDS to 10.46 ppm for the fully inserted (5,5)@[6]CPPDs complex, clearly indicating enhanced π–π interaction and severe aromatic disruption. Interestingly, a partial-insertion “hand-lever” conformation of the CNT(5,5) tube results in a near-zero NICS(0) value (0.0075 ppm), underscoring the importance of spatial overlap in influencing electronic delocalization.

A similar yet more pronounced effect is observed in the fullerene series. In [6]CPPAs complexes, both C₆₀ and C₇₀ induce substantial electronic perturbation, with isotropic NICS values of 8.96 and 28.04 ppm, respectively, and low anisotropy values (2.18 and 8.66 ppm), signifying aromaticity loss and weakened ring currents. The [6]CPPDs host responds even more dramatically to central fullerene inclusion: C₆₀@[6]CPPDs exhibits the highest NICS(0) value in the entire series (12.11 ppm), followed by C₇₀@[6]CPPDs (10.07 ppm), affirming strong aromaticity suppression due to deep guest insertion and extensive π-contact. In striking contrast, a “ball-in-a-bowl” conformation of C₆₀ within [6]CPPDs yields a markedly negative NICS(0) of − 5.83 ppm, suggesting preservation or even enhancement of aromatic character due to optimal curvature matching and minimal electronic disturbance. Overall, these trends reveal a clear relationship between guest geometry and host aromaticity: systems with deeper encapsulation and stronger π–π interactions exhibit substantial aromaticity loss, while geometrically favorable, non-invasive conformations retain or amplify aromatic delocalization. The data collectively support the utility of NICS metrics, particularly when combining isotropic and anisotropic values,in capturing the nuanced interplay between structure and electronic behavior in supramolecular complexes.

Electrostatic potential analysis

A molecule’s electrostatic potential (ESP) provides deep insights into its interaction energy, especially when considering a unit charge at a specific position in the studied system, excluding charge transfer and polarization effects. The ESP and HOMO-LUMO gap in these host-guest systems reveals a nuanced interplay of electronic stability, photoresponsivity, and structural compatibility²³. Additionally, incorporating ESP analysis into current systems enhances understanding of these interactions by shedding light on the interplay between the molecular structures and their surrounding environment, providing a holistic view of the forces governing host-guest complexes. By mapping ESP, we visualized electron density distributions, with red areas representing electron-dense regions and blue areas indicating electron-deficient regions. This color scaling ranges from highly electron-dense regions (red) to moderately dense (yellow/green) and electron-poor areas (blue). These color-coded maps provide a detailed understanding of where electrophilic or nucleophilic interactions are most likely. For instance, in Fig. 9, the ESP diagram of the free guest molecule and free host molecules reveals the regions of high electron density (−0.0295 eV) in red, while the areas of low electron density (+ 0.02953 eV) are in blue. The [6]CPPAs exhibit strong electron localization at the acetylene bridges, creating regions of high negative charge density, which suggests stronger π-π stacking or dipole interactions with guests. In contrast, [6]CPPDs show a more uniform charge distribution, with less intense electron density at the diazene bridges, indicating weaker but more evenly distributed binding interactions. Among the free CNTs, the CNT(3,3) displays strong electron-rich regions, suggesting localized electrostatic interactions but potential strain upon complexation. The CNT(4,4) exhibits a more balanced charge distribution, allowing for stabilized interactions, while the CNT(5,5) has a uniform charge distribution, implying van der Waals-driven interactions rather than electrostatic effects. For the fullerenes, C₆₀ shows minimal charge redistribution, relying on van der Waals interactions, whereas C₇₀ exhibits, more significant electroredistribution, indicating possible charge transfer interactions upon binding.

Moving to Figs. 10 and 11, which depict the ESP diagrams of the complex@[6]CPPAs and complex@[6]CPPDs, respectively, we observe similar patterns of electron distribution. In both host systems, the phenyl rings exhibit moderate electron density, while the hydrogen atoms on the rim of the rings serve as centers for electrophilic interactions. The bridges formed by multiple bonds between the phenyl rings are the most electron-rich areas, resulting in an electronegative interior within the rings. However, in nitrogen-containing [6]CPPDs rings, the electron-rich centers are more aligned, suggesting that incorporating electron-deficient moieties into these loops would yield favorable interactions. The nitrogen atoms in the [6]CPPDs rings also enhance the basicity of the phenyl hydrogen atoms compared to [6]CPPAs, further affecting the host-guest interactions.

The distortion observed in the geometry of (3,3)@host can be directly attributed to the strong interaction between the electron-rich acetylene bridges and the carbon nanotube guest. The lower atom count in the nanotube results in a stronger attraction, leading to structural strain within the host. Interestingly, the region where the guest is more attracted shows less electron density at the diazene bond than the rest of the host ring. This consistent electron cloud density suggests that the interactions between the host and guest might not involve significant π-π interactions or electron transfer. Instead, these interactions may be driven by weak van der Waals forces or hydrogen bonding. This conclusion could be supported by further analysis of frontier molecular orbitals and reduced density gradient plots.

Similarly, in the (4,4)@[6]CPPDs complex, although the geometry is not as distorted as in the (3,3)@[6]CPPDs complex, the diazene bond’s electron richness is still comparable to the host ring’s. However, the guest’s center in CNT (4,4) has a higher electron density than CNT(3,3), likely due to the more significant number of atoms in the structure. This trend continues in the CNT(5,5) complexes. In the hand-lever model of (5,5)@[6]CPPDs, there is a significant decrease in electron density around the ring and an increase in basicity. The asymmetric leaning of the CNT(5,5) guest over the host ring suggests a shift in the potential surface to achieve stable complexation. This observation aligns with findings from CNT complexes, where the host ring’s electron density remained unchanged except for the elevated guest.

In complexes like C₇₀@[6]CPPDs, the ESP analysis indicates a notable reduction in the potential distribution across the host ring, suggesting the involvement of both host and guest orbitals in complex formation. This could potentially indicate a charge transfer interaction. However, further analysis of the frontier molecular orbitals is necessary to confirm these findings.

The trends observed in electron density variations and potential shifts within host-guest complexes also align well with established interactions in supramolecular chemistry. For instance, the study of size-related π-π overlap phenomena in various ring sizes, especially in C₇₀ complexes, shows that the overlap occurs more prominently as the ring size decreases. The lone pairs from the nitrogen atoms in [6]CPPDs contribute to the ring current, enhancing the photoresponse.

Band gap analysis

In addition to ESP analysis, the HOMO-LUMO gaps of the host-guest complexes provide further insights into these systems’ electronic stability and photoresponsivity. The HOMO-LUMO gap or both complex@[6]CPPAs and complex@[6]CPPDs is shown in Fig. 12, and values are tabulated in Supporting Information Table S9.

For the CNT complexes in [6]CPPAs, the (3,3)@[6]CPPAs complex has a band gap of 2.09 eV, indicating a moderate level of electronic delocalization. As the nanotube size increases, the band gap decreases, with (4,4)@[6]CPPAs exhibiting a band gap of 2.2 eV and (5,5)@[6]CPPAs showing the lowest gap of 1.86 eV among the CNT-[6]CPPAs complexes. This trend suggests that larger guest molecules facilitate stronger host-guest interactions, leading to greater orbital overlap and charge delocalization.

A similar trend is observed in CNT complexes in [6]CPPDs but with consistently lower band gaps. The (3,3)@[6]CPPDs complex exhibits a band gap of 1.4 eV, significantly lower than its CPPA counterpart. The (4,4)@[6]CPPDs complex follows with a slightly increased gap of 1.49 eV, while the (5,5)@[6]CPPDs complex also maintains a narrow gap of 1.4 eV. The reduced band gaps across these complexes indicate greater charge mobility and electronic coupling within the [6]CPPDs framework.

For the fullerene complexes in [6]CPPAs, the C₆₀@[6]CPPAs complex has a band gap of 2.33 eV, while the C₇₀@[6]CPPAs complex exhibits a slightly lower band gap of 2.25 eV. This reduction in band gap suggests that C70, with its larger surface area, interacts more effectively with the [6]CPPAs host, leading to greater electronic delocalization. The fullerene complexes in [6]CPPDs follow a similar pattern but with consistently lower band gaps. The C₆₀@[6]CPPDs complex exhibits a band gap of 1.89 eV, while the C₇₀@[6]CPPDs complex has the lowest band gap among the fullerene-[6]CPPDs systems at 1.99 eV. This trend suggests that fullerenes exhibit better electronic coupling with the [6]CPPDs host, further reinforcing the impact of nitrogen incorporation on charge transfer properties. The Frontier Molecular Orbitals are visualized and plotted in Supporting Information Figures S9 to S22.TD-DFT calculations were done to predict the UV-spectrum, and λ_max analysis for hosts and host-guest complexes are provided in the Supporting Information (Figures S23 to S27). The λmax values for each complex are shown in Fig. 12.

A clear trend is that [6]CPPDs complexes generally exhibit higher λ_max values than their [6]CPPA counterparts, meaning [6]CPPDs absorb light at longer wavelengths. The highest λmax values are observed in (3,3)@[6]CPPDS (845 nm) and (5,5)@[6]CPPDS (847 nm), indicating that steric and electrostatic effects contribute to shifting absorption toward the near-infrared (NIR) region. The higher λ_max (718 nm) in the hand-lever model arises from charge delocalization, nitrogen-induced electrostatic effects, and enhanced π-π interactions, making this conformation more electronically active and optically responsive compared to the fully encapsulated (5,5)@[6]CPPDs complex. Similarly, in [6]CPPAs, (3,3)@[6]CPPAs (598 nm) and (5,5)@[6]CPPAs (670 nm) exhibit high λmax values, confirming that size-dependent electronic interactions significantly influence optical transitions. The fullerene complexes (C₆₀@[6]CPPAs, C₇₀@[6]CPPAs, C₆₀@[6]CPPDs, and C₇₀@[6]CPPDs) exhibit relatively consistent λ_max values (~ 540–655 nm), suggesting that their electronic transitions are primarily governed by π-π stacking and weak van der Waals interactions rather than significant charge transfer effects. Notably, the ball-in-a-bowl C₆₀@[6]CPPDs complex (636 nm) exhibits a slightly lower λmax than the standard C₆₀@[6]CPPDs (655 nm), reinforcing the hypothesis that binding mode plays a role in modulating electronic transitions.

Size-Selective and photoresponsive characters

In general, in the case of CNT complexes with [6]CPPAs, a size-dependent trend is observed, whereas larger CNT guests exhibit more substantial stabilization. Among the CNT@[6]CPPAs complexes, (5,5)@[6]CPPAs demonstrate the most favorable binding (ΔG = −36.68 kcal/mol, K_a = 3.1 × 10⁶), indicating a highly spontaneous interaction, followed by (4,4)@[6]CPPAs (ΔG = −26.69 kcal/mol, K_a = 1.4), which also exhibit strong thermodynamic preference. In contrast, (3,3)@[6]CPPAs (ΔG = −13.55 kcal/mol, K_a = 1.3 × 10⁻⁹) shows the weakest binding among the CNT complexes, suggesting that smaller CNTs do not fit as effectively within the [6]CPPAs cavity. The trend indicates that encapsulation becomes more efficient as the CNT size increases, leading to more stable host-guest interactions. In the case of CNT encapsulation within [6]CPPDs, a different trend follows, where intermediate-sized CNT guests exhibit the strongest stabilization, particularly (4,4)CNT. The (4,4)@[6]CPPDs complex shows the most favorable binding (ΔG = −38.56 kcal/mol, K_a = 7.8 × 10⁸), suggesting robust and selective binding. Meanwhile, (3,3)@[6]CPPDs (ΔG = −22.92 kcal/mol, K_a = 9.2 × 10⁻³) exhibit moderate stability but are significantly lower than (4,4). A notable deviation is observed in (5,5)@[6]CPPDS, which shows a positive ΔG (21.71 kcal/mol, K_a=8.72 × 10⁻³⁷), making its encapsulation thermodynamically unfavorable. This instability is likely due to steric hindrance. However, the hand-lever configuration in CNT(5,5) is thermodynamically more stable than the former (ΔG = −14.72 kcal/mol, K_a = 2.45 × 10⁻¹⁰), which is not fully immersed within the [6]CPPDs cavity.

In the case of fullerene@[6]CPPAs complexes, C₇₀@[6]CPPAs (ΔG = −25.19 kcal/mol, K_a =1.1 × 10⁻¹) exhibits stronger binding than C₆₀@[6]CPPAs (ΔG = −19.95 kcal/mol, K_a =1.1 × 10⁻¹), suggesting that C₇₀ fits better within the [6]CPPAs cavity and forms stronger π-π interactions. For fullerene@[6]CPPDs complexes, a striking difference is observed compared to [6]CPPAs. C₆₀@[6]CPPDs exhibits a positive ΔG (30.42 kcal/mol, K_a = 2.07 × 10⁻⁴²) making its encapsulation thermodynamically unfavorable. This suggests that steric hindrance and electronic factors prevent efficient binding within the rigid CPPDs cavity. The ball-in-a-bowl configuration observed for C₆₀@[6]CPPDs complexes suggests an alternative binding mode, where steric repulsion prevents complete encapsulation, but concave-convex interactions contribute to stabilization (ΔG = −18.10 kcal/mol, K_a = 6.58 × 10⁻⁷). In contrast, C₇₀@[6]CPPDs shows a more favorable interaction (ΔG = −19.20 kcal/mol, K_a= 4.94 × 10⁻⁰⁶), indicating that C₇₀ is better accommodated within [6]CPPDs than C₆₀. However, it still exhibits weaker stability compared to its [6]CPPAs counterpart. This implies that [6]CPPDs are less efficient than CPPAs in stabilizing fullerene guests, likely due to their electronic distribution and steric rigidity.

The ESP maps of [6]CPPAs host-guest complexes reveal key charge distribution patterns that influence binding strength, stability, and interaction mechanisms. The (3,3)@[6]CPPAs complex exhibits high electron localization at the acetylene bridges, with visible red regions indicating areas of strong electron density accumulation. However, the overall charge distribution remains more localized, suggesting that interactions are limited to specific areas rather than being uniformly spread across the host-guest interface. Moving to (4,4)@[6]CPPAs, the electron density becomes more balanced, with a noticeable reduction in highly localized charge regions, indicating an improved interaction between the host and guest. The (5,5)@[6]CPPAs complex exhibits the most uniform charge distribution, with reduced red regions and broader yellow-green areas, suggesting that the interaction is more delocalized across the entire system. This implies that π-π interactions are maximized in this configuration, leading to a more stable electronic structure.

For the fullerene@[6]CPPAs complexes, the ESP maps reveal significant electron redistribution at the host-guest interface. In C₆₀@[6]CPPAs, the charge density remains relatively uniform, with moderate electron accumulation near the fullerene surface, indicating van der Waals-driven interactions. However, in C₇₀@[6]CPPAs, more pronounced red regions appear along the fullerene-host interface, suggesting more excellent charge localization and stronger electrostatic interactions between the host and guest. This enhanced electron density alignment may facilitate improved orbital overlap, making the interaction stronger compared to C₆₀.

The ESP maps reveal charge distribution variations in [6]CPPDs host-guest complexes, highlighting differences in binding strength, interaction mechanisms, and structural adaptability. In (3,3)@[6]CPPDs, strong electron localization at the acetylene bridges suggests high electrostatic attraction, similar to CPPA, but with less strain due to nitrogen-induced charge alignment. As the guest size increases, (4,4)@[6]CPPDs exhibit balanced electron density distribution, leading to stable host-guest interactions. The electrostatic component weakens in (5,5)@[6]CPPDs, indicating a transition to van der Waals-driven interactions. The hand-lever model of (5,5)@[6]CPPDs further reduces host-guest electrostatic attraction, displaying an asymmetric charge shift to attain thermodynamic feasibility.

C₆₀@[6]CPPDs show minimal charge redistribution for fullerene complexes, confirming that van der Waals forces dominate encapsulation. However, stronger charge localization around C₆₀ suggests better binding affinity due to enhanced electrostatic stabilization in the ball-in-a-bowl model. In C₇₀@[6]CPPDs, significant charge redistribution and reduced electron density near the host ring indicate stronger π-π interactions and possible charge transfer effects, making it the most electronically responsive complex.

For (3,3)@[6]CPPAs, van der Waals forces dominate, particularly on the distorted side, while steric repulsion between the CNT and benzene rings prevents complete encapsulation. The (4,4)@[6]CPPDs complex exhibits stronger van der Waals interactions than its CPPAs counterpart, but steric repulsion within the ring leads to structural distortion. In (5,5)@[6]CPPAs, van der Waals interactions and minimal steric repulsion enable effective host-guest accommodation, supported by its flexible framework and firm binding. In contrast, (5,5)@[6]CPPDs exhibit higher steric repulsion and asymmetric charge localization due to nitrogen-induced electrostatic effects, leading to weaker binding. The hand-lever model in [6]CPPDs benefits from nitrogen-induced polarizability, enhancing electrostatic interactions and improving host-guest stabilization, making it a viable alternative for CNT encapsulation.

In fullerene complexes, C₆₀@[6]CPPAs show a uniform distribution of attractive forces, ensuring stable encapsulation with minimal repulsion. In contrast, C₆₀@[6]CPPDs exhibits strong steric repulsion, preventing symmetric immersion. The ball-in-a-bowl model of C₆₀@[6]CPPDs enhances π-π concave-convex interactions, leading to higher complexation energy and improved kinetic stability. Overall, [6]CPPDs exhibit stronger van der Waals interactions but greater steric repulsion than [6]CPPAs, affecting host-guest accommodation and stability. The findings reinforce how electronic effects, steric hindrance, and weak interactions drive supramolecular behavior in these systems.

A key observation found via HOMO-LUMO analysis is that [6]CPPDs generally exhibit lower band gaps (1.4–1.99 eV) compared to [6]CPPAs (1.86–2.33 eV), suggesting that [6]CPPDs are more electronically active and better suited for optoelectronic applications. Among the [6]CPPDs complexes, (3,3)@[6]CPPDs and (5,5)@[6]CPPDs exhibit the lowest band gaps (1.4 eV), indicating high electronic activity but reduced thermodynamic stability due to steric repulsion and weaker host-guest interactions. In [6]CPPAs, (5,5)@[6]CPPAs have the lowest band gap (1.86 eV), confirming that larger guests induce stronger host-guest interactions, leading to better electronic coupling. Additionally, the hand-lever model of (5,5)@[6]CPPDs exhibits an even lower band gap of 1.35 eV, suggesting that this alternative binding configuration enhances charge delocalization, potentially altering electronic properties in a way that could influence device applications. Meanwhile, the ball-in-a-bowl model of C₆₀@[6]CPPDs has a slightly higher band gap (1.95 eV) compared to the standard C₆₀@[6]CPPDs (1.89 eV), reinforcing the idea that concave-convex interactions enhance charge stabilization, affecting the electronic structure.

While [6]CPPAs primarily stabilize guests based on size compatibility, [6]CPPDs exhibit a greater influence from charge redistribution and host polarity, affecting the complexes’ overall stability and electronic behavior. These findings establish a fundamental contrast between the two host systems, with [6]CPPAs favoring geometric fit and [6]CPPDs displaying significant charge-driven interactions, influencing host-guest complexation trends. The comparison among these host-guest complexes is summarised and tabulated in Supporting Information Table S10.

The contrasting host–guest behaviors observed in [6]CPPAs and [6]CPPDs highlight their potential as functional materials in advanced supramolecular systems. The enhanced electronic activity of [6]CPPDs, particularly in configurations such as the hand-lever and ball-in-a-bowl complexes, accompanied by their reduced band gaps and localized charge redistribution, points to their utility in photoresponsive encapsulation–release systems and stimuli-controlled molecular switches, key components in molecular machines and innovative delivery platforms^51,52. The significant host-guest interaction anisotropy and tunable π–π interactions in nitrogen-rich CPPDs also pave the way for their integration into organic optoelectronic devices, including organic photovoltaics and molecular semiconductors, where their curvature and electronic modulation play crucial roles in charge transport and exciton dynamics⁵³. Recent studies have demonstrated the potential of nitrogen-doped cycloparaphenylenes for CO₂ capture through tunable non-covalent interactions, reporting significant complexation energies and spectroscopic responsiveness⁵⁴. In this context, our comparative investigation of [6]CPPAs and [6]CPPDs with fullerene and carbon nanotube guests offers compelling insights into how macrocyclic architecture and electronic modulation via diazene linkages can influence binding affinity, electron distribution, and interaction topology. The enhanced interaction anisotropy and polarizability observed in [6]CPPDs suggest that these systems could be extended to applications in selective gas adsorption or molecular sensing, where curvature, charge distribution, and responsive encapsulation are critical. The integration of energetic, topological, and electronic descriptors in this study demonstrates that [6]CPPDs, alongside their CPPAs counterparts, can be strategically developed as multifunctional platforms for applications in molecular electronics, sensing, and supramolecular nanotechnology. The broader implications of this research point to [6]CPPDs as a promising class of host molecules with the potential for various applications, such as molecular sensing, drug delivery, and optoelectronic devices. Findings from this study contribute to a deeper understanding of supramolecular chemistry and open avenues for developing advanced materials tailored for specific applications.

SpaceX’s Crew-10 astronauts will head home to Earth today (Aug. 7), and you can watch the action live.

The Crew-10 quartet’s Crew Dragon capsule, named Endurance, is scheduled to undock from the International Space Station (ISS) today at 12:05 p.m. EDT (1605 GMT) and splash down 24 hours later.

Large ocean animals can be protected throughout much of their lifecycle by huge Marine Protected Areas (MPAs), new research shows.

Scientists tracked sea turtles, manta rays and seabirds – all of which travel far and wide to forage, breed and migrate – in the Chagos Archipelago MPA in the Indian Ocean.

In total, 95% of tracking locations were recorded inside the MPA’s 640,000 square kilometre area – suggesting it is large enough to protect these wandering animals.

The study – by a team including Exeter and Heriot-Watt universities and ZSL – also assessed the impact of a smaller 100,000 square kilometre MPA and found seabirds would be less well protected in this scenario.

“Very large Marine Protected Areas (VLMPAs) are seen as essential for meeting international goals, such as the target for 30% protection by 2030,” said Dr Alice Trevail , from the Environment and Sustainability Institute at the University of Exeter’s Penryn Campus in Cornwall.

“However, the conservation value of VLMPAs – defined as anything over 100,000 square kilometres – is debated.

“Our results provide clear evidence for the value of the Chagos Archipelago VLMPA for protecting a diverse range of large and mobile marine species.”

The researchers used tracking data on hawksbill turtles, reef manta rays and three seabird species: red-footed boobies, brown boobies and wedge-tailed shearwaters.

“These large animals play a variety of important roles in marine ecosystems,” said co-author Dr Ruth Dunn, from Heriot-Watt University.

“For example, the Chagos Archipelago supports a huge number of seabirds, and the guano (droppings) from these birds help to fertilise coral reefs and other marine species.”

In their assessment of a hypothetical smaller VLMPA (100,000 km2), the team found 97% of manta and 94% of turtle locations would still be in protected waters.

However, just 59% of all seabird locations would be inside the MPA because they travel over a larger area.

With the anticipated change in sovereignty, as the Chagos Archipelago becomes part of Mauritius, the study’s findings are increasingly important. While providing compelling evidence for the value of the MPA, Dr Dunn said that they also indicate areas that are priorities for future long-term protection to ensure the viability of this marine megafauna community.

Ernesto Bertarelli, President of the Bertarelli Foundation – funders of the study – commented: “Discoveries like this are only possible when scientists from different disciplines work together. By doing so, this team of researchers has shown how truly large Marine Protected Areas can provide vital protection to vulnerable species throughout their lives.”

The paper, published in the Journal of Applied Ecology, is entitled: “Large marine protected areas can encompass movements of diverse megafauna.”

Wondering what you’re seeing when you look up at the moon tonight? Wonder no more, we’ve got all the information you need about the current lunar cycle.

What’s the lunar cycle, you ask? This is a series of eight unique phases of the moon’s visibility. The whole cycle takes about 29.5 days, according to NASA, and these different phases happen as the Sun lights up different parts of the moon whilst it orbits Earth. 

So, what’s happening with the moon tonight, Aug. 7?

What is today’s moon phase?

As of Thursday, Aug. 7, the moon phase is Waxing Gibbous. According to NASA’s Daily Moon Observation, the moon will be 96% lit up tonight.

There’s lots to see tonight, with your unaided eye alone, enjoy a glimpse of the Copernicus Crater, the Mare Fecunditatis, and the Oceanus Procellarum. With binoculars, you’ll also be able to see the Mare Humorum, the Archimedes Crater, and the Clavius Crater.

With a telescope, look towards the left (right if you’re in the Southern Hemisphere) to see the Reiner Gama and right (vice versa again) for the Rima Ariadaeus and Apollo 16.

When is the next full moon?

The next full moon will be on August 9. The last full moon was on July 10.

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What are moon phases?

According to NASA, moon phases are caused by the 29.5-day cycle of the moon’s orbit, which changes the angles between the Sun, Moon, and Earth. Moon phases are how the moon looks from Earth as it goes around us. We always see the same side of the moon, but how much of it is lit up by the Sun changes depending on where it is in its orbit. This is how we get full moons, half moons, and moons that appear completely invisible. There are eight main moon phases, and they follow a repeating cycle:

New Moon – The moon is between Earth and the sun, so the side we see is dark (in other words, it’s invisible to the eye).

Waxing Crescent – A small sliver of light appears on the right side (Northern Hemisphere).

First Quarter – Half of the moon is lit on the right side. It looks like a half-moon.

Waxing Gibbous – More than half is lit up, but it’s not quite full yet.

Full Moon – The whole face of the moon is illuminated and fully visible.

Waning Gibbous – The moon starts losing light on the right side.

Last Quarter (or Third Quarter) – Another half-moon, but now the left side is lit.

Waning Crescent – A thin sliver of light remains on the left side before going dark again.

A neurochemistry journal has retracted a paper that claimed prenatal vaccine exposure caused autism-like behaviors in rats. The retraction comes more than a year after the paper’s publication and amid criticism of its methodology.

The paper, which was originally published 10 January 2024 in Neurochemical Research, purported to show reduced sociability in rats born to mothers that received a human-sized dose of a COVID-19 mRNA vaccine while pregnant. Anti-vaccine advocates touted the result online. In addition, the work has been cited four times, according to Clarivate’s Web of Science.

But comments posted on social media and PubPeer since the paper’s publication have raised several questions about the work, including the dose of vaccine given to the rats, the proprietary software used for the analysis and the oddly similar data shown for different experimental conditions.

The dose is “so large, it seems like it’s maybe not done from the perspective of trying to do unbiased science,” Brian Lee, professor of epidemiology and biostatistics at Drexel University, told The Transmitter.

According to the 19 July 2025 retraction notice, a “post-publication review found inconsistencies in the number of subjects reported in the Methods and raw data. The Editor-in-Chief therefore no longer has confidence in the presented data.”

This is the second retraction for Mümin Alper Erdoğan, associate professor in the department of physiology at İzmir Katip Çelebi University. It is the first retraction for each of the other three authors, according to the Retraction Watch database. Only Erdoğan replied to The Transmitter’s emailed requests for comment. “I believe this decision was unjust and warrants open discussion,” he wrote, referring to the vaccine study retraction. Erdoğan did not respond to a follow-up request for an interview.

C

oncerns regarding this study appeared on PubPeer in January 2024, when sleuth Kevin Patrick, who posts under the pseudonym “Actinopolyspora biskrensis,” noted a variety of inconsistencies. Patrick has previously flagged issues with other papers by the same authors.

Patrick first pointed out that some of the error bars in Figure 2 were uneven, which wouldn’t happen if they were generated with the software the authors claimed to have used, he says. He also raised concerns about the authors’ AI-based behavioral analysis system, Scove Systems, that they used in the study. When the article was first published, the website for the software would not open, and there was very little information available publicly about the software.

Patrick says he wanted to see some evidence that the software works. Otherwise, “it’s a black box… we don’t know if any of the behavioral measurements that they use in that device are valid at all,” he says.

Patrick says he shared his concerns with the journal in January 2024.

At that time, the journal began an investigation and determined that no further action was needed, according to an email to The Transmitter from Tim Kersjes, head of research integrity and resolutions for Springer Nature.

In February 2025, sleuth and Columbia University mouse behavioralist Mu Yang posted on PubPeer under the pseudonym “Dysdera arabisenen” about two bar graphs, also in Figure 2, that were seemingly identical, even though they purportedly represented different data.

The data in both graphs derive from a three-chamber sociability test, in which the rat being tested is put in the middle of three connected chambers. The test is designed to measure different facets of sociability, Yang says.

One of the graphs in question claimed to provide evidence that rats preferred a chamber with another rat over an empty chamber, and the other that they preferred an unfamiliar rat to a familiar one.

“The two figures almost look identical…[but] these are very different scenarios,” Yang told The Transmitter.

Yang says the chambers were also surprisingly small—not much larger than chambers she and others routinely use in mouse experiments, despite rats being much larger animals.

“I think the peer review process failed awfully,” Patrick says. “Whoever read this paper, editors and the peer reviewers, obviously made no attempt to understand what was going on.” 

The journal became aware of these concerns in February and opened a new investigation, “including conducting a post-publication review of the raw data provided by the authors at that time,” Kersjes said. “After carefully considering the facts of this latest investigation, we concluded that retracting the paper was the correct action to take to maintain the validity of the scientific record.”

That’s the conclusion reached by a Rutgers-led team of researchers, whose recent study of fossil teeth from the extinct predator Dissacus praenuntius reveals how animals adapted to a period of extreme climate change known as the Paleocene-Eocene Thermal Maximum (PETM). The findings, published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, could help scientists predict how today’s wildlife might respond to modern global warming.

“What happened during the PETM very much mirrors what’s happening today and what will happen in the future,” said Andrew Schwartz, a doctoral student in the Department of Anthropology at the School of Arts and Sciences, who led the research. “We’re seeing the same patterns. Carbon dioxide levels are rising, temperatures are higher and ecosystems are being disrupted.”

Associate Professor Robert Scott of the Department of Anthropology is a co-author of the study.

Schwartz, Scott and another colleague used a technique called dental microwear texture analysis to study the tiny pits and scratches left on fossilized teeth. These marks reveal what kinds of food the animal was chewing in the weeks before it died.

The ancient omnivore was about the size of a jackal or a coyote and likely consumed a mix of meat and other food sources like fruits and insects. “They looked superficially like wolves with oversized heads,” Schwartz said, describing them as “super weird mammals.” “Their teeth were kind of like hyenas. But they had little tiny hooves on each of their toes.”

Before this period of rising temperatures, Dissacus had a diet similar to modern cheetahs, eating mostly tough flesh. But during and after this ancient period, its teeth showed signs of crunching harder materials, such as bones.

“We found that their dental microwear looked more like that of lions and hyenas,” Schwartz said. “That suggests they were eating more brittle food, which were probably bones, because their usual prey was smaller or less available.”

This dietary shift happened alongside a modest reduction in body size, likely because of food scarcity. While earlier hypotheses blamed shrinking animals on hotter temperatures alone, this latest research suggests that limited food played a bigger role, Schwartz said.

This period of rapid global warming lasted about 200,000 years, but the changes it triggered were fast and dramatic. Schwartz said studies of the past like his can offer practical lessons for today and what comes next.

“One of the best ways to know what’s going to happen in the future is to look back at the past,” he said. “How did animals change? How did ecosystems respond?”

The findings also highlight the importance of dietary flexibility, he said. Animals that can eat a variety of foods are more likely to survive environmental stress.

“In the short term, it’s great to be the best at what you do,” Schwartz said. “But in the long term, it’s risky. Generalists, meaning animals that are good at a lot of things, are more likely to survive when the environment changes.”

Such an insight may be helpful for modern conservation biologists, allowing them to identify which species today may be most vulnerable, he said. Animals with narrow diets, such as pandas, may struggle as their habitats shrink. But adaptable species, including jackals or raccoons, might fare better.

“We already see this happening,” Schwartz said. “In my earlier research, jackals in Africa started eating more bones and insects over time, probably because of habitat loss and climate stress.”

The study also showed that rapid climate warming as seen during the ancient past can lead to major changes in ecosystems, including shifts in available prey and changes in predator behavior. This may suggest that modern climate change could similarly disrupt food webs and force animals to adapt, or risk extinction, he said.

Even though Dissacus was a successful and adaptable animal that lived for about 15 million years, it eventually went extinct. Scientists think this happened because of changes in the environment and competition from other animals, Schwartz said.

Schwartz conducted his research using a combination of fieldwork and lab analysis, focusing on fossil specimens from the Bighorn Basin in Wyoming, a site with a rich and continuous fossil record spanning millions of years. Schwartz chose the location because it preserves a detailed sequence of environmental and ecological changes during the ancient period of climate warming.

Schwartz has been interested in paleontology, specifically dinosaurs, since he was a boy, journeying with his father, an amateur fossil hunter, on treks through New Jersey’s rivers and streams. Now, as a late-stage doctoral student, he hopes to use ancient fossils to answer urgent questions about the future.

He also wants to inspire the next generation of researchers.

“I love sharing this work,” he said. “If I see a kid in a museum looking at a dinosaur, I say, ‘Hey, I’m a paleontologist. You can do this, too.’”

In addition to Schwartz and Scott, Larisa DeSantis of Vanderbilt University is an author of the study.