Low-Temperature Effect on the Electronic Structure and Spectral-Fluorescent Properties of Highly Dipolar Merocyanines

Absorption and fluorescence spectra of a vinylogous series of reversely solvatochromic merocyanines based on benzimidazole and malononitrile have been studied in frozen ethanol solutions at 77 K. It is found that they possess negative thermochromism-in contrast to both positively solvatochromic merocyanines and negatively solvatochromic symmetrical ionic polymethines-and even stronger negative thermofluorochromism. It has been deduced from the spectral data that at low temperature their electronic structure becomes more dipolar, deviating substantially from the virtual ideal polymethine in both the ground and the excited states. At that, owing probably to the high polarity and ordering of frozen ethanol, the dipolarity of the studied merocyanines increases with the polymethine chain lengthening-the tendency not observed for them in common solvents. The conclusions, based on the spectral data analysis, have been verified by the (TD)DFT-PCM simulations of the dyes within the four-level scheme of electronic transitions.