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

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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-2-yl)thiophene (CT), and 2-(N-ethylcarbazol-2-yl)-9,9-diethylfluorene (CFl) 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 (S1 ← S0) is weakly allowed and polarized along the y-axis (short axis), whereas the S2 ← S0 electronic transition possesses a much larger oscillator strength and is polarized along the x-axis. The S2 ← S0 electronic transition can be correlated to the first absorption band of each derivative measured in n-hexane. The optimization (relaxation) of S1 and S2 electronic states has been done using the RCIS/6-31G* method. For the three oligomers, 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 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 S1 and S2. 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.