The photophysics of 1-hydroxypyrene, the acidity of its singlet excited state, and the nature of its photoionization process in polar media

Abstract

The photophysics of 1-hydroxypyrene, Py–OH, in various media has been examined by pulsed laser and steady state methods. In alkaline polar media the photophysics of the ionized form, Py–O⁻, the pyrenolate ion, is reported. From the data the pK_a is calculated to be 8.93 for the ground state and 4.5 for the excited state. However, the excited singlet state lifetime of Py–OH is too short to lead to significant dissociation into excited Py–O⁻* and H⁺, and no equilibrium between the species exists. Excitation of Py–OH leads to the excited singlet, the triplet, electrons (trapped), and a short-lived radical cation. The radical cation rapidly dissociates to give the pyrenyloxyl radical. Excitation of Py–O⁻ gives the excited singlet state, the triplet state, solvated electrons and the pyrenyloxyl radical. No evidence of a radical cation of Py–O⁻, i.e., ⁺˙Py–O⁻ is forthcoming, and only the neutral pyrenyloxyl radical is observed. Low temperature experiments in glasses are used to identify the various species produced.