Ramesh Jasti

Synthesis, Characterization, and Theory of [9]-, [12]-, and [18]Cycloparaphenylene: Carbon Nanohoop Structures

The first synthesis and characterization of [9]-, [12]-, and [18]cycloparaphenylene was demonstrated utilizing a novel aromatization reaction. We refer to these fascinating structures as “carbon nanohoops” due to their structural similarity to carbon nanotubes. Additionally, we have utilized computational methods to understand the unique properties of these fully conjugated macrocycles.

Synthesis, Characterization, and Crystal Structure of [6]Cycloparaphenylene†

macrocycles, represent the smallest structural unit of armchair carbon nanotubes (CNTs) and were termed “carbon nanohoops” by Jasti and Bertozzi in 2008. Although they possess a simple structure, the syntheses of CPPs have been a significant challenge, because the distorted aromatic rings are subjected to a considerable amount of strain energy. To our knowledge, the first synthetic endeavor targeting the CPPs was reported by Parekh and Guha in 1934. Modern cycloparaphenylene research can be traced back to the pioneering work by Vçgtle and co-workers, in which they suggested several ingenious strategies for the syntheses of the cycloparaphenylenes. More than seventy years after their original conceptualization, Jasti and Bertozzi reported the first synthesis of the CPPs. Following this seminal work, several clever strategies to the cycloparaphenylenes have been widely explored by Itami and co-workers and Yamago and coworkers. [n]CPPs have been successfully synthesized in many sizes (n = 7–16) and the shortest sidewall segments of chiral single-walled nanotubes (SWNTs) have also been prepared. Moreover, Itami has demonstrated the molecular structures of [9]and [12]CPP by X-ray crystallographic analysis. Computational studies have revealed that the strain energy of the [n]CPPs increases with decreasing n. 8] For example, the strain energy of [20]CPP is about 29 kcal mol , whereas [6]CPP, suggested to be the smallest CPP retaining a benzenoid structure, has a significantly higher strain energy of 97 kcalmol . Owing to the increasing strain energy, the smaller CPPs are extremely challenging synthetic targets. Very recently, we reported the selective synthesis of highly strained [7]cycloparaphenylene using cyclohexadienes as masked aromatic rings. A synthetic strategy to access even smaller cycloparaphenylenes would be a significant addition to this exciting area. Herein, we describe the selective synthesis, crystal structure, and optoelectronic characterization of [6]CPP (1), which is the smallest carbon nanohoop synthesized to date. Analysis of the X-ray structure revealed a fascinating packing geometry reminiscent of a carbon nanotube. To synthesize [6]cycloparaphenylene (1), we envisioned macrocycle precursor 9 as the penultimate intermediate in the sequence (Scheme 1). Macrocycle 9 contains two cyclohexadiene units as masked benzene rings to attenuate the strain energy for cyclization. We designed a sequential oxidative dearomatization/addition procedure to prepare dibromide 8 with high diastereoselectivity. The synthesis began with the oxidation of 4-bromo-4’[(trimethylsilyl)oxy]biphenyl (2) in the presence of phenyliodine(III) diacetate (PIDA) and water to generate ketone 3. 11] Deprotonation of alcohol 3 with Figure 1. [6]CPP is the shortest-possible subunit of a (6,6) CNT.

Selective Synthesis of Strained [7]Cycloparaphenylene: An Orange-Emitting Fluorophore

[n]Cycloparaphenylenes, which are short fragments of carbon nanotubes, have unique size-dependent optical properties. In this communication, we describe the first synthesis of [7]cycloparaphenylene ([7]CPP), the smallest cycloparaphenylene prepared to date. In order to access this structure, we have developed a synthetic route that capitalizes on successive orthogonal Suzuki-Miyaura coupling reactions. [7]CPP has 83 kcal/mol of strain energy and an orange emission at 592 nm.

Gram-scale synthesis and crystal structures of [8]- and [10]CPP, and the solid-state structure of C60@[10]CPP

A cost-effective gram-scale synthesis of [8]- and [10]cycloparaphenylenes (CPPs) has been developed for the first time. Both [8]- and [10]cycloparaphenylene organized into herringbone geometries in the crystalline state with well-defined cylindrical cavities of 1.1 and 1.4 nm, respectively. With large amounts of material available, the highly efficient convex–concave π–π interactions between [10]CPP and C60 in the solid-state was validated by X-ray diffraction analysis.

Bending Benzene: Syntheses of [n]Cycloparaphenylenes

Since the first successful synthesis in 2008, methods to prepare the [n]cycloparaphenylenes have evolved rapidly. The aim of this synopsis is to provide an overview of recent advancements in this emerging field. The optoelectronic properties and supramolecular chemistry of these unique structures are presented as well.

Selective Syntheses of [7]–[12]Cycloparaphenylenes Using Orthogonal Suzuki–Miyaura Cross-Coupling Reactions

The divergent, selective syntheses of [7]-[12]cycloparaphenylenes have been accomplished utilizing sequential, orthogonal Suzuki-Miyaura cross-coupling reactions from two late-stage intermediates. Quantum yields decrease dramatically as cycloparaphenylene size decreases, highlighting the unique photophysical behavior of the smaller cycloparaphenylenes.

Synthesis, Characterization, and Computational Studies of Cycloparaphenylene Dimers

Two novel arene-bridged cycloparaphenylene dimers (1 and 2) were prepared using a functionalized precursor, bromo-substituted macrocycle 7. The preferred conformations of these dimeric structures were evaluated computationally in the solid state, as well as in the gas and solution phases. In the solid state, the trans configuration of 1 is preferred by 34 kcal/mol due to the denser crystal packing structure that is achieved. In contrast, in the gas phase and in solution, the cis conformation is favored by 7 kcal/mol (dimer 1) and 10 kcal/mol (dimer 2), with a cis to trans activation barrier of 20 kcal/mol. The stabilization seen in the cis conformations is attributed to the increased van der Waals interactions between the two cycloparaphenylene rings. These calculations indicate that the cis conformation is accessible in solution, which is promising for future efforts toward the synthesis of short carbon nanotubes (CNTs) via cycloparaphenylene monomers. In addition, the optoelectronic properties of these dimeric cycloparaphenylenes were characterized both experimentally and computationally for the first time.

Self-Trapping of Excitons, Violation of Condon Approximation, and Efficient Fluorescence in Conjugated Cycloparaphenylenes

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.

Synthesis of Tetraphenyl-Substituted [12]Cycloparaphenylene: Toward a Rationally Designed Ultrashort Carbon Nanotube

The first phenyl-substituted [n]cycloparaphenylene (1) has been synthesized. The preparation of this structure addresses several challenges toward a more elaborate phenyl-substituted [n]cycloparaphenylene (2), a molecule that may lead to the homogeneous synthesis of armchair carbon nanotubes.

Photophysical and theoretical investigations of the [8]cycloparaphenylene radical cation and its charge-resonance dimer

Treatment of [8]cycloparaphenylene (CPP) with the oxidant triethyloxonium hexachloroantimonate afforded an isolable radical cation of the parent carbon nanohoop. The photophysical properties of [8]CPP˙+SbCl6− were investigated, showing the presence of two absorptions at 535 nm and 1115 nm. Time-dependent density functional theory (DFT) calculations were used to examine these optical absorptions, revealing a delocalized, quinoidal carbon nanohoop. Upon mixing with neutral [8]cycloparaphenylene, the formation of an unusually strong charge-resonance complex ([8]CPP2)˙+ was observed. Spectroscopic and computational studies were indicative of extensive intermolecular charge delocalization between the two carbon nanohoops as well.