Photoreactive dendrimers comprising a core portion, branching units and terminal groups, wherein at least one terminal group and/or branching unit is a photoreactive group and wherein the photoreactive groups include preferably cinnamates, coumarins, benzylidenephthalimidines, benzylideneacetophenones, diphenylacetylenes stilbazoles, uracyl, quinolinone, maleinimides, or cinnamylidene acetic acid derivatives and are able to undergo photocyclization, in particular Ä2+2Ü- photocyclization.