Compounds of the formula I or (I'), wherein R1 and R'1 for example are hydrogen, C3-C8cycloalkyl or C1-C12alkyl, R2 and R2' for example are hydrogen； unsubstituted C1-C20alkyl or substituted C1-C20alkyl； and R8 and R9 for example are hydrogen, C1-C12alkyl which optionally is substituted, or phenyl which optionally is substituted； exhibit an unexpectedly good performance in photopolymerization reactions.
