Ligand‐Controlled Synthesis of [3]‐ and [4]Cyclo‐9,9‐dimethyl‐2,7‐fluorenes through Triangle‐ and Square‐Shaped Platinum Intermediates

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.

Synthesis and Isolation of a Homochiral Nanohoop Composed of a Tröger's Base and Hexaparaphenylene

The synthesis of a new nanohoop containing a stereogenic Tröger's base skeleton tethered to a curved hexaparaphenylene ([6]CPP) is reported. The TB[6]CPP nanohoop possesses a stable C2 symmetrical structure, which promotes the allowed transition that gives rise to pale blue emission with a quantum yield of ~0.69, surpassing the value of the more symmetrical [8]CPP. Moreover, TB[6]CPP shows chiroptical properties including circular dichroism and circularly polarized luminescence with a moderate dissymmetry factor (|glum|) of ~2.1×10-3.

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 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.

Dynamic Au-C σ-Bonds Leading to an Efficient Synthesis of [n]Cycloparaphenylenes (n = 9-15) by Self-Assembly

The transmetalation of the digold(I) complex [Au₂Cl₂(dcpm)] (1) (dcpm = bis(dicyclohexylphosphino)methane) with oligophenylene diboronic acids gave the triangular macrocyclic complexes [Au₂(C₆H₄) _x (dcpm)]₃ (x = 3, 4, 5) with yields of over 70%. On the other hand, when the other digold(I) complex [Au₂Cl₂(dppm)] (1') (dppm = bis(diphenylphosphino)methane) was used, only a negligible amount of the triangular complex was obtained. The control experiments revealed that the dcpm ligand accelerated an intermolecular Au(I)-C σ-bond-exchange reaction and that this high reversibility is the origin of the selective formation of the triangular complexes. Structural analyses and theoretical calculations indicate that the dcpm ligand increases the electrophilicity of the Au atom in the complex, thus facilitating the exchange reaction, although the cyclohexyl group is an electron-donating group. Furthermore, the oxidative chlorination of the macrocyclic gold complexes afforded a series of [n]cycloparaphenylenes (n = 9, 12, 15) in 78-88% isolated yields. The reorganization of two different macrocyclic Au complexes gave a mixture of macrocyclic complexes incorporating different oligophenylene linkers, from which a mixture of [n]cycloparaphenylenes with various numbers of phenylene units was obtained in good yields.

Fluorescent cyclophanes and their applications

With the serendipitous discovery of crown ethers by Pedersen more than half a century ago and the subsequent introduction of host-guest chemistry and supramolecular chemistry by Cram and Lehn, respectively, followed by the design and synthesis of wholly synthetic cyclophanes-in particular, fluorescent cyclophanes, having rich structural characteristics and functions-have been the focus of considerable research activity during the past few decades. Cyclophanes with remarkable emissive properties have been investigated continuously over the years and employed in numerous applications across the field of science and technology. In this Review, we feature the recent developments in the chemistry of fluorescent cyclophanes, along with their design and synthesis. Their host-guest chemistry and applications related to their structure and properties are highlighted.

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.

Confining the Inner Space of Strained Carbon Nanorings

Strained aromatic macrocycles based on cycloparaphenylenes (CPPs) are the shortest repeating units of armchair single-walled carbon nanotubes. Since the development of several new synthetic methodologies for accessing these structures, their properties have been extensively studied. Besides the fundamental interest in these novel molecular scaffolds, their application in the field of materials science is an ongoing topic of research. Most of the reported CPP-type macrocycles display strong binding toward fullerenes, due to the perfect match between the convex and concave π-surfaces of fullerenes and CPPs, respectively. Highly functionalized CPP derivatives capable of supramolecular binding with other molecules are rarely reported. The synthesis of highly functionalized [n]cyclo-2,7-pyrenylenes leads to CPP-type macrocycles with a defined cavity capable of binding non-fullerene guests with high association constants.

[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.

Conjugated Molecular Nanotubes

Molecular compounds with permanent tubular architectures displaying radial π-conjugation are exceedingly rare. Their radial and axial delocalization presents them with unique optical and electronic properties, such as remarkable tuning of their Stokes shifts, and redox switching between global and local aromaticity. Although these tubular compounds display large internal void spaces, these attributes have not been extensively explored, thus presenting future opportunities in the development of materials. By using cutting-edge synthetic methodologies to bend aromatic surfaces, large opportunities in synthesis, property discovery, and applications are expected in new members of this family of conjugated molecular nanotubes.

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.

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.

Ultrafast Exciton Self-Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited-State Symmetry in Electron-Vibrational Coupling

Upon photon absorption, π-conjugated organics are apt to undergo ultrafast structural reorganization via electron-vibrational coupling during non-adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self-trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited-state symmetry-dependent electron-vibrational coupling. By accessing two high-lying excited states, one-photon and two-photon allowed states, a clear discrepancy in the initial time-resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited-state symmetry on ultrafast structural reorganization and exciton self-trapping in the emerging class of π-conjugated materials is provided.

Synthesis and Properties of Conjugated Nanohoops Incorporating Dibenzo[a,e]pentalenes: [2]DBP[12]CPPs

Conjugated nanohoops allow studying the effect of cyclic conjugation and bending on the incorporated π-systems. To date, no such system containing antiaromatic units has been reported. We herein present [12]cycloparaphenylenes incorporating two dibenzo[a,e]pentalene units: [2]DBP[12]CPP nanohoops. Dibenzo[a,e]pentalene is a nonalternant hydrocarbon with antiaromatic character. The syntheses and optoelectronic properties of two different [2]DBP[12]CPP nanohoops with electronically modifying substituents are reported, accompanied by TDDFT calculations.

Narrowing Segments of Helical Carbon Nanotubes with Curved Aromatic Panels

Rigid molecular cylinders with a 1 nm diameter were synthesized by assembling arylene panels with Pt-mediated macrocylization. Chrysenylene panels that previously participated in tetrameric macrocyclization were contorted by the addition of two benzo groups on the sides to form dibenzochrysenylene, which allowed for a reduction in the numbers of participating panels to three. Consequently, narrowed cyclochrysenylene congeners were obtained. The narrowed chiral cylinders possessed width-dependent chiroptical properties. The magnetic transition dipole moment was dictated by the radius of a ring-current-like circle that was formed by local electric transition dipole moments on the cylinder.

[4]Cyclofluorene: Unexpected Influence of Alkyl Chain Length

Presented here is the study of a new example of [4]cyclofluorene, with ethyl chains on the bridgeheads. Its molecular structure was established by solution NMR spectroscopy and single-crystal X-ray diffraction. Three successive oxidation processes and one reversible reduction were observed through cyclic voltammetry. The optical properties were characterized both in solution and thin film by UV/visible spectroscopy as well as stationary and time-resolved fluorescence. It was found that this [4]cyclofluorene displays different characteristics compared with the other [4]cyclofluorenes substituted by methyl or propyl chains: a simple modification of the chain length induces a non-negligible effect on the emission properties, which must be linked to the specific arrangement of the fluorene units. Furthermore, single-crystal X-ray diffraction reveals the formation of a pseudo-tubular solid-state arrangement of fully symmetrical ring structures, which was not observed for the other members of the [4]cyclofluorenes family. This finding could open the way to modulation of properties of cyclofluorenes through alkyl chain engineering.

Synthesis and Structures of Zigzag Shaped [12]Cyclo-p-phenylene Composed of Dinaphthofuran Units and Biphenyl Units

A [12]Cyclo-p-phenylene 9 composed of dinaphthofuran units and biphenyl units was synthesized through reductive elimination of the corresponding trinuclear complex by applying Yamago's method. The X-ray crystallographic analyses of 9 revealed that it adopts a zigzag conformation in the solid state. The UV-vis and fluorescence measurements of compound 9 indicated that it also preferentially took a zigzag conformation in the solution state.

Shortest Double-Walled Carbon Nanotubes Composed of Cycloparaphenylenes

The host-guest chemistry of cycloparaphenylenes (CPPs) of different sizes is described. [n]CPPs (n=5, 6, 7, 8, and 10) selectively interact with [n+5]CPPs, forming complexes [n+5]CPP⊃[n]CPP, which are the shortest double-walled armchair carbon nanotubes. The size selectivity is dictated by the difference in diameters of the CPPs (that is, 0.34-0.35 nm), which maximizes attractive van der Waals interactions. Theoretical calculations suggest that the orbital energies of the CPPs become perturbed upon complex formation, and orbital mixing between the two CPPs is predicted for large CPP pairs. The association constants in 1,1,2,2-[D₂ ]tetrachloroethane, estimated by ¹ H NMR titration, are approximately 10³  mol L^-1 at 50 °C. Van't Hoff plot analysis reveals that complexation is driven mainly by entropy owing to desolvation of the CPPs. [13]CPP also forms a complex with [4]cyclo-2,7-pyrenylene ([4]CPY), which is a π-extended [8]CPP. Theoretical calculations suggest that the formation of [13]CPP⊃[4]CPY is more exothermic than that of [13]CPP⊃[8]CPP. A ternary complex, [15]CPP⊃[10]CPP⊃C₆₀ , is also formed by mixing [15]CPP and [10]CPP⊃C₆₀ .

Pentagon-Embedded Cycloarylenes with Cylindrical Shapes

Cylinder-shaped graphitic networks in carbon nanotubes have attracted interest from scientists in various disciplines. The chemical synthesis of segments thereof is considered as a challenging and appealing subject in chemistry, and deepens our understanding of curved and conjugated arrays of hexagons. We herein report the synthesis of cylinder-shaped molecules containing non-hexagon bridges in their conjugated systems. Multiple pentagon units were embedded in the cylinder-shaped discrete molecules, and the stereoisomerism originating from their helical carbon arrangements was studied. Structural analysis by NMR, UV/Vis absorption spectroscopy, and single-crystal X-ray diffraction provided fundamental experimental information on the curved systems with conjugation across the pentagons. This study provides the first experimental guide for further explorations of anomalous non-hexagon arrays of graphitic carbon materials with cylindrical shapes.

Synthesis and physical properties of [4]cyclo-3,7-dibenzo[b,d]thiophene and its S,S-dioxide

Cyclic tetramers of 3,7-dibenzo[b,d]thiophene (DBT) and 3,7-dibenzo[b,d]thiophene-5,5-dioxide (DBTO), i.e., [4]cyclo-DBT ([4]CDBT) and [4]cyclo-DBTO ([4]CDBTO), respectively, are synthesized by the platinum-mediated assembly of bis-metallated DBT and DBTO and the subsequent reductive elimination of platinum. This is the first example of the synthesis of sulfur-containing cycloparaphenylene (CPP) derivatives. The structure of [4]CDBTO is unambiguously determined by the single crystal X-ray analysis. Theoretical and experimental analyses of their physical properties reveal the effect of the sulfur atom and sulfonyl group at the periphery of biphenyl units of the CPP.

An Obtuse-angled Corner Unit for Fluctuating Carbon Nanohoops

The invention of corner units was the key factor that allowed the synthesis of cyclo-para-phenylenes with strained curved π-systems. Despite only a few scarce instances of the development of corner units to date, a variety of structural congeners have been synthesized. These preceding corner units commonly possessed directing angles of ≤90°, which enabled the macrocyclization of multiple units, up to six. In this study, we introduce an obtuse-angled corner unit for the synthesis of cyclo-para-phenylene congeners. The corner unit with oxanorbornadiene possessed a directing angle of 126° and thus allowed for the macrocyclization of larger structures with up to seven units. Reductive aromatization was applicable to complete the cyclo-para-phenylene structures and afforded the congeners with multiple anthracenylene panels. Structural studies with experimental and theoretical methods revealed a fluctuating structure with an intrinsic non-belt shape.

Synthesis and properties of [8]-, [10]-, [12]-, and [16]cyclo-1,4-naphthylenes

The synthesis and properties of various [n]cyclo-1,4-naphthylenes ([n]CNs, n = 8, 10, 12, and 16) are described. Initially, extended L-shaped units, which could be converted into quater- or quinquenaphthylenes were prepared. Nickel- or palladium-mediated couplings of these extended L-shaped units, followed by reductive aromatization of the coupling products afforded [8]-, [10]-, [12]-, and [16]CNs. The size-dependent photophysical properties of these CNs were confirmed by measuring their UV-vis absorption and fluorescence spectra. The theoretical studies supported substantial effects of the number of naphthalene rings on the structural and photophysical properties of these CNs. A kinetic study on the thermal conversion of the C_s-symmetric conformer of [10]CN into its most stable D_5d-symmetric conformer indicated that ring strain affects the rotation barrier of the naphthalene rings in [10]CN.

Size Dependence of [n]Cycloparaphenylenes (n=5-12) in Electrochemical Oxidation

The oxidation processes of [n]cycloparaphenylenes ([n]CPPs) (n=5-12) were systematically investigated by cyclic and rotating disk electrode voltammetry. All CPPs underwent pseudo-reversible two-electron oxidation irrespective of ring size, forming the corresponding radical cations and then dications. The results were in sharp contrast to those observed for linear oligoparaphenylenes, which only undergo one-electron oxidation. The difference in the first and second oxidation potentials in the CPP oxidation was affected by the ring size and became more significant as the decrease of CPP size. In other words, while the first oxidation from neutral CPP to the radical cation occurred faster as the size of CPP becomes smaller, the second oxidation from the radical cation to dication exhibited opposite size dependence.

Synthesis and Crystal Structure of Highly Strained [4]Cyclofluorene: Green-Emitting Fluorophore

[4]Cyclo-9,9-dipropyl-2,7-fluorene ([4]CF) with the strain energy of 79.8 kcal/mol is synthesized in high quantum yield. Impressively, hoop-shaped [4]CF exhibits a green fluorescence emission around 512 nm offering a new explanation for the green band (g-band) in polyfluorenes. The solution-processed [4]CF-based organic light emitting diode (OLED) has also been fabricated with the a stronger green band emission. Strained semiconductors offer a promising approach to fabricating multifunctional optoelectronic materials in organic electronics and biomedicine.