Capping TTFtt enables facile transmetalation in three different oxidation states.

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7tetrathiolate (TTFtt 2−) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt 2−. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt 2− π-manifold from the Fe(II)-centered spin-crossover. The TTFtt 2− centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character. Results and Discussion Synthesis and Structural Parameters Complex 1 was synthesized via reaction with the deprotected proligand 2,3,6,7-tetrakis(2cyanoethylthio)tetrathiafulvalene (TTFtt(C2H4CN)4) in good yield. Complex 1 was insoluble in all solvents we investigated which precluded detailed characterization but is pure as indicated by combustion analysis and behaves as a suitable synthon for subsequent chemistry. Complex 1 can be doubly oxidized with [Cp2Fe][BAr F 4] to form 2 which is more soluble, enabling common solution characterization including 1 H NMR and cyclic voltammetry measurements (Figure S1-S2). Oxidation from 1 to 2 could be ligandcentered (TTFtt 4− →TTFtt 2−), metal-centered (2 Fe(II)→2 Fe(III)), or some intermediate case, but the data acquired for 2 supports a TTFtt 2− structure arising from ligandcentered oxidation (Chart 1B, see below). Compound 2 was structurally characterized via singlecrystal X-ray diffraction (SXRD) at 293 K (2-HT; Figure S3) and 100 K (2-LT; Figure 1). In both structures TTFtt 2− is bridged between two TPA-capped Fe centers with two outer-sphere BAr F 4 − counter anions. The most striking difference between these temperatures is markedly longer Fe bond lengths in 2-HT. The Fe-Npyridine and Fe-Namine bond lengths in 2-LT are 1.958(6)-1.979(6) and 2.017(6) Å (Figure 1), respectively. These values are consistent with Fe-N bonds in other low-spin complexes with a Fe-TPA moiety. 16,17 In 2-HT, these bonds are 0.18-0.19 and 0.244(11) Å longer than their counterparts at 100 K, respectively, and are consistent with high-spin Fe-TPA complexes. The shorter Fe bonds at lower temperature indicate that 2 exhibits a temperature dependent spincrossover as observed in related compounds. 16,21

ParagraphConducting organic materials, such as doped organic polymers, 1 molecular conductors, 2, 3 and emerging coordination polymers, 4 underpin technologies ranging from displays to flexible electronics. 5 Realizing high electrical conductivity in traditionally insulating organic materials necessitates tuning their electronic structure through chemical doping. 6 Furthermore, even materials that are intrinsically conductive, such as single-component molecular conductors, 7,8 require crystallinity for metallic behavior. However, commercial conducting polymers are often purposefully amorphous to aid in durability and processability. 9,10 Using molecular design to engender high conductivity in undoped amorphous materials would enable tunable and robust conductivity in many applications, but there are no intrinsically conducting organic materials which maintain high conductivity when disordered. Here we show that the completely amorphous coordination polymer Ni tetrathiafulvalene tetrathiolate (NiTTFtt) displays intrinsic metallic conductivity. Despite its disordered structure, NiTTFtt exhibits remarkably high electronic conductivity (1280 S/cm) and intrinsically glassy metallic behavior. Analysis with advanced theory shows that these properties are enabled by strong molecular overlap and correlation that are robust to structural perturbations. This unusual set of structural and electronic features results in remarkably stable organic conductivity which is maintained in air for weeks and at temperatures up to 140 °C. Our results demonstrate that molecular design can enable metallic conductivity even in heavily disordered materials. This both raises fundamental questions about how band-like transport can exist in the absence of periodic structure as well as suggests exciting new applications for these materials.

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Near-infrared (NIR)-emitting molecules are promising candidates for biological sensing and imaging applications; however, many NIR dyes are large conjugated systems which frequently have issues with stability, solubility, and tunability. Here, we report a novel class of compact and tunable fluorescent diradicaloid complexes which are air-, water-, light-, and temperature-stable. These properties arise from a compressed π manifold which promotes an intense ligand-centered π–π transition in the NIR II (1000–1700 nm) region and which subsequently emits at ∼1200 nm. This emission is among the brightest known for monomolecular lumiphores with deep NIR II (>1100 nm) emission, nearly an order of magnitude brighter than the commercially available NIR II dye IR 26. Furthermore, this fluorescence is electrochemically sensitive, with efficient switching upon addition of redox agents. The brightness, stability, and modularity of this system distinguish it as a promising candidate for the development of new technologies built around NIR emission.

While chemical systems containing hundreds to thousands of electrons remain beyond the reach of quantum devices, hybrid quantum-classical algorithms present a promising pathway toward a quantum advantage. Hybrid algorithms treat the exponentially scaling part of the calculation—the static correlation—on the quantum computer and the non-exponentially scaling part—the dynamic correlation—on the classical computer. While a variety of algorithms have been proposed, the dependence of many methods on the total wave function limits the development of easy-to-use classical post-processing implementations. Here, we present a novel combination of quantum and classical algorithms, which computes the all-electron energy of a strongly correlated molecular system on the classical computer from the 2-electron reduced density matrix (2-RDM) evaluated on the quantum device. Significantly, we circumvent the wave function in the all-electron calculations by using density matrix methods that only require input of the statically correlated 2-RDM. Although the algorithm is completely general, we test it with two classical density matrix methods, the anti-Hermitian contracted Schrödinger equation (ACSE) and multiconfiguration pair-density functional theories, using the recently developed quantum ACSE method for simulating the statically correlated 2-RDM. We obtain experimental accuracy for the relative energies of all three benzyne isomers and thereby demonstrate the ability of the developed algorithm to achieve chemically relevant and accurate results on noisy intermediate-scale quantum devices.

A central theme in chemistry is the understanding of the mechanisms that drive chemical transformations. A well-known, highly cited mechanism in organometallic chemistry is the superexchange mechanism in which unpaired electrons on two or more metal centers interact through an electron pair of the bridging ligand. We use a combination of novel synthesis and computation to show that such interactions may in fact occur by a more direct mechanism than superexchange that is based on direct quantum entanglement of the two metal centers. Specifically, we synthesize and experimentally characterize a novel cobalt dimer complex with benzoquinoid bridging ligands and investigate its electronic structure with the variational two-electron reduced density matrix method using large active spaces. The result draws novel connections between inorganic mechanisms and quantum entanglement, thereby opening new possibilities for the design of strongly correlated organometallic compounds whose magnetic and spin properties have applications in superconductors, energy storage, thermoelectrics, and spintronics.

Kohn-Sham density functional theory (DFT) has long struggled with the accurate description of strongly correlated and open shell systems and improvements have been minor even in the newest hybrid functionals. In this Letter we treat the static correlation in DFT when frontier orbitals are degenerate by the means of using a semidefinite programming (SDP) approach to minimize the system energy as a function of the N -representable, non-idempotent 1-electron reduced density matrix. While showing greatly improved singlet-triplet gaps for linear density approximation and generalized gradient approximation (GGA) functionals, the SDP procedure reveals flaws in modern meta and hybrid GGA functionals, which show no major improvements when provided with an accurate electron density.