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 S 0 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←S 0 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 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.