Superphanes: From Synthetic Breakthroughs to Supramolecular Hosts and Functional Molecular Materials

Conspectus

The development of host molecules has long been central to the progress of supramolecular chemistry and the design of advanced functional materials. Over the past several decades, a wide variety of host architectures─including crown ethers, cyclodextrins, cucurbiturils, calixarenes, calixpyrroles, pillararenes, beltarenes, covalent organic cages, and metal–organic cages─have been devised and exploited for molecular recognition, sensing, transport, self-assembly, and catalysis. Despite these advances, the continued search for structurally unique and functionally versatile molecular systems remains essential to achieving new levels of precision in host–guest chemistry and to driving the development of next-generation functional materials.

Superphanes, defined by two face-to-face benzene rings clamped by six bridging linkers, represent a structurally distinctive class of molecular constructs. First conceptualized by Vögtle in 1972 and synthesized by Boekelheide in 1979 as models for π–π interactions, superphanes remained largely unexplored for decades due to formidable synthetic challenges. Only two covalent superphanes without accessible voids and a handful of metal-coordinated analogues have been described, with investigations restricted primarily to synthesis and structural characterization. Importantly, their potential in host–guest chemistry and functional materials applications went unrealized, hindered by both the lack of practical synthetic access and the long-standing perception that these frameworks offered no accessible cavities.

Around the same time, our group, in parallel with Badjić and co-workers, independently developed dynamic imine-condensation strategies for accessing imine-based superphane-type architectures. Building on this foundation, our group further advanced superphane chemistry by establishing efficient gram-scale synthetic access through dynamic imine assembly, followed by postsynthetic stabilization via reduction or oxidation. Distinguished by six bridging linkers that generate a nearly enclosed three-dimensional cavity with extreme confinement, superphanes offer unique advantages, including guest incarceration, enhanced binding affinities, super cage effects, and a uniformly distributed array of binding sites. Their dense, symmetric architectures provide precise control over molecular recognition events that surpasses many traditional macrocycles and cages.

In this Account, we summarize recent breakthroughs in superphane chemistry, including scalable dynamic covalent syntheses, presynthetic functionalization, postsynthetic modifications, and fine-tuning of the cavity environment. We further highlight how these synthetic and design advances have enabled a wide spectrum of applications, ranging from fundamental host–guest chemistry to practical uses in environmental remediation, selective recovery of high-value resources, photocatalysis, energy storage, and desalination. Notably, superphane-based nonporous amorphous sorbent (NAS) materials have, for the first time, demonstrated performance competitive with state-of-the-art porous frameworks such as MOFs and COFs. Together, these examples underscore how structure–property relationships govern the supramolecular behavior and functional output of superphanes.

By tracing the conceptual evolution of superphanes from rigid cyclophanes to near-enclosed functional architectures with extreme confinement cavities, this Account aims to clarify the conceptual foundations of superphane chemistry while highlighting emerging opportunities. We anticipate that continued exploration at the intersection of supramolecular design, materials science, and molecular engineering will firmly establish superphanes as a distinct class of functional molecular systems. In doing so, they are poised to provide instructive design principles and transformative potential for the creation of next-generation functional molecular materials.