Understanding on absorption and fluorescence electronic transitions of carbazole-based conducting polymers: TD-DFT approaches

The electronic excitation transitions of carbazole-based oligomers, (Cz-co-Cz)(N), (Cz-co-Fl)(N) and (Cz-co-Th)(N) (N = 2-4) were investigated using density functional theory (DFT) and time-dependent (TD) DFT methods. Our results show that the calculated ground state geometries favor a more aromatic, planer structure, while the electronically excited geometries favor a quinoidic type structure. Absorption and fluorescence energies have been obtained from TD-B3LYP/SVP calculations performed on the S-1 optimized geometries and are in excellent agreement with experimental data. The experimental fluorescence excitation energies for (Cz-co-Cz)(4), (Cz-co-Fl)(4) and (Cz-co-Th)(4) (2.76, 2.63, and 2.25 eV, respectively) correspond closely with the predicted S1 transitions (2.84, 3.91 and 2.43 eV, respectively). We also report the predicted radiative lifetimes 0.52, 0.47, and 0.99 ns for (Cz-co-Cz)(N), (Cz-co-Fl)(N) and (Cz-co-Th)(N), discuss the origin of the small stoke shift of the carbazole based oligomers and the magnitude of bathochromic shifts. We conclude by discussing the benefits of theoretical calculations, which can provide critical structural and electronic understanding of excitation-relaxation phenomena that can be exploited in design of novel optical materials.