Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand.

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)2Pd(CN)4]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H2O,EtOH (A·H2O,Et), and H2O (A·H2O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H2O,Et and an abrupt single-step SCO for A·H2O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H2O,Et characterized by a unique FeII center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H2O shows a triclinic symmetry with two inequivalent FeII centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)2Pd(CN)4]·guest (cintrz = N-cinnamalidene 4-amino-1,2,4-triazole; A·guest; guest = 3H2O, 2H2O, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3H2O, 2H2O, and Ø can be exploited to regulate this balance. In A·3H2O, the dominant antiferroelastic interaction character between neighboring FeII sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin). Elastic interaction strain is responsible for stabilizing the {HS-LS} state and can be overcome by three mechanisms: (1) partial (2H2O) or complete (Ø) guest removal, (2) irradiation via the reverse light-induced excited spin-state trapping (LIESST) effect (λ = 830 nm), and (3) the application of external hydrostatic pressure. Combining experimental data with elastic models presents a clear interpretation that while guest molecules cause a negative chemical pressure, they also have consequences for the elastic interactions between metals beyond the simple chemical pressure picture typically proposed.

Hierarchical Spin-Crossover Cooperativity in Hybrid 1D Chains of FeII -1,2,4-Triazole Trimers Linked by [Au(CN)2 ]- Bridges.

Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4-triazole-based trimers coordinated into 1D chains by [Au(CN)2 ]- and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies. Detailed structural, vibrational, and theoretical studies conclusively confirmed that intra-trimer coupling was an order of magnitude greater than the intramolecular coupling, which was an order of magnitude greater than intermolecular coupling. As such, a clear hierarchy on the nature of elastic coupling in SCO materials was ascertained for the first time, which is a necessary step for the technological development of molecular switching materials.

Substituent effects on through-space intervalence charge transfer in cofacial metal-organic frameworks.

Electroactive metal-organic frameworks (MOFs) are an attractive class of materials owing to their multifunctional 3-dimensional structures, the properties of which can be modulated by changing the redox states of the components. In order to realise both fundamental and applied goals for these materials, a deeper understanding of the structure-function relationships that govern the charge transfer mechanisms is required. Chemical or electrochemical reduction of the framework [Zn(BPPFTzTz)(tdc)]·2DMF, hereafter denoted ZnFTzTz (where BPPFTzTz = 2,5-bis(3-fluoro-4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole), generates mixed-valence states with optical signatures indicative of through-space intervalence charge transfer (IVCT) between the cofacially stacked ligands. Fluorination of the TzTz ligands influences the IVCT band parameters relative to the unsubstituted parent system, as revealed through Marcus-Hush theory analysis and single crystal UV-Vis spectroscopy. Using a combined experimental, theoretical and density functional theory (DFT) analysis, important insights into the effects of structural modifications, such as ligand substitution, on the degree of electronic coupling and rate of electron transfer have been obtained.

Three Distinct Spin-Crossover Pathways in Halogen-Appended 2D Hofmann Frameworks.

We probe, here, a family of 2D Hofmann-type frameworks, [FeII(Pd(CN)4)(bztrzX)2]·nH2O [X·nH2O; X = F, Cl, Br; n = 1 (X = Cl, Br) and 3 (X = F); bztrzX = (E)-1-(2-Xphen-1-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine], with halogen-appended ligands. In all cases, there are two crystallographically distinct FeII sites, ({Fe1-Fe2}), driven by the presence of a range of host-host and host-guest interactions. We find that lattice modification through X variation influences the elastic coupling between the FeII sites, the emergence of ferroelastic or antiferroelastic interactions between these sites, and the relative spin-state stabilization/destabilization at each site. In Cl·H2O, the FeII sites show strong elastic coupling, as evidenced by both FeII sites undergoing a spin transition in a single cooperative step, as driven by the volume strain over the high-spin (HS)-to-low-spin (LS) transition. The FeII sites in F·3H2O are also elastically coupled; however, the change of the X atom characteristics and increased guest molecules in the pores result in an antiferroelastic interaction characteristic between Fe1 and Fe2 and a resultant two-step spin-state transition. The change of the X atom to Br in Br·H2O results in the FeII sites being decoupled due to halogen atom steric bulk, resulting in the independent spin-state transition of Fe1 and Fe2 sites and a two-step spin-state transition pathway. Uniquely, all three possible spin-state transition pathways of a two-site switching system are observed in this family [(1) {HS-HS} ↔ {HS-LS} ↔ {LS-LS} for Br·H2O, (2) {HS-HS} ↔ {LS-HS} ↔ {LS-LS} for F·3H2O, and (3) {HS-HS} ↔ {LS-LS} for Cl·H2O for {Fe1-Fe2}]. Overall, these findings broadly support recent theoretical models but highlight that additional structural and topological complexities are needed to form a holistic picture of the drivers of elastic frustration.

Guest Removal and External Pressure Variation Induce Spin Crossover in Halogen-Functionalized 2-D Hofmann Frameworks.

The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [FeIIPd(CN)4(thioX)2]·2H2O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloro- and bromo-functionalized ligands into the Hofmann-type frameworks (1Cl·2H2O and 2Br·2H2O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1Cl and 2Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1Cl·2H2O and 2Br·2H2O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements).

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal-Organic Frameworks.

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

Guest-Adaptable Spin Crossover Properties in a Dinuclear Species Underpinned by Supramolecular Interactions.

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introduce structural stabilizing supramolecular interactions into a dinuclear material and henceforth demonstrate a dynamic structural and spin crossover property interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A = [FeII2( o-NTrz)5(NCS)4]; 4-( o-nitrobenzyl)imino-1,2,4-triazole). Relatively uncommon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a single-crystal to single-crystal transformation with affiliated phase transition resulting in a reversible transformation from one- to two-step spin crossover (SCO) transition character. We additionally present the water-saturated product (A·3H2O), which distinctly shows an abrupt one-step SCO character with a 22 K wide thermal hysteresis loop. Detailed structure-property analysis highlights that the substantial structural malleability and guest-adaptable SCO properties of this dinuclear species are afforded by the supportive, yet flexible, supramolecular interaction pathways derived from the ligand functionalization.

Investigation of the High-Temperature Spin-Transition of a Mononuclear Iron(II) Complex Using X-ray Photoelectron Spectroscopy.

This study presents a new mononuclear complex (1) of the form [FeL](BF4)2, incorporating the thiazolylimine donor moiety, which was found to exhibit a high-temperature spin-transition. The effect of scan rate was investigated, with magnetic susceptibility being measured at 4, 2, and 1 K min-1. The magnetic susceptibility results were confirmed by variable temperature X-ray photoelectron spectroscopy (XPS) (100, 270, 400, and 500 K) and single crystal X-ray diffraction (150 and 400 K) experiments. A rare example of a high-temperature (400 K) single crystal structure of 1 has been reported. The high-spin fraction was calculated indirectly from XPS data, presenting a method for analyzing the spin-state in the surface layers of spin-crossover materials.

Guest Programmable Multistep Spin Crossover in a Porous 2-D Hofmann-Type Material.

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [FeII(bztrz)2(PdII(CN)4)]·n(guest) (bztrz = (E)-1-phenyl-N-(1,2,4-triazol-4-yl)methanimine, 1·n(guest)), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (1·(H2O,EtOH)), two- (1·3H2O) and three-stepped (1·âˆ¼2H2O) transitions and SCO-deactivation (1)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated.

Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics.

The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of ZnII centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N2 O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.

Commensurate CO2 Capture, and Shape Selectivity for HCCH over H2CCH2, in Zigzag Channels of a Robust Cu(I)(CN)(L) Metal-Organic Framework.

A novel copper(I) metal-organic framework (MOF), {[Cu(I)2(py-pzpypz)2(µ-CN)2]·MeCN}n (1·MeCN), with an unusual topology is shown to be robust, retaining crystallinity during desolvation to give 1, which has also been structurally characterized [py-pzpypz is 4-(4-pyridyl)-2,5-dipyrazylpyridine)]. Zigzag-shaped channels, which in 1·MeCN were occupied by disordered MeCN molecules, run along the c axis of 1, resulting in a significant solvent-accessible void space (9.6% of the unit cell volume). These tight zigzags, bordered by (Cu(I)CN)n chains, make 1 an ideal candidate for investigations into shape-based selectivity. MOF 1 shows a moderate enthalpy of adsorption for binding CO2 (-32 kJ mol(-1) at moderate loadings), which results in a good selectivity for CO2 over N2 of 4.8:1 under real-world operating conditions of a 15:85 CO2/N2 mixture at 1 bar. Furthermore, 1 was investigated for shape-based selectivity of small hydrocarbons, revealing preferential uptake of linear acetylene gas over ethylene and methane, partially due to kinetic trapping of the guests with larger kinetic diameters.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material.

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [FeIIPd(CN)4(R-1,2,4-trz)2]·nH2O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [FeIIPd(CN)4(thiome)2]·2H2O (A·2H2O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H2O due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in A·2H2O (0-0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in A·2H2O under hydrostatic pressure compared to a one-step SCO in A at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities.

Hysteretic Four-Step Spin Crossover within a Three-Dimensional Porous Hofmann-like Material.

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four-step hysteretic SCO framework. Single-crystal structure analysis of a porous 3D Hofmann-like material showed long-range ordering of spin states: HS, HS0.67 LS0.33 , HS0.5 LS0.5 , HS0.33 LS0.67 , and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host-host and host-guest interactions.

Strong interplay between the electron spin lifetime in chemically synthesized graphene multilayers and surface-bound oxygen.

The electron spin lifetime in an assembly of chemically synthesized graphene sheets was found to be extremely sensitive to oxygen. Introducing small concentrations of physisorbed O2 onto the graphene surface reduced the exceptionally long 140 ns electron spin lifetime by an order of magnitude. This effect was completely reversible: Removing the O2 by using a dynamic vacuum restored the spin lifetime. The presence of covalently bound oxygen also decreased the electron spin lifetime in graphene, although to a far lesser extent compared to physisorbed O2 . The conduction electrons in graphene were found to play a significant role by counter-balancing the spin depolarization caused by oxygen molecules. Our results highlight the importance of chemical environment control and device packing in practical graphene-based spintronic applications.

Reversible Guest Binding in a Non-Porous Fe(II) Coordination Polymer Host Toggles Spin Crossover.

Formation of either a dimetallic compound or a 1 D coordination polymer of adiponitrile adducts of [Fe(bpte)](2+) (bpte=[1,2-bis(pyridin-2-ylmethyl)thio]ethane) can be controlled by the choice of counteranion. The iron(II) atoms of the bis(adiponitrile)-bridged dimeric complex [Fe2 (bpte)2 (µ2 -(NC(CH2 )4 CN)2 ](SbF6 )4 (2) are low spin at room temperature, as are those in the polymeric adiponitrile-linked acetone solvate polymer {[Fe(bpte)(µ2 -NC(CH2 )4 CN)](BPh4 )2 ⋅Me2 CO} (3⋅Me2 CO). On heating 3⋅Me2 CO to 80 °C, the acetone is abruptly removed with an accompanying purple to dull lavender colour change corresponding to a conversion to a high-spin compound. Cooling reveals that the desolvate 3 shows hysteretic and abrupt spin crossover (SCO) S=0↔S=2 behaviour centred at 205 K. Non-porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3⋅Me2 CO concurrently reinstating a low-spin state.

The electronic, optical and magnetic consequences of delocalization in multifunctional donor-acceptor organic polymers.

Two organic polymers containing alternating electron donating triarylamine and electron accepting thiazolo[5,4-d]thiazole (TzTz) moieties have been synthesized and their redox states investigated. When donor and acceptor units are proximal (polymer )1, electron density is delocalized, leading to a small electrical and optical band gap; these are larger with the inclusion of an adjoining alkynyl-phenyl bridge (polymer 2), where electron density is more localized due to the rotation of the monomer units. As a result, 1 and 2 display different optical and fluorescence properties in their neutral states. Upon chemical and electrochemical redox reactions, radicals form in both 1 and 2, yielding magnetic materials that display temperature-independent paramagnetism, attributable to delocalization of radical spins along the polymeric backbones. The ability to convert between diamagnetic and paramagnetic states upon chemical oxidation and/or reduction allows for the materials to display switchable magnetism and fluorescence, imparting multifunctionality to these solid-state purely organic materials.

Experimental and computational studies of a multi-electron donor-acceptor ligand containing the thiazolo[5,4-d]thiazole core and its incorporation into a metal-organic framework.

A ligand containing the thiazolo[5,4-d]thiazole (TzTz) core (acceptor) with terminal triarylamine moieties (donors), N,N'-(thiazolo[5,4-d]thiazole-2,5-diylbis(4,1-phenylene))bis(N-(pyridine-4-yl)pyridin-4-amine (1), was designed as a donor-acceptor system for incorporation into electronically active metal-organic frameworks (MOFs). The capacity for the ligand to undergo multiple sequential oxidation and reduction processes was examined using UV/Vis-near-infrared spectroelectrochemistry (UV/Vis-NIR SEC) in combination with DFT calculations. The delocalized nature of the highest occupied molecular orbital (HOMO) was found to inhibit charge-transfer interactions between the terminal triarylamine moieties upon oxidation, whereas radical species localized on the TzTz core were formed upon reduction. Conversion of 1 to diamagnetic 2+ and 4+ species resulted in marked changes in the emission spectra. Incorporation of this highly delocalized multi-electron donor-acceptor ligand into a new two-dimensional MOF, [Zn(NO3 )2 (1)] (2), resulted in an inhibition of the oxidation processes, but retention of the reduction capability of 1. Changes in the electrochemistry of 1 upon integration into 2 are broadly consistent with the geometric and electronic constraints enforced by ligation.

An investigation of photo- and pressure-induced effects in a pair of isostructural two-dimensional spin-crossover framework materials.

Two new isostructural iron(II) spin-crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)2(NCX)2] (dpms = 4,4'-dipyridylmethyl sulfide; X = S (SCOF-6(S)), X = Se (SCOF-6(Se))) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF-6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high-spin (HS) to low-spin iron(II) sites over the temperature range 300-4 K (T1/2 = 75 K). In contrast, the NCSe(-) analogue, SCOF-6(Se), displays a complete SCO transition (T1/2 = 135 K). Photomagnetic characterizations reveal quantitative light- induced excited spin-state trapping (LIESST) of metastable HS iron(II) sites at 10 K. The temperature at which the photoinduced stored information is erased is 58 and 50 K for SCOF-6(S) and SCOF-6(Se), respectively. Variable-pressure magnetic measurements were performed on SCOF-6(S), revealing that with increasing pressure both the T1/2 value and the extent of spin conversion are increased; with pressures exceeding 5.2 kbar a complete thermal transition is achieved. This study confirms that kinetic trapping effects are responsible for hindering a complete thermally induced spin transition in SCOF-6(S) at ambient pressure due to an interplay between close T1/2 and T(LIESST) values.

Thermal- and light-induced spin-crossover bistability in a disrupted Hofmann-type 3D framework.

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe(II)M(II)(CN)4] (M(II) = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe(II)Pt(II)(CN)4] to form the material [Fe(H2O)2Fe(DPSe)2(Pt(CN)4)2]·3EtOH (DPSe = 4,4'-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron(II) environments, [Fe(II)N6] (Fe1) and [Fe(II)N4O2] (Fe2), (2) as a consequence of axial water coordination to Fe2, there are "holes" in the [Fe(II)Pt(II)(CN)4] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology such that only Fe1 is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (T(1/2)↓ = 120 K and T(1/2)↑ = 130 K) of this iron(II) site; Fe2 remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; T(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (T(1/2)↓ = 60 K and T(1/2)↑ = 66 K).