Time-dependent density-functional theory investigation of the formation of the charge transfer excited state for a series of aromatic donor–acceptor systems. Part I

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Abstract
Singlet excitation energies for a series of acceptor para-substituted N,N -dimethyl-anilines that are dual (4DMAB-CN, 3M4MAB-CN, MHD) and nondual (4AB-CN, 3M4AB-CN, 4MAB-CN, 3M4DMAB-CN, HHD, and MMD) fluorescent have been performed using the TDDFT method. The aim of this study is to investigate the influence of changing donor groups as well as the addition of methyl groups to the benzene moiety, on the fluorescence behavior of these molecules. Calculations of excitation energies have been performed with both B3LYP and MPW1PW91 functionals using a 6-311 * (2p,d) (Bg) basis set. For all systems, ground-state geometries were optimized using density-functional theory with the Becke three parameter Lee–Yang–Parr functional combined with a 6-31 G (d) (Sm) basis set. In addition, 4AB-CN, 4DMAB-CN, and MMD ground-state geometry has also been optimized using the MPW1PW91 functional with the Sm basis set. For all molecules, the potential energy surface (PES) has been investigated following the twisting intramolecular charge transfer (TICT) model proposed in the literature as a possible mechanism to explain the fluorescence behavior. Both 4AB-CN and HHD molecules have been computed to be nondual fluorescent in full agreement with experimental spectra. The single band observed in the gas-phase fluorescence spectra of 3M4DMAB-CN, and MMD is clearly understood by the form of the PES of the charge transferexcited state that presents a minimum for the perpendicular structure. The qualitative picture of the PES along the twisting angle is in full agreement with experimental observations. The dual fluorescence of 4DMAB-CN and 3M4MAB-CN is explained, within the TICT model, by a double mechanism proposed by Serrano et al. [J. Am. Chem. Soc. 117, 3189 (1995)], that involves the presence of two low-lying states close enough in energy. The nondual fluorescence of 4MAB-CN is explained by the height of the energy barrier (larger than for 4DMAB-CN and 3M4MAB-CN). Finally, the dual fluorescence of the MHD molecule can be fully understood by a double mechanism within the TICT model. 3M4AB-CN is computed nondual fluorescent like 4AB-CN and HHD, but no experimental data has been reported in the literature so far. Our calculations give new evidence in favor of the TICT model as an explanation for the occurrence of dual fluorescence.

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