Effect of donor terminal group and polymethine chain length on structure of merocyanine dyes in the crystal state

Abstract

X-ray diffraction study of the merocyanine series with different sign of solvatochromism but of the same structural type revealed the effects of donor-acceptor properties of terminal groups and polymethine chain length on their electronic structure in the solid state. The bond length alternation in the negatively solvatochromic dyes corresponds to the dipolar polyene limiting structure, contribution of which, in contrast to solutions, increases for higher vinylogues. Conversely, the nonpolar polyene structure prevails for the positively solvatochromic merocyanines, its contribution increases for higher vinylogues in both solid state and solutions. The DFT (in vacuo and PCM) calculations reproduce tendencies observed for the positively solvatochromic merocyanines but do not reflect the dipolarity growth in higher vinylogues of the negatively solvatochromic ones. The calculations results can be improved taking explicitly into account intermolecular interactions in the crystal.