External heavy-atom effects in thiacarbocyanine dyes
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The presence of a heavy atom in the meso position in the polymethine chain of a dye affects the intercombinational conversion rate constant more strongly in the case of the S*⇝ T, transition than in the case of theT ⇝ S0, transition, the difference being due to differences in the energy gaps for the two processes. The effect on the intercombinationalT ⇝ S0 transition is more pronounced than the effect on the nonradiativeT ⇝ S0+hνphos transition. The change in the rate constant for the nonradiative intercombinational transitions is determined by the magnitude of the Franck-Condon factors.
The effect of heavy-atom introduction into the benzthiazole ring of the polymethine dye is less pronounced than the effect of introduction into the meso position of this compound, the alteration in the rate constant for the intercombinational transition being determined by the second-power of the spin-orbital interaction matrix element. The Franck-Condon factors for theS* ⇝ T andT ⇝ S0 transitions remain constant here.
Photoisomerization of the dye proceeds exclusively from excited singlet states of the molecule.
KeywordsMatrix Element Conversion Rate Singlet State Heavy Atom Interaction Matrix
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