The present invention relates to unsymmetrical and symmetrical phosphorus-comprising heptazine derivatives, represented by the formula (1): ##STR00001## in which R.sub.a, R.sub.b and R.sub.c are, independently of one another, an azide group --N.sub.3 or an --N.dbd.PR.sub.1R.sub.2R.sub.3 group, with the proviso that at least one radical from R.sub.a, R.sub.b and R.sub.c is an --N.dbd.PR.sub.1R.sub.2R.sub.3 group, to a process for the preparation thereof and to the use thereof.