Search

The study reports two pairs of chiral multi‐resonant thermally activated delayed fluorescence (MR‐TADF) materials PCP‐DiKTa and Czp‐DiKTa by decorating a known MR‐TADF core, DiKTa, with different [2.2]paracyclophane (PCP) based planar chiral groups. PCP‐DiKTa shows narrow sky‐blue emission with a full width at half maximum (FWHM) of 44 nm, while the emission of Czp‐DiKTa is slightly broader with a FWHM of 66 nm and redshifted. Both emitters show high photoluminescence quantum yields of 93 and 99% for PCP‐DiKTa and Czp‐DiKTa, respectively. Enantiomerically pure samples of both compounds show chiroptical properties in the ground state while only Czp‐DiKTa exhibits chiroptical activity in the excited state, with dissymmetry factors (|gPL|) of 4 × 10−4. Organic light‐emitting diodes (OLEDs) with PCP‐DiKTa and Czp‐DiKTa show maximum external quantum efficiencies (EQEmax) of 25.7 and 29.2%, with λEL of 489 and 518 nm, and FWHMs of 53 and 69 nm, respectively. These EQEmax values are higher than those of other reported devices employing PCP‐based D‐A type emitters. This work demonstrates that the PCP moiety is not only a powerful building block to develop planar chiral emitters but one that is compatible with the fabrication of high efficiency devices. [ABSTRACT FROM AUTHOR]

The prototypical example of a (cyclo)phane, [2.2]paracyclophane (PCP), has proven to be a versatile stereogenic moiety within the design of circularly polarized thermally activated delayed fluorescence (CP‐TADF) emitters; however, the exploration of other cyclophanes within CP‐TADF emitter design has been largely neglected. Here, a comparative study of the photophysical and optoelectronic properties of two cyclophane emitters, (1,7)tBuCzpPhTrz and its isomer (1,4)tBuCzpPhTrz, is presented. The carbazolophane‐triazine compound (1,7)tBuCzpPhTrz, obtained via an unprecedented intramolecular rearrangement, is the first example of a planar chiral TADF emitter deviating from the PCP scaffold. Significant geometrical change of the enclosed carbazole in (1,7)tBuCzp results in an attenuation of the donor strength, while the merits of rigidity and steric bulk remain. In particular, the full width at half maximum (FWHM) of the photoluminescence spectrum in toluene of (1,7)tBuCzpPhTrz is reduced by 34% and blue‐shifted by 20 nm compared to that of (1,4)tBuCzpPhTrz. In doped films, the compounds reach high photoluminescence quantum yields (ΦPL) of 91 and 81%, respectively. The chiroptical properties reveal dissymmetry factors |gPL| of up to 5 × 10−4. These results demonstrate the impact of the cyclophane for the development of CP‐TADF materials and add to the currently limited scope of available planar chiral donors. [ABSTRACT FROM AUTHOR]

Multi‐resonant thermally activated delayed fluorescence (MR‐TADF) helicenes show great potential as chiral emitters due to their typically high photoluminescence quantum yield and that they emit circularly polarized luminescence. Here, a new propeller‐shaped chiral MR‐TADF helicene DiKTa3, integrating three DiKTa moieties, was designed and synthesized aiming to achieve co‐parallel electronic and magnitude transition dipole moments that would lead to a high dissymmetry factor, g. It emits at λPL of 491 nm, with a full width at half maximum of 52 nm and has a moderate ΔEST value of 0.24 eV in toluene. The separated (P,P,P) enantiomer shows an absorption dissymmetry factor |gabs| of 6.8×10−4 at 450 nm, which results from a lower symmetry conformation adopted by the compound in solution than the C3‐symmetric optimized structure. This work highlights the strong influence that geometry can have on the chiroptical properties of the emitter. [ABSTRACT FROM AUTHOR]

Self‐assembling features, chiroptical activity, and spin filtering properties are reported for 2,15‐ and 4,13‐disubstituted [6]helicenes decorated in their periphery with 3,4,5‐tris(dodecyloxy)‐N‐(4‐ethynylphenyl)benzamide moieties. The weak non‐covalent interaction between these units conditions the corresponding circularly polarized luminescence and spin polarization. The self‐assembly is overall weak for these [6]helicene derivatives that, despite the formation of H‐bonding interactions between the amide groups present in the peripheral moieties, shows very similar chiroptical properties both in the monomeric or aggregated states. This effect could be explained by considering the steric effect that these groups could generate in the growing of the corresponding aggregate formed. Importantly, the self‐assembling features also condition chiral induced spin selectivity (CISS effect), with experimental spin polarization (SP) values found between 35–40 % for both systems, as measured by magnetic‐conducting atomic force microscopy (AFM) technique. [ABSTRACT FROM AUTHOR]

Enantiopure helicenes have been extensively investigated due to their outstanding chiroptical properties, while helicene racemates are considered as chiroptically silent. Here, we describe a facile method to produce circularly polarized luminescence (CPL) from helicene racemates via supramolecular association with DNA in aqueous solution. Racemic cationic helicene derivatives are immobilized in the grooves of commercially available double‐stranded right‐handed DNA, and the discrimination of left‐ and right‐handed helicenes by chiral DNA is monitored by single molecule force spectroscopy. This subsequently leads to the generation of prominent CPL with dissymmetric factor |glum| of close to 0.01, which is approximate to enantiopure helicenes. The strategy developed in this work avoids the tedious and expensive chiral resolution process and provides a distinctive insight into the fabrication of CPL‐emitting systems. [ABSTRACT FROM AUTHOR]

Dynamic P/M (plus/minus) helical memory in chiral dissymmetric poly(diphenylacetylene)s (PDPA) is shown using a PDPA that bears the benzamide of (L)‐alanine methyl ester as pendant. For a single chiral polymer, it is possible to obtain either P or M helical structures in a specific solvent without the presence of any chiral external stimuli. To do that, it is necessary to combine the conformational control at the pendant group with a high steric hindrance at the backbone. In this case, by thermal annealing in low‐polar solvents, an anti‐conformer is stabilized at the pendant which commands a P helix in the PDPA. Next, solvent removal followed by addition of a polar solvent such as dimethyl sulfoxide (DMSO), results in the kinetic conformationally trapped P helix. However, in this medium, the preferred handedness and the thermodynamic macromolecular helix for poly‐(L)‐1 is M. This process also occurs in the opposite way. Electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) studies show that the dynamic memory effect is present both in ground and excited states. [ABSTRACT FROM AUTHOR]

The photophysical and chiroptical properties of a novel, chiral helicene‐NHC−Re(I) complex bearing an N‐(aza[6]helicenyl)‐benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene‐Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties. [ABSTRACT FROM AUTHOR]

2‐Amino[2.2]paracyclophane reacts with salicylaldehyde or 2‐hydroxyacetophenone to yield imines that then give access to a new series of boranils (8b–d) upon complexation with BF2. These novel boron‐containing compounds display both planar and axial chiralities and were examined experimentally and computationally. In particular, their photophysical and chiroptical properties were studied and compared to newly prepared, simpler boranils (9a–d) exhibiting axial chirality only. Less sophisticated chiral architectures were shown to demonstrate overall stronger circularly polarized luminescence (CPL) activity. [ABSTRACT FROM AUTHOR]

A novel enantiopure π‐allylruthenium(IV) precatalyst allowed the enantioselective and stereospecific allylations of indoles and gave access to indolin‐3‐ones, containing vicinal stereogenic centers. Facile separation of diastereoisomers exhibiting opposite circularly polarized luminescence (CPL) activities in diverse solvents, including water, demonstrated the potential of these sustainable transformations and of the newly prepared molecules. [ABSTRACT FROM AUTHOR]

The combination of physical properties sensitive to molecular chirality in a single system allows the observation of fascinating phenomena such as magneto‐chiral dichroism (MChD) and circularly polarized luminescence (CPL) having potential applications for optical data readout and display technology. Homochiral monodimensional coordination polymers of YbIII were designed from a 2,15‐bis‐ethynyl‐hexahelicenic scaffold decorated with two terminal 4‐pyridyl units. Thanks to the coordination of the chiral organic chromophore to Yb(hfac)3 units (hfac−=1,1,1,5,5,5‐hexafluoroacetylaconate), efficient NIR‐CPL activity is observed. Moreover, the specific crystal field around the YbIII induces a strong magnetic anisotropy which leads to a single‐molecule magnet (SMM) behaviour and a remarkable room temperature MChD. The MChD‐structural correlation is supported by computational investigations. [ABSTRACT FROM AUTHOR]

The calculation of magnetic transition dipole moments and rotatory strengths was implemented at the zeroth‐order regular approximation (ZORA) two‐component relativistic time‐dependent density functional theory (TDDFT) level. The circular dichroism of the spin‐forbidden ligand‐field transitions of tris(ethylenediamine)cobalt(III) computed in this way agrees very well with available measurements. Phosphorescence dissymmetry factors glum and the corresponding lifetimes are evaluated for three N‐heterocyclic‐carbene‐based iridium complexes, two of which contain helicene moieties, and for two platinahelicenes. The agreement with experimental data is satisfactory. The calculations reproduce the signs and order of magnitude of glum , and the large variations of phosphorescence lifetimes among the systems. The electron spin contribution to the magnetic transition dipole moment is shown to be important in all of the computations. [ABSTRACT FROM AUTHOR]

The first chiral helicene‐NHC gold(I) complexes efficient in enantioselective catalysis were prepared. The L‐shaped chiral ligand is composed of an imidazo[1,5‐a]pyridin‐3‐ylidene (IPy) scaffold laterally substituted by a configurationally stable [5]‐helicenoid unit. The chiral information was introduced in a key post‐functionalization step of a NHC‐gold(I) complex bearing a symmetrical anionic fluoreno[5]helicene substituent, leading to a racemic mixture of complexes featuring three correlated elements of chirality, namely central, axial and helical chirality. After HPLC enantiomeric resolution, X‐ray crystallography and theoretical calculations enabled structural and stereochemical characterization of these configurationally stable NHC‐gold(I) complexes. The high potential in asymmetric catalysis is demonstrated in the benchmark cycloisomerization of N‐tethered 1,6‐enynes with up to 95 : 5 er. [ABSTRACT FROM AUTHOR]

The chemical reduction of π‐conjugated bilayer nanographene 1 (C138H120) with K and Rb in the presence of 18‐crown‐6 affords [K+(18‐crown‐6)(THF)2][{K+(18‐crown‐6)}2(THF)0.5][C138H1223−] (2) and [Rb+(18‐crown‐6)2][{Rb+(18‐crown‐6)}2(C138H1223−)] (3). Whereas K+ cations are fully solvent‐separated from the trianionic core thus affording a "naked" 1.3− anion, Rb+ cations are coordinated to the negatively charged layers of 1.3−. According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site‐specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction‐induced site‐specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers. [ABSTRACT FROM AUTHOR]

Luminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with glum up to 7×10−3, one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone (glum ∼7×10−4). This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge‐transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor‐acceptor model. Interestingly, blue, green‐yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi‐color CP‐TADF emitters. [ABSTRACT FROM AUTHOR]

The experimental vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectra were measured for the enantiomers of [1]rotaxane 1. These experimental spectra have been analyzed using predicted VCD and ECD spectra for (S, Rmp) or (S, Smp) diastereomers using density functional theory. This comparison allowed for a definitive assignment of the absolute configuration of 1. [ABSTRACT FROM AUTHOR]

During the XIXth century, France has been the theater of many discoveries in stereochemistry thanks to prestigious physicists, chemists, and crystallographers. Among them, Louis Pasteur is certainly the father of molecular chirality (the so‐called molecular dissymmetry), and because of the exceptionally favored conditions of his famous discovery on the resolution of tartrates, his findings have often been attributed to serendipity. But let us not forget that "in the fields of observation, chance only favors the prepared minds." [ABSTRACT FROM AUTHOR]

We describe the preparation of helically chiral gold(I) complexes bearing a [5]helicenic‐N‐heterocyclic carbene ligand. They were successfully obtained as enantiopure compounds by semi‐preparative chiral HPLC and their structural, chiroptical, and photophysical properties were subsequently investigated. Notably, strong electronic circular dichroism, dual emission from singlet and triplet states, with the timescale of the latter up to the millisecond range at room temperature, and moderate circularly phosphorescence were observed. The σ‐donating and π‐accepting properties of the constituent helical ortho‐fused π‐conjugated carbene were investigated by classical quantitative analysis of the IR stretching frequencies and NMR characteristics of the corresponding Ir(CO)2Cl complex and selenourea. [ABSTRACT FROM AUTHOR]

The enantioselective synthesis of chiral [7]‐helical dispirodihydro[2,1‐c]indenofluorenes (DSF‐IFs) was achieved for the first time in good yields with high er values (er up to 99 : 1). The crucial step of the whole reaction sequence was the enantioselective intramolecular [2+2+2] cycloaddition of tethered triynediols to indenofluorenediols, which was catalyzed by a Rh/SEGPHOS® complex. Further transformations led to the corresponding DSF‐IFs. The prepared helically chiral DSF‐IFs combine circularly polarized luminescence (CPL) activity (glum=∼10−3) with exceptionally high fluorescence quantum yields (up to Φlum=0.97). [ABSTRACT FROM AUTHOR]

This work reports on the preparation and structural characterization of flavo[7]helicene 1 (flavin‐[7]helicene conjugate), which was subsequently characterized at the molecular level in either an aqueous environment or an organic phase, at the supramolecular level in the form of polymeric layers, and also embedded in a lipidic mesophase environment to study the resulting properties of such a hybrid relative to its parent molecules. The flavin benzo[g]pteridin‐2,4‐dione (isoalloxazine) was selected for conjugation because of its photoactivity and reversible redox behavior. Compound 1 was prepared from 2‐nitroso[6]helicene and 6‐methylamino‐3‐methyluracil, and characterized using common structural and spectroscopic tools: circular dichroism (CD), circularly polarized luminescence (CPL) spectroscopy, cyclic voltammetry (CV), and DFT quantum calculations. In addition, a methodology that allows the loading of 1 enantiomers into an internally nanostructured lipid (1‐monoolein) matrix was developed. [ABSTRACT FROM AUTHOR]

Mono‐ and di‐boranil‐substituted helicenes were prepared by BF2‐borylation of the corresponding anils, readily synthesized by condensation of 2‐amino‐ and 2,15‐diamino‐helicenes with 4‐(diethylamino)salicylaldehyde. After enantiomeric resolution using HPLC, their chiroptical properties including circularly polarized fluorescence in solution and in PMMA films were investigated and rationalized with the help of NMR, X‐ray and quantum‐chemical calculations. [ABSTRACT FROM AUTHOR]

The straightforward, multigram‐scale synthesis of the partially saturated H6‐fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5‐diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H6‐fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum‐chemical calculations, confirming their helicoidal nature. [ABSTRACT FROM AUTHOR]

Chiral zirconium(IV) double cage sandwich complex Zr(1)2 has been synthesized in one step from porphyrin cage H21. Zr(1)2 was obtained as a racemate, which was resolved by HPLC and the enantiomers were isolated in >99.5% ee. Their absolute configurations were assigned on the basis of X-ray crystallography and circular dichroism spectroscopy. Vibrational circular dichroism (VCD) experiments on the enantiomers of Zr(1)2 revealed that the chirality around the zirconium center is propagated throughout the whole cage structure. The axial conformational chirality of the double cage complex displayed a VCD fingerprint similar to the one observed previously for a related chiral cage compound with planar and point chirality. Zr(1)2 shows fluorescence, which is quenched when viologen guests bind in its cavities. The binding of viologen and dihydroxybenzene derivatives in the two cavities of Zr(1)2 occurs with negative allostery, the cooperativity factors α (= 4 K2/K') being as low as 0.0076 for the binding of N,N'-dimethylviologen. These allosteric effects are attributed to a pinching of the second cavity as a result of guest binding in the first cavity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

By attaching pyridine groups to a diaza[6]helicene, a helical, bis‐ditopic, bis‐N N‐coordinating ligand can be accessed. Dinuclear rhenium complexes featuring this bridging ligand, of the form [{Re(CO)3Cl}2(N N−N N)], have been prepared and resolved to give enantiopure complexes. These complexes are phosphorescent in solution at room temperature under one‐ and two‐photon excitation. Their experimental chiroptical properties (optical rotation, electronic circular dichroism and circularly polarized emission) have been measured. They show, for instance, emission dissymmetry factors of c.a. ±3x10−3. Quantum‐chemical calculations indicate the importance of stereochemistry on the optical activity, pointing towards further design improvements in such types of complexes. [ABSTRACT FROM AUTHOR]

Molecular designs merging circularly polarized luminescence (CPL) and thermally activated delayed fluorescence (CP‐TADF) using the concept of chiral perturbation appeared recently as a cornerstone for the development of efficient CP‐organic light emitting diodes (CP‐OLED). Such devices could strongly increase the energy efficiency and performances of conventional OLED displays, in which 50% of the emitted light is often lost due to the use of antiglare filters. In this context, herein, ten couples of enantiomers derived from novel chiral emitter designs are reported, exhibiting CPL, TADF, and aggregation induced enhancement emission properties (AIEE). Representing the first structure properties relationship investigation for CP‐TADF materials, this thorough experimental and theoretical work brings crucial findings on the key structural and electronic parameters (isomerism, nature of the carbazole substituents) governing the synergy between CPL and TADF properties. To conclude this study, the first top emission CP‐OLED is elaborated as a new approach of generating CP light in comparison with classical bottom‐emission CP‐OLED architecture. Indeed, the top‐emission configuration represents the only relevant device architecture for future microdisplay applications. Thereby, in addition to offer molecular guidelines to combine efficiently TADF and CPL properties, this study opens new avenues toward practical applications for CP‐OLEDs. [ABSTRACT FROM AUTHOR]

The first enantiopure chiral‐at‐rhenium complexes of the form fac‐ReX(CO)3(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems. [ABSTRACT FROM AUTHOR]

Phosphahelicenes with thiophosphinic acid and ester functions have been obtained by the oxidative photocyclisation of olefins bearing both a benzophenanthrene and a benzophosphole unit. When the method has been extended to olefins bearing a partially saturated benzophospholene unit, a divergent regioselectivity of the photocyclisation step has been observed, leading to new helicenes in which the phosphorus function is located on the external rim of the helical backbone. The observed regioselectivity correlates well with the free‐valence numbers of the atoms involved in the photocyclisation reaction (DFT calculations). [ABSTRACT FROM AUTHOR]

Novel [4, 6]helicenes (4a,b) bearing a fused imidazolium unit have been prepared from [4, 6]helicene‐2,3‐di‐n‐propyl‐amines 3a,b. The in situ formation of N‐heterocyclic carbene (NHC) derivatives followed by their complexation to iridium(I) or rhodium(I) gave access to complexes 1a, 1′a, and 1b, containing mono‐coordinated helicene‐NHC, chloro and COD (COD = 1,5‐cyclooctadiene) ligands. Ir and Rh complexes 1a and 1′a were characterized by X‐ray crystallography. HPLC and NMR analyses showed that Ir(I) complex 1b existed as a mixture of two diastereomeric complexes corresponding to enantiomeric pairs M‐(−)/P‐(+)‐1b1 and M‐(−)/P‐(+)‐1b2 which differ by the position of COD through space. The chiroptical properties (electronic circular dichroism and optical rotation) of the four stereoisomers were measured. These complexes were also tested as catalysts in a transfer hydrogenation reaction. [ABSTRACT FROM AUTHOR]

Herein, the experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism, optical rotations, and circularly polarized luminescence. Furthermore, detailed analysis of absorption and ECD spectra performed with the help of quantum‐chemical calculations allowed us to highlight general features of these helicenic phosphines. Finally, due to well‐suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light‐emitting diodes. Spirals of light: The experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported along with their theoretical analysis, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism (ECD), optical rotations, and circularly polarized luminescence. Due to well‐suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light‐emitting diodes. [ABSTRACT FROM AUTHOR]

The coordination of a helicene‐functionalized bipyridine to various lanthanide complexes (Ln = Y, Eu, Yb, Gd) led to the formation of an isostructural series. The photophysical study indicated that non‐emissive lanthanides act as heavy atoms strongly enhancing singlet oxygen generation. We also demonstrated that sensitization of the f‐f luminescence is in competition with singlet oxygen generation. The coordination of a helicene functionalized bipyridine to various lanthanide complexes (ln = Y, Eu, Yb, Gd) led to the formation of an isostructural series. The photophysical study indicated that lanthanides act as heavy atoms strongly enhancing the singlet oxygen generation. We also demonstrated that the sensitization of the f‐f luminescence is in competition with the singlet oxygen generation. [ABSTRACT FROM AUTHOR]

Introducing one or two alkynyl–iron moieties onto a carbo[6]helicene results in organometallic helicenes (2 a,b) that display strong chiroptical activity combined with efficient redox‐triggered switching. The neutral and oxidized forms have been studied in detail by electronic and vibrational circular dichroism, as well as by Raman optical activity (ROA) spectroscopy. The experimental results were analyzed and spectra were assigned with the help of first‐principles calculations. In particular, a recently developed method for ROA calculations under resonance conditions has been used to study the intricate resonance effects on the ROA spectrum of mono‐iron ethynylhelicene 2 a. Ironing in kinks: Introducing one or two alkynyl–iron moieties onto a carbo[6]helicene results in organometallic helicenes that display strong chiroptical activity combined with efficient redox‐triggered switching (see figure). The chiroptical properties and redox‐switching activity of iron alkynylhelicenes determined experimentally and theoretically are reported. [ABSTRACT FROM AUTHOR]

Abstract: Enantiopure P‐ and M‐carbo[6]helicenes substituted with one or two tetracyanobutadiene moieties at positions 2 and 15 have been prepared. Grafting of these electron‐accepting groups onto the π‐helical core resulted in strong charge‐transfer effects, which greatly affected the UV/Vis, electronic circular dichroism (ECD), and two‐photon absorption (TPA) responses. The ECD signal was found to be reversibly switched by applying a redox stimulus. [ABSTRACT FROM AUTHOR]

Abstract: Helicenes are polyaromatic compounds with chiral properties useful for many applications in optoelectronics, separation processes, chiral recognition and catalysis. Here we focused on the electrochemistry of carbo[n]helicenes (n=5,6,7). The cyclic voltammograms of racemic mixtures of target compounds in acetonitrile/0.1 M tetrabutylammonium perchlorate at a glassy carbon electrode reveal the diffusion‐controlled reactions in both anodic and cathodic potential regions. Electrochemical behaviors are different for individual helicenes, [7]helicene undergoes redox transformation easily in comparison to the other investigated compounds, which is in agreement with DFT (density functional theory) calculations. Generally, the multi‐component anodic process of helicenes is observable at potentials from +1.5 to +2.5 V, leading to the formation of deposited structures (layers) on the electrode surface. The helicenes were electrodeposited onto transparent indium tin oxide (ITO) electrodes and characterized by atomic force microscopy, UV/Vis, Raman spectroscopy and ellipsometry. Finally, the anodic deposition of P and M enantiomers of [6]helicene was performed using ITO substrates, resulting in the formation of enantiopure layers of nanometer thicknesses, as confirmed by circular dichroism spectroscopy. The discovered electrosynthetic procedure opens up a new possibility for the immobilization of chiral helicene layers onto solid supports. [ABSTRACT FROM AUTHOR]

Abstract: A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa‐peri‐hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X‐ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA‐stacked conformation, rarely observed in few‐layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications. [ABSTRACT FROM AUTHOR]

Abstract: The charged, electroactive bipyridine‐helicene‐ruthenium(III) complex [4].+,PF6− has been prepared from 3‐(2‐pyridyl)‐4‐aza[6]helicene and a Ru‐bis‐(β‐diketonato)‐bis‐acetonitrile precursor (β‐diketonato: 2,2,6,6‐tetramethyl‐3,5‐heptanedionato). Its chiroptical properties (electronic circular dichroism and optical rotation) were studied both experimentally and theoretically and suggest the presence of 2 diastereoisomers, namely (P,Δ)‐ and (P,Λ)‐[4].+,PF6− (denoted jointly as (P,Δ*)‐[4].+,PF6−) and their mirror‐images (M,Λ)‐ and (M,Δ)‐[4].+,PF6− ((M,Δ*)‐[4].+,PF6−). The electrochemical reduction of (P,Δ*)‐[4].+,PF6− to neutral complex (P,Δ*)‐4 was performed and revealed strong changes in the UV‐vis and electronic circular dichroism spectra. A reversible redox‐triggered chiroptical switching process was then achieved. [ABSTRACT FROM AUTHOR]

Abstract: In our effort towards measuring the parity violation energy difference between two enantiomers, a simple chiral oxorhenium complex 5 bearing enantiopure 2‐mercaptocyclohexan‐1‐ol has been prepared as a potential candidate species. Vibrational circular dichroism revealed a chiral environment surrounding the rhenium atom, even though the rhenium is not a stereogenic center itself, and enabled to assign the (1S,2S)‐(−) and (1R,2R)‐(+) absolute configuration for 5. For both compound 5 and complex 4, previously studied by us and bearing a propane‐2‐olato‐3‐thiolato ligand, relativistic calculations predict parity violating vibrational frequency differences of a few hundreds of millihertz, above the expected sensitivity attainable by a molecular beam Ramsey interferometer that we are constructing. [ABSTRACT FROM AUTHOR]

A fused π-helical N-heterocyclic carbene (NHC) system was prepared and examined through its diastereoisomerically pure cycloiridiated complexes. The latter display light-green phosphorescence with unusually long lifetimes and circular polarization that depends on both the helical NHC P/ M stereochemistry and the iridium Δ/Λ stereochemistry. These unprecedented features are attributed to extended π conjugation within the helical carbenic ligand and efficient helicene-NHC-Ir interaction. [ABSTRACT FROM AUTHOR]

The first stereocontrolled Cu-catalyzed sequential 1,6/1,4-asymmetric conjugate addition (ACA) of C-metalated hard nucleophiles to cyclic dienones is reported. The use of DiPPAM (diphenylphosphinoazomethinylate) followed by a phosphoramidite as the stereoinducing ligands facilitated both high ee values for the 1,6-ACA and high de values for the 1,4-ACA reaction components, which thus gave enantioenriched 1,3-dialkylated moieties. The absolute configurations were determined by using vibrational circular dichroism (VCD) and optical rotatory dispersion (ORD) spectroscopy, in combination with DFT calculations and X-ray analysis. Interestingly, DFT calculations for the mechanism of enantioselective 1,6-addition by using an unprecedented Cu-Zn bimetallic catalytic system confirmed this attribution. Lastly, exploring intramolecular cyclization avenues for enantioenriched 1,3-dialkylated products provided access to the challenging drimane skeleton. [ABSTRACT FROM AUTHOR]

Impact of the enantiopurity on organic photovoltaics (OPV) performance was investigated through the synthesis of racemic and enantiomerically pure naphthalimide end-capped helicenes and their application as non-fullerene molecular electron acceptors in OPV devices. A very strong increase of the device performance was observed by simply switching from the racemic to the enantiopure forms of these π-helical non-fullerene acceptors with power conversion efficiencies jumping from 0.4 to about 2.0 % in air-processed poly(3-hexylthiophene)-based devices, thus highlighting the key role of enantiopurity in the photovoltaic properties. [ABSTRACT FROM AUTHOR]

Four members of a new class of cycloborylated hexa-, octa-, and deca-helicenes ( 1 a- d) have been prepared in enantiopure form, along with two cycloplatinated deca-helicenes ( 1 d′, 1 d1), further extending the family of cycloplatinated hexa- and octa-helicenes reported previously. The azabora[ n]helicenes display intense electronic circular dichroism and large optical rotations; the dependence of the optical activity on the size of the helix ( n=6, 8, 10) and the number of boron atoms (1 or 2) has been examined in detail both experimentally and theoretically. The photophysical properties (nonpolarized and circularly polarized luminescence) of these new fluorescent organic helicenes have been measured and compared with the corresponding organometallic phosphorescent cycloplatinated derivatives ( 1 a1- d1). [ABSTRACT FROM AUTHOR]

Introduction of heterocycles into the helical skeleton of helicenes allows modulation of their redox, chiroptical, and photophysical properties. This paper describes the straightforward preparation and structural characterization by single-crystal X-ray diffraction of thiadiazole-[7]helicene, which was resolved into M and P enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands was structurally characterized, and it shows the presence of both M and P isomers coordinated to the metal center. The emission properties of the heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4 and 6.5 %, respectively. DFT calculations suggest quenching of the luminescence of enantiopure [7]helicenes through an intersystem-crossing mechanism arising from the relaxed excited S1 state. [ABSTRACT FROM AUTHOR]

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 mm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged. [ABSTRACT FROM AUTHOR]

The combination of a bis-alkynyl-helicene moiety with two iron centers leads to novel electroactive species displaying unprecedented redox-triggered chiroptical switching. Upon oxidation, strong changes of vibrational modes (either local or extended coupled modes) are detected by vibrational circular dichroism and Raman optical activity. Remarkably, the sign of the optical rotation at 1.54 μm (that is, at wavelengths typically used for telecommunications) changes upon oxidation while the topology and stereochemistry of the helicene remain unchanged. [ABSTRACT FROM AUTHOR]

Monometallic gold(I)-alkynyl-helicene complexes ( 1 a, b) and bimetallic gold(I)-alkynyl-helicene architectures featuring the presence ( 2 a, b) or absence ( 3 a, b) of aurophilic intramolecular interactions were prepared by using different types of phosphole ligands (mono-phosphole L1 or bis-phospholes L2, 3). The influence of the AuI d10 metal center(s) on the electronic, photophysical, and chiroptical properties of these unprecedented phosphole-gold(I)-alkynyl-helicene complexes was examined. Experimental and theoretical results highlight the importance of ligand-to-ligand-type charge transfers and the strong effect of the presence or absence of AuI-AuI interactions in 2 a, b. [ABSTRACT FROM AUTHOR]

Helicenes are fascinating molecules owing to their unusual properties and applications in many fields from catalysis to organic electronics. Herein, we report a straightforward pathway for the synthesis of helicene-like molecules on a gram scale in an enantiopure form. Thin-film materials with good propagating optical properties and very high chiroptical responses have been grown by using pulsed laser ablation without altering the structure or the enantiopurity of the molecules. Moreover, electronic and vibrational circular dichroism spectroscopies coupled with theoretical calculations enabled some dependences of the chiroptical properties with the structure to be highlighted, for example, effects of rigidification, aromatization, or the state of matter (liquid versus solid). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Acetylated and/or 3,5-dimethylphenylcarbamated riboflavins were prepared and the resulting riboflavin derivatives as well as natural riboflavin were regioselectively immobilized on silica gel through chemical bonding at the 5'-O- or 3-N-position of the riboflavin to develop novel chiral stationary phases (CSPs) for enantioseparation by high-performance (HPLC). The abilities of the obtained CSPs were significantly dependent on the structures of the riboflavin derivatives, the position of the chemical bonding on the silica gel, and the structures of the racemic compounds. The CSPs bonded at the 5'-O-position on the silica gel tended to well separate helicene derivatives, while the CSPs bonded at the 3-N-position composed of acetylated and 3,5-dimethylphenylcarbamated riboflavins showed a better resolving ability toward helicene derivatives and bulky aromatic racemic alcohols, respectively, and some of them were completely separated into the enantiomers. The observed difference in the abilities of these riboflavin-based CSPs is discussed based on the difference in their structures, including the substituents of riboflavin and the positions immobilized on the silica gel. Chirality 27:507-517, 2015. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Electronic circular dichroism and circularly polarized luminescence acid/base switching activity has been demonstrated in helicene-bipyridine proligand 1 a and in its 'rollover' cycloplatinated derivative 2 a. Whereas proligand 1 a displays a strong bathochromic shift (>160 nm) of the nonpolarized and circularly polarized luminescence upon protonation, complex 2 a displays slightly stronger emission. This strikingly different behavior between singlet emission in the organic helicene and triplet emission in the organometallic derivative has been rationalized by using quantum-chemical calculations. The very large bathochromic shift of the emission observed upon protonation of azahelicene-bipyridine 1 a has been attributed to the decrease in aromaticity (promoting a charge-transfer-type transition rather than a π-π* transition) as well as an increase in the HOMO-LUMO character of the transition and stabilization of the LUMO level upon protonation. [ABSTRACT FROM AUTHOR]