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Tounderstand chemical reactivity of molecules in condensed phasein real time, a structural dynamics technique capable of monitoringmolecular conformational motions on their intrinsic time scales, typicallyon femtoseconds to picoseconds, is needed. We have studied a strongphotoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS,pKa* ≈ 0) in pure methanol andobserved that excited-state proton transfer (ESPT) is absent, in sharpcontrast with our previous work on HPTS in aqueous solutions whereinESPT prevails following photoexcitation. Two transient vibrationalmarker bands at ∼1477 (1454) and 1532 (1528) cm–1appear in CH3OH (CD3OD), respectively, risingwithin the instrument response time of ∼140 fs and decayingwith 390–470 (490–1400) fs and ∼200 ps time constantsin CH3OH (CD3OD). We attribute the mode onsetto small-scale coherent proton motion along the pre-existing H-bondingchain between HPTS and methanol, and the two decay stages to the low-frequencyskeletal motion-modulated Franck–Condon relaxation within ∼1ps and subsequent rotational diffusion of H-bonding partners in solutionbefore fluorescence. The early time kinetic isotope effect (KIE) of∼3 upon methanol deuteration argues active proton motions particularlywithin the first few picoseconds when coherent skeletal motions areunderdamped. Pronounced quantum beats are observed for high-frequencymodes 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 impulsivelyexcited at ca. 96, 120, and 168 cm–1. The vividillustration of atomic motions of HPTS in varying H-bonding geometrywith neighboring methanol molecules unravels the multidimensionalenergy relaxation pathways immediately following photoexcitation,and provides compelling evidence that, in lieu of ESPT, the photoacidityof HPTS promptly activates characteristic low-frequency skeletal motionsto search phase space mainly concerning the phenolic end and to efficientlydissipate vibrational energy via skeletal deformation and proton shuttlingmotions within the intermediate, relatively confined excited-stateHPTS–methanol complex on a solvent-dependent dynamic potentialenergy surface. [ABSTRACT FROM AUTHOR]

Tracking molecular motions in real time remains a formidable challenge in science and engineering fields because the experimental methodology requires simultaneously high spatial and temporal resolutions. Building on early successes and future potential of femtosecond stimulated Raman spectroscopy (FSRS) as a structural dynamics technique, we present a comprehensive study of stimulated Raman line shapes of a photosensitive molecule in solution with tunable Raman pump and probe pulses. Following femtosecond 400 nm electronic excitation, the model photoacid pyranine exhibits dynamic and mode-dependent Raman line shapes when the Raman pump is tuned from the red side toward and across the excited-state absorption (ESA) band (e.g., from S 1 ) with varying resonance conditions. On the anti-Stokes FSRS side, low-frequency modes below ∼1000 cm -1 exhibit a line shape change from gain to dispersive to loss, whereas the dispersive intermediate is much less notable for high-frequency modes. The characteristic mode frequency blue shift involving vibrationally hot states in S 1 with time constants of ∼9.6 and 58.6 ps reveals the sensitivity of anti-Stokes FSRS to vibrational cooling and solvation. This work lays the foundation for expanding tunable FSRS technology on both the Stokes and anti-Stokes sides to investigate a variety of photoinduced processes in solution with sufficient resolution to expose functional motions and increased sensitivity to monitor vibrational cooling.