Intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye in polar solvents. A CS INDO MRCI study

Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy

“…This difference in Stokes shift is also accompanied by a commensurate difference in the fluorescence quantum yield, which varies by two orders of magnitude between nonpolar and polar solvents (27,28). There are currently two models for excited-state relaxation of DCM: (i) The first electronically excited state, S 1 , is a valence or locally excited (LE) state that undergoes fast nonradiative decay into a lower-lying CT state (CT state between the dimethylaniline and pyran rings) and emission from CT state dominates the fluorescence quantum yield (29)(30)(31)(32)(33)(34). This case is illustrated schematically in Fig.…”

“…1E. Some studies argue that the LE → CT surface crossing is accompanied by an excited-state isomerization or twisted intermediate (29,30,33,34). The other proposed model (ii), illustrated in Fig.…”

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

“…We confirm the coexistence of the LE and CT conformer states in the S 1 excited state and the relative energy ordering between the LE and ICT states from the FSRS results of DCM. This is the first to report the structural difference between the LE and CT conformers of DCM by two distinct Raman spectra in the fingerprint frequency range, which has been numerously proposed by many experimental and theoretical studies of DCM and related push-pull type fluorophores [12,13,24,25,28,30,34]. We also showed that the ultrafast conversion (0.3 ps) from the CT to LE conformer states in weakly polar CHCl 3 solution in addition to the conversion from the LE to ICT state (1.0 ps) in polar DMSO solution which has been numerously observed in time-resolved absorption and emission measurements [12,13,22,25,34,35].…”

“…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]. Similarly, a TICT state along the dimethylaminophenyl (DMAP) rotation was obtained by the complete active space self-consistent-field (CASSCF) calculations [27].…”

Molecular quantum mechanics

“…3, typical of states with large CT character [18,19], could imply that stabilization of the state reached by absorption in polar solvents leads to a structure of large ICT character or a new state of different conformation is produced having the NO 2 -or NO 2 -phenyl-group twisted by 90°( ''twisted intramolecular charge transfer", TICT process) [18,20,21]. At this stage of the work we have no clear evidence for the formation of this new state.…”