title: Tracking Ultrafast Vibrational Cooling during Excited-State Proton Transfer Reaction with Anti-Stokes and Stokes Femtosecond Stimulated Raman Spectroscopy.
abstract: Energy dissipation following photoexcitation is foundational to photophysics and chemistry. Consequently, understanding such processes on molecular time scales holds paramount importance. Femtosecond stimulated Raman spectroscopy (FSRS) has been used to study the molecular structure-function relationships but usually on the Stokes side. Here, we perform both Stokes and anti-Stokes FSRS to track energy dissipation and excited-state proton transfer (ESPT) for the photoacid pyranine in aqueous solution. We reveal biphasic vibrational cooling on fs-ps time scales during ESPT. Characteristic low-frequency motions (<800 cm-1) exhibit initial energy dissipation (∼2 ps) that correlates with functional events of forming contact ion pairs via H-bonds between photoacid and water, which lengthens to ∼9 ps in methanol where ESPT is inhibited. The interplay between photoinduced dissipative and reactive channels is implied. Thermal cooling to bulk solvent occurs on the ∼50 ps time scale. These results demonstrate the combined Stokes and anti-Stokes FSRS as a powerful toolset to elucidate structural dynamics.
authors.affiliation: School of Physical Science and Technology, ShanghaiTech University , Pudong, Shanghai 201210, People's Republic of China.
authors.forename: Weimin
authors.initials: W
authors.lastname: Liu
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Longteng
authors.initials: L
authors.lastname: Tang
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Breland G
authors.initials: BG
authors.lastname: Oscar
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Yanli
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Cheng
authors.initials: C
authors.lastname: Chen
authors.affiliation: Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
authors.forename: Chong
authors.identifier: http://orcid.org/0000-0002-8879-1825
authors.initials: C
authors.lastname: Fang