Photolysis of acylphosphine oxides II: The influence of methyl substitution in benzoyldiphenylphosphine oxides

Argon-saturated dilute solutions of 2-methyl, 4-methyl-, 2,4,6-trimethyl- and non-substituted benzoyldiphenylphosphine oxide (OTDPO, PTDPO, TMDPO and BDPO respectively) in various solvents were irradiated with flashes (duration, 20 ns) of 347 nm light. All compounds underwent α scission in times much shorter than the flash time: φ(α) = 0.5 – 0.6 (OTDPO, TMDPO and BDPO) and φ(α) - 1.0 (PTDPO). Diphenylphosphonyl radicals generated in this way characteristically absorb light around 330 nm. In the case of OTDPO, photoenolization competes with α scission, and transient spectra were observed which were assigned to the excited triplet (k = 3 × 107 s−1) and to the ground state (k = (3 × 105) - (1.5 × 104) s−1) of the enols. TMDPO did not undergo enolization. It is concluded that enolization involves triplets and that a triplet lifetime longer than a few nanoseconds is a prerequisite for its occurrence.