Antithetical product situation in the femtosecond and nanosecond photoionization of sterically hindered phenols in non-protic solvents

The photoionization of sterically hindered phenols in acetonitrile or n-butyl chloride solution gives apparently contradictory results performing biphotonic ionization with femtosecond as well as nanosecond UV lasers: in the first case phenol radical cations were found whereas in the latter one only phenoxyl radicals were detected. This is explained by non-resonant and resonant ionization pathways. Furthermore, the deprotonation rate of the phenol radical cations in acetonitrile in the presence of water traces could be determined as to be diffusion controlled ((3 to 7)×109 dm3 mol−1 s−1) which could not be observed till now because of its extreme pKa value of ≪−1.