New Cleavable Photoinitiator Architecture with Huge Molar Extinction Coefficients for Polymerization in the 340-450 nm Range.

A new Type I photoinitiator Tr_DMPA is described. It consists in three 2,2'-dimethoxy 2-phenyl acetophenone (DMPA) units grafted onto a truxene (Tr) scaffold. Compared to DMPA itself, the lowest electronic transition exhibits a pi pi* character and the corresponding molar extinction coefficients epsilon are increased from about 400 M-1 cm(-1) (at about 332 nm for DMPA) to 63 000 M-1 cm(-1) (at 338 nm for Tr_DMPA); such huge values are exceptional in Type I photoinitiators at this wavelength. Tr_DMPA undergoes a fast cleavage and efficiently initiate an acrylate polymerization upon a Xe-Hg lamp, a halogen lamp or a laser diode exposure at 405 nm (upon very low light intensities: 2-12 mW/cm(2)) in the 300-450 nm range. The polymerization of epoxides or divinylethers is also feasible in the presence of an iodonium salt at 405 nm where reference photoinitiators cannot efficiently operate. The chemical mechanisms analyzed by ESR, fluorescence, steady state photolysis, and laser flash photolysis experiments are discussed.