Polymorphism of Merocyanine Dyes Homologues with 1,3-Diethyl-2-thiobarbituric Acid Acceptor and p‑Dimethylaminobenzene Donor and Different Polymethine Chains Connecting Them

• Published in: Crystal growth & design Vol. 20; no. 1; pp. 167 - 177
• Main Authors: Rigin, Sergei, Tillotson, John, Perry, Joseph, Khrustalev, Victor N, Bogdanov, Georgii, Timofeeva, Tatiana V
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02.01.2020
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.9b00961

The crystallization from different solvents of homologues of 1,3-diethyl-2-thiobarbituric acid and p-dimethylaminobenzene with different lengths of the polymethine bridge between donor and acceptor groups (TB­[n]) produced three pairs of polymorphs with the number of CC bonds in the chain n = 1, 3, and 5. The homologues with the shortest (one methine unit) π-conjugated bridge were presented by two concomitant (obtained simultaneously) polymorphs. Bond length alternation values calculated using X-ray data and evaluation using NMR data showed that some homologues possesses a very high hyperpolarizability. It was shown that the increase of polarizability is related to the increase in the length of the π-conjugated bridge that was supported by experimental hyper-Rayleigh scattering data. Density functional theory calculations of HOMO/LUMO molecular energy levels indicated that compounds with a longer π-conjugated bridge have a higher thermal stability.