Photobasicity-Triggered Twisted Intramolecular Charge Transfer of Push-Pull Chromophores

Fluorogenic probes that undergo excited-state proton transfer (ESPT) and twisted intramolecular charge transfer (TICT) offer tun-able fluorescence properties for bioimaging and sensing applications. However, the relationship between ESPT and TICT remains poorly understood in push–pull chromophores. Despite extensive research on photoacids, photobases remain underutilized as fluo-rescence modulators, and the roles of solvent polarity, acidity, and donor–acceptor strength in governing photobasicity and TICT activation are not well established. We conducted photophysical experiments and (TD)-DFT calculations to explore how protona-tion and solvent interactions influence fluorescence behavior. Our findings reveal that while protonation consistently induces red-shifted absorption and emission, ESPT efficiency and TICT formation vary widely depending on molecular structure and solvent environment. This work provides new insights into photobasicity-driven fluorescence modulation, offering a foundation for de-signing next-generation probes with enhanced sensitivity to local acidity, viscosity, and microenvironmental factors.