Photoinduced Proton-Coupled Charge Transfer Drives Counterion Translocation

We report a light-triggered and directionally controlled migration of perchlorate (ClO₄⁻) anions between distinct binding sites within a synthetically designed BPym–OH dye molecule. The system undergoes a coupled excited-state intramolecular proton transfer (ESIPT) and a photoinduced charge-transfer (CT) process, which together constitute a proton-coupled charge transfer (PCCT) reaction. Combined steady-state and time-resolved spectroscopic analyses, supported by computational modeling, reveal that in a low-polarity toluene environment, the ESIPT-mediated redistribution of electronic density generates a much stronger electrostatic driving force for ion migration than the CT process alone. This enhanced driving force produces a pronounced red shift in the fluorescence spectrum, serving as a direct optical signature of ultrafast counterion motion. A new emission band centered at 750 nm and extending into the near-infrared region emerges after approximately 83 ps, following electrochromic modulation of the PCCT process. In contrast, the initial PCCT fluorescence appears at 640 nm with a rise time of less than 200 fs, indicating the onset of charge redistribution. These results enable direct visualization of light-driven counterion translocation and reveal the fundamental connection between proton-coupled charge transfer and ionic migration in molecular systems.