Electronic structures of the two structural isomers of novel tetrasilyl-substituted ethylenes, 5,5,6,6,11,11, 12,12-octaethyl-5,6,11,12-tetrasilanaphthacene (1) and 1,1＇-bis(3,4-benzo-2,2,5,5-tetraethyl-2,5-disilacyclopent-3-enylidene) (2), were studied from the viewpoint of electronic effect occurring from geometrical difference in the vicinity of the tetrasilylethene moiety. Characteristic weak bands at 350 (1) and 403 (2) nm of the absorption spectra in solution were attributed to the sigma(HOMO) --> pi*(LUMO) transition on the basis of the quantum chemical configuration interaction (CI) calculations by the semiempirical INDO/S model and ab initio Hartree-Fock theory. Moreover, the large bathochromic shift of 53 nm between 1 and 2 could be ascribed to the raising of the HOMO of 2, as is supported by cyclic voltammetry measurements: the small Si-C-Si bond angle of the tetrasilylethene moiety destabilizes the HOMO energy of 2 compared with that of 1.