{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)benzo

It has been obsd. exptl. that donor-acceptor systems of the same family 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-(N,N-diethylamino)benzonitrile (4DEAB-CN) and 4-(N,N-diisopropylamino)benzonitrile (4DIAB-CN) mols. is investigated using time-dependent d.-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 B3LYP level with the 6-31G(d) basis set. The formation of the charge-transfer excited state has been examd. from the point of view of the twisting intramol. charge-transfer (TICT) and planar intramol. charge-transfer (PICT) models previously proposed in the literature to explain the dual fluorescence behavior of this type of compd. Theor. vertical excitation energies of 4DEAB-CN (4.45 eV and 4.56 with the B3LYP and MPW1PW91 functionals, resp.) compare well with the exptl. value (4.35 eV) found in n-hexane. The abnormal La fluorescent band measured exptl. 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 obsd. for similar systems with pretwisted ground states (Jamorski, C.; Foresman, J. B.; Thilgen, C.; Luethi, H.-P. J. Chem. Phys. 2002, 116, 8761). Emission from the twisted conformer is calcd. at 3.40 eV (MPW1PW91) and 3.24 eV (B3LYP), both in good in agreement with the exptl. 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 mols. 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 obsd. when treating emission. Finally, the previous classification scheme previously established within the TICT model for this family of compds. (Joedicke Jamorski, C.; Luethi, H.-P. J. Chem. Phys. 2003, 119, 12852) is also found to hold for the present systems, confirming the general validity of this scheme. [on SciFinder(R)]

Références

Titre
{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)benzo
Type de publication
Article de revue
Année de publication
2004
Revue
J. Phys. Chem. B
Volume
108
Pagination
7132–7141
ISSN
1520-6106
Soumis le 12 avril 2018