The mol. and the electronic structures of bi(4,4-dimethyl-3-oxotetrahydrothiophen-2-ylidene), a compd. comprising the basic chromophore of thioindigo dyes, and its 2 vinylogs with 2 and 3 conjugated central double bonds, resp., were analyzed by B3LYP/6-31+G* calcns. and UPS. The calcd. structural parameters of these 3 compds. reveal no systematic variation. The first ionization potentials indicate no, or only a minor, increase in the energy of the HOMO with the size of the chromophore. These findings are in accord with the corresponding characteristic optical properties of such indigoid compds. For comparison, substituent effects on HOMO and LUMO energies were analyzed by semi-empirical PM3 calcns. in a series of a,w-substituted linear conjugated polyenes. Systems with sym. substitution by 2 donor and 2 acceptor groups simulate the properties of the 3 compds., having rather const. energies for these MOs and a narrow sepn. In addn., the long-wavelength absorption of the 3 compds., and probably of other indigoid compds., is caused by the small overlap d. of these MOs which are largely localized in different parts of the mol.