Heteroditopic Ligand Accommodating a Fused Phenanthroline and a Schiff Base Cavity as Molecular Spacer in the Study of Electron and Energy Transfer

Abstract

The synthesis and characterisation of the heteroditopic ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-5,6-(1,10-phenanthroline)diamine (DPSalH(2)) bearing a phenanthroline and a bis(salicylidene)diimine cavity are reported. This versatile ligand combines two of the most widely used ligands in coordination chemistry. Sequential metallation of the phenanthroline end with Ru(II) and the salophenic cavity with Cu(II) is described. Electrochemical behaviour of the supramolecular complexes [Ru(bpy)(2)(DPSalH(2))](2+) and [Ru(bpy)(2)(DPSalCu)](2+) are analysed in connection with UV/Vis and EPR spectroscopy. The data for the one-electron-reduced species and the singly oxidised species of the binuclear Ru(II)-Cu(II) complex confirmed the formation of metalloradical complexes. Density functional calculations on the free ligand and the copper-only complex indicate in both cases that the HOMOs and LUMOs are developed on the Schiff base cavity with minor contributions on the bipyridine end. These findings support a bichromophoric character for our ruthenium complexes in the ground state, a necessary condition in the design of supramolecular systems for the study of electron transfer. Photophysical studies indicate fast quenching of the triplet excited state in both complexes, which suggests strong intercomponent excited-state interactions. Evidence is presented that this quenching is due to intramolecular electron transfer, at least in the case of [Ru(bpy)(2)(DPSalH(2))](2+), for which a charge-separated state with a remarkable lifetime of about 30 mus was observed.

A Theoretical Investigation into the Photophysical Properties of Ruthenium Polypyridine‐Type Complexes

“…A DFT approach was first proposed by Daul et al [22] Recently published TDDFT results have concerned complexes of Re I , [23] Ru II [11] and Ir III . [24] We have recently reported [1] the synthesis and characterisation of the heteroditopic ligand N,N'-bis(3,5-di-tert-butylsalicylidene)-5,6-(1,10-phenanthroline)diamine, which can be viewed as a fused phenanthroline and a tetradentate Schiff base cavity. The properties-including electrochemistry, UV/Vis spectroscopy and photophysical studies-of the related complex derived from [Ru(bpy) 3 ] 2 + by substitution of one bpy by the ligand have been described.…”

“…The energy level diagram for the frontier spin-orbitals of the T 1 state-found to be a 3 B state-in connection with the MOs for the 1 A state are presented in Figure 7. They are derived through consideration of the optimised geometries for both states.…”

“…The large number of works devoted to the study of the ground state and the lowest excited states of this system has allowed an understanding of the photophysical properties to be gained. Laser excitation results in 1 MLCT metal-to-ligand excited singlet state(s). Ultrafast singlet!triplet excited state conversion (intersystem crossing) occurs with a time constant of a few tenths of a fs, [31] but about a hundred femtoseconds seems to be the overall timescale for formation of the 3 MLCT set.…”

“…We have recently found evidence of a remarkable long-lived excited state (30 ms) for a Ru II complex containing a heteroditopic ligand that can be viewed as a fused phenanthroline and salophen ligand. [1] To unravel this intriguing electronic property, we have used density functional theory (DFT) calculations to understand the groundstate properties of [(bpy) 2 2 + . The outstanding result is the presence of excited states lower in energy than the metal-to-ligand charge-transfer states, originating from intraligand charge transfer (ILCT) from the phenolic rings to the phenanthroline part of the coordinated LH 2 .…”

Influence of the Protonic State of an Imidazole‐Containing Ligand on the Electrochemical and Photophysical Properties of a Ruthenium(II)–Polypyridine‐Type Complex

“…En route towards these target systems, we have prepared and characterised different heteroditopic ligands. [17,18] Recently, we have reported on the synthesis of an imidazole-phenol containing ligand covalently linked to the [RuA C H T U N G T R E N N U N G (bpy) 3 ] 2 + chromophore as a biomimetic model of the His190/TyrZ pair of amino acid residues, which are involved in the catalytic cycle of light-driven water oxidation in photosystem II. [19] A photoinduced electron transfer to an external acceptor (MV 2 + ) was observed to generate the phenoxyl radical concomitantly to the reduction of the photogenerated Ru III .…”