Photoactive mono- and polynuclear Cu(i)–phenanthrolines. A viable alternative to Ru(ii)–polypyridines?

Over the past two decades the photochemical and photophysical properties of Cu(I)–phenanthrolines (Cu(NN)2+) have been investigated in detail. A high degree of control of the metal-to-ligand charge-transfer (MLCT) absorption and luminescence properties of Cu(NN)2+ is now possible, by means of a thorough choice and positioning of the phenanthroline ligand substituents. Exchange of Cu(I) with a variety of metal ions (Mn+) allows tuning of the electrochemical and photophysical properties of the M(NN)2n+ motif. This has prompted the design of fascinating multicomponent molecular architectures (catenates, knots, rotaxanes, dendrimers) in which photoinduced intercomponent processes like energy- and electron-transfer occur. The possibility of a fine tuning of the absorption and emission properties, long excited state lifetimes, and a characteristic structural flexibility, suggest Cu(I)–phenathrolines as interesting alternatives to Ru(II)–polypyridines, still the most popular family of complexes among inorganic photochemists.