Caught in the act: real-time observation of the solvent response that promotes excited-state proton transfer in pyranine

18 October 2022, Version 3
This content is a preprint and has not undergone peer review at the time of posting.


Photo-induced excited-state proton transfer (ESPT) reactions are of central importance in many biological and chemical processes. Based on the results of a joint study using optical pump THz probe (OPTP) spectroscopy and molecular dynamics simulations, we were able to elucidate the ultrafast changes in the solvation environment for three derivatives of pyranine: the photoacid HPTS, the methoxy derivative MPTS, and the photobase OPTS. Experimentally, we find damped oscillations in the THz signal at short times and our simulations enable their assignment to vibrational energy transfer beatings between the photoexcited chromophore and nearby solvent molecules. The simulations of HPTS reveal strikingly efficient sub-ps energy transfer into a particular solvent mode, that is active near 4 THz, and which can provide the requisite energy required for solvent reorganization promoting proton transfer. Similar oscillations are present in the THz signal for all three derivatives, however the signal is damped rapidly for HPTS (within 0.4 ps) and more slowly for MPTS (within 1.4 ps) and OPTS (within 2.0 ps). For HPTS, we also characterize an additional phonon-like propagation of the proton into the bulk (with 140 ps period and 83 ps damping time). Thermalization of the solvent occurs on a time scale exceeding 120 ps.


excited state proton transfer
optical pump terahertz probe
solvation dynamics
water solvent
vibrational energy transfer

Supplementary materials

Supporting Information for: Caught in the act: real-time observation of the solvent response that promotes excited-state proton transfer in pyranine
Experimental Fit of the OPTP Signals, Static TDDFT Calculation Details, Bilinearly-Coupled Harmonic Oscillator Model, Additional Simulation Data, Additional Convergence Tests, Estimation of Temperature Increase, Estimation of Heat Transport, Fluorescence Measurements of HPTS and OPTS, Long-Time OPTP Signal of HPTS, Additional Details of the Experimental Setup


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