Toward New Horizons in Verdazyl-Nitroxide High-Spin Systems: Thermally Robust Tetraradical with Quintet Ground State.

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but examples reported to date exhibit limited stability and processability. In this work, we designed the first tetraradical based on an oxoverdazyl core and nitronyl nitroxide radicals and successfully synthesized it using a palladium-catalyzed cross-coupling reaction of an oxoverdazyl radical bearing three iodo-phenylene moieties with a gold(I) nitronyl nitroxide-2-ide complex in the presence of a recently developed efficient catalytic system. The molecular and crystal structures of the tetraradical were confirmed by single crystal X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼125 °C in an inert atmosphere; in a toluene solution upon prolonged heating at 90 °C in air, no decomposition was observed. The resulting unique verdazyl-nitroxide conjugate was thoroughly studied using a range of experimental and theoretical techniques, such as SQUID magnetometry of polycrystalline powders, EPR spectroscopy in various matrices, cyclic voltammetry, and high-level quantum chemical calculations. All collected data confirm the high thermal stability of the resulting tetraradical and quintet multiplicity of its ground state, which makes the synthesis of this important paramagnet a new milestone in the field of creating high-spin systems.

A Nitronyl Nitroxide-Substituted Benzotriazinyl Tetraradical.

High-spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium-catalyzed cross-coupling reaction of a triiodo-derivative of the 1,2,4-benzotriazinyl radical with gold(I) nitronyl nitroxide-2-ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X-ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at -0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high-level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl-nitronylnitroxide tetraradical is a new milestone in the field of creating high-spin systems.

Two Discoveries in One Crystal: σ-Type Oxime Radical as an Unforeseen Building Block in Molecular Magnetics and Its Spatial Structure.

In the present work, the study of the unusual interaction between copper hexafluoroacetylacetonate and the diacetyliminoxyl radical resulted in two discoveries from different fields: the determination of the oxime radical spatial structure and the introduction of an oxime radical into the field of molecular magnetic material design. Oxime radicals are key plausible intermediates in the processes of oxidative CH-functionalization and in the synthesis of functionalized isoxazolines from oximes. Due to the lack of X-ray diffraction data for oxime radicals, the knowledge about their structure is based mainly on indirect approaches, spectroscopic methods (electron paramagnetic resonance and IR), and quantum chemical calculations. The structure of the oxime radical was determined for the first time by stabilizing the diacetyliminoxyl radical in the form of its complex with copper (II) hexafluoroacetylacetonate (Cu(hfac)2), followed by single-crystal X-ray diffraction analysis. Although oxime radicals are known to undergo oxidative coupling with acetylacetonate ligands in transition-metal complexes, a complex is formed with intact hfac ligands. X-ray diffraction studies have shown that the oxime radical is coordinated with copper ions through the oxygen atoms of the carbonyl groups without the direct involvement of the C═N-O• radical moiety. The structure of the coordinated diacetyliminoxyl is in good agreement with the density functional theory (DFT) prediction for free diacetyliminoxyl due to the very weak interaction of the radical molecule with copper ions. Remarkably, both weak ferromagnetic and antiferromagnetic interactions between Cu (II) and oxime radicals have been revealed by modeling the temperature dependence of magnetic susceptibility and confirmed by DFT calculations, rendering diacetyliminoxyl a promising building block for the design of molecular magnets.

Fullerene-based triplet spin labels: methodology aspects for pulsed dipolar EPR spectroscopy.

Triplet states of photoexcited organic molecules are promising spin labels with advanced spectroscopic properties for pulsed dipolar electron paramagnetic resonance (PD EPR) spectroscopy. Recently proposed triplet fullerene labels have shown great potential for double electron-electron resonance (DEER) distance measurements as "observer spins" due to a high quantum yield of the triplet state, hyperpolarization and relatively narrow EPR spectra. Here, we demonstrate the applicability of fullerene labels to other PD EPR techniques, such as relaxation induced dipolar modulation enhancement (RIDME) and laser induced magnetic dipolar spectroscopy (LaserIMD). In particular, a specific contaminating signal in LaserIMD experiments was observed, explained and mitigated. Comparative analyses of the signal-to-noise (SNR) ratios were performed for all employed methods. DEER on the fullerene-triarylmethyl pair shows the best performance, which allows state-of-the-art DEER acquisition at 100 nM with a SNR of ∼35 within reasonable 42 hours.

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization.

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin-spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1'-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

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.

Revealing the Structure of Transition Metal Complexes of Formaldoxime.

Aerobic reactions of iron(III), nickel(II), and manganese(II) chlorides with formaldoxime cyclotrimer (tfoH3) and 1,4,7-triazacyclononane (tacn) produce indefinitely stable complexes of general formula [M(tacn)(tfo)]Cl. Although the formation of formaldoxime complexes has been known since the end of 19th century and applied in spectrophotometric determination of d-metals (formaldoxime method), the structure of these coordination compounds remained elusive until now. According to the X-ray analysis, [M(tacn)(tfo)]+ cation has a distorted adamantane-like structure with the metal ion being coordinated by three oxygen atoms of deprotonated tfoH3 ligand. The metal has a formal +4 oxidation state, which is atypical for organic complexes of iron and nickel. Electronic structure of [M(tacn)(tfo)]+ cations was studied by XPS, NMR, cyclic (CV) and differential pulse (DPV) voltammetries, Mössbauer spectroscopy, and DFT calculations. Unusual stabilization of high-valent metal ion by tfo3- ligand was explained by the donation of electron density from the nitrogen atom to the antibonding orbital of the metal-oxygen bond via hyperconjugation as confirmed by the NBO analysis. All complexes [M(tacn)(tfo)]Cl exhibited high catalytic activity in the aerobic dehydrogenative dimerization of p-thiocresol under ambient conditions.

Fluorinated Organic Paramagnetic Building Blocks for Cross-Coupling Reactions.

New stable polyfluorinated nitroxide radicals for use in cross-coupling reactions, namely, N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-iodobenzene and N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-ethynylbenzene, were prepared from perfluoroiodobenzene. The reaction of the polyfluoro derivative with tert-butylamine under autoclaving conditions leading to the formation of N-tert-butyl-2,3,5,6-tetrafluoro-4-iodoaniline proved to be the key stage of the whole process. The fluorinated tert-butyl iodophenyl nitroxide was found to form in a solid state via N-O···I halogen bonds, a one-dimensional assembly of the radicals. The acceptor role of the nitroxide group in the halogen bonding changes to a donor role when the nitroxide reacts with Cu(hfac)2. In the last case, zero-dimensional assembly prevails, giving a three-spin complex with axial coordinated nitroxide groups and, as a consequence, causing ferromagnetic intramolecular exchange interactions between Cu(II) and radical spins.

Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl-Nitronyl Nitroxide Diradicals.

Thermally stable organic diradicals with a triplet ground state along with large singlet-triplet energy gap have significant potential for advanced technological applications. A series of phenylene-bridged diradicals with oxoverdazyl and nitronyl nitroxide units were synthesized via a palladium-catalyzed cross-coupling reaction of iodoverdazyls with a nitronyl nitroxide-2-ide gold(I) complex with high yields. The diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 °C. For the diradicals, both substantial AF (ΔEST ≈-64 cm-1 ) and FM (ΔEST ≥25 and 100 cm-1 ) intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para- or meta-phenylene) and by the type of oxoverdazyl block (C-linked or N-linked). Upon crystallization, diradicals with the triplet ground state form unique one-dimensional exchange-coupled chains with strong intra- and weak inter-diradical ferromagnetic coupling.

Radical Anions of Aromatic Carbonitriles as Reagents for Arylation of Fluorinated Benzonitriles.

The first example of phenylation of fluorobenzonitriles with the sodium salt of a benzonitrile radical anion in liquid ammonia is presented. The reaction regioselectivity corresponds to the ortho- and para-fluorine atom substitution in fluorobenzonitrile with the phenyl moiety of the benzonitrile radical anion and affords 2- and 4-cyanobiphenyls in 40-90% yields. 3-Methoxybenzonitrile as well as 1-cyanonaphthalene radical anions were also successfully subjected to this interaction forming 3'-methoxycyanobiphenyls and (1-naphthyl)benzonitriles, respectively. The radical anion acts as an ipso-C-nucleophile with consequent loss of the cyano group. The revealed new type of radical anion reactivity opens up the prospect of developing a general approach to fluorinated cyanobisarenes on the basis of an interaction of the cyanoarene radical anion with fluorinated substrates activated to aromatic nucleophilic substitution.

Triplet Fullerenes as Prospective Spin Labels for Nanoscale Distance Measurements by Pulsed Dipolar EPR Spectroscopy.

Precise nanoscale distance measurements by pulsed electron paramagnetic resonance (EPR) spectroscopy play a crucial role in structural studies of biomolecules. The properties of the spin labels used in this approach determine the sensitivity limits, attainable distances, and proximity to biological conditions. Herein, we propose and validate the use of photoexcited fullerenes as spin labels for pulsed dipolar (PD) EPR distance measurements. Hyperpolarization and the narrower spectrum of fullerenes compared to other triplets (e.g., porphyrins) boost the sensitivity, and superior relaxation properties allow PD EPR measurements up to a near-room temperature. This approach is demonstrated using fullerene-nitroxide and fullerene-triarylmethyl pairs, as well as a supramolecular complex of fullerene with nitroxide-labeled protein. Photoexcited triplet fullerenes can be considered as new spin labels with outstanding spectroscopic properties for future structural studies of biomolecules.

Determination of graphene's edge energy using hexagonal graphene quantum dots and PM7 method.

Graphene quantum dots (GQDs) are important for a variety of applications and designs, and the shapes of GQDs rely on the energy of their boundaries. Presently, many methods have been developed for the preparation of GQDs with the required boundaries, shapes and edge terminations. However, research on the properties of GQDs and their applications is limited due to the unavailability of these compounds in pure form. In the present computational study, the standard enthalpy of formation, the standard enthalpy of formation of edges and the standard enthalpy of hydrogenation are studied for hexagonal GQDs with purely zigzag and armchair edges in non-passivated and H-passivated forms using the semiempirical quantum chemistry method pm7. The standard enthalpy of formation of the edge is found to remain constant for GQDs studied in the range of 1 to 6 nm, and the enthalpies of edge C atoms are 32.4 and 35.5 kcal mol-1 for armchair and zigzag edges, respectively. In contrast to some literature data, the standard enthalpy of formation of hydrogenated edges is far from zero, and the values are 7.3 and 8.0 kcal mol-1 C for armchair and zigzag edges, respectively. The standard enthalpy of hydrogenation is found to be -10.2 and -9.72 eV nm-1 for the armchair and zigzag edges, respectively.

Substitution of a Fluorine Atom in Perfluorobenzonitrile by a Lithiated Nitronyl Nitroxide.

A 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) lithium derivative was found to react with perfluorobenzonitrile (2) substituting its para-fluorine atom to form 2-(4-cyanotetrafluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (3), a new nitronyl nitroxide containing a multifunctional framework of strong electron-withdrawing nature. This result shows the possibility of obtaining multifunctional nitronyl nitroxides via the interaction of paramagnetic lithium derivatives as C-nucleophiles with polyfluoroarenes activated for nucleophilic substitution. The reaction regioselectivity is supported by the data of quantum-chemical calculations, which also show that the reaction follows a concerted pathway without formation of an intermediate. Reduction of nitronyl nitroxide 3 in system NaNO2-AcOH yielded corresponding iminonitroxide 4. Characterization of persistent radicals 3 and 4 obtained by the SNF synthetic strategy includes X-ray crystal structures, electron spin resonance data, and static magnetic-susceptibility measurements. X-ray diffraction analysis of both nitronyl nitroxide and iminonitroxide revealed a complete match of the parameters of their crystal lattices.

Reaction of Paramagnetic Synthon, Lithiated 4,4,5,5-Tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, with Cyclic Aldonitrones of the Imidazole Series.

It was shown that dipole-stabilized paramagnetic carbanion lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5-dihydroimidazole 3-oxide and 2H-imidazole 1-oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono- and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X-ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a((14) N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a((14) N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.

One-pot synthesis of 4'-alkyl-4-cyanobiaryls on the basis of the terephthalonitrile dianion and neutral aromatic nitrile cross-coupling.

A convenient one-pot approach to alkylcyanobiaryls is described. The method is based on biaryl cross-coupling between the sodium salt of the terephthalonitrile dianion and a neutral aromatic nitrile in liquid ammonia, and successive alkylation of the long-lived anionic intermediate with alkyl bromides. The reaction is compatible with benzonitriles that contain methyl, methoxy and phenyl groups, fluorine atoms, and a 1-cyanonaphthalene residue. The variety of ω-substituted alkyl bromides, including an extra bromine atom, a double bond, cyano and ester groups, as well as a 1,3-dioxane fragment are suitable as alkylation reagents.

Photoswitching of a thermally unswitchable molecular magnet Cu(hfac)2L(i-Pr) evidenced by steady-state and time-resolved electron paramagnetic resonance.

Most photoswitchable molecular magnets exhibit thermally induced switching, as is typical of spin crossover (SCO), valence tautomerism and SCO-like phenomena. We report a rare case of a copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) that does not exhibit quantitative SCO-like behavior in the temperature range of its chemical stability (2-350 K); however, it can be switched to a metastable thermally inaccessible spin state via visible/near-IR light at cryogenic temperatures. By means of photogeneration, unique information on this otherwise unobservable spin state has been obtained using steady-state Q-band (34 GHz) and time-resolved W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In particular, we have found that the electronic structure and relaxation properties of the photoinduced state in Cu(hfac)2L(i-Pr) are very similar to those in its sister compound Cu(hfac)2L(n-Pr) that is thermally switchable and has been exhaustively characterized by many analytical methods, previously. The first observation of photoswitchable behavior in a thermally unswitchable copper-nitroxide based molecular magnet Cu(hfac)2L(i-Pr) paves the way for photoswitching applications of this and similar compounds in the remarkably broad temperature range of 2-350 K.

Preparation and magnetic properties of metal-complexes from N-t-butyl-N-oxidanyl-2-amino-(nitronyl nitroxide).

Metal complexation reactions of N-t-butyl-N-oxidanyl-2-amino(nitronyl nitroxide) diradical (1) with M(hfac)2 (M: Mn or Cu) were investigated. These reactions were found to be very sensitive to the type of metal ion employed. Complex [Mn(hfac)2·1], consisting of Mn(hfac)2 and diradical 1, was readily prepared by mixing the components. However, the reaction of Cu(hfac)2 with 1 or N-t-butyl-N-oxidanyl-2-amino(iminonitroxide) diradical (2) involved the reduction of the diradical to the N-t-butyl-N-oxidanide-2-amino(iminonitroxide) radical anion (3) and finally produced the polymer-chain complex [Cu2(hfac)2·32·Cu(hfac)2]n. The structures of these complexes were elucidated by X-ray analysis, and their magnetic properties were investigated in detail. The temperature dependence of χpT (χp: magnetic susceptibility) for [Mn(hfac)2·1] exhibited a strong antiferromagnetic interaction (H = -2JS1·S2, J/kB = -217 K) between the Mn(II) spin (S = 5/2) and the diradical 1 spin (S = 1). However, the χpT-T plots for [Cu2(hfac)2·32·Cu(hfac)2]n indicated the presence of several magnetic interactions: a large ferromagnetic interaction (J/kB = 510 K) between iminonitroxide 3 and the imino-coordinating Cu(II) atom, a moderately large ferromagnetic interaction (J/kB = 58 K) between the iminonitroxide and (iminonitroxide oxygen)-coordinating Cu(hfac)2, and a weak antiferromagnetic interaction (J/kB = -1.4 K) between the two Cu(hfac)-3 moieties within a Cu2O2 square.

Ultrafast photoswitching in a copper-nitroxide-based molecular magnet.

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper-nitroxide-based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper-nitroxide-based molecular magnets for the future design of ultrafast magnetic materials.

Molecular conformations and magnetic parameters of the compact trimethylenemethane-type triplet diradical.

The ESR spectrum of compact nitroxide (NO)-substituted nitronyl nitroxide (NN) triplet diradical N-tert-butyl-N-oxidanyl-2-amino-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) was recorded in solid argon matrix at 15 K. The zero-field splitting (ZFS) parameters of 1 were derived from the recorded ESR spectrum: |D| = 0.0248 cm(-1) and E = 0.0025 cm(-1). Quantum chemical calculations have been performed using DFT and multiconfigurational ab initio (CAS) methods in order to establish equilibrium geometries of the conformational isomers resulting from twisted conformations of NO and NN moieties. The ZFS parameters of 1 were calculated at these levels of theory to test validity of the calculated structures. The calculation results were analyzed using the measured ZFS parameters and magnetic and structural data from the previous studies (Suzuki, S.; et al. J. Am. Chem. Soc. 2010, 132, 15908; Tretyakov, E. V.; et al. Russ. Chem. Bull. 2011, 60, 2608). It was found that the ab initio method is most successful for accurate predictions of molecular and magnetic parameters. Diradical 1 has only one stable enantiomeric pair in pseudoeclipsed conformations. The two chiral isomers exist in racemic crystals 1 and in solid matrices with molecular parameters close to those attributed to a free molecule. The analysis of the spin density distribution suggests that one unpaired electron occupies NO group at the equilibrium geometry, whereas the torsion of NO group governs the spin density distribution of the second unpaired electron on a conjugated fragment in NN group. The increase in planarity by torsion of NO group enhances the trimethylenemethane-type properties and, therefore, gives rise to larger ferromagnetic exchange interaction. More planar equilibrium geometry and greater (three times) exchange interaction constant J were predicted for hypothetical diradical 1a, where bulky tert-butyl group is replaced by a methyl group in the nitroxide fragment.

Light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 with tert-butylpyrazolylnitroxides.

We report the study of light-induced magnetostructural anomalies in a polymer chain complex of Cu(hfac)2 (hfac = hexafluoroacetylacetonate) with an unusual acyclic tert-butylpyrazolylnitroxide radical (Ltert(Me)) using EPR. This complex ([Cu(hfac)2Ltert(Me)]n) belongs to the family of thermo- and photoswitchable molecular magnets "breathing crystals". Compared to previously studied breathing crystals with nitronyl nitroxides, [Cu(hfac)2Ltert(Me)]n shows much weaker absorption bands in the visible spectral region and therefore is superior for optical manipulation of the spin states. Illumination with light (λ ≈ 540 nm) at cryogenic temperatures leads to formation of a metastable weakly coupled spin state, which relaxes to the ground strongly coupled spin state on a time scale of hours. These phenomena are in many aspects similar to the light-induced excited spin state trapping (LIESST) well-known for spin-crossover compounds. Remarkably, the photoinduced spin state in [Cu(hfac)2Ltert(Me)]n is metastable at temperatures up to TLIESST ≈ 60 K, which is a significant improvement compared to that of previously studied breathing crystals with nitronyl nitroxides (TLIESST ≈ 20 K). We describe LIESST-like behavior observed in [Cu(hfac)2Ltert(Me)]n and discuss possible reasons for the increased stability of the photoinduced spin state.