Systems, including compositions, kits, and methods, particularly for photoluminescence applications. The systems may include, among others, (1) organic chelators, (2) complexes between the chelators and lanthanide ions, and (3) precursors, derivatives, and uses thereof. The chelators may include a 1,4,7,10-tetraazacyclododecane ring system, for example, having the formula: ##STR00001## Here, R.sup.1, R.sup.2, and R.sup.3 are substituents of the tetraazacyclododecane ring system, that is further substituted at the 10-position by a sensitizer Z that is typically a polyheterocyclic ring system. The organic chelator may be capable of forming a luminescent complex with a lanthanide ion, and is optionally further substituted by a reactive functional group or a conjugated substance. The resulting lanthanide complex may be useful in luminescence-based assays, such as energy transfer assays, among others.