Abstract
Ultrafast intramolecular proton transfers of 1,2-dihydroxyanthraquinone (alizarin-h2) and its deuterated product (alizarin-d2) in dimethyl sulfoxide (DMSO) have been investigated by femtosecond stimulated Raman spectroscopy. The population dynamics in the solute vibrational mode of νC=O and the coherent oscillations observed in all of the skeletal vibrational modes νC=O and νC=C clearly showed the ultrafast excited-state intramolecular proton transfer dynamics of 110 and 170 fs for alizarin-h2 and alizarin-d2, respectively. Interestingly, we have observed that the solvent vibrational modes νS=O and νCSC may also represent ultrafast structural dynamics at the frequencies for its “free” or “aggregated” species. From the kinetic analysis of the νS=O and νCSC modes of DMSO, the ultrafast changes in the solvation or intermolecular interactions between DMSO molecules initiated by the structural changes of solute molecules have been thoroughly investigated. We propose that the solvent vibrational modes νS=O and νCSC of DMSO can be used as a “sensor” for ultrafast chemical reactions accompanying the structural changes and subsequent solute-solvent interactions.
Highlights
Proton transfer reaction has been of great interest since it is considered as one of the fundamental chemical reactions in many chemical and biological systems
We propose that the solvent vibrational modes S1⁄4O and CSC of dimethyl sulfoxide (DMSO) can be used as a “sensor” for ultrafast chemical reactions accompanying the structural changes and subsequent solute-solvent interactions
We showed that the C1⁄4O mode of alizarin-h2 at 1627 cmÀ1 represents the excited-state vibration of the PT conformer and that two C1⁄4C modes of alizarin-h2 at 1521 and 1549 cmÀ1 are strongly coupled to the excited-state intramolecular proton transfer (ESIPT) reaction coordinate
Summary
Proton transfer reaction has been of great interest since it is considered as one of the fundamental chemical reactions in many chemical and biological systems. Nibbering and co-workers reported an ultrafast appearance of the free C1⁄4O mode of Coumarin 102 and the free OH mode of phenol upon photoexcitation, which implies instantaneous breakage of hydrogen bonding between the solute and solvent molecules.[37,38] The solvation dynamics including the ultrafast hydrogen bond cleavage has been observed in a recent time-resolved stimulated Raman work by Fang and co-workers.[39]. Alizarin is one of the model compounds for the ESIPT between the hydroxyl and adjacent carbonyl groups.[40–44] The structural changes of alizarin from a 9,10-keto (locally-excited; LE) to a 1,10-keto (PT) form with a huge Stokes’ shift of $7000 cmÀ1 have been investigated recently by FSRS, where the ultrafast ESIPT of a 70–80 fs time constant has been clearly evidenced in the population and relaxation dynamics of the major skeletal vibrations of C1⁄4C and C1⁄4O.45. The cross correlation time between these two pulses was measured as 60 fs with a 150 lm thick cover glass and 93 fs with a 0.5 mm quartz flow cell filled with solvent DMSO (see Fig. S1 in the supplementary material)
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