Electron structure and nature of electron transitions of squaraine and thiosquaraine as well as their 1,2-isomers

This paper presents the results of spectral measurements of UV–Vis absorption and 13C NMR spectra as well as quantum-chemical studies of molecular geometry and electron structure of squaraine, thiosquaraine and their corresponding 1,2-isomers. In the ground state 1,3- and 1,2-isomers have similar charge distributions and bond lengths within their chromophores but differ substantially in energy gap sizes and spectral properties, most prominently the maxima positions and intensities of long wavelength absorption bands. The replacement of oxygen atoms connected to the central cycle in squaraine with less electronegative sulfur atoms evokes a relatively intensive band in the short wavelength spectral region.