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 ( ), and hydrogen atoms (H • ). From Chemically Induced Dynamic Electron Polarization (CIDEP) effects and the effect of addition of N 2 O 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 (λ ex = 193 nm) gives rise to H-atom formation by O−H bond homolysis and by 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 . 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 (λ ex = 266 nm) can be completely quenched by addition of N 2 O. In this case H-atom formation is due entirely to the reaction of 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 1 PhOH* → [PhOH + ··· ] → [PhO • ···H • ] → PhO • + H • . Excitation to S 1 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 • .