Supramolecular Explorations: Exhibiting the Extent of Extended Cationic Cyclophanes

Abstract

Acting as hosts, cationic cyclophanes, consisting of π-electron-poor bipyridinium units, are capable of entering into strong donor-acceptor interactions to form host-guest complexes with various guests when the size and electronic constitution are appropriately matched. A synthetic protocol has been developed that utilizes catalytic quantities of tetrabutylammonium iodide to make a wide variety of cationic pyridinium-based cyclophanes in a quick and easy manner. Members of this class of cationic cyclophanes with boxlike geometries, dubbed Ex(n)Boxm(4+) for short, have been prepared by altering a number of variables: (i) n, the number of "horizontal" p-phenylene spacers between adjoining pyridinium units, to modulate the "length" of the cavity; (ii) m, the number of "vertical" p-phenylene spacers, to modulate the "width" of the cavity; and (iii) the aromatic linkers, namely, 1,4-di- and 1,3,5-trisubstituted units for the construction of macrocycles (ExBoxes) and macrobicycles (ExCages), respectively. This Account serves as an exploration of the properties that emerge from these structural modifications of the pyridinium-based hosts, coupled with a call for further investigation into the wealth of properties inherent in this class of compounds. By variation of only the aforementioned components, the role of these cationic receptors covers ground that spans (i) synthetic methodology, (ii) extraction and sequestration, (iii) catalysis, (iv) molecular electronics, (v) physical organic chemistry, and (vi) supramolecular chemistry. Ex(1)Box(4+) (or simply ExBox(4+)) has been shown to be a multipurpose receptor capable of binding a wide range of polycyclic aromatic hydrocarbons (PAHs), while also being a suitable component in switchable mechanically interlocked molecules. Additionally, the electronic properties of some host-guest complexes allow the development of artificial photosystems. Ex(2)Box(4+) boasts the ability to bind both π-electron-rich and -poor aromatic guests in different binding sites located within the same cavity. ExBox2(4+) forms complexes with C60 in which discrete arrays of aligned fullerenes result in single cocrystals, leading to improved material conductivities. When the substitution pattern of the Ex(n)Box(4+) series is changed to 1,3,5-trisubstituted benzenoid cores, the hexacationic cagelike compound, termed ExCage(6+), exhibits different kinetics of complexation with guests of varying sizes-a veritable playground for physical organic chemists. The organization of functionality with respect to structure becomes valuable as the number of analogues continues to grow. With each of these minor structural modifications, a wealth of properties emerge, begging the question as to what discoveries await and what properties will be realized with the continued exploration of this area of supramolecular chemistry based on a unique class of receptor molecules.

Perylene Bisimide Cyclophanes with High Binding Affinity for Large Planar Polycyclic Aromatic Hydrocarbons: Host–Guest Complexation versus Self‐Encapsulation of Side Arms

“…The new cyclophanes 2PBI (2,6-iPr)2 and 2PBI (2,6-Ph)2 were synthesized in two steps starting from literature knownP BIs PBI (2, and PBI (2,6-Ph)2 [14a] according to the route displayed in Scheme 1. In the first step, the alkaline hydrolysis of these PBIs was performed to obtain the corresponding perylene bisanhydrides PBA (2, 2 and PBA (2,6-Ph)2 in yields of 72 and 36 %, respectively.W hile % 10 equivalents of potassium hydroxide (KOH) were sufficient for the complete hydrolysis of PBI (2,6-iPr)2 , 250 equivalents of KOH weren ecessary to fully convert PBI (2,6-Ph)2 into its PBA derivative.…”

“…[10] Cyclophanes based on perylene bisimide (PBI) derivatives, one of the most widely appliedc lass of chromophores in supramolecular dye chemistry, [11] are of interest as hosts ystems since PBIs provide large p-surfaces to interact with other large-sized aromatic molecules. [13] Therefore, we have extended our efforts to developingn ew cyclophanes 2PBI (2, 2 and 2PBI (2,2 that comprise core-disubstituted, instead of core-tetrasubstituted as in 2PBI (4-tBu)4 ,P BI subunits.D i-instead of tetra-bay-functionalizationi sk nown to afford almost planar PBI scaffolds, [14] thus the cavitieso ft he new cyclophanes should not be distorted and accordingly may show strong binding affinity towards tiff PAHs. [13] It has been shown that the binding of aromatic hydrocarbons in this cyclophane is strongly dependent on the size of the guest molecule, and thusw ith increasingt he guests' p-surface the interacting p-surface area and the bindings trength are enhanced.…”

Shedding Light on the Nature of Host–Guest Interactions in PAHs-ExBox4+ Complexes

“…8 For that reason, different sort of compounds, able to recognize PAHs such as cyclodextrins, [9][10][11][12][13][14] calix[n]arenes, [15][16][17] colic acid, 18 and diazapyreniumbased metallocycles, 19,20 have been extensively synthesized. Recently, Stoddart and coworkers [21][22][23][24][25][26][27][28][29] reported the preparation, solid-state characterization, and HG binding affinities of a semi-rigid family of tetracationic cyclophanes, named ExBox 4+ (1), which are constituted by two π-electron-poor 4,4'-bipyridinium units tethered by two p-xylylene linkers (Figure 1a). Stoddart 21 have described the ability of 1 to scavenge eleven different electron-rich PAHs (azulene (2); anthracene (3); phenanthrene (4); pyrene (5); tetracene (6); tetraphene (7); chrysene (8); helicene (9); triphenylene (10); perylene (11); and coronene (12)) ( Fig-2 also explored the constitutional modifications of ExBox 4+ via efficient synthetic protocols, by increasing the size of the cavities, making them able to interact with large PAHs.…”

Cyclotris(paraquat‐p‐phenylenes)

“…Notably,the electrochemical properties of these macrocycles allow access to their corresponding open-shell species following either chemical or electrochemical reduction. [14] To explore extended derivatives of CBPQT 4+ ,w hilst retaining access to the rich and varied chemistry of viologen-based radicals,itisimportant to maintain the structural integrity [15] of the bipyridinium units so as to allow the convenient generation and isolation of the respective radical cations by chemical means. [14] To explore extended derivatives of CBPQT 4+ ,w hilst retaining access to the rich and varied chemistry of viologen-based radicals,itisimportant to maintain the structural integrity [15] of the bipyridinium units so as to allow the convenient generation and isolation of the respective radical cations by chemical means.…”