Toward a Charged Homo[2]catenane Employing Diazaperopyrenium Homophilic Recognition

Functionalization of a [2]Catenane with Donor-Acceptor Chromophores Using a Metal Template and Click Reactions

Synthesizing molecules with significant topological features, such as catenanes, tailored with specific groups to confer desired functionality, is essential for investigating various properties arising from the entanglement due to mechanical bonds. This investigation can pave the way for uncovering novel functional materials employing mechanically interlocked molecules (MIMs). In this direction, we have synthesized a π-donor (D) and π-acceptor (A) functionalized [2]catenane using a non-labile Co(III) metal ion as a template with pyridine-diamide templating center and utilizing click reaction for ring-closing. The donor group is a fluorene derivative, and the acceptor is a benzophenazine derivative, commonly employed in synthesizing conjugated polymers for various optoelectronic devices. Synthetically, the acceptor group was introduced into a macrocycle with a pyridine diamide unit. It was then threaded with a ligand having alkyne terminals to obtain the desired [2]pseudorotaxane utilizing cobalt ion as a template. Ring-closing was then performed with a di-azide functionalized molecule with the donor chromophore. The desired D-A functionalized [2]catenane was obtained after demetalation. All the starting materials, macrocycle, and entangled structures have been characterized by 1H-NMR, 13C-NMR, and mass spectroscopy. Some of these materials were also characterized by single-crystal X-ray analysis. The photophysical properties are studied by UV-visible and fluorescence spectroscopy.

Switchable Topologically Chiral [2]Catenane as Multiple Resonance Thermally Activated Delayed Fluorescence Emitter for Efficient Circularly Polarized Electroluminescence

Aiming at the fabrication of circularly polarized organic light-emitting diodes (CP-OLEDs) with high dissymmetry factors (gEL) and color purity through the employment of novel chiral source, topologically chiral [2]catenanes were first utilized as the key chiral skeleton to construct novel multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters. Impressively, the efficient chirality induction and unique switchable feature of topologically chiral [2]catenane not only lead to a high |gPL| value up to 1.6×10-2 but also facilitate in situ dynamic switching of the full-width at half-maximum (FWHM) and circularly polarized luminescence (CPL). Furthermore, the solution-processed CP-OLEDs based on the resultant topologically chiral emitters exhibit a narrow FWHM of 36 nm, maximum external quantum efficiency of 17.6 %, and CPEL with |gEL| of 2.1×10-3. This study demonstrates the successful construction of the first CP-MR-TADF emitters based on topological chirality with the highest |gPL| among the reported CP-MR-TADF emitters and excellent device performance to the best of our knowledge. Moreover, it endowed the MR-TADF emitter with distinctive switchable CPL performances, thus providing a novel design strategy as well as a promising platform for developing intelligent CP-OLEDs.

Advancements and strategic approaches in catenane synthesis

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

A Terrylene Bisimide based Universal Host for Aromatic Guests to Derive Contact Surface-Dependent Dispersion Energies

π-π interactions are among the most important intermolecular interactions in supramolecular systems. Here we determine experimentally a universal parameter for their strength that is simply based on the size of the interacting contact surfaces. Toward this goal we designed a new cyclophane based on terrylene bisimide (TBI) π-walls connected by para-xylylene spacer units. With its extended π-surface this cyclophane proved to be an excellent and universal host for the complexation of π-conjugated guests, including small and large polycyclic aromatic hydrocarbons (PAHs) as well as dye molecules. The observed binding constants range up to 108 M-1 and show a linear dependence on the 2D area size of the guest molecules. This correlation can be used for the prediction of binding constants and for the design of new host-guest systems based on the herewith derived universal Gibbs interaction energy parameter of 0.31 kJ/molÅ2 in chloroform.

Exciplex Emission and Förster Resonance Energy Transfer in Polycyclic Aromatic Hydrocarbon-Based Bischromophoric Cyclophanes and Homo[2]catenanes

Energy transfer and exciplex emission are not only crucial photophysical processes in many living organisms but also important for the development of smart photonic materials. We report, herein, the rationally designed synthesis and characterization of two highly charged bischromophoric homo[2]catenanes and one cyclophane incorporating a combination of polycyclic aromatic hydrocarbons, i.e., anthracene, pyrene, and perylene, which are intrinsically capable of supporting energy transfer and exciplex formation. The possible coconformations of the homo[2]catenanes, on account of their dynamic behavior, have been probed by Density Functional Theory calculations. The unique photophysical properties of these exotic molecules have been explored by steady-state and time-resolved absorption and fluorescence spectroscopies. The tetracationic pyrene-perylene cyclophane system exhibits emission emanating from a highly efficient Förster resonance energy transfer (FRET) mechanism which occurs in 48 ps, while the octacationic homo[2]catenane displays a weak exciplex photoluminescence following extremely fast (<0.3 ps) exciplex formation. The in-depth fundamental understanding of these photophysical processes involved in the fluorescence of bischromophoric cyclophanes and homo[2]catenanes paves the way for their use in future bioapplications and photonic devices.

Confinement-Driven Photophysics in Hydrazone-Based Hierarchical Materials

Confinement-imposed photophysics was probed for novel stimuli-responsive hydrazone-based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution-like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady-state and time-resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone-based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Thermally Controlled Exciplex Fluorescence in a Dynamic Homo[2]catenane

Motion-induced change in emission (MICE) is a phenomenon that can be employed to develop various types of probes, including temperature and viscosity sensors. Although MICE, arising from the conformational motion in particular compounds, has been studied extensively, this phenomenon has not been investigated in depth in mechanically interlocked molecules (MIMs) undergoing coconformational changes. Herein, we report the investigation of a thermoresponsive dynamic homo[2]catenane incorporating pyrene units and displaying relative circumrotational motions of its cyclophanes as evidenced by variable-temperature ¹H NMR spectroscopy and supported by its visualization through molecular dynamics simulations and quantum mechanics calculations. The relative coconformational motions induce a significant change in the fluorescence emission of the homo[2]catenane upon changes in temperature compared with its component cyclophanes. This variation in the exciplex emission of the homo[2]catenane is reversible as demonstrated by four complete cooling and heating cycles. This research opens up possibilities of using the coconformational changes in MIMs-based chromophores for probing fluctuations in temperature which could lead to applications in biomedicine or materials science.

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.

A light-responsive molecular switch based on cucurbit[7]uril and 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium dibromide displaying white light emission

By using ¹H NMR, ESI-MS and UV spectra, a novel light-responsive molecular switch constructed using 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium (1²⁺) and cucurbit[7]uril (Q[7]) is demonstrated. The E- to Z-isomerization of the double bond in 1²⁺ results in the transition of the switching states from the 1 : 2 complex E-1²⁺@Q[7]₂ to the stable 1 : 1 complex Z-1²⁺@Q[7]. In particular, both the 1 : 2 complex and the 1 : 1 complex can emit cold white fluorescence under UV light.

Macrocyclic host molecules with aromatic building blocks: the state of the art and progress

Macrocyclic host molecules play the central role in host-guest chemistry and supramolecular chemistry. The highly structural symmetry of macrocyclic host molecules can meet people's pursuit of aesthetics in molecular design, and generally means a balance of design, synthesis, properties and applications. For macrocyclic host molecules with highly symmetrical structures, building blocks, which could be described as repeat units as well, are the most fundamental elements for molecular design. The structural features and recognition ability of macrocyclic host molecules are determined by the building blocks and their connection patterns. Using different building blocks, different macrocyclic host molecules could be designed and synthesized. With decades of developments of host-guest chemistry and supramolecular chemistry, diverse macrocyclic host molecules with different building blocks have been designed and synthesized. Aromatic building blocks are a big family among the various building blocks used in constructing macrocyclic host molecules. In this feature article, the recent developments of macrocyclic host molecules with aromatic building blocks were summarized and discussed.

Synthesis, structure elucidation and functionalization of sulfonamide [2]catenanes

A pyrene-functionalized [2]catenane with switchable optical output was constructed through a novel sulfonamide [2]catenane synthesized by a self-templation approach.

Mechanical-Bond-Induced Exciplex Fluorescence in an Anthracene-Based Homo[2]catenane

Collisional intermolecular interactions between excited states form short-lived dimers and complexes that lead to the emergence of excimer/exciplex emission of lower energy, a phenomenon which must be differentiated from the photoluminescence (PL) arising from the monomeric molecules. Although the utilization of noncovalent bonding interactions, leading to the generation of excimer/exciplex PL, has been investigated extensively, precise control of the aggregates and their persistence at very low concentrations remains a rare phenomenon. In the search for a fresh approach, we sought to obtain exciplex PL from permanent structures by incorporating anthracene moieties into pyridinium-containing mechanically interlocked molecules. Beyond the optical properties of the anthracene moieties, their π-extended nature enforces [π···π] stacking that can overcome the Coulombic repulsion between the pyridinium units, affording an efficient synthesis of an octacationic homo[2]catenane. Notably, upon increasing the ionic strength by adding tetrabutylammonium hexafluorophosphate, the catenane yield increases significantly as a result of the decrease in Coulombic repulsions between the pyridinium units. Although the ground-state photophysical properties of the free cyclophane and the catenane are similar and show a charge-transfer band at ∼455 nm, their PL characters are distinct, denoting different excited states. The cyclophane emits at ∼562 nm (quantum yield ϕ_F = 3.6%, emission lifetime τ_s = 3 ns in MeCN), which is characteristic of a disubstituted anthracene-pyridinium linker. By contrast, the catenane displays an exciplex PL at low concentration (10^-8 M) with an emission band centered on 650 nm (ϕ_F = 0.5%, τ_s = 14 ns) in MeCN and at 675 nm in aqueous solution. Live-cell imaging performed in MIAPaCa-2 prostate cancer cells confirmed that the catenane exciplex emission can be detected at micromolar concentrations.

Symmetrical-Tetramethyl-Cucurbit[6]uril-Driven Movement of Cucurbit[7]uril Gives Rise to Heterowheel [4]Pseudorotaxanes

Two novel heterowheel [4]pseudorotaxanes consisting of cucurbit[7]uril (Q[7]) and symmetrical-tetramethyl-cucurbit[6]uril (TMeQ[6]) were constructed via the multirecognition mechanism, in which Q[7] can rotate freely around the horizontal axis, while TMeQ[6] cannot. In the construction process, due to strong repulsive forces between carbonyl portals of two neighboring wheels, the dethreading and movement of the wheels along the axle was observed. The dissociation of the [4]pseudorotaxanes was also discussed.