Abstract
The relaxation processes of the first excited singlet state of 3,3′-diethyloxadicarbocyanine iodide are investigated in this work. The viscosity effect of the medium is studied by applying the Oster—Nishijima relation which is modified to include photoisomerization. We show that, when the oscillator strength is constant and the spectral shift is small, the natural lifetime τ 0 in various solvents can be calculated from the spectrum of a reference solution. This is carried out by applying the well-known Strickler and Berg formula and introducing corrections for the refractive index and the spectral shift. The lifetime of the S 1 state is measured by means of picosecond fluorescence techniques and the photoisomerization is measured using classical laser flash photolysis. The Oster—Nishijima model applies well to unbranched alcohol solutions, from methanol to decanol. In glycerol, glycol or their mixtures with water all the relaxation processes except photoisomerization are accelerated. We suggest that a short range interaction occurs in these solvents, in keeping with the hypothesis that internal conversion via torsional movement and photoisomerization do not occur from the same molecular conformation.
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