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

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. © 2019 American Chemical Society.