RuIIMultinuclear Metallosupramolecular Rack-Type Architectures of Polytopic Hydrazone-Based Ligands: Synthesis, Structural Features, Absorption Spectra, Redox Behavior, and Near-Infrared Luminescence

Modulation of the structural information in shape-defined heterocyclic strands: the case of a (pyridine-hydrazone)2pyrazine ligand

Metal ions (Ag⁺, Cd²⁺, Eu³⁺, Sm³⁺) and protons can, through coordination and protonation, modulate in three specific ways the structural information contained in the pyrazine-based heterocyclic strand L (obtained from 2,5-bis(methylhydrazino)pyrazine and 2 equivalents of 2-pyridinecarboxaldehyde), thus generating two linear rod-like conformations and a bent one. This conformational diversity is associated with a structural one that consists of two diprotonated forms (H₂L(PF₆)₂ and H₂L(CF₃SO₃)₂), a polymeric architecture [AgL]_n(CF₃SO₃)_n, two rack-like complexes ([Eu₂H₂L₃(CF₃SO₃)₆](PF₆)₂ and [Sm₂H₂L₃(CF₃SO₃)₆](PF₆)₂) and a grid-like structure ([Cd₄L₄](CF₃SO₃)₈).

Recent developments in supramolecular complexes of azabenzenes containing one to four N atoms: synthetic strategies, structures, and magnetic properties

For the last couple of decades, azabenzene-based ligands have drawn much attention from inorganic chemists due to their ability to coordinate with different metal ions to form supramolecular clusters. These azabenzenes are weak σ donors and strong π acceptors and electron-deficient. Metallogrid complexes and non-grid oligomers are well-defined supramolecular clusters, formed by appropriate chelating ligands, and can show interesting optical, magnetic, and electronic properties. Self-assembly of [n × n] metallogrid complexes is dominated by the entropic factor while the formation of oligonuclear metal ion complexes is dominated by other effects like CFSE, electrostatic factors, ligand conformational characters, etc. Herein, the present article gives an overview of six-membered heterocyclic azine-based ligands and their potential for different metal ions to form polynuclear complexes. Moreover, their temperature-dependent magnetic properties and SCO phenomena are well described and tabulated.

Photoinduced Intercomponent Processes in Selectively Addressable Bichromophoric Dyads Made of Linearly Arranged Ru(II) Terpyridine and Expanded Pyridinium Components

Three new linearly arranged bichromophoric systems 1-3 have been prepared, and their photophysical properties have been studied, taking also advantage of femtosecond pump-probe transient absorption spectroscopy. The three compounds contain the same chromophores, that is a Ru(II)-terpy-like species and a fused expanded bipyridinium (FEBP) unit, separated by three different, variously methylated biphenylene-type bridges. The chromophores have been selected to be selectively addressable, and excitation involving the Ru-based or the FEBP-based dyes results in different excited-state decays. Upon Ru-based excitation at 570 nm, oxidative photoinduced electron transfer (OPET) takes place in 1-3 from the ³MLCT state; however, the charge-separated species does not accumulate, indicating that the charge recombination rate constant exceeds the OPET rate constant. Upon excitation of the organic dye at 400 nm, the FEBP-based ¹π-π* level is prepared, which undergoes a series of intercomponent decay events, including (i) electron-exchange energy transfer leading to the MLCT manifold (SS-EnT), which successively decays according to 570 nm excitation, and (ii) reductive photoinduced electron transfer (RPET), leading to the preparation of the charge-separated (CS) state. Reductive PET, involving the FEBP-based singlet state, is much faster than oxidative PET, involving the MLCT triplet state, essentially because of driving force reasons. The rate constant of CR is intermediate between the rate constants of OPET and RPET, and this makes 1-3 capable to selectively read the 400 nm excitation as an active input to prepare the CS state, whereas excitation at wavelengths longer than 480 nm is inefficient to accumulate the CS state. Moreover, intriguing differences between the rate constants of the various processes in 1-3 have been analyzed and interpreted according to the superexchange theory for electron transfer. This allowed us to uncover the role of the electron-transfer and hole-transfer superexchange pathways in promoting the various intercomponent photoinduced decay processes occurring in 1-3.

Self-Assembly of Cyclohelicate [M3 L3 ] Triangles Over [M4 L4 ] Squares, Despite Near-Linear Bis-terdentate L and Octahedral M

Self-assembly of 1:1:2 M^II (BF₄ )₂ (M=Zn or Fe), pyrazine-2,5-dicarbaldehyde (1) and 2-(2-aminoethyl)pyridine gave trimetallic triangle architectures rather than the anticipated tetrametallic [2×2] squares. Options for the nontrivial synthesis of 1 are considered, and synthetic details provided for both preferred routes. Rare cyclohelicate triangle architectures are observed for the pair of structurally characterized yellow-brown [Zn₃ L₃ ](BF₄ )₆ and dark green [Fe₃ L₃ ](BF₄ )₆ complexes of the neutral bis-terdentate Schiff base L. In order to form these pyrazine-edged triangles, the octahedral metal ions-with all 6 N-donors provided by the terdentate binding pockets of two L-are located 0.4-0.5 Å out of the plane of the bridging pyrazines, towards the center of the triangle. Density functional theory calculations confirm that simple particle counting entropic arguments, which predict triangles over squares, are correct here, with the triangles shown to be energetically favored over the corresponding squares. However, importantly, DFT analysis of these and related triangle versus square systems also show that vibrational contributions to entropy dominate and may significantly influence the preferred architecture, such that simple particle counting cannot in general be reliably employed to predict the observed architecture.

pH-Switchable "Off-On-Off" Near-Infrared Luminescence Based on a Dinuclear Ruthenium(II) Complex

The pH-switchable room-temperature near-infrared (NIR) phosphorescence emission based on ruthenium(II) polypyridyl complexes has been very rarely reported, even though it is very desirable for applications in sensing, switching, and logic molecular devices and bioimaging. Here we report a novel dinuclear ruthenium(II) complex in an aerated acetonitrile solution featuring a bright NIR emission centered at 760 nm with an absolute quantum yield of 1.03%, a large Stokes shift of 254 nm, and a long emission lifetime of 108.3 ± 0.4 ns. The complex in a Britton-Roberson buffer also exhibited pH-induced "off-on-off" NIR luminescent switches with a maximum intensity enhancement factor of 41 and one of the switching events occurring near the physiological pH range.

Synthesis and characterization of (6-{[2-(pyridin-2-yl)hydrazinylidene]methyl}pyridin-2-yl)methanol: a supramolecular and topological study

Hydrazones exhibit a versatile chemistry and are of interest for their potential use as functional molecular systems capable of undergoing reversible changes of configuration, i.e. E/Z isomerization. The title compound, C12H12N4O, has an E configuration with respect to the hydrazone C=N bond. The crystal packing is formed by N-H...N and O-H...N hydrogen bonds that give a two-dimensional layer structure and C-H...C interactions associated with layer stacking to produce the three-dimensional supramolecular structure. These intermolecular interactions were analyzed and quantified by the Hirshfeld surface method and the two-dimensional supramolecular arrangement was topologically simplified as a hcb network.

Interplay between singlet and triplet excited states in a conformationally locked donor–acceptor dyad

The synthesis and photophysical characterization of a palladium(ii) porphyrin – anthracene dyad bridged via short and conformationally rigid bicyclo[2.2.2]octadiene spacer were achieved.

Photophysical properties of an unusual bichromophoric species constructed from a cyclometalated Pt(ii) chromophore and a blue Bodipy-acetylacetonate species

Complex 1 exhibits a formally forbidden energy transfer from the metal-based triplet to the fluorescent Bodipy singlet state.

Dichloridobis[2-(1-hydrazinylideneeth-yl)pyrazine-κN(1)]zinc

In the structure of the title complex, [ZnCl(2)(C(6)H(8)N(4))(2)], the Zn(II) atom has a distorted octa-hedral geometry. Two cis Cl(-) ions and four N atoms belonging to two different 2-(1-hydrazinylideneeth-yl)pyrazine ligands coordinate the Zn(II) atom, forming two five-membered Zn-N-C-C-N rings. The dihedral angle between the planes of these metallocycles is 88.13 (4)°. The organic ligands are essentially planar (r.m.s. deviations from planarity = 0.072 and 0.040 Å). Inter-molecular N-H⋯N and N-H⋯Cl inter-actions join the mol-ecules into a three-dimensional framework.

Design, synthesis, and traveling wave ion mobility mass spectrometry characterization of iron(II)- and ruthenium(II)-terpyridine metallomacrocycles

New metallomacrocycles composed of 2,2':6',2″-terpyridine (tpy) ligands and Ru(II) or Fe(II) transition metal ions were prepared by stepwise directed assembly and characterized by 2D diffusion NMR spectroscopy (DOSY), electrospray ionization traveling wave ion mobility mass spectrometry (ESI TWIM MS), and molecular modeling. The supramolecular polymers synthesized include a homonuclear all-Ru hexamer as well as heteronuclear hexamer and nonamer with alternating Ru/Ru/Fe metal centers. ESI MS yields several charge states from each supramacromolecule. If ESI is interfaced with TWIM MS, overlapping charge states and the isomeric components of an individual charge state are separated based on their unique drift times through the TWIM region. From experimentally measured drift times, collision cross-sections can be deduced. The collision cross-sections obtained for the synthesized supramacromolecules are in good agreement with those predicted by molecular modeling for macrocyclic structures. Similarly, the hydrodynamic radii of the synthesized complexes derived from 2D DOSY NMR experiments agree excellently with the radii calculated for macrocyclic architectures, confirming the ESI TWIM MS finding. ESI TWIM MS and 2D DOSY NMR spectroscopy provide an alternative approach for the structural analysis of supramolecules that are difficult or impossible to crystallize, such as the large macrocyclic assemblies investigated. ESI TWIM MS will be particularly valuable for the characterization of supramolecular assemblies not available in the quantity or purity required for NMR studies.