Intramolecular Magnetic Exchange Interaction in Dichalcogenide Substituted Organic Diradical Dications

Organic diradical dications, due to reduced intermolecular interactions, exhibit a greater tendency to adopt high spin states in the solid phase compared to their neutral diradical counterparts. This characteristic makes them promising candidates for applications involving organic electronics. We present a theoretical study of a recently synthesized sulfur-based diradical dication, a unique system exhibiting a robust triplet ground state. Using a number of density functional theory (DFT)-based methods (e.g., standard broken-symmetry DFT, constrained DFT, spin-flip TDDFT) and wave function-based multireference CASSCF+NEVPT2 methods, we investigate its magnetic properties and explore the influence of chalcogen substitution on magnetic exchange coupling. An active space scanning method was adopted to overcome the difficulties in choosing the correct active space for multireference calculation. Our findings highlight the critical role of multireference methods in accurately capturing the magnetic behavior of highly π-conjugated systems. The study reveals a surprising variation in magnetic properties among sulfur, selenium, and tellurium-based diradical dications despite being elements of the same group. These results offer valuable insights into the design and tuning of magnetic properties in organic diradical dications.

The Ground State of Multispin Systems Based on Verdazyl and Nitrene Radicals: An EPR and Quantum-Chemical Study

In this study, low-temperature EPR spectroscopy and quantum-chemical techniques were employed to investigate multispin systems─1,5-diphenyl-3-(3-nitrenophenyl)-6-oxoverdazyl and 1,5-diphenyl-3-(4-nitrenophenyl)-6-oxoverdazyl─that contain a nitrene center at either a meta- or para-position, respectively. Ground states and magnetic zero-field splitting (ZFS) parameters of these multispin systems were determined by experimental and computational methods. The results indicated that the high-spin quartet state is a ground state, and the quartet-doublet energy gap is close to 10 kcal/mol for the para-position of the nitrene group, with ZFS parameters D = 0.292 cm-1 and E/D = 0.002 cm-1. In contrast, for the meta-position, the low-spin doublet state is favored with an energy gap of 1 kcal/mol. The observed difference in the ground states could be qualitatively explained by an analysis of the spin density distribution and by delocalization of the unpaired π-electron of the nitrene center. Overall, this study provides valuable insights into the electronic structures and magnetic parameters of such multispin systems.

Thermal curing mechanisms and cross-linking network structure of a novel silicon-containing arylacetylene resin with 2,7-diethynylnaphthalene unit

Silicon-containing arylacetylene resin and its composites have attracted great interest as emerging heat-resistant materials, but their curing mechanisms and products are still elusive. In this work, the influences of the terminal and inner acetylenes on the curing mechanisms of silicon-containing arylacetylene resin with 2,7-diethynylnaphthalene were first identified by density functional theory. Two reaction pathways were proposed and their products include polyenes, anthracene dimers, and benzene trimers. To gain a distinct observation of the cross-linking process, molecular dynamics simulations were used to construct a cross-linking polymerization model. The effects of the temperature on the cured structure were investigated by analyzing the characteristics of the cross-linked network. As expected, higher curing temperature will make the larger proportion of polyene chain and aromatic ring in the terminal alkyne-terminal alkyne route, meanwhile, for the inner alkyne-inner alkyne route, the short chains and a small amount of aromatic rings are major productions. Overall, our cross-linking method may provide an unique guidance for studying the cured structure of other thermosetting resins.

Enhancing Intramolecular Ferromagnetic Coupling in Tetrathiafulvalene-Nitronyl Nitroxide-Based Compounds through Spin Polarization Mechanism

Spin-polarized donor radicals based on tetrathiafulvalene (TTF) derivatives and nitronyl nitroxide (NN) radicals in which one-electron oxidation involves the HOMO instead of the SOMO are well known for exhibiting magnetoresistance. In particular, BTBN consists of one dibromo-TTF and one NN radical, which are linked by a phenyl coupler group. One of the key factors driving magnetoresistance is the presence of intramolecular ferromagnetic (FM) coupling between the oxidized π-donor (TTF+⋅, D unit) and NN (R unit). Here, a theoretical study is carried out to assess suitable candidates with enhanced FM coupling with respect BTBN, which is thus used as a reference. The study is conducted via in silico chemical modification of the substituents of the BTBN basic functional units (D and R radicals, C coupler) to benefit from the spin polarization mechanism to boost the intramolecular FM coupling, aiming to distort the BTBN radical arrangement within the molecular crystal as little as possible, in the event the material can be synthesized. NICSiso(1) and Wiberg's Bond Order are analyzed to further assist in identifying promising potential candidates, since the decrease in aromaticity is expected to enhance the diradical character and give rise to a larger magnetic coupling value. The most favorable diradical building block to replace the BTBN moiety results from using a hydroxyl-ethylene (-(H)C=C(OH)-) as a coupler preserving BTBN original radicals, namely, NN and TTF+⋅ units. This study aims at illustrating the feasibility of improving the intramolecular FM interaction between radical moieties, which is fully realized, as a first step towards the synthesis of new materials with (possibly) enhanced magnetoresistance properties.

Enhancing magnetic coupling through protonation of benzylideneaniline-bridged diradicals and comparison with stilbene- and azobenzene-based diradicals

Taking nitroxide radicals as spin sources, we explore the intramolecular magnetic coupling interactions of the trans- and cis-forms of benzylideneaniline (BA)-bridged diradicals, in which the central -CH[double bond, length as m-dash]N- unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) verify that the signs of their magnetic coupling constants J do not change, but the magnitudes significantly increase after protonation. In the structure, the better conjugation of the protonated trans diradical and two reduced CCNC and CCCN torsion angles of the protonated cis one make for a more efficient spin transport, promoting the spin polarization, thus leading to larger spin couplings. In terms of mechanism, the proton-induced magnetic enhancement should be attributed to strong participation of the coupler BA through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation, and the small HOMO-LUMO (HOMO: highest occupied molecular orbital) gap of the coupler BA through protonation is crucial in explaining such remarkable spin-coupling enhancement. Furthermore, different linking modes of the radical groups to the couplers are also considered to confirm our conclusions. In addition, we also make a comparison of the magnetic coupling strengths among their isoelectronic analogues of BA-, AB- and stilbene-bridged nitroxide diradicals before or after protonation, and find a linear correlation among them. It should be noted that the magnetic behaviors of all these diradicals obey the spin alternation rule and singly occupied molecular orbital (SOMO) effect. This work provides helpful information for the rational design of promising magnetic molecular switches.

Magnetic Modulation in Heteroallene and Heterocumulene Based tert-butyl Nitroxide Diradicals: Spin Delocalization and Conformation Play Crucial Roles

We have theoretically investigated the magnetic properties of heteroallene (>C=C=X-) and heterocumulene (>C=C=C=X-) based tert-butyl nitroxide diradicals (X is P/As). Calculation of magnetic exchange coupling constant (J) shows ferromagnetic interaction in heteroallene based diradicals. Whereas, in heterocumulene based diradicals, tuning of J value from antiferro- to ferro-magnetic state is observed from Z- to E- isomer. Delocalization of spin density from radical site to the coupler (in planar arrangement) is observed in spin distribution analysis which is also advocated by molecular orbital analysis. The typical feature of tert-butyl nitroxide radical creates spin delocalization along with spin polarization within the coupler. The J values of all the diradicals strongly depend on the dihedral angle between radical center and coupler. Magneto-structural correlation shows that the change in dihedral angle tunes the magnetic property for both the Z- and E- isomers of heterocumulenes depending on the spin accumulation on two nearby magnetic centers. The extent of spin delocalization and conformation of spin centers on the molecular axis are important for the different J values observed in our designed systems.

Verdazyls as Possible Building Blocks for Multifunctional Molecular Materials: A Case Study on 1,5-Diphenyl-3-(p-iodophenyl)-verdazyl Focusing on Magnetism, Electron Transfer and the Applicability of the Sonogashira-Hagihara Reaction

This work explores the use of Kuhn verdazyl radicals as building blocks in multifunctional molecular materials in an exemplary study, focusing on the magnetic and the electron transfer (ET) characteristics, but also addressing the question whether chemical modification by cross-coupling is possible. The ET in solution is studied spectroscopically, whereas solid state measurements afford information about the magnetic susceptibility or the conductivity of the given samples. The observed results are rationalized based on the chemical structures of the molecules, which have been obtained by X-ray crystallography. The crystallographically observed molecular structures as well as the interpretation based on the spectroscopic and physical measurements are backed up by DFT calculations. The measurements indicate that only weak, antiferromagnetic (AF) coupling is observed in Kuhn verdazyls owed to the low tendency to form face-to-face stacks, but also that steric reasons alone are not sufficient to explain this behavior. Furthermore, it is also demonstrated that ET reactions proceed rapidly in verdazyl/verdazylium redox couples and that Kuhn verdazyls are suited as donor molecules in ET reactions.

Spin coupling interactions in C[double bond, length as m-dash]C or B-B-cored porphyrin-mimetic graphene patch nitroxide diradicals

In view of the unique structures and promising applications of porphyrins and their derivatives, exploration of their various properties has continued to a hot topic. In this work, we combine porphyrin-mimetic graphene patches which are core-modified by a C[double bond, length as m-dash]C or a B-B unit and two nitroxide radical groups to construct a series of novel diradical molecules (the CC-cored or BB-cored molecules). The spin coupling constants (J) of diradicals were calculated at the (U)B3LYP/6-311G(d,p) level by considering the different linking modes of two nitroxide groups. The results indicate that different core modification considerably affects the J values of such diradicals, and the linking modes can tune the sizes and signs of J, changing their magnetic coupling interactions with different magnitudes and the signs of J from antiferromagnetic to ferromagnetic or vice versa. More interestingly and importantly, the spin coupling interactions of the CC-cored molecules can also be tuned by stretching the core unit C-C bond, suggesting the possibility of activating specific vibrational modes of the CC-cored diradicals by energy pulses to yield variable J coupling magnitudes. On the other hand, for the BB-cored molecules, two-electron reduction can switch or tune their magnetism from ferromagnetic to antiferromagnetic. The essence of all observations is further analyzed from the structural effects and orbital and spin density distributions. The findings about magnetic regulation in these core-modified porphyrin-mimetic graphene patch nitroxide diradicals further expand the field of molecular magnets and provide a rational theoretical basis for designing novel building blocks of magnetic functional molecular materials.

Magnetic and transport properties of conjugated and cumulated molecules: the π-system enlightens part of the story

We have investigated the intramolecular magnetic exchange coupling constants (J) for a series of nitronyl nitroxide diradicals connected by a range of linear conjugated and cumulene couplers focusing on the unusual π-interaction properties within the couplers.

Diradicalized biphenyl derivative carbon-based material molecules: exploring the tuning effects on magnetic couplings

While the conductance behavior of carbon-based couplers has been successfully investigated, insight into the magnetic properties of such carbon-based molecule coupled diradical systems is still scarce, and especially the structural effect of such couplers on the magnetic properties is poorly understood. The present work reports three different interference effects on the magnetic properties of carbon-based molecule coupled nitroxide diradicals: twisting, sideways group, and position effects. DFT calculations reveal that (i) torsion does not change their broken-symmetry singlet ground state and antiferromagnetic coupling, but decreases their magnetism; (ii) different linkages of two radical moieties result in different ground states and thus different magnetisms, depending on a combination of meta-sites and para-sites; (iii) the antiferromagnetic coupling with a broken-symmetry singlet ground state is not changed by adding sideways groups, but the coupling magnitude can be tuned by modifying the side-bridge. Discussions on geometries, magnetic properties, SOMO-SOMO splittings, and spin density distributions are made to clarify relevant magnetic behaviors. Clearly, the findings concerning the regulation of the diradicalized material molecules through modifying the carbon-based bridges provide a comprehensive understanding of the magnetism of such carbon-based diradicals and new prospects for the design of building blocks of magnetic functional molecular materials.

Effect of length on the transport and magnetic properties of diradical substituted molecular wires

Extended π-conjugated molecules are known to have interesting applications as conducting nanowires, memory devices, and diodes.

Structural, Magnetic and DFT studies on a Charge-Transfer Salt of a Tetrathiafulvalenepyridyl-(1,5-diisopropyl) verdazyl Diradical Cation

A new tetrathiafulvalene (TTF) donor covalently appended with a 1,5-diisopropylverdazyl radical through a cross-conjugated pyridyl linker (3) has been prepared and characterised. Reaction of 3 with tetracyanoquinonedimethane (TCNQ) afforded the 2:1 charge-transfer complex (3)₂ ⋅TCNQ (4), in which the IR and structural data are consistent with 0.25 e^- charge transfer from the TTF donor (D) to the TCNQ acceptor (A). The TTF and TCNQ molecules adopt a mixed-stack D⋅⋅⋅D⋅⋅⋅A arrangement that does not facilitate conduction. A solution EPR spectrum of 4 comprises a broad featureless singlet, which is consistent with the presence of a TCNQ radical anion. Theoretical studies were performed to probe the exchange interactions within selected fragments of 4 with and without charge transfer. In the absence of charge transfer, DFT calculations reveal weak antiferromagnetic exchange between verdazyl radicals within the (3)₂ monoradical unit. However, partial oxidation of the dimer (3)₂ to the diradical cation leads to an S= 1 / 2 ground state, in which the verdazyl radical spins are now aligned co-parallel as a consequence of antiferromagnetic exchange to the additional delocalised TTF-based spin containing unit. The magnetic properties of 4 are consistent with a net S= 1 / 2 spin state per formula unit with dominant antiferromagnetic interactions between spin-bearing building blocks.

Intra-molecular magnetic exchange interaction in the tripyridinium bis[tetrachloroferrate(iii)] chloride molecular magnet: a broken symmetry-DFT study

The investigation of magnetic ordering in magnetic molecular conductors is the subject of ongoing research studies in the fields of condensed matter physics, chemistry and material sciences. Following the photo-magnetic behavior already observed in individual (FeCl4)2(py·H)3Cl molecules at room temperature, magnetic ordering is studied in this work. Calculations are performed using spin projected broken symmetry via density functional theory within the scheme of the B3LYP approximation. The value of the intra-molecular magnetic coupling constant is obtained as J = 13.2062 kJ mol(-1). A field dependent magnetization experiment is conducted to validate the magnitude and the sign of the exchange constant. Reasonable consistency of experimental and theoretical results confirms the presence of a positive intra-molecular indirect exchange interaction. This work paves the way for possible introduction of a new molecule in the development of advanced molecular electronic devices.

Borazine: spin blocker or not?

Spin-blocker capacity of borazine is investigated formeta-BB,meta-NN andpara-BN structures highlighting the correlation between magnetic properties and aromaticity.

Diradicals acting through diamagnetic phenylene vinylene bridges: Raman spectroscopy as a probe to characterize spin delocalization

We present a complete Raman spectroscopic study in two structurally well-defined diradical species of different lengths incorporating oligo p-phenylene vinylene bridges between two polychlorinated triphenylmethyl radical units, a disposition that allows sizeable conjugation between the two radicals through and with the bridge. The spectroscopic data are interpreted and supported by quantum chemical calculations. We focus the attention on the Raman frequency changes, interpretable in terms of: (i) bridge length (conjugation length); (ii) bridge conformational structure; and (iii) electronic coupling between the terminal radical units with the bridge and through the bridge, which could delineate through-bond spin polarization, or spin delocalization. These items are addressed by using the "oligomer approach" in conjunction with pressure and temperature dependent Raman spectroscopic data. In summary, we have attempted to translate the well-known strategy to study the electron (charge) structure of π-conjugated molecules by Raman spectroscopy to the case of electron (spin) interactions via the spin delocalization mechanism.

An unusual single-crystal-to-single-crystal [2 + 2] photocyclisation reaction of a TTF-arylnitrile derivative

Irradiation of single crystals of a benzonitrile substituted tetrathiafulvalene with polychromatic light results in a [2 + 2] cycloaddition reaction.

THEORETICAL INVESTIGATION ON SECOND-ORDER NONLINEAR OPTICAL PROPERTIES AND REDOX-SWITCHING OF PHENYL NITRONYL-NITROXIDE RADICAL DERIVATIVES

The static second-order nonlinear optical (NLO) properties on a series of phenyl nitronyl-nitroxide (NN) radical derivatives were investigated using density functional theory (DFT). It is found that the first hyperpolarizabilities of these systems are sensitive to the substituents, and system 4a containing of a two-dimensional (2D) substituent possesses the largest βtot value of 83.134 × 10-30 esu. Meanwhile, the time-dependent (TD)-PBE1PBE calculations indicate that low transition energy can enhance the NLO responses. As a result of the reversible one-electron oxidizations of these systems, the redox switching of the NLO responses of systems 1a–5a have also been studied. The βtot value of system 2b′ is about 20.75 times larger than that of 2a due to the larger degree of conjugation, which provides a possibility of NLO switching. According to the analysis of the frontier molecular orbitals (FMO), oxidization reduces the HOMO–LUMO gap and changes charge transfer (CT) direction.

Photoswitching magnetic crossover in organic molecular systems

We have theoretically designed efficient photomagnetic diradicals using both substituted and unsubstituted cyclophanediene (CPD) and dihydropyrenes (DHP) as spacers. Nitronyl nitroxide (NN), oxoverdazyl (o-VER), and tetrathiafulvalene (TTF) are chosen as monoradical centers. Molecular geometries have been optimized by the density functional method UB3LYP using the 6-311G(d,p) basis set. Final single point calculations have been done with the 6-311++G(d,p) basis. Absorption wavelengths have been estimated from time-dependent density functional treatment using restricted spin-polarized density functionals (RB3LYP) and 6-31G basis. Both the substituted and unsubstituted CPD species with mixed monoradical centers are found to be antiferromagnetically coupled. Diradicals with the same centers but with DHP coupler exhibit strong ferromagnetic coupling. Also, photoexcitations of the diradicals are generally red-shifted by only a few nanometers from those of the 15,16-dimethyl pyrenes. This indicates that on photoexcitation a consistent magnetic crossover from an antiferromagnetic to a ferromagnetic regime is possible. The accompanying change in magnetic exchange coupling constant ΔJ is very large, varying from 445 to 1003 cm(-1). As far as organic molecular magnetism is concerned, this observation is entirely new and likely to be of technological significance.