Intramolecular Charge Transfer and Photoisomerization of the DCM Styrene Dye: A Theoretical Study

Abstract

Structures and properties of the low-lying states in 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM) have been investigated theoretically. Calculations show that the dimethylamino and dimethylanilino twisted conformations of DCM on the potential energy surface of the first excited state (S-1) have relatively high stabilities and remarkable intramolecular charge transfers (ICT). Both structures can serve as candidates for the red-shifted emissive state in polar solvent. In particular, the dimethylanilino twisted ICT state has been predicted to have a dipole moment increment of 20 Debye with respect to the ground state by CASSCF calculations, in good agreement with the suggested experimental values. The optimized geometry of the S-1 state exhibits a long central CC bond of 1.458 angstrom, which makes the trans-cis isomerization quite facile through intramolecular rotation around the central CC bond on the S-1 potential surface. The S-1 state is a precursor to the formation of the ICT emissive state and photoinduced trans cis isomerization. The S-1/S-0 crossing in polar solvent and avoid-crossing in the gas phase as well as in non-polar solvent are involved in the trans-cis isomerization process. The presence of an early S-1/S-0 crossing in the strong polar solvent reduces the isomerization efficiency.Research Grants Council of Hong Kong SAR [CityU 103106]; National Science Foundation [20173042, 20473062, 20233020, 20021002]; Ministry of Science and Technology [2004CB719902, 001CB1089]; Ministry of Educatio

Photophysical Properties of 4‐Dicyanomethylene‐2‐methyl‐6‐(p‐dimethylamino‐styryl)‐4H‐pyran Revisited: Fluorescence versus Photoisomerization

“…Indeed, in addition to the two configurational isomers, that is, E and Z , 1 can in principle exist in two conformers, obtained by rotation around the single bond in position 6 on the pyranyl moiety. Thus, depending on the reciprocal position of the two C=C double bonds, four isomers, namely s‐trans ‐( E ), s‐cis ‐( E ), s‐trans ‐( Z ), s‐cis ‐( Z ), can be defined (Figure 3, insets) [11b] . This variety of geometrical structures is of great importance, as it can be assumed that each particular isomer possesses peculiar features and contributes in a different way to the overall properties of 1 in solution.…”

“…This photoswitching ability would be extremely appealing, as it would not only influence its fluorescence properties, but also disclose new routes for fine tuning the optical properties of the fluorophore. However, among the very large number of examples involving DCM in the literature, only a limited number of qualitative studies have been reported on this phenomenon [10a, 11b, 13] . Moreover, aspects related to the complete conformational isomerism have never been deeply elucidated in experimental studies reported so far, and the conformation has been randomly presented in most reported structures.…”

Twisted Intramolecular Charge Transfer State of a “Push-Pull” Emitter

“…To corroborate the experimental Raman spectra of DCM with the possible structural changes during the ICT, we performed TDDFT simulations to find the locally optimized geometries and corresponding Raman spectra. Most of the (TD)DFT studies on DCM or similar push-pull emitters have focused on the evaluation of the absorption and emission spectra which are consistent with the experimental results [27][28][29][30][59][60][61]. Investigation of the vibrational spectrum changes of a molecule during the specific excited state processes may provide important clues to further development for the DFT methods of higher accuracy.…”

“…Despite the wide applications of the ICT process of the "push-pull" emitters, the details of the ICT process including the structural changes were not clearly understood yet. The "twisted" ICT (TICT) state with a perpendicular conformation of the donor group to the molecular plane [23][24][25][26][27] and the "planar" ICT (PICT) [11,13] with the parallel conformation between the donor and acceptor groups have been proposed by numerous theoretical approaches [25,[27][28][29][30]. Millie and co-workers suggested a TICT model with the perpendicularly twisted DMA group for DCM by conformation spectra-intermediate neglect of differential overlap (CS INDO) multi references configuration interaction (MRCI) simulations [28].…”

“…Millie and co-workers suggested a TICT model with the perpendicularly twisted DMA group for DCM by conformation spectra-intermediate neglect of differential overlap (CS INDO) multi references configuration interaction (MRCI) simulations [28]. Similarly, a TICT state along the dimethylaminophenyl (DMAP) rotation was obtained by the complete active space self-consistent-field (CASSCF) calculations [27]. On the other hand, the planar structures of DCM in both the ground state and the excited state were proposed by time-dependent density functional theory (TDDFT) simulations with the CAM-B3LYP functional [30].…”