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Photovoltaic systems for solar light harvesting offer a potential route to low power electricity. Ruthenium (Ru) complexes with a bpy (2,2′-bipyridine) ligand have traditionally been used in this research and in Grätzel cells, as they have excellent photophysical properties. Ru(tpy)₂²⁺ (2,2′:6′,2″-terpyridine) is a chromophore that has been used in multinuclear structures, but its room temperature excited-state lifetime is < 250 ps. Longer lifetimes have been obtained for Ru(II) terpyridine complexes where one tpy ligand is attached to a coplanar pyrimidine while the other tpy moiety has a 9-anthryl organic chromophore. The longer lifetimes are due to a bichromophoric effect (the triplet states of the metal-to-ligand charge-transfer (³MLCT) and of the anthracene are in equilibrium). In this case the Ru and anthryl chromophores are on the same ligand and synthesis is not easy.

Now, researchers from the Université de Montréal, Canada, and Università di Messina, Italy, have synthesised a new series of luminescent Ru(tpy)₂²⁺ species where the chromophores are separated by > 1 nm. The 9-anthryl chromophore (the energy reservoir) is in the tpy moiety not involved in the ³MLCT emitting level, while the 2-pyrimidyl-tpy subunit is involved in extended electron delocalisation.

In their method, the 9-anthryl chromophore is inserted into the 4′-position of the tpy moiety, giving a 4′-(9-anthryl)-2,2′:6′,2″-tpy Ru(II) complex. This started bichromophoric behaviour by lowering the ³MLCT state of Ru(tpy)₂²⁺ via the 2-pyrimidyl-tpy subunit, and increased the luminescence lifetimes because of the excited-state equilibrium. The luminescence lifetimes are long compared to those for Ru(tpy)₂²⁺.

This method gives separated chromophores on different ligands with long-lived excited states. It may allow special design of the Ru tpy moiety and the organic chromophore, and could result in compounds with specific photophysical properties.