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.

Graphical abstractEnergy-transfer rate-constants from tris(bipyridine)ruthenium(II) to chromate(III) complexes in aqueous depend on the molecular size of the ruthenium(II) complex. However, the rate constant is much more sensitive to the chromate(III) complex than to ruthenium(II) one. This result indicate that the ligand-congeniality between donor and acceptor in encounter complex plays a very important role in energy transfer even in aqueous solution.Figure optionsDownload full-size imageDownload as PowerPoint slide