Coplanar Dimers Research Articles

Glass Disorder Modulated Luminescence in Zero-Dimensional Antimony-Chloride Coplanar Dimers for Optical Anti-counterfeiting.

Zero-dimensional metal halides have received wide attention due to their structural diversity, strong quantum confinement, and associated excellent photoluminescence properties. A reversible and tunable luminescence would be desirable for applications such as anti-counterfeiting, information encryption, and artificial intelligence. Yet, these materials are underexplored, with little known about their luminescence tuning mechanisms. Here we report a pyramidal coplanar dimer, (TBA)Sb2Cl7 (TBA = tetrabutylammonium), showing broadband emission wavelength tuning (585-650 nm) by simple thermal treatment. We attribute the broad color change to structural disorder induced by varying the heat treatment temperatures. Increasing the heating temperature transitions the material from long-range ordered crystalline phase to highly disordered glassy phase. The latter exhibits stronger electron-phonon coupling, enhancing the self-trapped exciton emission efficiency. The work provides a new material platform for manifold optical anti-counterfeiting applications and sheds light on the emission color tuning mechanisms for further design of stimuli-responsive materials.

Two metal-free perovskite molecules with different 3D frameworks show reversible phase transition, dielectric anomaly and SHG effect.

Three-dimensional (3D) hybrid organic-inorganic perovskites (HOIPs) have attracted tremendous research interest due to their unique structure and promising applications. However, research on the design, synthesis and properties of this kind of metal-free crystalline material is still in the exploratory stage. Herein, two 3D perovskite molecules [1,4-3.2.2-dabcn]NH4Br3 (1) and [1,4-3.2.2-dabcn]NH4I3·0.5H2O (2) were obtained by reacting 1,4-diazabicyclo[3.2.2]nonane (1,4-3.2.2-dabcn) with NH4X (X = Br and I) in the corresponding concentrated halogen acids. The single X-ray diffraction results demonstrated that the inorganic framework structures in compounds 1 and 2 constructed with NH4Br and NH4I are completely different, caused by the radius of the bromide ion being smaller than that of the iodide ion. The 3D framework of compound 1 is constructed with a coplanar dimer [(NH4)2Br6]2- as the basic building unit, leading to the expanded 3D perovskite framework structure with a larger cavity to accommodate the 1,4-3.2.2-dabcn molecule. Nevertheless, compound 2 adopts a familiar 3D crystal framework structure with corner-sharing [(NH4)I6] octahedra, where the [1,4-3.2.2-dabcn] cations and water solvent molecule are confined in the cavities enclosed by the octahedra. Notably, both compounds exhibit reversible phase transition, dielectric anomaly and the second harmonic generation (SHG) effect. From the perspective of molecular design, this work is of great significance to guide the construction of new 3D metal-free perovskite molecular materials with reversible properties.

Interactions between temozolomide and guanine and its S and Se-substituted analogues

Temozolomide was paired with guanine, 6-selenoguanine, and 6-thioguanine, as well as the SH tautomer of the latter. The potential energy surface of each heterodimer was searched for all minima, using Dispersion-Corrected Density Functional Theory and MP2 methods. Among the dozens of minima, three categories were observed. Stacked geometries place the aromatic systems of the two molecules parallel to one another, while the two systems are roughly perpendicular to one another in a second category. Also found are coplanar structures held together by H-bonds. Dispersion proves to be a dominating attractive force for the stacked structures, less so for perpendicular, and smallest for the coplanar dimers. Geometries and energetics are relatively insensitive to S and Se substitution, but tautomerization reverses relative stabilities of different geometries.

Interactions of Nucleic Acid Bases with Temozolomide. Stacked, Perpendicular, and Coplanar Heterodimers.

Temozolomide (TMZ) was paired with each of the five nucleic acid bases, and the potential energy surface searched for all minima, in the context of dispersion-corrected density functional theory and MP2 methods. Three types of arrangements were observed, with competitive stabilities. Coplanar H-bonding structures, reminiscent of Watson-Crick base pairs were typically the lowest in energy, albeit by a small amount. Also very stable were perpendicular arrangements that included one or more H-bonds. The two monomers were stacked approximately parallel to one another in the third category, some of which contained weak and distorted H-bonds. Dispersion was found to be a dominating attractive force, largest for the stacked structures, and smallest for the coplanar dimers.

Hydrogen bonded and stacked geometries of the temozolomide dimer.

Dispersion-corrected density functional theory (DFT) and MP2 quantum chemical methods are used to examine homodimers of temozolomide (TMZ). Of the 12 dimer configurations found to be minima, the antarafacial stacked dimer is the most favored, it is lower in energy than coplanar dimers which are stabilized by H-bonds. The comparison between B3LYP and B3LYP-D binding energies points to dispersion as a primary factor in stabilizing the stacked geometries. CO(π) → CO(π*) charge transfers between amide groups in the global minimum are identified by NBO, as well as a pair of weak CH∙∙N H-bonds. AIM analysis of the electron density provides an alternative description which includes N∙∙O, N∙∙N, and C∙∙C noncovalent bonds.

Combined Effect of Hydrogen Bonding and π···π Stacking Interactions in the Assembly of Indium(III) Metal–Organic Materials: Structure-Directing and Aggregation-Induced Emission Behavior

Six novel luminescent materials In(III)-2,3-H2qldc/phen metal–organic assemblies (MOAs), namely, [In2(phen)3Cl6]·CH3CN (In1), [In2(phen)3Cl6]·CH3CN·2H2O (In2), [In2(phen)2Cl6] (In3), [In(3-qlc)2(phen)Cl] (In4), [In(3-qlc)2(2,2′-bipy)Cl]·2H2O (In5) and [In(3-qlc)3(phen)]·H2O (In6) (2,3-H2qldc = quinoline-2,3-dicarboxylic acid; 3-Hqlc = quinoline-3-carboxylic acid, which is formed from the decarboxylation reaction of 2,3-H2qldc; phen = 1,10-phenanthrolin; 2,2′-bipy = 2,2′-bipyridine) have been hydro(solvo)thermally synthesized and characterized by single-crystal X-ray diffraction, infrared (IR), elemental analysis, 1H NMR spectra, and thermogravimetric analysis (TGA). The crystal structures of In1–In6 indicate that the hydrogen bonding and π···π stacking interactions play critical roles in the formation of the extended supramolecular array. In1 displays the rare hexatomic ring 3D (4,5)-connected supramolecular architecture through twofold interpenetrating of 2D cell membrane-like structures. In2 exhibits a 3D 4-connected sra supramolecular architecture. In3 is built from binuclear [In2Cl2] clusters and possesses an interesting 2D (4,4)-connected sql supramolecular network. Due to existing hydrogen bonds forming coplanar dimers, In4 and In5 show more extended π-conjugated system and appear as 3D diamond-like supramolecular architectures with point symbol 66. In6 is coordinated with four almost vertical ligands and further form a 3D diamond-like supramolecular architecture via hydrogen bonding and π···π stacking interactions. Complexes In1–In6 exhibit tunable luminescence with emission maxima containing deep blue, blue, light blue, green, and yellow green at 298 and 77 K both in DMSO solvent and in the solid state. It is worth noting that In4 and In5 exhibit good aggregation-induced emission (AIE) properties in the solid state. Both of them show very weak luminescence in dimethyl sulfoxide (DMSO, good solvent) while their intensity increased enormously by the addition of water (H2O, poor solvent) due to aggregation.

2-Hydroxy-3-methoxybenzaldehyde (pyridine-2-carbonyl)hydrazone

The approximately planar mol­ecule of the title compound, C14H13N3O3, is in an E configuration with respect to the C=N bond, with a C—C=N—N torsion angle of −177.3 (2)°. There is an intra­molecular hydrogen bond involving the hydrazide N atom and the hydroxyl O atom on the benzene ring. Inter­molecular C—H⋯O weak inter­actions are also formed between two adjacent meth­oxy groups, linking two mol­ecules into a coplanar dimer. These dimers are further assembled into a zigzag framework along the [010] axis, using inter­molecular short C—H⋯O and long N—H⋯O hydrogen bonds, resulting in a two-dimensional supramolecular crystal structure.

The Nature of the Supramolecular Association of 1,2,5-Chalcogenadiazoles

Organochalcogen-nitrogen heterocycles such as the 1,2,5-chalcogenadiazoles have a distinct tendency to establish intermolecular links in the solid state through secondary bonding interactions E...N (E = S, Se, Te). The association of these molecules was examined in detail using relativistic density functional theory. Although there is an important electrostatic component, the interaction between these molecules is dominated by contributions arising from orbital mixing, which can be interpreted as the donation of a nitrogen lone pair into the chalcogen-centered antibonding orbitals. Because of its more polar character and lower-lying antibonding orbitals, the tellurium derivatives possess the strongest association energies; these are so large that the binding strength is comparable to that of some hydrogen bonds. In the absence of steric constraints, telluradiazoles associate in a coplanar fashion forming ribbon polymers. However, bulky susbstituents could be used to direct the formation of either helical chains or discrete dimers. In addition to its strength, the coplanar dimer is characterized by being rigid, yet no activation barrier is expected for the association/dissociation process. These attributes strongly indicate that tellurium-nitrogen heterocycles have great potential as building blocks in supramolecular architecture.

INTERMOLECULAR PROTON TRANSFER PROCESS IN SULFUR TAUTOMERIC HETEROCYCLES. I. A QUANTUM CHEMICAL APPROACH

Molecular structures of two tautomeric aromatic heterocycles, 2-mercaptobenzoxazole and 2-mercaptobenzothiazole, were analyzed from a quantum chemical approach by means of AM1 semiempirical molecular orbital calculations. Thiones are the preferential tautomeric structures of these compounds in solid phase and solutions, and the AM1 profiles of potential wells agree the experimental evidences. In spite of the good description of the thione and thiol structures by the AM1 approach, the experimental tautomerism observed at room temperature can not be explained by means of an intramolecular protonic transfer mechanism. The potential barrier AM1 calculations of both tautomers range from 40 to 50 kcal/mol, approximately. Therefore, we have postulated a tautomeric process through a geometrical model of co-planar dimers in these heterocyclic compounds. These dimers are more stable than the respective monomers and this model permit to assume a simultaneous intermolecular proton transfer process between sulfur and nitrogen centers in the thiocyanate (NCS) syntonic units. Our results give a coherent mechanistic explanation to several experimental observations in chemical reactions and spectroscopic data.

C-H ... O hydrogen bonding in solutions of methylated nucleic acid base analogs as revealed by NMR.

Formation and thermodynamic characteristics of C-H ... O hydrogen bonding of methylated uracils and caffeine have been studied by nmr along two lines. 1. The concentration and temperature dependencies of the PMR spectra of 1,3-dimethyluracil (m2 1,3Ura), 1,3-dimethylthymine (m2 1,3Thy), and 1,3,6-trimethyluracil (m3 1,3,6Ura) in chloroform at high concentrations of base analogs indicated the self-association of m2 1,3Ura and m2 1,3Thy via C(6)H ... O hydrogen bonding and the competitive formation of C-H ... O bonds between carbonyl oxygens and chloroform. The intermolecular interaction energy and the arrangement of molecules in the local minima of various m2 1,3Ura dimers were calculated by the method of atom-atom potentials. The deepest minimum for the m2 1,3Ura coplanar dimer corresponds to a C(6)-H ... O hydrogen-bond formation. 2. At low concentration of m2 1,3Ura and caffeine in CCl4, C(6)-H ... O bonding for m2 1,3Ura and C(8)-H ... O bonding for caffeine with oxygens of dimethyl sulfoxide (DMSO) and acetone were observed. The association constants of these complexes were obtained at different temperatures. The enthalpies delta H, of the m2 1,3Ura-DMSO, m2 1,3Ura-accetone, caffeine-DMSO, and caffeine-acetone complexes were -2 +/- 0.1 kcal/mol. The calculations showed that the deepest minimum of the caffeine-acetone coplanar complex corresponds to C(8)-H ... O bonding with energy of -3.5 kcal/mol and that of the m2 1,3Ura-acetone complexes corresponds to C(6)-H ... O bonding with energy of -3.4 kcal/mol. The approximate correction for the solvent effect provides good agreement of the experimental data with the calculations.