Assessment of time-dependent density-functional theory for the calculation of critical features in the absorption spectra of a series of aromatic donor-acceptor systems

Singlet and triplet vertical excitation energies of a series of acceptor parasubstituted N,N-dimethyl-anilines [NC-C6H4-N(CH3)(2), NC-C6H4-NH2, OHC-C6H4-N(CH3)(2), NC-C6H2(CH3)(2)-N(CH3)(2), (H2N)OC-C6H4-N(CH3)(2), (CH3)OC-C6H4-N(CH3)(2), O2N-C6H4-N(CH3)(2), named, respectively, 4DMAB-CN, 4AB-CN, 4DMAB-CHO, TMAB-CN, 4DMAB-CONH2, 4DMAB-COMe, and 4DMAB-NO2] have been calculated with TDDFT. Geometry optimization and excitation energy calculations have been performed, in most cases, with the B3LYP functional using a 6-31G(d) and a 6-311+G(2d,p) basis set (hereafter referred to as Sm and Bg, respectively). 4DMAB-CN and TMAB-CN have been investigated with particular care since gas-phase absorption spectra exist for those two molecules allowing thus a direct comparison with experimental results. The first and second singlet excited states of 4DMAB-CN, commonly named locally excited (LE) state and charge transfer (CT) state, are 0.1 and 0.04 eV higher than the experimental results at the B3LYP-Bg level, leading to a 0.06 eV underestimation of the gap between the two states. In the case of TMAB-CN, which is twisted in its ground state, B3LYP-(Sm/Bg) results show an error of 0.36 eV for the singlet CT state. Better agreement with experiment is obtained using the MPW1PW91 functional and Bg basis set with an underestimation of 0.17 eV for the singlet CT state and an overestimation of 0.16 eV for the second singlet state. Contrary to DFT/SCI results, the relative order and position of excitation energies of 4AB-CN and 4DMAB-CHO are well reproduced compared to solution spectra results. The singlet CT state using B3LYP and a Bg basis set is calculated 0.1 eV higher in energy than the experimental value obtained in isopentane for 4DMAB-CONH2, while the same excitation energy is predicted 0.08 and 0.28 eV too low compared to the gas-phase values for 4DMAB-COMe and 4DMAB-NO2, respectively. Finally, the CT excitation energy and its relative position to the LE state agrees with the acceptor strength concept. (C) 2002 American Institute of Physics.