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

Abstract

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-ethylcarbazol-2-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 S0 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 (ππ*) is weakly allowed and polarized along the y-axis (short axis) of the molecule. On the other hand, the S 2←S0 electronic transition of each oligomer 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 S2←S0 vertical transition energies in fair agreement with the absorption bands maxima measured in n-hexane. The optimization (relaxation) of S1 and S2 electronic states has been done using the RCIS/6-31G* method. For all the oligomers investigated, S2 is much more stabilized than S1 causing a crossing of the singlet excited states (S2 becomes lower in energy than S1). It is observed that the three dyads reach planarity in their S1 relaxed excited state. Electronic transition energies from the relaxed excited states have been obtained from ZINDO/S calculations performed on the optimized geometries of S1 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. © 2004 Elsevier B.V. All rights reserved.