Effects of External Pressure on the Light Absorption and Fluorescence of <I>s</I>-Tetracyanobenzene Complexes

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The effects of high pressure on the charge-transfer (CT) absorption and fluorescence spectra of tetracyanobenzene (TCNB) complexes at room temperature have been examined. The pressure-induced change in the absorption intensity indicated the reduction of the distance between donor and acceptor in the complex and the increase in the overlap between their electron clouds. The pressure-induced red shifts of the absorption and fluorescence spectra were almost equal to each other in the case of the TCNB-toluene system, but the former shift was larger than the latter shift in the case of the TCNB-mesitylene system. These results were interpreted in terms of the reduction of the donor-acceptor distance in the complex as well as in terms of electrical solute-solvent interactions. It has been confirmed that the fluorescence quantum yield and the decay time of the complex decrease with the increase in the pressure, and that the rate of the decrease of the former is much larger than that of the latter, in the case of TCNB-toluene system. These results have been interpreted as being due to the pressure-induced increase in the quenching process at the Franck-Condon excited state as well as in the course of the relaxation process from the excited Franck-Condon state to the equilibrium fluorescent state.
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