The comparative investigation on the optical properties and electronic structures of the alkoxy-tuned 1,3,4-oxadiazole derivatives

The optical properties and electronic structures of three series of 1,3,4-oxadiazole derivatives, 1,4-bis[(4-methylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD1), 1,4-bis[(4-alkoxyphenyl)-1,3,4-oxadiazolyl]phenylene (OXD3–n+1) and 5,5′-di-(4-methyl)-2,2′-p-(2,5-bisalkoxyphenylene)-bis-1,3,4-oxadiazole (OXD2–n+1) were investigated theoretically by quantum calculation theory method as well as experimentally by UV/vis absorption, photoluminescence (PL), excitation spectra and differential pulse polarograph (DPP). The maximum absorption and emission for these oxadiazole derivatives were increasingly red-shifted with the electron-donating abilities of the different position substituents, which was in accordance with the results from time-dependent density functional theory (TD-DFT) calculations. The excitation spectra for these oxadiazole derivatives in CH2Cl2 solution were extended in wide region from 250 nm to 420 nm, and the broad excitation band was attributed to the increasing effect of alkoxy group. The HOMO level, the LUMO level and energy gap (Eg, LUMO–HOMO) for these oxadiazole derivatives were determined by the experimental method and by DFT method, and the results showed that only HOMO had been evidently influenced by changing the different position substituents. We used the DFT methods to generate the optimized structures of their model compounds, OXD1, OXD3-1 and OXD2-1. It was found that the slight difference in bond length and dihedral angle for the model compounds was generated. This strongly localized character of the electron density was illustrated by the frontier orbital contour plots as the effect of alkoxy substituents. The calculated results showed that LUMO mainly localized on the central benzene ring and two oxadiazole rings and HOMO delocalized along the whole conjugated chain except for OXD2-1. It suggested that the spectral properties of these oxadiazole derivatives were determined by the electronic structures with the different effect of alkoxy substituents on the HOMO.