Solvent dependence of excited-state proton transfer from pyranine-derived photoacids

Abstract

Steady-state and time-resolved techniques were employed to study the excited-state proton-transfer (ESPT) rate of two newly synthesized 8-hydroxy-1,3,6-pyrenetrisulfonate (pyranine, HPTS) derived photoacids in three protic solvents, water, methanol and ethanol. The ESPT rate constant k_PT of tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, 1a, whose pK_a* ∼ −4, in water, methanol and ethanol is 3 × 10¹¹ s⁻¹, 8 × 10⁹ s⁻¹ and 5 × 10⁹ s⁻¹ respectively. (8-Hydroxy-N1,N3,N6-tris(2-hydroxyethyl)-N1,N3,N6-trimethylpyrene-1,3,6 trisulfonamide, 1b) is a weaker acid than 1a but still a strong photoacid with pK_a* ∼ −1 and the ESPT rate in water, methanol and ethanol is 7 × 10¹⁰ s⁻¹, 4 × 10⁸ s⁻¹ and 2 × 10⁸ s⁻¹. We qualitatively explain our kinetic results by a Marcus-like free-energy correlation which was found to have a general form suitable for describing proton transfer reactions in both the proton-adiabatic and the proton-non-adiabatic limits.