Photochemistry and photo-induced co-synergistic polymerisation activities of novel N,N,-dimethylaminobenzoates and benzamides

Three novel alkylamino ester and seven benzamide derivatives of N,N-dimethy laminobenzoic acid have been prepared and characterised and their behaviour examined as co-synergists in photocuring of acrylate monomers and prepolym ers using 2-propoxythioxanthone as the standard intiator. Absorption, fluor escence and photoreduction analysis have been undertaken in an attempt to r elate their photochemistry to their co-synergy in curing. The esters were f ound to exhibit longer wavelength absorptions than those of the amides and displayed a strong degree of charge-transfer in their lowest excited single t state. The lowest excited singlet states of all the compounds were found to be essentially pi pi* in character. All the co-synergists provided some degree of photoreduction for the initiator 2-propoxythioxanthone. In genera l, the amides, apart from compound 3, were found to be more effective in ph otoreduction than the esters. The photocuring data for the different co-syn ergists showed some inconsistencies and appeared to have no relationship wi th their photochemistry. This is in complete contrast with their primary in itiator counterparts where correlations of photochemical and photophysical properties relates well to their photoactivities. The role of the amine co- synergist appeared to be variable and dependent upon structure, electron do nation, absorption and perhaps inherent photolytic stability. Overall, the amine esters were found to exhibit enhanced activity compared with the amid es and in this way could be related to their greater degree of charge-trans fer character enhancing co-synergy to form a more reactive triplet-exciplex with the thioxanthone initiator. (C) 2000 Elsevier Science B.V. All rights reserved.