Fluorophore-Functionalized and Top-Covered Resorcin[4]arene Cavitands

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

Macrocyclic Arenes Functionalized with BODIPY: Rising Stars among Chemosensors and Smart Materials

Macrocycles play a crucial role in supramolecular chemistry and the family of macrocyclic arenes represents one of the most important types of hosts. Among them, calixarenes, resorcinarenes and pillararenes are the most commonly encountered macrocyclic arenes, and they have received considerable attention. Boron-dipyrromethene (BODIPY) dyes are fascinating compounds with multiple functionalization sites and outstanding luminescence properties including high fluorescence quantum yields, large molar absorption coefficients and remarkable photo- and chemical stability. The combination of macrocyclic arenes and BODIPY dyes has been demonstrated to be an effective strategy to construct chemosensors for various guests and smart materials with tailored properties. Herein, we firstly summarize the recent advances made so far in macrocyclic arenes substituted with BODIPY. This review only focuses on the three macrocyclic arenes of calixarenes, resorcinarenes and pillararenes, as there are no other macrocyclic arenes substituted BODIPY units at the present time. Hopefully, this review will not only afford a guide and useful information for those who are interested in developing novel chemosensors and smart materials, but also inspire new opportunities in this field.

Molecular Containers Derived from [60]Fullerene through Peroxide Chemistry

Molecular containers can keep guest molecules in a confined space that is completely separated from the solution. They have wide potential applications, including selective trapping of reactive intermediates, catalysis within the cavity, and molecular delivery. Numerous molecular containers have been prepared through covalent bonds, metal-ligand interactions and H-bonding or hydrophobic interactions. Fullerenes are all-carbon molecules with a spherical structure. Partial opening of the cage structure results in open-cage fullerenes, which can serve as molecular containers for various small molecules and atoms. Compared with classical molecular containers, open-cage fullerenes exhibit some unusual phenomena because of the unique structure of the fullerene cage. The synthesis of an open-cage fullerene with a large enough orifice as a molecular container requires consecutive cleavage of multiple fullerene skeleton bonds within a local area on the cage surface. In spite of the difficulty, remarkable progress has been achieved. Several reactions have been reported to cleave fullerene C-C bonds selectively to form open-cage fullerenes, some of which have been successfully used as molecular containers for molecules such as H₂O. The size and shape of the orifice play a key role in the encapsulation of the guest molecule. To date the focus in this area has been the preparation of open-cage fullerenes and encapsulation of small molecules. Little information has been reported about the functional properties of these host-guest systems. Potential applications of these systems need to be explored. This Account mainly presents our results on the encapsulation of small molecules in open-cage fullerenes prepared in my group. The preparation of our open-cage fullerenes is based on fullerene-mixed peroxides, which are briefly mentioned herein. The encapsulation and release of a single molecule of water is discussed in detail. Quantitative water encapsulation was achieved by heating the open-cage fullerene in a homogeneous CDCl₃/H₂O/EtOH mixture at 80 °C for 18 h. The kinetics of the water release process was studied by blackbody IR radiation-induced dissociation (BIRD) and theoretical calculations. The trapped water could also be released by H-bonding with HF. To control the encapsulation and release processes, we prepared open-cage fullerenes with a switchable stopper on the rim of the orifice. Besides H₂O, encapsulations of H₂, HF, CO, O₂, and H₂O₂ were also achieved by using different open-cage fullerenes. The encapsulation of CO is quite unusual in that the trapped CO is derived from a fullerene skeleton carbon that was pushed into the cavity by oxidation under ambient conditions at room temperature. The trapped O₂/H₂O₂ could be released slowly under mild conditions, and these systems are now being studied as a new type of oxygen-releasing materials for biomedical research. The present results demonstrate that open-cage fullerenes are suitable molecular containers for small molecules. Our future work will focus on optimizing the conditions for the preparation of open-cage fullerenes and applications of open-cage fullerenes in areas such as oxygen delivery for photodynamic therapy.

Endo or Exo? Structures of Gas-Phase Alkali Metal Cation/Aromatic Half-Belt Complexes

1,1,9,9-Tetramethyl[9](2,11)teropyrenophane (TM9TP), a belt-shaped molecule, has a sizable cavity that molecules or ions could occupy. In this study, the question of whether TM9TP forms gas-phase ion-molecule complexes with metal cations (K⁺ , Rb⁺ , Cs⁺ ) situated inside or outside the TM9TP cavity was addressed using both experimental and computational methods. Complexes were trapped in a Fourier transform ion cyclotron resonance mass spectrometer and their structures were explored by some novel physical chemistry/mass spectrometry methods. Blackbody infrared radiative dissociation kinetics reveal two populations of ions, a fast dissociating fraction and a persistent fraction. Infrared multiphoton dissociation spectra (vibrational spectra) provide very strong evidence that the most abundant population is a complex where the metal cation is inside the TM9TP cavity, endo-TM9TP. Red-shifted C-H stretching bands present in the gas-phase vibrational spectra of these ionic complexes show that there is an interaction between the metal cation and bridge C-H bonds due to the cation sitting inside the cavity of TM9TP. B3LYP/6-31+G(d,p) calculations showed the endo complexes to be the lowest in energy; about 60 kJ mol^-1 more thermodynamically stable and more than 120 kJ mol^-1 kinetically more stable than the exo complex.

Disentanglement of excited-state dynamics with implications for FRET measurements: two-dimensional electronic spectroscopy of a BODIPY-functionalized cavitand

Two-dimensional electronic spectroscopy of energy transfer and competing dynamics highlights how conformational changes create issues with lifetime-based FRET measurements.

Förster Resonance Energy Transfer (FRET) is the incoherent transfer of an electronic excitation from a donor fluorophore to a nearby acceptor. FRET has been applied as a probe of local chromophore environments and distances on the nanoscale by extrapolating transfer efficiencies from standard experimental parameters, such as fluorescence intensities or lifetimes. Competition from nonradiative relaxation processes is often assumed to be constant in these extrapolations, but in actuality, this competition depends on the donor and acceptor environments and can, therefore, be affected by conformational changes. To study the effects of nonradiative relaxation on FRET dynamics, we perform two-dimensional electronic spectroscopy (2DES) on a pair of azaboraindacene (BODIPY) dyes, attached to opposite arms of a resorcin[4]arene cavitand. Temperature-induced switching between two equilibrium conformations, vase at 294 K to kite at 193 K, increases the donor–acceptor distance from 0.5 nm to 3 nm, affecting both FRET efficiency and nonradiative relaxation. By disentangling different dynamics based on lifetimes extracted from a series of 2D spectra, we independently observe nonradiative relaxation, FRET, and residual fluorescence from the donor in both vase to kite conformations. We observe changes in both FRET rate and nonradiative relaxation when the molecule switches from vase to kite, and measure a significantly greater difference in transfer efficiency between conformations than would be determined by standard lifetime-based measurements. These observations show that changes in competing nonradiative processes must be taken into account when highly accurate measurements of FRET efficiency are desired.

Crystal structure of 2-(4-acetyl-2,6-dimethylphenyl)-5,6-dichloro-1H-isoindole-1,3(2H)-dione, C18H13Cl2NO3

C18H13Cl2NO3, monoclinic, P21/n (no. 14), a = 13.6464(2) Å, b = 18.1592(3) Å, c = 13.9829(2) Å, β = 107.557(1)°, V = 3303.66(9) Å3, Z = 8, Rgt(F) = 0.0291, wRref(F2) = 0.0768, T = 296(2) K.

Crystal structure of 2-(4-acetyl-2,6-dimethyl-phenyl)-5,6-dichloro-isoindole-1,3-dione, C18H13Cl2NO3

C18H13Cl2NO3, Mr = 362.19, monoclinic, P21/n, a = 13.6464(2) Å, b = 18.1592(3) Å, c = 13.9829(2) Å, β = 107.557(1)°, V = 3303.66(9) Å3, Z = 8, Mo K α radiation, μ = 0.41 mm−1, T = 296 K, 0.30 × 0.20 × 0.20 mm.

Molecular recognition in chemical and biological systems

Structure-based ligand design in medicinal chemistry and crop protection relies on the identification and quantification of weak noncovalent interactions and understanding the role of water. Small-molecule and protein structural database searches are important tools to retrieve existing knowledge. Thermodynamic profiling, combined with X-ray structural and computational studies, is the key to elucidate the energetics of the replacement of water by ligands. Biological receptor sites vary greatly in shape, conformational dynamics, and polarity, and require different ligand-design strategies, as shown for various case studies. Interactions between dipoles have become a central theme of molecular recognition. Orthogonal interactions, halogen bonding, and amide⋅⋅⋅π stacking provide new tools for innovative lead optimization. The combination of synthetic models and biological complexation studies is required to gather reliable information on weak noncovalent interactions and the role of water.

Development of redox-switchable resorcin[4]arene cavitands

CONSPECTUS: Within the framework of miniaturization of electromechanical devices, the development of a redox-switchable molecular gripper as a tool for nanorobotics is appealing both from an academic and from a practical perspective. Such a tool should be able to controllably grab a molecular cargo, translocate it over considerable distances and time scales, and release it. Resorcin[4]arene cavitands seem to be an ideal platform for the development of molecular grippers due to their ability to adopt two spatially well-defined conformations: an expanded kite and a contracted vase. Furthermore, they possess "legs" for functionalization and attachment to metal surfaces. While changes in temperature, pH, and metal-ion concentration were known to induce conformational switching, redox-switchable cavitands remained a challenge. In this Account, we describe our efforts toward the development of a new class of resorcin[4]arene cavitands that are redox-switchable. First, we introduced the naphthoquinone moiety as a redox-active wall component and showed that cavitands containing four quinone walls strongly prefer the open kite conformation in both the quinone and hydroquinone redox states, while cavitands that contain two quinone and two quinoxaline walls can adopt both the vase and the kite conformations depending on solvent but not on redox state. Next, in order to introduce a driving force for the conformational switching process in diquinone cavitands, we designed cavitands with hydrogen bond acceptor groups on the quinoxaline walls. These acceptors were sought to establish hydrogen bonds with the hydroquinone groups in the reduced redox state, thereby stabilizing the vase form. Oxidation to the quinone state would remove these interactions, switching the cavitand back to the kite form. Cavitands equipped with different hydrogen bond acceptor groups were synthesized and evaluated. We found that carboxamide moieties are best suited to assist redox-induced switching of conformational and binding properties. With the goal of further increasing association constants and reducing guest-exchange rates via steric congestion, we exchanged the naphthoquinone with the triptycene-quinone moiety. The congesting influence of the triptycene-quinone moiety on the binding properties was quantified both in the presence and in the absence of additional hydrogen bond interactions that stabilize the vase form. X-ray crystallographic studies provided insights into the solid-state structures of the cavitands in different solvents and redox states. A significant enhancement of association constants and reduction in guest release rates was observed in the reduced redox state compared with the top-open system, yielding redox-switchable cavitand baskets. These studies represent a step towards the development of redox-switchable molecular grippers on metal surfaces. Future challenges will consist in the development of cavitands that will no longer rely on an external proton source for the switching process, allowing redox-switching to be performed in purely aprotic media. Finally, suitable leg functionalization would enable the grippers to be interfaced with metal surfaces.