A template-directed protocol, which capitalizes on donor-acceptor interactions, is employed to synthesize a semi-rigid cyclophane (ExBox(4+)) that adopts a box-like geometry and is comprised of π-electron-poor 1,4-phenylene-bridged ("extended") bipyridinium units (ExBIPY(2+)). ExBox(4+) functions as a high-affinity scavenger of an array of different polycyclic aromatic hydrocarbons (PAHs), ranging from two to seven fused rings, as a result of its large, accommodating cavity (approximately 3.5 Å in width and 11.2 Å in length when considering the van der Waals radii) and its ability to form strong non-covalent bonding interactions with π-electron-rich PAHs in either organic or aqueous media. In all, 11 PAH guests were observed to form inclusion complexes with ExBox(4+), with coronene being the largest included guest. Single-crystal X-ray diffraction data for the 11 inclusion complexes ExBox(4+)⊂PAH as well as UV/vis spectroscopic data for 10 of the complexes provide evidence of the promiscuity of ExBox(4+) for the various PAHs. Nuclear magnetic resonance spectroscopy and isothermal titration calorimetric analyses of 10 of the inclusion complexes are employed to further characterize the host-guest interactions in solution and determine the degree with which ExBox(4+) binds each PAH compound. As a proof-of-concept, a batch of crude oil from Saudi Arabia was subjected to extraction with the water-soluble form of the PAH receptor, ExBox·4Cl, resulting in the isolation of different aromatic compounds after ExBox·4Cl was regenerated.
A monofunctionalised pillar[5]arene derivative carrying a viologen side chain which exhibits self-complexation in dilute dichloromethane solutions forms supramolecular daisy chain polymers and eventually organogels as its concentration is increased three-fold over the range from 0.1 to 100 mM.
Radically Organic Metals such as manganese are relatively stable over a wide range of oxidation states. In contrast, purely organic compounds are rarely susceptible to incremental addition or removal of electrons without accompanying fragmentation or coupling reactions. Barnes et al. (p. 429 ; see the Perspective by Benniston ) report a catenane (a compound comprising interlocked rings) in which the topological structure stabilizes six different states that successively differ by the presence or absence of one or two electrons in the framework. The hepta-oxidized state proved remarkably resilient to oxygen exposure.
The kinetics and thermodynamics of intramolecular electron transfer (IET) can be subjected to redox control in a bistable [2]rotaxane comprised of a dumbbell component containing an electron-rich 1,5-dioxynaphthalene (DNP) unit and an electron-poor phenylenebridged bipyridinium (P-BIPY 2þ ) unit and a cyclobis (paraquatp-phenylene) (CBPQT 4þ ) ring component. The [2]rotaxane exists in the ground-state co-conformation (GSCC) wherein the CBPQT 4þ ring encircles the DNP unit. Reduction of the CBPQT 4þ leads to the CBPQT 2ð•þÞ diradical dication while the P-BIPY 2þ unit is reduced to its P-BIPY •þ radical cation. A radical-state co-conformation (RSCC) results from movement of the CBPQT 2ð•þÞ ring along the dumbbell to surround the P-BIPY •þ unit. This shuttling event induces IET to occur between the pyridinium redox centers of the P-BIPY •þ unit, a property which is absent between these redox centers in the free dumbbell and in the 1∶1 complex formed between the CBPQT 2ð•þÞ ring and the radical cation of methyl-phenylene-viologen (MPV •þ ). Using electron paramagnetic resonance (EPR) spectroscopy, the process of IET was investigated by monitoring the line broadening at varying temperatures and determining the rate constant (k ET ¼ 1.33 × 10 7 s −1 ) and activation energy (ΔG ‡ ¼ 1.01 kcal mol −1 ) for electron transfer. These values were compared to the corresponding values predicted, using the optical absorption spectra and Marcus-Hush theory.T he simplest organic intervalence compounds consist (1) of two redox centers (X) which are connected by a covalent bridge (b) wherein the two centers possess oxidation states that range from being equal ( nþ0.5 X − b − X nþ0.5 ) to differing by as much as one unit ( n X − b − X nþ1 ). According to the Robin-Day classification (2), there are three main types of organic compounds, which can be defined by the degree of the electronic coupling element, H, that exists between the two interacting redox centers. Compounds that exhibit low electronic coupling between each X redox center have localized charges and exhibit no intramolecular electron transfer (IET) are classified as Class I systems ( n X − b − X nþ1 , H ¼ 0), whereas those that possess a strong electronic coupling between X redox centers and undergo complete delocalization of charge and fast IET are considered Class III systems ( nþ0.5 X − b − X nþ0.5 , H ≫ 0). In between these two classifications lie Class II systems, where the electronic coupling between X redox centers is moderate, leading to a mixture of charge between each unit as well as a measurable IET event.In the 1960s, Marcus (3-6) developed a theory for electron transfer in bridged transition metal-coordinated complexes by identifying the reorganizational energy, λ, required for solvent polarization and subsequent electron transfer between two metal centers. Shortly thereafter, Hush (7-9) proposed a theoretical method that could be applied to Class II intervalence compounds by focusing on the low-energy intervalence charge transfer (IT) bands in optical abso...
A functionalised cyclobis(paraquat-p-phenylene) attached by a rigid linker to a tetrathiafulvalene unit, which is incapable of self-complexation, forms preferentially a [c2]daisy chain which undergoes rapid disassociation and reassociation on the (1)H NMR time-scale above room temperature.
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