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

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 DO 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 DO, 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.