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