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) ₂ L ¹ ] ²⁺ 1 (L ¹ = 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 L ¹ which occurs on protonation. For [Ru(bipy) ₂ L ² ] ²⁺ 2 [L ² = 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 L ¹ or L ² as a reslt of the m.l.c.t. process. The luminescence of [Ru(bipy) ₂ (HL ³ )] ²⁺ 3 [HL ³ = 4-(4-hydroxyphenyl)-2,2′ -bipridine] is completely quenched on deprotonation of the pendant phenol group, while that of isomeric [Ru(bipy) ₂ (HL ⁴ )] ²⁺ 4 [HL ⁴ = 4-(3-hydroxyphenyl)-2,2′ -bipyridine] is only partially quenched, owing to the inability of the ligand π system of [L ⁴ ] ^- 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 HL ³ or HL ⁴ . The new compound HL ⁴ was crystallographically characterised revealing that molecules of HL ⁴ associate via OH · · · N hydrogen-bonding interactions to give a one-dimensional ribbon.