Mark J. Riley

Interpretation of the temperature dependent g values of the Cu(H2O)2+6 ion in several host lattices using a dynamic vibronic coupling model

The causes of the previously reported temperature dependence of the g values of the Cu(H2O)2+6 ions in Cu2+ doped Zn(H2O)6(GeF6) and the Tutton’s salts M2Zn(H2O)6(SO4)2, where M=K+, Rb+, NH+4, and Cs+, supplemented by new experimental measurements on the K+ salt, have been investigated. The ground state dynamics of the complexes have been modeled on the cubic E×e Jahn–Teller Hamiltonian perturbed by an orthorhombic lattice strain. For each compound, the vibronic energy levels and associated wave functions were calculated numerically, the overall g values at any temperature being given by a thermal average of the g values of the individual vibronic energy levels, because of rapid exchange between the levels. For the Tutton’s salts it was found that the low temperature g values are strongly influenced by the tetragonal component of the lattice strain, with this corresponding to an axial compression of the ligand field. The temperature dependence of the g tensors, on the other hand, was found to depend large...

Effects of vibronic coupling on the EPR spectra of copper(II) doped K2ZnF4

Abstract The single crystal g-values of ≈≈ 1% Cu2+ doped into tetragonal K2ZnF4 measured over a temperature range between 4 and 295 K are reported. The results are interpreted in terms of a predominantly dz2 ground state wavefunction for the CuF4−6 guest species, with a small admixture of dx2-y2 caused by vibronic coupling. To a first approximation both the magnitudes and temperature dependence of the g-values may be described using a model directly analogous to that conventionally used to represent the temperature dependence of the intensity of parity-forbidden electronic transitions. The generality of this model has been investigated by carrying out numerical calculations of the vibronic wavefunctions and energy levels, and values of the ligand-field and warping parameters of the “Mexican-hat” potential surface of the copper(II) guest complex have been derived. The results reported for several other systems where vibronic coupling had been thought to influence EPR parameters are also discussed using the present model.

Systematic study of spin crossover and structure in [Co(terpyRX)2](Y)2 systems (terpyRX = 4'-alkoxy-2,2':6',2''-terpyridine, X = 4, 8, 12, Y = BF4(-), ClO4(-), PF6(-), BPh4(-)).

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)(2)](Y)(2) x nH(2)O (X = 4, 8, 12 and Y = BF(4)(-), ClO(4)(-), PF(6)(-), BPh(4)(-)) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)(2)](ClO(4))(2), [Co(terpyR8)(2)](BF(4))(2) x H(2)O, and [Co(terpyR4)(2)](PF(6))(2) x 3 H(2)O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF(4)(-) family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)(2)](BF(4))(2) x H(2)O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn-Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).

Diluted Magnetic Semiconductor Nanowires Prepared by the Solution-Liquid-Solid Method

Wiry frame: Manganese-doped cadmium selenide (Mn-CdSe) colloidal nanowires (see picture) exhibit ferromagnetism and good conductivity without any changed to their optical properties. The nanowires are synthesized by a novel solution-liquid-solid approach that offers a low-cost route towards magnetically active quantum wires with excellent potential applications in electronics, photonics, and spintronics. (Figure Presented).

The reaction mechanism of the Ga(III)Zn(II) derivative of uteroferrin and corresponding biomimetics

Purple acid phosphatase from pig uterine fluid (uteroferrin), a representative of the diverse family of binuclear metallohydrolases, requires a heterovalent Fe(III)Fe(II) center for catalytic activity. The active-site structure and reaction mechanism of this enzyme were probed with a combination of methods including metal ion replacement and biomimetic studies. Specifically, the asymmetric ligand 2-bis{[(2-pyridylmethyl)-aminomethyl]-6-[(2-hydroxybenzyl)(2-pyridylmethyl)]aminomethyl}-4-methylphenol and two symmetric analogues that contain the softer and harder sites of the asymmetric unit were employed to assess the site selectivity of the trivalent and divalent metal ions using 71Ga NMR, mass spectrometry and X-ray crystallography. An exclusive preference of the harder site of the asymmetric ligand for the trivalent metal ion was observed. Comparison of the reactivities of the biomimetics with Ga(III)Zn(II) and Fe(III)Zn(II) centers indicates a higher turnover for the former, suggesting that the M(III)-bound hydroxide acts as the reaction-initiating nucleophile. Catalytically active Ga(III)Zn(II) and Fe(III)Zn(II) derivatives were also generated in the active site of uteroferrin. As in the case of the biomimetics, the Ga(III) derivative has increased reactivity, and a comparison of the pH dependence of the catalytic parameters of native uteroferrin and its metal ion derivatives supports a flexible mechanistic strategy whereby both the μ-(hydr)oxide and the terminal M(III)-bound hydroxide can act as nucleophiles, depending on the metal ion composition, the geometry of the second coordination sphere and the substrate.

Effects of Fluorination on Iridium(III) Complex Phosphorescence: Magnetic Circular Dichroism and Relativistic Time-Dependent Density Functional Theory

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.

The luminescence of Sm2+ in alkaline earth borophosphates

The temperature-dependent luminescence of Sm2+ ions in MBPO5 (M = Ca2+, Sr2+, Ba2+) was studied. At low temperature, Sm2+ in this series shows 4f(6) --> 4f(6) luminescence with only a single emission line observed for the D-5(0) --> F-7(0) transition, revealing that only one crystallographic cationic site is available for Sm2+ in all the hosts. With increasing temperature, the emission intensity of the D-5(0) --> F-7(0) transition increases whereas that of the D-5(0) --> 7F(1) transitions decreases. The D-5(1) --> F-7(0) transitions of Sm2+ were observed in BaBPO5 and its intensity increases with increasing temperature. At 450 K, a broad band of the 4f(5)5d --> 4f(6) luminescent transition of Sm2+ in SrBPO5 and BaBPO5 with maximum at similar to600 ran appears due to the thermal population. The lifetime of the D-5(0) --> F-7(0) transition Sm2+ was recorded at different temperatures, showing a single exponential decay for in SrBPO5 and BaBPO5,but a nonsingle-exponential decay in CaBPO5

Excitation avalanche in Ni2+-doped CsCdCl3

Abstract Nonlinear optical behaviour is observed in CsCdCl 3 :Ni 2+ and compared with a phenomenon reported from lanthanide doped compounds called a “photon avalanche”. Laser power and time dependences are correlated to an appropriate model. An excited-state excitation spectrum is shown.

Completion of the isomorphous Ln(trensal) series

The CeIII, PrIII, NdIII, GdIII and YbIII complexes of the heptadentate ligand 2,2´,2´´-tris(salicylideneimino) triethylamine, H3trensal (in its trianionic form), have been synthesized and characterized structurally by X-ray crystallography. These five [Ln(trensal)] structures complete a rare isomorphous and isostructural series of lanthanoid complexes in the trigonal P–3c1 space group with a ≈ 13.1 and c ≈ 16.5 A

Spectroscopic and catalytic characterization of a functional FeIIIFeII biomimetic for the active site of uteroferrin and protein cleavage

A mixed-valence complex, [Fe(III)Fe(II)L1(μ-OAc)(2)]BF(4)·H(2)O, where the ligand H(2)L1 = 2-{[[3-[((bis(pyridin-2-ylmethyl)amino)methyl)-2-hydroxy-5-methylbenzyl](pyridin-2-ylmethyl)amino]methyl]phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe(III)Fe(II) and Fe(III)(2) oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3/2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = -2JS(1)·S(2), where J = -5.6 cm(-1)) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe(III)Fe(II) complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin-lattice relaxation. However, a phosphate-bound Fe(III)(2) complex showed an EPR spectrum due to population of the S(tot) = 3 state (J= -3.5 cm(-1)). The phosphatase activity of the Fe(III)Fe(II) complex in hydrolysis of bis(2,4-dinitrophenyl)phosphate (k(cat.) = 1.88 × 10(-3) s(-1); K(m) = 4.63 × 10(-3) mol L(-1)) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe(III). It is interesting to note that aqueous solutions of [Fe(III)Fe(II)L1(μ-OAc)(2)](+) are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complexs catalytic promiscuity.