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 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 3MLCT 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 triplet state are isoenergetic, an equilibrium is established resulting in a long-lived room-temperature 3MLCT emission from [Ru]-pyrene (τ = 5.23 μs). At 77 K dual emission is observed from this bichromophore comprising pyrene phosphorescence and 3MLCT emission, the relative proportions of which vary with time after the laser pulse.

No abstract

The syntheses of linear and star-shaped light harvesting polymers with well defined structure and narrow molecular weight distribution are described. These polymers have ruthenium polybipyridine moieties as the energy trap cores and styrene functionalized coumarin monomers as the light absorbing antenna chromophores. The polymers have been made by reversible addition-fragmentation chain transfer (RAFT) polymerisation using di-or hexafunctional ruthenium-containing RAFT agents. The resulting ruthenium-containing polymers have narrow molecular weight distribution (polydispersity v 1.1) and exhibit energy transfer efficiencies of up to 60% between the coumarin donor dyes and the ruthenium acceptor chromophores.

The quenching (ηq) and subsequent cage-escape efficiency (ηce) have been measured for three arene-linked ruthenium trisbipyridine complexes quenched by methyl viologen (MV2+). The bichromophoric complexes are 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). The overall yield of MV•+ is given by the product ηqηce, which depends both on specific solvent effects and on the nature of the quenched excited state of the bichromophore, i.e., whether it is a metal-to-ligand charge transfer (3MLCT) or aromatic triplet state. In aqueous buffer the production of MV•+ is low for the three bichromophores (<10%). In methanol and acetonitrile MV•+ yields for [Ru]-anthracene and [Ru]-pyrene are >70%, reflecting the arene triplet character of the lowest excited state. This increase is due to an increase in the cage-escape efficiency (ηce) in these solvents. In contrast, for [Ru]-naphthalene the lowest excited state is 3MLCT in character and the yield of MV•+ in acetonitrile and methanol remains <10%. NMR spectra of the linking ethane group suggest that the bichromophores adopt different conformations in the different solvents, which could lead to the observed differences in ηce.

The oxidation of 4,4′-dimethyl-2,2′-bipyridine with potassium permanganate in water gives 2,2′-bipyridine-4,4′-dicarboxylic acid and 4′-methyl-2,2?-bipyridine-4-carboxylic acid. The latter acid is oxidized to the diacid by boiling nitric acid. Complexes of the type Ru ( bpy )2L2+ have been prepared where L is 2,2′-bipyridine-4,4′- dicarboxylic acid, diethyl 2,2′-bipyridine-4,4′-dicarboxylate, 4′- methyl-2,2′-bipyridine-4-carboxylic acid and ethyl 4′-methyl-2,2′- bipyridine-4-carboxylate. These complexes have been compared with [ Ru ( bpy )3]2+ as sensitizers for the photoreduction of water. Stern- Volmer analysis has been applied to the quenching of their luminescence by methylviologen (mv2+), [Co(sep)]3+ (sep is 1,3,6,8,10,13,16,19- octaazabicyclo [6.6.6] icosane ) and [Co( CLsar )]3+ ( CLsar is 1-chloro- 3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane ). Changes in the Stern-Volmer constants have been related to the free energy changes associated with the oxidative quenching and the overall charges of the ruthenium complexes. The rates of formation of hydrogen compared favourably in sacrificial cycles with the ruthenium complexes as sensitizers, mv2+, Co(sep)3+ as electron-transfer agents, platinum/poly(vinyl alcohol) as catalyst, and ethylenediaminetetraacetic acid as electron donor. The results obtained have been discussed in terms of variations in the efficiencies of cage escape in the oxidative quenching and competition between electron transfer and energy transfer.