Molecular complexes and main-chain organometallic polymers based on Janus bis(carbenes) fused to metalloporphyrins

Electrically conductive [Fe4S4]-based organometallic polymers

Tailoring the molecular components of hybrid organic-inorganic materials enables precise control over their electronic properties. Designing electrically conductive coordination materials, e.g. metal-organic frameworks (MOFs), has relied on single-metal nodes because the metal-oxo clusters present in the vast majority of MOFs are not suitable for electrical conduction due to their localized electron orbitals. Therefore, the development of metal-cluster nodes with delocalized bonding would greatly expand the structural and electrochemical tunability of conductive materials. Whereas the cuboidal [Fe4S4] cluster is a ubiquitous cofactor for electron transport in biological systems, few electrically conductive artificial materials employ the [Fe4S4] cluster as a building unit due to the lack of suitable bridging linkers. In this work, we bridge the [Fe4S4] clusters with ditopic N-heterocyclic carbene (NHC) linkers through charge-delocalized Fe-C bonds that enhance electronic communication between the clusters. [Fe4S4Cl2(ditopic NHC)] exhibits a high electrical conductivity of 1 mS cm-1 at 25 °C, surpassing the conductivity of related but less covalent materials. These results highlight that synthetic control over individual bonds is critical to the design of long-range behavior in semiconductors.

Mechanism of Enhanced Triplet-Triplet Upconversion in Organic Molecules

We use time-dependent density functional theory (TDDFT) to investigate the mechanism of efficient triplet-triplet upconversion (TTU) in certain organic materials. In particular, we focus on materials where some singlets are generated in a two-step spin-nonconserving process (T1 + T1 → T2 → S1). For this mechanism to contribute significantly, the intersystem crossing (ISC) from the high-lying triplet to the singlet (T2 → S1) must outcompete the internal conversion (IC) to the low-lying triplet (T2 → T1). By considering multiple families of materials, we show that the T2 → S1 ISC can be enhanced in a number of ways: the substitution of electron-donating (ED) and electron-withdrawing (EW) groups at appropriate positions; the substitution of bulky groups that distort the molecular geometry; and the substitution of heavy atoms that enhance the spin-orbit coupling (SOC). In the first two cases, the enhancements are consistent with El-Sayed's rule in that rapid T2 → S1 ISC requires significant differences in the characters of the S1 and the T2 wavefunctions. Together, these effects enable a wide tunability of T2 → S1 ISC rates over at least 5 orders of magnitude. Meanwhile, the T2 → T1 IC is inhibited in these systems due to the large T2 - T1 energy gap >0.5 eV, which entails a high energy barrier to the T2 → T1 IC and the prediction of a slow rate regardless of the substituents or the presence of heavy atoms. In this way, tuning the T2 → S1 ISC appears to provide an effective strategy to achieve systematic improvement of TTU materials.

Synthesis and disassembly of an organometallic polymer comprising redox-active Co4S4 clusters and Janus biscarbene linkers

Here, we show for the first time that main-chain organometallic polymers (MCOPs) can be prepared from Janus N-heterocyclic carbene (NHC) linkers and polynuclear cluster nodes. The crosslinked framework Co₄S₄-MCOP is synthesized via ligand displacement reactions and undergoes reversible electron transfer in the solid state. Discrete molecular cluster species can be excised from the framework by digesting the solid in solutions of excess monocarbene. Finally, we demonstrate a synthetic route to monodisperse framework particles via coordination modulation.

Janus-type homo-, hetero- and mixed valence-bimetallic complexes with one metal encapsulated in a cyclodextrin

Bis-azolium salts with one azolium capping a perbenzylated α-cyclodextrin have been designed to generate Janus-type bimetallic complexes with various combinations of copper, silver, gold or palladium salts. Encapsulation of one metal center inside the cavity allowed (trans)metalation and oxidation reactions to be controlled at selected positions. In particular, it was possible to oxidize Au^I into Au^III selectively on the position outside the cavity of the cyclodextrin on the bis-Au^I Janus complex.

N-Heterocyclic carbenes and their precursors in functionalised porous materials

Though N-heterocyclic carbenes (NHCs) have emerged as diverse and powerful discrete functional molecules in pharmaceutics, nanotechnology, and catalysis over decades, the heterogenization of NHCs and their precursors for broader applications in porous materials, like metal-organic frameworks (MOFs), porous coordination polymers (PCPs), covalent-organic frameworks (COFs), porous organic polymers (POPs), and porous organometallic cages (POMCs) was not extensively studied until the last ten years. By de novo or post-synthetic modification (PSM) methods, myriads of NHCs and their precursors containing building blocks were designed and integrated into MOFs, PCPs, COFs, POPs and POMCs to form various structures and porosities. Functionalisation with NHCs and their precursors significantly expands the scope of the potential applications of porous materials by tuning the pore surface chemical/physical properties, providing active sites for binding guest molecules and substrates and realizing recyclability. In this review, we summarise and discuss the recent progress on the synthetic methods, structural features, and promising applications of NHCs and their precursors in functionalised porous materials. At the end, a brief perspective on the encouraging future prospects and challenges in this contemporary field is presented. This review will serve as a guide for researchers to design and synthesize more novel porous materials functionalised with NHCs and their precursors.

Insights into the past and future of Janus-di-N-heterocyclic carbenes

Janus di-N-heterocyclic carbene (NHC) ligands are a subclass of poly-NHCs that feature coordination to two transition metals in a facially opposed manner. The combination of the structural features of Janus type ligands, with the properties conferred by the NHC ligands, has conferred Janus-di-NHCs with privileged attributes for their use in diverse areas of research, such as homogeneous catalysis, materials chemistry and supramolecular chemistry. In molecular chemistry, Janus di-NHCs constitute one of the most useful chemical platforms for constructing dimetallic structures, and this includes both homo- and hetero-dimetallic compounds. This review aims to cover the most relevant advances in the use of Janus-di-NHCs during the last 15 years, by classifying them according to their specific structural features.