A new efficient copper photocatalyst to photochemically induce the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction without the need for additional ligands is reported. The thioxanthone carboxylate moiety was combined with copper(II) ions by a simple ion exchange reaction to form a copper (II) thioxanthone carboxylate photocatalyst. In the catalytic process, intramolecular photoinduced electron transfer was stimulated to reduce copper(II) ions giving rise to a copper(I) species capable of catalyzing the CuAAC reaction. Temporal control over the click reaction was demonstrated by intermittent light on/off experiments. (C) 2015 Elsevier Ltd. All rights reserved.