Excited-State Electron Transfer in a Chromophore-Quencher Complex. Spectroscopic Identification of a Redox-Separated State

Photodriven electron-transfer dynamics in a series of heteroleptic Cu(I)-anthraquinone dyads

Solar fuels catalysis is a promising route to efficiently harvesting, storing, and utilizing abundant solar energy. To achieve this promise, however, molecular systems must be designed with sustainable components that can balance numerous photophysical and chemical processes. To that end, we report on the structural and photophysical characterization of a series of Cu(I)-anthraquinone-based electron donor-acceptor dyads. The dyads utilized a heteroleptic Cu(I) bis-diimine architecture with a copper(I) bis-phenanthroline chromophore donor and anthraquinone electron acceptor. We characterized the structures of the complexes using x-ray crystallography and density functional theory calculations and the photophysical properties via resonance Raman and optical transient absorption spectroscopy. The calculations and resonance Raman spectroscopy revealed that excitation of the Cu(I) metal-to-ligand charge-transfer (MLCT) transition transfers the electron to a delocalized ligand orbital. The optical transient absorption spectroscopy demonstrated that each dyad formed the oxidized copper-reduced anthraquinone charge-separated state. Unlike most Cu(I) bis-phenanthroline complexes where increasingly bulky substituents on the phenanthroline ligands lead to longer MLCT excited-state lifetimes, here, we observe a decrease in the long-lived charge-separated state lifetime with increasing steric bulk. The charge-separated state lifetimes were best explained in the context of electron-transfer theory rather than with the energy gap law, which is typical for MLCT excited states, despite the complete conjugation between the phenanthroline and anthraquinone moieties.

New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex

The new Ru(II)-anthraquinone complex [Ru(bpy)₂(qdpq)](PF₆)₂ (Ru-qdpq; bpy = 2,2'-bipyridine; qdpq = 2,3-di(2-pyridyl)naphtho[2,3-f]quinoxaline-7,12-quinone) possesses a strong ¹MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru(bpy)₂(qdppz)](PF₆)₂ (Ru-qdppz; qdppz = naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione), with λ_max = 450 nm. Ru-qdppz possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of Ru-qdpq results in the population of a relatively long-lived (τ = 19 ns) Ru(dπ) → qdpq(π*) ³MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. These observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Photoinduced charge accumulation by metal ion-coupled electron transfer

An oligotriarylamine (OTA) unit, a Ru(bpy)3(2+) photosensitizer moiety (Ru), and an anthraquinone (AQ) entity were combined to a molecular dyad (Ru-OTA) and a molecular triad (AQ-Ru-OTA). Pulsed laser excitation at 532 nm led to the formation of charge-separated states of the type Ru(-)-OTA(+) and AQ(-)-Ru-OTA(+) with lifetimes of ≤10 ns and 2.4 μs, respectively, in de-aerated CH3CN at 25 °C. Upon addition of Sc(OTf)3, very long-lived photoproducts were observed. Under steady-state irradiation conditions using a flux of (6.74 ± 0.21) × 10(15) photons per second at 450 nm, the formation of twofold oxidized oligotriarylamine (OTA(2+)) was detected in aerated CH3CN containing 0.02 M Sc(3+), as demonstrated unambiguously by comparison with UV-Vis absorption spectra obtained in the course of chemical oxidation with Cu(2+). Photodriven charge accumulation on the OTA unit of Ru-OTA and AQ-Ru-OTA is possible due to the lowering of the O2 reduction potential caused by the interaction of superoxide with the strong Lewis acid Sc(3+). The presence of the anthraquinone unit in AQ-Ru-OTA accelerates the rate-determining reaction step for charge accumulation by a factor of 10 compared to the Ru-OTA dyad. This is attributed to the formation of Sc(3+)-stabilized anthraquinone radical anion intermediates in the triad. Possible mechanistic pathways leading to charge accumulation are discussed. Photodriven charge accumulation is of key importance for solar fuels because their production will have to rely on multi-electron chemistry rather than single-electron reaction steps. Our study is the first to demonstrate that metal ion-coupled electron transfer (MCET) can be exploited to accumulate charges on a given molecular unit using visible light as an energy input. The approach of using a combination of intra- and intermolecular electron transfer reactions which are enabled by MCET is conceptually novel, and the fundamental insights gained from our study are relevant in the greater context of solar energy conversion.

Detection of a charge-separated catalyst precursor state in a linked photosensitizer-catalyst assembly

We have designed two new supramolecular assemblies based on Co(ii)-templated coordination of Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) analogues as photosensitizers and electron donors to a cobaloxime macrocycle, which are of interest as proton reduction catalysts. The self-assembled photocatalyst precursors were structurally characterized by Co K-edge X-ray absorption spectroscopy and solution-phase X-ray scattering. Visible light excitation of one of the assemblies has yielded instantaneous electron transfer and charge separation to form a transient Co(i) state which persists for 26 ps. The development of a linked photosensitizer-cobaloxime architecture supporting efficient Co(i) charge transfer is significant since it is mechanistically critical as the first photo-induced electron transfer step for hydrogen production, and has not been detected in previous photosensitizer-cobaloxime linked dyad assemblies. X-band EPR spectroscopy has revealed that the Co(ii) centres of both assemblies are high spin, in contrast to most previously described cobaloximes, and likely plays an important role in facilitating photoinduced charge separation. Based on the results obtained from ultrafast and nanosecond transient absorption optical spectroscopies, we propose that charge recombination occurs through multiple ligand states present within the photosensitizer modules. The studies presented here will enhance our understanding of supramolecular photocatalyst assembly and direct new designs for artificial photosynthesis.

Luminescent rhenium(i) diimine indole conjugates – photophysical, electrochemical and protein-binding properties

Two novel luminescent rhenium(I) diimine indole complexes have been designed and their properties studied; these conjugates can be recognised by indole-binding proteins including bovine serum albumin, lysozyme and tryptophanase.