低聚1,4-苯乙烯撑及其衍生物的结构和吸收光谱的理论研究

用B3LYP/6-31G对低聚物(PV)_n (PV＝1,4-phenylene vinylene, n＝2～8)和(PVSD)_n (PVSD＝2-(1,4-phenylene vinylene)-10-vinylene-spirocyclohexane-1,6-dibenzod,f](1,3)dioxepin, n＝1～4)体系进行了全优化并分析其结构特点, (PV)_n (n＝2～8)体系的结构中所有相邻C原子间形成的二面角均小于1°, 即所有原子有共面的趋势. 而在(PVSD)_n (n＝1～4)体系中低聚物的七元环处有较大的二面角, 约38°, 即在此处结构存在较大的扭曲. 这种扭曲结构对其光谱性质有较大的影响. 在优化结构的基础上分析了两系列低聚物的HOMO-LUMO能隙随n递增的变化规律和对光谱性质的影响, 推断高聚物的发光性质. 同时用ZINDO, TD-DFT方法计算这两系列低聚物的能隙和吸收光谱, 并将低聚物的这些性质与链长的倒数作图外推得到相应的高聚物的能带隙和吸收光谱最大吸收波长. 根据外推能带的曲线估算了聚合物的有效共轭链长. 结果说明, 在(PVSD)_n (n＝∞)中的扭曲结构特点导致其相对(PV)_n (n＝∞)的有效共轭链长变短, 能带隙变宽, 吸收光谱蓝移.

Theoretical Studies of Structures and Absorption Spectra of the Oligomers of 1,4-Phenylene Vinylene and Their Derivatives

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-dibenzod,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.