Photoinduced intercomponent energy transfer in covalently-linked dinuclear complexes containing Ru(II)-bipyridine and Ru(II)-biquinoline chromophores and aromatic and aliphatic spacers

Abstract

We have synthesized a number of dinuclear species containing both identical or different metal-based components by employing new bridging ligands having either aliphatic or aromatic spacers and taking advantage of the “complexes as metals and complexes as ligands” synthetic strategy. The bridging ligands are dpt-S-dpt (S is 1,4-cyclohexyl, 1,4-phenyl, 4,4′-biphenyl; dpt is 4-amino-3,5-bis(2-pyridyl)- 1,2,4-triazole; the connections between S and dpt are provided by amide links). The complexes synthesized are: [(bpy) 2Ru(dpt-S-dpt)Ru(bpy) 2](PF 6) 4 (bpy=2,2′-bipyridine; biq=2,2′-biquinoline; S=1,4-cyclohexyl ( 1), 1,4-phenyl ( 4), 4,4′-biphenyl ( 7)); [(biq) 2Ru(dpt-S-dpt)Ru(biq) 2](PF 6) 4 (S=1,4-cyclohexyl ( 2), 1,4-phenyl ( 5), 4,4′-biphenyl ( 8); [(bpy) 2Ru(dpt-S-dpt)-Ru(biq) 2](PF 6) 4 (S=1,4-cyclohexyl ( 3), 1,4-phenyl ( 6), 4,4′-biphenyl 3 9)). The absorption spectra, luminescence properties and redox behavior of all the compounds have been studied. In the complexes containing different metal-based components, photoinduced energy transfer occurs from the higher-lying Ru→bpy CT level, centered on a metal subunit, to the lower-lying Ru→biq CT excited state, centered on the other metal component. In fluid solution at room temperature, the energy transfer is suggested to be mediated by a two-step electron transfer mechanism, whereas direct energy transfer between the chromophores most likely occurs at 77 K in rigid matrix. At the moment we arc not able to say if the energy transfer at 77 K takes place via electron exchange or coulombic mechanisms. The results obtained indicate that the efficiency of the processes depends on the donor-acceptor distance, as expected, and that occasional π bonds which are present within the bridging ligands cannot be used for speeding up electron transfer in multicomponent systems if the main skeleton of the bridge is made by σ bonds.