Anthraquinone photoinitiators for free radical polymerisation: Structure dependence on photopolymerisation activity

The photophysical, photochemical and photopolymerisation properties have been studied for four derivatives of 2-substituted anthraquinone. Photopolymerisation activity is highly dependent on the configuration of the lowest triplet excited state. Fluorescence and phosphorescence analysis indicates efficient intersystem crossing to the triplet state and the pi-pi* or n-pi* nature of the lowest excited triplet state, The four derivative possess either an amide or an ester type substituent. The amide derivatives possess an excited triplet state with pi-pi* configuration, whereas the ester derivatives exhibit an n-pi* excited tripler state. Photoinduced polymerisation of lauryl acrylate monomer has been recorded by differential scanning photocalorimetry. Without an amine cosynergist, absolute quantum yields of photopolymerisation show that the anthraquinones with an ester type substituent and possessing an n-pi* configuration are more active. In the presence of a tertiary amine cosynergist the anthraquinone derivates with an amide substituent possessing a longer wavelength pi-pi* triplet state show enhanced activity. Nanosecond and microsecond hash photolysis studies and absolute quantum yields of photoreduction have been related to their photopolymerisation activity. The results indicate the presence of semi-anthraquinone and radical anion intermediates in the photoreduction step. The type of photoreduction reaction occurring is dependent on the nature of the excited state. The two types of derivative react predominantly via either direct hydrogen atom abstraction or electron transfer followed by proton transfer. The predominant method by which each molecule undergoes photoreduction has been demonstrated by steady state irradiations and flash photolysis studies. Copyright (C) 1996 Elsevier Science Ltd.