Elucidating the Ultrafast Dynamics of Photoinduced Charge Separation in Metalloporphyrin-Fullerene Dyads Across the Electromagnetic Spectrum

Metalloporphyrins are prominent building blocks in the synthetic toolbox of advanced photodriven molecular devices. When the central ion is paramagnetic, the relaxation pathways within the manifold of excited states are highly intricate so that unravelling the intramolecular energy and electron transfer processes is usually a very complex task. This fact is critically hampering the development of applications based on the enhanced coupling offered by the electronic exchange interaction. In this work, the dynamics of charge separation in a copper porphyrin-fullerene are studied with several complementary spectroscopic tools across the electromagnetic spectrum (from near-infrared to X-ray wavelengths), each of them providing specific diagnostics. Correlating the various rates clearly demonstrates that the lifetime of the photoinduced charge-separated state exceeds by about 10-fold that of the isolated photoexcited CuII porphyrin. As revealed by the spectral modifications in the XANES region, this stabilization is accompanied by a transient change in covalency around the CuII center, which is induced by an enhanced interaction with the C60 moiety. This experimental finding is further confirmed by state-of-the art calculations using DFT and TD-DFT including dispersion effects that explain the electrostatic and structural origins of this interaction, as the CuIIP cation becomes ruffled and approaches closer to the fullerene in the charge-separated state. From a methodological point of view, these results exemplify the potential of multielectron excitation features in transient X-ray spectra as future diagnostics of subfemtosecond electronic dynamics. From a practical point of view, this work is paving the way for elucidating out-of-equilibrium electron transfer events coupled to magnetic interaction processes on their intrinsic time-scales

Synthetic strategies to nanostructured photocatalysts for CO2 reduction to solar fuels and chemicals

Artificial photosynthesis represents one of the great scientific challenges of the 21st century, offering the possibility of clean energy through water photolysis and renewable chemicals through CO2 utilisation as a sustainable feedstock. Catalysis will undoubtedly play a key role in delivering technologies able to meet these goals, mediating solar energy via excited generate charge carriers to selectively activate molecular bonds under ambient conditions. This review describes recent synthetic approaches adopted to engineer nanostructured photocatalytic materials for efficient light harnessing, charge separation and the photoreduction of CO2 to higher hydrocarbons such as methane, methanol and even olefins

Streptavidin as a Scaffold for Light-Induced Long-Lived Charge Separation

Long-lived photo-driven charge separation is demonstrated by assembling a triad on a protein scaffold. For this purpose, a biotinylated triarylamine was added to a Ru II –streptavidin conjugate bearing a methyl viologen electron acceptor covalently linked to the N -terminus of streptavidin. To improve the rate and lifetime of the electron transfer, a negative patch consisting of up to three additional negatively charged amino acids was engineered through mutagenesis close to the biotin-binding pocket of streptavidin. Time-resolved laser spectroscopy revealed that the covalent attachment and the negative patch were beneficial for charge separation within the streptavidin hosted triad; the charge separated state was generated within the duration of the excitation laser pulse, and lifetimes up to 3120 ns could be achieved with the optimized supramolecular triad

The Carbene Cannibal : Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [(FeL2)-L-III](PF6) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate](-)). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer ((LMCT)-L-2) excited state (E0-0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [(FeL2)-L-II] and [(FeL2)-L-IV](2+), with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [(FeL2)-L-III](+), as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (Delta G(CS)degrees similar to -0.7 eV). This is in contrast to most systems investigated in the literature, where Delta C-CS degrees is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS

A surface-attached Ru complex operating as a rapid bistable molecular switch

An electrochemically bistable ruthenium polypyridyl complex was immobilised on platinum electrodesviaamide condensation with an amine-terminated self-assembled thiol monolayer and underwent rapid electron transfer-induced linkage isomerism

Mixed-Valence Properties of an Acetate-Bridged Dinuclear Ruthenium (II,III) Complex

The mixed-valence dinuclear ruthenium complex [Ru2(bpmp)(-OAc)2]2+ (where bpmp is the phenolate anion of 2,6-bis[bis(2-pyridylmethyl) aminomethyl]-4-methylphenol, H-bpmp) has been studied by UV-Vis-NIR, IR, and EPR spectroscopic and electrochemical techniques. The Ru2II,III complex undergoes reversible one-electron reduction (E1/2 = -0.61 V vs Fc+/0) and oxidation (E1/2 = 0.09 V vs Fc+/0), resulting in the Ru2II,II and Ru2III,III complexes, respectively. A comproportionation constant of Kc = 1.10 × 1012 (Gc = -68 kJ mol-1) indicates considerable stability of the mixed-valence state. The paramagnetic complex displays a rhombic EPR spectrum (g1 = 2.492; g2 = 2.242; g3 = 1.855) arising from a ground state in a S = 1/2 low spin system in a low symmetry environment. Three intense, distinguishable intervalence bands are observed in the NIR to mid-IR spectrum of [Ru2(bpmp)(-OAc)2]2+ at 3765 cm-1 ( = 1840 M-1cm-1), 5615 cm-1 ( = 10590 M-1cm-1 ), and 7735 cm-1 ( = 3410 M-1cm-1). All intervalence bands are symmetric but more narrow than predicted for the classical limit and independent of solvent polarity. The results of the spectroscopic and electrochemical characterization indicate that [Ru2(bpmp)(-OAc)2]2+ is either electronically delocalized (class III, Hab = 1880 cm-1) or at the borderline between localization and delocalization (class II-III, Hab 590 cm-1) with rapid electron transfer (kET > 4 × 1012 s-1) decoupled from solvent reorientation but with a residual activation barrier (Ea 440 cm-1) from inner reorganization