Positive Magneto-LC Effect in Conjugated Spin-Bearing Hexabenzocoronene

Evaluation of 1 H-NMR Spectroscopy-Based Quantification Methods of the Supramolecular Aggregation of a Molecular Photosensitizer

The supramolecular dimerization of a ruthenium polypyridyl precursor of a well-developed family of hydrogen evolving photocatalysts via π-π-interactions of the polyheteroaromatic bridging ligand was quantified with concentration dependent ¹ H-NMR-spectroscopy. The data sets were analyzed with different calculation and fit methods. A comparison between the results of direct calculation, linear and nonlinear approaches showed that the application of a global nonlinear fit procedure yields the best results. The presented methods are also applicable for dimerization processes in solution of other molecular moieties.

From spin-labelled fused polyaromatic compounds to magnetically active graphene nanostructures

Molecular design of magnetically active graphene nanoscale structures is an emerging field of research. The key goal of this research is to produce graphene nanoribbons and graphene quantum dots with specified electronic, optical and magnetic properties. The review considers methods for the synthesis of spin-labelled polycyclic aromatic hydrocarbons, which are homologous precursors of graphene nanostructures, and discusses the advances and prospects of the design of magnetically active graphene materials.

The bibliography includes 134 references.

Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core-Shell Columnar Liquid Crystals

Recently, we discovered a modular synthetic approach for constructing core-shell columnar liquid crystals (LCs) by supramolecular polymerization in LC media. In the present work, we successfully confirmed that our modular synthetic approach has the potential to be widely extended to the development of multifunctional columnar LCs. Herein, we constructed the first core-shell columnar LC that was proved to be orientable by both electric and magnetic fields by the supramolecular polymerization of ^NODisk_NH* in a nematic LC medium of 4-cyano-4'-pentyloxybiphenyl (5OCB). ^NODisk_NH* is a chiral benzenetricarboxamide derivative bearing 2,2,6,6-tetramethylpiperidine 1-oxyl termini, which is known to form a helical supramolecular polymer via a triple hydrogen-bonding array. ^NODisk_NH* alone formed a hydrogen-bonded liquid phase without any long-range structural ordering. However, a nematic LC medium of 5OCB, when mixed with ^NODisk_NH* at a molar ratio of 1:3, underwent a "structural order-increasing" mesophase transition, affording an optically active single LC phase with a hexagonally arranged core-shell columnar geometry in a temperature range from 113 to 51 °C. Unprecedentedly, this core-shell columnar LC can orient its columns both electrically and magnetically, resulting in unidirectional columnar ordering.

Planar Benzo[1,2- b:4,5- b']dithiophene Derivatives Decorated with Nitronyl and Imino Nitroxides

Four weakly antiferromagnetic interacting biradicals of benzo[1,2- b:4,5- b']dithiophene (BDT) and BDT extended with two thiophenes (BDTTh₂) linked with nitronyl and imino nitroxides (NN and IN) as BDT-NN, BDT-IN, BDTTh₂-NN, and BDTTh₂-IN were designed, synthesized, and characterized. Short intermolecular π-π distances were found (3.42 Å) for BDT-NN, whereas larger ones were found for BDT-IN (3.54 Å) and BDTTh₂-NN (3.67 Å), respectively. Intramolecular magnetic interaction ( J_intra,exp/ k_B) of BDT-NN (-26 K) is much larger than for BDT-IN (-5.3 K), while it is reduced for the dithiophene-extended molecule BDTTh₂-NN (-2.3 K). Intermolecular interactions ( zJ_inter,exp/ k_B) of BDT-NN (-6.5 K) and BDT-IN (-6.0 K) are stronger than for BDTTh₂-NN (-4.6 K). Such large intermolecular couplings resulting from good π-stacking mark BDT-IN and BDTTh₂-NN as promising crystalline materials with similar sized J_intra and J_inter. In addition, we also extracted a coupling within the chain of J_chain/ k_B = -2.2 K and a coupling between the chains of zJ_interchain = -1.5 K for BDTTh₂-NN by a Heisenberg chain model. Intra- and intermolecular interactions and spin densities were examined by DFT studies.

Unique Superparamagnetic-like Behavior Observed in Non-π-delocalized Nitroxide Diradical Compounds Showing Discotic Liquid Crystalline Phase

A unique superparamagnetic-like behavior and a large "positive magneto-LC effect" were observed in the solid phases and the hexagonal columnar (Col_h ) liquid crystalline (LC) phase, respectively, of novel achiral non-π-delocalized nitroxide diradical compounds (R,S)-1, which showed polymorphism in the solid phases (solids I and II). The SQUID magnetization measurement revealed that (1) (R,S)-1 containing a small amount of racemic diastereomers (R*,R*)-1 possessed an unusual and large temperature-independent magnetic susceptibility (χ_TIM >0) component in the original nanocrystalline solid I that was responsible for the observed superparamagnetic-like behavior under low magnetic fields and did not arise from the contamination by extrinsic magnetic metal or metal ion impurities, besides ordinary temperature-dependent paramagnetic susceptibility (χ_para >0) and temperature-independent diamagnetic susceptibility (χ_dia <0) components, (2) a large increase in molar magnetic susceptibility (χ_M ) (positive magneto-LC effect) that occurred at the solid I-to-liquid crystal transition upon heating was preserved as an additional χ_TIM increase in the resulting polymorphic nanocrystalline solid II by cooling, and (3) such unique magnetic phenomena were induced by thermal processing for (R,S)-1 or by adding a small amount of (R*,R*)-1 to (R,S)-1 as the impurity.

Dimethylcethrene: A Chiroptical Diradicaloid Photoswitch

We describe the synthesis and properties of 13,14-dimethylcethrene, a prototype of a chiral diradicaloid photochemical switch that can be transformed reversibly via conrotatory electrocyclization to its more stable closed form by light (630 nm) or heat and back to its open form by light (365 nm). This system illustrates how the chemical reactivity of a diradicaloid molecule can be translated into a switching function, which alters substantially all electronic parameters, namely, the HOMO-LUMO and the singlet-triplet (ST) energy gaps, and the degree of helical twist. As a result, distinct changes in the optical and chiroptical properties of this system were observed, which allowed us to monitor the switching process by a variety of spectroscopic techniques, including NMR, UV-vis, and CD. In comparison to the previously reported parent molecule cethrene, this system benefits from two methyl substituents installed in the fjord region, which account for the stability of the closed form against oxidation and racemization. The methyl substituents increase the ST energy gap of 13,14-dimethylcethrene by ∼4 kcal mol-1 in comparison to cethrene. Our DFT calculations reveal that the larger ST gap is a result of electronic and geometric effects of the methyl substituents and show the potential of related systems to act as magnetic switches at room temperature.

Molecular Mobility Effect on Magnetic Interactions in All-Organic Paramagnetic Liquid Crystal with Nitroxide Radical as a Hydrogen-Bonding Acceptor

We synthesized new chiral all-organic liquid crystalline (LC) compounds with nitroxide (NO) and hydroxy (OH) groups, which form intermolecular hydrogen bonds between the NO and OH groups. The LC compounds show hexagonal columnar phases at room temperature, which solidify as LC glasses at low temperature. The experimental magnetic susceptibility of each of the compounds in the LC and isotropic phases is larger than that theoretically estimated on the simple assumption about the amount of the spins, whereas it accords with the theoretical one in the LC glass state. It is called magneto-LC effects. The difference between experimental and theoretical magnetic susceptibilities gradually increases as temperature increases through the LC glass state-to-LC phase transition. It suggests that molecular mobility is one of the origins of the magneto-LC effects.

The Design of Radical Stacks: Nitronyl-Nitroxide-Substituted Heteropentacenes

The first alkyl chain-anchored heteropentacene, dithieno[2,3-d;2',3'-d']benzo-[1,2-b;3,4-b']dithiophene (DTmBDT), mono- or disubstituted with a nitronyl nitroxide group has been prepared through a cross-coupling synthetic procedure of the corresponding dibromo-derivative (Br2-DTmBDT) with a nitronyl nitroxide-2-ide gold(I) complex. The synthesized nitroxides possess high kinetic stability, which allowed us to investigate their structure and thermal, optical, electrochemical, and magnetic properties. Single-crystal X-ray diffraction of both mono- and diradicals revealed that the nitronyl nitroxide group lies almost in the same plane as the nearest side thiophene ring. Such arrangement favors formation of edge-to-edge dimers, which then form close π-stacks surrounded by interdigitating alkyl chains. Before melting, these nitronyl nitroxide radical substituted molecules undergo at least two different phase transitions (PTs): for the monoradical, PTs are reversible, accompanied by hysteresis, and occur near 13 and 83 °C; the diradical upon heating shows a reversible PT with hysteresis in the temperature range 2-11 °C and an irreversible PT near 135 °C. PTs of this type are absent in Br2-DTmBDT. Therefore, the step-by-step substitution of bromine atoms by nitronyl nitroxide groups changes the structural organization of DTmBDT and induces the emergence of PTs. This knowledge may facilitate crystal engineering of π-stacked paramagnets and related molecular spin devices.

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.

Induction of smectic polymorphism in bent-core derivatives of the 6-oxoverdazyl by partial fluorination of alkyl chains

A series of bent-core mesogens 2[m,n] derived from the 6-oxoverdazyl radical and containing partially fluorinated alkyl tails was prepared and investigated by thermal, optical, XRD and EPR methods.

Chiral all-organic nitroxide biradical liquid crystals showing remarkably large positive magneto-LC effects

The liquid crystalline chiral nitroxide biradical (S,S,S,S)-3 synthesized has shown much larger ‘positive magneto-LC effects’ in the chiral nematic (N*) phase than the monoradical (S,S)-1.

Positional Isomers of Tetramethoxypyrene-based Mono- and Biradicals

The positional isomers of tert-butylnitroxide (NO) substituted 4,5,9,10-tetramethoxypyrene-based mono- and biradical are synthesized. While the biradical 2,7-TMPNO in which two NO radical moieties are attached at the nodal plane of pyrene adopts a semiquinoid structure, the 1,6- and 1,8-isomers of the same exist in biradical form. The tuning of the antiferromagnetic exchange interactions is achieved by synthesizing the positional isomers of the biradical while maintaining the same radical moiety as well as the π spacer.

Electronic structure and stability of fluorophore-nitroxide radicals from ultrahigh vacuum to air exposure

Thin film processes of organic radicals remain widely unknown, although these materials may have a significant technological potential. In aiming at their use in applications, we explore the electronic structure of thin films of a nitronyl nitroxide radical attached to a fluorophore core. According to our findings, this molecule maintains its radical function and, consequently, its sensing capabilities in the thin films. The films are characterized by a high structural degree of the molecular arrangement, coupled to strong vacuum and air stability that make this fluorophore-nitroxide radical an extremely promising candidate for application in electronics. Our work also identifies a quantitative correlation between the results obtained by the simultaneous use of X-ray photoemission and electron paramagnetic resonance spectroscopy. This result can be used as a standard diagnostic tool in order to link the (in situ-measured) electronic structure with classical ex situ paramagnetic investigations.

3-Substituted 6-oxoverdazyl bent-core nematic radicals: synthesis and characterization

Bent-core mesogens containing the 6-oxoverdazyl as the central angular unit exhibit rare nematic behavior.

New advances in nanographene chemistry

This review discusses recent advancements in nanographene chemistry, focusing on the bottom-up synthesis of graphene molecules and graphene nanoribbons.

"Tschitschibabin type biradicals": benzenoid or quinoid?

Our findings provide a better understanding of the discrepancies related to the discussion in the literature on the ground state of Tschitschibabin's hydrocarbon. A series of phenylene bridged bisnitroxide molecules were designed and studied for the comparison. The simple theoretical and experimental methodologies have been developed and utilized to understand the singlet biradicaloids which exist in semi-quinoid form and exhibits characteristics of biradicaloid and quinoid form simultaneously.