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 (Δλ≈ 250 nm) and good fluorescence quantum yield (λ_em≈ 715 nm, φ_fl≈ 0.2 in CH₂Cl₂). Spectroscopic studies suggest that the probe is also involved in a minor equilibrium Ar−OH (λ_em≈ 715 nm) ↔ Ar−O⁻ (λ_em≈ 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 ≈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.