Simultaneous control of spectroscopic and electrochemical properties in functionalised electrochemiluminescent tris(2,2′-bipyridine)ruthenium(ii) complexes

Abstract

Using a combination of electrochemical, spectroscopic and computational techniques, we have explored the fundamental properties of a series of ruthenium diimine complexes designed for coupling with other molecules or surfaces for electrochemiluminescence (ECL) sensing applications. With appropriate choice of ligand functionality, it is possible to manipulate emission wavelengths while keeping the redox ability of the complex relatively constant. DFT calculations show that in the case of electron withdrawing substituents such as ester or amide, the excited state is located on the substituted bipyridine ligand whereas in the case of alkyl functionality it is localised on a bipyridine. The factors that dictate annihilation ECL efficiency are interrelated. For example, the same factors that determine ΔG for the annihilation reaction (i.e. the relative energies of the HOMO and LUMO) have a corresponding effect on the energy of the excited state product. As a result, most of the complexes populate the excited state with an efficiency (Φ(ex)) of close to 80% despite the relatively wide range of emission maxima. The quantum yield of emission (Φ(p)) and the possibility of competing side reactions are found to be the main determinants of ECL intensity.

Near‐Infrared Electrochemiluminescence from Bistridentate Ruthenium(II) Di(quinoline‐8‐yl)pyridine Complexes in Aqueous Media

“…The reductions associated with 2 occur at more negative potentials than 1 , probably due to the electron‐withdrawing effect of the carboxylic group compared to the ester. The irreversible electrochemistry associated with complex 2 is a consequence of the presence of a carboxylic acid moiety, as observed for other polypyridyl ruthenium complexes containing such groups …”

Co‐reactant Electrogenerated Chemiluminescence of Iridium(III) Complexes Containing an Acetylacetonate Ligand

“…For the complexes considered here, plots of the frontier MOs are given in Figure and S6. The MOs of [Ru(bpy) 3 ] 2+ are already well characterized, with a metal‐centered HOMO and ligand‐based LUMO . The triplet‐state spin density (Figure and S7) shares the same spatial extent as the singlet HOMO and LUMO, for which the lowest singlet‐triplet transition may be described as metal‐to‐ligand charge‐transfer (MLCT) .…”

Control of Excitation and Quenching in Multi‐colour Electrogenerated Chemiluminescence Systems through Choice of Co‐reactant

“…For these multi‐luminophore experiments, we replaced [Ru(bpy) 3 ] 2+ with [Ru(bpy) 2 (L)] 2+ (L= N 4 , N 4′ ‐bis((2S)‐1‐methoxy‐1‐oxopropan‐2‐yl)‐2,2′‐bipyridyl‐4,4′‐dicarboxamide)). The electron‐withdrawing amide groups of this complex impart a bathochromic shift ( λ max =666 nm),18 which provides much greater spectral separation from [Ir(ppy) 3 ] (Figure S3 in the Supporting Information). Although the amide groups also influence the ground and excited state redox properties (e.g., [Ru(bpy) 3 ] 2+ *: E ox =−1.22 V and E red =+0.36 V, whereas [Ru(bpy) 2 (L)] 2+ *: E ox =−0.90 V and E red =+0.39 V vs Fc 0/+ ),6h, 18 the differences are small when compared to the properties of [Ir(ppy) 3 ]* (see above), and it is the much greater reducing ability of the excited [Ir(ppy) 3 ]* species that renders it vulnerable to oxidative quenching.…”

“…The electron‐withdrawing amide groups of this complex impart a bathochromic shift ( λ max =666 nm),18 which provides much greater spectral separation from [Ir(ppy) 3 ] (Figure S3 in the Supporting Information). Although the amide groups also influence the ground and excited state redox properties (e.g., [Ru(bpy) 3 ] 2+ *: E ox =−1.22 V and E red =+0.36 V, whereas [Ru(bpy) 2 (L)] 2+ *: E ox =−0.90 V and E red =+0.39 V vs Fc 0/+ ),6h, 18 the differences are small when compared to the properties of [Ir(ppy) 3 ]* (see above), and it is the much greater reducing ability of the excited [Ir(ppy) 3 ]* species that renders it vulnerable to oxidative quenching. We have found that even a small decrease in the reducing ability, such as when a single fluorine group is added to the phenyl ring of one ligand, lessens the degree of the ‘switch off’ of ECL (Figure S5 in the Supporting Information).…”