Effect of ligand congeniality on energy transfer reaction between photo-excited tris(bipyridine)ruthenium(II) and chromate(III) complexes in aqueous solutions

Energy-transfer rate-constants from photo-excited [Ru(N–N)3]2+ (N–N = 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (4dmb), 5,5′-dimethyl-2,2′-bipyridine (5dmb)) to [Cr(O–O)3]3− (O–O2− = ox2− ((COO−)2), mal2− (CH2(COO−)2)) and [Cr(CN)6]3− in encounter complexes were evaluated in aqueous solutions containing alkali metal ion. The rate constant depends on the molecular size of the ruthenium(II) complex: 1.8 × 108 s−1 for [Ru(bpy)3]2+ (molecular radius, r = 5.8 Å), 1.4 × 108 s−1 for [Ru(5dmb)3]2+ (r = 6.1 Å) and 0.96 × 108 s−1 for [Ru(4dmb)3]2+ (r = 6.7 Å) in the system of [Ru(N–N)3]2+–[Cr(ox)3]3− in aqueous solution. However, the rate constant is much more sensitive to the chromate(III) complex than to ruthenium(II) complex; 1.8 × 108 s−1 and 0.43 × 108 s−1 for [Cr(ox)3]3− (r = 4.0 Å) and [Cr(mal)3]3− (r = 4.2 Å) in the [Ru(bpy)3]2+–[Cr(O–O)3]3− systems, respectively. We conclude that the congeniality between the donor’s and acceptor’s ligands in encounter complex plays an important role in energy transfer in aqueous solution.