Polymorphism of merocyanine dyes homologs with 1,3-diethyl-2-thiobarbituric acid and p-dimethylaminobenzene donor and acceptor and different polymethine chains connecting them

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