PHOTOPHYSICS OF A SIMPLE CYANIDE DYE

We have studied the photophysics of I,1'-diethyl-2,2'-cyanine p-toluen esulphonate (I) by absorption and fluorescence spectroscopy. Quantum y ields are reported for its fluorescence in a series of alcohol solvent s, and analyzed according to the model of Vogel and Rettig. We detect two rapidly equilibrating forms of I in solution, each with unique flu orescence, designated the V- and W-bands, respectively. We identify th ese forms with anti- and syn-I. Quantum chemical and molecular mechani cs calculations support this assignment. Observation of some V-band em ission on excitation of the W-isomer implies dual fluorescence from an ti-I. Time-resolved laser flash spectroscopy shows that recovery of th e ground state of I lags the decay of both fluorescences, which occurs on the picosecond time scale. We assign this effect to repopulation o f the ground state exclusively from a twisted intermediate funnel stat e which undergoes radiation-less deactivation only on the nanosecond t ime scale. N,N-dimethylaniline (DMA) quenches both fluorescences of I with a Stern-Volmer concentration dependence. N,N-dimethyl-p-toluidine (DMT) quenches the V-band but enhances W-band emission. We analyze th ese results in terms of pi-complexation of I by the quenchers. The W-f orm of I adsorbs to nanoparticulate AgBr which supports its assignment as the syn-isomer. Fluorescence quantum efficiency is enhanced on ads orption, but adsorbate fluorescence is quenched by both DMA and DMT. Q uenching shows a Perrin equation concentration dependence. We assign i t to long-range electron transfer between adsorbed quencher and excite d dye, which, in turn, we relate to the mechanism of super-sensitizati on in silver halide photography. (C) 1997 Elsevier Science S.A.