Liu Weimin, Tang Longteng, Oscar Breland G, Wang Yanli, Chen Cheng, Fang Chong
School of Physical Science and Technology, ShanghaiTech University , Pudong, Shanghai 201210, People's Republic of China.
Department of Chemistry, Oregon State University , Corvallis, Oregon 97331, United States.
J Phys Chem Lett. 2017 Mar 2;8(5):997-1003. doi: 10.1021/acs.jpclett.7b00322. Epub 2017 Feb 15.
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) 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.
光激发后的能量耗散是光物理和化学的基础。因此，在分子时间尺度上理解此类过程至关重要。飞秒受激拉曼光谱（FSRS）已被用于研究分子结构 - 功能关系，但通常是在斯托克斯侧。在此，我们进行斯托克斯和反斯托克斯FSRS，以追踪水溶液中光酸吡喃的能量耗散和激发态质子转移（ESPT）。我们揭示了ESPT过程中飞秒 - 皮秒时间尺度上的双相振动冷却。特征低频运动（<800 cm）表现出初始能量耗散（约2 ps），这与通过光酸和水之间的氢键形成接触离子对的功能事件相关，在甲醇中该过程延长至约9 ps，其中ESPT受到抑制。这暗示了光诱导耗散通道和反应通道之间的相互作用。向本体溶剂的热冷却发生在约50 ps的时间尺度上。这些结果表明，斯托克斯和反斯托克斯FSRS相结合是阐明结构动力学的强大工具集。
