Abstract
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.
Highlights
Additional ultrafast excited-state dynamics of curcumin representing the intramolecular charge transfer (ICT) in the S1 state were observed in the transient absorption measurements in methanol, dimethylformamide, and dimethyl sulfoxide (DMSO) [5]
The ultrafast ICT dynamics of curcumin in DMSO solution have been investigated by Femtosecond stimulated Raman spectroscopy (FSRS), where strong solvent interactions via hydrogen bonding exist
All the excited-state dynamics of curcumin in DMSO were probed by the solvent Raman bands of DMSO, the νCSC and νS=O
Summary
The spectral changes related to the intramolecular proton transfer reaction of curcumin may not be observed due to the symmetric nature of the molecular structure. The lifetimes and quantum yields of curcumin strongly increase with the increase of the solvent polarity in general, which is seemingly related to the changes in the intramolecular hydrogen bonding of the enol form of curcumin in solution [17,22]. Additional ultrafast excited-state dynamics of curcumin representing the intramolecular charge transfer (ICT) in the S1 state were observed in the transient absorption measurements in methanol, dimethylformamide, and dimethyl sulfoxide (DMSO) [5]. The detailed structural changes of curcumin during the ICT process in the S1 state or the solvation dynamics via hydrogen bonding facilitating the deactivation of the photoexcited curcumin have not been experimentally implemented, especially by time-resolved vibrational spectroscopy. The ultrafast changes in the νS=O and νCSC modes of solvent DMSO will be used to explain the solvation dynamics of DMSO before and during the ICT process of curcumin in the excited states
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