Low-Entropy-Penalty Synthesis of Giant Macrocycles for Good Self-Assembly and Emission Enhancement

Abstract

Abstract Macrocycles are key tools for molecular recognition and self-assembly. However, traditionally prevalent macrocyclic compounds exhibit specific cavities with diameters usually less than 1 nm, limiting their range of applications in supramolecular chemistry. The efficient synthesis of giant macrocycles remains a significant challenge because an increase in the monomer number results in cyclization-entropy loss. In this study, we developed a low-entropy-penalty synthesis strategy for producing giant macrocycles in high yields. In this process, long and rigid monomers possessing two reaction modules were condensed with paraformaldehyde via Friedel–Crafts reaction. A series of giant macrocycles with cavities of sizes ranging from 2.0 nm to 4.7 nm were successfully synthesized with cyclization yields of up to 72%. Experimental results and theoretical calculations revealed that extending the monomer length rather than increasing the monomer numbers could notably reduce the cyclization-entropy penalty and avoid configuration twists, thereby favoring the formation of giant macrocycles with large cavities. Significantly, the excellent self-assembly capacity of these giant macrocycles promoted their assembly into organogels in various solvents. The obtained xerogels exhibited enhanced photoluminescence quantum efficiencies of up to 83.1%. Mechanism investigation revealed that the excellent assembly capacity originated from the abundant π–π interactions sites of the giant macrocycles. The outstanding emission enhancement resulted from the restricted nonradiative decay processes of rotation/vibration and improved radiative decay process of fluorescence. This study provides an effective and general method for achieving giant macrocycles, thereby expanding the supramolecular toolbox for host–guest chemistry and assembly applications. Moreover, the intriguingly assembly and photophysical properties demonstrate the feasibility of developing novel and unique properties by expanding the macrocycle size.