Molecular and crystal structure of di-and hexamethine merocyanine dyes — derivatives of 3H-indole and malononitrile — manifesting positive solvatochromism
Single crystal X-ray diffraction and IR spectroscopy are applied to examine the molecular and crystal structures of 2-[(2E)-2-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidine)ethylidene]-malononitrile (1) and 2-[(2E, 4E,6E)-6-2-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidine)-2,4-hexadienylidene]-malononitrile (2). It is shown that both in solution and crystalline state the studied merocyanines completely retain the trans-conformation and approach a neutral polyene. It is found that the polyene character is increased in transition from di-to hexamethinemerocyanine, which is caused by a partial violation of conjugation with the nitrogen lone pair due to its slight deviation from the plane of the chromophore. It is demonstrated that the stretching vibration frequency of the nitrile group in the IR spectra of malononitrile merocyanines can be used to examine polyene-polymethine electron relaxations. Electronic and spatial structures of merocyanine molecules are calculated by AM1, PM3, and ab initio methods. Theoretical results are compared to experimental ones.
Keywordsmerocyanines single crystal X-ray diffraction conformation IR spectroscopy quantum chemical calculations
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