Nonlinear Optical and Transport Properties of Fullerene Crystals
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As fully conjugated, molecular organic systems, the fullerenes have attracted much attention for their potential applications as optical and, indeed, nonlinear optical materials1. Since they have become available in sizeable quantities, extensive investigations of their optical properties have been conducted, leading to the conclusion their molecular symmetry is a determining feature. The HOMO-LUMO transition is optically forbidden due to symmetry selection rules, becoming weakly allowed in solution through symmetry breaking interactions 2. Upon excitation, radiative relaxation is equally forbidden and the first excited singlet state decays predominantly via intersystem crossing, on a timescale of -lnsec, to the triplet state which is long lived 3. The effects of symmetry are similarly manifest in the vibrational spectroscopy of molecular fullerenes. The 174 degrees of vibrational freedom are reduced to 46 vibrational frequencies due to the icosahedral symmetry, of which 32 are silent 4. Although an understanding of the spectroscopic properties of molecular fullerenes was rapidly established, those of the solid state appear more complex. The speculation which has been directed towards the influence of packing on both the orbital symmetry and the intermolecular interaction has been fuelled by the variety of apparently conflicting results. Whereas band structure calculations have predicted an energy gap as low as 1.3eV 5 , the lowest energy, forbidden transition has been determined to lie at 1.85eV 6, remarkably similar to the HOMO-LUMO separation in molecular fullerenes 2. The vibrational spectrum is similarly unperturbed by the solid state environment 7. The observation of fluorescence in the solid is a 451 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
departure from observations in solution, but its low efficiency, coupled with its 1.2nsec decay time 8 are consistent with a rapid and efficient intersystem crossing from a molecular singlet to the triplet, as in solution. Indeed, such long lived triplets have been observed in ESR measurements and their ODMR measurements associate them with the fluorescence emission 9, 10. These triplets, however, were concluded to be delocalised over more than one molecule and, to add to the debate, the fluorescence has been associated with traps or defect states 11 . Transient excited state absorption measurements, which in solution clearly the evolution of the long lived triplet from the excited singlet on a timescale of -Insec, show a spectrum in the solid state which can not be correlated with either the molecular singlet or triplet, and which, over its lifetime, (>200nsec), shows no spectral evolution 12 and notably, no evidence of a species which decays on a nanosecond timescale. A proposal which may reconcile such conflicting results is that, whereas vacancies, defects and possibly surface molecules exhibit properties which may be associated with molecular fullerenes, excitation of the bulk material leads to the formation of a self-trapped exciton1 2
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