Direct Orbital Overlap Research Articles

Nearly linear orbital molecules on a pyrochlore lattice.

The interplay of spin-orbit coupling with other relevant parameters gives rise to the rich phase competition in complex ruthenates featuring octahedrally coordinated Ru4+. While locally, spin-orbit coupling stabilizes a nonmagnetic Jeff = 0 state, intersite interactions resolve one of two distinct phases at low temperatures: an excitonic magnet stabilized by the magnetic exchange of upper-lying Jeff = 1 states or Ru2 molecular orbital dimers driven by direct orbital overlap. Pyrochlore ruthenates A2Ru2O7 (A = rare earth, Y) are candidate excitonic magnets with geometrical frustration. We synthesized In2Ru2O7 with covalent In─O bonds. This pyrochlore ruthenate hosts a local Jeff = 0 state at high temperatures; however, at low temperatures, it forms a unique nonmagnetic ground state with nearly linear Ru─O─Ru molecules, in stark contrast to other A2Ru2O7 compounds. The disproportionation of covalent In─O bonds drives Ru2O molecule formation, quenching not only the local spin-orbit singlet but also geometrical frustration.

Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers.

We present an extensive study of tetranuclear transition-metal cluster compounds M4(NPtBu3)4 and [M4(NPtBu3)4][B(C6F5)4] (M = Ni, Cu; tBu = tert-butyl), which feature low-coordinate metal centers and direct metal-metal orbital overlap. X-ray diffraction, electrochemical, magnetic, spectroscopic, and computational analysis elucidate the nature of the bonding interactions in these clusters and the impact of these interactions on the electronic and magnetic properties. Direct orbital overlap results in strongly coupled, large-spin ground states in the [Ni4(NPtBu3)4]+/0 clusters and fully delocalized, spin-correlated electrons. Correlated electronic structure calculations confirm the presence of ferromagnetic ground states that arise from direct exchange between magnetic orbitals, and, in the case of the neutral cluster, itinerant electron magnetism similar to that in metallic ferromagnets. The cationic nickel cluster also possesses large magnetic anisotropy exemplified by a large, positive axial zero-field splitting parameter of D = +7.95 or +9.2 cm-1, as determined by magnetometry or electron paramagnetic resonance spectroscopy, respectively. The [Ni4(NPtBu3)4]+ cluster is also the first molecule with easy-plane magnetic anisotropy to exhibit zero-field slow magnetic relaxation, and under a small applied field, it exhibits relaxation exclusively through an Orbach mechanism with a spin relaxation barrier of 16 cm-1. The S = 1/2 complex [Cu4(NPtBu3)4]+ exhibits slow magnetic relaxation via a Raman process on the millisecond time scale, supporting the presence of slow relaxation via an Orbach process in the nickel analogue. Overall, this work highlights the unique electronic and magnetic properties that can be realized in metal clusters featuring direct metal-metal orbital interactions between low-coordinate metal centers.

Metal Bonding with 3d and 6d Orbitals: An EPR and ENDOR Spectroscopic Investigation of Ti3+-Al and Th3+-Al Heterobimetallic Complexes.

Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cptt2ThH3AlCTMS3 [TMS = Si(CH3)3; Cptt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp2TiH3AlCTMS3 (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, 1H and 27Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (aiso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th3+ complex is ∼5-fold larger than that in the titanium(3+) (Ti3+) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.

Exchange interaction mechanisms in 1,3,5,7-tetramethyl-2,6-diazaadamantane N,N’-dioxyl biradical

The exchange interaction parameters were calculated and the spin density distribution over the organic skeleton of the 1,3,5,7-tetramethyl-2,6-diazaadamantane N,N’-dioxyl biradical was studied based on the results of quantum chemical modeling of the biradical structure by the DFT method using various hybrid functionals (UB3LYP, LC-wPBE, UCAM-B3LYP, UHSEH1PBE) with the 6-311++G(2d,2p) basis set and by the UHF method with the same basis set. The characteristics of the direct orbital overlap between the N atoms of the two nitroxide groups were determined. The values of the J constant, obtained using different calculation methods, were found to be similar to each other. It was established that there is ferromagnetic exchange interaction between the radical sites in the system in question, which occurs predominantly according to the spin polarization mechanism in the 2,6-diazaadamantane core, and the various spin density transfer pathways through the C atoms of the organic skeleton were found to be nonequivalent. The direct overlap of the upper singly-occupied МОs with localization on the nitroxide groups led to a noticeable additional contribution of the antiferromagnetic exchange interaction. Despite the latter factor, the total contribution of these two mechanisms (spin polarization and direct through-space exchange) resulted in the triplet ground state in the biradical studied.

Comprehensive review on the development of high mobility in oxide thin film transistors

Oxide materials are one of the most advanced key technology in the thin film transistors (TFTs) for the high-end of device applications. Amorphous oxide semiconductors (AOSs) have leading technique for flat panel display (FPD), active matrix organic light emitting display (AMOLED) and active matrix liquid crystal display (AMLCD) due to their excellent electrical characteristics, such as field effect mobility (μ FE ), subthreshold swing (S.S) and threshold voltage (V th ). Covalent semiconductor like amorphous silicon (a-Si) is attributed to the anti-bonding and bonding states of Si hybridized orbitals. However, AOSs have not grain boundary and excellent performances originated from the unique characteristics of AOS which is the direct orbital overlap between s orbitals of neighboring metal cations. High mobility oxide TFTs have gained attractive attention during the last few years and today in display industries. It is progressively developed to increase the mobility either by exploring various oxide semiconductors or by adopting new TFT structures. Mobility of oxide thin film transistor has been rapidly increased from single digit to higher than 100 cm2/V·s in a decade. In this review, we discuss on the comprehensive review on the mobility of oxide TFTs in a decade and propose bandgap engineering and novel structure to enhance the electrical characteristics of oxide TFTs.

Direct observation of charge re-distribution in a MgB2 superconductor

To study the origin of negative thermal expansion effects near the superconducting transition temperature TC in MgB2, low-temperature high-energy synchrotron radiation x-ray diffraction was used to probe the charge redistribution near the boron atoms. Our results reveal that the in-plane hole-distribution of B− hops through the direct orbital overlap of Mg2+ along the c-axis at 50 K and is re-distributed out-of-plane. This study shows that the out-of-plane π-hole distribution plays a dominant role in the possible origin of superconductivity and negative thermal effects in MgB2.

Modulation of magnetic behavior vialigand-field effects in the trigonal clusters (PhL)Fe3L*3(L*= thf, py, PMe2Ph)

Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 = MeC(CH2NPh-o-NPh)3; L* = tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S = 6 or S = 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low-temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger σ-donating pyridine or tertiary phosphine ligands reduces the ground state to S = 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally-induced variation in molecular spin state between S = 2 and S = 4. Fits to these data provide thermodynamic parameters of ΔH = 406 cm−1 and Tc = 187 K for 2 and ΔH = 604 cm−1 and Tc = 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the σ orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.

Interaction between Encapsulated Excited Organic Molecules and Free Nitroxides: Communication Across a Molecular Wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron-electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. (1)H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron-electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall.

Density Functional Study on Metal Decoration onto a Metal-Organic Framework

We carried out density functional theory calculations on the adsorption of a Ti atom at the ZnO3, Zn-O2 and Hex sites in one of isoreticular metal-organic frameworks (IRMOFs). The binding energy is largest at the Hex site and smallest at the Zn-O3 site. Through the analyses of the orbitals and then density of states plots, we also found that the binding at the Hex site was due to direct orbital overlap between the Ti atom and the carbon atoms of the phenyl ring. When a Ti atom binds to the Zn-O3 site, the interactions of the d orbitals of the Ti atom with the adjacent oxygen atoms have anti-bonding characters. At the Zn-O2 site, however, the d orbitals of the Ti atom have bonding interactions with the oxygen atoms of the carboxylate groups. Thus, the binding energy is larger at the Zn-O2 site than at the Zn-O3 site.

Study on the Extent of Folding Back Conformation in Poly(aryl ether) Dendrimers by Intramolecular Electron Transfer and Exciplex Formation

A series of poly(aryl ether) dendrimers (Py−Gn−NHPh, n = 1−4) with only one pyrene chromophore at the periphery and an aniline group at the core were synthesized and the extent of folding back conformation was given by the intramolecular electron transfer and exciplex formation. Selective excitation of the periphery pyrene moiety of Py−Gn−NHPh in dichloromethane resulted in an electron-transfer process from aniline to pyrene, and as a consequence of the electron transfer, an intramolecular exciplex was formed. Since the exciplex formation requires a direct orbital overlap between the donor and the acceptor groups, this gives us a direct experimental observation for the folding back conformation of poly(aryl ether) dendrimers. The efficiencies and the rate constants of electron transfer in dichloromethane are measured to be 0.90, 0.83, 0.78, 0.68, and 3.6 × 108, 1.7 × 108, 1.2 × 108, and 6.6 × 107 s-1 for generations 1−4, respectively. The separations (center to center) between the core donor group and the...

Rhenium-based molecular rectangles as frameworks for ligand-centered mixed valency and optical electron transfer.

A series of six neutral, tetrametallic, molecular rectangles has been synthesized that have the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand. X-ray crystallographic studies of the six show that the rectangle frameworks, as defined by the metal atoms, range in size from 5.7 A x 7.2 A to 5.7 A x 19.8 A. The singly reduced rectangles are members of an unusual category of mixed-valence compounds in which the ligands themselves are the redox centers and interligand electronic communication is controlled by direct ligand orbital overlap rather than by superexchange through the metal ions. Despite nominally identical coordination-defined ligand positioning, the spectrally determined electronic strengths, H(ab)2, vary by roughly 100-fold. As shown by X-ray crystallography and computational modeling, the observed differences largely reflect detailed geometric configurational differences that can either facilitate or frustrate productive direct orbital overlap.

Direct Porphyrin−Aryl Orbital Overlaps in Some meso-Tetraarylporphyrins

Several meso-tetraarylporphyrins with ortho substituents such as −Cl, −Br, and −NO2 exemplify a hitherto unsuspected mesomeric interaction between the porphyrin and aryl groups. The interaction consists of direct orbital overlap between the porphyrin π system and the ortho substituents on the aryl groups. Interestingly, both the a1u and a2u HOMOs of the porphyrin ring have the correct symmetry for participation in this type of interaction. Nonlocal density functional calculations have been used to characterize and visualize the nature of this interaction. We have also investigated the effects of this interaction on porphyrin valence ionization potentials and the relative stabilities of the A1u and A2u cation radical states. An interesting consequence of this interaction is that the unpaired electron spin of porphyrin cation radicals can leak from the porphyrin macrocycle onto the aryl groups, an effect for which there is some experimental support from EPR measurements.

Factoring through-space and through-bond contributions to rates of photoinduced electron transfer in donor—spacer—acceptor molecules

Contributions from direct orbital overlap (through-space interactions) and superexchange (through-bond interactions) to the electronic coupling matrix elements for photoinduced charge separation and recombination in a series of linked donor—spacer—acceptor molecules were studied. The molecules consisted of a 4-piperidinyl-naphthalene-1,8-dicarboximide (ANI) electron donor and a N-(n-octyl)pyromellitimide (PI) electron acceptor attached to the 1,5- and 1,8-positions of either anthracene (ANC) or dibenzobicyclo(2.2.2)octatriene (DBO) spacers. For the 1,8-disubstituted compounds, ANI and PI are held approximately cofacial with a center-to-center distance of 5.3 Å, whereas for the 1,5-disubstituted compounds, the center-to-center distance increases to 13.5 Å. The through-bond interaction was investigated by replacing the ANC spacer with DBO. The charge separation and recombination reactions were examined in both toluene and tetrahydrofuran (THF). The results show that for the 1,8-disubstituted DBO system in both toluene and THF, charge separation and recombination is dominated by through-space interactions. For the 1,8-disubstituted ANC system in both toluene and THF, charge separation occurs by means of a direct, through-space interaction, while charge recombination occurs through the intermediacy of the ANI ANC + PI - ion pair in toluene and directly from ANI + ANC PI - to ground state in THF. For the 1,5-disubstituted molecules, possessing either the ANC or the DBO spacers, electron transfer from 1*ANI to PI was not kinetically competitive with the decay of 1*ANI to ground state in either toluene or THF.

Mechanism for the Direct Exchange Interaction of Europium in Compounds with Rocksalt Structure

From the strong decrease in the nearest-neighbor exchange interaction going from EuO to EuS, EuSe, EuTe, it has been concluded that this positive interaction must be caused by direct orbital overlap. Since the 4f cores are very small, this has to occur via partial occupation of outer orbits. The excitation to the 5d wave function by zero point lattice vibrations has been considered and is thought to be responsible for the ferromagnetism of these compounds. The order of magnitude has been estimated and agrees with experiment.