Oscillations of the lophyl radical concentration during the photo-decomposition of o-Cl-hexaarylbisimidazole

Hexaarylbisimidazole (HABI) is a well established photoinitiator in the graphics industry which undergoes in the absence of other reaction partners a fully reversible homolytic bond cleavage into two lophyl radicals L. Surprisingly we now found oscillations of the radical concentration at low temperature in a solution of o-Cl-HABI in toluene, which is an indication of a more complex photochemistry of the HABI/lophyl system. Furthermore this reaction is a very simple example of an oscillating chemical system working under non-flow conditions which involves only one initial reactant. A possible reaction mechanism based on kinetic simulations is proposed.