Article pubs.acs.org/JPCB
Ultrafast Intramolecular Proton Transfer of Alizarin Investigated by Femtosecond Stimulated Raman Spectroscopy Myungsam Jen,† Sebok Lee,† Kooknam Jeon,† Shafqat Hussain,‡,# and Yoonsoo Pang*,† †
Department of Chemistry and ‡Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of Korea S Supporting Information *
ABSTRACT: We report time-resolved stimulated Raman spectra of alizarin in DMSO solution with 403 nm excitation. Upon photoexcitation, the intramolecular proton transfer reaction of alizarin occurs in 70−80 fs, which is confirmed by both the population growth and the frequency and bandwidth changes of skeletal vibrational modes of alizarin. Interestingly, the vibrational frequencies of ν(C C) and ν(CO) show opposite shifts during the reaction, which may implicate changes in the resonance structure of anthraquinone and the attached carbonyl group. Vibrational relaxation in the potential surface of the proton transferred tautomer of alizarin and the population decay occurring with two distinct time scales were also observed in addition to the solvation dynamics of DMSO solvent molecules.
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INTRODUCTION Proton transfer reactions of molecules either occurring intermolecularly with solvent molecules1−3 or intramolecularly4−7 have been of great interest to researchers in chemistry and related disciplines since it is one of the fundamental chemical reactions. The intramolecular proton transfer reaction occurring in the excited states has been extensively investigated by many time-resolved spectroscopic techniques where the initiation of the chemical reaction can be controlled by ultrashort laser pulses.4−6,8,9 The ultrafast proton transfer reactions as fast as the time scales of ∼30 fs were reported by femtosecond transient absorption spectroscopy.4,5 Recently, a proton transfer reaction with a much faster ∼13 fs time constant was reported from time-resolved fluorescence measurements on 10-hydroxybenzo[h]quinolone by Kim and Joo.6 1,2-Dihydroxyanthraquinone (alizarin) is a natural dye extracted from the roots of madder plant and has been widely used as a pigment in textile fabrics and paintings. Alizarin has been used in many applications mainly due to its large Stokes’ shift which originates from intramolecular proton transfer in the excited state.10−13 Alizarin is also well-known for its antigenotoxic acitivity14,15 and has been widely used as a photosensitizer in dye-sensitized solar cells due to the ultrafast electron transfer to TiO2-based materials.16−18 Highly reversible and stable electrochemical properties of many anthraquinone derivatives have opened new applications in rechargeable batteries as renewable cathode materials.19−21 Photophysical properties of alizarin have been extensively studied experimentally and theoretically for its solvatochromic shifts in the ground and excited states.12,13,22−24 It is known that alizarin exists in two conformer forms of 9,10-keto (locally © XXXX American Chemical Society
excited; LE) and 1,10-keto (proton transferred; PT) in the ground and S1 excited states.12 In the ground state, the LE form exists lower in energy than the PT form and the barrier for the proton transfer is generally too high. However, the S1 state reached by the photoexcitation of LE conformer might exist higher in energy than the PT conformer and face almost barrier-less transformation to the PT conformer where the barrier height between the LE and PT forms shows a strong dependence on the solvent polarity.24 Excited state lifetimes of the PT conformer of alizarin in solution have been measured as 60−80 ps by transient absorption spectroscopy.25−28 The proton transfer rate of alizarin was not clearly measured in these works since it occurs on the ultrafast time scales shorter than or similar to the temporal resolution of transient absorption spectroscopy. Instead, the fast kinetic components of 350−400 fs were observed, which was considered as the vibrational cooling process in the LE conformer.27,28 The ultrafast ESIPT reactions in anthraquinone derivatives including 1-hydroxyanthraquinone and 1-chloroacetylaminoanthraquinone were observed in time-resolved measurements where the fast kinetic components of 50−120 fs were considered as the occurrence of the proton transfer reaction.10,29−31 A decade ago, the femtosecond stimulated Raman spectroscopy (FSRS) with both high temporal (
