Maleimides as electron-transfer photoinitiators: quantum yields of triplet states and radical-ion formation

The knowledge of quantum yields of triplet states and, moreover, of initiating radicals is a crucial prerequisite to evaluate any novel photoinitiator system. This work provides triplet quantum yields of N-methylmaleimide (0.03+/-0.01), N-ethylmaleimide (0.07+/-0.01) and N-propylmaleimide (0.05+/-0.02) determined by relative actinometry using laser flash photolysis and acetone sensitisation. Unsubstituted maleimide on the other hand shows rapid triplet state tautomerisation, not allowing the application of relative actinometry with triplet sensitisation. The triplet quantum yield was then determined by comparing the transient conductivity of the enolate formed with conductivity actinometry; it is unity. The yield of initiating radicals is only a fraction of the triplet yield, as the electron transfer reaction includes an efficient back donation. The radical ion yields measured span from 2% (thiocyanate) to 28% (allylthiourea) of the triplet quantum yield. (C) 2000 Elsevier Science S.A. All rights reserved.