Noncovalent Dualism in Perylene-Diimide-Based Keggin Anion Complexes: Theoretical and Experimental studies.

We report the synthesis and comprehensive characterization of organic-inorganic hybrid salts formed by bis-cationic N,N'-bis(2-(trimethylammonium)ethylene)perylene-3,4,9,10-tetracarboxylic acid bisimide (PTCD2+) and Keggin-type [XW12O40]n- (X = Si, n = 4; X = P, n = 3) polyoxometalates. (PTCD)3[PW12O40]2·3DMSO·2H2O (2) and (PTCD)2[SiW12O40]·DMSO·2H2O (3) were structurally characterized by single crystal X-ray diffraction. The cations in both structures exhibited infinite chainlike arrangements through π-π interactions, contrasting with the previously reported cation-anion stacking observed in naphthalene diimide derivatives. A detailed theoretical study employing topological analysis of the electron density distribution within the quantum theory of atoms in molecules approach provided further insights into this structural dualism. Atomic force microscopy analyses revealed the formation of self-assembled supramolecular structures on graphite from molecular monolayers (3 nm of thick) to submicrometer aggregates for 2. Hyperspectral Raman spectroscopy imaging revealed that such heterostructures are likely formed by an enhanced π-π interactions. Both complexes demonstrated interesting electrochemical behavior, photoluminescence and X-ray-induced luminescence. Electron spin resonance analysis confirmed charge separation in both compounds, with enhanced efficiency observed in compound 2. Our findings of these perylene-based organic-inorganic hybrid salts offer the potential for their application in optoelectronic devices and functional materials.

Correction: Optical property trends in a family of {Mo6I8} aquahydroxo complexes.

Correction for 'Optical property trends in a family of {Mo6I8} aquahydroxo complexes' by Margarita V. Marchuk et al., Dalton Trans., 2021, 50, 8794-8802, https://doi.org/10.1039/D1DT01293B.

Recombination of X-ray-Generated Radical Ion Pairs in Alkane Solution Assembles Optically Inaccessible Exciplexes from a Series of Perfluorinated para-Oligophenylenes with N,N-Dimethylaniline.

We demonstrate that a series of perfluorinated para-oligophenylenes C6F5-(C6F4)n-C6F5 (n = 1-3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7-4.6 × 104 M-1cm-1, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters.

A family of CuI-based 1D polymers showing colorful short-lived TADF and phosphorescence induced by photo- and X-ray irradiation.

Exploiting 2-(alkylsulfonyl)pyridines as 1,3-N,S-ligands, herein we have constructed 1D CuI-based coordination polymers (CPs) bearing unprecedented (CuI)n chains and possessing remarkable photophysical properties. At room temperature, these CPs show efficient TADF, phosphorescence or dual emission in the deep-blue to red range with outstandingly short decay times of 0.4-2.0 µs and good quantum performance. Owing to great structural diversity, the CPs demonstrate a variety of emissive mechanisms, spanning from TADF of 1(M + X)LCT type to 3CC and 3(M + X)LCT phosphorescence. Moreover, the designed compounds emit strong X-ray radioluminescence with the quantum efficiency of up to an impressive 55% relative to all-inorganic BGO scintillators. The presented findings push the boundaries in designing TADF and triplet emitters with very short decay times.

Improved Synthesis of (TBA)2[W6Br14] Paving the Way to Further Study of Bromide Cluster Complexes.

Octahedral cluster complexes of molybdenum and tungsten, [M6X8Y6]n- (M = Mo, W; X, Y = Cl, Br, I), are promising active components in various fields, including biomedicine and solar energy. Cluster complexes draw considerable attention due to their X-ray opacity, red/near-IR luminescence, and ability to convert triplet molecular oxygen to active singlet oxygen under UV and visible irradiation. Among the octahedral cluster complexes of molybdenum and tungsten, compounds with a {W6Br8}4+ core are the least studied. There are only a few examples of compounds with substituted terminal ligands, and their properties are not well understood. Among other things, this is due to more labor-intensive and expensive methods for obtaining the starting compounds in comparison with molybdenum counterparts. In this paper, we describe the synthesis of an octahedral cluster complex, (TBA)2[W6Br14] (TBA+ = tetrabutylammonium), in gram quantities, starting from simple substances─W, Br2, and Bi─in 70% yield. The formation of pentanuclear tungsten cluster complexes was recorded as a byproduct. Compounds with substituted terminal ligands (TBA)2[W6Br8Y6] (Y = NO3, Cl, I) were obtained. We also discuss the instability of (TBA)2[W6Br8(NO3)6] under light exposure, the optical properties of a series of compounds (TBA)2[W6Br8Y6] (Y = Cl, Br, I), and the effect of terminal ligands on the chemical shifts in 183W NMR spectra in dimethyl sulfoxide-d6. The presented approach to the synthesis of one of the main precursors of various bromide cluster complexes on a gram scale can stimulate the study of their properties and development of new functional materials based on them.

A new subclass of copper(I) hybrid emitters showing TADF with near-unity quantum yields and a strong solvatochromic effect.

We introduce here a new subclass of copper(I) hybrid emitters simultaneously containing [CuxIy]z- anions and Cu+ cations, separated in space by a Janus head ligand. When UV-irradiated at 298 K, these unique "Two-In-One" hybrids exhibit a short-lived green TADF with near-unity quantum yield and a strong solvatochromic effect. Moreover, they manifest a strong radioluminescence upon X-ray irradiation. These findings open up new possibilities for the design of highly performing TADF materials.

Overclocking Nitronyl Nitroxide Gold Derivatives in Cross-Coupling Reactions.

Nitronyl nitroxides are functional building blocks in cutting-edge research fields, such as the design of molecular magnets, the development of redox and photoswitchable molecular systems and the creation of redox-active components for organic and hybrid batteries. The key importance of the nitronyl nitroxide function is to translate molecular-level-optimized structures into nano-scale devices and new technologies. In spite of great importance, efficient and versatile synthetic approaches to these compounds still represent a challenge. Particularly, methods for the direct introduction of a nitronyl nitroxide moiety into aromatic systems possess many limitations. Here, we report gold derivatives of nitronyl nitroxide that can enter Pd(0)-catalysed cross-coupling reactions with various aryl bromides, affording the corresponding functionalized nitronyl nitroxides. Based on the high thermal stability and enhanced reactivity in catalytic transformation, a new reagent is suggested for the synthesis of radical systems via a universal cross-coupling approach.

Series of Fluorinated Benzimidazole-Substituted Nitronyl Nitroxides: Synthesis, Structure, Acidity, Redox Properties, and Magnetostructural Correlations.

A special series of nitronyl nitroxides was synthesized: 2-(benzimidazol-2'-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls mono-, di-, tri-, or tetrafluorinated on the benzene ring. The structure of all paramagnets was unambiguously confirmed by single-crystal X-ray diffraction. It was found that in crystals, the radicals are assembled into chains due to intermolecular H-bonds between the benzimidazole moiety (H-bond donor) and the nitronyl nitroxide group or benzimidazole ring (H-bond acceptor). The magnetic properties of nitronyl nitroxides depend on the type of binding of radicals by H-bonds. The magnetic motif of 4-fluoro-, 5-fluoro-, 4,6-difluoro-, 4,5,6-trifluoro-, 4,5,7-trifluoro-, and 4,5,6,7-tetrafluoro-derivatives, as well as the nonfluorinated compound, consists of ferromagnetic chains (J/kB ≈ 20-40 K) formed by the McConnell type I mechanism. In the 5,6-difluoro- and 4,5-difluoro-derivatives, the distances between the paramagnetic centers are large, as a result of which the exchange interactions are weak. According to cyclic voltammetry, paramagnets are oxidized reversibly, while their reduction is a quasi-reversible electron transfer (EC mechanism); experimental redox potentials of radicals correlate well with the calculated values. Quantum chemical assessment of the acidity of benzimidazolyl-substituted nitronyl nitroxides revealed that the introduction of fluorine atoms into the benzene ring enhances the acidity of the paramagnets by more than 5 orders of magnitude.

Geometrization for Energy Levels of Isotropic Hyperfine Hamiltonian Block and Related Central Spin Problems for an Arbitrarily Complex Set of Spin-1/2 Nuclei.

Description of interacting spin systems relies on understanding the spectral properties of the corresponding spin Hamiltonians. However, the eigenvalue problems arising here lead to algebraic problems too complex to be analytically tractable. This is already the case for the simplest nontrivial (Kmax−1) block for an isotropic hyperfine Hamiltonian for a radical with spin-12 nuclei, where n nuclei produce an n-th order algebraic equation with n independent parameters. Systems described by such blocks are now physically realizable, e.g., as radicals or radical pairs with polarized nuclear spins, appear as closed subensembles in more general radical settings, and have numerous counterparts in related central spin problems. We provide a simple geometrization of energy levels in this case: given n spin-12 nuclei with arbitrary positive couplings ai, take an n-dimensional hyper-ellipsoid with semiaxes ai, stretch it by a factor of n+1 along the spatial diagonal (1, 1, …, 1), read off the semiaxes of thus produced new hyper-ellipsoid qi, augment the set {qi} with q0=0, and obtain the sought n+1 energies as Ek=−12qk2+14∑iai. This procedure provides a way of seeing things that can only be solved numerically, giving a useful tool to gain insights that complement the numeric simulations usually inevitable here, and shows an intriguing connection to discrete Fourier transform and spectral properties of standard graphs.

Oxygen-Sensitive Photo- and Radioluminescent Polyurethane Nanoparticles Modified with Octahedral Iodide Tungsten Clusters.

The development of cancer treatment techniques able to cure tumors located deep in the body is an urgent task for scientists and physicians. One of the most promising methods is X-ray-induced photodynamic therapy (X-PDT), since X-rays have unlimited penetration through tissues. In this work, octahedral iodide tungsten clusters, combining the properties of a scintillator and photosensitizer, are considered as a key component of nanosized polyurethane (pU) particles in the production of materials promising for X-PDT. Cluster-containing pU nanoparticles obtained here demonstrate bright photo- and X-ray-induced emission in both solid and water dispersion, great efficiency in the generation of singlet oxygen, and high sensitivity regarding photoluminescence intensity in relation to oxygen concentration. Additionally, incorporation of the cluster complex into the pU matrix greatly increases its stability against hydrolysis in water and under X-rays.

Kinetic evidence for the transiently shifted acidity constant of histidine linked to paramagnetic tyrosine probed by intramolecular electron transfer in oxidized peptides.

The kinetics of electron transfer (ET) from tyrosine (Tyr) to short-lived histidine (His) radicals in peptides of different structures was monitored using time-resolved chemically induced dynamic nuclear polarization (CIDNP) to follow the reduction of the His radicals using NMR detection of the diamagnetic hyperpolarized reaction products. In aqueous solution over a wide pH range, His radicals were generated in situ in the photo-induced reaction with the photosensitizer, 3,3',4,4'-tetracarboxy benzophenone. Model simulations of the CIDNP kinetics provided pH-dependent rate constants of intra- and intermolecular ET, and the pH dependencies of the reaction under study were interpreted in terms of protonation states of the reactants and the product, His with either protonated or neutral imidazole. In some cases, an increase of pKa of imidazole in the presence of the short-lived radical center at a nearby Tyr residue was revealed. Interpretation of the obtained pH dependencies made is possible to quantify the degree of paramagnetic shift of the acidity constant of the imidazole of the His residue in the peptides with a Tyr residue in its paramagnetic state, and to correlate this degree with the intramolecular ET rate constant - a higher intramolecular ET rate constant corresponded to a greater acidity constant shift.

Optical property trends in a family of {Mo6I8} aquahydroxo complexes.

Luminescence is one of the key properties of octahedral molybdenum cluster complexes and the basis for most areas of their possible practical applications. Nevertheless, the factors affecting the optical properties of the clusters are insufficiently studied and establishing them will allow us to tune both absorption and emission more precisely. In this work, we obtained two new cationic [{Mo6I8}(H2O)4(OH)2](An)2·nH2O (An = NO3-, n = 3; An = OTs-, n = 2, OTs- - p-toluenesulfonate), and two neutral [{Mo6I8}(H2O)2(OH)4]·nH2O (n = 2, 12) aquahydroxo complexes. Due to the similar compositions of the clusters obtained, we determined the influence of crystal packing and ligand environment on the absorption and photo- and radioluminescence properties. Thus, the four-component nature of the cluster emission was established using Gaussian deconvolution of the photoluminescence spectra. It was shown that the influence of both ligand type and crystal density decreases when moving to the red (lower-energy) part of the spectra, with only the first two components located in the blue (higher-energy) part of the spectra being strongly affected. Also, it was found that protonation of two hydroxo ligands leads to a significant decrease in absorption in the visible spectral region.

Platform for High-Spin Molecules: A Verdazyl-Nitronyl Nitroxide Triradical with Quartet Ground State.

Thermally resistant air-stable organic triradicals with a quartet ground state and a large energy gap between spin states are still unique compounds. In this work, we succeeded to design and prepare the first highly stable triradical, consisting of oxoverdazyl and nitronyl nitroxide radical fragments, with a quartet ground state. The triradical and its diradical precursor were synthesized via a palladium-catalyzed cross-coupling reaction of diiodoverdazyl with nitronyl nitroxide-2-ide gold(I) complex. Both paramagnetic compounds were fully characterized by single-crystal X-ray diffraction analysis, superconducting quantum interference device magnetometry, EPR spectroscopy in various matrices, and cyclic voltammetry. In the diradical, the verdazyl and nitronyl nitroxide centers demonstrated full reversibility of redox process, while for the triradical, the electrochemical reduction and oxidation proceed at practically the same redox potentials, but become quasi-reversible. A series of high-level CASSCF/NEVPT2 calculations was performed to predict inter- and intramolecular exchange interactions in crystals of di- and triradicals and to establish their magnetic motifs. Based on the predicted magnetic motifs, the temperature dependences of the magnetic susceptibility were analyzed, and the singlet-triplet (135 ± 10 cm-1) and doublet-quartet (17 ± 2 and 152 ± 19 cm-1) splitting was found to be moderate. Unique high stability of synthesized verdazyl-nitronylnitroxide triradical opens new perspectives for further functionalization and design of high-spin systems with four or more spins.

A water-soluble octahedral molybdenum cluster complex as a potential agent for X-ray induced photodynamic therapy.

X-ray-induced photodynamic therapy (X-PDT) has recently evolved into a suitable modality to fight cancer. This technique, which exploits radiosensitizers producing reactive oxygen species, allows for a reduction of the radiation dose needed to eradicate cancer in the frame of the radiotherapy treatment of deep tumors. The use of transition metal complexes able to directly produce singlet oxygen, O2(1Δg), upon X-ray irradiation constitutes a promising route towards the optimization of the radiosensitizer's architecture. In our endeavour to conceive pertinent agents for X-PDT, we designed an octahedral molybdenum cluster complex (Mo6) with iodine inner ligands, and carboxylated apical ligands bearing ethylene oxide organic functions. The sodium salt of this complex is highly soluble in aqueous media and displays red luminescence which is efficiently quenched by oxygen to produce O2(1Δg) in a high quantum yield. Furthermore, due to its high radiodensity, the complex exhibits radioluminescence in aqueous media, with the same spectral features as for photoluminescence, indicating the production of O2(1Δg) upon X-ray irradiation. The uptake of the complex by Hep-2 and MRC-5 cells is negligible during the first hours of incubation, then considerably increases in connection with the hydrolysis of the apical ligands. The complex exhibits low toxicity in vitro and induces a radiotoxic effect, noticeable against cancerous Hep-2 cells but negligible against normal MRC-5 cells, at X-ray doses that do not affect cell viability otherwise. The first evaluation of in vivo toxicity of an Mo6 complex on a mouse model evidences a moderate and delayed toxic effect on kidneys, with an intravenous LD50 value of 390 ± 30 mg kg-1, possibly connected with hydrolysis-induced aggregation of the complex. Overall, this complex displays attractive features as a singlet oxygen radiosensitizer for X-PDT, highlighting the potential of transition metal cluster complexes towards this modality.

Simple rules for resolved level-crossing spectra in magnetic field effects on reaction yields.

In this work we derive conditions under which a level-crossing line in a magnetic field effect curve for a recombining radical pair will be equivalent to the electron spin resonance (ESR) spectrum and discuss three simple rules for qualitative prediction of the level-crossing spectra.

(Pyrrole-2,5-Diyl)-Bis(Nitronyl Nitroxide) and-Bis(Iminonitroxide): Specific Features of the Synthesis, Structure, and Magnetic Properties.

In contrast to diradicals connected by alternant hydrocarbons, only a few studies have addressed diradicals connected by nonalternant hydrocarbons and their heteroatom derivatives. Here, the synthesis, structure, and magnetic properties of pyrrole-2,5-diyl-linked bis(nitronyl nitroxide) and bis(iminonitroxide) diradicals are described. The diradicals show characteristic electron spin resonance spectra in dilute glassy solutions, from which conclusions about the presence of distinct conformations, their symmetry, and interspin distance were made. X-ray diffraction analysis of the diradicals revealed that paramagnetic moieties lie in the plane of the pyrrole ring, because of the formation of an intramolecular hydrogen bond, ONO…HN, with O…H distances of 2.15-2.23 Å. The N-O groups participating in the formation of H-bonds have greater bond lengths (~1.29 Å) as compared with nonparticipating groups (~1.27 Å). The nitronyl nitroxide and iminonitroxide diradicals showed an intramolecular antiferromagnetic interaction, with J = -77.3 and -22.2 cm-1, respectively (H = -2JS1S2).

Magnetic Properties of π-Conjugated Hybrid Phenoxyl-Nitroxide Radicals with Extended π-Spin Delocalization.

A series of stable and genuinely organic open-shell systems, π-conjugated phenoxyl-nitroxide free radicals (hybrid phenoxyl-nitroxide radicals), have been synthesized and their magnetic properties in the crystalline state investigated, revealing their usefulness as new building blocks for molecular magnetic materials. The salient electronic structure of the hybrid phenoxyl-nitroxide radicals is extended π-spin delocalization from the nitroxide moiety, mediating the localization effect intrinsic to nitroxide radicals. Five representative hybrid radicals containing an aliphatic, aromatic, and heteroaromatic substituent in the side part of the compact hybrid radical centers were synthesized, and their molecular/crystal structures in the crystalline state were determined by X-ray diffraction analyses. CW X-band ESR, 1H-ENDOR spectroscopy, and DFT calculations for the hybrid radicals confirmed that an unpaired spin delocalizes over the whole molecular frame including the nonconjugated fragments, suggesting the possibility of tuning their electronic properties through substituent effects in the crystalline state. Significant influence of the phenoxyl moiety on the electronic structure was analyzed in terms of the g-tensor calculations. The SQUID magnetization measurements revealed that the nitroxides bearing alkyl or aromatic substituents behave as 3D Curie-Weiss paramagnets with weak antiferromagnetic (AFM) (Θ = -1 to -2.6 K) or ferromagnetic (FM) (Θ = +0.33 K) spin-spin exchange interactions. On the other hand, heteroaromatically substituted hybrid phenoxyl-nitroxide showed significant AFM interactions with J/kB = -25.6 K. The analysis of the bulk magnetic properties based on the crystallographic data and DFT calculations revealed competition between the intermolecular AFM and FM interactions which originate from the C-O(phenoxyl)···Me(nitroxide) or (N)O-C(arom) infinite 1D head-to-tail chains and the C(arom)-C(arom) head-over-tail dimers forming 3D networks in their crystal lattices.

On algebraic properties of the sub-block of zero field hyperfine Hamiltonian with penultimate total spin projection for arbitrary hyperfine structure, and field dependence of radical pair recombination probability in the vicinity of zero field.

Basic algebraic arguments demonstrate that the probability of radical pair recombination in low field for an arbitrary pair with Hamiltonian confined to Zeeman and isotropic hyperfine interactions contains two additive contributions linear with applied field with equal, but opposite in sign, proportionality factors. Their weights are determined by the probability of having all nuclear spins along the field in the initial electron-singlet state of the pair, and in the case of equilibrium with respect to nuclear spins, the two contributions completely compensate the field dependences of each other, producing an additive term in the singlet yield with zero derivative. However, if the nuclear set is polarized, a linear skew proportional to polarization appears, introducing anisotropy in the intrinsically spherically symmetric system. The key element in this derivation is guaranteed nondegeneracy of the eigenvalues of the "penultimate" (Mmax - 1) block of the Hamiltonian for a radical with any number of distinct spin-1/2 nuclei in zero field, which leads to guaranteed applicability of the first-order nondegenerate perturbation theory with nonvanishing linear in field admixture of states, persisting all the way into the final expression for singlet yield for a pair with an arbitrary isotropic hyperfine structure. We argue that this behavior of the field dependence of recombination yield is representative of a radical pair of an arbitrarily complex hyperfine structure; this may be a possible mechanism for anisotropic response ("chemical compass") for an isotropic radical pair based system with isotropic-only internal interactions and anisotropy introduced via the initial state of nuclei, while such an approach may complement the usually needed numerical simulations.

Photochromism in oxalatoniobates.

Addition of 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or 2-aminopyridine (2-NH2-py) to aqueous solutions of (NH4)[NbO(C2O4)2(H2O)2]·3H2O (Nb-Ox) yields tris-oxalate complexes (bpyH2)(bpyH)[NbO(C2O4)3]·2H2O (1), (phenH)3[NbO(C2O4)3]·3H2O (2), and (2-NH2-pyH)3[NbO(C2O4)3]·2H2O (3), which were characterised by XRD, IR and EA. Bipyridinium salt 1 demonstrates remarkable photoactivity even under irradiation by daylight. The nature of the photoactivity was studied by diffuse reflectance (DR) spectroscopy, ESR and quantum-chemical calculations.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems.

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.