Ring–Chain Competition in Supramolecular Polymerization Directed by Molecular Recognition of the Bisporphyrin Cleft

Abstract

Increasing interest in innovative supramolecular materials has spurred efforts to develop head-to-tail monomers possessing a host moiety as a head and a guest moiety as a tail, making them capable of forming supramolecular polymers through intermolecular associations while avoiding intramolecular cyclization. This competition between the intramolecular cyclization and the intermolecular association is influenced by conformational entropy, relying on the flexibility of the linker chain that connects the host moiety to the guest moiety in a head-to-tail monomer. However, there are limited reports presenting a quantitative thermodynamic picture describing the conformational entropy in ring−chain equilibrium processes. Here, we report the quantitative evaluation of the role of conformational entropy in the ring−chain equilibrium mechanism of the supramolecular polymerization of a headto-tail monomer possessing a bisporphyrin host and a trinitrofluorenone guest connected with various alkyl chains as linkers. The supramolecular polymerization of the head-to-tail monomers was studied using UV/vis, NMR, and diffusion-ordered spectroscopy spectroscopic techniques and viscometry. The thermodynamic parameters of the ring−chain equilibrium were determined in the initial stage of the supramolecular polymerization. The conformational entropy, which relies on the flexibility of the linker, had a significant influence on the critical polymerization concentration. This quantitative discussion of ring−chain competition is expected to provide a foundation for the proper design of artificial head-to-tail monomers that form supramolecular polymers.

Host–Guest Complexation of Bisporphyrin Cleft and Electron-Deficient Aromatic Guests

“…Electron-deficient aromatic guest molecules are encapsulated within the electron-rich cleft-shaped cavity in a ratio of 1:1 through intermolecular charge transfer interactions. The strong host–guest interaction of 1 was appreciated for supramolecular polymerization. − However, due to a lack of crystal structure information and detailed thermodynamic analysis, our previous report in the host–guest complexation of 1 did not provide precise structural and mechanistic insights . Therefore, extensive investigation in the host–guest complexation of 1 was required.…”

Intermicellar Polymerization and Intramicellar Cyclization: A Supramolecular Ring–Chain Competition Reaction

“…The ring–chain competition between linear polymer chains and their cyclic counterparts is widely observed for monomers and even some oligomer units, for example, ditopic pyrimidinone, ribonuclease peptide, and oligomer units containing the bisporphyrin head and the trinitrofluorenone (TNF) tail. − ,− In this work, we found a novel supramolecular ring–chain competition phenomenon that exhibits the following uniqueness: (1) Different kinds of interactions: Polymerization and cyclization at the molecular level are related to the covalent interaction between end groups. − In this work, the supramolecular competition involves the noncovalent hydrophobic interaction. (2) The variable conformation of reaction units: In the classical JS model for the ring–chain competition at the molecular level, the chain conformation change is ignored.…”

“…Ring–chain equilibrium between intermolecular coupling and intramolecular cyclization, which is a common competitive reaction at the molecular level, affects the molecular structures and yields of products. − For instance, in the enzyme-catalyzed synthesis process of aliphatic and unsaturated polyesters, cyclic oligomers are formed concurrently with the corresponding linear chains, and the yield of cyclic oligomers depends on the monomer structure and the initial monomer concentration. ,, Even for the oligomer reaction units, the macrocyclization reaction can compete with their polymerization driven by the fast association and dissociation of reversible noncovalent interactions. − For such a system, it was found that the concentration of cyclic counterparts is influenced by the concentration and linker length of oligomer building blocks. − Thus, competition is crucial when attempting to obtain a high yield of linear or ring-like products or to build an exquisite buffering system. , However, the competitive reaction has rarely been observed in higher-level systems (higher level refers to the assembly of polymeric micellar subunits, which occurs at the nanoscale). − …”

Porphyrin-based supramolecular polymers

“…[51][52][53][54][55][56][57][58][59][60][61] The flat and expanded p aromatic structure of porphyrins allows for strong p-p interaction between the porphyrin units. [67][68][69][70][71][72][73][74][75] The p-p interactions often play a crucial role in the formation of porphyrin-based supramolecular polymers. However, p-p interaction mainly serves as a supplement unlike hydrogen bonding or coordination bonding because it is not an effective directional force to form linear structures.…”