Exploring Excited State Proton Transfer in Thin Films Under Vibrational Strong Coupling.

Molecular structure, solvent/matrix, the surrounding environment, and molecular vibrations can influence the excited-state proton transfer (ESPT) process. Theoretical studies predict that light-matter strong coupling can modify the energy barrier for proton transfer reactions. Here, we experimentally explore the role of vibrational strong coupling (VSC) on the ESPT using the weak photoacid, 7-hydroxy-1-naphthalenesulfonate (N8S) as the probe. The ESPT in N8S can occur through a solvent/matrix bridge connecting the proton-donating and accepting units. To check the role of VSC, we embedded N8S in a poly(vinyl alcohol) (PVA) matrix and strongly coupled the ─OH stretching vibrational modes of PVA. The steady-state and time-resolved emission analysis reveal that the quantum yield of RO^- emission and the ESPT rate constant enhance by a factor of two under VSC, compared to the noncavity, half-cavity, and off-resonance films. Our findings indicate that VSC can be a tool to control the proton transfer processes, opening avenues for developing physically tunable photoacid-based sensors.