Triplet-State ESR of Cyanine Dyes

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Abstract
ESR spectra have been obtained for four symmetric cyanine dyes in alcoholic solution at 77°K. Zero‐field‐splitting parameters show a regular dependence on chain length of the conjugated π‐electron system. Molecular‐orbital calculations of these parameters show that in some cases, at least, the triplet state is localized in the region of the polyene chains; the experimentally observed splittings and the variations with chain length are reproduced in the calculations. The correlation between properties in the singlet and triplet states is discussed.