Photoacid-induced aqueous acid-base reactions probed by femtosecond infrared spectroscopy.

Acid-base reactions involving an excited photoacid have typically been investigated at high base concentrations, but the mechanisms at low base concentrations require clarification. Herein, the dynamics of acid-base reactions induced by an excited photoacid, pyranine (DA), were investigated in the presence of azide ion (N3-) in D2O solution using femtosecond infrared spectroscopy. Specifically, the spectral characteristics of four species (DA, electronically excited DA (DA*), the conjugate base of DA* (A*-), and the conjugate base of DA (A-)) were probed in the spectral region of 1400-1670 cm-1 in the time range of 1 ps-1 μs. This broad timescale encompassed all the acid-base reactions initiated by photoexcitation at 400 nm; thus, reactions related to both DA* and A- could be probed. Furthermore, changes in the populations of N3- and DN3 were monitored using the absorption bands at 2042 and 2133 cm-1, respectively. Following excitation, approximately half of DA* relaxed to DA with a time constant of 0.44 ± 0.04 ns. The remainder underwent an acid-base reaction to produce A*-, which relaxed to A- with a time constant of 3.9 ± 0.3 ns. The acid-base reaction proceeded via two paths, namely, proton exchange with the added base or simple deuteron release to D2O (protolysis). Notably, all the acid-base reactions were well described by the rate constant at the steady-state limit. Thus, although the acid-base reactions at low base concentrations (< 0.1 M) were diffusion controlled, they could be described using a simple rate equation.