Ground and excited state properties of carbazole-based dyads:: correlation with their respective absorption and fluorescence spectra

The ground and excited states of covalently linked carbazole-based dimers were investigated theoretically and experimentally. Geometry optimizations of the ground state of N,N'-diethyl-2,2'-bicarbazole (CC), 2-(N-ethylcarbazol-2-yl)thiophene (CT), and 2-(N-ethylcarbazol2-yl)furan (CF) were carried out at the restricted Hartree-Fock level (RHF/6-31G*). It is found that CC and CT are nonplanar in their ground electronic states (S-0), whereas CF is completely planar in the So state. The nature and the energy of the first two singlet-singlet electronic transitions have been obtained by ZINDO/S semi-empirical calculations performed on the HF/6-31G* optimized geometries. For all the oligomers, the first electronic transition (pipi*) is weakly allowed and polarized along the Y-axis (short axis) of the molecule. On the other hand, the S-2 <-- S-0 electronic transition of each oligoiner possesses a much larger oscillator strength, is polarized along the x-axis, and is mainly described by the promotion of one electron from the HOMO to the LUMO. It is found that these calculations produce S-2 <-- S-0 vertical transition energies in fair agreement with the absorption bands maxima measured in n-hexane. The optimization (relaxation) of S-1 and S-2 electronic states has been done using the RCIS/6-31G* method. For all the oligomers investigated, S-2 is much more stabilized than S-1 causing a crossing of the singlet excited states (S-2 becomes lower in energy than S-1). It is observed that the three dyads reach planarity in their S-1 relaxed excited state. Electronic transition energies from the relaxed excited states have been obtained from ZINDO/S calculations performed on the optimized geometries of S-1 and S-2. It is found that the electronic transition energies from the first relaxed excited state are close to those determined experimentally from the fluorescence spectra recorded in n-hexane. (C) 2004 Elsevier B.V. All rights reserved.