Intramolecular Energy and Electron Transfer within a Diazaperopyrenium-Based Cyclophane.

Molecules capable of performing highly efficient energy transfer and ultrafast photoinduced electron transfer in well-defined multichromophoric structures are indispensable to the development of artificial photofunctional systems. Herein, we report on the synthesis, characterization, and photophysical properties of a rationally designed multichromophoric tetracationic cyclophane, DAPPBox4+, containing a diazaperopyrenium (DAPP2+) unit and an extended viologen (ExBIPY2+) unit, which are linked together by two p-xylylene bridges. Both 1H NMR spectroscopy and single-crystal X-ray diffraction analysis confirm the formation of an asymmetric, rigid, box-like cyclophane, DAPPBox4+. The solid-state superstructure of this cyclophane reveals a herringbone-type packing motif, leading to two types of π···π interactions: (i) between the ExBIPY2+ unit and the DAPP2+ unit (π···π distance of 3.7 Å) in the adjacent parallel cyclophane, as well as (ii) between the ExBIPY2+ unit (π···π distance of 3.2 Å) and phenylene ring in the closest orthogonal cyclophane. Moreover, the solution-phase photophysical properties of this cyclophane have been investigated by both steady-state and time-resolved absorption and emission spectroscopies. Upon photoexcitation of DAPPBox4+ at 330 nm, rapid and quantitative intramolecular energy transfer occurs from the 1*ExBIPY2+ unit to the DAPP2+ unit in 0.5 ps to yield 1*DAPP2+. The same excitation wavelength simultaneously populates a higher excited state of 1*DAPP2+ which then undergoes ultrafast intramolecular electron transfer from 1*DAPP2+ to ExBIPY2+ to yield the DAPP3+•-ExBIPY+• radical ion pair in τ = 1.5 ps. Selective excitation of DAPP2+ at 505 nm populates a lower excited state where electron transfer is kinetically unfavorable.

A Zwitterionic, 10 π Aromatic Hemisphere.

A new concept in anionic 10 π aromaticity is described by the embedding of a compensating charge within an aromatic cyclononatetraenide ring by the symmetric superposition of an alkyl ammonium bridge. This is accomplished by the methylation of azatriquinacene to give a quaternary ammonium salt, followed by oxidation to the tetraene and final deprotonation. The resulting zwitterion is a stable [9]annulene with strong aromaticity as shown by its degree of C-C bond equalization and a nucleus-independent chemical shift value lower than that of benzene. The solid-state structure shows an eclipsed stacking motif with the electron-poor ammonium methyl groups occupying the electron-rich cavity of the aromatic bowl.

Quantum Mechanical and Experimental Validation that Cyclobis(paraquat-p-phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene.

The promiscuous encapsulation of π-electron-rich guests by the π-electron-deficient host, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge-transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT(4+), is an emerald-green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas-phase calculations that reinterpreted this CT band in terms of an intermolecular side-on interaction of TTF with one of the bipyridinium (BIPY(2+)) units of CBPQT(4+), rather than the encapsulation of TTF inside the cavity of CBPQT(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT(4+) arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY(2+) units of a CBPQT(4+) ring residing on a separate [2]rotaxane in a side-on fashion. This [2]rotaxane has similar UV/Vis and (1)H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT(4+). The [2]rotaxane exists as an equimolar mixture of cis- and trans-isomers associated with the disubstituted TTF unit in its dumbbell component. Solid-state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT(4+).

Energy and Electron Transfer Dynamics within a Series of Perylene Diimide/Cyclophane Systems.

Artificial photosynthetic systems for solar energy conversion exploit both covalent and supramolecular chemistry to produce favorable arrangements of light-harvesting and redox-active chromophores in space. An understanding of the interplay between key processes for photosynthesis, namely light-harvesting, energy transfer, and photoinduced charge separation and the design of novel, self-assembling components capable of these processes are imperative for the realization of multifunctional integrated systems. We report our investigations on the potential of extended tetracationic cyclophane/perylene diimide systems as components for artificial photosynthetic applications. We show how the selection of appropriate heterocycles, as extending units, allows for tuning of the electron accumulation and photophysical properties of the extended tetracationic cyclophanes. Spectroscopic techniques confirm energy transfer between the extended tetracationic cyclophanes and perylene diimide is ultrafast and quantitative, while the heterocycle specifically influences the energy transfer related parameters and the acceptor excited state.

A Hafnium-Based Metal-Organic Framework as a Nature-Inspired Tandem Reaction Catalyst.

Tandem catalytic systems, often inspired by biological systems, offer many advantages in the formation of highly functionalized small molecules. Herein, a new metal-organic framework (MOF) with porphyrinic struts and Hf6 nodes is reported. This MOF demonstrates catalytic efficacy in the tandem oxidation and functionalization of styrene utilizing molecular oxygen as a terminal oxidant. The product, a protected 1,2-aminoalcohol, is formed selectively and with high efficiency using this recyclable heterogeneous catalyst. Significantly, the unusual regioselective transformation occurs only when an Fe-decorated Hf6 node and the Fe-porphyrin strut work in concert. This report is an example of concurrent orthogonal tandem catalysis.

Modulating the binding of polycyclic aromatic hydrocarbons inside a hexacationic cage by anion-π interactions.

We report the template-directed synthesis of BlueCage(6+), a macrobicyclic cyclophane composed of six pyridinium rings fused with two central triazines and bridged by three paraxylylene units. These moieties endow the cage with a remarkably electron-poor cavity, which makes it a powerful receptor for polycyclic aromatic hydrocarbons (PAHs). Upon forming a 1:1 complex with pyrene in acetonitrile, however, BlueCage⋅6 PF6 exhibits a lower association constant Ka than its progenitor ExCage⋅6 PF6. A close inspection reveals that the six PF6(-) counterions of BlueCage(6+) occupy the cavity in a fleeting manner as a consequence of anion-π interactions and, as a result, compete with the PAH guests. This conclusion is supported by a one order of magnitude increase in the Ka value for pyrene in BlueCage(6+) when the PF6(-) counterions are replaced by much bulkier anions. The presence of anion-π interactions is supported by X-ray crystallography, and confirms the presence of a PF6(-) counterion inside its cavity.

Adaptation of dynamic covalent systems of imine constituents to medium change by component redistribution under reversible phase separation.

A dynamic covalent library of interconverting imine constituents, dissolved in an acetonitrile/water mixture, undergoes constitutional reorganization upon phase separation induced by a physical stimulus (heat) or a chemical effector (inorganic salt, carbohydrate, organic solvent). The process has been made reversible, regenerating the initial library upon phase reunification. It represents the behavior of a dynamic covalent library upon reversible phase separation and its adaptation to a phase change, with up-regulation in each phase of the fittest constituents by component selection. Finally, the system exemplifies the splitting of a 2D (square) constitutional dynamic network into a 3D (cube) one.

The R3O+···H+ hydrogen bond: toward a tetracoordinate oxadionium(2+) ion.

Oxatriquinanes are tricyclic oxonium ions which are known to possess remarkable solvolytic stability compared to simple alkyl oxonium salts. Their rigid, hemispherical structure presents an oxygen at the apex of three fused five-membered rings. While trivalent oxygen species like these have been well described in the literature, the ability of oxygen to enter into a fourth covalent bonding relationship has been visited in theory and suggested by the outcome of certain reactions conducted in superacidic media, but has never been established by the characterization of a stable, persistent R(3)OH(2+) or R(4)O(2+) ion. In this study, the nucleophilicity of the oxatriquinane oxygen was evaluated first by a series of protonation studies using the Brønsted superacid H(CHB(11)Cl(11)) both in the solid state and in liquid HCl solution. The interaction of the oxatriquinane oxygen with a bridging carbocation was also examined. A strong case could be made for the occurrence of hydrogen bonding between H(CHB(11)Cl(11)) and oxatriquinane using IR spectroscopy. Under the most forcing protonation conditions, the oxatriquinane ring is cleaved to give a bridged, dicationic, protonated tetrahydrofuran-carbenium ion.

1,4,7-Trimethyloxatriquinane: S(N)2 reaction at tertiary carbon.

The synthesis of 1,4,7-trimethyloxatriquinane (1), a 3-fold tertiary alkyl oxonium salt, is described. Compound 1 is inert to solvolysis with alcohols, even at elevated temperatures, but undergoes facile substitution with the strongly nucleophilic azide anion. Since an S(N)1 pathway is excluded, the only reasonable mechanistic interpretation for the reaction between 1 and N(3)(-) is S(N)2, despite the fact that substitution is occurring at a tertiary carbon center. This finding is supported by computational modeling and a study of the reaction kinetics, and is also consistent with observed solvent and salt effects.

Effects of Methyl Branching on the Properties and Performance of Furandioate-Adipate Copolyesters of Bio-Based Secondary Diols.

Furandioate-adipate copolyesters are an emerging class of bio-based biodegradable polymers with great potential to replace fossil-derived terephthalic acid-based copolyesters such as poly(butylene adipate-co-terephthalate) (PBAT). Furandioate-adipate polyesters have almost exclusively been prepared with conventional primary (1°) alcohol diols, while secondary (2°) alcohol diol monomers have largely been overlooked until now, despite preliminary observations that using methyl-branched diols increases the T g of the resultant polyesters. Little is known of what impact the use of 2° alcohol diols has on other properties such as material strength, hydrophobicity, and rate of enzymatic hydrolysis-all key parameters for performance and end-of-life. To ascertain the effects of using 2° diols on the properties of furandioate-adipate copolyesters, a series of polymers from diethyl adipate (DEA) and 2,5-furandicarboxylic acid diethyl ester (FDEE) using different 1° and 2° alcohol diols was prepared. Longer transesterification times and greater excesses of diol (diol/diester molar ratio of 2:1) were found to be necessary to achieve M ws > 20 kDa using 2° alcohol diols. All copolyesters from 2° diols were entirely amorphous and exhibited higher T gs than their linear equivalents from 1° diols. Compared to linear poly(1,4-butyleneadipate-co-1,4-butylenefurandioate), methyl-branched, poly(2,5-hexamethyleneadipate-co-2,5-hexamethylenefurandioate) (0:7:0.3 furandioate/adipate ratio) displayed both higher modulus (67.8 vs 19.1 MPa) and higher extension at break (89.7 vs 44.5 mm). All other methyl-branched copolyesters displayed lower modulus but retained higher extension at break compared with their linear analogues. Enzymatic hydrolysis studies using Humicola insolens cutinase revealed that copolyesters from 2° alcohol diols have significantly decreased rates of biodegradation than their linear equivalents synthesized using 1° alcohol diols, allowing for fine-tuning of polymer stability. Hydrophobicity, as revealed by water contact angles, was also found to generally increase through the introduction of methyl branching, demonstrating potential for these materials in coatings applications.

Synthetic, non-intoxicating 8,9-dihydrocannabidiol for the mitigation of seizures.

There can be a fine line between therapeutic intervention and substance abuse, and this point is clearly exemplified in herbal cannabis and its products. Therapies involving cannabis have been the treatment of last resort for some cases of refractory epilepsy, and this has been among the strongest medical justifications for legalization of marijuana. In order to circumvent the narcotic effects of Δ9-tetrahydrocannabinol (THC), many studies have concentrated on its less intoxicating isomer cannabidiol (CBD). However, CBD, like all natural cannabinoids, is a controlled substance in most countries, and its conversion into THC can be easily performed using common chemicals. We describe here the anticonvulsant properties of 8,9-dihydrocannibidiol (H2CBD), a fully synthetic analogue of CBD that is prepared from inexpensive, non-cannabis derived precursors. H2CBD was found to have effectiveness comparable to CBD both for decreasing the number and reducing the severity of pentylenetetrazole-induced seizures in rats. Finally, H2CBD cannot be converted by any reasonable synthetic route into THC, and thus has the potential to act as a safe, noncontroversial drug for seizure mitigation.

Oxatriquinane and oxatriquinacene: extraordinary oxonium ions.

Oxatriquinane, a fused, tricyclic alkyl oxonium ion of unprecented stability, was synthesized in five steps from 1,4,7-cyclononatriene. It survives reflux in H2O, chromatography, and attack by alcohols, alkyl thiols, halide ions, and hindered amine bases. The X-ray crystal structure shows longer C-O bond distances and more acute C-O-C bond angles than any reported alkyloxonium salt. The corresponding oxatriquinene and oxatriquinacene were also synthesized and represent the first examples of stable allyl oxonium species.

Extreme oxatriquinanes and a record C-O bond length.

Oxatriquinanes are fused, tricyclic oxonium ions that are known to have exceptional stability compared to simple alkyl oxonium salts. C-O bonds in ethers are generally ∼1.43 Å in length, but oxatriquinane has been found to have C-O bond lengths of 1.54 Å. A search of the Cambridge Structural Database turned up no bona fide C-O bond length exceeding this value. Computational modelling of oxatriquinane alongside other alkyl oxonium ions indicated that the electronic consequences of molecular strain were primarily responsible for the observed bond elongation. We also show that substitution of the oxatriquinane ring system with alkyl groups of increasing steric demand pushes the C-O bond to unheard of distances, culminating in a tert-butyl derivative at a predicted 1.60 Å. Chemical synthesis and an X-ray crystallographic study of these compounds validated the results of the modelling work and, finally, an extraordinary 1.622 Å C-O bond was observed in 1,4,7-tri-tert-butyloxatriquinane.