Investigation of the structure, the optical properties, and the photophysics of some indolocarbazoles having terminal aromatic rings

Structures, optical properties, and photophysics of ladder indolo[3,2-b]carbazoles substituted symmetrically by phenylene and thiophene rings have been investigated theoretically and experimentally. The ground state optimized structures were obtained using the density functional theory (DFT) as approximated by the B3LYP functional and employing the 6-31G^* basis set. All derivatives were found nonplanar in their electronic ground states. The character and the energy of the singlet–singlet electronic transitions have been investigated by applying the time-dependent density functional theory (TDDFT) to the correspondingly optimized-ground-state geometries. The ab initio restricted configuration interaction (singles) method (RCIS/6-31G^*) was adopted to obtain the first singlet excited-state structures (S₁) of the molecule. TDDFT calculations performed on the S₁ optimized geometries was used to obtain emission energies. UV–vis and fluorescence spectroscopies were analyzed in conjunction with theoretical calculations. The computed excitation and emission energies were found in reasonable agreement with the experimental absorption and fluorescence spectra. Finally, the photophysical behavior of the indolocarbazoles have been studied by means of steady state and time resolved fluorescence. The overall data have allowed the determination of the rate constants for the radiative and nonradiative decay processes. Both theoretical and experimental data show that the replacement of phenylene rings by thiophene units induces a red shift in the absorption and fluorescence spectra. This behavior is interpreted in terms of the electron donor properties of the thiophene ring. On the other hand, the change of the substitutional pattern, from 2,8 to 3,9, causes a significant hypsochromic shift of the absorption and fluorescence bands.