Fullerenes with the maximum Clar number

The Clar number of a fullerene is the maximum number of independent resonant hexagons in the fullerene. It is known that the Clar number of a fullerene with n vertices is bounded above by [n/6]-2. We find that there are no fullerenes whose order n is congruent to 2 modulo 6 attaining this bound. In other words, the Clar number for a fullerene whose order n is congruent to 2 modulo 6 is bounded above by [n/6]-3. Moreover, we show that two experimentally produced fullerenes C80:1 (D5d) and C80:2 (D2) attain this bound. Finally, we present a graph-theoretical characterization for fullerenes, whose order n is congruent to 2 (respectively, 4) modulo 6, achieving the maximum Clar number [n/6]-3 (respectively, [n/6]-2)

The topology of fullerenes

Fullerenes are carbon molecules that form polyhedral cages. Their bond structures are exactly the planar cubic graphs that have only pentagon and hexagon faces. Strikingly, a number of chemical properties of a fullerene can be derived from its graph structure. A rich mathematics of cubic planar graphs and fullerene graphs has grown since they were studied by Goldberg, Coxeter, and others in the early 20th century, and many mathematical properties of fullerenes have found simple and beautiful solutions. Yet many interesting chemical and mathematical problems in the field remain open. In this paper, we present a general overview of recent topological and graph theoretical developments in fullerene research over the past two decades, describing both solved and open problems. WIREs Comput Mol Sci 2015, 5:96–145. doi: 10.1002/wcms.1207 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website

Fullerenes and their Nonlinear Optical Properties

Nonlinear optical properties of fullerenes are reviewed aifter a brief overview of their other physical properties. Available information on synthesis and purification of various fullerenes and fullerene derivatives is given first. This is followed by a review of their atomic and electronic Structure, and experimental results on their linear and nonlinear optical response Finally, we summarize theoretical results on their hyperpolarizobility in the background of available experimental information

Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.Comment: Review on superhard B-C-N-O phase

Comparing C-60 and C-70 as acceptor in organic solar cells : Influence of the electronic structure and aggregation size on the photovoltaic characteristics

The difference in aggregation size of the C-60 and C-70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly [2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C-70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 degrees C is approximately twice bigger when the C-60 is the acceptor. We attribute this effect to the tendency of C-60 in form smaller aggregate domains relatively to the C-70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C-60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C-60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C-60.Peer reviewe

ARCHITECTURE-ELECTRONIC PROPERTY RELATIONS ACROSS MOLECULAR SEMICONDUCTOR INTERFACES

Multicomponent organic films have increasing applications in photovoltaic technologies and other electronic devices. These applications depend strongly on the structural and electronic properties of the heterojunctions. This thesis reports a detailed investigation of these important aspects, such as structure control and structural-electronic correlation in molecular film heterojunctions, for two selected "donor-acceptor" model systems (TiOPc-C60 and TiOPc-C70) using STM/STS. The UHV-STM studies were started on a single component system, TiOPc deposited on Ag(111). Along with increasing deposition flux, TiOPc selectively forms three distinct ordered monolayer structures, namely honeycomb phase, hexagonal phase, and a misfit dislocation triangular network. Localized electrostatic intermolecular interactions can be utilized to stabilize kinetically accessible structures and cause different phase structures formed on surface. Molecular packing models for these phases are proposed based on STM measurements. By choosing different TiOPc monolayer phases as template for sequential C60 deposition, low-dimensional monolayer TiOPc-C60 interfaces have been prepared on Ag(111) and characterized with STM/STS. Thermally stable honeycomb and metastable hexagonal TiOPc templates rearrange upon C60 deposition to yield several binary film structures in the monolayer regime. These structures include phase-segregated TiOPc and C60 domains and co-crystalline TiOPc(2)C60(1) honeycomb network formed through a dynamic process balanced by intermolecular and molecular-substrate interactions. The least stable TiOPc phase, the dislocation network, turns out to be the most robust template for sequential C60 growth by forming nanophase-segregated TiOPc-C60 on the scale of 10 nm. The variations of C60 energy gap across the heterointerface created by depositing C60 on hexagonal TiOPc are evaluated with STS. Energy level shift on TiOPc-C60 co-crystal domain boundary is identified. This energy shift is correlated to an electron transport barrier from donor material (TiOPc) to acceptor (C60) in practical OPV cells. C70-TiOPc heterostructures are characterized and compared with those of C60. C70 present a greater variety of molecular configurations and related properties than those of C60because of the ellipsoid shape with lower symmetry and higher dipole polarizability. C70deposited on TiOPc honeycomb phase shows completely different growth mode from that of C60. The TiOPc honeycomb structure, functionalized as a dipole buffer layer, plays a substantial role on sequential C70 growth up to the fourth layer. Simple geometric effect and dipole-induced dipole interactions are considered to rationalize the intriguing C70a growth mode. The structural model for each layer is proposed. By employing fullerenes (C60 or C70) and TiOPc thin films as model system, I investigated the controlled formation of donor-acceptor molecular film architecture, measured the orientation and separation of donor-acceptor molecules along the domain boundaries, and correlated the structural information with the electronic structural information. These systematical works shed light on the optimization of molecular electronic devices from a fundamental microscopic perspective