Ground, Excited Structures And Photoelectronic Properties Of Poly(P-Phenylenevinylene) Oligomers With Biphenyl Bridge

Ground and excited state structures of several poly(p-phenylenevinylene) (PPV) oligomers that contain a biphenyl bridge were fully optimized by density functional theory (DFT) and the configuration interaction singles (CIS) method, respectively. The most rational geometric structures for the ground state, the excited state and frontier molecular orbitals were obtained. The absorption and emission properties of the oligomers were then calculated using the semi-empirical Zemer's intermediate neglect of differential overlap (ZINDO) and time-dependent density functional theory (TD-DFT). We analyzed the effect of an increase in the biphenyl chain length on frontier molecular orbitals and the energy gap. Results indicate that absorption and emission properties hardly change as the length of the biphenyl chain increases. A cross configuration between neighboring PPV oligomer chains and the degree of intrachain twist becomes increasingly obvious. Molecular symmetry is reduced and the pi-pi stacking effect is weakened among these types of conjugated molecules in a solid. This is perhaps the most important factor for an increase in the fluorescence quantum efficiency of light-emitting diodes.