Synthesis, Hole Doping, and Electrical Properties of a Semiconducting Azatriangulene-Based Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D COFs) containing heterotriangulenes have been theoretically identified as semiconductors with tunable, Dirac-cone-like band structures, which are expected to afford high charge-carrier mobilities ideal for next-generation flexible electronics. However, few bulk syntheses of these materials have been reported, and existing synthetic methods provide limited control of network purity and morphology. Here, we report transimination reactions between benzophenone-imine-protected azatriangulenes (OTPA) and benzodithiophene dialdehydes (BDT), which afforded a new semiconducting COF network, OTPA-BDT. The COFs were prepared as both polycrystalline powders and thin films with controlled crystallite orientation. The azatriangulene nodes are readily oxidized to stable radical cations upon exposure to an appropriate p-type dopant, tris(4-bromophenyl)ammoniumyl hexachloroantimonate, after which the network's crystallinity and orientation are maintained. Oriented, hole-doped OTPA-BDT COF films exhibit electrical conductivities of up to 1.2 × 10-1 S cm-1, which are among the highest reported for imine-linked 2D COFs to date.

Electronic Spin Qubit Candidates Arrayed within Layered Two-Dimensional Polymers.

Molecular electronic spin qubits are promising candidates for quantum information science applications because they can be reliably produced and engineered via chemical design. Embedding electronic spin qubits within two-dimensional polymers (2DPs) offers the possibility to systematically engineer inter-qubit interactions while maintaining long coherence times, both of which are prerequisites to their technological utility. Here, we introduce electronic spin qubits into a diamagnetic 2DP by n-doping naphthalene diimide subunits with varying amounts of CoCp2 and analyze their spin densities by quantitative electronic paramagnetic resonance spectroscopy. Low spin densities (e.g., 6.0 × 1012 spins mm-3) enable lengthy spin-lattice (T1) and spin-spin relaxation (T2) times across a range of temperatures, ranging from T1 values of 164 ms at 10 K to 30.2 µs at 296 K and T2 values of 2.36 µs at 10 K to 0.49 µs at 296 K for the lowest spin density sample examined. Higher spin densities and temperatures were both found to diminish T1 times, which we attribute to detrimental cross-relaxation from spin-spin dipolar interactions and spin-phonon coupling, respectively. Higher spin densities decreased T2 times and modulated the T2 temperature dependence. We attribute these differences to the competition between hyperfine and dipolar interactions for electron spin decoherence, with the dominant interaction transitioning from the former to the latter as spin density and temperature increase. Overall, this investigation demonstrates that dispersing electronic spin qubits within layered 2DPs enables chemical control of their inter-qubit interactions and spin decoherence times.

Diverse Reactivity of a Rhenium(V) Oxo Imido Complex: [2 + 2] Cycloadditions, Chalcogen Metathesis, Oxygen Atom Transfer, and Protic and Hydridic 1,2-Additions.

We present a wide range of reactivity studies focused on the rhenium(V) oxo imido complex (DippN)(O)Re(BDI) (1, Dipp = 2,6-diisopropylphenyl and BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate). This complex, which was previously shown to possess a highly polarized Re oxo moiety, has proven to be a potent nucleophile and a valuable precursor to a variety of rare structural motifs in rhenium coordination complexes. For example, the Re oxo moiety of 1 undergoes [2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide, and isothiocyanates at room temperature. In the case of CO2, the cycloadduct with 1 (a carbonate complex) undergoes the facile ejection of CO2, demonstrating that this binding process is reversible. In the case of isothiocyanate, chalcogen metathesis with 1 takes place readily as the inclusion of a second equivalent of substrate in the reaction mixture rapidly yields a dithiocarbamate complex. This metathesis process was extended to the reactivity of 1 with phosphine chalcogenides, leading to the isolation of terminal sulfido imido and selenido imido complexes. Attempts to complete this series and generate the analogous terminal telluride led to the formation of a bidentate tritelluride (Te32-) complex. Triethylphosphine could only undergo oxygen atom transfer (OAT) with 1 under pressing thermal conditions that also led to C-N cleavage of the BDI ligand. In contrast, OAT between 1 and CO or 2,6-xylylisocyanide (XylNC) was found to be much more facile, proceeding within seconds at room temperature. While the addition of excess CO led to a rhenium(III) imido dicarbonyl complex, we found that the addition of 2 equiv of XylNC was necessary to promote OAT, resulting in the isolation of a rare example of a stable metal isocyanate complex. Our experimental observations of CO and XylNC and their OAT reactions with 1 inspired a mechanistic computational study to probe the intermediates and kinetic barriers along these reaction pathways. Finally, we describe 1,2-additions of both protic and hydridic substrates with the Re oxo moiety of 1, which most notably led to the syntheses of an uncommon example of a terminal rhenium hydroxide complex and an oxo-bridged Re-O-Zr hetero-bi-metallic complex that was generated using Schwartz's reagent (Cp2ZrHCl). A brief discussion of a potential alternative route to 1 is also presented.

Competition between side-chain interactions dictates 2D polymer stacking order.

Interrogating the stacking of two-dimensional polymers (2DPs) as a function of chemical composition is important to leverage their properties. We explore the dependence of 2DP crystallinity and porosity on variable amounts of zwitterions contained within the pores and find that high zwitterion loadings consistently diminish 2DP materials quality. A competition between disruptive zwitterion electrostatic forces and alkyl stabilization directs the stacking order of each 2DP and demonstrates the contrasting effects of side chain composition on 2DP crystallinity and porosity.

Controlled n-Doping of Naphthalene-Diimide-Based 2D Polymers.

2D polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors is enhanced by controllably n-doping the NDI units using cobaltocene (CoCp2 ). Optical and transient microwave spectroscopy reveal that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp2 , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. While the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10-4  S cm-1 are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, the controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.