Simultaneous enhancement of phosphorescence and chirality by host–guest recognition of molecular tweezers

Manipulation of 1D and 2D self-assembly via geometry modulation of adamantane isocyanide Pt(II) complexes

Two cationic luminescent cyclometalated Pt(II) complexes with adamantane-based isocyanide ligands are reported. This work provides important insights for the manipulation of the 1D and 2D self-assembly of Pt(II) complexes by controlling the geometry.

Light-triggered transformation of stilbene supramolecular polymers: thermodynamic versus kinetic control

Light irradiation of stilbene supramolecular polymers produces [2+2] cycloadducts in the kinetically trapped state, which convert to the thermodynamically favorable state upon thermal annealing due to the shift of hydrogen bonds from intra- to inter-complexation modes.

Enhanced Near-infrared Phosphorescent Emission Modulated by Clipping of Metallotweezers in Aqueous Media

Near-infrared phosphorescent materials have received significant attention due to their potential applications in bioimaging and diagnostics. Although, many types of organic phosphors with near-infrared emission have been developed, the low phosphorescence efficiency in aqueous solution hampers their practical applications in biological systems. Hence, there is an urgent need to develop near-infrared phosphorescent materials with high emission efficiency in aqueous media. Metallotweezers, based on d8 transition metal complexes, emerge as the potential candidates for realizing this objective. Specifically, metallotweezers, featuring two positively charged platinum(II) terpyridine and neutral gold(III) diphenylpyridine pincers on diphenylpyridine spacer, have been designed and synthesized, respectively. The pre-organization effect, rendered by the rigid spacer, enables the resulting metallotweezers to complex with each other, resulting in the formation of clipping complex. The synergistic rigidifying and shielding effects of clipping structure results in enhanced phosphorescent emission intensity. Concurrently, due to phase segregation between the clipping units and the polyethylene glycol tail, the clipping complex undergoes self-assembly in aqueous solution, resulting in phosphorescent emission in the near-infrared region. Overall, non-covalent clipping of metallotweezers illustrated in this study presents a new and effective approach toward near-infrared phosphorescent materials.

Achieving Circularly Polarized Phosphorescence through Noncovalent Clipping of Metallotweezers

Circularly polarized phosphorescent materials, based on host-guest complexation, have received significant attention due to their outstanding emission performance in solutions. Recent studies have primarily focused on macrocyclic host-guest complexes. To broaden the scope of this research, there is a keen pursuit of developing novel chiral phosphorescent host-guest systems. Metallotweezers with square-planar d8 transition metal complexes emerge as promising candidates for achieving this objective. Specifically, metallotweezers, comprising platinum(II) terpyridine and gold(III) diphenylpyridine pincers on a diphenylpyridine scaffold, have been designed and synthesized. Due to the preorganization effect rendered by the diphenylpyridine scaffold, the resulting metallotweezers are capable of complexing with each other and forming quadruple stacking structures. The phosphorescent emission is enhanced owing to the synergistic rigidifying and shielding effects. Meanwhile, the steric effect of chiral (1R) pinene units on the platinum(II) terpyridine pincers results in a stereospecific twist for the quadruple stacking structures. Thus, the chirality transfers from the molecular to the supramolecular level. By a combination of phosphorescent enhancement and supramolecular chirality for the clipping complex, circularly polarized phosphorescent emission is achieved. Overall, noncovalent clipping of metallotweezers exemplified in the current study presents a novel and effective approach toward solution-processable circularly polarized phosphorescent materials.

A sequential light-harvesting system with thermosensitive colorimetric emission in both aqueous solution and hydrogel

Inspired by natural photosynthetic systems that feature both sequential energy transfer and temperature response, we herein report an artificial thermosensitive sequential light-harvesting system (LHS) based on an amphiphilic molecule TPEO. It self-assembles into fluorescent nanoparticles in water and shows tunable LCST behavior. By loading ESY as the first acceptor and NiR as the second acceptor into the nanoparticles, an artificial LHS with two-step FRET was successfully constructed. Interestingly, the system exhibits thermosensitive colorimetric fluorescence in both aqueous solution and hydrogel by taking advantage of a combination of LCST and sequential FRET.

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.

Circularly Polarized Phosphorescence of Benzils Achieved by Chiral Supramolecular Polymerization

In current approaches for circularly polarized phosphorescent materials, the crystallization of chiral phosphors suffers from poor processability, while integrating them into an amorphous polymer matrix results in unsatisfactory chiroptical signals due to the absence of chirality communication. Here, we have developed an innovative strategy through chiral supramolecular polymerization of benzil phosphors facilitated by intermolecular hydrogen bonds. The inherent film-forming capabilities of non-covalent supramolecular polymers obviate the need for an external polymer matrix. The pronounced helical asymmetry of benzil phosphors resulting from chiral supramolecular polymerization leads to enhanced circularly polarized phosphorescence compared to their non-hydrogen-bonded counterparts. The circularly polarized phosphorescent signals can be further modulated by varying the location of stereogenic centers or introducing halogen bonding to benzils. Incorporation of platinum(II) phosphor into the benzil supramolecular polymers induces both chirality and triplet-to-triplet energy transfer, leading to a change in circularly polarized phosphorescent color from yellow to red. In summary, chiral supramolecular polymerization of phosphors represents a novel and effective approach to circularly polarized phosphorescent materials.

Frustrated π-stacking

The properties of functional materials based on organic π-conjugated systems are governed extensively by intermolecular interactions between π-molecules. To establish clear relationships between supramolecular structures and functional properties, it is essential to attain structurally well-defined π-stacks, particularly in solution, as this enables the collection of valuable spectroscopic data. However, precise control and fine-tuning of π-stacks pose significant challenges due to the weak and bidirectional nature of π-π stacking interactions. This article introduces the concept of "frustrated π-stacking," strategically balancing attractive (π-π interaction) and repulsive (steric hindrance) forces in self-assembly to exert control over the sizes, sequences of π-stacks, and slip-stacked structures. These research efforts contribute to a deeper understanding of the correlation between π-stacks and their properties, thereby providing useful insights for the development of molecular materials with the desired performance.

Pathway Control of π-Conjugated Supramolecular Polymers by Incorporating Donor-Acceptor Functionality

Controlling the nanoscale orientation of π-conjugated systems remains challenging due to the complexity of multiple energy landscapes involved in the supramolecular assembly process. In this study, we have developed an effective strategy for programming the pathways of π-conjugated supramolecular polymers, by incorporating both electron-rich methoxy- or methanthiol-benzene as donor unit and electron-poor cyano-vinylenes as acceptor units on the monomeric structure. It leads to the formation of parallel-stacked supramolecular polymers as the metastable species through homomeric donor/acceptor packing, which convert to slip-stacked supramolecular polymers as the thermodynamically stable species facilitated by heteromeric donor-acceptor packing. By further investigating the external seed-induced kinetic-to-thermodynamic transformation behaviors, our findings suggest that the donor-acceptor functionality on the seed structure is crucial for accelerating pathway conversion. This is achieved by eliminating the initial lag phase in the supramolecular polymerization process. Overall, this study provides valuable insights into designing molecular structures that control aggregation pathways of π-conjugated nanostructures.

Dual supramolecular chirogenesis based on platinum(II) metallotweezers

Optically active platinum(II) metallotweezers demonstrate both self-complexation and host-guest complexation capabilities, leading to two distinct supramolecular chirogenic signals in the visible region.

Chirality-Controlled Supramolecular Donor-Acceptor Copolymerization with Distinct Energy Transfer Efficiency

Chirality delivers substantial value to the field of supramolecular polymers, not only serving as a probe to monitor the dynamic assembly process but providing access to chiroptical materials. The current study demonstrates that, for supramolecular donor-acceptor copolymers, their comonomer organization modes can be greatly influenced by stereocommunication at the molecular level. The enantiopure N-[(1R or 1S)-phenylethyl]benzamides are incorporated into two structurally similar comonomers, locating between the π-aromatic diethynylacene core and the alkyl chain peripheries. Parallel arrangement of the stereogenic methyl units brings steric hindrance between the homochiral comonomers, which is relieved for the heterochiral comonomers due to the adoption of staggered arrangement. It consequently steers randomly mixed organization for the homochiral supramolecular copolymers within the nanofibers. In comparison, the heterochiral counterparts form nanoparticles in an alternate donor-acceptor organization manner. The variation of comonomer arrangement modes gives rise to distinct energy transfer efficiency at the supramolecular level. Overall, the elaborate manipulation of stereogenic centers in the comonomer structures exerts significant impacts on the characteristics of supramolecular copolymers, which could be useful for chiral sensing, recognition, and optoelectronic applications.

Phosphorescent Host-Guest Complexes on the Basis of Polyhedral Oligomeric Silsesquioxane-Functionalized Metallotweezers

Phosphorescent host-guest systems have attracted considerable attention because of their intriguing properties and diverse applications. In this study, a polyhedral oligomeric silsesquioxane-functionalized gold(III) tweezer receptor has been designed and synthesized. It is capable of sandwiching platinum(II) terpyridine compounds into its cavity with a high noncovalent binding affinity (association constants: ∼10⁵ M^-1 in chloroform). The resulting heterometallic host-guest complexes exhibit enhanced phosphorescent emission compared with those of the individual species in chloroform, thanks to the prevention of vibration and rotation upon noncovalent complexation. They can further assemble into nanospheres in chloroform/diethyl ether (1:9, v/v) owing to phase segregation between the metallotweezer/guest motif and the peripheral polyhedral oligomeric silsesquioxane unit. When terpyridine platinum(II) chloride serves as the complementary guest, the resulting noncovalent system displays an intraligand emission at the individual host-guest complexed state yet excimeric emission at the supramolecular assembled state, yielding the phosphorescent solvatochromic behaviors. Overall, the polyhedral oligomeric silsesquioxane-functionalized metallotweezer combines guest encapsulation and supramolecular assembly capabilities, which provides new avenues for color-tunable phosphorescent materials.

Hierarchical lyotropic liquid crystalline behaviors of supramolecular polymers influenced by alkyl chain branching

The peripheral chain branching in monomeric structures influences the hierarchical supramolecular assembly and lyotropic liquid crystalline properties.