Efficient Thia‐Bridged Triarylamine Heterohelicenes: Synthesis, Resolution, and Absolute Configuration Determination

Organocatalytic hydrogen bond donor/Lewis base (HBD/LB) synthesis and chiroptical properties of thiabridged [5]helicenes

Thiabridged [5]helicenes are obtained from thioaryl-N-phthalimido benzo[a]phenothiazines using a hydrogen bond donor/Lewis base organocatalytic approach. Resolution of [5]helicenes using either (1S)-(-)-camphanic acid as a chiral auxiliary or CSP-HPLC is reported. Thiabridged [5]helicenes show an exceptional configurational stability with racemization energy barriers higher than 40 kcal mol-1. Electronic circular dichroism and TD-DFT calculations permit the assignment of the absolute configuration, demonstrating that the sign of optical rotation is not easily related to the M or P structure. Separated enantiomers show circularly polarized luminescence with high dissymmetry ratio.

Spin polarized current in chiral organic radical monolayers

The chirality-induced spin selectivity (CISS) effect is the capability of chiral molecules to act as spin filters, i.e. to selectively sort flowing electrons based on their spin states. The application of this captivating phenomenon holds great promise in the realm of molecular spintronics, where the primary focus lies in advancing technologies based on chiral molecules to regulate the injection and coherence of spin-polarized currents. In this context, we conducted a study to explore the spin filtering capabilities of a monolayer of the thia-bridged triarylamine hetero[4]helicene radical cation chemisorbed on a metallic surface. Magnetic-conductive atomic force microscopy revealed efficient electron spin filtering at exceptionally low potentials. Furthermore, we constructed a spintronic device by incorporating a monolayer of these molecules in between two electrodes, obtaining an asymmetric magnetoresistance trend with signal inversion in accordance with the handedness of the enantiomer involved, indicative of the presence of the CISS effect. Our findings underscore the significance of thia[4]azahelicene organic radicals as promising candidates for the development of quantum information operations based on the CISS effect as a tool to control the molecular spin states.

Strongly reducing helical phenothiazines as recyclable organophotoredox catalysts

Recyclable phenothiazine organophotoredox catalysts (PTHS 1-3, E1/2ox* = -2.34 to -2.40 V vs. SCE) have been developed. When the recycling performance was evaluated, PTHS-1 could be recovered at least four times without loss of its catalytic activity. These recyclable organophotoredox catalysts represent a promising tool for sustainable organic synthesis.

The influence of heteroatoms on the circularly polarized luminescence performance of [7]helicene derivatives: aromatic vs. non-aromatic five-membered rings

Helicenes are promising candidates for circularly polarized luminescence (CPL) materials, although the performance is poor due to the unsatisfactory dissymmetric factor (glum) and fluorescence quantum efficiency (ΦF). Herein, the influence of heteroatoms (C, Si, Ge, O, S and Se) on the electronic structures and chiroptical properties of [7]helicene derivatives is systematically investigated using density functional theory (DFT) and time-dependent DFT calculations combined with the thermal vibration correlation function theory. The results reveal that the non-radiative energy consumption processes for helicene systems are closely related to the variation of bond length upon electronic excitation. Moreover, by introducing five-membered rings and heteroatoms, the dipole-forbidden S1 → S0 emission of [7]helicene changes to dipole-allowed transition due to the rearrangement of occupied orbitals and lifting of the nearly degenerate orbitals, resulting in an enhancement of ΦF. As the heteroatomic radius increases, ΦF decreases while the glum increases. Compared with the derivatives containing aromatic five-membered rings ([7]H-O, [7]H-S, and [7]H-Se), the non-aromatic counterparts ([7]H-C, [7]H-Si, and [7]H-Ge) exhibit a balance in ΦF and glum values. The present study helps to clarify the relationship between structures and chiroptical properties and offers a feasible strategy for the future design of helicene-based CPL materials.

Acceptor-Induced Fluorescence of Phenolic Polymers Based on Triphenylamine Derivatives

Conductive polymers facilitate the electrical current flow through the transfer of electrons and holes. They show promise for novel photo-functional materials in photovoltaics. However, substantial electrostatic interactions between electron donors and acceptors induce polymer aggregation, limiting moldability and conductivity. In this study, robust donor polymers with high heat resistance were synthesized by bonding triphenylamine (TPA) derivatives and formaldehyde to phenolic groups. Resulting TPA-based phenolic polymers exhibited flexible structures and fluorescence due to charge transfer with acceptor molecules. Furthermore, TPA-based phenolic polymers' capacity to distinguish acceptor molecule sizes correlated with their molecular weight, reflecting upon donor-acceptor interactions. This novel optical trait in phenolic polymers holds potential for electronic components and conductive materials.

Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Three-component selective synthesis of phenothiazines and bis-phenothiazines under metal-free conditions

An iodine-containing reagent promoted three-component method for the selective synthesis of phenothiazines and bis-phenothiazines has been developed. The present protocol starts from simple and easily available cyclohexanones, elemental sulfur, and inorganic ammonium salts, selectively producing phenothiazines and bis-phenothiazines in satisfactory yields under aerobic conditions. This method has the advantages of simple and readily available starting materials and metal-free conditions, affording a facile and practical approach for the preparation of phenothiazines and bis-phenothiazines.

A double-helical S,C-bridged tetraphenyl-para-phenylenediamine and its persistent radical cation

A structurally constrained, double-helical S,C-bridged tetraphenyl-para-phenylenediamine (TPPD) has been synthesized. The stable radical cation of the S,C-bridged TPPD was generated by chemical oxidation, and the electron spin was found to be delocalized over the entire π-conjugated framework. The excellent conformational stability of the neutral molecule facilitated the separation of its enantiomers.

Resolution of a Configurationally Stable Hetero[4]helicene

We have developed an efficient chemical resolution of racemic hydroxy substituted dithia-aza[4]helicenes (DTA[4]H) 1(OH) using enantiopure acids as resolving agents. The better diastereomeric separation was achieved on esters prepared with (1S)-(−)-camphanic acid. Subsequent simple manipulations produced highly optically pure (≥ 99% enantiomeric excess) (P) and (M)-1(OH) in good yields. The role of the position where the chiral auxiliary is inserted (cape- vs. bay-zone) and the structure of the enantiopure acid used on successful resolution are discussed.

SET and HAT/PCET acid-mediated oxidation processes in helical shaped fused bis-phenothiazines

Helical shaped fused bis-phenothiazines 1-9 have been prepared and their red-ox behaviour quantitatively studied. Helicene radical cations (Hel.+ ) can be obtained either by UV-irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5-9 with peroxyl radicals (ROO. ) occurs with a 'classical' HAT process giving HelO. radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium-promoted proton-coupled electron transfer (PCET) process takes place with formation of HelO. radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1-4, lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium-promoted PCET mechanism with formation of the radical cations Hel.+ . Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel-OH group (BDEOH ), and kinetic constants (kinh ) of the reactions with ROO. species of helicenes 1-9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.

Stabilization of an Enantiopure Sub-monolayer of Helicene Radical Cations on a Au(111) Surface through Noncovalent Interactions

In the past few years, the chirality and magnetism of molecules have received notable interest for the development of novel molecular devices. Chiral helicenes combine both these properties, and thus their nanostructuration is the first step toward developing new multifunctional devices. Here, we present a novel strategy to deposit a sub‐monolayer of enantiopure thia[4]helicene radical cations on a pre‐functionalized Au(111) substrate. This approach results in both the paramagnetic character and the chemical structure of these molecules being maintained at the nanoscale, as demonstrated by in‐house characterizations. Furthermore, synchrotron‐based X‐ray natural circular dichroism confirmed that the handedness of the thia[4]helicene is preserved on the surface.

Low Temperature Dynamic Chromatography for the Separation of the Interconverting Conformational Enantiomers of the Benzodiazepines Clonazolam, Flubromazolam, Diclazepam and Flurazepam

Benzodiazepines (BZDs) are an important class of psychoactive drugs with hypnotic-sedative, myorelaxant, anxiolytic and anticonvulsant properties due to interaction with the GABAa receptor in the central nervous system of mammals. BZDs are interesting both in clinical and forensic toxicology for their pharmacological characteristics and potential of abuse. The presence of a non-planar diazepine ring generates chiral conformational stereoisomers, even in the absence of stereogenic centers. A conformational enrichment of BZD at the binding sites has been reported in the literature, thus making interesting a stereodynamic screening of a wide range of BZDs. Herein, we report the investigation of three stereolabile 1,4-benzodiazepine included in the class of “designer benzodiazepines” (e.g., diclazepam, a chloro-derivative of diazepam, and two triazolo-benzodiazepines, flubromazolam and clonazolam) and a commercially available BZD known as flurazepam, in order to study the kinetic of the “ring-flip” process that allows two conformational enantiomers to interconvert at high rate at room temperature. A combination of low temperature enantioselective dynamic chromatography on chiral stationary phase and computer simulations of the experimental chromatograms allowed us to measure activation energies of enantiomerization (ΔG‡) lower than 18.5 kcal/mol. The differences between compounds have been correlated to the pattern of substitutions on the 1,4-benzodiazepinic core.

Chromatographic separation of the interconverting enantiomers of imidazo- and triazole-fused benzodiazepines

The toolbox of medicinal chemists includes the 1,4-benzodiazepine scaffold as a "privileged scaffold" in drug discovery. Several biologically active small molecules containing a 1,4-benzodiazepine scaffold have been approved by the FDA for the treatment of various diseases, with most of them being used for their psychotropic effects. The therapeutic potential of 1,4-benzodiazepines has stimulated the interest of synthetic chemists in developing new synthetic strategies to a range of substituted analogues for biological evaluation. A structural variation of the classical benzodiazepine skeleton is observed e.g. in alprazolam, midazolam, and related benzodiazepines, which contain a 1,2,4-triazole or an imidazole ring fused to the benzodiazepine core. Irrespective of the presence of the fused heterocyclic ring, the seven-membered diazepine ring is far from planar, and its shape resembles a twist chair. Then, the unsymmetrical substitution pattern around the seven membered cycle renders these molecules chiral, as they lack any reflection-type symmetry element. However, chirality of this molecules is labile at room temperature, becausea simple ring flipping process converts one enantiomer into the other, and 1,4-benzodiazepines exist as a mixture of rapidly interconverting conformational enantiomers in solution at or near room temperature. Physical separation of the interconverting enantiomers of diazepam and of other related 1,4-benzodiazepin-2-ones can be accomplished by low temperature HPLC on chiral stationary phases (CSPs). If the HPLC column is cooled down to temperatures where the interconversion rate is sufficiently low, compared to the chromatographic separation rate, distinct separated peaks can be observed, provided the CSP is sufficiently enantioselctive. The apparent rate constants for the on-column enantiomerization and the corresponding free energy activation barriers were obtained by simulation of exchange-deformed HPLC profiles using a computer program based on the stochastic model. Here we report on the dynamic HPLC investigations carried out on a set of fused imidazo and triazolo-benzodiazepines (alprazolam, midazolam, triazolam and estazolam) The experimental dynamic chromatograms and the corresponding interconversion barriers reported in this paper show that the third fused heterocyclic ring increase the energy barrier by 2 kcal/mol.

Transition-Metal-Free Synthesis of N-Arylphenothiazines through an N- and S-Arylation Sequence

An efficient synthetic method of N-arylphenothiazines from o-sulfanylanilines under transition-metal-free conditions is disclosed. An N- and S-arylation sequence of o-sulfanylanilines enabled us to synthesize a wide variety of N-arylphenothiazines. In particular, one-pot synthesis of N-arylphenothiazines was accomplished from easily available modules through preparation of o-sulfanylanilines by thioamination of aryne intermediates and following N- and S-arylation sequence.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.

Chiral bis(benzo[1,2-b:4,3-b′]dithiophene) atropisomers: experimental and theoretical investigations of the stereochemical and chiroptical properties

Conformational chirality is a feature that may arise from the presence of a hindered rotation around a single bond that corresponds to a stereogenic axis.

A Spherically Shielded Triphenylamine and Its Persistent Radical Cation

This work reports the design and synthesis of a sterically protected triphenylamine scaffold which undergoes one-electron oxidation to form an amine-centered radical cation of remarkable stability. Several structural adjustments were made to tame the inherent reactivity of the radical cation. First, the parent propeller-shaped triphenylamine was planarized with sterically demanding bridging units and, second, protecting groups were deployed to block the reactive positions. The efficiently shielded triphenylamine core can be reversibly oxidized at moderate potentials (+0.38 V, vs. Fc/Fc+ in CH2 Cl2 ). Spectroelectrochemistry and chemical oxidation studies were employed to monitor the evolution of characteristic photophysical features. To obtain a better understanding of the impact of one-electron oxidation on structural and electronic properties, joint experimental and computational studies were conducted, including X-ray structural analysis, electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. The sterically shielded radical cation combines various desirable attributes: A characteristic and unobstructed absorption in the visible region, high stability which enables storage for weeks without spectroscopically traceable degradation, and a reliable oxidation/re-reduction process due to effective screening of the planarized triphenylamine core from its environment.

Ditocopheryl Sulfides and Disulfides: Synthesis and Antioxidant Profile

Symmetrical ditocopheryl disulfides (Toc)₂ S₂ and symmetrical and unsymmetrical ditocopheryl sulfides (Toc)₂ S were simply prepared under remarkably mild conditions with complete control of the regiochemistry by using δ-, γ-, and β-tocopheryl-N-thiophthalimides (Toc-NSPht) as common starting materials. The roles of sulfur atom(s), H-bond and aryl ring substitution pattern on the antioxidant profile of these new compounds, which were assembled by linking together two tocopheryl units, are also discussed.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

A double hetero[4]helicene composed of two phenothiazines: synthesis, structural properties, and cationic states

A novel double hetero[4]helicene has been prepared by the tandem oxidative C–N coupling of phenothiazine derivatives. This double helicene exhibited good electron donor properties and easily gave the stable cationic radical salt.

Thia-bridged triarylamine heterohelicene radical cations as redox-driven molecular switches

Remarkably stable radical cations as redox-driven molecular switches.

The quest for alternative routes to racemic and nonracemic azahelicene derivatives

A series of diverse aromatic azadienetriyne and azatriynes was synthesised. These compounds were subjected to transition metal-mediated [2+2+2] cycloisomerisation to form pentacyclic or hexacyclic helically chiral azahelicene or azahelicene-like structures mostly in moderate yields. Introducing stereogenic centre(s) into selected azatriynes, cyclisation proceeded in a stereoselective fashion providing aza[5]helicenes or aza[6]helicene-like compounds in up to a 100:0 diastereomeric ratio. Gibbs energy differences between corresponding pairs of diastereomers (calculated at the DFT(B3LYP)/TZV+P level) were in good agreement with the experimental data and allowed for the prediction of the stereochemical outcome of the reaction. This study presents for the first time asymmetric synthesis of azahelicene derivatives in high optical purities.