Twisted intramolecular charge transfer of nitroaromatic push-pull chromophores.

The structural changes during the intramolecular charge transfer (ICT) of nitroaromatic chromophores, 4-dimethylamino-4'-nitrobiphenyl (DNBP) and 4-dimethylamino-4'-nitrostilbene (DNS) were investigated by femtosecond stimulated Raman spectroscopy (FSRS) with both high spectral and temporal resolutions. The kinetically resolved Raman spectra of DNBP and DNS in the locally-excited and charge-transferred states of the S1 state appear distinct, especially in the skeletal vibrational modes of biphenyl and stilbene including ν8a and νC=C. The ν8a of two phenyls and the νC=C of the central ethylene group (only for stilbene), which are strongly coupled in the planar geometries, are broken with the twist of nitrophenyl group with the ICT. Time-resolved vibrational spectroscopy measurements and the time-dependent density functional theory simulations support the ultrafast ICT dynamics of 220-480 fs with the twist of nitrophenyl group occurring in the S1 state of the nitroaromatic chromophores. While the ICT of DNBP occurs via a barrier-less pathway, the ICT coordinates of DNS are strongly coupled to several low-frequency out-of-phase deformation modes relevant to the twist of the nitrophenyl group.

Intramolecular charge transfer of a push-pull chromophore with restricted internal rotation of an electron donor.

Intramolecular charge transfer (ICT) of 4-(dicyanomethylene)-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)vinyl]-4H-pyran (LD688) in DMSO solution was investigated by femtosecond stimulated Raman spectroscopy (FSRS) with 403 nm excitation. The molecular structure of LD688 is similar to that of a well-known push-pull chromophore, 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), except that the internal rotation of the electron-donating dimethylamino group is restricted with the introduction of the julolidine moiety. Upon photo-excitation, LD688 shows an ultrafast (1.0 ps) ICT followed by the vibrational relaxation (3-8 ps) in the charge-transfer (CT) state. Two distinct Raman spectra of LD688 in the locally excited (LE) and CT state of the S1 state were retrieved from FSRS measurements. Based on the time-dependent density functional theory (TDDFT) simulations, a "twisted" julolidine geometry of LD688 was proposed for the ICT state, which was further confirmed in comparison to the spectral changes of several push-pull chromophores with the π-conjugated backbone of stilbene, biphenyl, styrylpyran, styrylpyridinium, and styrene in terms of the skeletal vibrational modes of ν19b,py, νCC,ph, and νCN.

Intramolecular Charge Transfer of Curcumin and Solvation Dynamics of DMSO Probed by Time-Resolved Raman Spectroscopy.

Intramolecular charge transfer (ICT) of curcumin in dimethyl sulfoxide (DMSO) solution in the excited state was investigated by femtosecond electronic and vibrational spectroscopy. Excited-state Raman spectra of curcumin in the locally-excited and charge-transferred (CT) state of the S1 excited state were separated due to high temporal (<50 fs) and spectral (<10 cm-1) resolutions of femtosecond stimulated Raman spectroscopy. The ultrafast (0.6-0.8 ps) ICT and subsequent vibrational relaxation (6-9 ps) in the CT state were ubiquitously observed in the ground- and excited-state vibrational modes of the solute curcumin and the νCSC and νS=O modes of solvent DMSO. The ICT of curcumin in the excited state was preceded by the disruption of the solvation shells, including the breakage of hydrogen bonding between curcumin and DMSO molecules, which occurs at the ultrafast (20-50 fs) time scales.