Divinylanthracene-Containing Tetracationic Organic Cyclophane with Near-Infrared Photoluminescence.

Near-infrared (NIR) light is known to have outstanding optical penetration in biological tissues and to be non-invasive to cells compared with visible light. These characteristics make NIR-specific light optimal for numerous biological applications, such as the sensing of biomolecules or in theranostics. Over the years, significant progress has been achieved in the synthesis of fluorescent cyclophanes for sensing, bioimaging, and making optoelectronic materials. The preparation of NIR-emissive porphyrin-free cyclophanes is, however, still challenging. In an attempt for fluorescence emissions to reach into the NIR spectral region, employing organic tetracationic cyclophanes, we have inserted two 9,10-divinylanthracene units between two of the pyridinium units in cyclobis(paraquat-p-phenylene). Steady-state absorption, fluorescence, and transient-absorption spectroscopies reveal the deep-red and NIR photoluminescence of this cyclophane. This tetracationic cyclophane is highly soluble in water and has been employed successfully as a probe for live-cell imaging in a breast cancer cell line (MCF-7).

Triplet-Triplet Annihilation Upconversion in a Porphyrinic Molecular Container.

Triplet-triplet annihilation-based molecular photon upconversion (TTA-UC) is a photophysical phenomenon that can yield high-energy emitting photons from low-energy incident light. TTA-UC is believed to fuse two triplet excitons into a singlet exciton through several consecutive energy-conversion processes. When organic aromatic dyes─i.e., sensitizers and annihilators─are used in TTA-UC, intermolecular distances, as well as relative orientations between the two chromophores, are important in an attempt to attain high upconversion efficiencies. Herein, we demonstrate a host-guest strategy─e.g., a cage-like molecular container incorporating two porphyrinic sensitizers and encapsulating two perylene emitters inside its cavity─to harness photon upconversion. Central to this design is tailoring the cavity size (9.6-10.4 Å) of the molecular container so that it can host two annihilators with a suitable [π···π] distance (3.2-3.5 Å). The formation of a complex with a host:guest ratio of 1:2 between a porphyrinic molecular container and perylene was confirmed by NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry (ITC) as well as by DFT calculations. We have obtained TTA-UC yielding blue emission at 470 nm when the complex is excited with low-energy photons. This proof-of-concept demonstrates that TTA-UC can take place in one supermolecule by bringing together the sensitizers and annihilators. Our investigations open up some new opportunities for addressing several issues associated with supramolecular photon upconversion, such as sample concentrations, molecular aggregation, and penetration depths, which have relevance to biological imaging applications.

An electric molecular motor.

Macroscopic electric motors continue to have a large impact on almost every aspect of modern society. Consequently, the effort towards developing molecular motors1-3 that can be driven by electricity could not be more timely. Here we describe an electric molecular motor based on a [3]catenane4,5, in which two cyclobis(paraquat-p-phenylene)6 (CBPQT4+) rings are powered by electricity in solution to circumrotate unidirectionally around a 50-membered loop. The constitution of the loop ensures that both rings undergo highly (85%) unidirectional movement under the guidance of a flashing energy ratchet7,8, whereas the interactions between the two rings give rise to a two-dimensional potential energy surface (PES) similar to that shown by FOF1 ATP synthase9. The unidirectionality is powered by an oscillating10 voltage11,12 or external modulation of the redox potential13. Initially, we focused our attention on the homologous [2]catenane, only to find that the kinetic asymmetry was insufficient to support unidirectional movement of the sole ring. Accordingly, we incorporated a second CBPQT4+ ring to provide further symmetry breaking by interactions between the two mobile rings. This demonstration of electrically driven continual circumrotatory motion of two rings around a loop in a [3]catenane is free from the production of waste products and represents an important step towards surface-bound14 electric molecular motors.

Polyrotaxanes and the pump paradigm.

The year 2022 marks the 30th anniversary of the first reports of polyrotaxanes in the scientific literature. During the past three decades, many combinations of molecular rings and polymer chains have been synthesised and characterised. Until recently, however, the permutations of polyrotaxanes available to researchers were limited by synthetic methods which typically relied on an innate affinity between the molecular rings and polymer chains. With the advent of oligorotaxane-forming molecular pumps in 2015, it has now become possible to pump multiple rings against their will onto oligomer and polymer chains which have little or no affinity for the rings. These molecular pumps, which can recruit rings actively from solution to form precise polyrotaxanes, represent a major breakthrough in the field. This Tutorial Review highlights key milestones in the synthesis and investigation of polyrotaxanes along with recent developments in the synthesis and theory relating to molecular pumps. Polyrotaxane properties, arising from their topologies, have allowed them to steal a march on traditional polymers in a wide range of applications in materials, electronic and biological science, from slide-ring gels to robust coatings on cell phones, from molecular wires to flexible binders for battery anodes, from efficient multivalent protein binders to bio-cleavable polyplexes for cellular DNA delivery. Molecular pumps have the potential to blaze a contemporary trail for the synthesis of precise mechanically interlocked materials, especially those dependent on non-equilibrium chemistry and those related to energy storage and nanomedicine.

Precise Non-Equilibrium Polypropylene Glycol Polyrotaxanes.

Traditionally, the synthesis of polyrotaxanes has been limited by synthetic methods that rely on an innate affinity between the rings and the polymer chains. The use of rotaxane-forming molecular pumps allows this limitation to be circumvented in the production of non-equilibrium polyrotaxanes in which rings are trapped on polymer chains for which they have little or no affinity. Pumping cassettes, each composed of a bipyridinium unit linked (i) by a bismethylene bridge to a terminal 2,6-dimethylpyridinium cationic unit and (ii) by a methylene group to an isopropylphenylene steric barrier, were attached using copper-catalyzed azide-alkyne cycloadditions to the ends of a polypropylene glycol (PPG) chain of number-average molecular weight Mn ≈ 2200. Using a one-pot electrosynthetic protocol, a series of PPG-based polyrotaxanes with cyclobis(paraquat-p-phenylene) as the rings were synthesized. Despite the steric bulk of the PPG backbone, it was found to be a suitable collecting chain for threading up to 10 rings. The pumping of two rings is sufficient to render these hydrophobic polymers soluble in aqueous solution. Their hydrodynamic diameters and diffusion constants vary according to the number of pumped rings. The non-equilibrium nature of these polyrotaxanes is manifested in their gradual degradation and dethreading at elevated temperatures.

Alkoxy-Substituted Quadrupolar Fluorescent Dyes.

Polar and polarizable π-conjugated organic molecules containing push-pull chromophores have been investigated extensively in the past. Identifying unique backbones and building blocks for fluorescent dyes is a timely exercise. Here, we report the synthesis and characterization of a series of fluorescent dyes containing quadrupolar A-D-A constitutions (where A = acceptor and D = donor), which exhibit fluorescence emission at a variety of different wavelengths. We have investigated the effects of different electron-withdrawing groups, located at both termini of a para-terphenylene backbone, by steady-state UV/vis and fluorescence spectroscopy. Pyridine and substituted pyridinium units are also introduced during the construction of the quadrupolar backbones. Depending on the quadrupolarity, fluorescence emission wavelengths cover from 380 to 557 nm. Time-resolved absorption and emission spectroscopy reveal that the photophysical properties of those quadrupolar dyes result from intramolecular charge transfer. One of the dyes we have investigated is a symmetrical box-like tetracationic cyclophane. Its water-soluble tetrachloride, which is non-cytotoxic to cells up to a loading concentration of 1 µM, has been employed in live-cell imaging. When taken up by cells, the tetrachloride emits a green fluorescence emission without any hint of photobleaching or disruption of normal cell behavior. We envision that our design strategy of modifying molecules through the functionalization of the quadrupolar building blocks as chromophores will lead to future generations of fluorescent dyes in which these A-D-A constitutional fragments are incorporated into more complex molecules and polymers for broader photophysical and biological applications.

Active mechanisorption driven by pumping cassettes.

Over the past century, adsorption has been investigated extensively in equilibrium systems, with a focus on the van der Waals interactions associated with physisorption and electronic interactions in the case of chemisorption. In this study, we demonstrate mechanisorption, which results from nonequilibrium pumping to form mechanical bonds between the adsorbent and the adsorbate. This active mode of adsorption has been realized on surfaces of metal-organic frameworks grafted with arrays of molecular pumps. Adsorbates are transported from one well-defined compartment, the bulk, to another well-defined compartment, the interface, thereby creating large potential gradients in the form of chemical capacitors wherein energy is stored in metastable states. Mechanisorption extends, in a fundamental manner, the scope and potential of adsorption phenomena and offers a transformative approach to control chemistry at surfaces and interfaces.

Molecular Pumps and Motors.

Pumps and motors are essential components of the world as we know it. From the complex proteins that sustain our cells, to the mechanical marvels that power industries, much we take for granted is only possible because of pumps and motors. Although molecular pumps and motors have supported life for eons, it is only recently that chemists have made progress toward designing and building artificial forms of the microscopic machinery present in nature. The advent of artificial molecular machines has granted scientists an unprecedented level of control over the relative motion of components of molecules through the development of kinetically controlled, away-from-thermodynamic equilibrium chemistry. We outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective. A key insight connecting biomolecular and artificial molecular machines is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium at every instant. The operation of molecular motors and pumps can be described by trajectory thermodynamics, a theory based on the work of Onsager, which is grounded on the firm foundation of the principle of microscopic reversibility. Free energy derived from thermodynamically non-equilibrium reactions kinetically favors some reaction pathways over others. By designing molecules with kinetic asymmetry, one can engineer potential landscapes to harness external energy to drive the formation and maintenance of geometries of component parts of molecules away-from-equilibrium, that would be impossible to achieve by standard synthetic approaches.