Investigation of the photochemical properties of an important class of photobase generators: the O-acyloximes

Abstract

The influence of the molecular structure and the required conditions for bond breaking in the triplet state of 10 carefully selected O-acyloximes (eight of them are original) usable as photobase generators are deeply investigated for the first time. Ground state absorption and fluorescence spectra revealed that the transitions involved exhibit a ππ∗ character. A clear nonvertical character in triplet–triplet energy transfer was found in compounds exhibiting a flexible arylCNO moiety. An important and new point is concerned with this effect which strongly affects the triplet sensitized decomposition process, due to the possible formation of a highly vibrationally excited triplet state from which dissociation can occur. Laser experiments enabled to detect the primary free radicals formed from both direct and sensitized dissociation and to measure the relative quantum yields in iminyl radicals. Comparison of these laser experiments with steady state quantum yields of photolysis demonstrates the involvement of in-cage reactions that influence the overall efficiency of base generation, depending on the acyl moiety. All these facts were rationalized in a global mechanism of photodissociation and found to explain well the results of photoinitiated thermal crosslinking experiments performed in a model media with the starting oxime esters.