Electronic Structures of Novel Tetrasilyl-Substituted Ethylenes:  5,5,6,6,11,11,12,12-Octaethyl-5,6,11,12-tetrasilanaphthacene and 1,1‘-Bis(3,4-benzo-2,2,5,5-tetraethyl-2,5-disilacyclopent-3-enylidene)

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 (<BO>1</BO>) and 1,1‘-bis(3,4-benzo-2,2,5,5-tetraethyl-2,5-disilacyclopent-3-enylidene) (<BO>2</BO>), were studied from the viewpoint of electronic effect occurring from geometrical difference in the vicinity of the tetrasilylethene moiety. Characteristic weak bands at 350 (<BO>1</BO>) and 403 (<BO>2</BO>) nm of the absorption spectra in solution were attributed to the σ(HOMO) → π*(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 <BO>1</BO> and <BO>2</BO> could be ascribed to the raising of the HOMO of <BO>2</BO>, as is supported by cyclic voltammetry measurements:  the small Si−C−Si bond angle of the tetrasilylethene moiety destabilizes the HOMO energy of <BO>2</BO> compared with that of <BO>1</BO>.