The
p-hydroxyphenacyl group
1 is an effective photoremovable protecting group, because it undergoes an unusual photo-Favorskii rearrangement concomitant with the fast release (<1 ns) of its substrates in aqueous solution. The reaction mechanism of the diethyl phosphate derivative
1a was studied by picosecond pump-probe spectroscopy, nanosecond laser flash photolysis, and step-scan FTIR techniques. The primary photoproduct is a triplet biradical,
33, with a lifetime of about 0.6 ns. The release of diethyl phosphate determines the lifetime of the triplet state T
1(
1a),
![](/images/gifchars/tau.gif)
(T
1) = 60 ps in wholly aqueous solution. Formation of a new photoproduct,
p-hydroxybenzyl alcohol (
6), was observed at moderate water concentrations in acetonitrile. It is formed by CO elimination from the elusive spirodione intermediate (
4), followed by hydration of the resulting
p-quinone methide (
5). Computational studies show that CO elimination from the spirodione is a very facile process.