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 (epsilon = 16460 L mol(-1) cm(-1)) absorption band between 500-700 nm which are assigned to an n(amido, RNH-) -> pi* (dppz) inter-ligand charge transfer band offering an extraordinarily low energy charge separating absorption with potential for imaging and energy application.