Very Strongly Ferromagnetically Coupled Diradicals from Mixed Radical Centers: Nitronyl Nitroxide Coupled to Oxoverdazyl via Polyene Spacers

Antiaromatic Molecules as Magnetic Couplers: A Computational Quest

In this study, we investigate a set of organic diradical structures in which two oxo-verdazyl radicals are selected as radical spin centers that are connected (coupled) via six coupler molecules (CM), resulting in various magnetic (ferromagnetic (FM) or antiferromagnetic (AFM)) characteristics, as reflected by their exchange coupling constants (J). We have designed 12 diradicals with 6-antiaromatic couplers coupled with bis-oxo-verdazyl diradicals with meta-meta (m-m) and para-meta (p-m) positional connectivities. The nature of the magnetic coupling (ferromagnetic, nonmagnetic, or antiferromagnetic) and the magnitude of the exchange constant J depend on the type of coupler, the connecting point between each radical center and CM, the degree of aromaticity of the coupler, and the length of the through-bond distance between radical centers. The computed magnetic exchange coupling constants J for these diradicals at the B3LYP/6-311++G(d,p) and MN12SX/6-311++G(d,p) levels of theory are large for many of these structures, indicating strong ferromagnetic coupling (with positive J values). In some cases, magnetic couplings are observed with J > 1000 cm-1 (B3LYP/6-311++G(d,p)) and strong antiferromagnetic coupling (with negative J values) with J < -1000 cm-1 (B3LYP/6-311++G(d,p)). Similarly, in some cases, magnetic couplings are observed with J > 289 cm-1 (MN12SX/6-311++G(d,p)) and strong antiferromagnetic coupling (with negative J values) with J < -568 cm-1 (MN12SX/6-311++G(d,p)). Furthermore, while numerous studies have reported that the degree of aromaticity of molecular couplers often favors strong ferromagnetic coupling, displaying the high-spin character of diradicals in their ground states, the couplers chosen in this study are characterized as antiaromatic or nonaromatic. The current investigation provides evidence that, remarkably, antiaromatic couplers are able to enhance stability by favoring electronic diradical structures with very strong ferromagnetic coupling when the length of the through-bond distance and connectivity pattern between radical centers are selected in such a way that the FM coupling is optimized. The findings in this study offer new strategies in the design of novel organic materials with interesting magnetic properties for practical applications.

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.

Endohedral σ-Diradical Nitrogen-Vacancy Diamond Nanoclusters with a Confined Magnetic Space and Strong Electronic Spin Couplings

The electronic properties and their modulations for the nitrogen-vacancy (NV) centers in various nanoscale diamonds are of profound current interest because of their potential applications. However, although the NV centers as chromophores in diamond are the most widely studied, surprisingly, little is known about their magnetic spin coupling properties up to now. Here, we for the first time show, using the spin-polarized DFT calculations, that the NV centers can act as unique endohedral σ-diradical magnets in diamond nanoclusters and exhibit quite strong ferromagnetic (FM) or antiferromagnetic (AFM) spin coupling characteristics due to their unique endotetrahedral structures with favorable radical-radical contacts. Although the neutral NV center (NV⁰) in its doublet ground state exhibits quite strong AFM spin coupling among three radical C-sites (i.e., an AFM triradical center), interestingly, excess electron injection can convert it to a FM diradical magnet (i.e., the triplet ground state NV^-) with almost unchanged J-coupling magnitude, and the J-coupling of the nanocluster can be noticeably enhanced by F-termination of the surface due to triradical spin delocalization mediated by excess electron. However, interior modification (one C in the endotetrahedron core is substituted by N or B or is hydrogenated) can assign the nanocluster perfect AFM diradical character. The spin coupling strength presents a quasilinear correlation with the distance between the two C radicals in the NV core for the same size of the clusters and a high linear correlation with the energy difference between two singly occupied molecular orbitals. Clearly, the FM and AFM couplings as well as their switching behavior in such NV defect diamond nanoclusters featuring the endohedral σ-diradicals are a novel type of promising magnetic material motifs. These findings open up promising spintronic application prospects of the NV diamonds and provide helpful information for the design of inorganic magnetic materials and logic devices.

Intriguing strain-governed magnetic phase transitions in 2D vanadium porphyrin sheets

The strain effect on the magnetic response of 2D materials as spintronic devices is always important in actual applications. Due to the intriguing electronic and magnetic properties of two-dimensional (2D) vanadium porphyrin (V-PP) sheets, we studied the strain-induced magnetic coupling changes in 2D V-PP sheets by using the density functional theory method and found intriguing magnetic variation characters. The calculated results indicate that biaxial strain can modulate the magnetic moments of the central transition metal vanadium atoms and more importantly can induce phase transitions among three magnetic modes with four magnetic states (ferromagnetic (FM), ferrimagnetic (FIM), and two antiferromagnetic (AFM: AFM1 featuring a parallel spin lattice versus AFM2 featuring a crossing spin lattice)) with unique conversion pathways due to their different responses to the strain. As the compressive strain increases, the magnetic characteristics of 2D-VPP transitions as FM → FIM → AFM1 with two critical points (-4.7% and -6%), while the tensile strain can induce the original FM coupling to transition to another AFM state (FM → AFM2) at 5.3%. Analyses of the density of states, spin densities, and Bader charges reveal that the rich magnetic response properties of the system originate from the electron transfer between the central V and the porphyrin ligand induced by strain. This work provides intriguing information regarding the strain-induced magnetic phase transition mechanism and also presents a viable development direction to design 2D porphyrin magnetic semiconductors and spintronic devices.

Tuning the magnetic properties of a diamagnetic di-Blatter's zwitterion to antiferro- and ferromagnetically coupled diradicals

In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), a molecule containing two Blatter's moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from the zwitterionic ground state to the diradicalized state. With a larger coupler, ferromagnetic interactions are realized based on density functional theory (DFT) and wave-function theory (WFT) based complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2) methods. An analysis based on the extent of spin contamination, diradical character, CASSCF orbital occupation number, Head-Gordon's index, HOMO-LUMO and SOMOs energy gaps is demonstrated that marks the transition of the ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on a TPHA skeleton with diradicaloid state and, in some cases, even a triplet ground state.

Effective modulation of intramolecular ferromagnetic interaction of diradicals by functionalization of cross-conjugated coupler

Cross-conjugated molecules are an interesting class of conjugated systems possessing a spatially separated HOMO and LUMO. Most previous studies have taken advantage of this property by using it in organic semiconductor applications. Herein, we undertake a new investigation on the use of this type of molecule, in particular benzo[1,2-d;4,5-d']bisoxazole (BBO), as a coupler for organic diradicals. BBO has two sites available for adding a substituent and a spin center (SC) which are along its 4,8- and 2,6-axes. Functionalizations using electron donating (ED) and electron withdrawing (EW) groups were imposed to tune its FMOs and it was found that the longer 2,6-axis is an ideal site with a broader LUMO range via substituent effects. Diradicalization of these BBOs using nitronyl nitroxide (NN) and nitroxide (NO) as SCs was done using the remaining available axis. The calculated J values are linearly dependent on the LUMO energy of the coupler, but with 4,8-NH2-2,6-SC as an outlier. This exceptional case is related to 4,8-NH2-2,6-SC having the lowest BBO-NN dihedral angle. Moreover, the diradicals 4,8-X-2,6-SC (with X = H, NH2, CH3) have higher J values than 2,6-X-4,8-SC (with X = H, NH2, CH3), which is counterintuitive because the latter have a shorter coupling path. These diradicals are positioned to the right of the intersection of their trend lines, which implies that diradicals with LUMO values to the right of this intersection have the tendency to attain J values that are higher than those diradicals with a shorter coupling path. 4,8-NH2-2,6-SC even surpasses the projected JMax values which we associate with the highest attainable J values due to LUMO tuning via substituent effects. These results provide useful insights, especially into the interplay between the LUMO and the dihedral angle and how these affect magnetism in diradicals. In conclusion, we found that BBO can be a good candidate as an effective coupler for diradicals with tunable J values via incorporation of ED and EW groups. This first approach to studying the application of cross-conjugated molecules as couplers also paves the way for new candidates for the development of more effective diradical systems.

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.

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.

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.

Unusually large coupling constants in diradicals obtained from excitation of mixed radical centers: a theoretical study on potential photomagnets

Three sets of heterosubstituted, interconvertible, cyclophanediene (CPD), and dihydropyrenes (DDPs) and one such set involving dinitrilepyrenes were examined by UB3LYP broken-symmetry methodology with 6-311++g(d,p) bases. Nitronyl nitroxide and oxoverdazyl (with both N and C terminals) are monoradical centers, whereas CPD and DDP moieties serve as couplers. The photoexcited CPD converts to DDP. The calculated exchange coupling constant (J) for o-VER(N)-DDP-NN is surprisingly high, 6412 cm(-1), and much larger than 28.9 cm(-1) for the CPD species, but the unsubstituted DDP is known to transform readily into pyrene, with the loss of reversibility. Nevertheless, o-VER(N)-(15,16-dinitrile)DDP-NN also has a large J value, 589.4 cm(-1). The corresponding CPD species has J = 53.3 cm(-1). We predict that the latter CPD and DDP diradicals are potential molecules to synthesize photomagnetic materials. The o-VER(N)-DDP-NN can also be an excellent photomagnetic switch at a considerably low temperature.