Investigate the stability and opto-electronic properties of carbazole and its derivatives via quantum-chemical calculations

Abstract

DFT/TD-DFT calculations were employed to clarify the relationship between molecular structure and electronic properties of carbazole and four derivatives. Ground-state singlet (S₀) geometries were optimized with TPSS/6-311++G(d,p) (no imaginary frequencies), while triplet-state (T₁) and other open shell species were treated with UTPSS using the same basis set; absorption spectra were simulated via TD-DFT. The computed results show that donor–acceptor groups and substituent positions (2,7 versus 3,6; N-9 substitution by Cl) strongly modulate the HOMO/LUMO levels, the first singlet and triplet excitation energies (E_S1, E_T1), and the singlet–triplet energy gap (ΔE_S–T). Overall, introducing substituents at the 3,6-positions of the carbazole core to form a D–π–A system (compound c) reduces ΔE_S–T to 0.58 eV and produces a pronounced red shift. iIn addition, the Cl substituent can diminish conjugation and intramolecular charge transfer (compound e), whereas compound d still absorbs in the visible region (~420 nm) with enhanced intensity. These findings suggest that carefully choosing both the position and nature of substituents on the carbazole scaffold is an effective strategy for optimizing materials for optoelectronic applications.