Article pubs.acs.org/JPCA
Early Time Excited-State Structural Evolution of Pyranine in Methanol Revealed by Femtosecond Stimulated Raman Spectroscopy Yanli Wang, Weimin Liu, Longteng Tang, Breland Oscar, Fangyuan Han, and Chong Fang* Department of Chemistry, Oregon State University, Corvallis, Oregon 97331, United States S Supporting Information *
ABSTRACT: To understand chemical reactivity of molecules in condensed phase in real time, a structural dynamics technique capable of monitoring molecular conformational motions on their intrinsic time scales, typically on femtoseconds to picoseconds, is needed. We have studied a strong photoacid pyranine (8-hydroxypyrene-1,3,6trisulfonic acid, HPTS, pKa* ≈ 0) in pure methanol and observed that excited-state proton transfer (ESPT) is absent, in sharp contrast with our previous work on HPTS in aqueous solutions wherein ESPT prevails following photoexcitation. Two transient vibrational marker bands at ∼1477 (1454) and 1532 (1528) cm−1 appear in CH3OH (CD3OD), respectively, rising within the instrument response time of ∼140 fs and decaying with 390−470 (490−1400) fs and ∼200 ps time constants in CH3OH (CD3OD). We attribute the mode onset to small-scale coherent proton motion along the pre-existing H-bonding chain between HPTS and methanol, and the two decay stages to the low-frequency skeletal motionmodulated Franck−Condon relaxation within ∼1 ps and subsequent rotational diffusion of H-bonding partners in solution before fluorescence. The early time kinetic isotope effect (KIE) of ∼3 upon methanol deuteration argues active proton motions particularly within the first few picoseconds when coherent skeletal motions are underdamped. Pronounced quantum beats are observed for high-frequency modes consisting of strong phenolic COH rocking (1532 cm−1) or H-out-of-plane wagging motions (952 cm−1) due to anharmonic coupling to coherent low-frequency modes impulsively excited at ca. 96, 120, and 168 cm−1. The vivid illustration of atomic motions of HPTS in varying H-bonding geometry with neighboring methanol molecules unravels the multidimensional energy relaxation pathways immediately following photoexcitation, and provides compelling evidence that, in lieu of ESPT, the photoacidity of HPTS promptly activates characteristic low-frequency skeletal motions to search phase space mainly concerning the phenolic end and to efficiently dissipate vibrational energy via skeletal deformation and proton shuttling motions within the intermediate, relatively confined excited-state HPTS−methanol complex on a solvent-dependent dynamic potential energy surface. polar and nonpolar liquids,7,12−26 since a well-defined time zero to trigger PT along H-bonds can be applied with an ultrafast laser pulse. Dissociation of a photoacid by light irradiation in aqueous solutions generally involves PT to form a transient contact ion pair (CIP) that consists of the deprotonated species and the hydronium ion.12,20,25 The proton dissociation processes include breaking and formation of the H-bonds with solvent rearrangement in hundreds of femtoseconds (fs) to a few picoseconds (ps), proton dissociation/transfer and relaxation in tens to hundreds of ps, and finally diffusion and geminate recombination of the dissociated proton with the conjugated photobase followed by fluorescence observed in nanoseconds (ns).7,13,23
I. INTRODUCTION Proton transfer is one of the fundamental and fastest chemical reactions in nature and participates in numerous processes of biological significance.1−3 In order to understand the choreography of protons in condensed phase, hydrogenbonding (H-bonding) systems have become the research focus for decades because they not only provide important insights into acid−base reactions but also present a paradigm to dissect the chemical reaction coordinate. Hydrogen bonds (Hbonds) are generally weaker than ionic or covalent bonds but stronger than intermolecular interactions (van der Waals forces). The dissociation and subsequent re-formation of Hbonds all play critical roles in chemical and biological processes.4−11 In particular, proton transfer between Brønsted acids and bases occurs along a type of the H-bond A−H···B ↔ A−···H+−B, where the H-bonding interaction determines the reaction coordinate of the proton.5,8,11−14 Photoacids, with an electronic excited-state pKa much lower than that of the ground state (S0), have been widely used to study both excited-state proton transfer (ESPT) and H-bonding interaction dynamics in © XXXX American Chemical Society
Special Issue: Prof. John C. Wright Festschrift Received: December 15, 2012 Revised: April 29, 2013
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dx.doi.org/10.1021/jp312351r | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
order to fully elucidate the proton choreography when the ESPT channel is essentially blocked. In our previous work, the ultrafast structural dynamics of HPTS ESPT to water and to carboxylate bases in water solution14,26 have been studied using femtosecond stimulated Raman spectroscopy (FSRS)38−41 on the fs to ps time scale in the vibrational frequency range of 100−1800 cm−1. FSRS is a novel spectroscopic technique that allows rapid acquisition of stimulated Raman spectra with simultaneously high temporal (
