Spectroscopic and photophysical properties of carbazole-based triads

Abstract

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 S 1 ← S 0 and S 2 ← S 0 electronic transitions of CPC, CFC, and CCC are weakly allowed, whereas the S 3 ← S 0 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 S 1 ← S 0 . 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, S 3 is much more stabilized than S 1 and S 2 , causing a crossing of the singlet excited states (S 3 becomes lower in energy than S 1 and S 2 ). Emission energies from the first relaxed excited states (S 1 → S 0 ) have been obtained from ZINDO/S calculations performed on the optimized S 1 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 S 1 relaxed excited states confirm this statement. Finally, the photophysical properties of CTC are mainly governed by nonradiative processes (k nr ), whereas the variation in the photophysics of CPC, CFC, and CCC mainly involves radiative processes (k F ).Résumé : Une étude de la spectroscopie et de la photophysique de quatre dérivés du carbazole, soit 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), et 2,7-bis(N-octylcarbazol-2-yl)-N-octylcarbazole (CCC) est reportée. À partir de calculs ZINDO/S effectués sur les géométries à l'état fondamental optimisées (HF/6-31G*), il est démontré que les transitions électro-niques S 1 ← S 0 et S 2 ← S 0 du CPC, du CFC et du CCC sont faiblement permises. Par contre, la transition électro-nique S 3 ← S 0 de ces molécules est fortement permise (axe x) et possède une valeur élevée de sa force oscillatrice. D'autre part, pour le CTC, la transition élect...

Conformationally-restricted bicarbazoles with phenylene bridges displaying deep-blue emission and high triplet energies: systematic structure–property relationships

“…27 The optical and computational data of the N-octyl analogue of 9 have been reported and they agree with the present data. 28 In particular, the authors note that the fluorescence spectra of this molecule in solution is sharp with clear substructure indicating a more planarised excited state which agrees with our DFT results. The dihedral angle between carbazole and the p-phenylene bridge for the compounds 7-10 is τ = 36−37° in all cases.…”

Polycarbazoles: 25 Years of Progress

“…Nevertheless, this type of materials could be very interesting for electroactive and photoactive devices, since carbazole units linked at the 2-and 7-positions should lead to materials having a longer conjugation length than all other known carbazole derivatives. [123][124][125] However, the synthesis of 2,7-carbazole-based materials is not as straightforward as that of 3,6-carbazole-based materials. Indeed, both the 2-and 7-positions are in the meta position of the amino group of the carbazole unit.…”

Molecular Simulation of Excimer Fluorescence in Polystyrene and Poly(vinylcarbazole)

“…In N-methyl carbazole, ZINDO/S calculations on RCIS/6-31G* geometries report 346.8 and 331.7 nm for S 0 f S 1 and S 0 f S 2 transitions, respectively, i.e., less than 5 kcal/mol between these levels. 28 Thus, internal relaxation is expected to occur much more rapidly than fluorescence. In addition, the S 1 and S 2 states are expected to mix for intermediate fragment separation.…”