Compounds of the formula (I), (II), and wherein R.sub.1, R.sub.2, R'.sub.2 and R'''.sub.2 for example are C.sub.1-C.sub.20alkyl; R.sub.3, R.sub.4, and R.sub.5 for example independently of one another are hydrogen or a defined substituent provided that at least one of R.sub.3, R.sub.4 or R.sub.5 is other than hydrogen or C.sub.1-C.sub.20alkyl; R.sub.6, R.sub.7, R.sub.8, R'.sub.6, R'.sub.7, R'.sub.8, R''.sub.6, R''.sub.7, R'''6 and R'''7 for example independently of one another have one of the meanings as given for R.sub.3, R.sub.4, and R.sub.5; and R.sub.9 for example is C.sub.1-C.sub.20alkyl; exhibit an unexpectedly good performance in photopolymerization reactions. ##STR00001##