Compounds of the formula (I), wherein R.sub.1, R.sub.2 and R.sub.10 independently of one another are C.sub.1-C.sub.20alkyl, phenyl, C.sub.1-C.sub.12alkylphenyl or phenyl-C.sub.1-C.sub.6alkyl; R.sub.3 and R.sub.4 independently of one another are hydrogen, C.sub.1-C.sub.20alkyl, NR.sub.6R.sub.7 or SR.sub.8, provided that at least one of R.sub.3 or R.sub.4 is NR.sub.6R.sub.7 or SR.sub.8; R.sub.5 is hydrogen or C.sub.1-C.sub.20alkyl; R.sub.6 and R.sub.7 independently of one another are C.sub.1-C.sub.20alkyl, or R.sub.6 and R.sub.7 together with the N-atom to which they are attached form a 5 or 6 membered ring, which optionally is interrupted by O, S or NR.sub.9 and which optionally additionally is substituted by one or more C.sub.1-C.sub.4alkyl; R.sub.8 is phenyl, biphenylyl, naphthyl, anthryl or phenanthryl, all of which optionally are substituted by one or more C.sub.1-C.sub.4alkyl; and R.sub.9 is hydrogen, C.sub.1-C.sub.20alkyl, C.sub.2-C.sub.4hydroxyalkyl or phenyl; exhibit an unexpectedly good performance in photopolymerization reactions.