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)(+)(ClO4)(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 tripler 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 (MLCT)-M-3 state, whereas for [Ru]-anthracene the energy is localized on the anthracene triplet state. Since the [Ru]-centered 3MLCT state and the lowest energy pyrene tripler state are isoenergetic, an equilibrium is established resulting in a long-lived room-temperature 3MLCT emission from [Ru]-pyrene (tau = 5.23 mu s). At 77 K dual emission is observed from this bichromophore comprising pyrene phosphorescence and (MLCT)-M-3 emission, the relative proportions of which vary with time after the laser pulse.