Fullertubes: A 30-Year Story of Prediction, Experimental Validation, and Applications for a Long-Missing Family of Soluble Carbon Molecules

ConspectusDuring the last 30 years, theoretical scientists imagined segmental families of monolayer carbon tubules with fullerene-based end-caps. These fullertube molecules would possess structural features of both fullerenes (hemispherical end-caps) and tubular belts of single-walled carbon nanotubes (SWCNTs). Yet, their experimental verification remained elusive for decades. It was not until 2020-2023 that segmental families of fullertubes were finally confirmed in the lab. The shocking irony is that these fullertubes were unwittingly coproduced alongside fullerenes (e.g., C60, C70, C84) in both flame and electric arc soot since the 1990s. Yet, nobody knew these "hidden" families of fullertubes were experimentally present in their extracted soot due to their low abundance and the absence of isolation methodology.This eruption of fullertube discoveries in 2020-2023 was brought to fruition by structural data, both DFT and experimental. This "Treasure Trove" of new molecules during this four-year window occurred with only microgram quantities. Typically, milligram levels of purified samples are required for X-ray crystallography and 13C NMR structural analysis. The breakthrough for experimentally verifying the missing fullertubes was an aminopropanol reagent to selectively react with and remove spheroidal carbon (e.g., C60, C70, C84) as hydrophilic derivatives. In contrast, there was suppressed reaction with fullertubes, which remained in organic solvent. It is well established that high symmetry (3-, 5-, and 6-fold) hemispheres for C60-Ih and other fullerenes and metallofullerenes are prerequisite end-caps for fullertubes. For the case of [5,5] C130 fullertubes, this requirement results in only eight 3-, 5-, and 6-fold symmetry structural isomers possible from a total of 39,393 possible isolated pentagon rule (IPR) isomers. From this C130 list of 8 candidate isolated pentagon rule (IPR) high symmetry isomers, surprisingly only one structure matched the DFT polarizability versus chromatographic retention parameter (a new gold standard for isomer identification). The simultaneous emergence of DFT computations of other properties (e.g., total energy, HOMO-LUMO gap, UV-vis) for large carbon molecules provided support for structural determination. Experimental approaches (e.g., mass spectrometry, UV-vis, XPS, Raman, and STEM) provided additional layers of structural elucidation at the microgram level. For the first time, we developed a chemical isolation protocol that would allow the preparation and isolation of soluble pristine fullertubes in the range of C90-C200. To date, applications of SWCNTs for use in nanoscale computer applications requires purities greater than 99.999%. Although this stringent mandate has not yet been demonstrated using SWCNT samples, this high level of purity appears achievable for metallic [5,5] D5d-C120 and semiconductor [10,0] D5h-C120 [10,766] fullertubes. Moreover, commercial production of pristine fullertubes should easily be feasible by the flame method due to its continuous operation and inexpensive feedstock. For application development, theoretical and electrochemical experimental data show that fullertubes exhibit high catalytic activity in oxygen reduction reactions. In the medical sector, pristine fullertube dispersions exhibit antimicrobial effects on Mycobacterium smegmatis and M. abscessus.

Fullerene-Functionalized Halogen-Bonding Heteroditopic Hosts for Ion-Pair Recognition

Despite their hydrophobic surfaces with localized π-holes and rigid well-defined architectures providing a scaffold for preorganizing binding motifs, fullerenes remain unexplored as potential supramolecular host platforms for the recognition of anions. Herein, we present the first example of the rational design, synthesis, and unique recognition properties of novel fullerene-functionalized halogen-bonding (XB) heteroditopic ion-pair receptors containing cation and anion binding domains spatially separated by C60. Fullerene spatial separation of the XB donors and the crown ether complexed potassium cation resulted in a rare example of an artificial receptor containing two anion binding sites with opposing preferences for hard and soft halides. Importantly, the incorporation of the C60 motif into the heteroditopic receptor structure has a significant effect on the halide binding selectivity, which is further amplified upon K+ cation binding. The potassium cation complexed fullerene-based receptors exhibit enhanced selectivity for the soft polarizable iodide ion which is assisted by the C60 scaffold preorganizing the potent XB-based binding domains, anion-π interactions, and the exceptional polarizability of the fullerene moiety, as evidenced from DFT calculations. These observations serve to highlight the unique properties of fullerene surfaces for proximal charged guest binding with potential applications in construction of selective molecular sensors and modulating the properties of solar cell devices.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

Data-driven tailoring of molecular dipole polarizability and frontier orbital energies in chemical compound space

Understanding correlations - or lack thereof - between molecular properties is crucial for enabling fast and accurate molecular design strategies. In this contribution, we explore the relation between two key quantities describing the electronic structure and chemical properties of molecular systems: the energy gap between the frontier orbitals and the dipole polarizability. Based on the recently introduced QM7-X dataset, augmented with accurate molecular polarizability calculations as well as analysis of functional group compositions, we show that polarizability and HOMO-LUMO gap are uncorrelated when considering sufficiently extended subsets of the chemical compound space. The relation between these two properties is further analyzed on specific examples of molecules with similar composition as well as homooligomers. Remarkably, the freedom brought by the lack of correlation between molecular polarizability and HOMO-LUMO gap enables the design of novel materials, as we demonstrate on the example of organic photodetector candidates.

Highly Hydroxylated Buckminsterfullerene Complexes with an Endohedral Iodide Anion

The properties of the complexes I^-@C₆₀(OH)₂₀ and I^-@C₆₀(OH)₃₀ have been determined using the quantum chemical DFT PBE0 method to assess the possibility of synthesis of highly hydroxylated fullerene endo-iodides for the use in radiotherapy of different organs. The arrangement of the hydroxyl groups corresponds to the location of halogen atoms in (D_5d)-C₆₀F₂₀ and (D_3d)-C₆₀Cl₃₀. Negative-charged iodine is localized in the center of the fullerene cavity and has no significant influence on the IR spectra of hydroxylated fullerenes. The exothermic effect of iodide anion encapsulation in C₆₀(OH)_n (0.33 eV at n = 20; 0.03 eV at n = 30) is smaller than in C₆₀ (0.78 eV). The energies of the lower excited singlet states computed by the TD DFT method for I^-@C₆₀(OH)_n (2.11 eV at n = 20; 1.77 eV at n = 30) are larger than those for I^-@C₆₀ (1.36 eV). They indicate a low reactivity, making it possible to synthesize such complexes for medical use.

Introducing a novel C50N10 azafullerene with chained nitrogen atoms on a buckyball pole: structure, stability, vibration, and electronic properties

CONTEXT

Fullerenes are of high significance due to their unique chemical properties and various applications in technology, particularly materials science, drug delivery, electronics, and nanoelectronics. In the recent years, many attempts have been focused to introduce new heteroatom-doped fullerenes having advanced chemical properties and tunable electronic traits, which make them a potential candidate for application in many branches of sciences. In this study, a novel C₅₀N₁₀ azafullerene with a fascinating structure of chained nitrogen atoms on a buckyball pole, with different electronic and optical properties compared to its other analogs, is introduced and trace of N-N substructures on the surface of C₆₀ fullerene cage is investigated. For this molecule, four structural isomers including 3 structures with chain N atoms on a fullerene buckyball pole (NP isomers) and one isomer with separated N atoms (SN isomer) have been studied. All isomers have been studied with and without symmetry constraints, and the symmetry influence on the structure and stability of each isomer has been investigated. Although the studied NP structures have lower stability than the SN isomer, some reasons (such as their more all-carbon hexagonal rings, breaking some of their N-N bonds for partial opening of the cage and creating bigger rings in order to get rid of the unfavorable strain, as well as decreasing the N-N lone pair repulsions) lead to the acceptable stability of these structures with the bonded N atoms. The results of atomization energy and vibrational frequency calculations indicate that isomers with the bonded N atoms have acceptable stabilities and do not decompose into their constituent components. Investigation on the structural parameters demonstrates important roles of the number of all-carbon hexagonal rings, the number of N-N junction, and the molecule symmetry in the stability of the structures with the bonded N atoms. Study on the electronic and optical properties indicates that the target structures exhibit high electronic polarizability, relatively small HOMO/LUMO gap, high first- and second-order hyperpolarizability, and also large third-order nonlinear optical properties.

METHODS

All calculations have been performed using Gaussian G09 software using density functional theory (DFT) approach. Three-parameter Beck hybrid exchange functional (B3) hybridized with nonlocal correlation functional of Lee, Yang, and Parr (LYP) has been employed as the level of DFT calculations. All optimizations have been performed at double-zeta polarized (DZP) split valence 6-31G(d,p) and also at split valence TZP 6-311G(d,p) basis sets. The global minimum structures have been confirmed by frequency calculations at the same level of optimizations. The natural bond orbital (NBO) analyses, frontier orbital surfaces imaging, atomic charges, and charge transfer analyses have been achieved by GenNBO program package.

Gigantic C120 Fullertubes: Prediction and Experimental Evidence for Isomerically Purified Metallic [5,5] C120-D5d(1) and Nonmetallic [10,0] C120-D5h(10766)

We report the first experimental characterization of isomerically pure and pristine C₁₂₀ fullertubes, [5,5] C₁₂₀-D_5d(1) and [10,0] C₁₂₀-D_5h(10766). These new molecules represent the highest aspect ratio fullertubes isolated to date; for example, the prior largest empty cage fullertube was [5,5] C₁₀₀-D_5d(1). This increase of 20 carbon atoms represents a gigantic leap in comparison to three decades of C₆₀-C₉₀ fullerene research. Moreover, the [10,0] C₁₂₀-D_5d(10766) fullertube has an end-cap derived from C₈₀-I_h and is a new fullertube whose C₄₀ end-cap has not yet been isolated experimentally. Theoretical and experimental analyses of anisotropic polarizability and UV-vis assign C₁₂₀ isomer I as a [5,5] C₁₂₀-D_5d(1) fullertube. C₁₂₀ isomer II matches a [10,0] C₁₂₀-D_5h(10766) fullertube. These structural assignments are further supported by Raman data showing metallic character for [5,5] C₁₂₀-D_5d(1) and nonmetallic character for C₁₂₀-D_5h(10766). STM imaging reveals a tubular structure with an aspect ratio consistent with a [5,5] C₁₂₀-D_5d(1) fullertube. With microgram quantities not amenable to crystallography, we demonstrate that DFT anisotropic polarizability, augmented by long-accepted experimental analyses (HPLC retention time, UV-vis, Raman, and STM) can be synergistically used (with DFT) to down select, predict, and assign C₁₂₀ fullertube candidate structures. From 10 774 mathematically possible IPR C₁₂₀ structures, this anisotropic polarizability paradigm is quite favorable to distinguish tubular structures from carbon soot. Identification of isomers I and II was surprisingly facile, i.e., two purified isomers for two possible structures of widely distinguishing features. These metallic and nonmetallic C₁₂₀ fullertube isomers open the door to both fundamental research and application development.

Physics-Inspired Structural Representations for Molecules and Materials

The first step in the construction of a regression model or a data-driven analysis, aiming to predict or elucidate the relationship between the atomic-scale structure of matter and its properties, involves transforming the Cartesian coordinates of the atoms into a suitable representation. The development of atomic-scale representations has played, and continues to play, a central role in the success of machine-learning methods for chemistry and materials science. This review summarizes the current understanding of the nature and characteristics of the most commonly used structural and chemical descriptions of atomistic structures, highlighting the deep underlying connections between different frameworks and the ideas that lead to computationally efficient and universally applicable models. It emphasizes the link between properties, structures, their physical chemistry, and their mathematical description, provides examples of recent applications to a diverse set of chemical and materials science problems, and outlines the open questions and the most promising research directions in the field.

Functional Carbazole-Fullerene Complexes: A New Perspective of Carbazoles Acting as Nano-Octopus to Capture Globular Fullerenes

Fullerene host-guest constructs have attracted increasing attention owing to their molecular-level hybrid arrangements. However, the usage of simple carbazolic derivatives to bind with fullerenes is rare. In this research, three novel carbazolic derivatives, containing a tunable bridging linker and carbazole units for the capturing of fullerenes, are rationally designed. Unlike the general concave-convex interactions, fullerenes could interact with the planar carbazole subunits to form 2-dimensional hexagonal/quadrilateral cocrystals with alternating stacking patterns of 1 : 1 or 1 : 2 stoichiometry, as well as the controllable fullerene packing modes. At the meanwhile, good electron-transporting performances and significant photovoltaic effects were realized when a continuous C_60⋅⋅⋅ C₆₀ interaction channel existed. The results indicate that the introduction of such carbazolic system into fullerene receptor would provide new insights into novel fullerene host-guest architectures for versatile applications.

Molecular Collision Diameters and Electronic Polarizabilities: Inherent Relationship and Fast Evaluation

The collision diameter σ for a large set of molecular species is related to the static electronic polarizability α_el. A remarkable correlation between these quantities conceptually similar to the analogous one previously identified for atoms is revealed. Our recommended model is the function σ(α_el) = p₁ + p₂α_el^1/3, where p₁ = 0.768 Å and p₂ = 2.168 are the fitting parameters providing the best overall match to the reference data for collision diameter (181 data points). The obtained correlation allows one to easily find the collision diameter of molecules from the known polarizability and vice versa. These findings can be useful for many applications, where there is a need for inexpensive assessments of the collision diameters or electronic polarizabilities, for example, when developing the transport property databases for modeling of chemically reacting flows.

Prediction of unexpected B n P n structures: promising materials for non-linear optical devices and photocatalytic activities

In the present work, a modern method of crystal structure prediction, namely USPEX conjugated with density functional theory (DFT) calculations, was used to predict the new stable structures of B n P n (n = 12, 24) clusters. Since B12N12 and B24N24 fullerenes have been synthesized experimentally, it motivated us to explore the structural prediction of B12P12 and B24P24 clusters. All new structures were predicted to be energetically favorable with negative binding energy in the range from -4.7 to -4.8 eV per atom, suggesting good experimental feasibility for the synthesis of these structures. Our search for the most stable structure of B n P n clusters led us to classify the predicted structures into two completely distinct structures such as α-B n P n and β-B n P n phases. In α-B n P n , each phosphorus atom is doped into a boron atom, whereas B atoms form a B n unit. On the other hand, each boron atom in the β-phase was bonded to a phosphorus atom to make a fullerene-like cage structure. Besides, theoretical simulations determined that α-B n P n structures, especially α-B24P24, show superior oxidation resistance and also, both α-B n P n and β-B n P n exhibit better thermal stability; the upper limit temperature that structures can tolerance is 900 K. The electronic properties of new compounds illustrate a higher degree of absorption in the UV and visible-region with the absorption coefficient larger than 105 cm-1, which suggests a wide range of opportunities for advanced optoelectronic applications. The β-B n P n phase has suitable band alignments in the visible-light excitation region, which will produce enhanced photocatalytic activities. On the other hand, α-B n P n structures with modest band gap exhibit large second hyperpolarizability, which are anticipated to have excellent potential as second-order non-linear optical (NLO) materials.

Semiconducting and Metallic [5,5] Fullertube Nanowires: Characterization of Pristine D5h(1)-C90 and D5d(1)-C100

Although fullerenes were discovered nearly 35 years ago, scientists still struggle to isolate "single molecule" tubular fullerenes larger than C₉₀. In similar fashion, there is a paucity of reports for pristine single-walled carbon nanotubes (SWNTs). In spite of Herculean efforts, the isolation and properties of pristine members of these carbonaceous classes remain largely unfulfilled. For example, the low abundance of spherical and tubular higher fullerenes in electric-arc extracts (<0.01-0.5%) and multiplicity of structural isomers remain a major challenge. Recently, a new isolation protocol for highly tubular fullerenes, also called f ullertubes, was reported. Herein, we describe spectroscopic characterization including ¹³C NMR, XPS, and Raman results for purified [5,5] fullertube family members, D_5h-C₉₀ and D_5d-C₁₀₀. In addition, DFT computational HOMO-LUMO gaps, polarizability indices, and electron density maps were also obtained. The Raman and ¹³C NMR results are consistent with semiconducting and metallic properties for D_5h-C₉₀ and D_5d-C₁₀₀, respectively. Our report suggests that short [5,5] fullertubes with aspect ratios of only ∼1.5-2 are metallic and could exhibit unique electronic properties.

Simplified Ab Initio Molecular Dynamics-Based Raman Spectral Simulations

We describe a simplified approach to simulating Raman spectra from ab initio molecular dynamics (AIMD) calculations. The protocol relies on on-the-fly calculations of approximate molecular polarizabilities using the well-known sum over orbitals (as opposed to states) method. This approach bypasses the more accurate but computationally expensive approach to calculating molecular polarizabilities along AIMD trajectories, i.e., solving the coupled perturbed Hartree-Fock/Kohn-Sham equations. We demonstrate the advantages and limitations of our method through a few case studies targeting molecular systems of interest to surface- and/or tip-enhanced Raman spectroscopy practitioners.

Structure–property correlations of the nonlinear optical properties of a few bipodal D–π–A molecules – an experimental and theoretical approach

The nonlinear optical (NLO) properties of a series of pyran based bipodal D–π–A molecules have been studied experimentally and theoretically.

C70 Fullerene Cage as a Novel Catalyst for Efficient Proton Transfer Reactions between Small Molecules: A Theoretical study

When acids are supplied with an excess electron (or placed in an Ar or the more polarizable N₂ matrix) in the presence of species such as NH₃, the formation of ion-pairs is a likely outcome. Using density functional theory and first-principles calculations, however, we show that, without supplying an external electron or an electric field, or introducing photo-excitation and -ionization, a single molecule of HCl or HBr in the presence of a single molecule of water inside a C₇₀ fullerene cage is susceptible to cleavage of the σ-bond of the Brønsted-Lowry acid into X⁻ and H⁺ ions, with concomitant transfer of the proton along the reaction coordinate. This leads to the formation of an X⁻···⁺HOH₂ (X = Cl, Br) conjugate acid-base ion-pair, similar to the structure in water of a Zundel ion. This process is unlikely to occur in other fullerene derivatives in the presence of H₂O without significantly affecting the geometry of the carbon cage, suggesting that the interior of C₇₀ is an ideal catalytic platform for proton transfer reactions and the design of related novel materials. By contrast, when a single molecule of HF is reacted with a single molecule of H₂O inside the C₇₀ cage, partial proton transfers from HF to H₂O is an immediate consequence, as recently observed experimentally. The geometrical, energetic, electron density, orbital, optoelectronic and vibrational characteristics supporting these observations are presented. In contrast with the views that have been advanced in several recent studies, we show that the encaged species experiences significant non-covalent interaction with the interior of the cage. We also show that the inability of current experiments to detect many infrared active vibrational bands of the endo species in these systems is likely to be a consequence of the substantial electrostatic screening effect of the cage.

Accurate molecular polarizabilities with coupled cluster theory and machine learning

The molecular dipole polarizability describes the tendency of a molecule to change its dipole moment in response to an applied electric field. This quantity governs key intra- and intermolecular interactions, such as induction and dispersion; plays a vital role in determining the spectroscopic signatures of molecules; and is an essential ingredient in polarizable force fields. Compared with other ground-state properties, an accurate prediction of the molecular polarizability is considerably more difficult, as this response quantity is quite sensitive to the underlying electronic structure description. In this work, we present highly accurate quantum mechanical calculations of the static dipole polarizability tensors of 7,211 small organic molecules computed using linear response coupled cluster singles and doubles theory (LR-CCSD). Using a symmetry-adapted machine-learning approach, we demonstrate that it is possible to predict the LR-CCSD molecular polarizabilities of these small molecules with an error that is an order of magnitude smaller than that of hybrid density functional theory (DFT) at a negligible computational cost. The resultant model is robust and transferable, yielding molecular polarizabilities for a diverse set of 52 larger molecules (including challenging conjugated systems, carbohydrates, small drugs, amino acids, nucleobases, and hydrocarbon isomers) at an accuracy that exceeds that of hybrid DFT. The atom-centered decomposition implicit in our machine-learning approach offers some insight into the shortcomings of DFT in the prediction of this fundamental quantity of interest.

The Emergence of Anion-π Catalysis

The objective of this Account is to summarize the first five years of anion-π catalysis. The general idea of anion-π catalysis is to stabilize anionic transition states on aromatic surfaces. This is complementary to the stabilization of cationic transition states on aromatic surfaces, a mode of action that occurs in nature and is increasingly used in chemistry. Anion-π catalysis, however, rarely occurs in nature and has been unexplored in chemistry. Probably because the attraction of anions to π surfaces as such is counterintuitive, anion-π interactions in general are much younger than cation-π interactions and remain under-recognized until today. Anion-π catalysis has emerged from early findings that anion-π interactions can mediate the transport of anions across lipid bilayer membranes. With this evidence for stabilization in the ground state secured, there was no reason to believe that anion-π interactions could not also stabilize anionic transition states. As an attractive reaction to develop anion-π catalysis, the addition of malonic acid half thioesters to enolate acceptors was selected. This choice was also made because without enzymes decarboxylation is preferred and anion-π interactions promised to catalyze selectively the disfavored but relevant enolate addition. Concerning anion-π catalysts, we started with naphthalene diimides (NDIs) because their intrinsic quadrupole moment is highly positive. The NDI scaffold was used to address questions such as the positioning of substrates on the catalytic π surface or the dependence of activity on the π acidity of this π surface. With the basics in place, the next milestone was the creation of anion-π enzymes, that is, enzymes that operate with an interaction rarely used in biology, at least on intrinsically π-acidic or highly polarizable aromatic amino-acid side chains. Electric-field-assisted anion-π catalysis addresses topics such as heterogeneous catalysis on electrodes and remote control of activity by voltage. On π-stacked foldamers, anion-(π) _n-π catalysis was discovered. Fullerenes emerged as the scaffold of choice to explore contributions from polarizability. On fullerenes, anionic transition states are stabilized by large macrodipoles that appear only in response to their presence. With this growing collection of anion-π catalysts, several reactions beyond enolate addition have been explored so far. Initial efforts focused on asymmetric anion-π catalysis. Increasing enantioselectivity with increasing π acidity of the active π surface has been exemplified for enamine and iminium chemistry and for anion-π transaminase mimics. However, the delocalized nature of anion-π interactions calls for the stabilization of charge displacements over longer distances. The first step in this direction was the formation of cyclohexane rings with five stereogenic centers from achiral acyclic substrates on π-acidic surfaces. Moreover, the intrinsically disfavored exo transition state of anionic Diels-Alder reactions is stabilized selectively on π-acidic surfaces; endo products and otherwise preferred Michael addition products are completely suppressed. Taken together, we hope that these results on catalyst design and reaction scope will establish anion-π catalysis as a general principle in catalysis in the broadest sense.

Remote Control of Anion-π Catalysis on Fullerene-Centered Catalytic Triads

The design, synthesis and evaluation of catalytic triads composed of a central C60 fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene-fullerene-amine triads display the highest selectivity in anion-π catalysis observed so far, whereas NDI-fullerene-amine triads are not much better than fullerene-amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene-fullerene-amine triads. Activation and deactivation of the fullerene-centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self-induced, the anionic intermediates and transition states create their own anion-π catalyst.

Computational study for the circular redox reaction of N2O with CO catalyzed by fullerometallic cations C60Fe+ and C70Fe<sup/>

We applied density functional calculations to study the circular redox reaction mechanism of N₂O with CO catalyzed by fullerometallic cations C₆₀Fe⁺ and C₇₀Fe⁺. The on-top sites of six-membered rings (η⁶) of fullerene cages are the most preferred binding sites for Fe⁺ cation, and the hexagon to pentagon migration of Fe⁺ is unlikely under ambient thermodynamic conditions. The initial ion/molecule reaction, N₂O rearrangement and N₂ abstraction on the considered fullerometallic cations are easier than those on the bare Fe⁺ cation in the gas phase. Generally, our results indicate that fullerometallic ions, C₆₀Fe⁺ and C₇₀Fe⁺, are more favorable substrates for redox reaction of N₂O with CO in comparison to the other previously studied carbon nanostructures such as graphene and nanotubes.

Polarizability as a tool to determine the electrostatic shielding effect of nanocarbon cages: a polarizability distribution study on noble gas endohedral fullerenes

A noble gas atom can be treated as a sensor to determine the dielectric properties of nanocages including fullerenes.

Comment on "Fullerene-based materials for solar cell applications: design of novel acceptors for efficient polymer solar cells--a DFT study" by A. Mohajeri and A. Omidvar, Phys. Chem. Chem. Phys., 2015, 17, 22367

Though a linear correlation has been recently reported between mean polarizabilities of the fullerene derivatives and open-circuit voltages of organic solar cells based on them, we demonstrate that there is no general dependence between these two values and some related quantities (anisotropy of polarizability and LUMO levels).

Effect of particle size on the surface activity of TiC–Ni composite coating via the interfacial valence electron localization

Electron work functions and open potentials of electrodeposited Ni, TiC micro- and nano-particle-reinforced Ni matrix coatings were studied to investigate the effects of TiC particle size on the surface activity and corrosion tendency of the coatings.

Activation energies and information entropies of helium penetration through fullerene walls. Insights into the formation of endofullerenes nX@C60/70 (n = 1 and 2) from the information entropy approach

In the present study, we calculate the activation barriers and information entropies of helium penetration into the C₆₀ and C₇₀ fullerenes resulting in the singly and doubly filled endofullerenes He_n@C_60/70 (n = 1 and 2).

Interfacial valence electron localization and the corrosion resistance of Al-SiC nanocomposite

Microstructural inhomogeneity generally deteriorates the corrosion resistance of materials due to the galvanic effect and interfacial issues. However, the situation may change for nanostructured materials. This article reports our studies on the corrosion behavior of SiC nanoparticle-reinforced Al6061 matrix composite. It was observed that the corrosion resistance of Al6061 increased when SiC nanoparticles were added. Overall electron work function (EWF) of the Al-SiC nanocomposite increased, along with an increase in the corrosion potential. The electron localization function of the Al-SiC nanocomposite was calculated and the results revealed that valence electrons were localized in the region of SiC-Al interface, resulting in an increase in the overall work function and thus building a higher barrier to hinder electrons in the nano-composite to participate in corrosion reactions.

Counting the Isomers and Estimation of Anisotropy of Polarizability of the Selected C60 and C70 Bisadducts Promising for Organic Solar Cells

Currently, bisadducts of C60 and C70 fullerenes are widely studied as electron-acceptor materials for organic solar cells. These compounds are usually used as mixtures of the positional isomers. However, as recently shown, the separate use of the purified isomers with lowest anisotropies of polarizability may enhance solar cell output parameters. To predict the structures of the compounds appropriate for this purpose, we calculated anisotropies of polarizability of four classes of fullerene bisadducts, namely, bis-[60]PCBM, [60]OQMF, bis-[70]PCBM, and [70]OQMF (18, 16, 41, and 42 positional isomers, respectively). As found, the anisotropies quadratically correlate with the interaddend distances in fullerene bisadducts, whereas there are no obvious correlations between the structures and lowest unoccupied molecular orbital levels, traditionally used for assessing the efficiency of candidates for organic solar cell electron acceptors. According to our calculations, bisadducts bis-[60]PCBM-ee-1, [60]OQMF-cis-3.2, [60]OQMF-trans-4.2, cc(1.1)cc(2'.1)-bis-[70]PCBM, and cc1cc(2'.1)-[70]OQMF have the lowest anisotropies of polarizability. These compounds have a primary interest for synthesis, purification, and further separate testing in solar cells. The structures of these adducts have a common feature, which we describe with the "not so close and not so far" rule: the distances between the addends in the most isotropic fullerene bisaddicts should be medium among the possible values. These are ee, ef, cis-3, and trans-4 positions in the case of the C60 bisadducts and cc bonds placed on the different poles and the same hemisphere of the C70 skeleton.

Spin-dependent recombination probed through the dielectric polarizability

Despite residing in an energetically and structurally disordered landscape, the spin degree of freedom remains a robust quantity in organic semiconductor materials due to the weak coupling of spin and orbital states. This enforces spin-selectivity in recombination processes which plays a crucial role in optoelectronic devices, for example, in the spin-dependent recombination of weakly bound electron-hole pairs, or charge-transfer states, which form in a photovoltaic blend. Here, we implement a detection scheme to probe the spin-selective recombination of these states through changes in their dielectric polarizability under magnetic resonance. Using this technique, we access a regime in which the usual mixing of spin-singlet and spin-triplet states due to hyperfine fields is suppressed by microwave driving. We present a quantitative model for this behaviour which allows us to estimate the spin-dependent recombination rate, and draw parallels with the Majorana-Brossel resonances observed in atomic physics experiments.

Electrical detection of ortho-para conversion in fullerene-encapsulated water

Water exists in two spin isomers, ortho and para, that have different nuclear spin states. In bulk water, rapid proton exchange and hindered molecular rotation obscure the direct observation of two spin isomers. The supramolecular endofullerene H2O@C60 provides freely rotating, isolated water molecules even at cryogenic temperatures. Here we show that the bulk dielectric constant of this substance depends on the ortho/para ratio, and changes slowly in time after a sudden temperature jump, due to nuclear spin conversion. The attribution of the effect to ortho-para conversion is validated by comparison with nuclear magnetic resonance and quantum theory. The change in dielectric constant is consistent with an electric dipole moment of 0.51±0.05 Debye for an encapsulated water molecule, indicating the partial shielding of the water dipole by the encapsulating cage. The dependence of bulk dielectric constant on nuclear spin isomer composition appears to be a previously unreported physical phenomenon.

Fullerene-based materials for solar cell applications: design of novel acceptors for efficient polymer solar cells--a DFT study

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer solar cells (PSCs) hold promise for their potential to be used as low-cost and efficient solar energy converters. PSCs have been commonly made from bicontinuous polymer:fullerene composites or so-called bulk heterojunctions. The conjugated polymer donors and the fullerene derivative acceptors are the key materials for high performance PSCs. In the present study, we have performed density functional theory calculations to investigate the electronic structures and magnetic properties of several representative C60 fullerene derivatives, seeking ways to improve their efficiency as acceptors of photovoltaic devices. In our survey, we have successfully correlated the LUMO energy level as well as chemical hardness, hyper-hardness, nucleus-independent chemical shift, and static dipole polarizability of PC60BM-like fullerene derivative acceptors with the experimental open circuit voltage of the photovoltaic device based on the P3HT:fullerene blend. The obtained structure-property correlations allow finding the best fullerene acceptor match for the P3HT donor. For this purpose, four new fullerene derivatives are proposed and the output parameters for the corresponding P3HT-based devices are predicted. It is found that the proposed fullerene derivatives exhibit better photovoltaic properties than the traditional PC60BM acceptor. The present study opens the way for manipulating fullerene derivatives and developing promising acceptors for solar cell applications.

Information Entropy of Fullerenes

The reasons for the formation of the highly symmetric C60 molecule under nonequilibrium conditions are widely discussed as it dominates over numerous similar fullerene structures. In such conditions, evolution of structure rather than energy defines the processes. We have first studied the diversity of fullerenes in terms of information entropy. Sorting 2079 structures from An Atlas of Fullerenes [ Fowler , P. W. ; Manolopoulos , D. E. An Atlas of Fullerenes ; Oxford : Clarendon , 1995 . ], we have found that the information entropies of only 14 fullerenes (<1% of the studied structures) lie between the values of C60 and C70, the two most abundant fullerenes. Interestingly, buckminsterfullerene is the only fullerene with zero information entropy, i.e., an exclusive compound among the other members of the fullerene family. Such an efficient sorting demonstrates possible relevance of information entropy to chemical processes. For this reason, we have introduced an algorithm for calculating changes in information entropy at chemical transformations. The preliminary calculations of changes in information entropy at the selected fullerene reactions show good agreement with thermochemical data.

Dispersive interactions in solution complexes

Dispersive interactions are known to play a major role in molecular associations in the gas phase and in the solid state. In solution, however, their significance has been disputed in recent years on the basis of several arguments. A major problem until now has been the separation of dispersive and hydrophobic effects, which are both maximized in water due the low polarizability of this most important medium. Analyses of complexes between porphyrins and systematically varied substrates in water have allowed us to discriminate dispersive from hydrophobic effects, as the latter turned out to be negligible for complexations with flat surfaces such as porphyrins. Also, for the first time, it has become possible to obtain binding free energy increments ΔΔG for a multitude of organic residues including halogen, amide, amino, ether, carbonyl, ester, nitro, sulfur, unsatured, and cyclopropane groups, which turned out to be additive. Binding contributions for saturated residues are unmeasurably small, with ΔΔG > 1 kJ/mol, but they increase to, e.g., ΔΔG = 5 kJ/mol for a nitro group, a value not far from, e.g., that of a stacking pyridine ring. Stacking interactions of heteroarenes with porphyrins depend essentially on the size of the arenes, in line with polarizabilities, and seem to be rather independent of the position of nitrogen within the rings. Measurements of halogen derivatives indicate that complexes with porphyrins, cyclodextrins, and pillarenes as hosts in different media consistently show increasing stability from fluorine to iodine as the substituent. This, and the observed sequence with other substrates, is in line with the expected increase in dispersive forces with increasing polarizability. Induced dipoles, which also would increase with polarizability, can be ruled out as providing the driving source in view of the data with halides: the observed stability sequence is opposite the change of electronegativity from fluorine to iodine. The same holds for the solvent effect observed in ethanol-water mixtures. Dispersive contributions vary not only with the polarizability of the used media but also with the interacting receptor sites; it has been shown that for cucurbiturils the polarizability inside the cavity is extremely low, which also explains why hydrophobic effects are maximized with these hosts. Complexations with other known host compounds, however, such as those between cryptands or cavitands with, e.g., noble gases, bear the signature of dominating dispersive forces. Some recent examples illustrate that such van der Waals forces can also play an important role in complexations with proteins. Again, a clue for this is the increase in ΔG for inhibitor binding by 7 kJ/mol for, e.g., a bromine in comparison to a fluorine derivative.