Taking Advantage of Disorder: Small-Molecule Organic Glasses for Radiation Detection and Particle Discrimination.

A series of fluorescent silyl-fluorene molecules were synthesized and studied with respect to their photophysical properties and response toward ionizing neutron and gamma-ray radiation. Optically transparent and stable organic glasses were prepared from these materials using a bulk melt-casting procedure. The prepared organic glass monoliths provided fluorescence quantum yields and radiation detection properties exceeding the highest-performing benchmark materials such as solution-grown trans-stilbene crystals. Co-melts based on blends of two different glass-forming compounds were prepared with the goal of enhancing the stability of the amorphous state. Accelerated aging experiments on co-melt mixtures ranging from 0% to 100% of each component indicated improved resistance to recrystallization in the glass blends, able to remain fully amorphous for >1 month at 60 °C. Secondary dopants comprising singlet fluorophores or iridium organometallic compounds provided further improved detection efficiency, as evaluated by light yield and neutron/gamma particle discrimination measurements. Optimized singlet and triplet doping levels were determined to be 0.05 wt % 1,4-bis(2-methylstyryl)benzene singlet fluorophore and 0.28 wt % Ir3+, respectively.

Guest-Induced Emergent Properties in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) are crystalline nanoporous materials comprised of organic electron donors linked to metal ions by strong coordination bonds. Applications such as gas storage and separations are currently receiving considerable attention, but if the unique properties of MOFs could be extended to electronics, magnetics, and photonics, the impact on material science would greatly increase. Recently, we obtained "emergent properties," such as electronic conductivity and energy transfer, by infiltrating MOF pores with "guest" molecules that interact with the framework electronic structure. In this Perspective, we define a path to emergent properties based on the Guest@MOF concept, using zinc-carboxylate and copper-paddlewheel MOFs for illustration. Energy transfer and light harvesting are discussed for zinc carboxylate frameworks infiltrated with triplet-scavenging organometallic compounds and thiophene- and fullerene-infiltrated MOF-177. In addition, we discuss the mechanism of charge transport in TCNQ-infiltrated HKUST-1, the first MOF with electrical conductivity approaching conducting organic polymers. These examples show that guest molecules in MOF pores should be considered not merely as impurities or analytes to be sensed but also as an important aspect of rational design.

Charge-transfer guest interactions in luminescent MOFs: implications for solid-state temperature and environmental sensing.

One of the ongoing goals in the field of porous materials is the design and synthesis of materials that possess chemical structures amenable for use in sensing applications. We describe the preparation, luminescence characteristics, and environmental sensing properties of variants of the aluminum-based MOF [Al(OH)(O(2)C-C(10)H(6)-CO(2))](∞). Careful activation of the open framework complex, 1, yielded a dynamic structural transformation to a non-porous form, 2, that exhibited strong inter-linker interactions and red-shifted emission characteristics indicative of dimer formation. We also demonstrate the formation of highly luminescent ground-state charge-transfer (CT) complexes between 1 and the electron-donating amines dimethylamine (DMA) (1a), and N,N-diethylaniline (DEA) (1b), both of which exhibit dual-emission characteristics and a ratiometric luminescence response that is sensitive to temperature and solvent polarity. Steady-state and time-resolved measurements on 1a, 1b, and 2 indicate that the MOF structures stabilize ground-state CT interactions that are distinct from the weakly-bound exciplexes formed in comparable mixtures of purely organic components. The spectra for 1a and 1b also indicate different temperature dependencies that correspond to thermally-activated complex formation (ΔH(f) = +1.1 ± 0.2 kcal mol(-1)) in 1a and static quenching effects (ΔH(f) = -2.2 ± 0.3 kcal mol(-1)) in 1b. The addition of ethanol, isopropanol, toluene, or chloroform to suspensions of 1b indicate destabilization of the CT state with increasing solvent polarity, which suggests the generalized application of this or related materials in sensor applications.

Assessing the purity of metal-organic frameworks using photoluminescence: MOF-5, ZnO quantum dots, and framework decomposition.

Photoluminescence (PL) spectroscopy was used to characterize nanoscale ZnO impurities, amine-donor charge-transfer exciplexes, and framework decomposition in samples of MOF-5 prepared by various methods. The combined results cast doubt on previous reports describing MOF-5 as a semiconductor and demonstrate that PL as a tool for characterizing MOF purity possesses advantages such as simplicity, speed, and sensitivity over currently employed powder XRD MOF characterization methods.

Isostructural single-chain and single-molecule magnets.

Isostructural single-chain magnet (SCM) and single-molecule magnets (SMM) with formulas [Mn(6)X(2)(salox)(6)O(2)(N(3))(8)] (X = Mn(II) (1), Cd(II) (2); H(2)salox = salicylaldoxime) have been synthesized and magnetically characterized. Complexes 1 and 2 possess significantly different magnetization reversal barriers of U(eff) = 100.3 and 57.0 K, in spite of comparable uniaxial anisotropies (D) and ground state spin values (S). These observations are indicative of the intrinsic spin dynamics in these structurally related yet magnetically distinct SCM/SMM systems.

Magnetic quantum tunneling: insights from simple molecule-based magnets.

This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 Ni(II)(4), Mn(III)(3) (S = 2 and 6) and Mn(III)(6) (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn(III)(3) units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn(6)) has barely increased in the 15 years since the first studies of Mn(12)-acetate, and why the tiny Mn(3) molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn(3) and Ni(4)). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn(6) possesses the record anisotropy barrier, and Mn(3) is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

Large spin-state changes in isostructural cyanate- and azide-bridged Mn(3)(III)Mn(2)(II) single-molecule magnets.

We prepared three structurally related Mn(3)(III)Mn(2)(II) complexes that possess S approximately 1-11 spin ground states as a result of variations in the geometry and identity of mu(2)-eta(1):eta(1) bridging groups. These complexes function as single-molecule magnets yet demonstrate other interesting behavior such as quasi-classical magnetization hysteresis and comparable magnetization reversal barriers (U(eff)).

Nanomodulation of molecular nanomagnets.

Detailed synthetic, structural, and magnetic characterizations for a family of six [Mn(3)Zn(2)](13+) complexes are presented. These complexes have planar [Mn(3)(III)-(mu(3)-oxo)](7+) core magnetic units and have formulas represented by [cation](3)[Mn(3)Zn(2)(R-salox)(3)O(N(3))(6)X(2)], where [cation](+) = [NEt(4)](3)(+) or [AsPh(4)](3)(+); R = H or Me; and X = Cl(-), Br(-), I(-), or N(3)(-). Least-squares fits to the magnetic susceptibility data for these complexes indicate large negative values of the axial zero field splitting (ZFS) parameter D (approximately -1.1 K) and spin ground states ranging from a highly spin-mixed S approximately 1 to a reasonably isolated S = 6 (DeltaE(S = 5) = 69.2 K). The strength and magnitude of the intramolecular exchange interactions have been observed to change with the crystal packing as a result of systematic variations in the co-crystallizing cation, terminal ion, and oximate ligand. Alternating current susceptibility data were collected from 1.8-7 K at 10-997 Hz, revealing strong frequency-dependent peaks in the out-of-phase susceptibility (chi''(M)) for ferromagnetic S = 6 complexes 1, 2, and 6. Fitting of these data to the Arrhenius equation gave U(eff) = 44.0 K and tau(0) = 3.8 x 10(-8) s for [NEt(4)](3)[Mn(3)Zn(2)(salox)(3)O(N(3))(6)Cl(2)] (1), and U(eff) = 45.6 K and tau(0) = 2.1 x 10(-7) s for [NEt(4)](3)[Mn(3)Zn(2)(Me-salox)(3)O(N(3))(6)Cl(2)] (6). The enhanced relaxation behavior in complex 6 is associated with stronger ferromagnetic exchange interactions and a more isolated S = 6 ground state than in 1 and 2. Comprehensive high-frequency electron paramagnetic resonance (HFEPR) experiments were conducted on single crystals of complexes 1, 2, and 6, revealing sharp absorption peaks and allowing for the precise determination of ZFS parameters. Similar experiments on [AsPh(4)](3)[Mn(3)Zn(2)(salox)(3)O(N(3))(6)Cl(2)] (4) resulted in the observation of a broad absorption peak, consistent with the highly spin-mixed ground state. Single crystal magnetization hysteresis measurements on complexes 1 and 2 indicate SMM behavior via temperature- and sweep-rate dependent hysteresis loops and the observance of very sharp quantum tunneling resonances. Additionally, the Hamiltonian parameters derived from the magnetic data, HFEPR, and hysteresis measurements are in good agreement and highlight the relationships between superexchange, spin-orbit interactions, and the varied relaxation behavior in these complexes.

Single-molecule-magnet behavior and spin changes affected by crystal packing effects.

Five Mn 3Zn 2 heterometallic complexes have been synthesized and structurally and magnetically characterized. Spin ground states up to S = 6 have been observed for these complexes and are shown to depend on the cocrystallizing cation and on the terminal ligand. Large axial zero-field interactions ( D = -1.16 K) are the result of near-parallel alignment of the Mn (III) Jahn-Teller axes. High-frequency electron paramagnetic resonance, single-crystal magnetization hysteresis, and alternating current susceptibility measurements are presented to characterize [NEt 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6X 2] [X (-) = Cl (-) ( 1), Br (-) ( 2)] and [AsPh 4] 3[Mn 3Zn 2(salox) 3O(N 3) 6Cl 2] ( 3) and reveal that 1 and 2 are single-molecule magnets ( U eff = 44 K), while 3 is not.

Heterometallic integer-spin analogues of S = 9/2 Mn4 cubane single-molecule magnets.

A family of distorted heterometallic cubanes, [Mn (III) 3Ni (II)(hmp) 3O(N 3) 3(O 2CR) 3], where O 2CR (-) is benzoate ( 1), 3-phenylpropionate ( 2), 1-adamantanecarboxylate ( 3), or acetate ( 4) and hmp (-) is the anion of 2-pyridinemethanol, was synthesized and structurally as well as magnetically characterized. These complexes have a distorted-cubane core structure similar to that found in the S = 9/2 Mn 4 cubane family of complexes. Complexes 1, 3, and 4 crystallize in rhombohedral, hexagonal, and cubic space groups, respectively, and have C 3 molecular symmetry, while complex 2 crystallizes in the monoclinic space group Cc with local C 1 symmetry. Magnetic susceptibility and magnetization hysteresis measurements and high-frequency electron paramagnetic resonance (HFEPR) spectroscopy established that complexes 1-4 have S = 5 spin ground states with axial zero-field splitting (ZFS) parameters ( D) ranging from -0.20 to -0.33 cm (-1). Magnetization versus direct-current field sweeps below 1.1 K revealed hysteresis loops with magnetization relaxation, definitely indicating that complexes 1-4 are single-molecule magnets that exhibit quantum tunneling of magnetization (QTM) through an anisotropy barrier. Complex 2 exhibits the smallest coercive field and fastest magnetization tunneling rate, suggesting a significant rhombic ZFS parameter ( E), as expected from the low C 1 symmetry. This was confirmed by HFEPR spectroscopy studies on single crystals that gave the following parameter values for complex 2: gz = 1.98, gx = gy = 1.95, D = -0.17 cm (-1), B 4 (0) = -6.68 x 10 (-5) cm (-1), E = 6.68 x 10 (-3) cm (-1), and B 4 (2) = -1.00 x 10 (-4) cm (-1). Single-crystal HFEPR data for complex 1 gave g z = 2.02, gx = gy = 1.95, D = -0.23 cm (-1), and B 4 (0) = -5.68 x 10 (-5) cm (-1), in keeping with the C 3 site symmetry of this Mn 3Ni complex. The combined results highlight the importance of spin-parity effects and molecular symmetry, which determine the QTM rates.

Charge-separation in uranium diazomethane complexes leading to C-H activation and chemical transformation.

The reaction of diphenyldiazomethane with [((t-BuArO)3tacn)UIII] (1) results in an eta(2)-bound diphenyldiazomethane uranium complex. This complex exhibits unusual electronic properties as a charge-separated species with a radical anionic open-shell ligand, [((t-BuArO)3tacn)UIV(eta2-NNCPh2)] (2). Treating Ph2CN2 with a uranium complex that contains a sterically more demanding adamantane functionalized ligand, [((AdArO)3tacn)UIII] (3) results in an unprecedented C-H activation and nitrogen insertion to produce a five-membered heterocyclic indazole complex, [((AdArO)3tacn)UIV(eta(2)-3-phen(Ind))] (5). X-ray crystallography and spectroscopic characterization of these two compounds show that the [((t-BuArO)3tacn)UIV(eta(2)-NNCPh2)] compound is a U(IV) complex with a radical anionic ligand, whereas [((AdArO)3tacn)UIV(eta(2)-3-phen(Ind))] is a U(IV) f (2) species with a closed-shell ligand.

Approximating the product spectrum and product concentrations in continuous photochemical reactions.

Several approximations to a common photochemical rate law, in which the rate is proportional to the fraction of light absorbed by the reactant chromophore, have been developed to permit the product spectrum to be determined from a sequence of spectra during irradiation that exhibit isosbestic points. The methods were tested on the photolysis of [Cr(NH(3))(6)](3+) and [Cr(en)(3)](3+) (en = ethylenediamine) in water, and [Fe(Et(2)dtc)(3)], tris(diethyldithiocarbamato)iron(III), in CHCl(3).

Heterometallic cubane single-molecule magnets.

Two new heterometallic cubane molecules have been synthesized. High-frequency electron paramagnetic resonance and magnetization measurements indicate that [Mn(3)Ni(hmp)(3)O(N(3))(3)(C(7)H(5)O(2))(3)] (1) displays a well-isolated S = 5 ground state (DeltaE > 120 K), with g = 2.0, D = -0.23 cm(-1), and ferromagnetic Mn-Mn exchange interactions competing with antiferromagnetic Ni-Mn interactions. [Mn(3)Zn(hmp)(3)O(N(3))(3)(C(3)H(5)O(2))(3)] (2) possesses a S = 6 ground state (DeltaE > 105 K), with g = 2.0, D = -0.14 cm(-1), and ferromagnetic Mn-Mn exchange interactions. Magnetization vs magnetic field data for oriented single crystals of 1 and 2 indicate that both complexes are single-molecule magnets.

Synthesis and characterization of two quaternary thorium chalcophosphates: Cs4Th2P6S18 and Rb7Th2P6Se21.

Two new thorium chalcophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction, diffuse reflectance, and Raman spectroscopy: Cs4Th2P6S18 (I); Rb7Th2P6Se21 (II). Compound I crystallizes as colorless blocks in the triclinic space group P1 (No. 2) with a = 12.303(4) A, b = 12.471(4) A, c = 12.541(4) A, alpha = 114.607(8) degrees, beta = 102.547(6) degrees, gamma = 99.889(7) degrees, and Z = 2. The structure consists of (Th2P6S18)(4-) layers separated by layers of cesium cations and only contains the (P2S6)(4-) building block. Compound II crystallizes as red blocks in the triclinic space group P1 (No. 2) with a = 11.531(3) A, b = 12.359(4) A, c = 16.161(5) A, alpha = 87.289(6) degrees, beta = 75.903(6) degrees, gamma = 88.041(6) degrees, and Z = 2. The structure consists of linear chains of (Th2P6Se21)(7-) separated by rubidium cations. Compound II contains both the (PSe4)(3-) and (P2Se6)(4-) building blocks. Both structures may be derived from two known rare earth structures where a rare earth site is replaced by an alkali or actinide metal to form these novel structures. Optical band gap measurements show that compound I has a band gap of 2.8 eV and compound II has a band gap of 2.0 eV. Solid-state Raman spectroscopy of compound I shows the vibrations expected for the (P2S6)(4-) unit. Raman spectroscopy of compound II shows the vibrations expected for both (PSe4)(3-) and (P2Se6)(4-) units. Our work shows the remarkable diversity of the actinide chalcophosphate system and demonstrates the phase space is still ripe to discover new structures.