Early photophysical events of a ruthenium(ii) molecular dyad capable of performing photochemical water oxidation and of its model compounds

Abstract

The early photophysical events occurring in the dinuclear metal complex [(ttb-terpy)(I)Ru(μ-dntpz)Ru(bpy)₂]³⁺ (2; ttb-terpy = 4,4′,4′′-tri-tert-butyl-terpy; bpy = 2,2′-bipyridine; dntpz = 2,5-di-(1,8-dinaphthyrid-2-yl)pyrazine) – a species containing the chromophoric {(bpy)₂Ru(μ-dntpz)}²⁺ subunit and the catalytic {(I)(ttb-terpy)Ru(μ-dntpz)}⁺ unit, already reported to be able to perform photocatalytic water oxidation – have been studied by ultrafast pump–probe spectroscopy in acetonitrile solution. The model species [Ru(bpy)₂(dntpz)]²⁺ (1), [(bpy)₂Ru(μ-dntpz)Ru(bpy)₂]⁴⁺ (3), and [(ttb-terpy)(I)Ru((μ-dntpz)Ru[(ttb-terpy)(I)]²⁺ (4) have also been studied. For completeness, the absorption spectra, redox behavior of 1–4 and the spectroelectrochemistry of the dinuclear species 2–4 have been investigated. The usual ³MLCT (metal-to-ligand charge transfer) decay, characterized by relatively long lifetimes on the ns timescale, takes place in 1 and 3, whose lowest-energy level involves a {(bpy)₂Ru(dntpz)}²⁺ unit, whereas for 2 and 4, whose lowest-energy excited state involves a ³MLCT centered on the {(I)(ttb-terpy)Ru(μ-dntpz)}⁺ subunit, the excited-state lifetimes are on the ps timescale, possibly involving population of a low-lying ³MC (metal-centered) level. Compound 2 also exhibits a fast process, with a time constant of 170 fs, which is attributed to intercomponent energy transfer from the MLCT state centered in the {(bpy)₂Ru(μ-dntpz)}²⁺ unit to the MLCT state involving the {(I)(ttb-terpy)Ru(μ-dntpz)}⁺ unit. Both the intercomponent energy transfer and the MLCT-to-MC activation process take place from non-equilibrated MLCT states.