Ruthenium tris-(bipyridyl) complexes with pendant protonatable and deprotonatable moieties: pH sensitivity of electronic spectral and luminescence properties

Abstract

The effect of pH on the electronic absorption spectra and luminescence behaviour of four complexes consisting of {Ru(bipy)₃}²⁺ chromophores (bipy = 2,2′-bipyridine) bearing pendant pH-sensitive functional groups has been investigated by pH titrations. For [Ru(bipy)₂L¹]²⁺1 (L¹ = 2,2′∶4′,4″-terpyridine) protonation of the pendant pyridyl group (pK_a = 3.2) results in a red-shift of the lowest-energy metal-to-ligand charge-transfer (m.l.c.t.) band by 870 cm^-1 due to the lowering of the π* level of L¹ which occurs on protonation. For [Ru(bipy)₂L²]²⁺2 [L² = 4(4-pyridyl)-2,2′∶4′,4″-terpyridine] both pendant pyridyl sites protonate simultaneously (pK_a = 3.6); the red-shift of the lowest-energy m.l.c.t. band is 1570 cm^-1, approximately double that which occurred for 1. The pK_a* values of 1 and 2 were estimated to be 5.0 and 5.2 respectively, indicating that their excited states are more basic than the ground state due to the presence of an additional electron on L¹ or L² as a reslt of the m.l.c.t. process. The luminescence of [Ru(bipy)₂(HL³)]²⁺3 [HL³ = 4-(4-hydroxyphenyl)-2,2′ -bipridine] is completely quenched on deprotonation of the pendant phenol group, while that of isomeric [Ru(bipy)₂(HL⁴)]²⁺4 [HL⁴ = 4-(3-hydroxyphenyl)-2,2′ -bipyridine] is only partially quenched, owing to the inability of the ligand π system of [L⁴]^- to allow internal charge transfer of the phenolate negative charge to the metal centre across a meta-substituted linkage. The pK_a values for 3 and 4 are 8.6 and 8.9 respectively, and their pK_a* values are essentially the same as the pK_a values because the m.l.c.t. excited states involve the ancillary bipy ligands rather than HL³ or HL⁴. The new compound HL⁴ was crystallographically characterised revealing that molecules of HL⁴ associate via OH · · · N hydrogen-bonding interactions to give a one-dimensional ribbon.

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