Kenneth Wärnmark

Iron sensitizer converts light to electrons with 92% yield

Solar energy conversion in photovoltaics or photocatalysis involves light harvesting, or sensitization, of a semiconductor or catalyst as a first step. Rare elements are frequently used for this purpose, but they are obviously not ideal for large-scale implementation. Great efforts have been made to replace the widely used ruthenium with more abundant analogues like iron, but without much success due to the very short-lived excited states of the resulting iron complexes. Here, we describe the development of an iron-nitrogen-heterocyclic-carbene sensitizer with an excited-state lifetime that is nearly a thousand-fold longer than that of traditional iron polypyridyl complexes. By the use of electron paramagnetic resonance, transient absorption spectroscopy, transient terahertz spectroscopy and quantum chemical calculations, we show that the iron complex generates photoelectrons in the conduction band of titanium dioxide with a quantum yield of 92% from the (3)MLCT (metal-to-ligand charge transfer) state. These results open up possibilities to develop solar energy-converting materials based on abundant elements.

Towards longer-lived metal-to-ligand charge transfer states of iron(ii) complexes: an N-heterocyclic carbene approach.

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex.

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses.

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Exceptional Excited-State Lifetime of an Iron(II)-N-Heterocyclic Carbene Complex Explained.

Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron-N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe(II) complexes. In addition, a shallow (3)MLCT potential energy surface with a low-energy transition path from the (3)MLCT to (3)MC and facile crossing from the (3)MC state to the ground state are identified as key features for the excited-state deactivation.

Stereoisomerically controlled inorganic architectures: synthesis of enantio- and diastereo-merically pure ruthenium–palladium molecular rods from enantiopure building blocks

The construction of nanometre scale RuIIPdIIRuIIπ-conjugated molecular rods, Λ, Λ-1 and Δ, Δ-1, from enantiomerically pure building blocks, provides a method for obtaining enantio- and diastereo-merically pure RuII-based molecular architectures.

A BIS(CROWN-ETHER) ANALOGUE OF TROGER'S BASE : RECOGNITION OF ACHIRAL AND CHIRAL PRIMARY BISAMMONIUM SALTS

A C2-symmetric bis(18-crown-6) analogue (2) of Trogers base (1) has been synthesized. In a series of achiral primary bisammonium salts, compound 2 binds heptane-1,7-diylbis(ammonium chloride) most strongly. Moderate enantioselective discrimination in the complexation of l-cystine dimethyl ester dihydrochloride with rac-2 was observed.

A low-spin Fe( iii ) complex with 100-ps ligand-to-metal charge transfer photoluminescence

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts

Building a detailed understanding of the structure-function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.

Stereoisomerically controlled inorganic architectures: synthesis of extended enantio- and diastereo-merically pure tris-ruthenium disks from enantiopure building blocks

Extended enantio- and diastereo-merically pure D3-symmetric disks, ΛΛΛ-1 and ΔΔΔ-1, are synthesised from enantiomerically pure building blocks.

Twisted Amide Analogues of Tröger’s Base

Lets do the twist: The first twisted bis-amide is obtained by the benzylic oxidation of Trogers base (TB). Kinetic studies of its acidic hydrolysis reveal that the hydrolysis is to a large extent funneled through doubly protonated species.