Luminescent probes for indole-binding proteins derived from ruthenium(II) polypyridine complexes

Abstract

We report here the synthesis, characterisation, electrochemical, photophysical and protein-binding properties of four luminescent ruthenium(II) polypyridine indole complexes [Ru(bpy) 2(L1)](PF 6) 2 ( 1), [Ru(bpy) 2(L2)](PF 6) 2 ( 2), [Ru(L1) 3](PF 6) 2 ( 1a), and [Ru(L2) 3](PF 6) 2 ( 2a) (bpy = 2,2′-bipyridine; L1 = 4-( N-(2-indol-3-ylethyl)amido)-4′-methyl-2,2′-bipyridine; L2 = 4-( N-(6- N-(2-indol-3-ylethyl)hexanamidyl)amido)-4′-methyl-2,2′-bipyridine). Their indole-free counterparts, [Ru(bpy) 2(L3)](PF 6) 2 ( 3) and [Ru(L3) 3](PF 6) 2 ( 3a) (L3 = 4-( N-(ethyl)amido)-4′-methyl-2,2′-bipyridine), have also been synthesised for comparison purposes. Cyclic voltammetric studies revealed ruthenium-based oxidation at ca. +1.3 V versus SCE and diimine-based reductions at ca. −1.20 to −2.28 V. The indole moieties of complexes 1, 2, 1a and 2a displayed an irreversible wave at ca. +1.1 V versus SCE. All the ruthenium(II) complexes exhibited intense and long-lived orange-red triplet metal-to-ligand charge-transfer 3MLCT (dπ(Ru) → π*(L1–L3)) luminescence upon visible-light irradiation in fluid solutions at 298 K and in alcohol glass at 77 K. The binding of the indole-containing complexes to bovine serum album (BSA) has been studied by quenching experiments and emission titrations.