The mechanism of 4-chlorophenol (4CP) photolysis was investigated with
the aid of Fourier Transform Electron Paramagnetic Resonance (FT-EPR)
and pulsed-laser photolysis combined with High Performance Liquid Chr
omatography (HPLC) detection of stable (diamagnetic) products. With FT
-EPR transient free radicals produced by pulsed-laser excitation of so
lutions of 4CP in alcohols could be identified. Time profiles of the F
T-EPR spectra provided information on reaction kinetics and Chemically
Induced Dynamic Electron Polarization (CIDEP) effects. It was found t
hat 4CP photolysis in alcohols leads to the simultaneous formation of
the phenoxyl radical and radicals produced by hydrogen abstraction fro
m the solvent. CIDEP patterns establish that these radicals are formed
in a reaction sequence involving a triplet state precursor and radica
l pair intermediate. Results of earlier transient optical absorption m
easurements indicate that the triplet precursor must be the carbene 4-
oxocyclohexa-2,5-dienylidene. This assignment is supported by the find
ing that photolysis of quinone diazide in a hydrogen-donating solvent
gives the same free radical products as those obtained from 4CP. The f
ormation of the phenoxyl radical intermediate accounts for the finding
that photolysis of deoxygenated solutions of 4CP in alcohols gives ph
enol as stable diamagnetic product. By contrast, photolysis of aerated
and deoxygenated aqueous solutions of 4CP produces benzoquinone and h
ydroquinone as primary products, respectively.