Structural and magnetic modulation of a purely organic open framework by selective guest inclusion

Exploring the Luminescence, Redox, and Magnetic Properties in a Multivariate Metal-Organic Radical Framework

Persistent neutral organic radicals are excellent building blocks for the design of functional molecular materials due to their unique electronic, magnetic, and optical properties. Among them, triphenylmethyl radical derivatives have attracted a lot of interest as luminescent doublet emitters. Although neutral organic radicals have been underexplored as linkers for building metal-organic frameworks (MOFs), they hold great potential as organic elements that could introduce additional electronic properties within these frameworks. Herein, we report the synthesis and characterization of a novel multicomponent metal-organic radical framework (PTMTCR@NR-Zn MORF), which is constructed from the combination of luminescent perchlorotriphenylmethyl tricarboxylic acid radical (PTMTCR) and nonemissive nonradical (PTMTCNR) organic linkers and Zn(II) ions. The PTMTCR@NR-Zn MORF structure is layered with microporous one-dimensional channels embedded within these layers. Kelvin probe force microscopy further confirmed the presence of both organic nonradical and radical linkers in the framework. The luminescence properties of the PTMTCR ligand (first studied in solution and in the solid state) were maintained in the radical-containing PTMTCR@NR-Zn MORF at room temperature as fluorescence solid-state quenching is suppressed thanks to the isolation of the luminescent radical linkers. In addition, magnetic and electrochemical properties were introduced to the framework due to the incorporation of the paramagnetic organic radical ligands. This work paves the way for the design of stimuli-responsive hybrid materials with tunable luminescence, electrochemical, and magnetic properties by the proper combination of closed- and open-shell organic linkers within the same framework.

Energy-Efficient Iodine Uptake by a Molecular Host⋅Guest Crystal

Recently, porous organic crystals (POC) based on macrocycles have shown exceptional sorption and separation properties. Yet, the impact of guest presence inside a macrocycle prior to adsorption has not been studied. Here we show that the inclusion of trimethoxybenzyl-azaphosphatrane in the macrocycle cucurbit[8]uril (CB[8]) affords molecular porous host⋅guest crystals (PHGC-1) with radically new properties. Unactivated hydrated PHGC-1 adsorbed iodine spontaneously and selectively at room temperature and atmospheric pressure. The absence of (i) heat for material synthesis, (ii) moisture sensitivity, and (iii) energy-intensive steps for pore activation are attractive attributes for decreasing the energy costs. 1 H NMR and DOSY were instrumental for monitoring the H2 O/I2 exchange. PHGC-1 crystals are non-centrosymmetric and I2 -doped crystals showed markedly different second harmonic generation (SHG), which suggests that iodine doping could be used to modulate the non-linear optical properties of porous organic crystals.

Understanding the g-tensors of perchlorotriphenylmethyl and Finland-type trityl radicals

The 285 GHz EPR spectra of perchlorotriphenylmethyl and tetrathiatriarylmethyl radicals in frozen solution have been accurately measured. The relationship between their molecular structures and their g-tensors has been investigated with the aid of DFT calculations, revealing that the degree of spin density delocalization away from the central methylene carbon is an important determining factor of the g-anisotropy. In particular, the small amount of spin densities on the Cl or S heteroatoms at the 2 and 6 positions with respect to the central carbon have the strongest influence. Furthermore, the amount of spin densities on these heteroatoms and thus the anisotropy can be modulated by the protonation (esterification) state of the carboxylate groups at the 4 position. These results provide unique insights into the g-anisotropy of persistent trityl radicals and how it can be tuned.

Trityl radicals in perfluorocarbon emulsions as stable, sensitive, and biocompatible oximetry probes

EPR oximetry with the use of trityl radicals can enable sensitive O₂ measurement in biological cells and tissues. However, in vitro cellular and in vivo biological applications are limited by rapid trityl probe degradation or biological clearance and the need to enhance probe O₂ sensitivity. We synthesized novel perfluorocarbon (PFC) emulsions, ∼200nm droplet size, containing esterified perchlorinated triphenyl methyl (PTM) radicals dispersed in physiological aqueous buffers. These formulations exhibit excellent EPR signal stability, over 20-fold greater than free PTM probes, with high oxygen sensitivity ∼17mG/mmHg enabling pO₂ measurement in aqueous solutions or cell suspensions with sensitivity >0.5mmHg. Thus, PFC-PTM probes hold great promise to enable combined O₂ delivery and sensing as needed to restore or enhance tissue oxygenation in disease.

Hydrogen bonding, π-π stacking and van der Waals forces-dominated layered regions in the crystal structure of 4-amino-pyridinium hydrogen (9-phosphono-non-yl)phospho-nate

The asymmetric unit of the title mol-ecular salt, [C5H7N2+][(HO)2OP(CH2)9PO2(OH)-], consists of one 4-amino-pyridinium cation and one hydrogen (9-phos-phono-non-yl)phospho-nate anion, both in general positions. As expected, the 4-amino-pyridinium moieties are protonated exclusively at their endocyclic nitro-gen atom due to a mesomeric stabilization by the imine form which would not be given in the corresponding double-protonated dicationic species. In the crystal, the phosphonyl (-PO3H2) and hydrogen phospho-nate (-PO3H) groups of the anions form two-dimensional O-H⋯O hydrogen-bonded networks in the ab plane built from 24-membered hydrogen-bonded ring motifs with the graph-set descriptor R66(24). These networks are pairwise linked by the anions' alkyl-ene chains. The 4-amino-pyridinium cations are stacked in parallel displaced face-to-face arrangements and connect neighboring anionic substructures via medium-strong charge-supported N-H⋯O hydrogen bonds along the c axis. The resulting three-dimensional hydrogen-bonded network shows clearly separated hydro-philic and hydro-phobic structural domains.

An Ideal Detector Composed of Two-Dimensional Cd(II)-Triazole Frameworks for Nitro-Compound Explosives and Potassium Dichromate

The two-dimensional (2D) metal-organic framework (MOF) [Cd(TPTZ)(H2O)2(HCOOH)(IPA)2]n (1; TPTZ = {4-[4-(1H-1,2,4-triazol-1-yl)phenyl]phenyl}-1H-1,2,4-triazole, IPA = isophthalic acid) has been constructed with the π-electron-rich aromatic ligand TPTZ, auxiliary ligand IPA, and the metal Cd(2+) ion with a d(10) configuration under solvothermal conditions. Complex 1 exhibits a strong ligand-originated photoluminescence emission, which is selectively sensitive toward electron-deficient nitroaromatic compounds, such as nitrobenzene (NB), 1,3-dinitrobenzene (m-DNB), and 1,4-dinitrobenzene (p-DNB), and nitro-aliphatic compounds, such as nitromethane (NM) and tris(hydroxymethyl)nitromethane. This property makes complex 1 a potential fluorescence sensor for these chemicals. Single-crystal X-ray diffraction studies revealed that dinuclear cadmium building units were further bridged by TPTZ ligands to give a four-connected uninodal net with the Schläfli symbol of [4.6(3).4.6(3).6(2).6(4)].

Organic multishell isostructural host-guest crystals: fullerenes C(60) inside a nitroxide open framework

The dinitroxide biradical crystallizes forming hexagonal open frameworks with one-dimensional corrugated channels filled with crystallization solvent. The large pockets constitutive of the channels allowed the inclusion of C(60) in the paramagnetic network. The rapidity and high fidelity of crystal growth were used to prepare isostructural multilayer host-guest crystals successively stained with C(60).

The perchlorotriphenylmethyl (PTM) radical

In spite of the considerable understanding and development of perchlorotriphenylmethyl (PTM) radical derivatives, the preparation of crystals of the pure unsubstituted PTM radical, C19Cl15, suitable for single-crystal X-ray diffraction has remained a challenge since its discovery, and only two studies dealing with the crystal structure of the unsubstituted PTM radical have been published. In one study, the radical forms clathrates with aromatic solvents [Veciana, Carilla, Miravitlles & Molins (1987).J. Chem. Soc. Chem. Commun.pp. 812–814], and in the other the structure was determinedab initiofrom powder X-ray diffraction data [Rius, Miravitlles, Molins, Crespo & Veciana (1990).Mol. Cryst. Liq. Cryst.187, 155–163]. We report here the preparation of PTM crystals for single-crystal X-ray diffraction and their resolution. The structure, which shows monoclinic symmetry (C2/c), revealed a nonsymmetric molecular propeller conformation (D3symmetry) caused by the steric strain between theortho-Cl atoms, which protect the central C atom (sp2-hybridization and major spin density) and give high chemical and thermal persistence to the PTM. The supramolecular structure of PTM shows short Cl...Cl intermolecular interactions and can be described in terms of layers formed by rows of molecules positioned in a head-to-tail manner along thecaxis.

Playing with organic radicals as building blocks for functional molecular materials

The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals-nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl-so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references).

Three-dimensional porous metal-radical frameworks based on triphenylmethyl radicals

Lanthanide coordination polymers {[Ln(PTMTC)(EtOH)(2)H(2)O]·x H(2)O, y EtOH} [Ln=Tb (1), Gd (2), and Eu (3)] and {[Ln(αH-PTMTC)(EtOH)(2)H(2)O]·x H(2)O, y EtOH} [Ln=Tb (1'), Gd (2'), and Eu (3')] have been prepared by reacting Ln(III) ions with tricarboxylate-perchlorotriphenylmethyl/methane ligands that have a radical (PTMTC(3-)) or closed-shell (αH-PTMTC(3-)) character, respectively. X-ray diffraction analyses reveal 3D architectures that combine helical 1D channels and a fairly rare (6,3) connectivity described with the (4(2).8)·(4(4).6(2).8(5).10(4)) Schäfli symbol. Such 3D architectures make these polymers porous solids upon departure of the non-coordinated guest-solvent molecules as confirmed by the XRD structure of the guest-free [Tb(PTMTC)(EtOH)(2)H(2)O] and [Tb(αH-PTMTC)(EtOH)(2)H(2)O] materials. Accessible voids represent 40% of the cell volume. Metal-centered luminescence was observed in Tb(III) and Eu(III) coordination polymers 1' and 3', although the Ln(III)-ion luminescence was quenched when radical ligands were involved. The magnetic properties of all these compounds were investigated, and the nature of the {Ln-radical} (in 1 and 2) and the {radical-radical} exchange interactions (in 3) were assessed by comparing the behaviors for the radical-based coordination polymers 1-3 with those of the compounds with the diamagnetic ligand set. Whilst antiferromagnetic {radical-radical} interactions were found in 3, ferromagnetic {Ln-radical} interactions propagated in the 3D architectures of 1 and 2.

Polychlorinated trityl radicals for dynamic nuclear polarization: the role of chlorine nuclei

Polychlorinated trityl radicals bearing carboxylate substituents are water soluble persistent radicals that can be used for dynamic nuclear polarization. In contrast to other trityl radicals, the polarization mechanism differs from the classical solid effect. DFT calculations performed to rationalize this behaviour support the hypothesis that polarization is transferred from the unpaired electron to chlorine nuclei and from these to carbon by spin diffusion. The marked differences observed between neutral and anionic forms of the radical will be discussed.