Speaker
Description
Proton transfer processes lie at the heart of many (bio)chemical reactions. Excited state intramolecular proton transfer (ESIPT) systems are of particular interest due to possible application as e.g. photoluminescence sensors or white-light emitting materials. In ESIPT, the proton transfer process is initiated through excitation into a 𝛑𝛑* state with UV or visible light and leads to fluorescence from the product excited state. The Stokes-shifted fluorescence makes ESIPT systems a promising candidate for optoelectronic applications [1]. To investigate the coupled nuclear and electronic motion during and following ESIPT, we here consider an ESIPT model system, 10-hydroxybenzo[h]quinoline (HBQ). In the ground state, HBQ in solution mainly exists in its enol form. Upon excitation from the enol ground state, ESIPT to a keto excited state takes place within 13 fs [2]. We probed the coupled nuclear and electronic structure of HBQ using resonant inelastic X-ray scattering (RIXS) spectroscopy at the N and O K-edges. The atom-specificity of the X-ray probe allows for directly probing the local electronic structure around the proton donor (O) and acceptor (N) atoms. Here, we present a combined experimental and computational approach to elucidate the coupled electronic and nuclear motion during and following ESIPT.
We simulated RIXS spectra along the excited state trajectory of the proton transfer using excited state ab initio molecular dynamics simulations combined with time-dependent density functional theory calculations. The results highlight the potential of RIXS to observe the coupling between different electronic states via off-diagonal peaks containing complementary information to XANES and UV/vis spectra [3]. The excited state spectra specifically show how the time-dependent changes in the intramolecular hydrogen bond are encoded in the dynamics of the RIXS spectral features during and following ESIPT at the proton donor and acceptor sites. Further, the calculations show that some of the RIXS features can be used as a ruler probing the proton transfer distance during the ultrafast chemical process.
The computational results complement our experimental study using femtosecond X-ray pulses at the newly commissioned chemRIXS endstation at LCLS-II. We recorded RIXS spectra at the O and N K-edges of the enol ground state and of the keto excited state following 390 nm excitation. The experimental data highlight the differences in electronic structure at the proton donor and acceptor sites and their changes following proton transfer. The experimental results are in good agreement with the computational data providing detailed insights into a prototypical proton transfer process.
[1] Kwon, J. E., & Park, S. Y. Advanced Materials, 23(32), 3615–3642, 2011.
[2] Kim, C.H, and Joo, T. Phys. Chem. Chem. Phys., 2009, 11, 10266–10269.
[3] Nimmrich, A., Govind, N. and Khalil, M., J.Phys. Chem. Lett., 2024, 15 (51), 12652-12662.
