Blue rhenium tricarbonyl DPPZ complexes - low energy charge-transfer absorption at tissue-penetrating wavelengths.

fac-[Re(CO)3(dppz)] complexes of amido ligands generated in situ under basic conditions display an intense (ε = 16460 L mol-1 cm-1) absorption band between 500-700 nm which are assigned to an n(amido,RNH-) → π* (dppz) inter-ligand charge transfer band offering an extraordinarily low energy charge separating absorption with potential for imaging and energy application.