560 Russian Chemical Bulletin, Vol. 45, No. 3, March, 1996 Orientational dynamics of C70 molecules in chlorobenzene D. V. Khudiakov, L V. Rubtsov,* ;I. A. Nadtochenko, and A. P. Moravskii Institute of Chemical Physics m Chernogolovka, Russian Academy of Sciences, 14 prosp, lnstitutskii, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax: + 7 (096) 515 3588 Dynamics of anisotropy relaxation of C70 singlet excited molecules in chlorobenzene was measured at room temperature by the picosecond transient grating technique. The time- dependent diffraction efficiency exhibits a two-stage decay: a fast component (xl = 12:1:5 ps), which is comparable with the corresponding signal of C60 in chlorobenzene (~ = 8+2 ps), and a slow one (x2 = 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 inter- actions between molecules upon collisions, agrees satisfactorily with the experiment. The influence of dielectric friction on the orientational mobility of C70 in chlorobenzene was estimated. Key words: fullerenes, orientational relaxation, picosecond polarization spectroscopy. Considerable attention has been given to fullerenes, which is related to the unique properties of these mole- cules, e.g., the unusually high mobility of C60 and C70 molecules in the solid and liquid phases. I-s Orienta- tional relaxation of C60 in solutions was studied by t3C NMR, t-s ESR, 6 and fluorescence polarization 7 spectroscopies. The orientational relaxation time of C60 in tetrachloroethane ! measured by t3C NMR spectros- copy at 283 K was 15.5 ps, 16 ps in carbon disulfide z at 298 K, and 16.9 ps in deuterated toluene 3 at 303 K. As far as we know, no studies on the orientational dynamics of C70 in solutions have been performed so far. We have previously studied s the orientational dynam- ics of a C60 molecule in several organic solvents by the picosecond transient grating technique. The observed signals of anisotropy relaxation of singlet excited C60 molecules were assigned to the orientational dynamics of the molecular framework. The times of orientational relaxation obtained turned out to be considerably lower than those predicted by the Einstein--Stokes--Debye (ESD) equation. Good agreement with experiment was obtained within the framework of the microscopic Hynes--Kapral--Weinberg (HKW) theory. 9 Unusually short times of orientational relaxation of a C6o molecule were observed in decalin and toluene solutions, in which the orientational motion is likely inertial and is not determined by diffusion. In this work, the orientational mobility of C70 in chlorobenzene was studied by the picosecond transient phase grating technique. Experimental Details of the picosecond setup were described previously, s Unlike C60, the extinction coefficient of the triplet excited state of a C70 molecule (crl_rz) is approximately twice that of the singlet excited state Est s~ g 2" 103 tool -I L cm -I (see Refs. 10--12). Therefore, the ptcosecond transient phase grat- ing technique with crossed polarization of exciting beams was used to prevent the effect of long-lived triplet and heat grat- ings. A neodymium-glass laser with passive mode synchroniza- tion was used as a source of picosecond pulses. The generation wavelength was 1055 nm, and the pulse duration after an optical modulator and an amplifying system was 6.2 ps. Light of the second harmonic light with wavelength 528 nm was used for excitation of the C70 molecule. To create the transient phase grating, two light excitation pulses simultaneously converged on the sample. When the polarizations of excitation pulses are perpendicular, their interference results only in the distribution of directions of the polarization vector, 13 due to which only the phase grating of moment directions of the singlet excited C70 molecule transi- tion is induced in the sample. Excited C70 molecules were probed by light of the first harmonic at 1055 nm. The probe pulse was polarized at an angle of 45* to the polarization of excitation pulses and diffracted on the phase grating formed. Each point in the kinetic dependence was recorded during one laser pulse in an automatic mode, and the time delay of the probe pulse was established relative to excitation pulses. The diffraction signal was recorded and recovered, taking into account intensities of excitation and probe pulses.. The excitation pulse energy was I to 5 p.I. The probe pulse energy was approximately tenfold lower. Sizes of the grating and probe areas were 250 to 300 tam and 200 tam, respectively. The diffraction signal was registered by a photodiode, ampli- Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 601--604, March, 1996 1066-5285/96/4503-0560 $15.00 ,~ 1996 Plenum Publishing Corporation