Ultrafast and reversibly cyclization/cycloreversion of 4-imino-4H-Quinolizine-1-Carbonitrile: A new strategy for fluorescence modulation in pH sensing in living cells

Abstract Reversible chemically stimulated cyclization/cycloreversion reactions are of great interest and highly demanding for developing molecular switches, such as fluorescent probes. Herein, ultrafast and reversible cyclization/cycloreversion reactions of 4-imino-4H-quinolizine-1-carbonitrile (IMQU) derivatives, induced by H+/OH−, were developed as a new strategy for pH sensing. 1H NMR, HRMS techniques and quantum chemical methods applied to the pH sensing behavior of IMQUs reveal its legitimate mechanism. The IMQU derivatives targeted lysosomes and displayed bright emission, large Stokes shifts, robust fatigue resistance, and superior photostability over commercially available Lyso-Tracker Red. A good correlation was found between pKa values and natural atomic charges at the C-1 atom of the aromatic substituents, which should enable rational tuning of the pKa values.