Pyrilocyanines. 24. Symmetrical tetra-tert-butyl substituted pyrilo-2-cyanines

2,4-di-tert-butyl-6-methylpyrilium and -thiopyrilium salts were synthesized. From them were obtained symmetrical tetra-tert-butyl substituted α-pyrilocarbo- and dicarbocyanines and their sulfur analogs. The α-pyrilocyanines were converted to symmetrical α-pyridocyanines. The effects of heteroresidue structure, length of polymethine chain, and solvent on the location and shape of the absorption bands of these dyes were studied. Their experimental spectral properties were compared with the results of quantum chemical calculations of average band location, second-power changes of bond order upon excitation, and theoretical electron donor capabilities