Synthesis and Properties of Bis(naphthofuran)-Fused [7]Helicene Derivatives

Novel benzo[1,2-b:4,3-b′]dinaphthofuran (oxa[7]helicene) with various substituents in the central ring were designed and successfully synthesized based on the late stage central ring formation strategy from naphthofuranoneby using an efficient McMurry coupling or Friedel-Crafts acylation. The photochemical properties of helicenes were evaluated by UV-Visible absorption spectroscopy and fluorescence spectroscopy, while electrochemical behaviour including their energy profile were investigated by cyclic voltammetry and DFT calculations. According to the computational study, substituents on the central ring could influence both electron delocalization and MO distribution of oxa[7]helicenes resulting in the change in charge transporting ability. In comparison to the parent carbo[7]helicene, the furan unit in conjunction with substituents on helical system caused the bathochromic shift with different absorption pattern and exhibited higher fluorescence quantum yield (ϕ_f up to 0.63). These new materials showed promising characteristics in optoelectronic application.     

Synthesis and characterization of a new hexacyclic helicene

A new helically chiral hexacyclic system, bearing an acetoxymethyl group, was prepared from a simple naphthalene building block in good yield and purity, via a four-step sequence involving palladium-catalysed Heck couplings and oxidative photocyclizations. Suitable crystals of the latter indicate that its conformation closely resembles that of unsubstituted [6]helicene, whose idealized symmetry is C₂. The optical properties of the hexacyclic helicene were investigated and show interesting behaviour.     

A Quintuple [6]Helicene with a Corannulene Core as a C5‐Symmetric Propeller‐Shaped π‐System

The synthesis and structural analysis of a quintuple [6]helicene with a corannulene core is reported. The compound was synthesized from corannulene in three steps including a five‐fold intramolecular direct arylation. X‐ray crystallographic analysis revealed a C₅‐symmetric propeller‐shaped structure and one‐dimensional alignment in the solid state. The enantiomers of the quintuple [6]helicene were successfully separated by HPLC, and the chirality of the two fractions was identified by CD spectroscopy. A kinetic study yielded a racemization barrier of 34.2 kcal mol^?1, which is slightly lower than that of pristine [6]helicene. DFT calculations indicate a rapid bowl‐to‐bowl inversion of the corannulene moiety and a step‐by‐step chiral inversion pathway for the five [6]helicene moieties.     

Synthesis and Properties of Spiro-double Sila[7]helicene: The LUMO Spiro-conjugation

A double helicene with a spiro-Si linker ( 4 ) was synthesized by four successive nucleophilic substitutions on SiCl₄. Its (P,P), (M,M) and (P,M) isomers were isolated and characterized by single crystal X-ray analysis. Due to the central spirosilabi[fluorene] moiety, the two helicene units in 4 are symmetrically and nearly perpendicularly arranged. (P,P)- 4 and (M,M)- 4 exhibit unique optical properties attributable to the LUMO spiro-conjugation between the two sila[7]helicene units.     

Stereoselective Syntheses,Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes

We report a molecular design and concept using π-system elongation and steric effects from helicenes surrounding a triphenylene core toward stable chiral polycyclic aromatic hydrocarbons (PAHs) with a maximal π-distortion to tackle their aromaticity, supramolecular and molecular properties. The selective syntheses, and the structural, conformational and chiroptical properties of two diastereomeric large multi-helicenes of formula C₉₀H₄₈ having a triphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable, enantiospecific Yamamoto-type cyclotrimerizations of racemic or enantiopure 9,10-dibromo[7]helicene. Both molecules have an extremely distorted triphenylene core, and one of them exhibits the largest torsion angle recorded so far for a benzene ring (twist=36.9°).     

Assembly of π‐Conjugated Phosphole Azahelicene Derivatives into Chiral Coordination Complexes: An Experimental and Theoretical Study

Aza[n]helicene phosphole derivatives have been prepared from aza[n]helicene diynes by the Fagan–Nugent route. Their photophysical properties (UV/Vis absorption and emission behavior) have been evaluated. Their behavior as P,N chelates towards coordination to Pd^II and Cu^I has been investigated: metal–bis(aza[n]helicene phosphole) assemblies are formed by a highly stereoselective coordination process, as demonstrated by X‐ray crystallography. An aza[6]helicene phosphole bearing an enantiopure helicene part has been obtained, which allows the preparation of enantiopure Pd^II and Cu^I complexes with original topologies and high molar rotation (MR) and circular dichroism (CD). The structure–property relationship established from the experimental data has been studied in detail by theoretical studies (TDDFT calculations of UV/Vis, CD, and MR). Aza[n]helicene phosphole derivatives show π conjugation extended over the entire molecule, and its influence on the MR of aza[6]helicene phosphole 5 c has been demonstrated. Finally, it has been shown that the nature of the metal (coordination geometry and electronic interaction) can have a great impact on the amplitude of the chiroptical properties in metal–bis(aza[n]helicene phosphole) assemblies.     

Stereoselective Syntheses,Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes

We report a molecular design and concept using π‐system elongation and steric effects from helicenes surrounding a triphenylene core toward stable chiral polycyclic aromatic hydrocarbons (PAHs) with a maximal π‐distortion to tackle their aromaticity, supramolecular and molecular properties. The selective syntheses, and the structural, conformational and chiroptical properties of two diastereomeric large multi‐helicenes of formula C₉₀H₄₈ having a triphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable, enantiospecific Yamamoto‐type cyclotrimerizations of racemic or enantiopure 9,10‐dibromo[7]helicene. Both molecules have an extremely distorted triphenylene core, and one of them exhibits the largest torsion angle recorded so far for a benzene ring (twist=36.9°).     

Synthesis of polymers containing regularly distributed tetrathia‐[7]‐elicene units along the backbone

A tetrathia‐[7]‐helicene bearing in the 2 and 13 positions cyanovinyl groups was used as comonomer in the Michael‐type polyaddition reaction with N,N′‐bis(β‐mercaptoethyl)piperazine. This led to a new polymer bearing tetrathia‐[7]‐helicene units regularly distributed along the polymer backbone, which may be regarded as the first example of a new family of potentially useful nonlinear optical materials. All products were structurally characterized by ¹H and ¹³C NMR spectroscopy. Differential scanning calorimetry characterizations revealed the presence, in both monomeric and polymeric helicenes, of glass‐transition like temperatures, associated to some conformational variation of the helicene units. The optical properties, the film formation and the morphology of the polymer‐containing tetratia‐[7]‐helicenes were also investigated. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Synthesis of Optically Pure Helicene Metallocenes

Two types of ruthenocenes and a ferrocene coordinated by rac ‐9H ‐cyclopenta[1,2‐c :4,3‐c ′]diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono‐helicene ruthenocene 1 and its iron analogue 1_Fe with one [7]helicene ligand bound to the central metal, and bis‐helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac ‐ 2 and meso ‐ 2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.     

S-shaped para-Quinodimethane-Embedded Double [6]Helicene and Its Charged Species Showing Open-Shell Diradical Character

Helicenes and extended helical π-conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S-shaped double [6]helicene 1 , which has two embedded para-quinodimethane (p-QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open-shell diradical character. Its model compound, C-shaped single [6]helicene 2 containing one p-QDM unit, was also synthesized and compared. Their ground-state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double-helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti/syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open-shell diradical character (y₀=12.1 %) and a small singlet–triplet gap. On the other hand, compound 2 has a typical closed-shell nature. The dication and dianion of 1 also show open-shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open-shell diradical character and magnetic activity.     

Synthesis,Crystal Structure and Photophysical Properties of Pyrene–Helicene Hybrids

Synthesis of helically chiral aromatics resulting from fusion of pyrene and [4]‐ or [5]helicene has been accomplished using photoredox catalysis employing a Cu‐based sensitizer as the key step. Photocyclisation experiments for the synthesis of the target compounds were carried out in batch and using continuous flow strategies. The solid‐state structures, UV/Vis absorption spectra and fluorescence spectra of the pyrene–helicene hybrids were investigated and compared to that of the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton. The studies demonstrated that pyrene–helicene hybrids adopt co‐planar or stacked arrangements in the solid state, in contrast to the solid‐state structure of the parent [5]helicene. The UV/Vis and fluorescence spectra of the pyrene–helicene hybrids exhibited strong red‐shifts when compared to the parent [5]helicene. DFT calculations suggest that the strategy of extending the π surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps.     

Toward Zigzag-edged Helical Nanographene Based on [7]Helicene

Due to their unique chemical and physical properties, zigzag-edged nanographenes have attracted increasing interest in recent years. Herein, a novel zigzag-edged nanographene ( 6 ) containing a [7]helicene subunit was designed and synthesized. However, because of the high reactivities of zigzag edges, compound 1 with a diketone structure was obtained owing to the oxidation of 6 . The helical carbon skeleton of 1 is unambiguously revealed by single-crystal X-ray crystallography analysis. The photophysical properties of the precursor and helical diketone 1 are studied by UV-vis absorption spectroscopy. The electrochemical property of 1 is investigated by cyclic voltammetry, which was further studied by density functional theory (DFT) calculations (ΔE_g^Cal=2.94 eV). The work reported here not only represents the synthesis of an unprecedented [7]helicene-embedded nanographene, but also provides the possibility for the synthesis of helical nanographenes with rich zigzag edges.     

Synthesis of Terminally Fluorinated [7]Helicenes and Their Application to Photochemical Domino Reactions

The intramolecular Diels−Alder reactions of helicenes deform their π-conjugated screw-shaped skeletons. In particular, terminally tetrafluorinated [7]helicene (F₄-[7]helicene) undergoes a photoinduced Diels−Alder reaction followed by a photoinduced double fluorine atom transfer. Herein, we thoroughly investigated this photochemical domino process by decreasing the level of fluorine substitution. F₃-[7]Helicenes bearing two fluorine atoms at the dienophile terminal underwent photoinduced Diels−Alder reactions, but the whole domino process became slow. F₂-[7]Helicene, which is difluorinated only at the dienophile terminal, was also photolabile. As a result, two fluorine atoms were sufficient for the photochemical domino reaction to occur. X-ray crystallographic analysis revealed that F₂-[7]helicene was less compressed than F₄-[7]helicene, indicating that terminal polyfluorination enhanced the intramolecular arene−fluoroarene stacking interactions and thus promoted the transformations.     

Synthesis and Characterization of a Helicene‐Based Imidazolium Salt and Its Application in Organic Molecular Electronics

Herein we demonstrate the synthesis of a helicene‐based imidazolium salt. The salt was prepared by starting from racemic 2‐methyl[6]helicene, which undergoes radical bromination to yield 2‐(bromomethyl)[6]helicene. Subsequent treatment with 1‐butylimidazole leads to the corresponding salt 1‐butyl‐3‐(2‐methyl[6]helicenyl)‐imidazolium bromide. The prepared salt was subsequently characterized by using NMR spectroscopy and X‐ray analysis, various optical spectrometric techniques, and computational chemistry tools. Finally, the imidazolium salt was immobilized onto a SiO₂ substrate as a crystalline or amorphous deposit. The deposited layers were used for the development of organic molecular semiconductor devices and the construction of a fully reversible humidity sensor.     

Closed Pentaaza[9]helicene and Hexathia[9]/[5]helicene: Oxidative Fusion Reactions of ortho‐Phenylene‐Bridged Cyclic Hexapyrroles and Hexathiophenes

Oxidative fusion reactions of ortho ‐phenylene‐bridged cyclic hexapyrroles and hexathiophenes furnished novel closed helicenes in a selective manner. X‐Ray diffraction analysis unambiguously revealed the structures to be a closed pentaaza[9]helicene, the longest azahelicene reported so far, and an unexpected double‐helical structure of hexathia[9]/[5]helicene, whose formation was assumed to result from multiple oxidative fusion along with a 1,2‐aryl shift. The pentaaza[9]helicene exhibited well‐defined emission with high fluorescence quantum yield (Φ _F=0.31) among the known [9]helicenes. Chiral resolution of the racemic pentaaza[9]helicene and hexathia[9]/[5]helicene were achieved by chiral‐phase HPLC and the enantiomers were characterized by circular dichroism spectra and DFT calculations.     

Ruthenium‐Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

The properties of mono‐ and bis‐Ru–vinyl[6]helicene complexes ( 2 a and 2 b , respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π‐helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([ 2 a ] ^{. +}, [ 2 b ] ^{. +}, and [ 2 b ]²⁺), are analyzed with the aid of quantum‐chemical calculations. The extended π‐conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a , b , introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru–vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal‐based helicene derivatives. Finally, the redox‐triggered chiroptical switching activity of 2 a , b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations.     

Synthesis,Structure, Properties,and Application of a Carbazole‐Based Diaza[7]helicene in a Deep‐Blue‐Emitting OLED

A carbazole‐based diaza[7]helicene, 2,12‐dihexyl‐2,12‐diaza[7]helicene ( 1 ), was synthesized by a photochemical synthesis and its use as a deep‐blue dopant emitter in an organic light‐emitting diode (OLED) was examined. Compound 1 exhibited good solubility and excellent thermal stability with a high decomposition temperature (T_d=372.1 °C) and a high glass‐transition temperature (T_g, up to 203.0 °C). Single‐crystal structural analysis of the crystalline clathrate ( 1 )₂ ? cyclohexane along with a theoretical investigation revealed a non‐planar‐fused structure of compound 1 , which prevented the close‐packing of molecules in the solid state and kept the molecule in a good amorphous state, which allowed the optimization of the properties of the OLED. A device with a structure of ITO/NPB (50 nm)/CBP:5 % 1 (30 nm)/BCP (20 nm)/Mg:Ag (100 nm)/Ag (50 nm) showed saturated blue light with Commission Internationale de L’Eclairage (CIE) coordinates of (0.15, 0.10); the maximum luminance efficiency and brightness were 0.22 cd A^?1 (0.09 Lm W^?1) and 2365 cd m^?2, respectively. This new class of helicenes, based on carbazole frameworks, not only opens new possibilities for utilizing helicene derivatives in deep‐blue‐emitting OLEDs but may also have potential applications in many other fields, such as molecular recognition and organic nonlinear optical materials.     

Synthesis of a Blue-Emissive Azaborathia[9]helicene by Silicon-Boron Exchange from Unusual Atropisomeric Teraryls

We report the concise synthesis and chir(optical) properties of an azaborathia[9]helicene consisting of two thienoazaborole motifs. The key intermediate, a highly congested teraryl with nearly parallel isoquinoline moieties, was generated as a mixture of atropisomers upon fusion of the central thiophene ring of the dithienothiophene moiety. These diastereomers were characterized by single crystal X-ray analysis revealing intriguing interactions in the solid state. Subsequent insertion of boron into the aromatic scaffold via silicon-boron exchange involving triisopropylsilyl groups fixed the helical geometry, thereby establishing a novel method for the preparation of azaboroles. The ligand exchange at boron in the final step afforded the blue emitter displaying a fluorescence quantum yield of 0.17 in CH₂Cl₂ and excellent configurational stability. Detailed structural and theoretical investigation of unusual atropisomers and the helicene provide insights into their isomerization processes.     

两个二茂铁基查尔酮衍生物的合成及超快三阶非线性光学响应

合成两种新颖的含二茂铁基团的查尔酮衍生物：1-二茂铁基-3-(4-叔丁基苯基)丙烯酮(a)和1-二茂铁基-3-联苯基丙烯酮(b),经₁H NMR、₁₃C NMR、HR-MS对其结构进行了表征;测定了该两化合物的热力学性质;采用量子化学方法计算了它们的分子轨道能量和极化率,给出了轨道电子云图;采用紫外-可见吸收光谱和Z-扫描技术测定了其线性和非线性光学性质。结果表明：与原料相比,化合物a和b的紫外吸收明显增加,且b的吸收波长更长;化合物a和b均存在分子内电荷转移现象,表现出超快三阶非线性光学响应。     

Structural,elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn2P2

The present study explores the structural, elastic, electronic and optical properties of the newly synthesized monoclinic Zintl phase BaIn₂P₂ using a pseudopotential plane-wave method in the framework of density functional theory within the generalized gradient approximation. The calculated lattice constants and internal coordinates are in very good agreement with the experimental findings. Independent single-crystal elastic constants as well as numerical estimations of the bulk modulus, the shear modulus, Young's modulus, Poisson's ratio, Pugh's indicator of brittle/ductile behaviour and the Debye temperature for the corresponding polycrystalline phase were obtained. The elastic anisotropy of BaIn₂P₂ was investigated using three different indexes. The calculated electronic band structure and the total and site-projected l-decomposed densities of states reveal that this compound is a direct narrow-band-gap semiconductor. Under the influence of hydrostatic pressure, the direct D–D band gap transforms into an indirect B-D band gap at 4.08 GPa, then into a B–Γ band gap at 10.56 GPa. Optical macroscopic constants, namely, the dielectric function, refractive index, extinction coefficient, reflectivity coefficient, absorption coefficient and energy-loss function, for polarized incident radiation along the [100], [010] and [001] directions were investigated.