Carbo[n]helicenes Restricted to Enantiomerize: An Insight into the Design Process of Configurationally Stable Functional Chiral PAHs

Ultra-Narrowband Circularly Polarized Luminescence from Multiple 1,4-Azaborine-Embedded Helical Nanographenes

In this manuscript we present a strategy to achieve ultranarrowband circularly polarized luminescence (CPL) from multiple 1,4-azaborine-embedded helical nanographenes. The impact of number and position of boron and nitrogen atoms in the rigid core of the molecule on optical properties─including absorption and emission maxima, photoluminescence quantum yield, Stokes shift, excited singlet-triplet energy gap and full width at half-maximum (fwhm) for CPL and fluorescence─was investigated. The molecules reported here exhibits ultranarrowband fluorescence (fwhm 16-17.5 nm in toluene) and CPL (fwhm 18-19 nm in toluene). To the best of our knowledge, this is among the narrowest CPL for any organic molecule reported to date. Quantum chemical calculations, including computed CPL spectra involving vibronic contributions, provide valuable insights for future molecular design aimed at achieving narrowband CPL.

A Double Twisted Nanographene with a Contorted Pyrene Core

The mature synthetic methodologies enable us to rationally design and produce chiral nanographenes (NGs), most of which consist of multiple helical motifs. However, inherent chirality originating from twisted geometry has just emerged to be employed in chiral NGs. Herein, we report a red-emissive chiral NG constituted of orthogonally arranged two-fold twisted π-skeletons at a contorted pyrene core which contributes to optical transitions of S0→S1 and vice versa. The thus-obtained NG exhibited a robustness on its redox properties through 2e- uptake/release. The chemical oxidation generated stable radical cation whose absorption covers near-infrared I and II regions. Overall, the contorted pyrene core governs electronic nature of the chiral NG. The twist operation on NGs would be, therefore, a design strategy to alter conventional chirality induction on NGs.

Helically twisted nanoribbons via stereospecific annulative π-extension reaction employing [7]helicene as a molecular wrench

Over the past decade, significant progress has been made in synthesizing atomically precise carbon nanostructures, particularly graphene nanoribbons (NRs), employing advanced synthetic methodologies. Despite these advancements, achieving control over the stereochemistry of twisted NRs has proven to be a formidable challenge. This manuscript presents a strategic approach to achieve absolute control over the single-handed helical conformation in a cove-edged NR. This strategy leverages enantiopure helicenes as a molecular wrench, intricately influencing the overall conformation of the NR. [7]helicenes stitched to the terminal K-regions of a conjugated pyrene NR through a stereospecific annulative π-extension reaction to produce a helically twisted NR with an end-to-end twist of 171°. Furthermore, a detailed investigation of the impact of twisting on the conformational population was studied by quantum chemical calculations.

Stereochemical Control of Redox CoII/CoIII-Cages with Switchable Cotton Effects Based on Labile-Static States

The structural dynamics of artificial assemblies, in aspects such as molecular recognition and structural transformation, provide us with a blueprint to achieve bioinspired applications. Here, we describe the assembly of redox-switchable chiral metal-organic cages Λ8/Δ8-[Pd6(CoIIL3)8]28+ and Λ8/Δ8-[Pd6(CoIIIL3)8]36+. These isomeric cages demonstrate an on-off chirality logic gate controlled by their chemical and stereostructural dynamics tunable through redox transitions between the labile CoII-state and static CoIII-state with a distinct Cotton effect. The transition between different states is enabled by a reversible redox process and chiral recognition originating in the tris-chelate Co-centers. All cages in two states are thoroughly characterized by NMR, ESI-MS, CV, CD, and X-ray crystallographic analysis, which clarify their redox-switching behaviors upon chemical reduction/oxidation. The stereochemical lability of the CoII-center endows the Λ8/Δ8-CoII-cages with efficient chiral-induction by enantiomeric guests, leading to enantiomeric isomerization to switch between Λ8/Δ8-CoII-cages, which can be stabilized by oxidation to their chemically inert forms of Λ8/Δ8-CoIII-cages. Kinetic studies reveal that the isomerization rate of the Δ8-CoIII-cage is at least an order of magnitude slower than that of the Δ8-CoII-cage even at an elevated temperature, while its activation energy is 16 kcal mol-1 higher than that of the CoII-cage.

Cyclic Azahelicene Dimers Showing Bright Circularly Polarized Luminescence and Selective Fluoride Recognition

Although helicenes are promising molecules, the synthetic difficulty and tediousness have often been problems, and only small amounts of optically pure helicenes have been obtained by using chiral HPLC in most cases. Herein, aza[7]helicenes or closed-aza[7]helicenes with (1R)-menthyl substituents were selectively synthesized via the intramolecular Scholl reaction, and the diastereomeric pairs were separated by silica gel column chromatography. The optically pure helicenes were further transformed into the corresponding cyclic dimers, and the chiroptical properties were investigated. The rigid π-frameworks of the dimers led to the high molar extinction coefficients and fluorescence quantum yields, while the twisted helicene moieties induced clear Cotton effects and CPL in the visible region, and the high CPL brightness (BCPL) was achieved. Furthermore, the cyclic dimers were found to have the macrocyclic cavity with the two NH groups suitable for the selective binding of a fluoride anion, which induced significantly redshifted fluorescence and CPL in the red region.

Unusually Short H⋅⋅⋅H Contacts in Intramolecularly Cyclized Helically Fused Anthracenes

The intramolecular coupling of dichloro-substituted helically fused anthracenes using the Yamamoto coupling yielded cyclized products with sterically congested molecular structures. The X-ray analysis and DFT calculations showed that the aromatic framework adopted a nonplanar structure with a twisted conformation about the newly formed single bond, which acts as a chiral axis. Interestingly, the X-ray structure obtained through the Hirshfeld atom refinement revealed short interatomic distances between the inner hydrogen atoms (1.648-1.692 Å), much shorter than the sum of their van der Waals radii. Owing to these unusually short contacts, the 1H NMR spectrum exhibited a significant deshielding (12.5 ppm) and a large nuclear Overhauser effect (44 %). Additionally, the IR spectrum displayed a high-frequency shift of the C-H stretching vibration. These observations, along with the noncovalent interaction plot indicative of a characteristic steric environment, strongly support the presence of steric hindrance. Moreover, dynamic NMR measurement of the mesityl-substituted derivative yielded a barrier to helical inversion of 84 kJ mol-1. The optical properties and crystal packing of the cyclized products are also reported.

Synthesis of Aza[n]helicenes up to n = 19: Hydrogen-Bond-Assisted Solubility and Benzannulation Strategy

Synthetic challenges toward anomalous structures and electronic states often involve handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes, which benefit from both the suppressed elevation of highest occupied molecular orbital (HOMO) energies and high solubility due to hydrogen bonding with solvent molecules to overcome these challenges. This strategy enabled the synthesis of six new aza[n]helicenes ([n]AHs) of different lengths (n = 9-19) from acyclic precursors via one-pot intramolecular oxidative fusion reactions. The structures of all of the synthesized aza[n]helicenes were determined by X-ray diffraction (XRD) analysis, and their electrochemical potentials were measured by cyclic voltammetry. Among the synthesized aza[n]helicenes, [17]AH and [19]AH are the first heterohelicenes with a triple-layered helix. The noncovalent interaction (NCI) plots confirm the existence of an effective π-π interaction between the layers. The absorption and fluorescence spectra red-shifted as the helical lengths increased, without any distinct saturation points. The optical resolutions of N-butylated [9]AH, [11]AH, [13]AH, and [15]AH were accomplished, and their circular dichroism (CD) and circularly polarized luminescence (CPL) were measured. Thus, the structural, (chir)optical, and electrochemical properties of the aza[n]helicenes were comprehensively analyzed.

Copper-catalysed perarylation of cyclopentadiene: synthesis of hexaarylcyclopentadienes

While hexaphenylsilacyclopentadiene (hexaphenylsilole) is viewed as an archetypal Aggregation-Induced Emission (AIE) luminogen, its isostructural hydrocarbon surrogate hexaphenylcyclopentadiene has strikingly never been investigated in this context, most probably due to a lack of synthetic availability. Herein, we report a straightforward synthesis of hexaphenylcyclopentadiene, via the direct perarylation of cyclopentadiene upon copper(i) catalysis under microwave activation, with the formation of six new C-C bonds in a single synthetic operation. Using zirconocene dichloride as a convenient source of cyclopentadiene and a variety of aryl iodides as coupling partners, this copper-catalysed cross-coupling reaction gave rise to a series of unprecedented hexaarylcyclopentadienes. The latter are direct precursors of extended π-conjugated polycyclic compounds, and their cyclodehydrogenation under Scholl reaction conditions yielded helicenic 17,17-diarylcyclopenta[l,l']diphenanthrenes. These structurally complex polyannelated fluorene derivatives can now be prepared in only two synthetic steps from cyclopentadiene.

Nonalternant Nanographenes Containing N-Centered Cyclopenta[ef]heptalene and Aza[7]Helicene Units

Introducing helical subunits into negatively curved π-systems has a significant effect on both the molecular geometry and photophysical properties; however, the synthesis of these helical π-systems embedded with nonbenzenoid subunits remains challenging due to the high strain deriving from both the curvature and helix. Here, we report a family of nonalternant nanographenes containing a nitrogen (N)-doped cyclopenta[ef]heptalene unit. Among them, CPH-2 and CPH-3 can be viewed as hybrids of benzoannulated cyclopenta[ef]heptalene and aza[7]helicene. The crystal structures revealed a saddle geometry for CPH-1, a saddle-helix hybrid for CPH-2, and a twist-helix hybrid for CPH-3. Experimental measurements and theoretical calculations indicate that the saddle moieties in CPHs undergo flexible conformational changes at room temperature, while the aza[7]helicene subunit exhibits a dramatically increased racemization energy barrier (78.2 kcal mol-1 for CPH-2, 143.2 kcal mol-1 for CPH-3). The combination of the nitrogen lone electron pairs of the N-doped cyclopenta[ef]heptalene unit with the twisted helix fragments results in rich photophysics with distinctive fluorescence and phosphorescence in CPH-1 and CPH-2 and the similar energy fluorescence and phosphorescence in CPH-3. Both enantiopure CPH-2 and CPH-3 display distinct circular dichroism (CD) signals in the UV-vis range. Notably, compared to the reported fully π-extended helical nanographenes, CPH-3 exhibits excellent chiroptical properties with a |gabs| value of 1.0 × 10-2 and a |glum| value of 7.0 × 10-3; these values are among the highest for helical nanographenes.

Significance of Vibronic Coupling that Shapes Circularly Polarized Luminescence of Double Helicenes

The circularly polarized luminescence (CPL) spectra of S- and X-shaped double helicenes exhibit distinct vibrational structures and overall shape variations. In this study, we conducted an in-depth investigation into the vibronic effects influencing the CPL spectra of two double helicenes, namely DPC and DNH. Employing state-of-the-art computations utilizing the FC-HT1|VH model at the CAM-B3LYP/def2-TZVP level, we unveiled the paramount impact of Franck-Condon (FC), Herzberg-Teller (HT), and Duschinsky effects on their chiroptical responses. Our research underscores the pivotal role of structural deformations associated with the S1-to-S0 electronic transition in molding CPL spectra and wavelength-dependent dissymmetry (g) factor values, as well as the significance of HT effects in shaping and enhancing CPL responses. This extensive investigation not only advances our comprehension of the vibronic characteristics in configurationally distinct double helicenes but also offers valuable insights for the design of chiral molecules featuring controllable or finely-tunable CPL responses.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

Enantiomerization of five-membered-heterocycle-embedded helicenes: A DFT study

In this work, DFT theoretical calculations were employed to investigate the enantiomerization of helicenes embedded with five-membered heterocycles. The original benzene rings in the helicene backbone were replaced by heterocycles such as furan, thiophene, pyrrole, or phosphole to create [n]helicenes with n ranging from 4 to 7. The impact of the type, position, and number of heterocycles on the enantiomerization barrier was systematically evaluated. Notably, the enantiomerization barrier was found to be significantly dependent on the rotatory angle and the position of the heterocycles, particularly for [4, 5]helicenes. With less rotatory angle of heterocycle, the enantiomerization barrier of helicenes was revealed to be lower, while when the heterocycle was close to the central part of the helicene chain, the barrier was also lower. Furthermore, the number of thiophene rings also had a marked effect on enantiomerization, showing a decrease of the barrier with more thiophene rings placed on the helicenes backbone. We expect this work would deliver new perspective on the relative studies for the helicene conformational conversion.

π-Extended Diaza[7]helicenes with Dual Negatively Curved Heptagons: Extensive Synthesis and Spontaneous Resolution into Strippable Homochiral Lamellae with Helical Symmetry

We report a unique category of π-extended diaza[7]helicenes with double negative curvatures. This is achieved by two-fold regioselective heptagonal cyclization of the oligoarylene-carbazole precursors through either intramolecular C-H arylation or Scholl reaction. The fusion of two heptagonal rings in the helical skeleton dramatically increases the intramolecular strain and forces the two terminal carbazole moieties to stack in a compressed fashion. The presence of the deformable negatively curved heptagonal rings endows the resulting diaza[7]helicenes with dynamic chiral skeletons, aggregation-induced emission feature and relatively low racemization barrier of ca. 25.6 kcal mol-1 . Further π-extension on the carbazole moieties subsequently leads to a more sophisticated C2 -symmetric homochiral triple helicene. Notably, these π-extended diaza[7]helicenes show structure-dependent stacking upon crystallization, switching from heterochiral packing to intra-layer homochiral stacking. Interestingly, the C2 -symmetric triple helicene molecules spontaneously resolve into a homochiral lamellar structure with 31 helix symmetry. Upon ultrasonication in a nonsolvent, the crystals can be readily exfoliated into large-area ultrathin nanosheets with height of ca. 4.4 nm corresponding to two layers of stacked triple helicene molecules and relatively thicker nanosheets constituted by even-numbered molecular lamellae. Moreover, regular hexagonal thin platelets with size larger than 30 μm can be readily fabricated by flash aggregation.

Controlling helicene's pitch by molecular tethering

We applied post-cyclization annulation to introduce a series of tethered S-shaped double [4]helicenes in which the intramolecular tether imposes a specific helical handedness. Introducing a tether and then shortening the tether length incrementally increase the pitch angle of [4]helicene, thus enabling a quantitative study of the effects of helicene's pitch on its electronic and (chiro)optical properties.

Can Magnetic Dipole Transition Moment Be Engineered?

The development of chiral compounds with enhanced chiroptical properties is an important challenge to improve device applications. To that end, an optimization of the electric and magnetic dipole transition moments of the molecule is necessary. Nevertheless, the relationship between chemical structure and such quantum mechanical properties is not always clear. That is the case of magnetic dipole transition moment (m) for which no general trends for its optimization have been suggested. In this work we propose a general rationalization for improving the magnitude of m in different families of chiral compounds. Performing a clustering analysis of hundreds of transitions, we have been able to identify a single group in which |m| value is maximized along the helix axis. More interestingly, we have found an accurate linear relationship (up to R2 =0.994) between the maximum value of this parameter and the area of the inner cavity of the helix, thus resembling classical behavior of solenoids. This research provides a tool for the rationalized synthesis of compounds with improved chiroptical responses.

Boosting quantum yields and circularly polarized luminescence of penta- and hexahelicenes by doping with two BN-groups

The incorporation of boron-nitrogen (BN) units into polycyclic aromatic hydrocarbons (PAHs) as an isoelectronic replacement of two carbon atoms can significantly improve their optical properties, while the geometries are mostly retained. We report the first non-π-extended penta- and hexahelicenes comprising two aromatic 1,2-azaborinine rings. Comparing them with their all-carbon analogs regarding structural, spectral and (chir)optical properties allowed us to quantify the impact of the heteroatoms. In particular, BN-hexahelicene BN[6] exhibited a crystal structure congruent with its analog CC[6], but displayed a fivefold higher fluorescence quantum yield (φfl = 0.17) and an outstanding luminescence dissymmetry factor (|glum| = 1.33 × 10-2). Such an unusual magnification of both properties at the same time makes BN-helicenes suitable candidates as circularly polarized luminescence emitters for applications in materials science.

Synthesis of Closed-Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms

Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed-heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed-[7]helicene dimers connected at the β-position of one of the pyrrole units with remarkably elongated C-C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed-helicene monomer and dimer was achieved. The optically pure dimer was photo-dissociated into the monomers associated with a turn-on of circularly polarized luminescence (CPL).

Push-pull [7]helicene diimide: excited-state charge transfer and solvatochromic circularly polarised luminescence

In this communication we describe a helically chiral push-pull molecule named 9,10-dimethoxy-[7]helicene diimide, displaying fluorescence (FL) and circularly polarised luminescence (CPL) over nearly the entire visible spectrum dependent on solvent polarity. The synthesised molecule exhibits an unusual solvent polarity dependence of FL quantum yield and nonradiative rate constant, as well as remarkable gabs and glum values along with high configurational stability.

Synthesis of Nonsymmetric NBN-Embedded [6]- and [7]Helicenes with Amplified Activities

Two C1-symmetric heterohelicenes were constructed by nonsymmetrically extending the ortho-fused structures of a C2v-symmetric NBN-embedded phenalene derivative and featured intense luminescence, large Stokes shifts, and successive reversible redox behaviors. Increasing one fused phenyl unit in such a helical structure led to a 10-fold-enhanced dissymmetry factor. Their strong double hydrogen-bond-donating capability makes them distinctly red-shifted in absorption, emission, and CD and CPL spectra upon the addition of fluoride anion.

Heterocyclic [9]Helicenes Exhibiting Bright Circularly Polarized Luminescence

We describe a concise synthetic strategy for the preparation of heterocyclic [9]helicenes and a simple preparative-scale protocol for the optical resolution of the resulting M- and P-enantiomers. The helicenes were characterized by single-crystal X-ray diffraction along with a range of spectroscopic and computational techniques. A fluorescence quantum yield of up to 65 % was observed, and the chiroptical properties of both M- and P-helicenes revealed large dissymmetry factors. The circularly polarized luminescence brightness reaches up to 17 M-1  cm-1 , as measured experimentally and verified computationally, which makes this the highest circularly polarized luminescence brightness among heterocyclic helicenes. We describe how chiroptical properties (both circular dichroism and circularly polarized luminescence) can be described and predicted using quantum chemical calculations. The synthetic approach also reveals by-products that originate from internal oxidation reactions, presumably mediated by the close proximity of the π-surfaces in the helicene structure.

Dinor[7]helicene and Beyond: Divergent Synthesis of Chiral Diradicaloids with Variable Open-Shell Character

Diradicaloid helicenes constructed formally by non-benzenoid double π-extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid-state structures, spectroscopic methods, and computational analyses. The open-shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet-triplet gaps. Compounds 1-3 displayed narrow optical gaps of 0.79-1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm.

Substitution Effects on the Photophysical and Photoredox Properties of Tetraaza[7]helicenes

A series of substituted derivatives of tetraaza[7]helicenes were synthesized and the influence of the substitution on their photophysical and photoredox-catalytic properties was studied. The combination of their high fluorescence quantum yields of up to 0.65 and their circularly polarized luminescence (CPL) activity results in CPL brightness values (BCPL ) that are among the highest recorded for [7]helicenes so far. A sulfonylation/hetarylation reaction using cyanopyridines as substrates for photoinduced electron transfer (PET) from the excited helicenes was conducted to test for viability in photoredox catalysis. DFT calculations predict the introduction of electron withdrawing substituents to yield more oxidizing catalysts.

A C-H activation-based enantioselective synthesis of lower carbo[n]helicenes

The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4-6) is still lacking. We report here that Pd-catalysed enantioselective C-H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C-H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

Synthesis and Excited-State Properties of Donor-Acceptor Azahelical Coumarins

A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).

Synthesis of Bioctacene-Incorporated Nanographene with Near-Infrared Chiroptical Properties

We report the synthesis of a hexabenzoperihexacene (HBPH) with two incorporated octacene substructures, which was unambiguously characterized by single-crystal X-ray analysis. The theoretical isomerization barrier of the (P,P)-/(P,M)-forms was estimated to be 38.4 kcal mol^-1 , and resolution was achieved by chiral HPLC. Notably, the enantiomers exhibited opposite circular dichroism responses up to the near-infrared (NIR) region (830 nm) with a high g_abs value of 0.017 at 616 nm. Moreover, HBPH demonstrated NIR emission with a maximum at 798 nm and an absolute PLQY of 41 %. The excited-state photophysical properties of HBPH were investigated by ultrafast transient absorption spectroscopy, revealing an intriguing feature that was attributed to the rotational and/or conformational dynamics of HBPH after excitation. These results provide new insight into the design of chiral nanographene with NIR optical properties for potential chiroptical applications.

Divergent Synthesis of Helical Ketone Enabled by Rearrangement of Spiro Carbocation

An acid-mediated electrophilic cyclization of 2-alkynyl-1,1'-biphenyls for the divergent synthesis of angular, bent, and zigzag fused nonplanar conjugated organic molecules was realized. The key feature of this reaction is a Wagner-Meerwein-type rearrangement via a spiro carbocation intermediate, which was formed by electrophilic cyclization of the 9H-fluoren-9-one derivative at the meta position. The products can be advanced to helical fluorenes, which exhibit high fluorescence quantum yields.

Efficient Narrowband Circularly Polarized Light Emitters Based on 1,4-B,N-embedded Rigid Donor-Acceptor Helicenes

Narrow-band emission is essential for applicable circularly polarized luminescence (CPL) active materials in ultrahigh-definition CP-OLEDs. One of the most promising classes of CPL active molecules, helicenes, however, typically exhibit broad emission with a large Stokes shift. We present, herein, a design strategy capitalizing on intramolecular donor-acceptor interactions between nitrogen and boron atoms to address this issue. 1,4-B,N-embedded configurationally stable single- and double helicenes were synthesized straightforwardly. Both helicenes show unprecedentedly narrow fluorescence and CPL bands (full width at half maximum between 17-28 nm, 0.07-0.13 eV) along with high fluorescence quantum yields (72-85 %). Quantum chemical calculations revealed that the relative localization of the natural transition orbitals, mainly on the rigid core of the molecule, and small values of root-mean-square displacements between S₀ and S₁ state geometries, contribute to the narrower emission.

(BO)2 -Doped Tetrathia[7]helicene: A Configurationally Stable Blue Emitter

Helicenes combine two central themes in chemistry: extended π-conjugation and chirality. Hetero-atom doping preserves both characteristics and allows modulation of the electronic structure of a helicene. Herein, we report the (BO)2 -doped tetrathia[7]helicene 1, which was prepared from 2-methoxy-3,3'-bithiophene in four steps. 1 is formally derived by substituting two (Mes)B-O moieties in place of (H)C=C(H) fragments in two benzene rings of the parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)° between the two terminal thiophene rings. The (P)-/(M)-1 enantiomers were separated by chiral HPLC and are configurationally stable at room temperature. The experimentally determined enantiomerization barrier of 27.4±0.1 kcal mol-1 is lower than that of tetrathia[7]helicene (39.4±0.1 kcal mol-1 ). The circular dichroism spectra of (P)- and (M)-1 show a perfect mirror-image relationship. 1 is a blue emitter (λem =411 nm) with a photoluminescence quantum efficiency of ΦPL =6 % (cf. tetrathia[7]helicene: λem ≈405 nm, ΦPL =5 %).

Synthesis of Highly Luminescent Chiral Nanographene

Chiral nanographenes with both high fluorescence quantum yields (Φ_F ) and large dissymmetry factors (g_lum ) are essential to the development of circularly polarized luminescence (CPL) materials. However, most studies have been focused on the improvement of g_lum , whereas how to design highly emissive chiral nanographenes is still unclear. In this work, we propose a new design strategy to achieve chiral nanographenes with high Φ_F by helical π-extension of strongly luminescent chromophores while maintaining the frontier molecular orbital (FMO) distribution pattern. Chiral nanographene with perylene as the core and two dibenzo[6]helicene fragments as the wings has been synthesized, which exhibits a record high Φ_F of 93 % among the reported chiral nanographenes and excellent CPL brightness (B_CPL ) of 32 M^-1  cm^-1 .

Aryne Atropisomers: Chiral Arynes for the Enantiospecific Synthesis of Atropisomers and Nanographene Atropisomers

The basics about arynes and their applications in synthetic organic chemistry are briefly presented, and the concept of atropisomerism is defined, highlighting that it is a time-dependent form of isomerism and chirality. It is remembered that racemization is a macroscopic and statistical irreversible process, while enantiomerization is a nanoscopic reversible process that occurs at the molecular scale, with racemization being twice as fast as enantiomerization. The concept of aryne atropisomers is introduced with a naive question: Can synthetically useful nonracemic aryne atropisomers having a triple bond ortho to the stereogenic single bond exist in solution? It was found that such aryne atropisomers can be generated in solution from easily available ortho-iodoaryl triflate precursors and excess trimethylsilylmethylmagnesium chloride. Analysis of the barriers to enantiomerization of some aryne atropisomers by computational modeling revealed the key contribution to the configurational stability of the H atom in tris-ortho-substituted biphenyl-based atropisomers. Using a specially designed prototype of aryne atropisomer, for which the barrier to enantiomerization was accurately evaluated by advanced computational modeling, the kinetic parameters of its reaction with furan were experimentally determined. From these measurements, it was concluded that any aryne atropisomer with a barrier to enantiomerization ΔG_enant^⧧ equal to or higher than 50 kJ mol^-1 would lead to fully enantiospecific reactions. The synthetic applications of two structurally distinct aryne atropisomers built on a 1-phenylnaphthalene platform are described: one has the aryne triple bond embedded in the naphthyl moiety, and the other has the aryne triple bond embedded in the phenyl moiety. Both aryne atropisomers allowed for the fully enantiospecific, and possibly overall enantioselective, syntheses of original atropisomers based on standard aryne chemistry. For instance, reactions with anthracene and perylene afforded triptycene and nanographene atropisomers, respectively, in high enantiomeric excesses. A bis(aryne) atropisomer synthetic equivalent prepared from either enantiomer of BINOL is described for 3D bidirectional reactions with a single handedness. Its 2-fold reactions with anthracene and perylene afforded the corresponding severely congested bis(benzotriptycene) (99% ee) nanocarbon atropisomer and bis(anthra[1,2,3,4-ghi]perylene) (98% ee) nanographene atropisomer, respectively. This allowed the discovery of bis(twistacene) atropisomers as a new class of polycyclic aromatic hydrocarbons (PAH) with multiple stereogenicities. Cross reactions with the bis(aryne) atropisomer synthetic equivalent and two different arynophiles proved feasible, providing a nanographene atropisomer with a benzotriptycene unit and an anthra[1,2,3,4-ghi]perylene unit assembled around a stereogenic axis as a unique chiral PAH (99% ee). Overall, because the concept is simple and its implementation is easy, aryne atropisomers is an attractive approach to the synthesis of atropisomers in a broad meaning. Applications to the synthesis of large PAH atropisomers with single handedness are particularly promising.

[5]-Helistatins: Tubulin-Binding Helicenes with Antimitotic Activity

Helicenes are high interest synthetic targets with unique conjugated helical structures that have found important technological applications. Despite this interest, helicenes have had limited impact in chemical biology. Herein, we disclose a first-in-class antimitotic helicene, helistatin 1 (HA-1), where the helicene scaffold acts as a structural mimic of colchicine, a known antimitotic drug. The synthesis proceeds via sequential Pd-catalyzed coupling reactions and a π-Lewis acid cycloisomerization mediated by PtCl2. HA-1 was found to block microtubule polymerization in both cell-free and live cell assays. Not only does this demonstrate the feasibility of using helicenes as bioactive scaffolds against protein targets, but also suggests wider potential for the use of helicenes as isosteres of biaryls or cis-stilbenes-themselves common drug and natural product scaffolds. Overall, this study further supports future opportunities for helicenes for a range of chemical biological applications.

Helically Twisted Nanoribbons Based on Emissive Near-Infrared Responsive Quaterrylene Bisimides

Graphene nanoribbons (GNRs) have the potential for next-generation functional devices. So far, GNRs with defined stereochemistry are rarely reported in literature and their optical response is usually bound to the ultraviolet or visible spectral region, while covering the near-infrared (NIR) regime is still challenging. Herein, we report two novel quaterrylene bisimides with either one- or twofold-twisted π-backbones enabled by the steric congestion of a fourfold bay arylation leading to an end-to-end twist of up to 76°. The strong interlocking effect of the π-stacked aryl substituents introduces a rigidification of the chromophore unambiguously proven by single-crystal X-ray analysis. This leads to unexpectedly strong NIR emissions at 862 and 903 nm with quantum yields of 1.5 and 0.9%, respectively, further ensuring high solubility as well as resolvable and highly stable atropo-enantiomers. Circular dichroism spectroscopy of these enantiopure chiral compounds reveals a strong Cotton effect Δε of up to 67 M^-1 cm^-1 centered far in the NIR region at 849 nm.

C- and S-Shaped Perylene Diimide Heterohelicenes: Modular Synthesis and Spiral-Stair-Like π-Stacking

A number of C- and S-shaped perylene diimide (PDI) heterohelicenes with high dipole moments were synthesized from simple perylene tetrabutylester (PTE). Taking advantage of the weak coordination ability of the sterically crowded peri ester groups in PTE, efficient Rh(III)-catalyzed 2,8- and 2,11-bisiodinations of the perylene core were realized. The 2,8- and 2,11-diiodinated PTEs and PDIs represent key synthons for further ortho-π-extensions. In contrast to most helical π-skeletons that feature loose molecular packings, enantiomerically pure C-shaped PDI azahelicenes adopt unique spiral-stair-like π-stacking superstructures.

Enantioselective Recognition of Helicenes by a Tailored Chiral Benzo[ghi]perylene Trisimide π-Scaffold

Enantioselective molecular recognition of chiral molecules that lack specific interaction sites for hydrogen bonding or Lewis acid-base interactions remains challenging. Here we introduce the concept of tailored chiral π-surfaces toward the maximization of shape complementarity. As we demonstrate for helicenes it is indeed possible by pure van-der-Waals interactions (π-π interactions and CH-π interactions) to accomplish enantioselective binding. This is shown for a novel benzo[ghi]perylene trisimide (BPTI) receptor whose π-scaffold is contorted into a chiral plane by functionalization with 1,1'-bi-2-naphthol (BINOL). Complexation experiments of enantiopure (P)-BPTI with (P)- and (M)-[6]helicene afforded binding constants of 10 700 M-1 and 550 M-1 , respectively, thereby demonstrating the pronounced enantiodifferentiation by the homochiral π-scaffold of the BPTI host. The enantioselective recognition is even observable by the naked eye due to a specific exciplex-type emission originating from the interacting homochiral π-scaffolds of electron-rich [6]helicene and electron-poor BPTI.

Synthesis, Stereochemical and Photophysical Properties of Functionalized Thiahelicenes

We report on the synthesis of a novel class of functionalized thia[6]helicenes and a thia[5]helicene, containing a benzothiophene unit and a second heteroatom embedded in the helix (i.e., nitrogen and oxygen) or a pyrene or a spirobifluorene moiety. These systems are obtained through straightforward and general procedures that involve: (i) palladium-catalyzed annulation of iodo-atropoisomers with internal alkynes and (ii) Suzuki coupling of iodo-atropoisomers with phenyl boronic acid followed by a Mallory-type reaction. Both experimental and theoretical studies on the configurational stability of some selected thia[6]helicenes confirmed their stability toward racemization at room temperature, while the pyrene-based thia[5]helicene was found to be unstable. Moreover, the configuration assignment for one representative thiahelicene was established through the comparison between experimental and theoretical circular dichroism (CD) spectra. A systematic study of the photophysical properties of both thiahelicenes and the corresponding atropoisomers has been carried out to provide a complete overview on the new molecules proposed in this work. The obtained data showed regular trends in all the thiahelicene series with spectroscopic traits in line with those previously observed for similar heterohelicenes.

Enhanced N-directed electrophilic C-H borylation generates BN-[5]- and [6]helicenes with improved photophysical properties

Helicenes are chiral polycyclic aromatic hydrocarbons (PAHs) of significant interest, e.g. in supramolecular chemistry, materials science and asymmetric catalysis. Herein an enhanced N-directed electrophilic C-H borylation methodology has been developed that provides access to azaborine containing helicenes (BN-helicenes). This borylation process proceeds via protonation of an aminoborane with bistriflimidic acid. DFT calculations reveal the borenium cation formed by protonation to be more electrophilic than the product derived from aminoborane activation with BBr3. The synthesised helicenes include BN-analogues of archetypal all carbon [5]- and [6]helicenes. The replacement of a CC with a BN unit (that has a longer bond) on the outer helix increases the strain in the BN congeners and the racemization half-life for a BN-[5]helicene relative to the all carbon [5]helicene. BN incorporation also increases the fluorescence efficiency of the helicenes, a direct effect of BN incorporation altering the distribution of the key frontier orbitals across the helical backbone relative to carbo-helicenes.

Pyrene-Fused [7]Helicenes Connected Via Hexagonal and Heptagonal Rings: Stereospecific Synthesis and Chiroptical Properties

In this manuscript, we portrayed a stereospecific synthesis of C₂- and C₁-symmetric pyrene-fused [7]helicene compounds 1 and 2, respectively. Compounds 1 and 2 were synthesized via a one-pot Suzuki coupling-C-H activation and two-step Suzuki coupling-Scholl reaction, respectively, with complete retention of configuration. The synthesized molecules differ in the fusing mode of [7]helicene units with pyrene via six- and seven-membered rings for 1 and 2, respectively. There was a significant difference in the functional properties and enantiomerization barrier of both compounds because of their distinct molecular symmetry as well as fusing mode to pyrene moiety. The heptagon-containing molecule 2 showed remarkable photophysical and chiroptical properties with commendable configurational stability compared to 1 and pristine [7]helicene as well as its [5]helicene congener.

Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular "Metallic" Möbius Strip

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol-1 (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity.

Chiral Distorted Hexa-peri-hexabenzocoronenes Bearing a Nonagon-Embedded Carbohelicene

A new family of chiral saddle-helix hybrid nanographenes is reported. The first hexa-peri-hexabenzocoronene (HBC) analogues bearing a nine-membered carbocycle are presented. Furthermore, for the first time, π-extended carbo[n]helicenes containing a nine-membered ring as part of the helical moiety have been synthesized. The combination of a [5]helicene moiety and a nonagon ring in a single chiral motif induces a tremendous distortion from planarity into the nanographenic structures compared to other saddle-helix hybrids such as heptagon- and octagon-containing π-extended carbo[5]helicenes. In fact, the interplanar angle of the two terminal rings reaches the largest angle (134.8°) of a carbohelicene reported to date, thus being by far the most twisted helicene yet prepared. Photophysical properties evaluation showed improved absorption dissymmetry factors (|gabs |=4.2×10-3 ) in the new family of nonagon-containing π-extended carbo[5]helicenes.

Effects of Temperature on Enantiomerization Energy and Distribution of Isomers in the Chiral Cu13 Cluster

In this study, we report the lowest energy structure of bare Cu₁₃ nanoclusters as a pair of enantiomers at room temperature. Moreover, we compute the enantiomerization energy for the interconversion from minus to plus structures in the chiral putative global minimum for temperatures ranging from 20 to 1300 K. Additionally, employing nanothermodynamics, we compute the probabilities of occurrence for each particular isomer as a function of temperature. To achieve that, we explore the free energy surface of the Cu₁₃ cluster, employing a genetic algorithm coupled with density functional theory. Moreover, we discuss the energetic ordering of isomers computed with various density functionals. Based on the computed thermal population, our results show that the chiral putative global minimum strongly dominates at room temperature.