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

High turnover photocatalytic hydrogen formation with an Fe(iii) N-heterocyclic carbene photosensitiser

Herein we report the first high turnover photocatalytic hydrogen formation reaction based on an earth-abundant FeIII-NHC photosensitiser. The reaction occurs via reductive quenching of the 2LMCT excited state that can be directly excited with green light and employs either Pt-colloids or [Co(dmgH)2pyCl] as proton reduction catalysts and [HNEt3][BF4] and triethanolamine/triethylamine as proton and electron donors. The outstanding photostability of the FeIII-NHC complex enables turnover numbers >1000 without degradation

Evaluation of two- and three-dimensional electrode platforms for the electrochemical characterization of organometallic catalysts incorporated in non-conducting metal–organic frameworks

The development of a reliable platform for the electrochemical characterization of a redox-active molecular diiron complex, [FeFe], immobilized in a non-conducting metal organic framework (MOF), UiO-66, based on glassy-carbon electrodes is reported. Voltammetric data with appreciable current responses can be obtained by the use of multiwalled carbon nanotubes (MWCNT) or mesoporous carbon (CB) additives that function as conductive scaffolds to interface the MOF crystals in “three-dimensional” electrodes. In the investigated UiO-66-[FeFe] sample, the low abundance of [FeFe] in the MOF and the intrinsic insulating properties of UiO-66 prevent charge transport through the framework, and consequently, only [FeFe] units that are in direct physical contact with the electrode material are electrochemically addressable

A bistable electrochromic material based on a hysteretic molecular switch immobilized on nanoparticulate metal oxide

Combination of an electrochemically bistable Ru polypyridyl complex (2) with the metal oxide semiconductor Sb:SnO2 formed the basis of an electrochromic hybrid material characterised by a hysteretic response to applied potential. The electrochemical bistability of the molecular component arises from redox-triggered linkage isomerisation where an ambidentate ligand changes reversibly between N- and O-coordination in the Ru(II) and Ru(III) states, respectively. Reversible oxidation and reduction result in a pronounced electrochromic effect (change in UV-Vis absorption at the metal-to-ligand-charge-transfer (MLCT) band) and the potentials for interconversion between the states are separated by approx. 0.5 V due to the linkage isomerisation reactions. With a carboxylate anchoring group on the auxiliary ligand, the bistable molecular switch was immobilised on the surface of nanoparticulate Sb:SnO2 films. This resulted in an electrode material featuring a hysteretic electrochemical response where oxidation state and colour of the electrode depended on the electrochemical history of the system

Time-Resolved Laser Spectroscopy in Molecular Devices for Solar Energy Conversion

A complete characterization of solar energy conversion devices and the processes underlying their function is a challenge, and require a multitude of different experimental methods. This chapter discusses investigations of molecular solar cells and solar fuels devices by time-resolved laser spectroscopic methods. These methods have established important concepts we now use for understanding the function of devices for solar energy conversion into primary products. We give examples of scientific insight provided by ultrafast methods using detection in the regions from X-ray to THz radiation, and particularly highlight the case where the use of different methods has provided complementary information. Charge collection and solar fuel catalysis on the other hand occur on longer time scales, which opens for the use of time-resolved magnetic resonance and microwave conductivity methods. We also point out that, with suitable precautions, time-resolved laser spectroscopy is able to give information relevant for in operando device condition