Synthesis and spectral properties of cyanine dyes—Derivatives of 10,10-dimethyl-7,8,9,10-tetrahydro-6H-pyrido[1,2-a]indolium

The electronic absorption spectra of merocyanines and symmetric cationic polymethine dyes based on 10,10-dimethyl-7,8,9,10-tetrahydro-6H-pyrido[1,2-a]indolium have been investigated in a wide range of solvent polarity. An important feature of these compounds is a cyclic group connecting the nitrogen atom of indole nucleus with the polymethine chain. The explored cationic cyanine dyes, in comparison with the derivatives of 3H-indole, are characterized by less bond order and charge alternation in their chromophore in the ground state. It causes a decrease of both vibronic and intermolecular solute–solvent interactions and, consequently, a decrease of their solvatochromic range, a long-wavelength shift, a narrowing, and an increase of the peak intensity of their spectral bands. The synthesized merocyanines get the greater dipolarity of the ground state, as compared to their non-cyclic analogues. It leads to a color deepening and an increase of the absorption band intensity. Besides, it was revealed that the reversion of solvatochromism for merocyanines with N,N-diethylthiobarbituric moiety containing a cyclic group takes place. The listed regularities fail for the short-chain polymethine dyes. Presumably, it is caused by steric hindrances resulting in non-planarity of their molecules.