A fresh look into the time-resolved fluorescence of 8-hydroxy-1,3,6-pyrenetrisulfonate with the use of the fluorescence up-conversion technique

Steady-state and time-resolved fluorescence techniques were used to study the excited-state-proton-transfer (ESPT) process of 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) in H₂O and D₂O. In this contribution we use the fluorescence up-conversion technique with a time resolution of ∼100 fs to monitor the short-time components of HPTS ROH (protonated) and RO⁻ (deprotonated) signals. The ESPT rate constant, k_PT, for HPTS in H₂O and D₂O is rather small 10¹⁰s⁻¹ and 3.3 × 10⁹s⁻¹, respectively. In the time-resolved fluorescence signal of the deprotonated form we find a rise-component of 2.5 ps which we assign to slow charge rearrangement as was already suggested by Spry and Fayer [Spry, D. B.; Fayer, M. D. Charge Redistribution and Photoacidity: Neutral Versus Cationic Photoacids. J. Chem. Phys. 2008, 128, 084508-1–084508-9]. Already in the time-window of 0.2–1.2 ns, the proton geminate recombination (GR) fluorescence tail of the ROH form decays as t^−α where α ≈ 3/2, as predicted by the diffusion-assisted GR model, but for much longer times (t > 5 ns). We also found that the rotation-relaxation time of the ROH form is about τ_or = 80 ps in H₂O, shorter than previously reported, whereas in methanol solution, with much lower viscosity, it is much larger – τ_or = 190 ps. We explain this large difference of τ_or by counter-ion association on all the three sulfonate groups of HPTS.