Some embodiments of the present invention may be said to be directed to metal complexes of Formula I, wherein at least one of X1, X2, X3, R1, R2, R3, R4 and R5 is what may be characterized as an antenna capable of providing (e.g., absorbing and/or emitting) an appropriate electromagnetic signal. Some embodiments of the present invention are directed to ligands corresponding to metal complexes of Formula I. Some embodiments of the invention are directed to methods of determining renal function using at least one metal complex of Formula I.