Exciton dynamics in alternating polyfluorene/fullerene blends

Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.     

富勒烯形成机理的研究进展

富勒烯的形成机理多年来一直是富勒烯科学研究的焦点问题,科学家们用大量的理论计算和实验工作来探索富勒烯的形成过程,并提出了多种模型和可能机理．根据富勒烯形成途径的不同,这些机理可分为“自下而上”、“自上而下”或“先上后下”3种生长方式,但是由于缺乏足够的实验证据,至今还没有一种机理得到实验事实的完全证明并被普遍接受．本文就近年来有关富勒烯形成机理的研究进行了归纳阐述,并概述了其中影响较大的几种机理．     

Transport and recombination dynamics studies of polymer/fullerene based solar cells

We have studied the electron/hole transport and recombination dynamics in blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene], (MDMO-PPV) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) at room temperature, as a function of laser excitation density and PCBM concentration. The experimental results of these studies indicate the important role played by hole-trap states in MDMO-PPV. Electron and hole transport are not balanced within the blend. PCBM is a less disordered material than MDMO-PPV and electron transport dominates the response of the solar cell device.     

Theoretical investigation of nonadiabatic and internal temperature effects on the collision-induced multifragmentation dynamics of Na5+ cluster ions

In a continued effort to disentangle adiabatic, nonadiabatic, and internal temperature effects in the collision-induced multifragmentation of alkali-metal cluster ions at moderate energies, we report a theoretical study of this process for the Na5++He encounter in the 100 eV (center-of-mass) collision energy range. The investigation makes use of a diatomics-in-molecules based nonadiabatic molecular-dynamics (NAMD) method. All of the ten electronic 1A' molecular states of the cluster that can be formed by assembling ground-state monomers are considered explicitly. Cross sections for the corresponding 12 possible fragmentation channels are determined. As in the Na4++He case, we find that a few-channel characteristic of adiabatic fragmentation in the electronic ground state dominates. This owes primarily to the dominance of impulsive adiabatic mechanisms. Nonetheless, two significant nonadiabatic transitions take place: electronic excitation during the collision and electronic deexcitation in the postcollision stage. A large amount of the electronic excitation subsequently relaxes into the electronic ground state during the postcollision stage. This important intramolecular vibrational relaxation (IVR)-type mechanism enhances the population of channels characteristic of adiabatic fragmentation in the electronic ground state. The populations of the fragmentation channels are quite sensitive to the internal cluster temperature. This is discussed in terms of the conditions of occurrence of the fragmentation mechanisms and their competition. Comparisons with experimental results are presented and discussed.     

Above the surface multifragmentation of surface scattered fullerenes

C(60) (-) ions were scattered from a gold surface at impact energies of 80-900 eV. The C(n) (-) fragments abundance distribution (odd and even) and the sharp fragmentation threshold observed, point at a prompt shattering event. The measured angle and energy distributions of the C(n) (-) fragments (n=2-12) provide clear evidence for a multifragmentation process where the superheated fullerenes leave the surface "intact" and disintegrate away from the surface.     

Molecular dynamics simulation of the glass transition temperature of fullerene filled cis-1,4-polybutadiene nanocomposites

t In this work,the effect of the fullerene (C60) weight fraction and PB-C60 interaction on the glass transition temperature (Tg) of polymer chains has been systemically investigated by adopting the united atom model of cis-1,4-poly(butadiene) (cis-PB).Various chain dynamics properties,such as atom translational mobility,bond/segment reorientation dynamics,torsional dynamics,conformational transition rate and dynamic heterogeneity of the cis-PB chains,are analyzed in detail.It is found that Tg could be affected by the C60 weight fraction due to its inhibition effect on the mobility of the cis-PB chains.However,Tg is different,which depends on different dynamics scales.Among the chain dynamics properties,Tg is the lowest from atom translational mobility,while it is the highest from the dynamic heterogeneity.In addition,Tg can be more clearly distinguished from the dynamic heterogeneity;however,the conformational transition rate seems to be not very sensitive to the C60 weight fraction compared with others.For pure cis-PB chains,Tg and the activation energy in this work can be compared with those of other polymers.In addition,the temperature dependence of the dynamic properties has different Arrhenius behaviors above and below Tg.The activation energy below Tg is lower than that above Tg.This work can help to understand the effect of the C60 on the dynamic properties and glass transition temperature of the cis-PB chains from different scales.     

Classical molecular dynamics study of [60]fullerene interactions with silica and polyester surfaces

This study examines the interaction of neutral and charged fullerenes with model silica and polyester surfaces. Molecular dynamics simulations at 298 K indicate that van der Waals forces are sufficiently strong in most cases to cause physisorption of the neutral fullerene particle onto the surfaces. The fullerenes are unable to penetrate the rigid silica surface but are generally able to at least partially infiltrate the flexible polymer surface by opening surface cavities. The introduction of charge to the fullerene generally leads to an increase in both the separation distance and Work of Separation with silica. However, the charged fullerenes generally exhibit significantly closer and stronger interactions with polyester films, with a distinct tendency to absorb into the "bulk" of the polymer. The separation distance and Work of Separation of C60 with each of the surfaces also depend greatly on the sign, magnitude, and localization of the charge on the particle. Cross-linking of the polyester can improve resistance to the neutral fullerene. Functionalization of the polyester surface (F and OH substituents) has been shown to prevent the C60 from approaching as close to the polyester surface. Fluorination leads to improved resistance to positively charged fullerenes, compared to the unmodified polyester. However, hydroxylation generally enables greater adhesion of charged fullerenes to the surface due to H-bonding and electrostatic attraction.     

Experimental and theoretical analysis of the reorientational dynamics of fullerene C70 in various aromatic solvents

A previous study of C70 in deuterated chlorobenzene generated evidence suggesting C70 was experiencing unique reorientational behavior at given temperatures. The present study explores the possibility that this behavior is present across other solvents. The 13C spin-lattice relaxation rates for four carbon resonances in C70 were analyzed in benzene-d6, chlorobenzene-d5, and o-dichlorobenzene-d4, and as a function of temperature, to probe the reorientational dynamics of this fullerene. Anisotropic behavior was observed at the lowest (283 K) and highest temperatures (323 K), isotropic diffusion was seen between 293 and 303 K, and quasi-isotropic at 313 K. When anisotropic motion was present, diffusion about the figure axis was seen to be higher than diffusion of the figure axis. Experimentally obtained diffusion coefficients generated reorientational correlation times that were in excellent agreement with experimental values. Theoretical predictions generated by a modified Gierer-Wirtz model provided acceptable predictions of the diffusion constants; with DX usually being more closely reproduced and DZ values generally being underestimated. Overall, the results indicate that the factors affecting rotational behavior are complex and that multiple solvent factors are necessary to characterize the overall motion of C70 in these solvents. Although a solvent's viscosity is normally sufficient to characterize the tumbling motion, the spinning motion is less sensitive to solvent viscosity but more responsive to solvent structure. The balance and collective influence of these factors ultimately determines the overall rotational behavior.     

Extraction of fullerene mixture from fullerene soot with organic solvents

Investigation of extraction of fullerene mixture from the fullerene soot obtained by plasma erosion of graphite rod in helium atmosphere with different solvents such as α-chloronaphthalene, o-dichlorobenzene, o-xylene, toluene, benzene, carbon tetrachloride, and n-hexane at 25°C was carried out. Completeness and effectiveness of extraction as well as relative content of light (C₆₀, C₇₀) and heavy (C₇₆, C₇₈, C₈₄) fullerenes in the extract were evaluated.     

The effect of fullerene derivative on polaronic charge transfer in poly(3-hexylthiophene)/fullerene compound

The effect of a structure of a fullerene derivative on electronic properties of poly(3-hexylthiophene)/fullerene composite was studied at 3 cm wave band EPR in wide (77-320 K) temperature region. All the systems with different fullerene derivatives demonstrate sum spectrum of small polarons with different mobilities and contributions depending on the structure of the fullerene derivative. Both the spin-lattice and spin-spin relaxation times were determined separately by using the steady-state saturation method. The treatment of polymer matrix by fullerene derivative changes its dimensionality and conformation. This leads to the change in the spin relaxation and susceptibility as well as in the rate of polaron diffusion along polymer chain.     

Charge-transfer complexes of fullerene C70 and ternary amines in chlorobenzene. Picosecond dynamics of charge recombination

The kinetics of charge recombination in radical ion pairs C₇₀ ^–/Am⁺ (Am isN, N, NN-tetramethyl-p-phenylenediamine,p-methoxy-N,N-dimethylaniline,p-methyl-N,N-di-methylaniline,N,N-diethylaniline,N,N-dimethylaniline, and triphenylamine) in chlorobenzene was studied by the picosecond laser photolysis technique. The radical ion states are the products of excitation of charge-transfer complexes between C₇₀ and amines and are also formed by quenching of the singlet excited state of C₇₀ by the amine. The rate constant of electron transfer in the radical ion pair decreases as the free Gibbs energy (G) of the reaction increases and reflects the Marcus-inverted region of the dependence of the rate constant on G. The C₇₀/Am and C₆₀/Am systems are compared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1151–1158, May, 1996.     

The structure of fullerene films and their metallocene doping

Hexagonal close-packed (HCP) C₆₀ and C₇₀ films have been prepared by the Langmuir method and examined by electron microscopy and electron-diffraction analysis. It has been shown that the vacuum deposition of a C₆₀+C₇₀ mixture results in the formation of a film with small sized grains and a distorted C₆₀-HCP structure. The simultaneous deposition of C₆₀ and ferrocene results in the formation of a film with a changed morphology and an electron-diffraction pattern that contains a variable amount of ferrocene depending on the experimental conditions. The electron-diffraction pattern corresponds to the presence of the known molecular complex C₆₀[(C₅H₅)₂Fe]₂. The analogous simultaneous deposition of fullerene C₆₀ and cobaltocene results in the formation of a C₆₀ film stable in air and water, which contains carbon and cobalt (from the data of X-ray fluorescence, electron microscopy and microdiffraction). It has a different morphology and different diffraction patterns than pure C₆₀ films and, depending on the cobaltocene content (relative to that of fullerene), appears to be a fullerite film doped with various amounts of cobaltocenium fullende, which is an ionic compound.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1379–1383, August, 1994.The work was financially supported by the Russian Foundation for Basic Research (Projects 93-03-4676 and 93-03-18368).     

Superconducting fullerene nanowhiskers

We synthesized superconducting fullerene nanowhiskers (C(60)NWs) by potassium (K) intercalation. They showed large superconducting volume fractions, as high as 80%. The superconducting transition temperature at 17 K was independent of the K content (x) in the range between 1.6 and 6.0 in K-doped C(60) nanowhiskers (K(x)C(60)NWs), while the superconducting volume fractions changed with x. The highest shielding fraction of a full shielding volume was observed in the material of K(3.3)C(60)NW by heating at 200 °C. On the other hand, that of a K-doped fullerene (K-C(60)) crystal was less than 1%. We report the superconducting behaviors of our newly synthesized K(x)C(60)NWs in comparison to those of K(x)C(60) crystals, which show superconductivity at 19 K in K(3)C(60). The lattice structures are also discussed, based on the x-ray diffraction (XRD) analyses.     

Preferential separation of fullerene[84] from fullerene mixtures by encapsulation

The encapsulation of fullerenes with a cyclotriveratrylene derivative, capable to self assemble into a dimer by means of three strong 4-ureidopyrimidinone quadruple hydrogen bonds is described. The system shows preference for C84, allowing its easy enrichment directly from fullerene mixtures.     

The reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine: formation of an open-cage fullerene derivative.

A thermal reaction of fullerene C(60) with 4,6-dimethyl-1,2,3-triazine (4) in o-dichlorobenzene gave azacyclohexadiene-fused fullerene derivative 5, by the reaction with intermediate azete 11, and then, after flash chromatography over SiO(2), open-cage fullerene derivative 6 having an eight-membered ring orifice on the C(60) cage. Compound 6 is assumed to be formed via addition of diradical intermediate 13 to C(60). Compound 6 underwent a further photochemical reaction with singlet oxygen with the cleavage of one of the double bonds at the rim of the orifice to afford triketone derivative 8 having a 12-membered ring orifice.     

Progress in fullerene fluorination

Fluorofullerenes hold the promise of being useful synthetic intermediates, because of the ease with which they undergo nucleophilic substitution. However, the progress in this field has been hampered by the difficulty in controlling fluorination for the preparation of fluorofullerenes of either low, or specific fluorine content. In this paper, previous work is described together with the latest developments involving the use of metal fluorides as fluorinating species. The availability of specific fluorofullerenes has made it possible to make the first detailed studies of the nucleophilic substitution reactions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 852–861, May, 1998.     

Saturation number of fullerene graphs

Saturation number of a graph G is the minimum possible size of a maximal matching in G. We establish improved upper and lower bounds on the saturation number in fullerene graphs and discuss their sharpness and quality.     

Advances in fullerene chemistry

The review outlines the key results obtained in experimental and theoretical studies on fullerene chemistry in the Russian Federation and, in particular, at the A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences.Devoted to fiftieth anniversary of the A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1749–1770, September, 2004.