[ n ]‐Cyclo‐9,9‐dibutyl‐2,7‐fluorene ( n =4, 5): Nanoring Size Influence in Carbon‐Bridged Cyclo‐ para ‐phenylenes

π-Conjugated Nanohoops: A New Generation of Curved Materials for Organic Electronics

Nanohoops, cyclic association of π-conjugated systems to form a hoop-shaped molecule, have been widely developed in the last 15 years. Beyond the synthetic challenge, the strong interest towards these molecules arises from their radially oriented π-orbitals, which provide singular properties to these fascinating structures. Thanks to their particular cylindrical arrangement, this new generation of curved molecules have been already used in many applications such as host-guest complexation, biosensing, bioimaging, solid-state emission and catalysis. However, their potential in organic electronics has only started to be explored. From the first incorporation as an emitter in a fluorescent organic light emitting diode (OLED), to the recent first incorporation as a host in phosphorescent OLEDs or as charge transporter in organic field-effect transistors and in organic photovoltaics, this field has shown important breakthroughs in recent years. These findings have revealed that curved materials can play a key role in the future and can even be more efficient than their linear counterparts. This can have important repercussions for the future of electronics. Time has now come to overview the different nanohoops used to date in electronic devices in order to stimulate the future molecular designs of functional materials based on these macrocycles.

C-H-activated Csp2-Csp3 diastereoselective gridization enables ultraviolet-emitting stereo-molecular nanohydrocarbons with mulitple H···H interactions

Gridization is an emerging molecular integration technology that enables the creation of multifunctional organic semiconductors through precise linkages. While Friedel-Crafts gridization of fluorenols is potent, direct linkage among fluorene molecules poses a challenge. Herein, we report an achiral Pd-PPh3-cataylized diastereoselective (>99:1 d.r.) gridization based on the C-H-activation of fluorene to give dimeric and trimeric windmill-type nanogrids (DWGs and TWGs). These non-conjugated stereo-nanogrids showcase intramolecular multiple H…H interactions with a low field shift to 8.51 ppm and circularly polarized luminescence with high luminescent dissymmetry factors (|gPL | = 0.012). Significantly, the nondoped organic light-emitting diodes (OLEDs) utilizing cis-trans-TWG1 emitter present an ultraviolet electroluminescent peak at ~386 nm (CIE: 0.17, 0.04) with a maximum external quantum efficiency of 4.17%, marking the highest record among nondoped ultraviolet OLEDs based on hydrocarbon compounds and the pioneering ultraviolet OLEDs based on macrocycles. These nanohydrocarbon offer potential nanoscafflolds for ultraviolet light-emitting optoelectronic applications.

Grafting Electron-Accepting Fragments on [4]cyclo-2,7-carbazole Scaffold: Tuning the Structural and Electronic Properties of Nanohoops

Since the first applications of nanohoops in organic electronics appear promising, the time has come to go deeper into their rational design in order to reach high-efficiency materials. To do so, systematic studies dealing with the incorporation of electron-rich and/or electron-poor functional units on nanohoops have to be performed. Herein, the synthesis, the electrochemical, photophysical, thermal, and structural properties of two [4]cyclo-2,7-carbazoles, [4]C-Py-Cbz, and [4]C-Pm-Cbz, possessing electron-withdrawing units on their nitrogen atoms (pyridine or pyrimidine) are reported. The synthesis of these nanohoops is first optimized and a high yield above 50% is reached. Through a structure-properties relationship study, it is shown that the substituent has a significant impact on some physicochemical properties (eg HOMO/LUMO levels) while others are kept unchanged (eg fluorescence). Incorporation in electronic devices shows that the most electrically efficient Organic Field-Effect transistors are obtained with [4]C-Py-Cbz although this compound does not present the best-organized semiconductor layer. These experimental data are finally confronted with the electronic couplings between the nanohoops determined at the DFT level and have highlighted the origin in the difference of charge transport properties. [4]C-Py-Cbz has the advantage of a more 2D-like transport character than [4]C-Pm-Cbz, which alleviates the impact of defects and structural organization.

Synthesis and Photophysical Properties of Silole-Fused Cycloparaphenylenes

Herein, we report the introduction of a silole unit into cycloparaphenylenes (CPPs), and two compounds [12]Si3CPP and [16]Si4CPP are obtained by a platinum- and gold-mediated cyclooligomerization strategy. Their optical and electronic properties are studied by UV-vis absorption and fluorescence spectra, which show red shifts and higher photoluminescence quantum yields (PLQYs) compared with the corresponding CPPs.

Modulation of [8]CPP properties by bridging two phenylene units

We report the synthesis and characterization of two new fluorophores, consisting of a [8]cyclo-para-phenylene core in which two phenylenes are bridged by either a nitrogen atom or a carbonyl group. The nitrogen bridge increases the HOMO-LUMO gap, whereas the carbonyl bridge decreases it. These results provide guidelines to control the electronic properties of nanohoops.

A Fourfold Gold(I)-Aryl Macrocycle with Hyperbolic Geometry and its Reductive Elimination to a Carbon Nanoring Host

An ethylene glycol-decorated [6]cyclo-meta-phenylene (CMP) macrocycle was synthesized and utilized as a subunit to construct a fourfold Au^I ₂ -aryl metallacycle with an overall square arrangement. The corners consist of rigid dinuclear gold(I) complexes previously known to form only triangular metallacycles. The interplay between the conformational flexibility of the [6]CMP macrocycle and the rigid dinuclear gold(I) moieties enable the square geometry, as revealed by single-crystal X-ray diffraction. The formation of the gold complex shows size-selectivity compared to an alternative route using platinum(II) corner motifs. Upon reductive elimination, an all-organic ether-decorated carbon nanoring was obtained. Investigation as a host for the complexation of large guest molecules with a suitable convex π-surfaces was accomplished using isothermal NMR binding titrations. Association constants for [6]cycloparaphenylene ([6]CPP), [7]CPP, C₆₀ , and C₇₀ were determined.

Conjugated Nanohoops with Dibenzo[a,e]pentalenes as Non-alternant and Antiaromatic π-Systems

Conjugated nanohoops are excellent candidates to study structure-property relationships, as optoelectronic materials and as hosts for supramolecular chemistry. While carbon nanohoops containing aromatics are well studied, antiaromatic units had not been incorporated until recently by our group using dibenzo[a,e]pentalene (DBP). The non-alternant electronic character of the DBP units significantly influences the optoelectronic properties of such nanohoops. We herein summarize our synthetic strategies to DBP-containing nanohoops, their structural and electronic properties, chirality and host-guest chemistry. We demonstrate how incorporating antiaromatic units leads to unique properties and opens new synthetic avenues, making such nanohoops attractive as potential electronic materials.

Covalent nanosynthesis of fluorene-based macrocycles and organic nanogrids

Gridization is an alternative way to create macromolecules of various sizes in addition to linear and dendritic polymerization as well as cyclization. Organic nanogrids are an expanding family of macrocycle-like closed structures at the nanoscale, but with a series of well-defined extension edges and vertices. Cyclic nanogrids can be used as nanoscale building blocks for the fabrication of not only rotaxanes, catenanes, knots, 3D cages, but also nanopolymers, covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and complex molecular cross-scale architectures. In this review, the history of fluorene-based macrocycles has first been explored, followed by the development of the synthetic methodologies; in particular, fluorene-based nanogrids are highlighted owing to their features and applications. Typically, fluorenes are fused arenes with a hybrid entity between tetrahedral Csp³ and Csp². Four ingenious connection modes of fluorene-based macrocycles, including 2,7-, 3,6-, 9,9-, and 2,9-linkages, fully demonstrate the geometric possibilities of the macrocycles and nanogrids. Such fluorene-based nanogrids will give birth to organic intelligence.

Naphthodithiophene Diimide Based Chiral π-Conjugated Nanopillar Molecules

The synthesis, structures, and properties of [4]cyclonaphthodithiophene diimides ([4]C-NDTIs) are described. NDTIs as important n-type building blocks were catenated in the α-positions of thiophene rings via an unusual electrochemical-oxidation-promoted macrocyclization route. The thiophene-thiophene junction in [4]C-NDTIs results in an ideal pillar shape. This interesting topology, along with appealing electronic and optical properties inherited from the NDTI units, endows the [4]C-NDTIs with both near-infrared (NIR) light absorptions, strong excitonic coupling, and tight encapsulation of C₆₀ . Stable orientations of the NDTI units in the nanopillars lead to stable inherent chirality, which enables detailed circular dichroism studies on the impact of isomeric structures on π-conjugation. Remarkably, the [4]C-NDTIs maintain the strong π-π stacking abilities of NDTI units and thus adopt two-dimensional (2D) lattice arrays at the molecular level. These nanopillar molecules have great potential to mimic natural photosynthetic systems for the development of multifunctional organic materials.

[n]Cyclodibenzopentalenes as Antiaromatic Curved Nanocarbons with High Strain and Strong Fullerene Binding

Conjugated nanohoops provide a platform to study structure-property relationships; they are attractive hosts for supramolecular chemistry as well as promising candidates as new organic materials. We herein present [n]cyclodibenzopentalenes ([n]CDBPs) as antiaromatic analogues of [n]cycloparaphenylenes. Platinum-mediated macrocyclization of dibenzopentalene boronic esters provided the trimer and tetramer with strain energies of up to 80 kcal mol^-1. In the solid state, the cylindrical [4]CDBP molecules align to form columnar structures. The larger hoop [4]CDBP binds both fullerenes C₆₀ and C₇₀ with temperature-dependent exchange behavior, providing higher activation energies for the exchange compared to [10]CPP. The antiaromatic character of the [n]CDBPs paired with the cyclic conjugation leads to high HOMO energies and lowered LUMO energies with band gaps below 2 eV. This work presents a new class of the antiaromatic and nonalternant curved nanocarbons with intriguing supramolecular and ambipolar optoelectronic properties.

Confined Spaces in [n]Cyclo-2,7-pyrenylenes

A set of strained aromatic macrocycles based on [n]cyclo-2,7-(4,5,9,10-tetrahydro)pyrenylenes is presented with size-dependent photophysical properties. The K-region of pyrene was functionalized with ethylene glycol groups to decorate the outer rim and thereby confine the space inside the macrocycle. This confined space is especially pronounced for n=5, which leads to an internal binding of up to 8.0×104  m-1 between the ether-decorated [5]cyclo-2,7-pyrenylene and shape-complementary crown ether-cation complexes. Both the ether-decorated [n]cyclo-pyrenylenes as well as one of their host-guest complexes have been structurally characterized by single-crystal X-ray analysis. In combination with computational methods the structural and thermodynamic reasons for the exceptionally strong binding have been elucidated. The presented rim confinement strategy makes cycloparaphenylenes an attractive supramolecular host family with a favorable, size-independent read-out signature and binding capabilities extending beyond fullerene guests.

[4]Cyclo-N-alkyl-2,7-carbazoles: Influence of the Alkyl Chain Length on the Structural, Electronic, and Charge Transport Properties

Macrocycles possessing radially oriented π-orbitals have experienced a fantastic development. However, their incorporation in organic electronic devices remains very scarce. In this work, we aim at bridging the gap between organic electronics and nanorings by reporting the first detailed structure-properties-device performance relationship study of organic functional materials based on a nanoring system. Three [4]cyclo-N-alkyl-2,7-carbazoles bearing different alkyl chains on their nitrogen atoms have been synthesized and characterized by combined experimental and theoretical approaches. This study includes electrochemical, photophysical, thermal, and structural solid-state measurements and charge transport properties investigations. An optimized protocol of the Pt approach has been developed to synthesize the [4]cyclocarbazoles in high yield (52-64%), of great interest for further development of nanorings, especially in materials science. The charge transport properties of [4]cyclocarbazoles and model compound [8]cycloparaphenylene ([8]CPP) have been studied. Although no field effect (FE) mobility was recorded for the benchmark [8]CPP, FE mobility values of ca. 10^-5 cm²·V^-1·s^-1 were recorded for the [4]cyclocarbazoles. The characteristics (threshold voltage V_TH, subthreshold swing SS, trapping energy ΔE) recorded for the three [4]cyclocarbazoles appear to be modulated by the alkyl chain length borne by the nitrogen atoms. Remarkably, the space-charge-limited current mobilities measured for the [4]cyclocarbazoles are about 3 orders of magnitude higher than that of [8]CPP (1.37/2.78 × 10^-4 cm²·V^-1·s^-1 for the [4]cyclocarbazoles vs 1.21 × 10^-7 cm²·V^-1·s^-1 for [8]CPP), highlighting the strong effect of nitrogen bridges on the charge transport properties. The whole study opens the way to the use of nanorings in electronics, which is now the next step of their development.

Conjugated Nanohoops Incorporating Donor, Acceptor, Hetero- or Polycyclic Aromatics

In the last 13 years several synthetic strategies were developed that provide access to [n]cycloparaphenylenes ([n]CPPs) and related conjugated nanohoops. A number of potential applications emerged, including optoelectronic devices, and their use as templates for carbon nanomaterials and in supramolecular chemistry. To tune the structural or optoelectronic properties of carbon nanohoops beyond the size‐dependent effect known for [n]CPPs, a variety of aromatic rings other than benzene were introduced. In this Review, we provide an overview of the syntheses, properties, and applications of conjugated nanohoops beyond [n]CPPs with intrinsic donor/acceptor structure or such that contain acceptor, donor, heteroaromatic or polycyclic aromatic units within the hoop as well as conjugated nanobelts.