Highly Luminescent Aceno[6]helicenones by Intramolecular Radical Cyclization.

Helicenes and helicenoid structures are promising candidates for future applications exploiting circularly polarized light. Ideal candidates should possess near-quantitative photoluminescence quantum yield, a high luminescence dissymmetry factor and an adjustable HOMO-LUMO gap. However, carbo[n]helicenes are poorly luminescent compounds and they absorb light mainly in the ultraviolet region.

Self-Assembled Tetranuclear Square Complex of Chromium(III) Bridged by Radical Pyrazine: A Molecular Model for Metal-Organic Magnets.

The attractive electronic properties of metal-pyrazine materials─electrical conductivity, magnetic order, and strong magnetic coupling─can be tuned in a wide range depending on the metal employed, as well as its ligand-imposed redox environment. Using solvent-directed synthesis to control the dimensionality of such systems, a discrete tetranuclear chromium(III) complex, exhibiting a rare example of bridging radical pyrazine, has been prepared from chromium(II) triflate and neutral pyrazine. The strong antiferromagnetic interaction between Cr ( = 3/2) and radical pyrazine ( = 1/2) spins, theoretically estimated at about -932 K, leads to a thermally isolated = 4 ground state, which remains the only populated state observable even at room temperature.

Carbonyl mediated fluorescence in aceno[]helicenones and fluoreno[]helicenes.

Helicenes are very attractive chiral non-planar polycyclic aromatic hydrocarbons possessing strong chiroptical properties. However, most of the helicenes absorb light mainly in the ultraviolet region, with only a small segment in the blue part of the visible spectrum. Furthermore, carbo[]helicenes exhibit only weak luminescence that limits their utilization.

Coronenes with push-pull geometries from macrocycle-forming Perkin condensations.

Although the Perkin reaction has been successful in producing ester-substituted conjugated macrocycles with four or six building blocks, macrocycles composed of only two elements remained elusive until now. Through the development of a building block derived from phenanthrene with two glyoxylic acid substituents in a pincer-like arrangement, formation of a two-block macrocycle was induced when paired with a complementary phenylenediacetic acid unit. The addition of ether functions to the phenanthrene building block not only improved the yields, but led to macrocycles with push-pull geometries.

Light-Induced, Structural Matrix Guided Stepwise Spin-State Switching in 3d-5d Molecular Assembly.

A new iron(II) molecular complex {[W(CN)][Fe(bik*)]}BF·7HO·1.5CHOH (.7HO·1.

Reactions of Cadmium(II) Halides and Di-2-Pyridyl Ketone Oxime: One-Dimensional Coordination Polymers.

The coordination chemistry of 2-pyridyl ketoximes continues to attract the interest of many inorganic chemistry groups around the world for a variety of reasons. Cadmium(II) complexes of such ligands have provided models of solvent extraction of this toxic metal ion from aqueous environments using 2-pyridyl ketoxime extractants. Di-2-pyridyl ketone oxime (dpkoxH) is a unique member of this family of ligands because its substituent on the oxime carbon bears another potential donor site, i.

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane.

A rigid propeller-shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.

The effect of substituents and molecular aggregation on the room temperature phosphorescence of a twisted π-system.

Considering the relevance of room temperature phosphorescent (RTP) materials, we discuss the influence of donor and acceptor groups substituted on to a twisted three-fold symmetric hydrocarbon homotruxene, which presents a persistent RTP, even in the absence of donor or acceptor moieties, under ambient conditions as a result of the twisted π-system. Compared to a fluorine acceptor, a donor methoxy group increases the phosphorescence decay rate in solution, while in the solid-state, molecular aggregation and packing yield a very persistent phosphorescence visible by the eye. The RTP of the intrinsically apolar homotruxene is found to be modulated by polar substituents, whose main impact on the solid-state emission is due to altered packing in the crystal.

Metal-to-metal electron transfer in a cyanido-bridged {FeCo} square complex followed by X-ray diffraction and absorption techniques.

The switching properties of a cyanido-bridged Fe/Co square molecule were investigated by single-crystal X-ray diffraction and X-ray absorption spectroscopy at both Fe and Co K-edges. Combining these two techniques, a complete picture of the thermal-, light- and X-ray-induced metal-to-metal electron transfer is obtained, illustrating the concerted role played by the Fe and Co sites.

Indium(III)/2-benzoylpyridine chemistry: interesting indium(III) bromide-assisted transformations of the ligand.

Reactions of 2-benzoylpyridine, (py)(ph)CO, with InX (X = Cl, Br) in EtOH at room temperature have been studied. The InCl/(py)(ph)CO system has provided access to complex [InCl{(py)(ph)CO}(EtOH)]·{(py)(ph)CO} (1) and the byproduct {(pyH)(ph)CO}Cl (2). The reaction of InBr with (py)(ph)CO has led to a mixture of (L)[InBr{(py)(ph)CO}] (3) and [InBr{(py)(ph)CH(O)}(EtOH)] (4), where L is the 9-oxo-indolo[1,2-]pyridinium cation and (py)(ph)CH(O) is the anion of (pyridin-2-yl)methanol.

Halogenation of a twisted non-polar π-system as a tool to modulate phosphorescence at room temperature.

Halogenation of a twisted three-fold symmetric hydrocarbon with F, Cl or Br leads to strong modulation of triplet-triplet annihilation and dual phosphorescence, one thermally activated and the other very persistent and visible by eye, with different relative contributions depending on the halide. The room temperature phosphorescence is highly unusual given the absence of lone-pair-contributing heteroatoms. The interplay between the spin-orbit coupling matrix elements and the spatial configuration of the triplet state induces efficient intersystem crossing and thus room temperature phosphorescence even without relying on heteroatomic electron lone pairs.

Indium(III) in the "Periodic Table" of Di(2-pyridyl) Ketone: An Unprecedented Transformation of the Ligand and Solid-State In NMR Spectroscopy as a Valuable Structural Tool.

Reactions of di(2-pyridyl) ketone, (py)CO, with indium(III) halides in CHNO have been studied, and a new transformation of the ligand has been revealed. In the presence of In, the C═O bond of (py)CO is subjected to nucleophilic attack by the carbanion :CHNO, yielding the dinuclear complexes [InX{(py)C(CHNO)(O)}] (X = Cl, ; X = Br, ; X = I, ) in moderate to good yields. The alkoxo oxygens of the two η:η:η-(py)C(CHNO)(O) ligands doubly bridge the In centers and create a {In(μ-OR)} core.

Asymmetric Dinuclear Lanthanide(III) Complexes from the Use of a Ligand Derived from 2-Acetylpyridine and Picolinoylhydrazide: Synthetic, Structural and Magnetic Studies.

A family of four Ln(III) complexes has been synthesized with the general formula [Ln(NO)(L)(S)] (Ln = Gd, Tb, Er, and S = HO; , and , respectively/Ln = Dy, S = MeOH, complex ), where HL is the flexible ditopic ligand '-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide. The structures of isostructural MeOH/HO solvates of these complexes were determined by single-crystal X-ray diffraction. The two Ln ions are doubly bridged by the deprotonated oxygen atoms of two "head-to-head" 2.

A heteroleptic diradical Cr(iii) complex with extended spin delocalization and large intramolecular magnetic exchange.

Successive chemical reductions of the heteroleptic complex [(tpy)Cr(tphz)] (tpy = terpyridine; tphz = tetrapyridophenazine) give rise to the mono- and di-radical redox isomers, [(tpy)Cr(tphz˙)] and [(tpy˙)Cr(tphz˙)], respectively. As designed, the optimized overlap of the involved magnetic orbitals leads to extremely strong magnetic interactions between the S = 3/2 metal ion and S = 1/2 radical spins, affording well isolated S = 1 and S = 1/2 ground states at room temperature.

Slow Dynamics of the Spin-Crossover Process in an Apparent High-Spin Mononuclear Fe Complex.

A mononuclear Fe complex that shows a high-spin (S=2) paramagnetic behavior at all temperatures (with standard temperature-scan rates, ≈1 K min ) has, in fact, a low-spin (S=0) ground state below 100 K. This low-spin state is not easily accessible due to the extremely slow dynamics of the spin-crossover process-a full relaxation from the metastable high-spin state to the low-spin ground state takes more than 5 h below 80 K. Bidirectional photo-switching of the Fe state is achieved reproducibly by two selective irradiations (at 530-590 and 830-850 nm).

Using Redox-Active π Bridging Ligand as a Control Switch of Intramolecular Magnetic Interactions.

Intramolecular magnetic interactions in the dinuclear complexes [(tpy)Ni(tphz)Ni(tpy)] ( n = 4, 3, and 2; tpy, terpyridine; tphz, tetrapyridophenazine) were tailored by changing the oxidation state of the pyrazine-based bridging ligand. While its neutral form mediates a weak antiferromagnetic (AF) coupling between the two S = 1 Ni(II), its reduced form, tphz, promotes a remarkably large ferromagnetic exchange of +214(5) K with Ni(II) spins. Reducing twice the bridging ligand affords weak Ni-Ni interactions, in marked contrast to the Co(II) analogue.

Persistent Solid-State Phosphorescence and Delayed Fluorescence at Room Temperature by a Twisted Hydrocarbon.

The dehydrating cyclotrimerization of 1-tetralone in the presence of titanium tetrachloride at high temperatures leads to homotruxene, a nonplanar arene in which the twist angles between its three outer benzene rings and the central benzene are stabilized by ethylene bridges. This non-planar configuration allows for pronounced spin-orbit coupling and a high triplet energy, leading to room-temperature phosphorescence in air with a lifetime of 0.38 s and a quantum yield of 5.

Cyclic tris-[5]helicenes with single and triple twisted Möbius topologies and Möbius aromaticity.

A number of singly (180°) twisted, largely single-stranded and thus conformationally rather fragile, Möbius molecules have been synthesized within the last 15 years, which are aromatic with 4 electrons, thus violating the Hückel rule. Annulenes with significantly higher twist ( 540°) that retain a full cyclic conjugation path have been elusive, mainly because of the high strain and loss of orbital overlap. Recently, a topological strategy was devised to project the "twist" into "writhe", thus reducing the strain.

Atomic Scale Evidence of the Switching Mechanism in a Photomagnetic CoFe Dinuclear Prussian Blue Analogue.

Molecular complexes based on Prussian Blue analogues have recently attracted considerable interest for their unique bistable properties combined to ultimately reduced dimensions. Here, we investigate the first dinuclear FeCo complex exhibiting both thermal and photomagnetic bistability in the solid state. Through an experimental and theoretical approach combining local techniques-X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and ligand field multiplet calculations-we were able to evidence the changes occurring at the atomic scale in the electronic and magnetic properties.

Magnetic Bistability in Crystalline Organic Radicals: The Interplay of H-bonding, Pancake Bonding, and Electrostatics in 4-(2'-Benzimidazolyl)-1,2,3,5-dithiadiazolyl.

The neutral radical 4-(2'-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA) exhibits a first order phase transition around 270 K without symmetry breaking, preserving its orthorhombic Pbca space group between 340 and 100 K. Associated with this reversible single-crystal-to-single-crystal phase transition, thermal hysteresis of the magnetic susceptibility is observed. The low temperature (LT) phase is diamagnetic owing to pancake bonding between the π-radicals.

Spin State Chemistry: Modulation of Ligand p K by Spin State Switching in a [2×2] Iron(II) Grid-Type Complex.

The iron(II) [2×2] grid complex Fe-8H has been synthesized and characterized. It undergoes spin-crossover (SCO) upon deprotonation of the hydrazine-based terpyridine-like ligand. The deprotonation patterns have been determined by X-ray crystallography and H NMR spectroscopy and discussed in relation to the spin state of the iron(II) centers, which influences greatly the p K of the ligand.

Molecule-based microelectromechanical sensors.

Incorporating functional molecules into sensor devices is an emerging area in molecular electronics that aims at exploiting the sensitivity of different molecules to their environment and turning it into an electrical signal. Among the emergent and integrated sensors, microelectromechanical systems (MEMS) are promising for their extreme sensitivity to mechanical events. However, to bring new functions to these devices, the functionalization of their surface with molecules is required.

A Redox-Active Bridging Ligand to Promote Spin Delocalization, High-Spin Complexes, and Magnetic Multi-Switchability.

A dinuclear Co complex, [Co (tphz)(tpy) ] (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox-active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin-crossover with T ≈470 K for the pristine compound (n=4) to single-molecule magnet with an S =5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high-spin species even at room temperature.

Isomeric Column-Forming Esters and Imides with Varying Curvatures of the Aromatic Plane.

Dibenzo[a,j]coronene-tetracarboxylic alkyl esters and imides with either a centrosymmetric bis-peri substitution pattern or a polar bis-ortho substitution pattern form hexagonal columnar mesophases, which in the case of the imides persist at room temperature. The bis-peri isomers are obtained via a two-fold oxidative photocyclization; the bis-ortho isomers are accessed via a glyoxylic Perkin reaction of triphenylene and naphthalene building blocks. Steric congestion between the substituents and the adjacent benzo protrusion in the bis-ortho esters and imides leads to bending of the aromatic plane, which thus avoids twisting.