Optical Phenomena in Molecule-Based Magnetic Materials.

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

SHG-active luminescent thermometers based on chiral cyclometalated dicyanidoiridate(iii) complexes.

Multifunctional optical materials can be realized by combining stimuli-responsive photoluminescence (PL), e.g., optical thermometry, with non-linear optical (NLO) effects, such as second-harmonic generation (SHG). We report a novel approach towards SHG-active luminescent thermometers achieved by constructing unique iridium(iii) complexes, cis-[IrIII(CN)2(R,R-pinppy)2]- (R,R-pinppy = (R,R)-2-phenyl-4,5-pinenopyridine), bearing both a chiral 2-phenylpyridine derivative and cyanido ligands, the latter enabling the formation of a series of molecular materials: (TBA)[IrIII(CN)2(R,R-pinppy)2]·2MeCN (1) (TBA+ = tetrabutylammonium) and (nBu-DABCO)2[IrIII(CN)2(R,R-pinppy)2](i)·MeCN (2) (nBu-DABCO+ = 1-(n-butyl)-1,4-diazabicyclo-[2.2.2]octan-1-ium) hybrid salts, (TBA)2{[LaIII(NO3)3(H2O)0.5]2[IrIII(CN)2(R,R-pinppy)2]2} (3) square molecules, and {[LaIII(NO3)2(dmf)3][IrIII(CN)2(R,R-pinppy)2]}·MeCN (4) coordination chains. Thanks to the chiral pinene group, 1-4 crystallize in non-centrosymmetric space groups leading to SHG activity, while the N,C-coordination of ppy-type ligands to Ir(iii) centers generates visible charge-transfer (CT) photoluminescence. The PL characteristics are distinctly temperature-dependent which was utilized in achieving ratiometric optical thermometry below 220 K. The PL phenomena were rationalized by DFT/TD-DFT calculations indicating an MLCT-type of the emission in obtained Ir(iii) complexes with the rich vibronic structure providing a few emission bands that variously depend on temperature due to the role of thermally activated vibrations. As these crucial vibrational modes depend on the crystal lattice, the thermometry performance differs within 1-4 being the most efficient in 4 while the SHG is by far the best also for 4. This proves that pinene-functionalized cyclometalated dicyanidoiridates(iii) are great prerequisites for tunable PL-NLO conjunction with the most effective multifunctionality ensured by the insertion of these anions into bimetallic frameworks.

Synchronous Switching of Dielectric Constant and Photoluminescence in Cyanidonitridorhenate-Based Crystals.

Switching of multiple physical properties by external stimuli in dynamic materials enables applications in, e.g., smart sensors, biomedical tools, as well as data-storage devices. Among stimuli-responsive materials, inorganic-organic molecular hybrids exhibiting thermal order-disorder phase transitions were tested as promising molecular switches of electrical characteristics, including dielectric constant. We aimed at broadening the multifunctional potential of such hybrid materials towards the switching of not only electrical but also other physical properties, e.g., light emission. We report two ionic salts based on luminescent tetracyanidonitridorhenate(V) anions bearing two different diamine ligands, 1,2-diaminoethane (1) and 1,3-diaminopropane (2), both crystallizing with polar N-methyl-dabconium cations. They exhibit an order-disorder phase transition related to the heating-induced turning-on of the rotation of polar cations. This leads to a unique synchronous switching of the dielectric constant as well as metal-complex-centered photoluminescence, as demonstrated by changes in, e.g., emission lifetime. The roles of organic cations, non-trivial Re(V) complexes, and their interaction in achieving the coupled thermal switching of electrical and optical properties are discussed utilizing experimental and theoretical approaches.

Neutral dicyanidoferrate(II) metalloligands for the rational design of dysprosium(III) single-molecule magnets.

Diamagnetic cis-dicyanidoferrate(II) complexes bearing blocking aromatic diimines, cis-[FeII(CN)2(LNN)2]0 (LNN = 2,2'-bipyridine, 1,10'-phenanthroline) serve as metalloligands to DyIII centres leading to a rigid cyanido-bridged chain of vertex-sharing {DyIII2FeII2} squares which constrains the equatorial plane of embedded 4f metal ions. This results in a novel convenient route to rationally designed single-molecule magnets as the magnetic anisotropy of DyIII centres can be efficiently generated by inserting aromatic N-oxide ligands on labile axial positions.

Solvent- and Temperature-Driven Photoluminescence Modulation in Porous Hofmann-Type SrII-ReV Metal-Organic Frameworks.

A unique family of three-dimensional (3D) luminescent SrII-ReV metal-organic frameworks (MOFs), {[SrII(MeOH)5][ReV(CN)4(N)(bpen)0.5]·MeOH}n [1·MeOH; N3- = nitrido ligand, bpen = 1,2-bis(4-pyridyl)ethane, and MeOH = methanol], {[SrII(MeOH)4][ReV(CN)4(N)(bpee)0.5]·2MeOH}n [2·MeOH; bpee = 1,2-bis(4-pyridyl)ethylene], and {[SrII(bpy)0.5(MeOH)2][ReV(CN)4(N)(bpy)0.5]}n (3·MeOH; bpy = 4,4'-bipyridine), is reported. They are obtained by the molecular self-assembly of Sr2+ ions with tetracyanidonitridorhenate(V) metalloligands, [ReV(CN)4(N)]2-, and pyridine-based organic spacers (L = bpen, bpee, bpy). Such a combination of molecular precursors results in bimetallic SrII-ReV cyanido-bridged layers further bonded by organic ligands into pillared Hofmann-type coordination skeletons. Because of the formation of {ReV-(L)-ReV} moieties providing emissive metal-to-ligand charge-transfer states, 1·MeOH-3·MeOH exhibit solid-state room-temperature photoluminescence tunable from green to orange by the applied organic ligand. The most stable MOF of 3·MeOH, based on the alternating {ReV-(bpy)-ReV} and {SrII-(bpy)-SrII} linkages, exhibits three interconvertible, variously solvated phases, methanol-solvated 3·MeOH, hydrated {[SrII(bpy)0.5(H2O)2][ReV(CN)4(N)(bpy)0.5]·0.6H2O}n (3·H2O), and desolvated {[SrII(bpy)0.5][ReV(CN)4(N)(bpy)0.5]}n (3). Their formation was correlated with water and methanol vapor sorption properties investigated for 3·H2O. The solvent content affects the luminescence mainly by tuning the emission energy within the series of 3·MeOH, 3·H2O, and 3. All of the obtained compounds exhibit temperature-driven modulation of luminescence, including the shift of the emission maximum and lifetime. The thermochromic luminescent response was found to be sensitive to the presence and type of solvent in the crystal lattice. This work shows that the construction of [ReV(CN)4(N)]2--based MOFs is an efficient route toward advanced solid luminophores tunable by external stimuli such as solvent or temperature.

Tuning of the phase transition between site selective SCO and intermetallic ET in trimetallic magnetic cyanido-bridged clusters.

A series of crystalline phases composed of trimetallic 3d-5d-5d' {Fe9[Re(CN)8]6-x[W(CN)8]x(MeOH)24}·yMeOH (x = 1 (1), 2 (2), 3 (3), 4 (4) and 5 (5); y = 10-15) clusters were obtained by altering the octacyanidometalate composition. The temperature dependent studies involving SC XRD, SQUID magnetic measurements, IR spectroscopy and 57Fe Mössbauer spectroscopy revealed reversible phase transition with the retention of single crystal character in each congener. The transition was assisted by reversible spin-crossover (SCO) HSFeII↔LSFeII transition at the central Fe1(ii) site for Fe9Re5W1 (1), Fe9Re4W2 (2), Fe9Re3W3 (3) and Fe9Re2W4 (4). In contrast, the tungsten-rich congener Fe9Re1W5 (5) exhibited nontrivial behavior with the SCO transition being stopped halfway through the cooling process, to be completed with single electron transfer (ET) from the external Fe2(ii) center towards one of the neighboring W(v) sites. The critical temperature Tc of SCO has been systematically increased from 193 K (1) to 247 K (4). All experimental data indicate the domination of the Fe(ii)-W(v) valence states in all crystals 1-5, however, with increasing quantity of [W(CN)8]3- (and decreasing quantity of [Re(CN)8]3-), the valence equilibrium Fe(ii)-W(v) ↔ Fe(iii)-W(iv) was systematically shifted to the right, starting from congener 3. The overall electronic configuration at low temperatures and variable amounts and location of spin carriers along the whole series suggest the remarkable competition between magnetic super-exchange Fe(ii)-CN-W(v) interactions and intermolecular interactions. The observed behavior is in line with the information collected previously for the bimetallic congeners Fe9Re6 and Fe9W6, to shed light on the role of the mixed tri-metallic composition in changing the properties observed for the relevant bimetallic cyanido-bridged skeletons.

Structural Disorder in High-Spin {CoII9WV6} (Core)-[Pyridine N-Oxides] (Shell) Architectures.

The combinations of Co(II), octacyanidotungstate(V), and monodentate pyridine N-oxide (pyNO) or 4-phenylpyridine N-oxide (4-phpyNO) led to crystallization of novel crystalline phases {CoII[CoII8(pyNO)12(MeOH)12][WV(CN)8]6} (1) and {CoII[CoII8(4-phpyNO)7(MeOH)17][WV(CN)8]6}·7MeOH·(4-phpyNO)3 (2). In both architectures, metal-cyanide clusters are coordinated by N-oxide ligands in a simple monodentate manner to give the spherical objects of over 1 nm core diameter and about 2.2 nm (1) and 3 nm (2) of the total diameter, terminated with the aromatic rings. The supramolecular architecture is dominated by dense and rich π-π interaction systems. Both structures are characterized by a significant structural disorder in ligand shell, described with the suitable probability models. For 1, the π-π interactions between the pyNO ligands attached to the same metal centers are suggested for the first time. In 2, 4-phpyNO acts as monodentate ligand and as the crystallization molecule. Magnetic studies indicate the high-spin ground state due to the ferromagnetic interactions Co(II)-W(V) through the cyanido bridges. Due to the high symmetry of the clusters, no signature of slow magnetic relaxation was observed. The characterization is completed by solid-state IR and UV-Vis-NIR spectroscopy. The conditions for the stable M9M'6-based crystals formation are synthetically discussed in terms of the type of capping ligands: monodentate, bridging, and chelating. The potential of the related polynuclear forms toward the magnetism-based functional properties is critically indicated.

Bulky ligands shape the separation between the large spin carriers to condition field-induced slow magnetic relaxation.

Crystal engineering of magnetic relaxation in supramolecular networks based on almost isotropic cyanido-bridged {Mn9[W(CN)8]6L8(solv)8} clusters decorated by bulky 4,4'-di-tert-butyl-2,2'-bipyridine (But2bpy) and 4,7-diphenyl-1,10-phenanthroline (Ph2phen) ligands is presented. The three new compounds {MnII9[WV(CN)8]6(But2bpy)8(MeOH)8}·Pri2O·13MeOH (1), {MnII9[WV(CN)8]6(But2bpy)8(MeOH)6(H2O)2}·4Pri2O·2H2O (1a), and {MnII9[WV(CN)8]6(Ph2phen)8(MeOH)8}·29MeOH·6H2O (2) were characterized structurally and magnetically. Compound 1 exhibits unequivocal domination of repulsive intercluster contacts operating between the side But groups leading to intercluster distances exceeding 10 Å in all three dimensions. Compound 1a reveals a 1-dimensional (1D) supramolecular chain structure with very close intercluster distances of 6.7 Å realized through the direct W-CNHO-Mn hydrogen bonds along the chain, further isolated by the above repulsive ButBut synthons. Compound 2 shows significant separation in all three directions with the intercluster distances close to 10, 12 and 13.5 Å. However, in contrast to 1, these separations are accompanied by indirect hydrogen bond arrays and local π-π interactions of potential to assist in the transfer of weak magnetic interactions. The dc magnetic data show the signature of S = 39/2 in the ground state, which is typical in this group of compounds. The high-spin clusters are accompanied by different intercluster interactions, illustrated by the effective zJ' values of +0.010 cm-1 (1), +0.008 cm-1 (1a) and +0.001 cm-1 (2). The low temperature ac susceptibility measurements revealed a temperature- and field-dependent magnetic relaxation time for all 1, 1a and 2 compounds (τ1, τ1a-fast, and τ2-fast in the range 10-3-10-4 s). In contrast and only in the case of 1a and 2, an additional temperature independent slow process was detected (τ1a-slow and τ2-slow located between 0.1 s and 1 s). The magnetic relaxations were correlated with the obtained supramolecular networks, indicating the significant role of dipolar fields, weak non-covalent interactions, hydrogen bonds and π-π interactions.