Ultrafast intermolecular proton transfer to a proton scavenger in an organic solvent

Abstract

Proton transfer reactions are functionally important in numerous chemical and biological processes. To unravel proton scavengers in action with atomistic details, we studied excited-state proton transfer (ESPT) from photoacid pyranine to the weak base acetate in methanol using transient absorption and wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS). Proton transfer is inhibited in neat methanol, but coherent proton motions and the formation of a charge-separated state occur on the sub-picosecond (sub-ps) timescale, accompanied by chromophore solvation wherein the longitudinal relaxation time of methanol (∼9 ps) dominates. With acetate ions added, bimolecular diffusion-controlled ESPT from the photoacid to acetate occurs on the ∼30 ps timescale, followed by ∼600 ps diffusion-assisted charge separation and solvation in the methanol H-bonding network. Besides intensity dynamics, frequency redshift and blueshift of the transient ∼285 and 1525 cm⁻¹ modes track ESPT after 400 nm photoexcitation. Tunable FSRS exploits resonance Raman enhancement with optimal wavelengths, extends the detection window of excited-state vibrational modes to low frequency, and enables a deeper mechanistic understanding of the proton transfer reaction to proton scavengers in an organic solvent.