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.

Multicomponent Assembled Systems Based on Platinum(II) Terpyridine Complexes

Platinum(II) terpyridine complexes have received tremendous attention in recent years because of their square-planar geometry and fascinating photophysics. Bottom-up self-assembly represents an intriguing approach to construct well-ordered supramolecular architectures with tunable optical and electronic properties. Until now, much effort has been devoted to the fabrication of monocomponent platinum(II) terpyridine-based assemblies. The next step is to develop multicomponent coassembled systems via the combination of platinum(II) terpyridine complexes with other π-organic and -organometallic molecules. The implementation of electron/energy transfer processes renders advanced functionality to the resulting coassemblies. For the fabrication of discrete multicomponent architectures, a feasible protocol is to construct preorganized molecular tweezers and macrocycles with the involvement of platinum(II) terpyridine complexes as the panel units. In view of their planar surface and positively charged character, such supramolecular receptors are capable of encapsulating electron-rich polyaromatic hydrocarbons and organometallic guests via donor-acceptor charge-transfer and/or metal-metal interactions. Intermolecular hydrogen bonds can be further incorporated between the molecular tweezers receptor and the polyaromatic hydrocarbon guests, giving rise to the strengthened binding affinity and sensitive stimuli-responsiveness. On this basis, multilayer donor-acceptor stacks have been obtained via the precise control over the number of pincers, which feature enhanced complexation strength and superior functionality. Moreover, platinum(II) terpyridine-based macrocycles are more suitable for guest accommodation than the corresponding molecular tweezers receptors in light of their definite size and constrained environment. Stimuli-responsive elements can be conveniently implemented into the rigid spacers of the molecular tweezers and macrocyclic receptors, facilitating the capture and release of the sandwiched guests in a highly controlled manner. On the other hand, long-range-ordered supramolecular polymers have been successfully fabricated with linear, hyperbranched, and cross-linked topologies by employing platinum(II) terpyridine-based molecular tweezers/guest recognition motifs as the non-covalent connecting unit. The degree of polymerization of the resulting donor-acceptor-type supramolecular polymers can be efficiently modulated by incorporating intermolecular hydrogen bonds between the molecular tweezers receptor and the complementary guest unit. An alternative approach toward extended multicomponent donor-acceptor assemblies is to mimic the structure of Magnus' green salt. A delicate balance of non-covalent driving forces between homo- and heterocomplexation processes and a deeper understanding of thermodynamic and kinetic behaviors play the decisive roles in the final arrangement of the coassembled structures. Overall, multicomponent coassembly of platinum(II) terpyridine complexes into well-ordered nanostructures would open up a new avenue toward functional supramolecular materials that are especially promising for sensing, optoelectronics, and catalytic applications.