Ideal polymethine state of merocyanines in the crystal

• Published in: Structural chemistry 2022-02, Vol.33 (1), p.169-178
• Main Authors: Shishkina, Svitlana V., Dyakonenko, Viktoriya V., Ishchenko, Alexander A., Kulinich, Andrii V.
• Format: Article
• Language: English
• Subjects: Algorithms; Analysis; Carbon compounds; Chains; Chemistry; Chemistry and Materials Science; Chromophores; Computer Applications in Chemistry; Crystallography; Crystals; Dimers; Dyes; Molecular orbitals; NMR; Nuclear magnetic resonance; Organic compounds; Original Research; Physical Chemistry; Solid state; Theoretical and Computational Chemistry
• ISSN: 1040-0400; 1572-9001
• DOI: 10.1007/s11224-021-01834-7

The concept of the ideal polymethine state lies at the heart of the theory of color of π-conjugated organic compounds. For merocyanines—neutral donor–acceptor polymethine dyes—an attainment of a structure close to the cyanine limit is usually traced in solutions by their solvatochromic behavior or 1 H NMR spectra. Herein, we report an X-ray crystallographic study of a vinylogous series of merocyanines characterized by a close uniformity of the C − C-bond length in the polymethine chain in the solid state. It reveals that for merocyanines in the crystal the ideal polymethine state can be achieved via a judicious choice of the donor–acceptor properties of their end-groups. The polymethine chain length is found to have only a slight impact on the electronic symmetry of such dyes in the solid state, as distinct from both less and more dipolar merocyanines, the bond length alternation in the chromophore of which increases for higher vinylogues. The interaction energies between merocyanine molecules in the stacking dimers in the crystal have been analyzed using the Localized Molecular Orbital Energy Decomposition Analysis (LMOEDA) algorithm.