Theoretical analysis of [5.7](n)cyclacenes: closed-shell cyclacene isomers

The [5.7](n)cyclacenes represent a novel class of all sp(2)-hybridized carbon structures. In contrast to the isomeric [n]cyclacenes, [5.7](n)cyclacenes are predicted at the B3LYP/6-31G* level of theory to have stable, closed-shell singlet ground state configurations. Predicted geometries, electronic structures, band gaps, nucleus-independent chemical shift (NICS) values, and strain energies for this new family of cyclic conjugated molecules are presented.     

DFT calculations on [6.8]cyclacenes and CpCo-capped [4.8]cyclacenes

Two different types of cyclacenes, utilizing eight-membered and six-/four-membered rings as building blocks ([6.8](n) and CpCo-capped [4.8](n)cyclacenes), have been theoretically investigated with respect to their geometries, relative energies, and magnetic properties (aromaticity).     

Open-shell singlet character of cyclacenes and short zigzag nanotubes

The electronic ground states of [n]cyclacenes, as well as short-zigzag nanotubes, computed at unrestricted broken spin-symmetry density functional theory (UBS-DFT), were found to be open-shell singlets, rather than triplets. Computations for [6]cyclacene at complete active space self-consistent field (CASSCF) and multireference perturbation theory (MRMP2) levels support this conclusion. Along with strain, the radical character of the open-shell singlet with antiferromagnetically coupled electron spins may contribute to the difficulties in synthesizing [n]cyclacenes.     

Exploring a Route to Cyclic Acenes by On‐Surface Synthesis

A route to generate cyclacenes by on‐surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels–Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning‐tunneling‐microscopy(STM)‐based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO‐functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.     

Polyacene and cyclacene geometries and electronic structures: bond equalization, vanishing band gaps, and triplet ground states contrast with polyacetylene

The ground-state geometries and excited singlet and lowest triplet energies of polyacenes from benzene through nonacene are predicted with B3LYP/6-31G* calculations and compared to experimental data where available. The results are compared to these data for cyclacenes and polyenes. The polyacenes and cyclacenes have geometries consisting of two fully delocalized nonalternating ribbons joined by relatively long bonds. Polyacenes are predicted to have smaller band gaps than the corresponding polyenes and triplet ground states for nine or more benzene rings. The fully delocalized nonalternating nature of polyacenes differs from the bond alternation resulting from Peierls distortion in polyenes. The differences are rationalized in terms of a simple MO model, and the results are compared to extensive prior theoretical work in the literature. Predictions about the electronic structure of analogues containing polyacene units are made.     

Modelling a Linker Mix‐and‐Match Approach for Controlling the Optical Excitation Gaps and Band Alignment of Zeolitic Imidazolate Frameworks

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO₂ reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.     

Bridging the Green Gap: Metal–Organic Framework Heteromultilayers Assembled from Porphyrinic Linkers Identified by Using Computational Screening

In organic photovoltaics, porphyrins (PPs) are among the most promising compounds owing to their large absorption cross-section, wide spectral range, and stability. Nevertheless, a precise adjustment of absorption band positions to reach a full coverage of the so-called green gap has not been achieved yet. We demonstrate that a tuning of the PP Q- and Soret bands can be carried out by using a computational approach for which substitution patterns are optimized in silico. The most promising candidate structures were then synthesized. The experimental UV/Vis data for the solvated compounds were in excellent agreement with the theoretical predictions. By attaching further functionalities, which allow the use of PP chromophores as linkers for the assembly of metal-organic frameworks (MOFs), we were able to exploit packing effects resulting in pronounced redshifts, which allowed further optimization of the photophysical properties of PP assemblies. Finally, we use a layer-by-layer method to assemble the PP linkers into surface-mounted MOFs (SURMOFs), thus obtaining high optical quality, homogeneous and crystalline multilayer films. Experimental results are in full accord with the calculations, demonstrating the huge potential of computational screening methods in tailoring MOF and SURMOF photophysical properties.     

Hydrogen-bonded inclusion compounds with reversed polarity: anionic metal-complexes and cationic organic linkers

Synthesized and structurally characterized is a new series of soft-host frameworks assembled by charge-assisted hydrogen bonds between an anionic metal complex (MC) and cationic organic linkers (OL), specifically [Co(en)(ox)(2)](-) and diprotonated 4,4'-bipyridinium (H(2)bpy) or 1,2-bis(4-pyridinium)ethylene (H(2)bpye). While frameworks built of cationic complexes and anionic organic linkers are already well-known, the seven new compounds described here represent the first series of frameworks with reversed polarity, that is, made of anionic complexes and cationic organic linkers. The compounds have a general formula [OL][MC](2)·n(guest), where the guest molecules 4,4'-biphenol (bp), 4-methoxyphenol (mp), 1,4-dimethoxybenzene (dmb), 1,6-dimethoxynaphtalene (dmn), and 4-nitroanisole (na). Structurally the compounds can be described as pillared-layer frameworks with layers constructed of MC anions and linked together by hydrogen-bonded cationic OL pillars. The guest molecules occupy the galleries between the pillars while their steric, electronic, and π-π and hydrogen-bonding capabilities influence the overall structure of the soft frameworks.     

Stability and electronic properties of binary systems involving hydrogen and halogen bonded [12]cyclacenes: a DFT study

The [n]cyclacene as a class of aromatic nanobelts can be considered for synthesizing new compounds. Here, the binary systems involving hydrogen and halogen bonded [12]cyclacenes are investigated at M06-2X/6-311++G(d,p) level of theory. The calculations are performed in gas phase and in water and acetone solvents. Because of their opportunities for the various applications in materials science, some electronic properties such as HOMO–LUMO gap, electronic chemical potential, first ionization energy, electron affinity, electrophilicity index, chemical hardness, and softness are examined. The role of hydrogen and halogen bonds on stability of dyads is also studied by atoms in molecules (AIM) theory.

     

Structure determination and characterization of two rare-earth molybdenum borate compounds: LnMoBO(6) (Ln = La, Ce)

The structural, optical, and electronic properties of two rare-earth molybdenum borate compounds, LnMoBO(6) (Ln = La, Ce), have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectral measurements, as well as calculations of energy band structures, density of states, and optical response functions by the density functional method. The title compounds, which crystallize in monoclinic space group P2(1)/c, possess a similar network of interconnected [Ce(2)(MoO(4))(2)](2+) chains and [BO(2)](-) wavy chains. Novel 1D molybdenum oxide chains are contained in their three-dimensional (3D) networks. The calculated results of crystal energy band structure by the density functional theory (DFT) method show that the solid-state compound LaMoBO(6) is a semiconductor with indirect band gaps.     

A robust, porous, cationic silver(i) 3,5-diphenyl-1,2,4-triazolate framework with a uninodal 4(9).6(6) net

The reactions of silver(i) salts with 3,5-diphenyl-1,2,4-triazole result in three-dimensional, porous, cationic metal-organic frameworks based on novel 6-connected, trigonal-prismatic, pentanuclear silver-triazolate clusters, interconnected by linear N-Ag-N linkers; this robust water- and organic-insoluble structure exhibits both nonpolar-solvent adsorption and anion-exchange properties.     

Modulating Electronic Coupling Using O- and S-donor Linkers

Structures of compounds having two dimolybdenum units Mo2(DAniF)3+ (DAniF = N,N'-di-p-anisylformamidinate) connected by unsubstituted oxamidate (1) and dithiooxamidate (2) linkers are isomorphous, and the cores of the molecules are planar because of two intramolecular hydrogen bonds within the linkers. Molecular mechanics calculations show a barrier of rotation along the C-C bond of approximately 10 kcal x mol(-1), which suggests that planar conformations are also expected in solution. Changing the two oxygen atoms in the linker of 1 to sulfur atoms results in a significant enhancement of the electronic coupling between the dimetal units (Delta(E1/2) = 204 mV for 1 and 407 mV for 2). The electronic spectrum of 2 shows an intense low energy (600 nm) metal-to-ligand charge transfer (MLCT) band, whereas that for 1 shows only a weak absorption band at 460 nm. DFT calculations on models 1' and 2', in which the anisyl groups were replaced by hydrogen atoms, show that the energy of the pi* orbital of the linker is much lower for 2'. This allows dpi-ddelta interactions from the electrons in the delta orbitals of the Mo2 unit to the sulfur atom that in turn facilitates an electron hopping pathway.     

Neutral coordination polymers based on a metal-mono(dithiolene) complex: synthesis, crystal structure and supramolecular chemistry of [Zn(dmit)(4,4'-bpy)]n, [Zn(dmit)(4,4'-bpe)]n and [Zn(dmit)(bix)]n (4,4'-bpy = 4,4'-bipyridine, 4,4'-bpe = trans-1,2-bis(4-pyridyl)ethene, bix = 1,4-bis(imidazole-1-ylmethyl)-benzene

This article describes a unique synthetic route that enables a neutral mono(dithiolene)metal unit, {Zn(dmit)}, to link with three different organic molecules, resulting in the isolation of a new class of neutral coordination polymers. The species {Zn(dmit)} coordinates with 4,4'-bipyridine (4,4'-bpy), trans-1,2-bis(4-pyridyl)ethene (4,4'-bpe) and 1,4-bis(imidazole-1-ylmethyl)-benzene (bix) as linkers giving rise to the formation of coordination polymers [Zn(dmit)(4,4'-bpy)](n) (1), [Zn(dmit)(4,4'-bpe)](n) (2) and [Zn(dmit)(bix)](n) (3) respectively. Compounds 1-3 were characterized by elemental analyses, IR, diffuse reflectance and single crystal X-ray diffraction studies. Compounds 1 and 3 crystallize in the monoclinic space group P2(1)/n, whereby compound 2 crystallizes in triclinic space group P1[combining macron]. In the present study, we chose three linkers 4,4'-bpy, 4,4'-bpe and bix (see , respectively, for their structural drawings), that differ in terms of their molecular dimensions. The crystal structures of compounds 1-3 are described here in terms of their supramolecular diversities that include π-π interactions, not only among aromatic stacking (compounds 1 and 3), but also between an aromatic ring and an ethylenic double bond (compound 2). The electronic absorption spectroscopy of compounds 1-3 support these intermolecular π-π interactions.     

AM1 treatment of cyclacene tubes

Semiempirical molecular orbital calculations at the level of AM1 type were performed on the nanotubes generated from the Hückel type cyclacenes. It has been found that the cryptoannulenic effect exhibited by the parent cyclacenes tends to disappear as the tube length increases.     

Peripherally B and N substituted cyclacenes

AM1 (RHF) type semiempirical quantum chemical calculations have been applied to cyclacenes whose fusion points and peri positions of one of their peripheral circuits are substituted with nitrogen and boron, respectively. The structures have been found to be stable but endothermic (except for R=8 and 9) in nature. The nitrogen and boron (the fusion points peri positions, respectively,) substitution have been found to have stabilizing effect on the parent unsubstituted cyclacenes. Some geometrical and physicochemical data are also reported.     

Structures,Magnetic Properties,and Aromaticity of Cyclacenes

Two [2 n ]trannulene rings coupled through n C−C bonds dictate the properties and aromaticity of [n]cyclacenes (one possible valence isomer is shown schematically). Theoretical calculations indicate that [n]cyclacenes with even values of n constitute a novel class of organic semiconductors and magnetic materials.     

Bi‐anchoring Organic Dyes that Contain Benzimidazole Branches for Dye‐Sensitized Solar Cells: Effects of π Spacer and Peripheral Donor Groups

Benzimidazole‐branched bi‐anchoring organic dyes that contained triphenylamine/phenothiazine donors, 2‐cyanoacrylic acid acceptors, and various π linkers were synthesized and examined as sensitizers for dye‐sensitized solar cells. The structure–activity relationships in these dyes were systematically investigated by using absorption spectroscopy, cyclic voltammetry, and density functional theory calculations. The wavelength of the absorption peak was more‐heavily influenced by the nature of the π linker than by the nature of the donor. For a given donor, the absorption maximum (λ_max) was red‐shifted on changing the π linker from phenyl to 2,2′‐bithiophene, whilst the dyes that contained triphenylamine units displayed higher molar extinction coefficients (?) than their analogous phenothiazine‐based triphenylamine dyes, which led to good light‐harvesting properties in the triphenylamine‐based dyes. Electrochemical data for the dyes indicated that the triphenylamine‐based dyes possessed relatively low‐lying HOMOs, which could be beneficial for suppressing back electron transfer from the conduction band of TiO₂ to the oxidized dyes, owing to facile regeneration of the oxidized dye by the electrolyte. The best performance in the DSSCs was observed for a dye that possessed a triphenylamine donor and 2,2′‐bithiophene π linkers. Electron impedance spectroscopy (EIS) studies revealed that the use of triphenylamine as the donor and phenyl or 2,2′‐bithiophene as the π linkers was beneficial for disrupting the dark current and charge‐recombination kinetics, which led to a long electron lifetime of the injected electrons in the conduction band of TiO₂.     

Polyunsaturated dicarboxylate tethers connecting dimolybdenum redox and chromophoric centers: syntheses,structures, and electrochemistry

Compounds with two quadruply bonded Mo(2)(4+) units, Mo(2)(DAniF)(3) (DAniF = N,N '-di-p-anisylformamidinate), linked by unsaturated dicarboxylate dianions of various lengths have been prepared and their spectroscopic and electrochemical properties studied. As identified by the dicarboxylate linkers, these compounds are maleate (7), allene-1,3-dicarboxylate (10), cis,cis-muconate (11), trans,trans-muconate (12), octa-2,4,6-trans,trans,trans-hexatriene-1,8-dioate (tamuate, 13), and deca-2,4,6,8-trans,trans,trans,trans-octatetraene-1,10-dioate (texate, 14). The latter three molecules complete the five-membered (all trans) series [Mo(2)(DAniF)(3)](O(2)C(CH=CH)(n)CO(2))[Mo(2)(DAniF)(3)] (n = 0-4). Several unsymmetrical paddlewheel compounds of the type Mo(2)(DAniF)(3)(O(2)CX) (X = C triple bond CH (3), CH=CH(2) (4), (E)-CH=CH-CH=CH(2) (5)) have also been prepared for comparison to the molecules in which there are linked Mo(2) units. The precursors [Mo(2)(DAniF)(3)(MeCN)(2)](BPh(4)), [1]BPh(4), and Mo(2)(DAniF)(3)Cl(MeCN) (2) have also been isolated and characterized. The structures of all new molecules have been established by X-ray crystallography, including the methyl esters of various carboxylates used as ligands. All of the linked molecules have been examined by cyclic and differential pulse voltammetry, and deltaE(1/2) values, the separation between successive Mo(2)(4+)/Mo(2)(5+) oxidations, have been determined. Those compounds with highly unsaturated, fully conjugated linkers demonstrate electrochemical communication from end-to-end that is more persistent over distance than is accounted for by an electrostatic interaction alone, implying that the pi system of these dicarboxylate linkers is mediating communication. In the series [Mo(2)(DAniF)(3)](O(2)C(CH=CH)(n)CO(2))[Mo(2)(DAniF)(3)] (n = 0-4), the first oxidation potential shifts progressively to less positive values due to an increasing contribution of the polyolefinic alpha,omega-dicarboxylate to the molecular orbital undergoing oxidation. This first oxidation potential approaches a limiting value of 63 mV (vs Ag/AgCl) as n becomes infinitely long. Compound 11 can be photoisomerized to 12 in a process that is affected by the presence of the Mo(2)(4+) units, as the analogous rearrangement of dimethyl cis,cis-muconate is faster.     

Kinetics of hydrogen bonding between anthracene urea derivatives and anions in the excited state

Urea compounds are useful anion sensors due to their hydrogen-bonding capabilities. We investigated the emissive properties of complexes consisting of urea-anthracene (nPUA, n = 1, 2) host compounds and acetate anions held as guests through intermolecular hydrogen bonding. The kinetics of a new emission band formed by conformational changes in the excited singlet state were revealed. The new band is thought to arise from a charge-transfer interaction between the anthracene and urea moieties after intermolecular hydrogen-bond reconfiguration in the excited state, with rate constants of 2.4 × 10(9) and 4.0 × 10(7) s(-1) for 1PUA and 2PUA, respectively.     

Probing the Nature of the Cluster Effect Observed with Synthetic Multivalent Galactosides and Peanut Agglutinin Lectin

We designed a set of multi‐galactosides with valencies ranging from one to seven and different spacer‐arm lengths. The compounds display a high structural homology for a strict assessment of multivalent phenomena. The multimers were first evaluated by an enzyme‐linked lectin assay (ELLA) toward the peanut agglutinin (PNA). The binding affinity was shown to be dependent on the spacer‐arm length, and cluster effects were observed for the galactosides bearing the shortest and the longest linkers. The latter compounds were shown to be much more potent PNA cross‐linkers in a “sandwich assay”. Dynamic light scattering (DLS) experiments also revealed the formation of soluble aggregates between heptavalent derivatives with medium or long linkers and the labeled PNA. ELLA experiments performed with valency‐controlled clusters and labeled lectins are therefore not always devoid from aggregative processes. The precise nature of the multivalent interaction observed by ELLA for the compounds bearing the shortest linkers, which are unable to form PNA aggregates, was further investigated by atomic force microscopy (AFM). The galactosides were grafted onto the tip of a cantilever and the PNA lectin onto a gold surface. Similar unbinding forces were registered when the valency of the ligands was increased, thus showing that the multimers cannot interact more strongly with PNA. Multiple binding events to the PNA were also never observed, thus confirming that a chelate binding mode does not operate with the multivalent galactosides, probably because the linkers are too short. Altogether, these results suggest that the cluster effect that operates in ELLA with the multimers is not related to additional PNA stabilizations and can be ascribed to local concentration effects that favor a dynamic turnover of the tethered galactosides in the PNA binding sites.