Methylene-Bridged [6]-, [8]-, and [10]Cycloparaphenylenes: Size-Dependent Properties and Paratropic Belt Currents

Abstract

Cycloparaphenylenes (CPPs) and carbon nanobelts (CNBs) represent some of the most iconic cyclic molecular nanocarbons in recent chemistry owing to their unique properties derived from rigid, strained, and cyclic π-conjugated systems. In the last decade, the synthesis of various sizes of CPPs and CNBs has been achieved that allowed not only for investigating their size-dependent properties and strategically using such properties in various applications but also understanding the fundamental features of cyclic π-conjugated systems and molecular nanocarbons in general. Herein, we report on the synthesis, size-dependent properties, and paratropic belt currents of methylene-bridged [n]cycloparaphenylenes ([n]MCPP, n = 6, 8, 10). [8]MCPP and [10]MCPP were synthesized by the same strategy we developed for [6]MCPP synthesis. With readily available ethoxy-substituted pillar[8]arene and pillar[10]arene as precursors, [8]MCPP and [10]MCPP were successfully synthesized in three steps consisting of de-ethylation, triflation, and nickel-mediated aryl–aryl coupling. The structural and electronic properties of MCPPs were investigated by nuclear magnetic resonance analyses, absorption/fluorescence measurements, X-ray crystallographic analyses, and computational studies, revealing their interesting size-dependent properties. The differences in the size dependency between MCPPs and CPPs reflect the belt-form features of MCPPs, namely, methylene-bridging effects on MCPPs. Moreover, an interesting paratropic belt current along the MCPP backbone has been uncovered both experimentally and theoretically. The 1H NMR chemical shifts of MCPPs confirmed the presence of a paratropic belt current, whose strength rapidly decreases with increasing nanobelt size.

Carbon Nanobelts: Brief History and Perspective

“…In 2020, we synthesized a methylene-bridged [6]­CPP ([6]­MCPP), which is a substructure of heackelite CNTs, by 6-fold Ni-mediated Yamamoto coupling of triflated pillar[6]­arene . Our group applied the same synthetic method to pillar[8]­arene and pillar[10]­arene to synthesize [8]­MCPP and [10]­MCPP and to prism[6]­arene to synthesize methylene-bridged [6]­cyclonaphthylene ([6]­MCN) in 2023 . In 2021, the group of Chen also reported the synthesis of [6]­MCPP through an intramolecular coupling reaction of a methylene-bridged fluorene-based macrocycyle .…”

Tuning optical properties of π-conjugated double nanohoops under external electric field stimuli-responsiveness

“…In recent years, these molecules and their derivatives have gained significant attention in various applications such as fluorescent functional molecules, organic optoelectronic device materials, and supramolecular compounds owing to their distinctive optical and electronic properties. 30–34 Nakano group reported the diradical characteristics and third-order NLO properties of open-shell carbon nanobelts and carbon nanotubes. 35–37 Qiu group reported second-order NLO properties based on Pb( ii )CPP.…”

Axially Chiral Nonbenzenoid Nanographene with Second Harmonic Generation Property

“…It is known that the configurations, electronic properties, and bandgaps of nonbenzenoid NGs can be tuned by replacing the 6-membered rings with 5/7-membered rings. The configurations of nonbenzenoid NGs can be planar − or contorted − or curved − or helical, − or nanobelt, or bilayer, depending on the number of nonbenzenoid rings and how these nonbenzenoid rings are incorporated into the NGs. As a result, axially chiral NGs entailing nonbenzenoid rings are expected to show unusual and unique chiral characteristics compared with their benzenoid counterparts.…”