Phospha-cyanines in Their Ideal Polymethine State: Synthesis and Structure-Property Relationships.

We report the synthesis and full characterization of a family of phosphorus-containing polymethine cyanines (phospha-cyanines). The compounds are easily prepared in two steps, starting from readily available phosphanes. The impact of the P-substituents and the counterions on the structural and optical properties is investigated through a joint experimental/theoretical approach. Based on the study of the single-crystal X-ray diffraction structures, all phospha-cyanines present a bond length alternation close to zero, independently of the substituents and counterions, which indicates an ideal polymethine state. All these compounds display the typical cyanine-like UV-vis absorption with an intense and sharp transition with a vibronic shoulder despite possessing a reverse electronic configuration compared to "classical" cyanines. Time-dependent density-functional theory calculations allowed us to fully rationalize the optical properties (absorption/emission wavelengths, luminescence quantum yields). Interestingly, due to the tetrahedral shape of the P atom, the optical properties are independent of the counterion, which is in marked contrast with N-analogues, which enables predictive engineering of the phospha-cyanines regardless of the medium in which they are used.

Thiophene-Bipyridine Appended Diketopyrrolopyrrole Ligands and Platinum(II) Complexes.

Straightforward palladium(II) catalyzed direct cross-coupling reaction between decyl, (S)-2-methyl-butyl, and dodecyl N-substituted diketopyrrolopyrrole thiophene (DPPT), including a 3-methoxy-thiophene derivative, and 6-bromo-2,2'-bipyridine afforded a series of mono- and bis-bipyridine substituted DPPT ligands 1-3. Complexation reactions with PtCl2(DMSO)2 provided ortho-metalated platinum(II) complexes 1-Pt and 2-Pt, together with the N^N^O complex 3d-Pt(N^N^O) resulted from the O-Me activation of the intermediary complex 3d-Pt(N^N). The ligand 1b and the mononuclear complexes 1a-Pt and 1b-Pt have been structurally characterized by single crystal X-ray structure, evidencing the establishment of numerous intermolecular π-π interactions in the solid state. Moreover, in the crystal structure of the model complex DMTB-Pt(N^N^O) (DMTB = 3,4-dimethoxy-(2,2'-bipyridine)) the chelating tridentate N^N^O mode is clearly evidenced. The chiral ligand 1b and its mononuclear complex 1b-Pt do not show any CD signal in solution, but they are CD active in the solid state with bisignate bands in the low energy region, opposite in sign between the ligand and the complex, suggesting helical supramolecular arrangement of the dpp chromophore in the solid state. Photophysical investigations demonstrate that all of the ligands are fluorescent with high quantum yields, while the emission is quenched for the complexes, except partially in 3d-Pt(N^N), very likely through an intersystem crossing mechanism promoted by the heavy metal. Density functional theory calculations support the differences observed between the absorption properties of the ligands, ortho- and non-ortho-metalated complexes. The highly fluorescent bipyridine ligands reported herein open the way toward multifunctional transition metal complexes and their use in organic electronics.

Hysteresis Photomodulation via Single-Crystal-to-Single-Crystal Isomerization of a Photochromic Chain of Dysprosium Single-Molecule Magnets.

A one-dimensional coordination solid 1c is synthesized by reaction of a bispyridyl dithienylethene (DTE) photochromic unit with the highly anisotropic dysprosium-based single-molecule magnet [Dy(Tppy)F(pyridine)2]PF6. Slow magnetic relaxation characteristics are retained in the chain compound 1c, and photoisomerization of the bridging DTE ligand induces a single-crystal-to-single-crystal transformation that can be monitored using photocrystallography. Notably, the resulting chain compound 1o exhibits faster low-temperature relaxation than that of 1c, which is apparent in magnetic hysteresis data collected for both compounds as high as 4 K. Ab initio calculations suggest that this photomodulation of the magnetic relaxation behavior is due to crystal packing changes rather than changes to the crystal field splitting upon ligand isomerization.

Cationic Two-Photon Lanthanide Bioprobes Able to Accumulate in Live Cells.

An original cationic water-soluble cyclen-based Eu(III) complex [EuL(1)](+) featuring a chromophore-functionalized antenna to increase the two-photon (2P) absorption properties was synthesized. The photophysical properties were thoroughly studied in various solvents and rationalized with the help of theoretical calculations. The complex exhibits an optimized 2P absorption cross section. Finally, 2P microscopy imaging experiments on living T24 human cancer cells highlighted the spontaneous internalization and the biological stability of this 2P bioprobe in vitro. Macrocyclic-based antennas open new perspectives for future optimization of the photophysical properties and allows envisaging the design of Eu, Tb, Yb, and Sm bioprobes. This result also opens the way for the design of functional two-photon Ln complexes able to monitor intracellular physicochemical parameters.

Mechanism of Magnetostructural Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from ab Initio Quantum Chemistry Calculations.

The gradual magnetostructural transition in breathing crystals based on copper(II) and pyrazolyl-substituted nitronyl nitroxides has been analyzed by means of DDCI quantum chemistry calculations. The magnetic coupling constants (J) within the spin triads of Cu(hfac)2L(Bu)·0.5C8H18 have been evaluated for the X-ray structures reported at different temperatures. The coupling is strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. The intercluster magnetic coupling (J') is antiferromagnetic and shows a marked dependence on temperature. The magnetostructural transition can be reproduced using the calculated J values for each structure in the simulation of the magnetic susceptibility. However, the µ(T) curve can be improved nicely by considering the coexistence of two phases in the transition region, whose ratio varies with temperature corresponding to both the weakly and strongly coupled spin states. These results complement a recent VT-FTIR study on the parent Cu(hfac)2L(Pr) compound with a gradual magnetostructural transition.

Calculation of magnetic couplings in hydrogen-bonded Cu(II) complexes using density functional theory.

The performance of recent density functionals for computation of molecular magnetic coupling constants (J) in hydrogen-bonded systems is evaluated. A survey of six Cu(II) dinuclear complexes is considered. The global accuracy trend is GGAs

Addressing Through-H Magnetic Interactions: A Comprehensive ab Initio Analysis of This Efficient Coupler.

The exchange coupling in a structuraly characterized Cu(II)2 complex is analyzed to highlight the role of H bonds in the generation of efficient magnetic interactions. The interest for complementary insights which are not accessible through DFT calculations (Desplanches, C. et al. J. Am. Chem. Soc. 2002, 124, 5197) has driven this state-of-the-art ab initio inspection. The wave function expansion based upon localized orbitals allows us to selectively turn on specific mechanisms and quantitatively evaluate their roles in the exchange interactions. Our singlet-triplet splitting calculations demonstrate the enhancement of the magnetic coupling through a concerted oxygen-to-metal charge transfer and electronic redistribution within the OH bond of the OH···O magnetic linker. This mechanism accounts for ∼35% of the total experimentally measured singlet-triplet energy difference. This analysis strongly suggests that H bonds might be particularly useful not only in the establishment of intermolecular contacts but also within the basic units of magnetic materials.

Ruthenium complexes of C,C'-bis(ethynyl)carboranes: an investigation of electronic interactions mediated by spherical pseudo-aromatic spacers.

The complexes [Ru(1-C[triple bond]C-1,10-C2B8H9)(dppe)Cp*] (3a), [Ru(1-C[triple bond]C-1,12-C2B10H11)(dppe)Cp*] (3b), [{Ru(dppe)Cp*}2{mu-1,10-(C[triple bond]C)2-1,10-C2B8H8}] (4a) and [{Ru(dppe)Cp*}2{mu-1,12-(C[triple bond]C)2-1,12-C2B10H10}] (4b), which form a representative series of mono- and bimetallic acetylide complexes featuring 10- and 12-vertex carboranes embedded within the diethynyl bridging ligand, have been prepared and structurally characterized. In addition, these compounds have been examined spectroscopically (UV-vis-NIR, IR) in all accessible redox states. The significant separation of the two, one-electron anodic waves observed in the cyclic voltammograms of the bimetallic complexes 4a and 4b is largely independent of the nature of the electrolyte and is attributed to stabilization of the intermediate redox products [4a]+ and [4b]+ through interactions between the metal centers across a distance of ca. 12.5 A. The mono-oxidized bimetallic complexes [4a]+ and [4b]+ exhibit spectroscopic properties consistent with a description of these species in terms of valence-localized (class II) mixed-valence compounds, including a unique low-energy electronic absorption band, attributed to an IVCT-type transition that tails into the IR region. DFT calculations with model systems [4a-H]+ and [4b-H]+ featuring simplified ligand sets reproduce the observed spectroscopic data and localized electronic structures for the mixed-valence cations [4a]+ and [4b]+.

Antiferromagnetic behavior based on quasi-orthogonal MOs: synthesis and charaterization of a Cu3 oxidase model.

We report the synthesis and structural and magnetic characterization of an original Cu(3) oxidase model. The Schiff base ligand used in the synthesis derives from condensation of acetylacetone with glycine amino acid. The K[Cu(3)(L)(3)(micro(3)-OH)].(H(2)O)(2) complex crystallizes at room temperature in the tetragonal P43212 space group with a = 20.540(3) A and c = 15.866(6) A and consists of triangular Cu(3) units. The magnetic behavior interpretation suggests the presence of spin frustration, which has been investigated by means of ab initio DDCI calculations. It is shown that the system should be viewed as a "ménage à trois" spin-coupled pattern mediated by a central hydroxo group, lifting the doublet degeneracy by approximately 8 cm-1.