Structural, electronic, and optical properties of novel indolocarbazole-based conjugated derivatives

A study of the structure, electronic, and optical properties of new indolo[3,2-b]carbazoles is reported. Geometry optimizations of the ground state of the derivatives were carried out using the density functional theory (DFT) with the B3LYP functional and the 6-31G∗ basis set. Molecules disubstituted with phenyl or thiophene units at molecular ends were found nonplanar in their electronic ground states (S₀), whereas indolocarbazoles having phenylenevinylenes at molecular ends are nearly planar. The electronic excitation transitions of the indolocarbazoles were investigated using the time-dependent (TD) DFT method performed on the ground state optimized geometries. For all the derivatives, excitation to the S₁ state corresponds mainly to LUMO ← HOMO transition, whereas the second electronic transition mainly originates from the LUMO ← HOMO − 1 excitation. The excitation energies are found in fair agreement with the absorption energies of the indolocarbazoles. The optimization (relaxation) of the first singlet excited electronic state (S₁) has been done using the restricted configuration interaction (singles) (RCIS/6-31G∗) approach. The electronically excited geometries favor a more quinoidic type structure. Emission energies have been obtained from TDDFT calculations performed on the S₁ optimized geometries and are in fair agreement with experimental data obtained from fluorescence spectra. The change from phenyl to thiophene rings as well as the incorporation of vinyl units between the phenyl and the indocarbazole moieties induce a significant decrease in the excitation and emission energies.