Dual Luminescence and Long-Lived Charge-Separated States in Donor-Acceptor Assemblies Based on Tetrathiafulvalene-Fused Ruthenium(II)-Polypyridine Complexes

Abstract

: The creation of long-lived charge-separated states in donor − acceptor assemblies has been the goal of many studies aimed at mimicking the primary processes in photosynthesis. Here we present such assemblies based on tetrathiafulvalene (TTF) as electron donor and a dipyridophenazine (dppz) unit as electron acceptor in the form of a fused ligand (TTF-dppz) coordinated to ruthenium(II) via the dipyrido coordination site and with 2,2'-bipyridine (bpy) as auxiliary ligand, namely [Ru(bpy) 3 − x (TTF-dppz) x ] 2+ (x = 1 − 3). For x = 2, irradiation into the metal to dppz charge transfer transition results in electron transfer from TTF to ruthenium, thus creating a charge-separated state best described by [(TTF + -dppz)Ru(dppz − -TTF)(bpy)] 2+ with a lifetime of 2.5 µ s in dichloromethane.Keywords: Charge-separated states ⋅ Intraligand charge transfer ⋅ Metal-ligand-charge-transfer ⋅ Ruthenium(II)-polypyridine complexes ⋅ Tetrathiafulvalene step is always given by the absorption of a photon, which promotes the chromophore to an electronically and often vibrationally excited state. This initially excited state is most often extremely short-lived, and the system decays via vibrational relaxation, internal conversion, intersystem crossing, luminescence, and energy and electron transfer back to the ground state or via some chemical reaction to a given photochemical product.A key issue of photophysics and photochemistry is the creation of long-lived charge-separated states using so-called donor − acceptor (DA) assemblies, in which the excitation of either the donor to D*A or the acceptor to DA* is followed by the transfer of an electron from D to A to form a state best described by D + A − . As an extension of the simple dyad, the donor and the acceptor may be linked by a photophysically active bridge to afford the triad DBA. In this case, the excitation of the bridge to DB*A results in double electron transfer, namely from the HOMO of D to the SOMO-1 of B* and from the SOMO of B* to the LUMO of A, thus forming a species of the form D + BA − , which in general has a longer lifetime of the charge-separated state than that in the corresponding dyad due to the weaker electronic coupling between D and A. Such triads, composed of tetrathiafulvalene, porphyrins and fullerenes have been successfully used in the creation of artificial photosynthetic systems. [6] Tertrathiafulvalene derivatives, on the one hand, constitute a class of versatile electron donors for a number of interesting applications, [7] in fields such as molecular electronics [8] and in organic conductors and superconductors. [9] Of particular interest in the context of this paper is that they are known to quench the luminescence of almost any chromophore in their vicinity through reductive electron transfer quenching. [10] Ruthenium(ii) polypyridine complexes, on the other hand, are known not only for the luminescence from metal-to-ligand-chargetransfer (MLCT) states but also for their use as sensitizers for light-induced electron transfer in solar ene...

Cyclometalated Iridium(III) Complexes as Photosensitizers for Long‐Range Electron Transfer: Occurrence of a Coulomb Barrier

“…A similar Coulomb barrier phenomenon was observed in ruthenium‐sensitized proton‐coupled electron transfer across a guanidinium–carboxylate salt bridges,33 and in a Ru(bpy) 3 2+ ‐tetrathiafulvalene donor–acceptor molecule 34…”

Effect of Bridge Alteration on Ground- and Excited-State Properties of Ruthenium(II) Complexes with Electron-Donor-Substituted Dipyrido[3,2-a:2′,3′-c]phenazine Ligands

“…A secondary MLCT transition involving reduction of the bpy ligand was also observed, characterized by enhancement of bpy modes at 1561, 1488, and 1315 cm –1 at an excitation wavelength of 448 nm. [Ru­(bpy) 3 ] 2+ has an MLCT transition at 455 nm, so it would be expected to see maximum enhancement of the bpy modes at 458 nm rather than 448 nm. Goze et al studied similar Ru­(II) dppz D–A compounds, as discussed previously .…”

“…[Ru(bpy) 3 ] 2+ has an MLCT transition at 455 nm, 43 so it would be expected to see maximum enhancement of the bpy modes at 458 nm rather than 448 nm. Goze et al studied similar Ru(II) dppz D−A compounds, as discussed previously.…”

Current advances in fused tetrathiafulvalene donor–acceptor systems

“…The lifetime of this ligand-to-ligand charge-separated state (LLCS) is B2 ms at room temperature (Scheme 3), see references for in-depth details. 26,27 This LLCS is long-lived presumably due to the spatial separation of the charges located on different ligands as well as the Ru 2+ acting as a ''blocker'' for charge recombination.…”