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.

中文翻译：

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低温对高双极性花菁的电子结构和光谱荧光性质的影响

已经研究了在苯乙咪唑和丙二腈下于乙烯基酒精溶液中于77 K的乙烯基系列反向溶剂化变色花青的吸收和​​荧光光谱。发现它们具有负热致变色—与正溶剂化变色花青和负溶剂变色对称离子聚次甲基相反-甚至更强的负热荧光色度。从光谱数据可以推断出，在低温下，它们的电子结构变得更加偶极，在基态和激发态下都与虚拟的理想聚次甲基基本偏离。那时，可能由于冷冻乙醇的极性高和有序，所研究的花菁的双极性随聚次甲基链的延长而增加，这是在普通溶剂中未观察到的趋势。