Excited State Dual Proton Transfer on a β-Carboline Fluorophore and Its Variable Response to Water and D₂O

The excited state proton transfer (ESPT), a fundamental reaction in chemical and biological systems, is known for its diverse applications. Recent developments in these reactions have examined excited-state multiple proton transfer (ESMPT) involving two or more protons via inter-/intra-molecular mode. This work reports the proton transfer ability of a β-carboline probe, TrySPy, bearing dual intramolecular hydrogen bonds. The molecule was designed as a hybrid of known fluorophores, TryPy and TrySy, and can be synthesized in one step. Preliminary studies revealed a rigid structure of the compound with increased hydrogen bonding and high relative photoluminescence quantum yield (PLQY ∼ 99). The probe works effectively in the cellular environment and can differentiate between water and D₂O by slowing down the proton transfer (PT) process_. In water, the fast PT does not allow emission from the enol form (N-N*), and the emission is observed at 520 nm due to its N-ZPT* form. However, in D₂O, replacing the OH group with OD promotes aggregation of the enol form, with emission at ∼450 nm. The mechanistic model proposed for this work relies on the non-cascaded excited-state intramolecular double proton transfer (ESIDPT) mechanism. This study expands the scope of the ESIDPT systems in the domain of biologically important fused heterocyclic systems.