Conformational analysis and electronic transition of carbazole-based oligomers as explained by density functional theory

The conformational analysis and vertical excitation energies of carbazole-based molecules were carried out by B3LYP calculations with three basis sets: 6-31G, 6-31G(d) and 6-311G(d,p). Carbazole-based molecules were separated into two types, carbazole combined with five-membered (Cz-co-Thiophene (Cz-co-Th)N and Cz-co-Furan (Cz-co-F)N) and six-membered (Cz-co-Fluorene (Cz-co-Fl)N and Cz-co-Phenyl (Cz-co-P)N) aromatic rings. The analyses of potential energy surfaces and torsional angles for carbazole-based oligomers reveal that these copolymerized with five-membered aromatic rings are more planar than those copolymerized with six-membered ring. The obtained results indicate that TD-PBE1PBE and TD-B3LYP methods predict the excitation energies with the 6-311++G(2d,2p) basis set based on B3LYP/6-311G(d,p) optimized geometries for (Cz)N, (Cz-co-Fl)N and (Cz-co-Th)N in good agreement with experimental data. An analysis of the vertical singlet-singlet transition was also performed and the results indicated that for (Cz)N, (Cz-co-Fl)N, (Cz-co-P)N, (Cz-co-F)N and (Cz-co-Th)N oligomers, S1transition plays an important role. Excitation to the S0 → S1 state corresponds almost exclusively to the promotion of an electron from the HOMO to the LUMO. The electronic transition derived by TD-DFT method gives useful structural and electronic information for designing novel conducting polymer materials.