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.
Keywords: excited-state dynamics; femtosecond stimulated Raman spectroscopy; hydrogen bonding; intramolecular charge transfer; solvation dynamics.