PHOTODISSOCIATION OF HEXAARYLBIIMIDAZOLE .2. DIRECT AND SENSITIZED DISSOCIATION

Photodissociation of a widely used photopolymerization initiator, 2-chlorohexaarylbiimidazole (omicron-Cl-HABI), is studied in dichloromethane solution in the absence and presence of the visible light photosensitizing dye, 2,5-bis[(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-1-yl) methylene]cyclopentanone (JAW). Laser flash photolysis at 480 nm is employed. In the absence of JAW, the omicron-Cl-HABI dissociates into triarylimidazolyl radicals (L.). In the presence of JAW, an increase in L. formation is observed. The mechanism of this photosensitization of dissociation is explored. It is concluded that this increase occurs by the dissociation of the omicron-Cl-HABI radical anion formed by electron transfer from the excited singlet state of JAW to omicron-Cl-HABI. The observed formation of L. radicals exhibits a linear dependence on omicron-Cl-HABI concentration. The rate constant of electron transfer obtained from this dependence is equal to (2.2 +/- 0.4) x 10(9) M-1 s-1. No reaction between the excited triplet state of JAW and omicron-Cl-HABI is found.