Phenothiazine-Bridged Cyclic Porphyrin Dimers as High-Affinity Hosts for Fullerenes and Linear Array of C60 in Self-Assembled Porphyrin Nanotube

A Bis-Porphyrin Cavitand Breathing-In to Constrict Bucky Balls

Bis-porphyrin cages have long been exploited to bind fullerenes selectively for various applications. The major consideration for an effective binding here had been the cavity size. Herein, we structurally demonstrate that a bis-Ni-porphyrin cavitand having even a smaller cavity can host a larger fullerene by a breathing and ruffling mechanism. It has also been shown that both the electronic and steric influence at the meso- positions of the porphyrin in fact dictate the binding character. The smaller cavity of 2NiD exhibits preferential binding for C70 over C60; however, surprisingly, the larger cavities in 2HD and 2NiTD display stronger affinities for C60 over the larger fullerene. We show here that the structural elasticity infused both by the metalloporphyrins and the connecting bridges play a major role in directing the binding. These conclusions have adequately been supported by structural and spectroscopic investigations. Additionally, the suitability of one of the conjugates for photoinduced charge-separation has been investigated using ultrafast transient absorption measurements. 2NiD⊃C60 has a charge separation timescale of ~0.8 ps, while charge recombination occurs at a longer timescale of ~920 ps.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

Recent update on the electroactive oligopyrrolic macrocyclic hosts with a Bucky-ball heart

Supramolecular chemistry is a multidisciplinary research area mostly associated with the investigation of host-guest interactions within intricate three-dimensional (3D) molecular architectures held together reversibly by various non-covalent interactions. Continuous efforts to develop such kinds of complex host-guest systems with designer oligopyrrolic macrocyclic receptors are a rapidly growing research domain, which is deeply involved in applied supramolecular chemistry research. These host-guest supramolecular complexes can be constructed by combining suitable electron-rich oligopyrrolic donors (as a host) with complementary electron-poor guests (as acceptors), held together by the ionic force of attraction triggered by intermolecular charge/electron transfer (CT/ET) transitions. Some of these resulting CT/ET ensembles are potential candidates for the construction of efficient optoelectronic materials, optical sensors, molecular switches, etc. In this Feature Article we aim to focus on these supramolecular ensembles composed by size and shape complementary electroactive oligopyrrolic molecular containers, which are suitable for spherical guest (e.g., buckminsterfullerene) complexation. We also provide a "state-of-the-art" overview on plausible applications of these particular host-guest systems. Our aim is to cover only specific electron-rich tetrathiafulvalene (TTF)-based oligopyrrolic receptors, e.g., TTF-calix[4]pyrroles, TTF-cryptands, TTF-porphyrins and exTTF-porphyrin-based molecular motifs reported to date, along with a brief outlining of their "functional behaviour" in materials chemistry research.

Phenothiazine-Embedded Hexaphyrins

Unprecedented nonaromatic stable phenothiazine-embedded porphyrinoids were synthesized by incorporating phenothiazine subunits into the hexaphyrin framework. The crystal structure revealed that the macrocycle adopted a twisted conformation wherein the phenothiazine units maintained their planarity, which was an impediment in π-delocalization throughout the macrocyclic core. The macrocycles exhibited distinct absorption bands in the visible-near-infrared region, and electrochemical studies indicated their electron-rich nature. Theoretical studies were consistent with the experimental observations.

Helical Nanofibers Formed by Palladium-Mediated Assembly of Organic Homochiral Macrocycles Containing Binaphthyl and Pyridyl Units

Herein, we report the synthesis and characterization of homochiral macrocycles, in which molecular rigidity, combined with the presence of multiple functional groups, allow for the assembly of helical nanostructures. 1,1'-bi-2-naphthol (Binol) units are used as robust chirality inducers, and pyridyl units embedded within the molecular frameworks allow the assembly, upon coordination with Pd(II) metal ions, of the macrocyclic building blocks. CD and NMR spectroscopies show the formation of ordered 1D assembly in solution. AFM studies indicate that the molecular systems are capable of forming nanoscale structures. The effective transfer of chiral information results in helical nanofibers, with lengths ranging from a few hundreds of nanometers to some micrometers. AFM line profiles reveal a helical longitudinal period of about 50 nm and a transverse width of 25 to 45 nm after deconvolution.

Porphyrin bearing phenothiazine pincers as hosts for fullerene binding via concave–convex complementarity: synthesis and complexation study

Free base porphyrin hosts, m-(PTZ)4-H2P and p-(PTZ)4-H2P, tethered with four phenothiazine moieties at the meso-position via a flexible ethoxy phenyl linker were synthesized and used for the selective complexation of fullerenes, C60 and C70.

Semiconducting Supramolecular Organic Frameworks Assembled from a Near-Infrared Fluorescent Macrocyclic Probe and Fullerenes

We report here a new extended tetrathiafulvalene (exTTF)-porphyrin scaffold, 2, that acts as a ball-and-socket receptor for C₆₀ and C₇₀. Supramolecular interactions between 2 and these fullerenes serve to stabilize 3D supramolecular organic frameworks (SOFs) in the solid state formally comprising peapod-like linear assemblies. The SOFs prepared via self-assembly in this way act as "tunable functional materials", wherein the complementary geometry of the components and the choice of fullerene play crucial roles in defining the conductance properties. The highest electrical conductivity (σ = 1.3 × 10^-8 S cm^-1 at 298 K) was observed in the case of the C₇₀-based SOF. In contrast, low conductivity was seen for the SOF based on pristine 2 (σ = 5.9 × 10^-11 S cm^-1 at 298 K). The conductivity seen for the C₇₀-based SOF approaches that seen for other TTF- and fullerene-based supramolecular materials despite the fact that the present systems are metal-free and constructed entirely from neutral building blocks. Transient absorption spectroscopic measurements corroborated the formation of charge-transfer states (i.e., 2^δ+/C₆₀^δ- and 2^δ+/C₇₀^δ-, respectively) rather than fully charge separated states (i.e., 2^•+/C₆₀^•- and 2^•+/C₇₀^•-, respectively) both in solution (toluene and benzonitrile) and in the solid state at 298 K. Such findings are considered consistent with an ability to transfer charges effectively over long distances within the present SOFs, rather than, for example, the formation of energetically trapped ionic species.

Chrysene-Bridged Porphyrin Tweezers: Chiral Receptors for Fullerenes

Bis(bicyclic) molecules dimethanochrysene and diethanochrysene were prepared by Diels-Alder reaction of the naphthodiyne equivalent with cyclopentadiene and 1,3-cyclohexadiene, respectively. Reaction of dimethanochrysene and 7,9-diphenyl-8H-cyclopent[a]acenaphthylen-8-one resulted in the generation of a fluorescent hydrocarbon in unexpected multistep pericyclic reactions. Syn-oriented diethanochrysene-connected bisporphyrin tweezers was prepared from the reaction of chrysene-bridged syn-dipyrrole with tripyrranedicarbaldehyde. The structure of the receptor and its 1 : 1 complex with C₆₀ or C₇₀ was determined by X-ray diffraction analysis. The dihedral angles of the bicyclo[2.2.2]octadiene moieties were narrowed by complexation of the receptor with the fullerenes. The binding affinities of the tweezers with C₆₀ and C₇₀ were calculated to be 2.7(4)×10⁴ and 8.01(7)×10⁴  M^-1 , respectively by UV-vis and fluorescence spectroscopy.

Highly stable fullerene-based porous molecular crystals with open metal sites

The synthesis of conventional porous crystals involves building a framework using reversible chemical bond formation, which can result in hydrolytic instability. In contrast, porous molecular crystals assemble using only weak intermolecular interactions, which generally do not provide the same environmental stability. Here, we report that the simple co-crystallization of a phthalocyanine derivative and a fullerene (C₆₀ or C₇₀) forms porous molecular crystals with environmental stability towards high temperature and hot aqueous base or acid. Moreover, by using diamond anvil cells and synchrotron single-crystal measurements, stability towards extreme pressure (>4 GPa) is demonstrated, with the stabilizing fullerene held between two phthalocyanines and the hold tightening at high pressure. Access to open metal centres within the porous molecular co-crystal is demonstrated by in situ crystallographic analysis of the chemisorption of pyridine, oxygen and carbon monoxide. This suggests strategies for the formation of highly stable and potentially functional porous materials using only weak van der Waals intermolecular interactions.

Discovery of two types of new porphyrin–C70 co-crystals: influence of intermolecular contact on the inherent resistance

Two types of supramolecular aggregates based on C₇₀ were synthesised and their inherent resistances were analysed.

One-flask synthesis of dibenzotetraaza[14]annulene cyclic congeners bearing buta-1,3-diyne bridges

Glaser–Hay and Glaser–Eglinton coupling conditions were applied to the direct synthesis of a cyclic strapped ligand and its corresponding dimer in 44% and 30% yields respectively.

Synthesis of a tethered dibenzotetramethyltetraaza[14]annulene macrocycle and the di-nickel(ii) derivative

Selective reaction of 2,6-isophthaloyl chloride at the methine positions of two dibenzotetramethyltetraaza[14]annulene (tmtaa) units results in tethering of the macrocycles by either one or two 2,6-isophthaloyl bridging groups.

Theoretical characterization of supramolecular complexes formed by fullerenes and dimeric porphyrins

Intramolecular stacking is very strong in dimeric porphyrins. However, in solution they are able to inhibit folding and can trap fullerenes with very high association constants. Diabatic interaction energies can be a useful approach to evaluate the strength of porphyrin/fullerene supramolecular complexes.

Solid-state inclusion of C60 and C70 in a co-polymer induced by metal–ligand coordination of a Zn–porphyrin cage with a bis-pyridyl perylene derivative

We have conducted X-ray characterization of two unprecedented supramolecular solids involving three different components: a suitable Zn(ii)–bisporphyrin cage, a bis-pyridyl perylene derivative and a fullerene.

Singlet oxygen generation properties of an inclusion complex of cyclic free-base porphyrin dimer and fullerene C60

We demonstrate that a cyclic free-base porphyrin dimer and its inclusion complex with fullerene C₆₀ possess the ability to generate singlet oxygen (¹O₂) under visible light irradiation.

Synthesis of Cyclic Porphyrin Trimers through Alkyne Metathesis Cyclooligomerization and Their Host-Guest Binding Study

Cyclic porphyrin trimers were synthesized through one-step cyclooligomerization via alkyne metathesis from diyne monomers. These macrocycles show interesting host-guest binding interactions with fullerenes, selectively binding C70 (6 × 10(3) M(-1)) over C60 and C84 (no binding observed). The fullerene-encapsulated host-guest complex can undergo guest or host exchange in the presence of another guest (2,4,6-tri(4-pyridyl)-1,3,5-triazine) or host (cage COP5) molecule with higher binding affinity.

Dye-sensitized solar cell based on an inclusion complex of a cyclic porphyrin dimer bearing four 4-pyridyl groups and fullerene C60

Cyclic free-base porphyrin dimers linked by butadiyne or phenothiazine bearing four 4-pyridyl groups and their inclusion complexes with fullerene C₆₀ have been applied to dye-sensitized solar cells as a new class of porphyrin dye sensitizers.

Chiral nanostructuring of multivalent macrocycles in solution and on surfaces

We describe the design and synthesis of a novel functionality-rich, homochiral macrocycle, possessing the overall molecular D2 symmetry, in which multivalency is introduced into the covalent framework by means of four suitably positioned pyridine moieties. The macrocycle synthesis is carried out with functionalized, enantiopure 1,1'-binaphthyl synthons as the source of chirality by means of a room temperature esterification reaction as the cyclization procedure. Upon addition of Pd(2+), coordination of the pyridine moieties occurs both intra and intermolecularly, to afford chiral ordered mono and dimeric macrocycles or multimeric aggregates depending on the solvents and conditions used. The metal binding event takes place in combination with a significant macrocyclic conformational rearrangement detected by circular dichroism spectroscopy. When in combination with a third component (C60), the macrocycle-Pd(2+) hybrid undergoes surface-confined nanostructuring into chiral nanofibres.

Linear assembly of a porphyrin–C60 complex confined in vertical nanocylinders of amphiphilic block copolymer films

Linear assemblies of a 1 : 1 porphyrin–fullerene C₆₀ complex were formed in vertical cylindrical polyether nanodomains of amphiphilic block copolymer films.

Photophysical insights into fullerene–porphyrazine supramolecular interactions in solution

This communication reports supramolecular interactions of a porphyrazine derivative, namely, 2,7,12,17-tetra-tert-butyl-5,10,15,20-tetraaza-21H,23H-porphine (1) with C₆₀ and C₇₀ in toluene and dichlorobenzene.