Electronic Angular Momentum Effects in the Photophysical Behavior of Fullerenes
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ABSTRACT The intensity of the electronic origin in the emission spectrum of C7 0 exhibits a strong solvent sensitivity. The intensity of this peak increases relative to the vibronic features as the dielectric constant of the solvent increases. This solvent dependence is explained by an extension of Platt's Perimeter Free Electron Orbital model. The fullerenes, in particular C6 0 and C70 , possess a nearly spherical symmetry that gives rise to the presence of significant electronic orbital angular momentum. The unexpected spectral sensitivity is shown to arise from quenching of the electronic angular momentum by the solvent environment.
INTRODUCTION Several groups have examined the fluorescence of C7 0 [1-8]. The reported fluorescence spectra show significant vibronic intensity differences that are dependent on the solvent. The room temperature spectra reported by Williams and Verhoeven [2] and Sun and Bunker [3] reveal dramatic intensity differences between the origin and the first vibronic feature in hexane, benzene, toluene, and dichloromethane. Palewska et al [4] noted the pronounced difference in intensity between a vibronic feature of their fluorescence spectrum taken in n-hexane at 77K and the same band in Arbogast and Foote's [5] fluorescence spectrum taken in 3:1 MCH/3-methylpentane glass at the same temperature.
The solvent-dependent behavior of the C70 emission is easier to study in phosphorescence rather than fluorescence simply because the fluorescence structure broadens in solvents of increasing dielectric constant and the phosphorescence remains well-resolved. We have observed a giant solvent-induced intensity change in the phosphorescence of C7 0 ; the relative intensity of the 0-0 transition increases dramatically as the dielectric constant of the solvent increases. We have been able to understand this behavior by extending Platt's Perimeter Free Electron Orbital (PFEO) model, originally developed to describe the photophysical behavior of planar aromatic cata-condensed hydrocarbons (general formula: C4n+2H2n+4) [9]. Platt's model successfully explains the characteristically weak intensity of the absorbance origin of these symmetric molecules in terms of their intrinsic forbiddeness by selection rules based on the conservation of orbital angular momentum. Angular momentum is a good quantum number only for the most symmetric molecules. Certainly the fullerenes fall into this category. C60 and C70 in particular possess a nearly spherical symmetry that gives rise to the presence of significant angular momentum. The unexpected spectral sensitivity we have observed is shown to arise from environmental induced quenching of the electronic angular momentum, similar to the angular momentum quenching that occurs when substituents are added to cata-condensed hydrocarbons.
221 Mat. Res. Soc. Symp. Proc. Vol. 359 01995 Materials Research Society
EXPERIMENTAL SECTION Spectrophotometric grade toluene, methylcyclohexane (MCH), and iodoethane were used
to prepare 107•M solutions of C70. The solutions were degas
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