Synthesis of benzidine-based conjugated organic materials bearing donor-acceptor groups: DFT studies and photovoltaic applications

Herein, tetraethoxymethyl, diphenyl, and dipyrenyl units have been fused with biphenyl group, via diimine bond, resulting in 3 (2 novel) benzidine derivatives 2, 4a-b. The derivatives were successfully synthesized by condensation of three different aldehydes (formaldehyde, benzaldehyde 3a, and pyrenecarboxaldehyde 3b) with benzidine in high yields. Structural identifications of the newly obtained compounds are determined by NMR, IR, UV–Vis, and HRMS spectroscopic techniques. Then, the photovoltaic properties of the organic compounds were investigated. To obtain insight into the geometric, electronic, and spectroscopic features of compounds 2, 4a-b, theoretical calculations were performed utilizing the B3LYP/6–311G(2d,2p) basis set. Theoretical 1H and 13C NMR resonance signals of compounds 2, 4a-b were estimated in the gas phase and the IEFPCM model in solvent field calculations utilizing the GIAO method at the B3LYP/6–311G(2d,2p) level and showed excellent correlation with the experimental results. A comparison of computed and experimental vibrational frequencies was performed, and significant bands were allocated. The results showed that experimental and theoretical IR frequencies had a good correlation (R2=0.9997 (2), 0.9996 (4a), and 0.9940 (4b)). To explain the relationship between the molecular structure of the compounds and photovoltaic properties, density functional theory (DFT) and its time-dependent form (TD-DFT) were utilized. The HOMO-LUMO energies and energy gap (Eg) helped understand optimal energy levels for electron absorption and transfer. The calculated band-gap of compounds 2, 4a-b were 4.705, 3.679, and 3.006 eV, respectively. The power conversion efficiency (PCE) values for the compounds 2, 4a-b were also calculated as 2.25, 2.70, and 2.80%, respectively. Compared to compounds 2 and 4a, dipyrenyl-bearing benzidine derivative 4b showed a larger redshift in the absorption wavelength with a remarkable high PCE value along with a low Eg value, which can be attributed to the greater stability of 4b due to the extended π-conjugation.