Deprotonation dynamics and stokes shift of pyranine (HPTS)

The short and intermediate time scale dynamics of the photoacid pyranine (1-hydroxy-3,6,8-pyrenetrisulfonic acid, commonly referred to as HPTS) are studied with visible pump-probe spectroscopy in various solvents to elucidate the nature of its proton-transfer kinetics in water. The observed time dependences of HPTS are compared with those of the methoxy derivative, MPTS. A global fitting procedure is employed to model both the spectral shift (Stokes shift) caused by solvent reorganization and deprotonation of pyranine in water. Three distinct time-dependent features can be clearly identified. They are the Stokes shift (1 ps in H2O and 1.5 ps in D2O), followed by the deprotonation processes, which gives rise to a biexponential decay of the protonated species with time constants (in H2O) of 3 and 88 ps. By the use of a model previously discussed in the literature, the biexponential process can be interpreted as an initial deprotonation step followed by the longer time scale process which separates the resulting ion pair. The results presented here are consistent with some of the previous reports but unambiguously identify and quantitatively measure the Stokes shift as a separate and distinct phenomenon from the deprotonation process, in contrast to other reports that have suggested that all short time (a few picoseconds) dynamics are merely a Stokes shift.