[n]Cycloparaphenylenes as Compatible Fluorophores for Melt Electrowriting.

Fluorescent probes are an indispensable tool in the realm of bioimaging technologies, providing valuable insights into the assessment of biomaterial integrity and structural properties. However, incorporating fluorophores into scaffolds made from melt electrowriting (MEW) poses a challenge due to the sustained, elevated temperatures that this processing technique requires. In this context, [n]cycloparaphenylenes ([n]CPPs) serve as excellent fluorophores for MEW processing with the additional benefit of customizable emissions profiles with the same excitation wavelength. Three fluorescent blends are used with distinct [n]CPPs with emission wavelengths of either 466, 494, or 533 nm, identifying 0.01 wt% as the preferred concentration. It is discovered that [n]CPPs disperse well within poly(ε-caprolactone) (PCL) and maintain their fluorescence even after a week of continuous heating at 80 °C. The [n]CPP-PCL blends show no cytotoxicity and support counterstaining with commonly used DAPI (Ex/Em: 359 nm/457 nm), rhodamine- (Ex/Em: 542/565 nm), and fluorescein-tagged (Ex/Em: 490/515 nm) phalloidin stains. Using different color [n]CPP-PCL blends, different MEW fibers are sequentially deposited into a semi-woven scaffold and onto a solution electrospun membrane composed of [8]CPP-PCL as a contrasting substrate for the [10]CPP-PCL MEW fibers. In general, [n]CPPs are potent fluorophores for MEW, providing new imaging options for this technology.

Appending Coronene Diimide with Carbon Nanohoops Allows for Rapid Intersystem Crossing in Neat Film.

The development of innovative triplet materials plays a significant role in various applications. Although effective tuning of triplet formation by intersystem crossing (ISC) has been well established in solution, the modulation of ISC processes in the solid state remains a challenge due to the presence of other exciton decay channels through intermolecular interactions. The cyclic structure of cycloparaphenylenes (CPPs) offers a unique platform to tune the intermolecular packing, which leads to controllable exciton dynamics in the solid state. Herein, by integrating an electron deficient coronene diimide (CDI) unit into the CPP framework, a donor-acceptor type of conjugated macrocycle (CDI-CPP) featuring intramolecular charge-transfer (CT) interaction was designed and synthesized. Effective intermolecular CT interaction resulting from a slipped herringbone packing was confirmed by X-ray crystallography. Transient spectroscopy studies showed that CDI-CPP undergoes ISC in both solution and the film state, with triplet generation time constants of 4.5 ns and 238 ps, respectively. The rapid triplet formation through ISC in the film state can be ascribed to the cooperation between intra- and intermolecular charge-transfer interactions. Our results highlight that intermolecular CT interaction has a pronounced effect on the ISC process in the solid state, and shed light on the use of the characteristic structure of CPPs to manipulate intermolecular CT interactions.

A High-Yielding Active Template Click Reaction (AT-CuAAC) for the Synthesis of Mechanically Interlocked Nanohoops.

Mechanically interlocked molecules (MIMs) represent an exciting yet underexplored area of research in the context of carbon nanoscience. Recently, work from our group and others has shown that small carbon nanotube fragments-[n]cycloparaphenylenes ([n]CPPs) and related nanohoop macrocycles-may be integrated into mechanically interlocked architectures by leveraging supramolecular interactions, covalent tethers, or metal-ion templates. Still, available synthetic methods are typically difficult and low yielding, and general methods that allow for the creation of a wide variety of these structures are limited. Here we report an efficient route to interlocked nanohoop structures via the active template Cu-catalyzed azide-alkyne cycloaddition (AT-CuAAC) reaction. With the appropriate choice of substituents, a macrocyclic precursor to 2,2'-bipyridyl embedded [9]CPP (bipy[9]CPP) participates in the AT-CuAAC reaction to provide [2]rotaxanes in near-quantitative yield, which can then be converted into the fully π-conjugated catenane structures. Through this approach, two nanohoop[2]catenanes are synthesized which consist of a bipy[9]CPP catenated with either Tz[10]CPP or Tz[12]CPP (where Tz denotes a 1,2,3-triazole moiety replacing one phenylene ring in the [n]CPP backbone).

Synthesis and Properties of Fluorenone-Containing Cycloparaphenylenes and Their Late-Stage Transformation.

Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor-acceptor chromophores were obtained by incorporating fluorenone or 2-(9H-fluoren-9-ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late-stage functionalization of the fluorenone-based rings by high-yielding Knoevenagel condensations. The structures were confirmed by X-ray crystallographic analyses, which revealed that replacing a phenylene for a fused-ring-system acceptor introduces additional strain. The donor-acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi-redox systems undergoing reversible or quasi-reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units.

Synthesis and metalation of polycatechol nanohoops derived from fluorocycloparaphenylenes.

Due to their unique topology and distinct physical properties, cycloparaphenylenes (CPPs) are attractive building blocks for new materials synthesis. While both noncovalent interactions and irreversible covalent bonds have been used to link CPP monomers into extended materials, a coordination chemistry approach remains less explored. Here we show that nucleophilic aromatic substitution reactions can be leveraged to rapidly introduce donor groups (-OR, -SR) onto polyfluorinated CPP rings. Demethylation of methoxide-substituted CPPs produces polycatechol nanohoop ligands that are readily metalated to produce well-defined, multimetallic CPP complexes. As catechols are recurring motifs throughout coordination chemistry and dynamic covalent chemistry, the polycatechol nanohoops reported here open the door to new strategies for the bottom-up synthesis of atomically precise CPP-based materials.

Tunable Macrocyclic Polyparaphenylene Nanolassos via Copper-Free Click Chemistry.

Deriving diverse compound libraries from a single substrate in high yields remains to be a challenge in cycloparaphenylene chemistry. In here, a strategy for the late-stage functionalization of shape-persistent alkyne-containing cycloparaphenylene has been explored using readily available azides. The copper-free [3+2]azide-alkyne cycloaddition provided high yields (>90 %) in a single reaction step. Systematic variation of the azides from electron-rich to -deficient shines light on how peripheral substitution influences the characteristics of the resulting adducts. We find that among the most affected properties are the molecular shape, the oxidation potential, excited state features, and affinities towards different fullerenes. Joint experimental and theoretical results are presented including calculations with the state-of-the-art, artificial intelligence-enhanced quantum mechanical method 1 (AIQM1).

Experimental and theoretical elucidation of SPAAC kinetics for strained alkyne-containing cycloparaphenylenes.

Tuning strained alkyne reactivity via organic synthesis has evolved into a burgeoning field of study largely focused on cyclooctyne, wherein physical organic chemistry helps guide rational molecular design to produce molecules with intriguing properties. Concurrent research in the field of carbon nanomaterials has produced new types of strained alkyne macrocycles, such as cycloparaphenyleneacetylenes, that possess uniquely curved aromatic π systems but hover on the edge of stability. In 2018, we introduced a strained alkyne scaffold that marries the synthetic accessibility and stability of cyclooctyne with the curved π system of carbon nanomaterials. These molecules are strained alkyne-containing cycloparaphenylenes (or [n+1]CPPs), which have been shown to possess size-dependent reactivity as well as the classic characteristics of the unfunctionalized parent CPP, such as a tunable HOMO-LUMO gap and bright fluorescence for large sizes. Herein, we elaborate further on this scaffold, introducing two modifications to the original design and fully characterizing the kinetics of the strain-promoted azide-alkyne cycloaddition (SPAAC) for each [n+1]CPP with a model azide. Additionally, we explain how electronic (the incorporation of fluorine atoms) and strain (a meta linkage which heightens local strain at the alkyne) modulations affect SPAAC reactivity via the distortion-interaction computational model. Altogether, these results indicate that through a modular synthesis and rational chemical design, we have developed a new family of tunable and inherently fluorescent strained alkyne carbon nanomaterials.

Active template strategy for the preparation of π-conjugated interlocked nanocarbons.

Mechanically interlocked carbon nanostructures represent a relatively unexplored frontier in carbon nanoscience due to the difficulty in preparing these unusual topological materials. Here we illustrate an active-template method in which a [n]cycloparaphenylene precursor macrocycle is decorated with two convergent pyridine donors that coordinate to a metal ion. The metal ion catalyses alkyne-alkyne cross-coupling reactions within the central cavity of the macrocycle, and the resultant interlocked products can be converted into fully π-conjugated structures in subsequent synthetic steps. Specifically, we report the synthesis of a family of catenanes that comprise two or three mutually interpenetrating [n]cycloparaphenylene-derived macrocycles of various sizes. Additionally, a fully π-conjugated [3]rotaxane was synthesized by the same method. The development of synthetic methods to access mechanically interlocked carbon nanostructures of varying topology can help elucidate the implications of mechanical bonding for this emerging class of nanomaterials and allow structure-property relationships to be established.

Splitting the Ring: Impact of Ortho and Meta Pi Conjugation Pathways through Disjointed [8]Cycloparaphenylene Electronic Materials.

In this report, we describe the synthesis and electronic properties of small-molecule and polymeric [8]cycloparaphenylenes ([8]CPPs) with disjointed pi-conjugated substituents. Arylene-ethynylene linkers were installed on opposite sides of the [8]CPP nanohoop as separated by three phenyl units on either side such that the monomer systems have syn (C2 symmetry) and anti (C1 symmetry) conformers with a small energy gap (0.1-0.6 kcal/mol). This disjoined substitution pattern necessarily forces delocalization through and around the CPP radial structure. We demonstrate new electronic states from this radial/linear mixing in both the small molecules and the pi extended polymers. Quantum chemical calculations reveal that these electronic processes arise from multiple operative radial/linear conjugation pathways, as the disjoint pattern results in both ortho and meta connections to the CPP ring. These results affirm the unique nature of hybrid radial and linear pi electron delocalization operative in these new conjugation pathways.

Polyyne [3]Rotaxanes: Synthesis via Dicobalt Carbonyl Complexes and Enhanced Stability.

New strategies for synthesizing polyyne polyrotaxanes are being developed as an approach to stable carbyne "insulated molecular wires". Here we report an active metal template route to polyyne [3]rotaxanes, using dicobalt carbonyl masked alkyne equivalents. We synthesized two [3]rotaxanes, both with the same C28 polyyne dumbbell component, one with a phenanthroline-based macrocycle and one using a 2,6-pyridyl cycloparaphenylene nanohoop. The thermal stabilities of the two rotaxanes were compared with that of the naked polyyne dumbbell in decalin at 80 °C, and the nanohoop rotaxane was found to be 4.5 times more stable.

Subcellular Targeted Nanohoop for One- and Two-Photon Live Cell Imaging.

Fluorophores are powerful tools for interrogating biological systems. Carbon nanotubes (CNTs) have long been attractive materials for biological imaging due to their near-infrared excitation and bright, tunable optical properties. The difficulty in synthesizing and functionalizing these materials with precision, however, has hampered progress in this area. Carbon nanohoops, which are macrocyclic CNT substructures, are carbon nanostructures that possess ideal photophysical characteristics of nanomaterials, while maintaining the precise synthesis of small molecules. However, much work remains to advance the nanohoop class of fluorophores as biological imaging agents. Herein, we report an intracellular targeted nanohoop. This fluorescent nanostructure is noncytotoxic at concentrations up to 50 µM, and cellular uptake investigations indicate internalization through endocytic pathways. Additionally, we employ this nanohoop for two-photon fluorescence imaging, demonstrating a high two-photon absorption cross-section (65 GM) and photostability comparable to a commercial probe. This work further motivates continued investigations into carbon nanohoop photophysics and their biological imaging applications.

Controlled Polymerization of Norbornene Cycloparaphenylenes Expands Carbon Nanomaterials Design Space.

Carbon-based materials-such as graphene nanoribbons, fullerenes, and carbon nanotubes-elicit significant excitement due to their wide-ranging properties and many possible applications. However, the lack of methods for precise synthesis, functionalization, and assembly of complex carbon materials has hindered efforts to define structure-property relationships and develop new carbon materials with unique properties. To overcome this challenge, we employed a combination of bottom-up organic synthesis and controlled polymer synthesis. We designed norbornene-functionalized cycloparaphenylenes (CPPs), a family of macrocycles that map onto armchair carbon nanotubes of varying diameters. Through ring-opening metathesis polymerization, we accessed homopolymers as well as block and statistical copolymers constructed from "carbon nanohoops" with a high degree of structural control. These soluble, sp2-carbon-dense polymers exhibit tunable fluorescence emission and supramolecular responses based on composition and sequence. This work represents an important advance toward bridging the gap between small molecules and functional carbon-based materials.

Nanohoop Rotaxane Design to Enhance the Selectivity of Reaction-Based Probes: A Proof-of-Principle Study.

Mechanical interlocking of a nanohoop fluorophore and a reactive thread couples the benefits of a reaction-based probe with a sterically congested active site for enhanced selectivity. Advantageously, the thread design uses dual function stoppers that act as both a quencher and a trigger for sensing. In progress toward expanding this approach to biologically relevant analytes, this system is used to demonstrate steric differentiation and provide a selective turn-on fluorescent response with size selectivity for HS- rather than larger thiolates.

Stretching [8]cycloparaphenylene with encapsulated potassium cations: structural and theoretical insights into core perturbation upon four-fold reduction and complexation.

The consequences of four-electron addition to [8]cycloparaphenylene ([8]CPP, 1) have been evaluated crystallographically, revealing a significant core deformation. The structural analysis exposes an elliptical distortion observed upon electron transfer, with the deformation parameter (D.P.) increased by 28% in comparison with neutral [8]CPP. The C-C bond length alteration pattern also indicates a quinoidal structural rearrangement upon four-fold reduction. The large internal cavity of [8]CPP4- allows the encapsulation of two {K+(THF)2} cationic moieties with two additional cations bound externally in the solid-state structure of [{K+(THF)2}4([8]CPP4-)]. The experimental structural data have been used as a benchmark for the comprehensive theoretical description of the geometric changes and electronic properties of the highly-charged [8]CPP4- nanohoop in comparison with its neutral parent. While neutral [8]CPP and the [8]CPP2- anion clearly show aromatic behavior of all six-membered rings, subsequent addition of two more electrons completely reverses their aromatic character to afford the highly-antiaromatic [8]CPP4- anion, as evidenced by structural, topological, and magnetic descriptors. The disentanglement of electron transfer from metal binding effects allowed their contributions to the overall core perturbation of the negatively-charged [8]CPP to be revealed. Consequently, the internal coordination of potassium cations is identified as the main driving force for drastic elliptic distortion of the macrocyclic framework upon reduction.

Spin-Crossover Properties of an Iron(II) Coordination Nanohoop.

Addition of the bipyridyl-embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H2 B(pyz)2 }] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H2 B(pyz)2 }2 ] (1). The molecular structure of 1 shows that the nanohoop ligand contains a non-planar bipy unit. Magnetic susceptibility measurements indicate spin-crossover (SCO) behaviour with a T1/2 of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H2 B(pyz)2 }2 ] (2), which contains a conventional bipy ligand. A computational study of 1 and 2 reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop.

Synthesis, Characterization, and Computational Investigation of Bright Orange-Emitting Benzothiadiazole [10]Cycloparaphenylene.

Conjugated aromatic macrocycles are attractive due to their unique photophysical and optoelectronic properties. In particular, the cyclic radially oriented π-system of cycloparaphenylenes (CPPs) gives rise to photophysical properties unlike any other small molecule or carbon nanomaterial. CPPs have tunable emission, possess large extinction coefficients, wide effective Stokes shifts, and high quantum yields. However, accessing bright CPPs with emissions beyond 500 nm remains difficult. Herein, we present a novel and bright orange-emitting CPP-based fluorophore showing a dramatic 105 nm red-shift in emission and striking 237 nm effective Stokes shift while retaining a large quantum yield of 0.59. We postulate, and experimentally and theoretically support, that the quantum yield remains large due to the lack of intramolecular charge transfer.

Exploration of the Solid-State Sorption Properties of Shape-Persistent Macrocyclic Nanocarbons as Bulk Materials and Small Aggregates.

Porous molecular materials combine benefits such as convenient processability and the possibility for atom-precise structural fine-tuning which makes them remarkable candidates for specialty applications in the areas of gas separation, catalysis, and sensing. In order to realize the full potential of these materials and guide future molecular design, knowledge of the transition from molecular properties into materials behavior is essential. In this work, the class of compounds termed cycloparaphenylenes (CPPs)-shape-persistent macrocycles with built-in cavities and radially oriented π-systems-was selected as a conceptually simple class of intrinsically porous nanocarbons to serve as a platform for studying the transition from analyte sorption properties of small aggregates to those of bulk materials. In our detailed investigation, two series of CPPs were probed: previously reported hoop-shaped [n]CPPs and a novel family of all-phenylene figure-8 shaped (lemniscal) bismacrocycles, termed spiro[n,n]CPPs. A series of nanocarbons with different macrocycle sizes and heteroatom content have been prepared by atom-precise organic synthetic methods, and their structural, photophysical, and electronic attributes were disclosed. Detailed experimental studies (X-ray crystallography, gas sorption, and quartz-crystal microbalance measurements) and quantum chemical calculations provided ample evidence for the importance of the solid-state arrangement on the porosity and analyte uptake ability of intrinsically porous molecular nanocarbons. We demonstrate that this molecular design principle, i.e., incorporation of sterically demanding spiro junctions into the backbone of nanohoops, enables the manipulation of solid-state morphology without significantly changing the nature and size of the macrocyclic cavities. As a result, the novel spiro[n,n]CPPs showed a remarkable performance as high affinity material for vapor analyte sensing.

Active Metal Template Synthesis and Characterization of a Nanohoop [c2]Daisy Chain Rotaxane.

Molecules and materials that demonstrate large amplitude responses to minor changes in their local environment play an important role in the development of new forms of nanotechnology. Molecular daisy chains are a type of a mechanically interlocked molecule that are particularly sensitive to such changes in which, in the presence of certain stimuli, the molecular linkage enables muscle-like movement between a reduced-length contracted form and an increased-length expanded form. To date, all reported syntheses of molecular daisy chains are accomplished via passive-template methods, resulting in a majority of structures being switchable only through the addition of an exogenous stimuli such as metal ions or changes in pH. Here, we describe a new approach to these structural motifs that exploits a multi-component active-metal template synthesis to mechanically interlock two pi-rich nanohoop macrocycles into a molecular daisy chain that undergoes large conformational changes using thermal energy.

Linear and Radial Conjugation in Extended π-Electron Systems.

We describe the synthesis and electronic properties of new π-conjugated small molecules and polymers that combine the linear intramolecular conjugation pathways commonly associated with organic electronic materials with the emerging properties of radial conjugation found in cycloparaphenylenes (CPPs) and other curved π-surfaces. Using arylene ethynylenes as prototypical linear segments and [6]/[8]CPP as the radial segments, we demonstrate the formation of new electronic states that are not simply additive responses from the individual components. Quantum chemical calculations of model oligomeric structures reveal these electronic processes to arise from the hybrid nature of wave function delocalization over the linear and radial contributors in the photophysically relevant electronic states.

Structural deformation and host-guest properties of doubly-reduced cycloparaphenylenes, [n]CPPs2- (n = 6, 8, 10, and 12).

Chemical reduction of several cycloparaphenylenes (CPPs) ranging in size from [8]CPP to [12]CPP has been investigated with potassium metal in THF. The X-ray diffraction characterization of the resulting doubly-reduced [n]CPPs provided a unique series of carbon nanohoops with increasing dimensions and core flexibility for the first comprehensive structural analysis. The consequences of electron acquisition by a [n]CPP core have been analyzed in comparison with the neutral parents. The addition of two electrons to the cyclic carbon framework of [n]CPPs leads to the characteristic elliptic core distortion and facilitates the internal encapsulation of sizable cationic guests. Molecular and solid-state structure changes, alkali metal binding and unique size-dependent host abilities of the [n]CPP2- series with n = 6-12 are discussed. This in-depth analysis opens new perspectives in supramolecular chemistry of [n]CPPs and promotes their applications in size-selective guest encapsulation and chemical separation.