Spectroscopic and photophysical properties of carbazole-based triads

A study of the spectroscopy and photophysics of four carbazole-based triads, namely, 1,4-bis(N-octylcarbazol-2-yl)phenylene (CPC), 2,5-bis(N-octylcarbazol-2-yl)thiophene (CTC), 2,7-bis(N-octylcarbazol-2-yl)-9,9-dihexylfluorene (CFC), and 2,7-bis(N-octylcarbazol-2-yl)-N-octylcarbazole (CCC) are reported. From ZINDO/S calculations performed on the optimized ground state geometries (HF/6-31G*), the S1 ← S0 and S2 ← S0 electronic transitions of CPC, CFC, and CCC are weakly allowed, whereas the S3 ← S0 electronic transition is dipole-allowed (x axis) and possesses a high value of the oscillator strength. On the other hand, for CTC, the dipole-allowed electronic transition is S1 ← S0. For the four derivatives, the dipole-allowed transition corresponds mainly to the promotion of an electron from the HOMO to the LUMO. The first absorption band of each derivative can be assigned to the dipole-allowed transition. The geometry optimization of the excited states has been done with RCIS/6-31G* ab initio calculations. For CPC, CFC, and CCC, S3 is much more stabilized than S1 and S2, causing a crossing of the singlet excited states (S3 becomes lower in energy than S1 and S2). Emission energies from the first relaxed excited states (S1 → S0) have been obtained from ZINDO/S calculations performed on the optimized S1 geometries (RCIS/6-31G*). It is found that these energies are in very good agreement with those determined from the fluorescence spectra. Going from CPC to CTC, the replacement of a phenylene ring by a thiophene unit induces a red shift in the absorption and fluorescence spectra. This behavior is interpreted in terms of the electron donor properties of the thiophene ring. The increase of the oligomer chain (CFC and CCC compared to CPC) also induces bathochromic shifts in the spectra because of a longer electronic delocalization along the molecular frame. From fluorescence data, it is observed that a more planar conformation is favored in the relaxed excited states. RCIS/6-31G* ab initio calculations performed on the S1 relaxed excited states confirm this statement. Finally, the photophysical properties of CTC are mainly governed by nonradiative processes (knr), whereas the variation in the photophysics of CPC, CFC, and CCC mainly involves radiative processes (kF).Key words: electronic spectroscopy, fluorescence, photophysics, ab initio calculations, excited states.