Photoinduced energy transfer in a rod-like dinuclear Ru(II) complex containing bis-pyridyl-1,3,5-triazine ligands

Three new rod-like dinuclear Ru(II) polypyridine compounds have been prepared and characterized and their absorption spectra, redox behaviour and photophysical properties have been investigated, by conventional steady-state absorption and luminescence spectroscopic methods and by time-resolved methods operating in the nanosecond and femtosecond time regimes. All the new species contain 1,4-bis(2,2':6',2 ''-terpyridin-4'-yl)benzene (4) as bridging ligand and 2,4-bis(2-pyridyl)-6-p-bromophenyl-1,3,5-triazine and/or 4'-(p-bromophenyl)-2,2':6',2 ''-terpyridine as peripheral ligands. In particular, 1 and 2 are symmetric dyads since the Ru(II) ions carry identical peripheral ligands (the triazine-based ligands in 1 and the terpyridine-based ligands in the case of 2), whereas 3 is a non-symmetric dyad since its two Ru(II) centers carry two different peripheral ligands (for the structural formulae of the compounds, see Chart 1). The absorption spectra and redox behaviour of the new compounds indicate, that each subunit of the dyads maintain its own peculiar properties in the dinuclear system, a requisite confirming the supramolecular nature of the systems. All the species exhibit metal-to-ligand charge-transfer (MLCT) emission, both at room temperature influid solution and at 77 K in rigid matrix. Luminescence lifetime and transient absorption spectroscopy reveal that efficient and fast photoinduced energy transfer takes place in the non-symmetric dyad 3 from the subunit containing the terpyridine peripheral ligand to the subunit containing the triazine-based peripheral ligand. However, given the complexity of the transient spectra and the overlapping timescales of the processes of the symmetric and non-symmetric systems, any detailed analysis of the ultrafast results aimed at the identification of specific intercomponent energy transfer steps, and therefore to kinetically characterize the directional energy transfer, appears to be speculative.