In 2016, Li et al. reported the first discrete organic radical with magnetic bistability: the crystalline diradical dication salt of 1,4-di(bisphenylamino)-2,3,5,6-tetramethylbenzene, whose magnetic susceptibility shows a reproducible thermal hysteresis loop (midpoints 118 K on cooling and 131 K on heating, width 13 K at 1 K/min) attributed to a temperature-induced modulation of the intramolecular singlet-triplet exchange gap (2J changing from -1.06 to -0.54 kcal/mol between the low- and high-temperature phases). The authors left two questions open: how to generalize this intramolecular-exchange bistability to other molecular scaffolds by rational design, creating organic analogues of metal-ion spin-crossover complexes, and what kinetic mechanism underlies the unexplained scan-rate dependence of the hysteresis width. No further example of the same mechanism has been reported since: the bulk organic bistable systems described to date operate by intermolecular aggregation (radical dimers, pimers, charge-transfer stacks, or packing transitions), while intramolecular spin bistability has otherwise been realized only in on-surface or matrix-isolated single-molecule regimes without cooperative bulk hysteresis, and no kinetic study of the prototype compound is known. Both strands therefore remain open, with the 2016 salt still the unique member of its mechanism class.

Background

Magnetic bistability is the ability of a material to persist in two distinct magnetic states over a range of external conditions, so that its magnetization traces a hysteresis loop as temperature or magnetic field is cycled; it is the physical basis of molecular switches, thermal sensors, and memory elements. The best-developed molecular examples are metal-ion spin-crossover (SCO) complexes, in which a transition-metal center (typically Fe(II)) is switched thermally between a low-spin and a high-spin electronic state in a cooperative, first-order transition that produces thermal hysteresis in the magnetic susceptibility. A long-standing goal of molecule-based magnetism has been an all-organic counterpart: a metal-free molecule whose magnetic state switches thermally with hysteresis.

Organic radicals (molecules with unpaired electrons) are the natural candidates, but in essentially all examples known before 2016 the bistability was intermolecular: crystals of dithiazolyl radicals switch hysteretically between a paramagnetic radical phase and a diamagnetic sigma-bonded dimer phase (Lekin et al., JACS 2010), and crystals of nitroxide biradicals show spin-transition-like steps governed by molecular packing (Nishimaki and Ishida, JACS 2010). In such systems the two magnetic states differ by whether adjacent molecules aggregate, so the 'switch' is a crystal-wide supramolecular event rather than a property of an individual molecule.

Li et al. (JACS 2016, 138, 10092-10095) reported the first discrete exception. Two-electron oxidation of 1,4-di(bisphenylamino)-2,3,5,6-tetramethylbenzene, crystallized with the weakly coordinating alkoxyaluminate anion [Al(ORMe)4]−[\mathrm{Al(OR_{Me})_4}]^- (ORMe=OC(CF3)2Me\mathrm{OR_{Me}=OC(CF_3)_2Me}; such bulky, weakly interacting anions spatially separate the cations in the crystal), gives the diradical dication salt 12+⋅2[Al(ORMe)4]−1^{2+}\cdot 2[\mathrm{Al(OR_{Me})_4}]^-. SQUID (superconducting quantum-interference device) magnetometry of the product χMT\chi_M T (molar magnetic susceptibility times temperature) shows a reproducible thermal hysteresis loop with midpoint T1/2=118T_{1/2}=118 K on cooling and 131131 K on heating, a width of 13 K at a temperature-scan rate of 1 K/min, and a high-temperature paramagnetic limit corresponding to about 93% S=1S=1 Curie spins. Fitting the susceptibility of each phase to the Bleaney-Bowers model, the standard expression for two exchange-coupled S=1/2S=1/2 spins with singlet-triplet gap 2J2J (negative meaning a singlet ground state), gives 2J=−1.062J=-1.06 kcal/mol in the low-temperature (LT) phase and 2J=−0.542J=-0.54 kcal/mol in the high-temperature (HT) phase: both phases are open-shell singlets thermally populating triplet states, but with quantitatively different intramolecular exchange couplings. Single-crystal X-ray diffraction at 100 K (LT) and 200 K (HT) shows the dications are spatially isolated by the anions (intramolecular N···N distance 5.6 Å versus the shortest intermolecular 10.1 Å) with small phase-dependent geometry changes (bond-length alternation and torsions). Electron paramagnetic resonance (EPR) spectra show triplet-state resonances with zero-field-splitting parameter D≈10.5D\approx 10.5 mT (the splitting of a triplet's sublevels in zero field, whose magnitude reflects the dipolar spin-spin distance), from which a spin-spin separation of about 6.4 Å is inferred, matching the intramolecular distance. Broken-symmetry density-functional theory (a DFT technique comparing high-spin and spin-contaminated 'broken-symmetry' solutions to extract exchange couplings) gives an open-shell singlet ground state with a singlet-triplet gap of -0.4 kcal/mol, and the calculated energy difference between the two experimental geometries (0.79 kcal/mol) is comparable to the measured change in singlet-triplet gap (0.52 kcal/mol). Together these results identify the bistability mechanism as intramolecular: a small thermal geometry change of the isolated molecule modulates the singlet-triplet exchange between its two radical centers, making 12+1^{2+} a molecular-scale organic analogue of metal-ion spin crossover.

Two issues were explicitly left open. First, generalization: the compound is a single prototype, and the authors state that 'a new class of organic analogues of metal ion based spin-crossover complexes may be accessible by rational designs of discrete magnetic bistable diradicals'; no design rules exist for choosing a scaffold, substituents, or counterions that would deliver a second bistable diradical with a chosen transition temperature and hysteresis width. Second, kinetics: the measured hysteresis width depends on the temperature scan rate (documented in the supporting information; faster scans give wider loops), but the activation barriers and dynamical pathway of the phase transition were not determined, so what physically controls hysteresis width and switching speed in such discrete-diradical crystals is unknown.

Problem Statement

The problem, as posed by the 2016 prototype discovery and left open since, is to turn the single example of intramolecularly driven magnetic bistability in a discrete organic diradical into a controllable materials class - establishing metal-free molecular magnetic-bistability materials (organic analogues of metal-ion spin-crossover complexes) with predictable switching temperatures, hysteresis widths, and switching kinetics - and comprises two linked components that jointly serve this objective: (i) By rational structural design, realize and fully characterize at least one further discrete organic diradical - a crystalline, molecularly isolated (non-aggregated) species - that exhibits thermally driven magnetic bistability with a reproducible hysteresis loop in its magnetic susceptibility arising from intramolecular modulation of the singlet-triplet exchange between two spin states of the same molecule (not from intermolecular radical dimerization, pi-stacking, or packing/order-disorder transitions), together with the structure-property relations - derived by comparing this further diradical with the prototype - by which structural modification tunes the transition temperature, the hysteresis width, and the phase-specific singlet-triplet gaps relative to the prototype values (T1/2=118/131T_{1/2}=118/131 K, 13 K width at 1 K/min, 2J=−1.06/−0.542J=-1.06/-0.54 kcal/mol). (ii) Determine the kinetic mechanism of the hysteretic structural/electronic phase transition in such discrete-diradical crystals - the activation barriers of the forward and reverse transitions and the dynamical pathway, including the role of the lattice and counterions in the cooperativity - at a level that quantitatively explains the observed scan-rate dependence of the hysteresis width and identifies what controls the switching speed.

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