Photoelectron Spectra and Electronic Structures of a Series of Vinylogous Thioindigoid Compounds

The molecular and the electronic structures of bi(4,4-dimethyl-3-oxotetrahydrothiophen-2-ylidene) (1), a compound comprising the basic chromophore of thioindigo dyes, and its two vinylogs 2 and 3 with two and three conjugated central double bonds, respectively, have been analyzed by B3LYP/6–31+G* calculations and UV photoelectron spectroscopy. The calculated structural parameters of 1–3 reveal no systematic variation. The first ionization potentials indicate no, or only a minor, increase in the energy of the highest occupied molecular orbital (HOMO) with the size of the chromophore. These findings are in accord with the corresponding characteristic optical properties of such indigoid compounds. For comparison, substituent effects on highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were analyzed by semi-empirical PM3 calculations in a series of α,ω-substituted linear conjugated polyenes. Systems with symmetrical substitution by two donor and two acceptor groups simulate the properties of 1–3, having rather constant energies for these molecular orbitals and a narrow separation. In addition, the long-wavelength absorption of 1–3, and probably of other indigoid compounds, is caused by the small overlap density of these molecular orbitals which are largely localized in different parts of the molecule.