ON/OFF metal-triggered molecular tweezers for fullerene recognition

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.

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

Nanographene-Fused Expanded Carbaporphyrin Tweezers

A nanographene-fused expanded carbaporphyrin (5) and its BF2 complex (6) were synthesized. Single-crystal X-ray structures revealed that 5 and 6 are connected by two hexa-peri-hexabenzocoronene (HBC) units and two dipyrromethene or BODIPY units, respectively. As prepared, 5 and 6 both show nonaromatic character with figure-of-eight carbaoctaphyrin (1.1.1.0.1.1.1.0) cores and adopt tweezers-like conformations characterized by a partially confined space between the two constituent HBC units. The distance between the HBC centers is >10 Å, while the dihedral angles between the two HBC planes are 30.5 and 35.2° for 5 and 6, respectively. The interactions between 5 and 6 and fullerene C60 were studied both in organic media and in the solid state. Proton NMR spectral titrations of 5 and 6 with C60 revealed a 1:1 binding mode for both macrocycles. In toluene-d8, the corresponding binding constants were determined to be 1141 ± 17 and 994 ± 10 M-1 for 5 and 6, respectively. Single-crystal X-ray diffraction structural analyses confirmed the formation of 1:1 fullerene inclusion complexes in the solid state. The C60 guests in both complexes are found within triangular pockets composed of two HBC units from the tweezers-like receptor most closely associated with the bound fullerene, as well as an HBC unit from an adjacent host. Femtosecond transient absorption measurements revealed subpicosecond ultrafast charge separation between 5 (and 6) and C60 in the complexes. To the best of our knowledge, the present report provides the first example wherein a nanographene building block is incorporated into the core of a porphyrinic framework.

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).

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Chemically Driven Rotatory Molecular Machines

Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light-induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines.

Self-Resetting Bistable Redox Molecular Machines for Fullerene Recognition

Addressing control over molecular machines resulting in variable output modulation by mimicking nature mechanisms is a current hot topic. The exploitation of reversibility in thiol/disulfide motifs in chemical systems flanked by nonplanar corannulene moieties capable to recognize fullerenes is presented herein. Two redox-based machines have been conceived for this purpose: an ON/OFF switch that activates its binding properties upon dimerization and a self-resetting (i.e., with an automated backward process) host that substantially modulates its affinity.