Synthesis and Properties of Bismerocyanines Linked by a 1,8-Naphthylene Skeleton. Novel Solvatochromism Based on Change of Intramolecular Excitonic Coupling Mode

A newly prepared bismerocyanine linked by a 1,8-naphthylene skeleton has two rotational isomers, syn and anti conformers, with respect to orientation of the two merocyanine chromophores. The NMR studies revealed that a polar solvent such as acetonitrile enhanced a preference for the syn conformer to bring about much larger bathochromic shift of the UV/vis absorption band than that of a 1-naphthyl-substituted merocyanine. The INDO/S-CI calculation indicated that the UV/vis absorption bands of the syn and anti conformers are hypsochromically and bathochromically shifted, respectively, compared to that of the 1-naphthylmerocyanine. In accordance with the prediction based on an exciton coupling theory, the anti conformer fluoresces while the syn conformer does not. A polarographic analysis indicated a positive shift of reduction potential and a negative shift of oxidation potential compared to the corresponding value of the 1-naphthylmerocyanine. The shift in the reduction potential of the bismerocyanine relative to the 1-naphthylmerocyanine in acetonitrile was larger than those in chloroform. This was attributed to the difference in the syn/anti ratio.

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