Theoretical study on the structures and electronic spectra of novel quinoid 1,3,4-oxadiazole derivatives

The B3LYP and PBE1PBE methods with the 6-31G∗ basis set have been used to compute the ground state geometries, electronic structures and IR spectra of novel quinoid 1,3,4-oxadiazole derivatives. The lowest singlet excited state geometries of these compounds were optimized by the configuration interaction with single excitations (CIS) method. The absorption and emission spectra were calculated by time-dependent density functional theory (TDDFT) on the basis of the ground and excited state optimized geometries, respectively. Solvent effects on the excitation energies were computed through the integral equation formalism of the polarizable continuum model (IEFPCM). The quinoid structure in ααα′α′-tetra(4-tert-butylphenyl)-1,3,4-oxadiazolequinodimethane (TPOQ) was further identified by calculating the structural parameters of the ground state geometry and its IR spectra. Through the analysis of charge distribution in model compounds bearing aromatic groups with different electron-donor abilities, an intramolecular charge transfer has been confirmed, which plays an important role in the light-emitting properties. TDDFT calculations revealed that the maximal absorptions of all these quinoid derivatives originate from the π-π∗ transition from HOMO to LUMO. The calculated absorption and emission spectral properties of TPOQ are in good agreement with the experimental results.