Raman resonance of electron donor/acceptor complex.

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The resonance Raman excitation profiles of a number of charge transfer transitions in electron donor/acceptor complexes with tetracyanoethylene as acceptor in solution at room temperature are reported and compared with absorption and fluorescence spectra of these complexes. All complexes show distinct anomalies which cannot be accounted for by existing theories unless they are extended. In particular, the excitation profiles peak at the low energy side of the absorption profiles by amounts of the order of 1000-2000 cm-1 and also, in the cases where two charge transfer bands are present, resonance occurs independently for the two bands. The latter observation suggests that the two bands are due to distinct species of the complex with differing geometrical configurations. The former observations is interpreted, in connection with the known asymmetry of the absorption profile and the large Stokes gap between absorption and fluorescence peaks, as arising from the relatively stronger contributions of the pure electronic and vibronic levels in the Stokes gap to the Raman scattering cross-section of the complex, and a frequency dependent damping of the vibronic transitions contributing to resonance. This provides important physical insights into the nature of charge transfer transitions of electron donor/acceptor complexes in general. In our discussion we also refer to similar anomalies in the work of others on the resonance Raman effect of the iodine visible absorption band in solution.

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