Absorption and emission properties of carbazole-based dyads studied from experimental and theoretical investigations

The ground and excited states of covalently linked 2-carbazole-based dimers were investigated by theoretical methods and by UV-vis and fluorescence spectroscopies. Geometry optimizations of the ground state of N,N'-diethyl-2,2'-bicarbazole (CC), 2-(N-ethylcarbazol-2yl)thiophene (CT), and 2-(N-ethylcarbazol-2-yl)-9,9-diethylfluorene (CFI) were carried out at the restricted Hartree-Fock level (RHF/6-31G*). The three derivatives are non-planar in their ground electronic states. The nature and the energy of the first five 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 (S-1 <-- S-0) is weakly allowed and polarized along the y-axis (short axis), whereas the S-2 <-- S-0 electronic transition possesses a much larger, oscillator strength and is polarized along the x-axis. The S-2 <-- S-0 electronic transition can be correlated to the first absorption hand of each derivative 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 the three oligomers, S2 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 almost planarity in their first 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. Finally, the fluorescence quantum yield and lifetime have been obtained for the three compounds dissolved in n-hexane. The photophysical properties of these derivatives are discussed in terms of the relative importance of radiative and non-radiative processes. (C) 2004 Published by Elsevier B.V.