Structure and intermolecular interactions of the fully negative solvatochromic merocyanine in the crystal phase

Abstract

Herein we report an X-ray crystallographic characterization of tetramethine merocyanine based on benzimidazole and thiobarbituric acid, which shows negative solvatochromism in the entire polarity range of organic solvents. In the crystal, there are three types of non-equivalent dye molecules (A, B, C) and one water molecule in the independent part of the unit cell. The former constitute the AACBBC π-stacked columns, while the latter serves as a hydrogen-bonded bridge between the B and C molecules of the adjacent stacks. The negative bond-length alternation (BLA) values for the polymethine chain of all merocyanine species indicate a greater contribution of the zwitterionic limiting structure in their electronic structure in the crystal. The dipolarity is somewhat lesser for the molecule A, which does not form H-bonds with water. The DFT-B3LYP calculations of the studied merocyanine, its monohydrate, and the π-stacked dimer in vacuo and in a model environment (IEFPCM) have shown that a single hydrogen bond should have lesser effect on its electronic structure in the crystal than π-stacking and other multiple close-range interactions.