Quenching of pH‐Responsive Luminescence of a Benzoindolizine Sensor by an Ultrafast Hydrogen Shift

Fluorescent‐sensor design requires consideration of how photochemical dynamics control properties of a sensing state. Transient absorption (TA) spectroscopy reveals an ultrafast net [1,3]‐hydrogen shift following excitation of a protonated methoxy benzoindolizine (bzi) sensor in solution. These photochemical dynamics explain a quenched pH‐responsive fluorescence shift and dramatically reduced fluorescence quantum yield relative to other (e. g. methyl) bzi compounds that do not tautomerize. Calculations predict the energetic and structural feasibility for rearrangement in protonated bzi compounds, such that interaction between the pi‐network and strongly electron‐donating methoxyl must lower the barrier for suprafacial H or H⁺ shift across an allylic moiety. As bzi compounds broadly exhibit pH‐responsive emission shifts, chemical interactions that modulate this electronic interaction and suppress tautomerization could be used to facilitate binding‐ or surface‐specific acid‐responsive sensing.