Hideki Tachi

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.

Non-vertical energy transfer to oximes: role of structural changes

Energy transfer to aromatic ketone oximes can exhibit non-vertical behaviour depending on the planarity of ground state equilibrium geometry, a fact discussed in terms of structural changes of ground and triplet states.

Photo-initiated thermal cross-linking behavior of acetophenone O-acryloyloxime-methyl methacrylate copolymer films in the presence of quinones

Abstract We have investigated the photo-initiated thermal cross-linking systems of acetophenone O-acryloyloxime (AAPO)–methyl methacrylate (MMA) copolymer with quinones such as benzoquinone (BQ), 1,4-naphthoquinone (NQ) and 2-dodecylthio-1,4-benzoquinone (DTBQ). Quinones have been found to act as sensitizers of photolysis of acyloxyimino (AOI) groups. It is clarified that photo-cross-linking of AAPO–MMA copolymer films is due to the reaction of polymer radicals formed in the photolysis of AOI groups with quinones and that their post-thermal cross-linking is due to that of the resultant amino groups with quinones. NQ was most effective in the photo-cross-linking among the quinones. DTBQ was most effective for photo-assisted thermal cross-linking.

Novel photolabile crosslinkers based on O-acyloxime moiety

Controlled decrosslinking is an attractive technique in the fields of functional materials and polymer recycling. Photo-triggered decrosslinking is especially advantageous for easy remote activation and spatiotemporal control. However, only a few photolabile units have been used for the cleavage in crosslinkers. Herein, 1,4-diacetylbenzene 1,4-bis(O-methacryloyl)dioxime and 1,3,5-triacetylbenzene 1,3,5-tris(O-methacryloyl)trioxime are proposed as novel crosslinkers. They have a benzene ring bearing two or three arms in which O-acyloxime moiety is introduced as a novel type of photolabile unit. These crosslinkers were prepared from aromatic ketones in two steps. Photopolymerization of methyl acrylate and the crosslinkers was performed using diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as a photoinitiator and UV light above 310 nm. Obtained polymer films were crosslinked and insoluble in tetrahydrofuran (THF). These films were further irradiated at 254 nm of light. The photolysis of O-acyloxime units in these crosslinkers were confirmed by UV and IR spectral measurements. The precedence of decrosslinking on 254 nm irradiation was evidenced by the decrease in thickness of the films after soaking in THF. Soluble fraction of the decrosslinked polymers were subjected to size exclusion chromatography, and their molecular weights were found to be similar to that obtained by photopolymerized film in the absence of the crosslinker. These results demonstrated that the proposed compounds are potential candidates for novel photolabile crosslinkers.