Igor V. Rubtsov

Rotational reorientation dynamics of C60 in various solvents. Picosecond transient grating experiments

Abstract The picosecond transient grating technique has been used to study the rotational reorientation of C 60 in various solvents: in toluene 7± 1.5 ps, o -dichlorobenzene 10.3± 1.5 ps, o -xylene 13±2 ps and in decalin 3.5± 1.5 ps. The data obtained cannot be described by hydrodynamic Debye theory. Rough-sphere fluid theory predicts the correct values for C 60 rotation in toluene, o -dichlorobenzene and in decalin. The deviations for o -xylene are probably connected with the specifics of the local solvent structure or with the stronger interaction of C 60 with solvent molecules. The rotation of C 60 in decalin is rapid and approaches the rotation in the gas phase determined by inertia.

Photophysical properties of C@60: picosecond study of intersystem crossing

Abstract The picosecond transient absorption spectrum of C 60 in toluene was obtained in the region 420-1100 nm and a new transient absorption band near 1000 nm was discovered. This band was attributed to the S 1 —S x transition, probably between configurations (h u ) 9 (t 1g ) 1 ← (h u ) 9 (t 1u ) 1 . The intersystem crossing time τ= 830±190 ps was estimated from the S 1 —S x absorption decay and from the creation of the T 1 -T x absorption band.

Photochemical properties of C60. Triplet-excited C60 quenching by electron acceptors TCNQ and TCNE in solution. Laser photolysis study

Abstract The quenching of triplet-excited 3C60 by electron acceptors TCNQ and TCNE has been established in nonpolar toluene and moderately polar benzonitrile solutions. The quenching constants kq are 5.5 × 109 in toluene, 2.2 × 109 in benzonitrile for TCNQ and 7.9 × 105 in toluene, 4.2 × 108 (M s)−1 in benzonitrile for TCNE. The 3C60 quenching can be explained by triplet exciplex formation. The time-resolved spectra of the C60/TCNE system exhibit a well pronounced charge-transfer excited state [Cδ60 … TCNEδ−] in benzonitrile.

Orientational behavior of C70 molecules in chlorobenzene

Abstract The orientational behavior of C 70 singlet excited molecules in a chlorobenzene solution has been measured at room temperature by the picosecond transient grating technique. A two-stage decay signal was observed: a fast part ( τ = 12 ± 5 ps) which is comparable with the corresponding signal of C 60 in chlorobenzene ( τ = 8 ± 2 ps), and a slow part ( τ = 30 ± 5 ps). The data obtained are analyzed in terms of the rotation of C 70 molecules. The influence of the dielectric friction on the C 70 rotation is reported.

Dynamics of geminate charge separation in liquid methylcyclohexane studied by the photoassisted ion pair separation technique

Abstract A laser photoassisted ion pair separation technique has been developed to study the kinetics of geminate charge recombination in the picosecond domain in hydrocarbon liquids. Recombination of geminate pairs in liquid methylcyclohexane has been studied by this technique. The electrons were created by two-photon ionization of the anthracene and tetramethyl-p-phenylenediamine (TMPD) admixture using a picosecond UV laser pulse at 270 or 360 nm and then were excited by an IR laser pulse at 1080 nm. The diffusion model of photostimulated ion pair dissociation has been extended to the picosecond time regime in order to describe the observed dependence of the IR enhanced photoconductivity of the solutions on the time delay between the UV and IR pulses. The average initial separation between a geminate trapped electron and its sibling cation was obtained from the experimental decay curves of the IR enhanced photocharge. The separation in the geminate electron-cation pair was found to increase with initial excess excitation energy, ΔE , of the solute above its ionization threshold. It is concluded that the hot electron, the precursor of the trapped geminate electron, is initially injected into the liquid with kinetic energy that increases with increasing the ΔE energy.

EFFECT OF TRANSMEMBRANE POTENTIAL ON CHARGE PHOTOSEPARATION IN LIPOSOMES

Abstract— An unexpected transmembrane potential effect on the recombination rate of the pheophytin or bacteriopheophytin anion‐radicals (dissolved in membrane) and ascorbic ion‐radicals (dissolved in aqueous interior) has been established in liposomes. The influence of transmembrane potential on the recombination rate of Ru3+ (dissolved in inner volume) and (C18H37)V+ or (C14H29)V+ (dissolved in membrane) was observed. The potential was created by a potassium concentration gradient between inner and outer volumes of liposomes in the presence of valinomycin. The effect of the potential was considered on the bases of: (1) it was determined by the diffusional drift of the hydrophobic radicals in a radial direction in the membrane, according to the direction of the electric field; (2) the electric field changed the rate constant of the electron transfer, owing to the effects on the free energy and electronic coupling. Our results show the first explanation to be preferable.

Orientational dynamics of C70 molecules in chlorobenzene

Dynamics of anisotropy relaxation of C70 singlet excited molecules in chlorobenzene was measured at room temperature by the picosecond transient grating technique. The time-ependent diffraction efficiency exhibits a two-stage decay: a fast component (τ1 = 12±5 ps), which is comparable with the corresponding signal of C60 in chlorobenzene (τ = 8±2 ps), and a slow one (τ2 = 30±5 ps). It is supposed that relaxation of anisotropy is related to the orientational mobility of excited C70 molecules relative to two axes of the molecular framework. The results obtained cannot be described by the Einstein-Stokes-Debye theory. The Hynes-Kapral-Weinberg theory, which takes into account microscopic interactions between molecules upon collisions, agrees satisfactorily with the experiment. The influence of dielectric friction on the orientational mobility of C70 in chlorobenzene was estimated.

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, N′N′-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 (ΔG) of the reaction increases and reflects the Marcus-inverted region of the dependence of the rate constant on ΔG. The C70/Am and C60/Am systems are compared.

Nonlinear optical response of C60 in solvents: picosecond transient grating experiments

Time-resolved resonant nonlinear optical response of C60 in a chlorobenzene solution was measured for 528 nm excitation and 1055, 528, and 351 nm probing for zzzz and zzyy configurations. The slow part of the signal (8 +/- 2 ps) was attributed to the orientational motion of C60 excited molecules.

Quenching excited triplet C60 fullerene by tetracyanoethylene in benzonitrile

Quenching excited triplet3C60 fullerene by tetracyanoethylene (TCNE) in a benzonitrile solution proceeds with a rate constant equal to (4.2±0.3) · 1018 (M · s)−1. The formation of a radical ion pair [C60+ · · · TCNE−] was observed.