Conjugated organic molecules with open-shell diradical character (y) possess two weakly paired electron spins interacting across their constituent π-systems. These materials provide fundamental insight into the nature of electron pairing, enabling the utilization of the spin degree of freedom within emerging technologies. However, materials systems that synergistically offer high modularity, tunable y, high chemical stability, and interrelated (opto)electronic functionalities remain limited. Here, we report the facile synthesis of donor-acceptor-donor diradicaloids comprised of a central electron-deficient 6,7,8,9-tetrachloro-[1,2,5]thiadiazolo[3,4-b]phenazine acceptor flanked by electron-rich thiophene-based donors. Nuclear magnetic resonance and electron paramagnetic resonance spectroscopies, and theoretical investigations that account for the multiconfigurational nature of these species, connect a narrowing of the singlet-triplet splitting (∆EST), extension of π-conjugation, and electronic correlations with the evolution of diradical character. These data demonstrate that the differences in structural, electronic, spin, magnetic, physicochemical, and transport properties of the materials can be modulated, while the inherent multireference nature of the electronic structure can be predicted using optimally tuned long-range corrected Mixed-Reference Spin-Flip Time-Dependent Density Functional Theory. These insights enable facile access to a broader range of open-shell materials and facilitate the manipulation of important properties such as electronic structure, topology, exchange, and interrelated optoelectronic and transport functionalities.

Several physical processes produce similar effects in electron paramagnetic resonance (EPR) spectroscopy that are often collectively characterized as exchange. One common description of the exchange effects is based on stochastic changes in the EPR frequency. This general treatment has been adapted and modified for each specific type of exchange. But the general treatment can provide insights into the mechanisms and appearance of exchange.

Molecular spin systems that can be chemically tuned, coherently controlled, and readily integrated within devices remain central to the realization of emerging quantum technologies. Organic high-spin materials are prime candidates owing to their similarity in electronic structure to leading solid-state defect-based systems, light element composition, and the potential for entanglement and qubit operations mediated through spin-spin exchange. However, the inherent instability of these species precludes their rational design, development, and application. Here, the first example of an organic high-spin qubit based on a conjugated polymer semiconductor comprised of alternating dithienosilole and thiadiazoloquinoxaline heterocycles is demonstrated. It is shown that electron spins within the macromolecule demonstrate high-fidelity coherent control of the superposition state with room temperature coherence and solid-state relaxation times that are competitive with or exceed other synthetic molecular qubits. These attributes, along with robust stability, chemical tunability, rich interrelated optoelectronic functionalities, and solution processability, offer a fundamentally new approach to integrating quantum phenomena within functional device platforms.

An analytical approach is demonstrated for the direct calculation of the mean distance between a pair of spins from pulse dipolar spectroscopy measurements by electron paramagnetic resonance. This direct approach uses the Mellin integral transform and does not measure the spectrum of distances between spins, offering substantial increases in sensitivity. The approach provides accurate a priori prediction of the uncertainty for a mean distance measurement, based only on experimental measurement parameters and the final signal-to-noise ratio. The feasibility of this approach is confirmed by comparison with the standard indirect approaches using a set of in silico measurements. The new approach demonstrates comparable accuracy and possibilities for enhancing sensitivity.

The distribution of inter-label distances obtained by electron paramagnetic resonance (EPR) pulse dipolar spectroscopies (PDS), such as DEER aka PELDOR, gives a valuable characterization of structure on the nanometer scale. The impact of random experimental noise on such experiments is examined for three independent methods for analysing PDS data: the model-free method with Tikhonov regularization, model-free with Mellin-transformation, and a model-based method. All three methods show negative bias for the mean distance and positive bias for the distribution width. Both biases grow with increasing noise levels. The estimated confidence bands and the uncertainties obtained from a single experimental measurement by the standard bootstrapping or χ2-surface scanning approaches are inconsistent and can exclude the true distance distribution. Yet, both approaches can provide quite valuable support for hypothesis testing in PDS studies.

The reduction of the carcinogen chromate has been proposed to lead to three Cr(III)-containing DNA lesions: binary adducts (Cr(III) and DNA), interstrand crosslinks, and ternary adducts (Cr(III) linking DNA to a small molecule or protein). Although the structures of binary adducts have recently been elucidated, the structures of interstrand crosslinks and ternary adducts are not known. Analysis of Cr(III) binding to an oligonucleotide duplex containing a 5'-CG site allows elucidation of the structure of an oxide- or hydroxide-bridged binuclear Cr(III) assembly bridging the two strands of DNA. One Cr(III) is directly coordinated by the N-7 atom of a guanine residue, and the complex straddles the helix to form a hydrogen bond between another guanine residue and a Cr(III)-bound aquo ligand. No involvement of the phosphate backbone was observed. The properties and stability of this Cr-O(H)-Cr-bridged complex differ significantly from those reported for Cr-induced interstrand crosslinks, suggesting that interstrand crosslinks resulting from chromate reduction may be organic in nature.

We present an X-band pulse EPR spectrometer with high throughput and excellent sensitivity in the 8.5-11.5GHz range. It is designed for high stability and low noise Fourier Transform measurements for applications in pulse dipolar spectroscopy, pulse hyperfine spectroscopy, and spin relaxation from cryogenic temperatures to room temperature. An arbitrary waveform generator is used to generate pulses of any frequency and shape for multiple resonance experiments or for uniform broadband excitation with bandwidths exceeding 350 MHz. We illustrate the capabilities and performance of the spectrometer by measurements on free radicals and biradicals in solids and liquids. Relaxation times of radicals in liquid solution are measured for fewer than 30,000,000 spins (less than 3 nanomoles per liter). Non-uniform acquisition provides higher throughput for mixtures of radicals with quite different relaxation rates. Conventional DEER measurements on a rigid biradical have good modulation depth. Broadband SIFTER with chirped adiabatic WURST pulses demonstrates versatility for the latest broadband pulse schemes. A broadband ESEEM measurement correlates ESEEM and EPR frequencies which characterize the conformation of a nitroxide radical. The entire EPR spectrum with a width approaching 300 MHz was excited and detected throughout the measurement. The spectrometer supports the operator in tuning, setting up experiments and monitoring their progress so that even novice users consistently can obtain optimal results.

Open-shell conjugated polymers (CPs) offer new opportunities to integrate the spin degree of freedom within emerging technologies. Central to their realization are strong acceptors that stabilize unpaired spins within the x-conjugated backbones. Here, we demonstrate a high-spin CP composed of alternating benzo[1,2-b:4,5-b ']dithiophene donors and a new, strongly electron-withdrawing 6,7,8,9-tetrachloro-[1,2,5]thiadiazolo[3,4-b]phenazine acceptor. A comparative study with a 6,7dimethyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) acceptor demonstrates that annulation and chlorination of the TQ framework facilitates a transition between closed-shell aromatic and high-spin quinoidal forms. This is accompanied by a concomitant reduction of the bandgap, high electron affinity, delocalization of spin density, and n-type conduction. These insights enable access to a broader range of open-shell CPs and the manipulation of important properties such as topology, exchange interactions, and carrier polarity.

The impact of g-tensor anisotropy on the dipole–dipole interaction (DDI) of Kramers paramagnetic centers (PCs) with spins of ½ is theoretically considered in the point dipole limit. The magnetic moment may be expressed in terms of the tensor G = gT.g and the signature of the g-tensor (sign of its determinant). This means that the DDI spin Hamiltonian depends only on the G of the PCs involved and on their respective g-tensor signatures. Abragam’s dipole alphabet for isotropic PCs consists of six letters by Abragam (The principles of nuclear magnetism, Clarendon Press, Oxford, 1961), each letter a product of a spin operator and a spatial coordinate. Pairs of letters correspond to zero-, single-, and double-quantum coherences. In the case of like anisotropic spins, the dipole alphabet has the same structure but with different coordinate factors that depend on the tensor G and its orientation in the laboratory frame. In the general case, anisotropic spins have nine letters in their dipole alphabet. Analytic expressions for all the letters are obtained. The DDI spin Hamiltonian for anisotropic PCs can contain terms having the appearance of isotropic exchange-like and quadrupolar-like interactions resulting entirely from the DDI.

A copper porphyrin and two copper phthalocyanine complexes were incorporated into a hierarchically porous carbon support formed by a sol-gel resorcinol-formaldehyde polymerization templated by F-127 block copolymer. The F127 self-organization leads to formation of mesostructure that on carbonization produces mesopores. Striking differences in catalytic activity for iodobenzene/imidazole cross coupling were observed between Alcian Blue-8gx (CuPc-8gx) (active) and copper phthalocyanine disulfonate (CuPc2S) (inactive). FTIR and EPR spectroscopy confirmed that phthalocyanine remained in the carbon after it was formed by heating to 500 ?, but that no complex survived heating to 800 ?. Electron microscopy (SEM and TEM) showed that the two dyes were differently distributed within the carbon support and found very different sizes of copper nanoparticles: 1.5 nm for CuPc-8gx and 86 nm for CuPc2S. Based on quenching of pyrene fluorescence by the two dyes it was concluded that CuPc-8gx was incorporated into F127 micelles. During polymerization the CuPc-8gx remained associated with F127 and so was incorporated into the polymer and subsequently the carbon. CuPc2S was not associated with F127 micelles, was not incorporated into the polymer and instead precipitated out during drying. This led to large CuPc2S nanoparticles, which resulted in very large copper nanoparticles with no catalytic activity.

We report the experimental observation of a spectral manifestation of a magnetic polariton that was theoretically predicted last year. This unprecedented manifestation is demonstrated not only for 15N-enriched peroxylamine disulfonate, a radical that adheres strictly to the assumptions of the theory, but also for a radical, 4-oxo-2,2,6,6-tetramethylpiperidine-d16;1-15N-1-oxyl, that departs somewhat from the assumptions, as well as the Galvinoxyl radical that represents a severe departure. The magnetic polariton is likely to be of interest to physical chemists in other fields because of the intrinsic advantage of a finite basis set in developing theories.

The development of open-shell organic molecules that magnetically order at room temperature,which can be practically applied, remains a grand challenge in chemistry, physics, and materials science. Despite the exploration of vast chemical space, design paradigms for organic paramagnetic centers generally result in unpaired electron spins that are unstable or isotropic. Here, a high-spin conjugated polymer is demonstrated, which is composed of alternating cyclopentadithiophene and benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole heterocycles, in which macromolecular structure and topology coalesce to promote the spin center generation and intermolecular exchange coupling. Electron paramagnetic resonance (EPR) spectroscopy is consistent with spatially localized spins, while magnetic susceptibility measurements show clear anisotropic spin ordering and exchange interactions that persist at room temperature. The application of long-range π-correlations for spin center generation promotes remarkable stability. This work offers a fundamentally new approach to the implementation of this long-sought-after physical phenomenon within organic materials and the integration of manifold properties within emerging technologies.

Donor-acceptor (DA) conjugated polymers (CPs) with narrow bandgaps and open-shell (diradical) character represent an emerging class of materials whose rich behavior emanates from their collective electronic properties and diminished electron pairing. However, the structural and electronic heterogeneities that define these materials complicate bandgap control at low energies and connections linking topology, exchange interactions, and (opto)electronic functionality remain nascent. To address these challenges, we demonstrate structurally rigid and strongly π-conjugated copolymers comprised of a solubilizing thiadiazoloquinoxaline acceptor and cyclopenta[2,1-b:3,4-b′]dithiophene or dithieno[3,2-b:2′,3′-d]thiophene donors. Atom-specific substitution modulates local aromatic character within the donor resulting in dramatic differences in structural, physicochemical, electronic, and magnetic properties of the polymers. These long-range π-mediated interactions facilitate control between low-spin aromatic and high-spin quinoidal forms. This work provides a strategy to understand the evolution of the electronic structure within DA CPs, control the ground state spin multiplicity, tune spin-spin interactions, and articulate the emergence of their novel properties.

Pulse dipolar spectroscopy (PDS) in Electron Paramagnetic Resonance (EPR) is the method of choice for determining the distance distribution function for mono-, bi- or multi- spin-labeled macromolecules and nanostructures. PDS acquisition schemes conventionally use uniform sampling of the dipolar trace, but non-uniform sampling (NUS) schemes can decrease the total measurement time or increase the accuracy of the resulting distance distributions. NUS requires optimization of the data acquisition scheme, as well as changes in data processing algorithms to accommodate the non-uniformly sampled data. We investigate in silico the applicability of the NUS approach in PDS, considering its effect on random, truncation and sampling noise in the experimental data. Each type of noise in the time-domain data propagates differently and non-uniformly into the distance spectrum as errors in the distance distribution. NUS schemes seem to be a valid approach for increasing sensitivity and/or throughput in PDS by decreasing and redistributing noise in the distance spectrum so that it has less impact on the distance spectrum.

Pulsed Dipolar Spectroscopy (PDS) methods of Electron Paramagnetic Resonance (EPR) were used to detect and characterize reversible non-covalent dimers of Human Serum Albumin (HSA), the most abundant protein in human plasma. The spin labels, MTSL and OX063, were attached to Cys-34 and these chemical modifications of Cys-34 did affect the dimerization of HSA, indicating that other post-translational modifications can modulate dimer formation. At physiologically relevant concentrations, HSA does form weak, non-covalent dimers with a well-defined structure. Dimer formation is readily reversible into monomers. Dimerization is very relevant to the role of HSA in the transport, binding, and other physiological processes.

Electron paramagnetic resonance (EPR) spectroscopy, also known as electron spin resonance spectroscopy (ESR), utilizes absorption of microwave radiation by unpaired electrons in a magnetic field. The interaction between the unpaired electron(s) and nearby magnetic nuclei helps identify paramagnetic species and can provide information about the motion of the molecule and the local polarity, pH, viscosity, concentration, and accessibility to other paramagnetic species. This mini-review discusses the fundamental underpinnings of EPR needed to correctly interpret EPR spectra. We describe various types of EPR spectra encountered by chemical engineers, and use application examples drawn from the chemical engineering literature to illustrate the information available from the technique. Few chemical engineering departments or even chemistry departments have EPR instruments, which contributes to the significant barrier that prevents this being adopted as a routine measurement technique. However, in 2016 and 2017, Web of Science indexed 7000 articles that applied EPR spectroscopy. A bibliometric map categorized the keywords in four categories based on co-occurrences: magnetic properties, films, and luminescence; crystal structure, complexes, and ligands; nanoparticles, oxidation, and degradation; and, systems, radicals, and H2O2.