A Step Toward ESIPT-Based Mitochondrial Probe That Responds to ATP Level

An excited state intramolecular proton transfer (ESIPT) probe with a benzoindolium terminal group has been synthesized, whose fluorescence shows large Stokes' shift (Delta lambda approximate to 250 nm) and good fluorescence quantum yield (lambda em approximate to 715 nm, phi fl approximate to 0.2 in CH2Cl2). Spectroscopic studies suggest that the probe is also involved in a minor equilibrium Ar-OH (lambda em approximate to 715 nm) <-> Ar-O- (lambda em approximate to 610 nm) + H+, resulting from deprotonation of phenolic proton. This made it possible for two-channel responses. When being used to stain biological cells, the probe exhibits excellent selectivity toward intracellular mitochondria but gives unusually strong emission from approximate to 600 nm. Near-infrared (NIR) emission is only observable when cellular ATP production is inhibited. The study thus illustrated a unique reaction-based probe for detecting ATP in the intracellular organelle.