Fluorescence Lifetime Based Probes for Water Mediated Proton Transfer and Weak Bases

A new concept for fluorescent probes capable of reporting on the concentrations of weak bases in aqueous solution was recently reported, and suggested as a potential tool for studying cellular buffer systems and metabolite pools (Jakobsen et al., Chem Sci, 2025, 16, 7450). These probes are based on proton coupled electron transfer (PCET) quenching of a phenol substituted diazaoxatriangulenium (DAOTA⁺) fluorophore, promoted by the weak bases acting as proton acceptors for the phenol group. The fluorescence lifetime is reduced as function of the proton transfer (PT) rate and base concentration. Herein, we report the synthesis of a new series of related DAOTA+ probes designed to elucidate the involvement of water bridges in PT. The 5-hydroxy-3-benzoic acid substituted DAOTA⁺ probe showcases intramolecular PT that can only happen by water bridging between the carboxylate proton acceptor and phenol donor groups. This exemplifies how the PCET based PT probes can be designed to investigate the role of Grotthuss-like water bridges in acid/base reactions. Furthermore, the negatively charged carboxylate group, through electrostatic repulsion, prohibits ground state association between the probe and negatively charged bases. These new probes showcase a further development of dynamic PCET probes for studies of PT rates in complex systems.