Delayed photoacidity produced through the triplet–triplet annihilation of a neutral pyranine derivative

Abstract

A novel pyranine derivative, ^EtHPTA-OH, was synthesized via the substitution of the anionic sulfonate groups with neutral diethylsulfonamide groups. The photophysical and photochemical properties of ^EtHPTA-OH were studied using photoluminescence quenching and transient absorption spectroscopy. The singlet state of ^EtHPTA-OH was found to be highly photoacidic (pK_a* = 8.74 in acetonitrile). A series of aniline and pyridine bases were used to investigate excited-state proton transfer (ESPT) from singlet ^EtHPTA-OH, and rate constants for singlet quenching via ESPT were determined (k_q = 5.18 × 10⁹ to 1.05 × 10¹⁰ M⁻¹ s⁻¹). ^EtHPTA-OH was also found to exhibit a long-lived triplet state which reacts through a triplet–triplet annihilation (TTA) process to reform singlet ^EtHPTA-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 ^EtHPTA-OH.