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

Abstract

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.

Excitation energies in density functional theory: An evaluation and a diagnostic test

“…Excitations of CT character are significantly underestimated; DMABN is the exception, consistent with Ref. 24. As anticipated, CT errors are largest for the tripeptide, with errors up to 5 eV.…”

“…The observed correlation of the errors with ⌳ provides a simple explanation as to why the dipeptide CT ͑small ⌳͒ is poorly described by GGA and hybrid functionals, 13 whereas the DMABN CT ͑large ⌳͒ is well described. 24 It is inappropriate to say that these functionals fail to describe CT excitations. The degree of CT-in the sense of how much the occupied and virtual orbitals overlap-must be quantified using ⌳ or some related quantity before judgment can be made.…”

“…The situation is complicated by the fact that for some molecules, CT excitations are accurately reproduced. [24][25][26] In practical calculations, it can therefore be difficult to judge the reliability of a TDDFT CT excitation.…”

TD-DFT investigation on the electronic spectra of novel N-methylmaleimides linked with indolizine ring system

“…Our TD-DFT calculations had good agreements with the MS-CASPT2 results [20] for the π-π* transition energies within 0.1eV deviations, however showed poor agreements for n-π* transition with more than 1eV deviation from the MS-CASPT2 results. Such large discrepancies in n-π* excitation energies using TD-DFT were reported elsewhere by Lüthi [42] et al and Tozer & Handy [43]. Turning to indolizine, the four occupied near-frontier MOs and the three unoccupied MOs have all π-character, in exception of LUMO+2 with σ-character.…”

Absorption and fluorescence properties of oligothiophene biomarkers from long-range-corrected time-dependent density functional theory

“…[6][7][8][9][10][11][12][13] This shortcoming is not a failure of TDDFT itself (which is formally an exact theory), nor is it a breakdown of the linear-response approximation. As in the case for ground-state DFT, this limitation arises from approximations to the (still unknown) exact exchange-correlation functional.…”