No 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...

No abstract

À partir de données recueillies en 1978, nous analysons les représentations spatiales et les aspirations résidentielles de propriétaires et de locataires de Montréal et de Québec, de la banlieue et de la ville centre. Si les rapports entre l’intimité et la sphère publique se sont modifiés à la faveur des moyens de communication actuels et si proximités géographique et affective se sont découplées, c’est dans le prolongement de tendances amorcées depuis longtemps.Beginning with data gathered in 1978, this paper analyses the spatial representations and residential aspirations of home-owners and tenants in Montréal and Québec City, from both the suburban and downtown areas. While the relationship between intimacy and the public sphere has shifted due to modern means of communication, and while geographical and affective proximity have become disconnected, this represents a continuation of trends that have been under way for a long time

No abstract