The structures of oligomers (PV)_n (PV＝1,4-phenylene vinylene, n＝2～8) and (PVSD)_n (PVSD＝ 2-(1,4-phenylene vinylene)-10-vinylene-spirocyclohexane-1,6-dibenzo[d,f](1,3)dioxepin, n＝1～4) were optimized by B3LYP/6-31G. The structures of (PV)_n (n＝2～8) tend to be coplanar, with the dihedral angles of no larger than 1°. (PVSD)_n (n＝1～4) are twisted in seven-membered-rings, with large dihedral angles about 38° between phenyl rings. The relations between the HOMO-LUMO gaps of oligomers and their chain length were analyzed on the basis of the optimized structures. This relationship is useful to study optical properties of their polymers. The ZINDO and TD-DFT were also used to study the energy gaps. To the importance, the absorption spectra were obtained by both means. The polymeric absorption wavelengths were estimated by extrapolating those of the oligomers to the inverse of chain length to zero. The effective conjugation length was obtained during extrapolation. The result shows that the twisted structure of (PVSD)_n (n＝∞) breaks the conjugation length. It has broader energy band than that of (PV)_n (n＝∞), that is, the absorption band was blue shifted.

Key words: 1,4-phenylene vinylene, density functional theory, optimization, absorption spectrum