Methods for Determining the Photoacidity of Photoacids

In certain cases of well-behaved photoacids, typically having piT values in the range of 0-3, Forster cycle predictions may be tested against piT values found by two direct experimental methods. The first is titration of the excited photoacid with a stronger mineral acid until an end-poinf is reached [9]. The titration of the excited photoacid is monitored by following the change in the relative quantum yield of the photoacid or the photobase as a function of the solution pH. Taking the steady-state fluorescence spectra of the photoacid at each titration point is 

The second independent method for finding K is by direct time-resolved measurements of the proton-dissociation and proton-recombination reaction rates of the excited photoacid. These measurements have traditionally utilized time-resolved fluorescence and absorption spectroscopy. They were originally developed by Weller [7-11] and Forster [3-6] and have been widely in use in photoacid research [17,18, 27]. 

Assuming homogenous proton recombination and unidirectional dissociation reaction one has, for the excited-state equilibrium constant  

Recently, the usefulness of fs-resolved mid-IR measurements of some vibrational markers of the photoacid and the photobase was demonstrated by Nibber-ing et al. [97-100]. Direct mid-IR absorption spectroscopy has thus proved to be an additional tool for directly monitoring the proton-transfer kinetics of photoacids while in the excited state. 

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