Excited-State Processes in Ruthenium(II) Bipyridine Complexes Containing Covalently Bound Arenes

The photophysical properties of a series of ruthenium trisbipyridine complexes covalently linked to aromatic chromophores of the type [Ru(bpy) 2 (4-methyl-4‘-(2-arylethyl)-2,2‘-bipyridine)] 2+ (ClO 4 ) 2, where aryl = 2-naphthyl ([Ru]-naphthalene), 1-pyrenyl ([Ru]-pyrene), and 9-anthryl ([Ru]-anthracene) have been investigated at room temperature and at 77 K. The photophysical properties of these bichromophores are determined by intramolecular energy-transfer processes that are governed by the relative positions of the various singlet and triplet energy levels. As a result, fluorescence from each of the pendant aromatic chromophores is completely quenched following their photoexcitation. For [Ru]-naphthalene the initial excitation energy is localized on the [Ru]-centered 3 MLCT state, whereas for [Ru]-anthracene the energy is localized on the anthracene triplet state. Since the [Ru]-centered 3 MLCT state and the lowest energy pyrene triplet state are isoenergetic, an equilibrium is established resulting in a long-lived room-temperature 3 MLCT emission from [Ru]-pyrene (τ = 5.23 μs). At 77 K dual emission is observed from this bichromophore comprising pyrene phosphorescence and 3 MLCT emission, the relative proportions of which vary with time after the laser pulse.