FT-EPR study of the wavelength dependence of the photochemistry of phenols

The excitation wavelength dependence of the photochemistry of phenol, p-cresol, and tyrosine in neutral and acidic aqueous solution was examined using Fourier Transform Electron paramagnetic Resonance (FT-EPR). Photoexcitation gives rise to formation of phenoxyl, p-cresyl, or tyrosyl radicals (PhO.) hydrated electrons (e(aq)(-)), and hydrogen atoms (H-.). From Chemically Induced Dynamic Electron Polarization (CIDEP) effects and the effect of addition of N2O on the spectra it can be deduced that the mechanism of hydrogen atom formation depends on the wavelength of excitation. Excitation into the S-2 absorption band (lambda(ex) = 193 nm) gives rise to H-atom formation by O-H bond homolysis and by e(aq)(-) capture by protons. The electron capture reaction produces a net absorption signal contribution in the FT-EPR spectrum of H-. due to transfer of spin polarization from e(aq)(-), This demonstrates that the reaction occurs with H+ in the bulk of the solution. The spectrum of H-. produced by excitation into the S-1 band (lambda(ex) = 266 nm) can be completely quenched by addition of N2O. In this case H-atom formation is due entirely to the reaction of e(aq)(-) with H+. The excitation energy apparently is insufficient to cause O-H bond cleavage. The spectra show no evidence of an in-cage H-atom formation mechanism such as (PhOH)-Ph-1* --> [PhOH(+...)e(aq)(-)] --> [(PhOH.)-H-....] --> PhO. + H-.. Excitation to S, also leads to triplet-state formation. FT-EPR spectra show that triplet-state phenol (p-cresol) reacts with ground-state phenol (p-cresol) to give a cyclohexadienyl-type radical and PhO..