Rhodium-Catalyzed Synthesis, Crystal Structures, and Photophysical Properties of [6]Cycloparaphenylene Tetracarboxylates

Carbon Nanomaterial Fluorescent Probes and Their Biological Applications

Fluorescent carbon nanomaterials have broadly useful chemical and photophysical attributes that are conducive to applications in biology. In this review, we focus on materials whose photophysics allow for the use of these materials in biomedical and environmental applications, with emphasis on imaging, biosensing, and cargo delivery. The review focuses primarily on graphitic carbon nanomaterials including graphene and its derivatives, carbon nanotubes, as well as carbon dots and carbon nanohoops. Recent advances in and future prospects of these fields are discussed at depth, and where appropriate, references to reviews pertaining to older literature are provided.

Continuous-Flow Synthesis of Cycloparaphenylene Building Blocks on a Large Scale

The synthesis of [n]cycloparaphenylenes ([n]CPPs) and similar nanohoops is usually based on combining building blocks to a macrocyclic precursor, which is then aromatized in the final step. Access to those building blocks in large amounts will simplify the synthesis and studies of CPPs as novel functional materials for applications. Herein, we report a continuous-flow synthesis of key CPP building blocks by using versatile synthesis techniques such as electrochemical oxidation, lithiations and Suzuki cross-couplings in self-built reactors on up-to kilogram scale.

A Modular Synthesis of Substituted Cycloparaphenylenes

Herein, we report a modular synthesis providing access to substituted cycloparaphenylenes (CPPs) of different sizes. A key synthon introducing two geminal ester units was efficiently prepared by [2+2+2] cycloaddition. This building block can be conveniently converted to macrocyclic precursors controlling the ring size of the final CPP. Efficient reductive aromatization through single-electron transfer provided the substituted nanohoops in a straightforward manner. The tBu ester substitution pattern enables a tube-like arrangement in the solid-state governed by van der Waals interactions that exhibits one of the tightest packings of CPPs in tube direction, thus opening new avenues in the crystal design of CPPs.

Construction of a double-walled carbon nanoring

The nanoring structure of cycloparaphenylenes (CPPs) can be considered as the shortest fragment of a carbon nanotube. Herein, we successfully prepared a double-walled carbon nanoring of [6]CPP⊂[12]CPP, which can be regarded as the shortest double-walled carbon nanotube. [6]CPP⊂[12]CPP was constructed through the supramolecular assembly, and its crystallographic structure was unambiguously determined by single-crystal X-ray diffraction. The host-guest interaction and charge transfer in [6]CPP⊂[12]CPP were disclosed by UV-Vis absorption, fluorescence, and electrochemical studies. Electron paramagnetic resonance (EPR) spectroscopy disclosed the stability of the [6]CPP⊂[12]CPP cation radical, whose unpaired spin was fully delocalized on the inner [6]CPP and well protected by outer [12]CPP. Moreover, [6]CPP⊂[12]CPP shows highly enhanced photoconductivity and photocurrent under light irradiation compared to those of pristine monomers. The self-assembly behavior of [6]CPP⊂[12]CPP was also studied, and it was found that [6]CPP⊂[12]CPP molecules tend to form a square rod structure in the DMF solution. Thus, these results demonstrate that this double-walled carbon nanoring material has a great potential application in photoelectronic devices and organic semiconductors.

Photophysical property change of N-(5-bromo-salicylidene)-3-aminoethylpyridine monohydrated crystals via dehydration phase transition

The crystals of N-salicylideneaniline (SA) and SA derivatives are classic functional materials that exhibit reversible colour changes (photochromism) and/or excited-state intramolecular proton transfer (ESIPT) fluorescence emission under ultraviolet (UV) light irradiation. In this study, a novel SA derivative was synthesised with an extended alkyl chain, N-(5-bromo-salicylidene)-3-aminoethylpyridine (5Br-SAEP). The photophysical properties of 5Br-SAEP were characterised in the crystalline state. The monohydrated crystal (1H) of 5Br-SAEP was dehydrated to form the anhydrous crystal (1A) at a relative humidity of less than 76%. The photochromic activity was switched by the dehydration phase transition from the non-photochromic 1H to the photochromic 1A. The quantum yield of fluorescence decreased significantly from 8% in 1H to 3% in 1A. The in situ change of photophysical properties occurred due to the change in the crystal structure. This indicated the potential of the solvated crystals of the SAEP derivatives for applications in novel switching or smart materials.

A Macrocyclic Gold(I)-Biphenylene Complex: Triangular Molecular Structure with Twisted Au2 (diphosphine) Corners and Reductive Elimination of [6]Cycloparaphenylene

The digold(I) complex [Au₂ Cl₂ (Cy₂ PCH₂ PCy₂ )] reacts with 4,4'-diphenylene diboronic acid to form a triangular macrocyclic complex with twisted Au-P-C-P-Au groups at the three corners. The synthesis of the complex and its chemical oxidation produced [6]cycloparaphenylene ([6]CPP) in 59 % overall yield.

Synthesis, Structures, and Properties of Highly Strained Cyclophenylene-Ethynylenes with Axial and Helical Chirality

Single and double cyclophenylene-ethynylenes (CPEs) with axial and helical chirality have been synthesized by the Sonogashira cross-coupling of di- and tetraethynyl biphenyls with a U-shaped prearomatic diiodoparaphenylene followed by reductive aromatization. X-ray crystallographic analyses and DFT calculations revealed that the CPEs possess highly twisted bent structures. Bend angles on the edge of the paraphenylene units were close to the value of [5]cycloparaphenylene (CPP)-the smallest CPP to date. The double and single CPEs possessed stable chirality despite flexible biphenyl structures because of the high strain in the diethynyl-paraphenylene moiety. In both the single and double CPEs, orbital interactions along the biphenyl axis were observed by DFT calculations in LUMO and LUMO+2 of the single CPE and LUMO+1 of the double CPE, which likely cause lowering of these orbital energies. Concerning chiroptical properties: boosting of the g_abs value was observed in the biphenyl-based double CPE, as well as the binaphthyl-based single CPE, compared to the biphenyl-based single CPE.

Enantioselective Synthesis of Planar Chiral Zigzag-Type Cyclophenylene Belts by Rhodium-Catalyzed Alkyne Cyclotrimerization

Planar chiral zigzag-type [8]- and [12]cyclophenylene (CP) belts have been synthesized in good yields with high ee values of 98% and 83%, respectively, by the rhodium-catalyzed enantioselective intramolecular sequential cyclotrimerizations of the corresponding cyclic polyynes. The observed high enantioselectivity arises from the regioselective formation of a rhodacycle intermediate from an unsymmetric triyne unit. The X-ray crystal structural analysis of the racemic planar chiral zigzag-type [8]CP belt revealed that the uneven molecules mesh with each other to form a one-dimensional columnar packing structure, in which one column contains single enantiomers, giving two types of chiral columns [(S)- and (R)-form columns] arranged alternately. The ring strain of the zigzag-type [8]CP belt was smaller than that of the armchair-type [8]CPP belt despite its smaller ring size, due to the presence of the strain-relieving m-terphenyl moieties. The effect of the number of the benzene rings of the zigzag-type CP belts on absorption and emission peaks was small due to interruption of π-conjugation at the m-phenylene moieties. However, the bending effect on the absolute fluorescence quantum yield as well as absorption and emission peaks was significant. Concerning chiroptical properties, the modest anisotropy dissymmetry factors of ECD and CPL were observed in the [8]CP belt.

Site-selective [2 + 2 + n] cycloadditions for rapid, scalable access to alkynylated polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are attractive synthetic building blocks for more complex conjugated nanocarbons, but their use for this purpose requires appreciable quantities of a PAH with reactive functional groups. Despite tremendous recent advances, most synthetic methods cannot satisfy these demands. Here we present a general and scalable [2 + 2 + n] (n = 1 or 2) cycloaddition strategy to access PAHs that are decorated with synthetically versatile alkynyl groups and its application to seven structurally diverse PAH ring systems (thirteen new alkynylated PAHs in total). The critical discovery is the site-selectivity of an Ir-catalyzed [2 + 2 + 2] cycloaddition, which preferentially cyclizes tethered diyne units with preservation of other (peripheral) alkynyl groups. The potential for generalization of the site-selectivity to other [2 + 2 + n] reactions is demonstrated by identification of a Cp₂Zr-mediated [2 + 2 + 1]/metallacycle transfer sequence for synthesis of an alkynylated, selenophene-annulated PAH. The new PAHs are excellent synthons for macrocyclic conjugated nanocarbons. As a proof of concept, four were subjected to alkyne metathesis catalysis to afford large, PAH-containing arylene ethylene macrocycles, which possess a range of cavity sizes reaching well into the nanometer regime. Notably, these high-yielding macrocyclizations establish that synthetically convenient pentynyl groups can be effective for metathesis since the 4-octyne byproduct is sequestered by 5 Å MS. Most importantly, this work is a demonstration of how site-selective reactions can be harnessed to rapidly build up structural complexity in a practical, scalable fashion.

An orthogonal [2 + 2 + n] cycloaddition/alkyne metathesis reaction sequence enables streamlined access to conjugated macrocyclic nanocarbons.