Time-Dependent Density Functional Theory (TDDFT) Study of the Excited Charge-Transfer State Formation of a Series of Aromatic Donor−Acceptor Systems

Abstract

Singlet excitation energy calculations for a series of acceptor para-substituted N,N-dimethyl-anilines that are dual (4-(N,N-dimethylamino)benzonitrile, 4DMAB-CN, 4-(N,N-dimethylamino)benzaldhyde, 4DMAB-CHO, 1-methyl-7-cyano-2,3,4,5-tetrahydro-1H-1-benzazepine, NMC7) and nondual (4-aminobenzonitrile, 4AB-CN, 3-(N,N-dimethylamino)benzonitrile, 3DMAB-CN, and 4-nitro(N,N-dimethyl) aniline, 4DMAB-NO(2)) fluorescent have been performed using time-dependent density functional theory (TDDFT). The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) (Bg) basis set have been used to compute excitation energies. Ground-state geometries were optimized using density functional theory (DFT) with both B3LYP and MPW1PW91 functionals combined with a 6-31G(d) basis set. For most of the molecules presented in this study, potential energy surfaces have been computed according to the coordinates related to the three following mechanisms proposed in the literature: twisting, wagging, and planar intramolecular charge transfer (ICT). Comparison of the three models for the different molecules leads to the conclusion that only the twisting ICT model is able to explain the low frequency, strongly solvent-dependent energy band present in the fluorescence spectra. According to this model, the 4AB-CN molecule is calculated to be nondual fluorescent in agreement with the experimental spectra. The single band observed in the fluorescence spectra of TMAB-CN (4-(N,N-dimethylamino)-3,5-(dimethyl)benzonitrile) is due to a large stabilization of the charge-transfer excited state along the twisting coordinate. The nondual fluorescence of the 4DMAB-NO(2) molecule is explained by the same mechanism. In the case of 3DMAB-CN, the single observed emission, which is solvent-dependent, has been assigned to the lowest charge-transfer excited state. The dual fluorescence of 4DMAB-CN and 4DMAB-CHO is explained within the twisting ICT model by a double mechanism (already proposed by Serrano et al.: Serrano-Andrés, L.; Merchán, M.; Roos, B. J.; Lindh, R. J. Am. Chem. Soc. 1995, 117, 3189) that involves the presence of two low-lying states close enough in energy. The observation of dual fluorescence in NMC7, that has been one of the origins of the planar ICT model put forward by Zachariasse et al. (Zachariasse, K.; van der Haar, T.; Hebecker, A.; Leinhos, U.; Kühnle, W. Pure Appl. Chem. 1993, 65, 1745), could be fully understood by a double mechanism within the twisting ICT model. Within the set of investigated molecules, our calculations confirm that the twisting ICT model is the only mechanism acceptable to explain the dual and nondual fluorescence phenomenon. Our calculations are in complete agreement with experimental data.

Properties of oligothiophene polycations

“…The second estimation of ε g was derived from the excitation energy values calculated with time‐dependent DFT (TD‐DFT). This methodology, which has been widely applied to study the UV–Vis spectra of conjugated organic compounds, provides a robust and efficient description of low‐lying molecular states …”

“…This methodology, which has been widely applied to study the UV-Vis spectra of conjugated organic compounds, provides a robust and efficient description of low-lying molecular states. [42][43][44] It is well known that the behavior of n-oligothiophenes depends on n, being different for small n values (i.e., typically n 10) than for n > 10. The influence of n on the electronic properties of π-conjugated polymers was recently reviewed and discussed.…”

Pyrrolidine‐based dye‐sensitized solar cells: A time‐dependent density functional theory investigation of the excited state electronic properties

“…The continuous improvements in central processing unit (CPU) resources now allow the study, at correlated levels of approximation, of the absorption spectra of large molecular species such as the dyes of DSSCs. For UV/Vis calculations, one of the most popular approach remains the time‐dependent density functional theory (TDDFT), that commonly provides accurate results for a reasonable computational effort, especially when hybrid functionals are used 15–20. “Cyanine‐like” structures, such as PYR derivatives, show a large CT character electronic excitations (and sometimes a multideterminental nature) which makes them difficult to study with DFT that tends to overestimate CT effects.…”

Improvement of the efficiency of thiophene-bridged compounds for dye-sensitized solar cells

“…The progresses in CPU resources now allow to compute the absorption spectra of large molecular species, such as the DSSCs dyes [35][36][37][38]. For such calculations, one of the most popular schemes remains the time-dependent density functional theory (TDDFT) as it provides results that qualitatively agree with experimental data for a reasonable computational effort, especially when hybrid functionals are used [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. Nevertheless, it is known that ''cyaninelike" structures such as triphenylamine derivatives may be problematic to study with DFT [55][56][57][58].…”