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A series of hemicyanine dimers, (B3, B5, and B12) whose two chromophores are linked by different numbers of methylene groups, are synthesized. Negative slovatochromic behaviors are observed, and the fluorescence quantum yield is sensitive to the polarity of solvent. In chloroform, the fluorescence intensity is much more intensive than in other polar solvents, and the decay lifetime is much longer. This fluorescence sensitivity to solvent is due to the twisted intramolecular charge transfer (TICT) state formation, which can be blocked in weak polar solvent. The two chromophores in one dimer molecule are correlated and the TICT formation of dimer is relatively difficult compared with monomer. When the linkage is long enough, this correlation can be reduced. The dynamic fluorescence spectra of the chloroform solutions of these dyes support our assumption. The time-resolved fluorescence studies of their LB films provide a new perspective on the excited-state deactive process. The two-dimensional Forster energy transfer between the chromphores within monolayer is assumed to play an important role of their excited states in these LB samples. The difference of the proportions of energy transfer is supposed to be the responsible for the difference of their photoelectro conversion efficiencies.

the journal of physical chemistry b

Photophysical Studies on the Mono- and Dichromophoric Hemicyanine Dyes II. Solvent Effects and Dynamic Fluorescence Spectra Study in Chloroform and in LB Films