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A novel pyranine derivative, Et HPTA-OH, was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of Et HPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of Et HPTA-OH was found to be highly photoacidic (p K a * = 8.74 in acetonitrile). A series of aniline and pyridine bases were used to investigate excited-state proton transfer (ESPT) from singlet Et HPTA-OH, and rate constants for singlet quenching via ESPT were determined ( k q = 5.18 × 10 9 to 1.05 × 10 10 M −1 s −1 ). Et HPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet–triplet annihilation (TTA) process to reform singlet Et HPTA-OH on timescales of up to 80 μs. Detection of ESPT photoproducts on timescales comparable to that of TTA singlet regeneration provides strong evidence for photoacidic behavior stemming from the regenerated singlet Et HPTA-OH.