Time-dependent density-functional theory investigation of the fluorescence behavior as a function of alkyl chain size for the 4-(N,N-dimethylamino)benzonitrile-like donor-acceptor systems 4-(N,N-diethylamino)benzonitrile and 4-(N,N-diisopropylamino)benzonitrile

It has been observed experimentally that donor-acceptor systems of the same familly as 4-(N,N-dimethylamino)benzonitrile (4DMAB-CN) exhibit increased dual fluorescence activity as their alkyl chain length becomes longer. In the present study, the dual fluorescence activity of 4-(NN-diethylamino)benzonitrile (4DEAB-CN) and 4-(NN-diisopropylamino)benzonitrile (4DIAB-CN) molecules is investigated using time-dependent density-functional theory (TDDFT). Absorption and emission energies have been computed with the MPW1PW91 and the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid functionals in combination with the 6-311+G(2d,p) basis set. The ground-state geometry has been optimized at the 133LYP level with the 6-31G(d) basis set. The formation of the charge-transfer excited state has been examined from the point of view of the twisting intramolecular charge-transfer (TICT) and planar intramolecular charge-transfer (PICT) models previously proposed in the literature to explain the dual fluorescence behavior of this type of compound. Theoretical vertical excitation energies of 4DEAB-CN (4.45 eV and 4.56 with the B3LYP and MPW1PW91 functionals, respectively) compare well with the experimental value (4.35 eV) found in n-hexane. The abnormal La fluorescent band measured experimentally at 3.27 eV in n-hexane is well-produced by the MPW1PW91 vertical excitation energy (3.34 eV) computed for the twisted structure. For 4DIAB-CN, the MPW1PW91 absorption energy (4.21 eV) is in better agreement with that of the corresponding vapor phase (4.40 eV) than is the B3LYP result, as has already been observed for similar systems with pretwisted ground states (Jamorski, C.; Foresman, J. B.; Thilgen, C.; Luthi, H.-P. J. Chem. Phys. 2002, 116, 8761). Emission from the twisted conformer is calculated at 3.40 eV (MPW1PW91) and 3.24 eV (133LYP), both in good in agreement with the experimental gas-phase emission value (3.33 eV). No indication of a low-energy charge-transfer excited state is found when an investigation of potential energy surfaces is carried out within the geometrical constraints of the PICT model for the molecules studied here. It is found that, although TDDFT and DFT/MRCI methods yield results of comparable quality regarding absorption energies (DFT/MRCI results being better in the case of 4DIAB-CN), differences are observed when treating emission. Finally, the previous classification scheme previously established within the TICT model for this family of compounds (Joedicke Jamorski, C.; Luthi, H.-P. J. Chem. Phys. 2003, 119, 12852) is also found to hold for the present systems, confirming the general validity of this scheme.