A propargyl-appended Eu(III) complex shows unanticipated reactivity towards NaN3 in the Cu(I) catalysed click reaction yielding an unsubstituted 1,2,3-triazole. The resulting complex exhibits pH-responsive 1H NMR and Eu(III) luminescence behaviour in the physiological pH range consistent with deprotonation of the NH-acidic bound triazole.