The design and construction of non-natural products have fascinated and perplexed organic chemists for years. Their assembly, akin to what has been accomplished for the total synthesis of natural products, has stretched the limits of what can be prepared in the laboratory. Unlike many natural products, however, carbon-rich structures often lack heteroatoms, further complicating their construction. Consider some of the classical molecules in this genre: cubane and dodecahedrane. While highly symmetric, their assembly is far from trivial. These fascinating hydrocarbon targets have fueled the development of carbon-carbon bond-forming reactions, as new methods are needed to access these types of compounds. Among these carbon-rich structures, polycyclic aromatics such as helicenes, fullerenes, and some fullerenes share common ground due to the distortion of one or more aromatic rings out of planarity. Recently added to this group are the [n]cycloparaphenylenes ([n]CPPs), "carbon nanohoops". Here, a linear string of benzene rings connected at the para positions is wrapped back upon itself to form a cyclic structure. Clearly a simple linear p-oligophenylene cannot be cyclized in this manner without extremely harsh reaction conditions. In order to access these structures using solution-phase organic chemistry, clever synthetic strategies that can compensate for this severe distortion are required. Although cycloparaphenylenes can be considered the smallest possible fragment of an armchair carbon nanotube (CNT), they were envisioned as synthetic targets long before CNTs were discovered in 1991. CPP synthesis was first attempted in 1934, almost 70 years before Iijima's first report on CNTs. The long-forgotten targets reemerged in 1993 with a report from Vögtle, though he ultimately was unsuccessful in achieving their synthesis. More than a decade later, in 2008, CPPs succumbed to total synthesis by Jasti and Bertozzi, allowing access to three different-sized carbon nanohoops in milligram quantities. Since then, the Jasti group has embraced the smallest CPPs as inspiring synthetic targets, challenging us to develop new methodology to construct increasingly strained macrocycles. Having recently synthesized [5]-, [6]- and [7]CPP, the three smallest nanohoops synthesized to date, we have been able to realize a variety of new physical phenomena unique to these structures. Perhaps most significantly, unlike linear p-phenylenes and inorganic quantum dots, the HOMO-LUMO gaps of the CPPs narrow with decreasing CPP size. The smallest CPPs discussed in this Account illustrate this feature exceptionally well, as their HOMO-LUMO gaps become narrower than those of even the longest p-polyphenylenes. The smaller CPPs are fascinating from a structural standpoint as well because of the high amount of distortion in each benzene ring. From the synthesis of [7]CPP (84 kcal/mol of strain energy) to that of [5]CPP (119 kcal/mol of strain energy), our laboratory has been able to test the boundaries of synthetic and physical organic chemistry. In ...

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.

[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.

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.

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Studies on the phase segregation of unimolecular block copolymers (BCPs) are limited by a lack of reliable, versatile methods for the synthesis of such polymers on the preparative scale. Herein, we describe an advancement of Iterative Exponential Growth (IEG) wherein chiral allyl-based IEG oligomers are subjected to thiol-ene reactions and converted into unimolecular BCPs. With this strategy we have synthesized uniform BCPs with molar masses up to 12.1 kDa on ∼1 g scale. BCPs composed of decane-based side chains and either triethyleneglycol- or thioglycerol-based side chains phase-segregate into hexagonal cylinder morphologies. The assembly is not driven by side-chain crystallization, but is instead the result of amorphous BCP assembly.

Nitroxides occupy a privileged position among plausible metal-free magnetic resonance imaging (MRI) contrast agents (CAs) due to their inherently low-toxicity profiles; nevertheless, their translational development has been hindered by a lack of appropriate contrast sensitivity. Nanostructured materials with high nitroxide densities, where each individual nitroxide within a macromolecular construct contributes to the image contrast, could address this limitation, but the synthesis of such materials remains challenging. Here, we report a modular and scalable synthetic

Deciphering the significance of length, sequence, and stereochemistry in block copolymer self-assembly remains an ongoing challenge. A dearth of methods to access uniform block co-oligomers/polymers with precise stereochemical sequences has precluded such studies. Here, we develop iterative exponential growth (IEG) methods for the synthesis of a small library of unimolecular stereoisomeric diblock 32-mers. X-ray scattering reveals that stereochemistry modulates the phase behavior of these polymers, which we rationalize based on simulations carried out on a theoretical model system. This work demonstrates that stereochemical sequence can play a crucial role in unimolecular polymer self-assembly.