Helicene Core Research Articles

Theoretical investigation of the electronic and second-order non-linear optical properties of [n]helicene derivatives

Designing and synthesizing nonlinear optical materials with excellent performance has always been a persistent goal pursued by material chemists. In paper, based on the reported [6]heliceno-bis(naphthalimides) 2, four new molecules are designed by introducing 2-dicyanomethylene-1,3-dithiole on the two middle benzene rings of the naphthalimide unit and replacing CH3 units on the R1 and R2 substituents with NH2/NO2 moieties, or their combination. The electronic spectra, reorganization energy and second-order nonlinear optical properties of the reported derivatives 1 and 2, as well as four newly designed derivatives, have been systematically investigated by means of (time-dependent) density functional theory calculations. Frontier molecular orbital analysis, electron excitation analysis, reorganization energy, and the calculation of the static and dynamic first hyperpolarizability all indicate that the studied system is greatly influenced by the number of benzene rings in helicene core and the terminal substituents attached at both ends of helicene core. Further analysis of depolarization, hyperpolarizability density and unit sphere representation reveals the nature of the nonlinear optical response of the studied system. Considering that the studied derivatives all have large static first hyperpolarizabilities (e.g., 210 × 10-30 esu for derivative 5) and small reorganization energy (e.g., 0.092 eV of hole reorganization energy and 0.118 eV of electron reorganization energy for derivative 3), they are expected to become potential nonlinear optical materials with excellent performance and bipolar transport materials. This work contributes to the rational design of high-performance molecules based on [n]helicenes derivatives and further explores their potential applications in optical materials.

Versatile polyamides with new triphenylamine monomer based on dinaphtho furan as core for electrochromic/electrofluorochromic device, TNP detection and memory storage

In order to obtain a versatile electrochromic (EC) and electrofluorochromic (EFC) material with good processability, stable redox properties, high optical contrast and fast response, a novel diamine monomer, N1, N1'-(dinaphtho[2,1-b:1',2'-d] furan-5,9-diyl) bis(N1-(4-methoxyphenyl) benzene- 1,4-diamine) (DNFTPA-NH2), which introducing an oxa[5]helicene core into two electron-rich triphenylamine (TPA) blocks is designed and synthesized. Subsequently, four polyamides (PAs) were obtained via polycondensation of diamine with dicarboxylic acids, denoted as DNFTPA-CA, DNFTPA-OA, DNFTPA-NA and DNFTPA-SA, respectively. The four PAs show excellent solution processability and thermal stability, and cyclic voltammograms (CV) with excellent redox reversibility. Among them, DNFTPA-SA shows marked optical contrast of 34 % at 831 nm and rapid switching time (color/bleach, 3.23s/0.39s) between 0 V and 1.0 V, with no significant decay after 5000 s cycle. DNFTPA-CA exhibits high fluorescence (PL) quantum efficiency of 27 %, which is sensitive to explosive trinitrophenol (TNP) with a low detection limit of 7.24 × 10−2 μM. Given the practical application, it is deemed necessary to prepare EC/EFC device. The findings demonstrate the value of PAs for practical applications in EC/EFC device. Moreover, PAs used in memory devices with sandwich structure of ITO/PAs/Al demonstrate non-volatile flash-type behavior with excellent stability within 10000 s and current switching ratios up to 103. Thus, it provides a facile strategy for the design of polymers in the devices of EC/EFC, PL sensing detection and memristor.

Expanded [2,1][n]Carbohelicenes with 15- and 17-Benzene Rings.

Expanded carbohelicenes with structures fused to 15- and 17-benzene were successfully synthesized. Establishing a new synthetic strategy is crucial to realize the development of longer expanded [2,1][n]helicenes with a kekulene-like projection drawing structure. This article describes the sequential integration of the π-elongating Wittig reaction of functionalized phenanthrene units and ring-fusing Yamamoto coupling for the synthesis of [2,1][15]helicenes and [2,1][17]helicenes. X-ray crystallographic structures, photophysical properties, and density functional theory (DFT) calculations revealed the unique characteristics of the synthesized expanded helicenes. Furthermore, because of the high enantiomerization barrier derived from a wide-range intrahelix π-π interaction, the optical resolution of [2,1][17]helicene was successfully achieved, and chiroptical properties such as circular dichroism and circularly polarized luminescence were elucidated for the first time as enantiomers of pristine [2,1][n]helicene core.

Naphthalimide-Annulated [n]Helicenes: Red Circularly Polarized Light Emitters

Two [n]heliceno-bis(naphthalimides) 1 and 2 (n = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S0-S1 transition and high fluorescence quantum yields (73% for 1 and 69% for 2), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness (BCPL) up to 66.5 M-1 cm-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |m| of 2 is responsible for its high absorbance dissymmetry (gabs) and luminescence dissymmetry (glum) factor.

A π-Extended Pentadecabenzo[9]Helicene.

A novel chiral nanographene (i.e. EP9H) with a pentadecabenzo[9]helicene core fragment has been synthesized and fully characterized. Single-crystal X-ray diffraction unambiguously confirms the helical structure. The fluorescence emission of EP9H is located in the near infrared region (λem =684 nm) with a medium quantum yield (0.10) for helicene derivatives. Cyclic voltammetry reveals its seven quasi-reversible redox states from -2 to +5. Furthermore, enantiopure EP9H displays distinct CD signals in a broad spectral range from 300 to 700 nm. Notably, compared to the reported small organic molecules, EP9H displays an outstanding |glum | value of 4.50×10-2 and BCPL as 304 M-1 cm-1 .

A π‐Extended Pentadecabenzo[9]Helicene

AbstractA novel chiral nanographene (i.e.EP9H) with a pentadecabenzo[9]helicene core fragment has been synthesized and fully characterized. Single‐crystal X‐ray diffraction unambiguously confirms the helical structure. The fluorescence emission ofEP9His located in the near infrared region (λem=684 nm) with a medium quantum yield (0.10) for helicene derivatives. Cyclic voltammetry reveals its seven quasi‐reversible redox states from −2 to +5. Furthermore, enantiopureEP9Hdisplays distinct CD signals in a broad spectral range from 300 to 700 nm. Notably, compared to the reported small organic molecules,EP9Hdisplays an outstanding |glum| value of 4.50×10−2andBCPLas 304 M−1cm−1.

Low-Temperature Luminescence in Organic Helicenes: Singlet versus Triplet State Circularly Polarized Emission.

The experimental measurement of the photophysical and chiroptical properties of helicene-based π-conjugated emitters with electron-accepting (-CN, -py, -NO2) or donating (TMS, NMe2, NH2) moieties is reported at low temperature (77 K). The samples exhibit strong circularly polarized phosphorescence in frozen solution of 2-MeTHF, with a luminescence dissymmetry factor reaching 1.6 × 10-2 and a lifetime of over 0.46 s for the most active molecule, the nitro compound. The theoretical investigation shows that although the singlet (S1) and triplet (T1) excited-state emissions mainly arise from the helicene core, the rotatory strengths of the spin-allowed versus spin-forbidden emission have opposite signs. Further analysis of the spin-orbit coupling matrix elements shows that there is no strong mixing between S1 and T1, justifying the different signs of the rotatory strengths. In the case of the nitro compound, the enhanced phosphorescence emission is due to an efficient intersystem crossing.

Mutual Monomer Orientation To Bias the Supramolecular Polymerization of [6]Helicenes and the Resulting Circularly Polarized Light and Spin Filtering Properties.

We report on the synthesis and self-assembly of 2,15- and 4,13-disubstituted carbo[6]helicenes 1 and 2 bearing 3,4,5-tridodecyloxybenzamide groups. The self-assembly of these [6]helicenes is strongly influenced by the substitution pattern in the helicene core that affects the mutual orientation of the monomeric units in the aggregated form. Thus, the 2,15-substituted derivative 1 undergoes an isodesmic supramolecular polymerization forming globular nanoparticles that maintain circularly polarized light (CPL) with glum values as high as 2 × 10–2. Unlike carbo[6]helicene 1, the 4,13-substituted derivative 2 follows a cooperative mechanism generating helical one-dimensional fibers. As a result of this helical organization, [6]helicene 2 exhibits a unique modification in its ECD spectral pattern showing sign inversion at low energy, accompanied by a sign change of the CPL with glum values of 1.2 × 10–3, thus unveiling an example of CPL inversion upon supramolecular polymerization. These helical supramolecular structures with high chiroptical activity, when deposited on conductive surfaces, revealed highly efficient electron-spin filtering abilities, with electron spin polarizations up to 80% for 1 and 60% for 2, as measured by magnetic conducting atomic force microscopy.

New aza[5]helicene derivative for selective Fe(III) fluorescence sensing in aqueous media and its application in water samples

A new iron(III) fluorescence sensor AH2 was developed based on aza[5]helicene core structure this was the first report of aza[5]helicene derivative as fluorescence dye in sensing application. AH2 was designed and synthesized by using sample conditions with only a two-step reaction, i.e., Cu(II)-catalyzed aerobic oxidation of hydrazide/hetero Diels-Alder reaction with diene followed by oxidation. This AH2 compound showed excellent selectivity toward Fe(III) over other metals including the well-known interference ions of Fe(II) and Cu(II). It had a large Stokes shift in aqueous buffer solution with physiological pH. The solvent effect study showed positive solvatochromism with increasing solvent polarity. Additionally, this sensor had potential to detect Fe(III) in real water samples including drinking water, pond water, canal water and river water.

Tunable construction of transition metal-coordinated helicene cages

We report a facile and tailored method to prepare globally twisted chiral molecular cages through tunable coordination of bis-bipyridine-terminated helicene ligands to a series of transition metals including Fe(II), Co(II), Ni(II) and Zn(II). This system shows an efficient remote transfer of stereogenecity from the helicene core to the bipyridine-metal coordination sites and subsequently the entire cages. While the Fe(II), Co(II) and Ni(II)-derived M2L3 (M for metal and L for ligand) cages exhibit quasi-reversible redox features, the Zn(II) analogues reveal prominent yellow circularly polarized luminescence. Interestingly, with the addition of Na2SO4, the Zn2L3 cages reassemble into sextuple-stranded Zn6L6(SO4)4 cages in which three Zn2L2 units are bound together by four sulfates and further coalesced by offset inter-ligand π-π interactions.

Low-Cost Hole-Transporting Materials Based on Carbohelicene for High-Performance Perovskite Solar Cells.

Two hole-transporting materials (HTMs) based on carbohelicene cores, CH1 and CH2, are developed and used in fabricating efficient and stable perovskite solar cells (PSCs). Owing to the rigid conformation of the helicene core, both compounds possess unique CH-π interactions in the crystalline packing pattern and good phase stability, which are distinct from the π-π intermolecular interactions of conventional planar and spiro-type molecules. PSCs based on CH1 and CH2 as HTMs deliver excellent device efficiencies of 19.36 and 18.71%, respectively, outperforming the control device fabricated with spiro-OMeTAD (18.45%). Furthermore, both PSCs exhibit better ambient stability, with 90% of initial performance retained after aging with a 50-60% relative humidity at 25 °C for 500 h. Due to the low production cost of both compounds, these newly designed carbohelicene-type HTMs have the potential for the future commercialization of PSCs.

Peripherally Donor-Installed 7,8-Diaza[5]helicenes as a Platform for Helical Luminophores

AbstractA new family of emissive donor–acceptor–donor (D-A-D) compounds has been developed by introducing two electron donors into the 7,8-diaza[5]helicene core as a novel helical electron acceptor. The X-ray crystallographic analyses revealed the uniquely twisted and helical structures of these compounds in the solid states. Notably, some D-A-D compounds developed herein display distinct mechanochromic luminescence (MCL) in the solid state, and a D-A-D helicene shows circularly polarized luminescence (CPL) with a relatively high luminescence dissymmetry factor g lum of ca. 10–3. Time-resolved spectroscopic analysis revealed the aspects of thermally activated delayed fluorescence characters of the helicenes. Furthermore, the emissive helicenes were applied to organic light-emitting diodes as emitters.

Metal‐Based Multihelicenic Architectures

AbstractThe preparation of multihelicenic systems by conventional organic synthesis is a challenging task and under continuous development. In parallel, using complexing units grafted to or incorporated within the helicene core and taking advantage of coordination/organometallic chemistry constitutes a powerful strategy to obtain multihelicenic structures. This Minireview focuses on the state‐of‐the‐art preparation of metal‐based multihelicenic architectures such as coordination‐driven supramolecular assemblies and organometallic architectures. Their properties and their applications are presented.

Metal-Based Multihelicenic Architectures.

The preparation of multihelicenic systems by conventional organic synthesis is a challenging task and under continuous development. In parallel, using complexing units grafted to or incorporated within the helicene core and taking advantage of coordination/organometallic chemistry constitutes a powerful strategy to obtain multihelicenic structures. This Minireview focuses on the state-of-the-art preparation of metal-based multihelicenic architectures such as coordination-driven supramolecular assemblies and organometallic architectures. Their properties and their applications are presented.

Charging OBO-Fused Double [5]Helicene with Electrons.

Chemical reduction of OBO‐fused double[5]helicene with Group 1 metals (Na and K) has been investigated for the first time. Two doubly‐reduced products have been isolated and structurally characterized by single‐crystal X‐ray diffraction, revealing a solvent‐separated ion triplet (SSIT) with Na+ ions and a contact‐ion pair (CIP) with K+ ion. As the key structural outcome, the X‐ray crystallographic analysis discloses the consequences of adding two electrons to the double helicene core in the SSIT without metal binding and reveals the preferential binding site in the CIP with K+ counterions. In both products, an increase in the twisting of the double helicene core upon charging was observed. The negative charge localization at the central core has been identified by theoretical calculations, which are in full agreement with X‐ray crystallographic and NMR spectroscopic results. Notably, it was confirmed that the two‐electron reduction of OBO‐fused double[5]helicene is reversible.

Charging OBO‐Fused Double [5]Helicene with Electrons

AbstractChemical reduction of OBO‐fused double[5]helicene with Group 1 metals (Na and K) has been investigated for the first time. Two doubly‐reduced products have been isolated and structurally characterized by single‐crystal X‐ray diffraction, revealing a solvent‐separated ion triplet (SSIT) with Na+ions and a contact‐ion pair (CIP) with K+ion. As the key structural outcome, the X‐ray crystallographic analysis discloses the consequences of adding two electrons to the double helicene core in the SSIT without metal binding and reveals the preferential binding site in the CIP with K+counterions. In both products, an increase in the twisting of the double helicene core upon charging was observed. The negative charge localization at the central core has been identified by theoretical calculations, which are in full agreement with X‐ray crystallographic and NMR spectroscopic results. Notably, it was confirmed that the two‐electron reduction of OBO‐fused double[5]helicene is reversible.

Helicenes as Chiroptical Photoswitches

AbstractIn this Concept Article, we analyze known helicene systems that function as chiroptical photoswitches from a stereochemical perspective and classify them based on the type of transformation of the helicene core that takes place during the switching process. In every case, we inspect the stereodynamics of each of the two forms of the switch and the stereospecificity of their interconversion, and we discuss the chemical working principles and chiroptical responses of the switches. We hope that our analysis will motivate the design of new types of helicene photoswitches to expand the structural diversity of this, at present rare but beautiful, class of compounds.

Substituent‐Controlled and Rhodium‐Catalyzed Intramolecular [2+2+2] and [2+1+2+1] Cycloadditions of Electron‐Deficient Triynes

It has been established that the ratio of [2+2+2] and [2+1+2+1] cycloaddition products in the reactions of electron‐deficient 2‐phenylnaphthalene‐linked triynes, possessing carbonyl groups at the alkyne termini, with a cationic rhodium(I)/bisphosphine complex are dominated by substituents on the carbonyl groups rather than the ligands used. Thus, a triyne, possessing the bulky and electron‐withdrawing isobutanoyl and pivaloyl groups at the alkyne termini, exclusively afforded the [2+1+2+1] cycloaddition products, on the contrary, a triyne, possessing the highly coordinating dimethylcarbamoyl groups at the alkyne termini, exclusively afforded the [2+2+2] cycloaddition product. Additionally, helicity stability of the [2+2+2] cycloaddition product, dibenzo[7]helicenes, possessing two adjacent carbonyl groups, was examined. The dibenzo[7]helicenes showed lower racemization barrier than [7]helicenes presumably due to low aromaticity of the two benzene rings in the middle of the two triphenylene skeletons, which may allow flexible bending of two terminal phenanthrene moieties to form the parallel transition state in the racemization process. Furthermore, steric repulsion between the substituents on the carbonyl groups and the benzene rings outside the helicene core affects the helicity stability, which may also affect the ee values of the reaction products.

Development of New Chiral Phosphine Ligands with Helical Environments and Their Application in Asymmetric Catalytic Reactions

The design and development of new chiral ligands to enable precise stereocontrol in a wide variety of reactions is one of the most important branches of organic synthesis. To date, the development of hybrid ligands containing both σ-donating and π-donating groups has attracted considerable attention, with unprecedented reactivities and stereoselectivities being observed. Therefore to develop efficient hybrid chiral ligands with novel structural motifs, we envisage that helicene would be a suitable π-donor efficiently to construct a helical environment around a metal center. In this context, we herein describe our recent efforts to develop a series of novel chiral [5]helicene-derived phosphine ligands (L1, with a 7,8-dihydro[5]helicene core structure, and L2, with a fully aromatic [5]helicene core structure). The prepared ligands, and in particular L1, were found highly effective in the asymmetric allylation of 1,3-diphenylallyl acetate with indoles and etherification with alcohols. Furthermore, in the asymmetric Suzuki-Miyaura coupling reaction, L2 exhibited excellent enantioselectivities. Finally, density functional theory studies were employed to propose a model that accounts for the origin of such high enantioselectivity in these reactions.

Azahelicenes from the Oxidative Photocyclization of Boron Hydroxamate Complexes.

Aromatic hydroxamic acids (Ar-CO-NOH-Ar') were used as bidentate chelating ligands to generate the corresponding boron hydroxamate complexes, which were subsequently transformed into nitrogen-containing helicenes (azahelicenes) using an oxidative photocyclization method that is frequently used for stilbene-type (Ar-CH=CH-Ar') precursors of carbohelicenes. The nitrogen atom of the hydroxamate linker was thus directly embedded into the helicene core without using nitrogen-containing aromatic rings in the stilbene-type precursors. In a batch photoreaction, aza[4]helicenes were readily and efficiently prepared, but aza[6]helicenes underwent severe decomposition upon irradiation. Alternatively, a continuous flow photoreactor was employed to furnish an amide-type aza[6]helicene.