Boosting Circularly Polarized Luminescence Performance by a Double π-Helix and Heteroannulation

Building metal-induced inherent chirality: chiral stability/phosphorescence enhancement via a ring expansion strategy of Pt(ii) complexes

Chirality is indeed a fundamental aspect of nature, particularly in chemistry and biology. Herein, we have demonstrated a straightforward strategy to create a novel type of metal-induced inherent chirality by expanding coordination rings. Typically, tetracoordinated d8 Pt(ii) complexes with two five-membered coordination rings are 2D square-planar, resulting in the absence of chirality. However, if one of the coordination rings is expanded to six or seven members, the molecular structures of Pt(ii) complexes would transition from a 2D plane to a 3D boat-like configuration. This not only facilitates the construction of inherently chiral scaffolds but also addresses the problem of emission aggregation-caused quenching. Furthermore, the strategy of ring expansion can enhance the chiral stability and phosphorescence quantum yield (up to 81.0%). Thus, the present work introduces a new structural motif with promising potential for application as chiroptical and luminescent materials.

Synthesis and Chiroptical Properties of a Saddle-Shaped Quadruple Helicene with a Cyclooctatetrapyrrole Core

A quadruple helicene with a cyclooctatetrapyrrole core was synthesized via a simple Ullmann reaction. Its single-crystal structure exhibits a saddle shape. The compound shows fluorescence emission with λem = 575 nm and good chiroptical properties, including a dissymmetric absorption factor (|gabs|) of 5.84 × 10-3 and a dissymmetric emission factor (|glum|) of 1.21 × 10-2.

Inversion of Circularly Polarized Luminescence in the Left-Handed Chitosan-Templated Co-assemblies

Circularly polarized luminescence (CPL) materials are attractive due to their unique applications in fields such as 3D displays, information encryption, and chiroptical switches. Natural biomolecules-based CPL materials are gaining plenty of attention due to their chiral diversity and sustainability. However, it is still challenging to construct CPL materials with opposite CPL signs from a single natural biomolecule due to its inherent chirality. Here, chiral assemblies with opposite CPL signs using chitosan oligosaccharide (COS) and achiral luminescent dyes are successfully prepared. It is found that COS can serve as a chiral template to induce the ordered assembly of the dyes along the polymer chain through electrostatic attraction interaction. It is demonstrated experimentally that the structural planarity of the dye molecules is crucial for the formation of chiral co-assemblies. Interestingly, the left-handed COS-templated co-assemblies can emit CPL with opposite handedness, which is controlled by the helicity degree of the co-assemblies. This study not only deepens the understanding of the complex assembly of natural biomacromolecules but also provides new insights into the design and construction of CPL materials.

Advances in Chiral Macrocycles: Molecular Design and Applications

Chiral macrocycles have recently emerged as promising materials for enantioselective recognition, asymmetric catalysis, and circularly polarized luminescence (CPL) due to their terminal-free structure, preorganized chiral cavities, and unique host-guest and self-assembly properties. This review summarizes recent advances in the design and synthesis of chiral macrocycles with central, axial, helical, and planar chirality, each imparting distinct structural and chiroptical characteristics. We highlight key strategies for constructing these macrocycles and their applications in optoelectronic and catalytic systems. Emphasis is placed on the balance between rigidity and flexibility in macrocycle design, essential for effective molecular recognition, adaptable catalysis, and CPL. We conclude with perspectives on future opportunities, anticipating ongoing developments in chiral macrocycle research.

A Helical Tubular Dyad of [9]Cycloparaphenylene: Synthesis, Chiroptical Properties and Post-Functionalization

The bottom-up synthesis of discrete tubular molecules that mimic the structural features of carbon nanotubes has been a long-standing pursuit for synthetic chemists. As the shortest segments of armchair-type carbon nanotubes, cycloparaphenylenes are regarded as ideal macrocyclic building blocks for achieving this goal. Here we report the synthesis of a helical tubular molecule featuring three diyne linkers between two site-specifically functionalized [9]cycloparaphenylenes. Its C3-symmetrical, radially conjugated structure and solid-state packing have been elucidated by spectroscopic and crystallographic characterizations. Notably, the resolved enantiomers display a circularly polarized luminescence brightness value of 1.47×103 M-1 cm-1, which is among the highest values for chiral organic molecules. Furthermore, the diyne-linked molecule could be directly converted into a thiophene-linked helical molecule, demonstrating the post-functionalization approach for the construction of chiral tubular molecules from cycloparaphenylenes.

Stepwise amplification of circularly polarized luminescence in indium-based metal halides by regulating their structural dimension

The pursuit of chiral lead-free metal halides with both high photoluminescence quantum yield (PLQY) and large luminescence dissymmetry factor (glum) remains a priority for designing efficient circularly polarized light sources. However, a tradeoff exists between PLQY and glum in chiral materials due to the mismatched electric (μ) and magnetic transition dipole moment (m). Herein, we address this contradiction and develop the efficient circularly polarized luminescence (CPL) emitters through structural dimension modulation. By tuning the size and polarization of chiral organic cations and employing the cascade cationic insertion strategy, 0D, 1D and 3D indium-based chiral metal halides are constructed. These hybrids exhibit self-trapped excitons emission with near-unity PLQY, while the |glum| boosts exponentially from 10-3 to nearly 10-1 as the structural dimension increases from 0D to 3D, and the highest |glum| of 0.89 × 10-1 has been achieved. Structural analysis and theoretical calculation indicate the increased structural dimension promotes the formation of helical structure and enlarges magnetic transition dipole moment, thus resulting in improved CPL performance. Our research provides valuable insights on the relationship between glum and structural dimension, thus will advance the development of efficient CPL-active materials for practical applications.

Chiral Resolution and Chiroptical Properties of Hindered Tetraphenylethylene Helicates

Hindered tetraphenylethylene (hTPE) helicates are resolved into two left-handed (M) and right-handed (P) isomers by linkage and removal of chiral auxiliary (1R,2S,5R)-menthol, furnishing gram-scale hTPE enantiomers via flash silica column chromatography. hTPE helicate enantiomers bearing electron-accepting cyano and electron-donating triphenylamine groups can emit deep-blue CPL signals with a fluorescence quantum yield surpassing 50%. Full-color and white-light emission were achieved by blending them with dyes in a poly(methyl methacrylate) (PMMA) film.

Chiroptical Amplification of [7]-Helicene Nanographene by Additional Helical Chirality

Nanographenes have captivated scientific interest since the pioneering discovery of graphene. Recently, attention has shifted towards exploring chiral and nonplanar nanographenes, for their distinct optical, chiroptical, and electronic properties. Despite the growing acceptance of helicenes, the research on inducing helical chirality on π-extended derivatives to boost chiroptical properties remains unattended. In our study, we introduce a new π-extended [7]-helicene resulting from the condensation of diamines with 3,6-dibromophenanthrene-9,10-dione, complemented by two hexabenzocoronene arms in the periphery. Notably, the nanographene containing binaphtho-[1,4]diazocine, compared to the corresponding phenazine, exhibits a remarkable average 2.5, 5, and 10-fold enhancements in quantum yield, dissymmetry factor, and brightness, respectively, when measured in five different solvents. These improvements underscore the significance of the induced helical chirality by the antiaromatic binaphtho-[1,4]diazocine in influencing the chiroptical properties of the helical nanographene. Our research represents a significant stride toward unlocking the potential of π-extended helicenes and lays the groundwork for further exploration in designing and synthesizing new chiral nanomaterials.

Regioselective late-stage functionalization of tetraphenylenes: rapid construction of double helical architectures and potential hole transport materials

Herein we report a novel approach for diversification of tetraphenylene via regioselective late-stage iodination followed by atom(s) insertion into the resulting cyclic iodonium salts. Thus, the steric hindrance effect of tert-butyl facilitates the regioselective synthesis of two cyclic iodonium salts of 2,7,10,15-tetra-tert-butyltetraphenylene. In addition, two more cyclic iodonium salts of 2,7,10,15-tetranitrotetraphenylene were also readily synthesized due to the meta-position induced effect of the electron-withdrawing NO2 group. Subsequent functionalization of these tetraphenylene-based cyclic iodonium salts via diverse atom(s) insertion processes led to several tetraphenylene-based [8 + n] and [n + 8 + n] fused rings including fascinating double helical architectures. This newly developed late-stage functionalization methodology was also successfully applied to rapid synthesis of potential hole transport materials, thereby demonstrating its robust synthetic value in both tetraphenylene chemistry and materials science.

Adding multiple electrons to helicenes: how they respond?

Helicenes of increasing dimensions and complexity have recently burst into the scene due to their unique structures coupled with interesting chiral, optical, and conducting properties. The helicene-related research has quickly progressed from fundamental curiosity to a diverse range of applications in organic catalysis, optoelectronic devices, chiroptical switches, sensors, and energy storage. The in-depth understanding of electron accepting properties of helicenes should further advance their materials chemistry applications, however, previous reports only relied on spectrocopic and electrochemical studies, while their structural changes weren't extensively discussed. Therefore, we initiated a broad investigation of chemical reduction behaviour of helicenes ranging in size and properties coupled with X-ray diffraction characterization of the reduced products. The responses of helicenes with different structures to the stepwise electron addition were investigated using a combination of X-ray crystallography, spectroscopic methods, and calculations. This study revealed topology- and charge-dependent consequences of chemical reduction ranging from reversible geometry perturbation to irreversible core transformation and site-specific reactivity of helicenes in addition to original alkali metal coordination patterns. This overview is focused on the crystallographically confirmed examples stemming from chemical reduction reactions of different helicenes with alkali metals. The opened discussion should stimulate further exploration of reactivity and complexation of novel π-expanded and heteroatom-doped helicenes based on the revealed structure-property correlations, thus advancing their applications as intriguing new materials.

Rational Control of Maximum EMI/CPL Intensity and Wavelength of Bora[6]helicene via Polarity and Vibronic Effects

Solvent polarity control as an efficient methodology to regulate the chiroptical properties, including spectral shape, width, intensity, wavelength, etc., has emerged as a novel frontier in optical materials design. However, the underling relationship connecting polarity to the optical property remains unclear. Herein, using state-of-the-art computations and the FC|VG model, the solvent effect on the chiroptical properties of bora[6]helicene was accurately and systematically computed to shed light on this issue. It is found that the vibronic coupling is crucial in explaining the spectral shape, width, and relative intensity of different peaks. Moreover, the intensity and position of the emission (EMI) and circularly polarized luminescence (CPL) are closely related to the polarity of the solvent. Intriguingly, we got a series of good linear relationships between polarity and EMI|CPL (|r| ≥ 0.95). Thus, this parameter can be used as a potential descriptor to estimate the intensity and position of EMI|CPL, leading to new strategies for designing fully colored fluorescent materials.

Room-Temperature Liquid Crystalline Tetraphenylethylene-Surfactant Complex with Chiral Supramolecular Structure and Tunable Circularly Polarized Luminescence

A novel room-temperature liquid crystal of tetraphenylethylene derivative (TPE-DHAB) was synthesized using an ionic self-assembly strategy. The TPE-DHAB complex exhibits typical aggregation-induced emission properties and a unique helical supramolecular structure. Moreover, the generation and handedness inversion of circularly polarized luminescence (CPL) can be achieved through further chiral solvation, providing a facile approach to fabricate room-temperature liquid crystalline materials with controllable supramolecular structures and tunable CPL properties through a synergistic strategy of ionic self-assembly and chiral solvation process.

Tether-entangled conjugated helices

A new design concept, tether-entangled conjugated helices (TECHs), is introduced for helical polyaromatic molecules. TECHs consist of a linear polyaromatic ladder backbone and periodically entangling tethers with the same planar chirality. By limiting the length of tether, all tethers synchronously bend and twist the backbone with the same manner, and change it into a helical ribbon with a determinate helical chirality. The 3D helical features are customizable via modular synthesis by using two types of synthons, the planar chiral tethering unit (C 2 symmetry) and the docking unit (C 2h symmetry), and no post chiral resolution is needed. Moreover, TECHs possess persistent chiral properties due to the covalent locking of helical configuration by tethers. Concave-type and convex-type oligomeric TECHs are prepared as a proof-of-concept. Unconventional double-helix π-dimers are observed in the single crystals of concave-type TECHs. Theoretical studies indicate the smaller binding energies in double-helix π-dimers than conventional planar π-dimers. A concentration-depend emission is found for concave-type TECHs, probably due to the formation of double-helix π-dimers in the excited state. All TECHs show strong circularly polarized luminescence (CPL) with dissymmetric factors (|g lum|) generally over 10-3. Among them, the (P)-T4-tBu shows the highest |g lum| of 1.0 × 10-2 and a high CPL brightness of 316 M-1 cm-1.

Adaptive Helical Chirality in Supramolecular Microcrystals for Circularly Polarized Lasing

Chiral organic molecules offer a promising platform for exploring circularly polarized lasing, which, however, faces a great challenge that the spatial separation of molecular chiral and luminescent centers limits chiroptical activity. Here we develop a helically chiral supramolecular system with completely overlapped chiral and luminescent units for realizing high-performance circularly polarized lasing. Adaptive helical chirality is obtained by incorporating chiral agents into organic microcrystals. Benefiting from the efficient coupling of stimulated emission with the adaptive helical chirality, the supramolecular microcrystals enable high-performance circularly polarized lasing emission with dissymmetry factors up to ~0.7. This work opens up the way to rational design of chiral organic materials for circularly polarized lasing.

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.

Supramolecular Polymerization as a Tool to Reveal the Magnetic Transition Dipole Moment of Heptazines

Heptazine derivatives have attracted significant interest due to their small S1-T1 gap, which contributes to their unique electronic and optical properties. However, the nature of the lowest excited state remains ambiguous. In the present study, we characterize the lowest optical transition of heptazine by its magnetic transition dipole moment. To measure the magnetic transition dipole moment, the flat heptazine must be chiroptically active, which is difficult to achieve for single heptazine molecules. Therefore, we used supramolecular polymerization as an approach to make homochiral stacks of heptazine derivatives. Upon formation of the supramolecular polymers, the preferred helical stacking of heptazine introduces circular polarization of absorption and fluorescence. The magnetic transition dipole moments for the S1 ← S0 and S1 → S0 are determined to be 0.35 and 0.36 Bohr magneton, respectively. These high values of magnetic transition dipole moments support the intramolecular charge transfer nature of the lowest excited state from nitrogen to carbon in heptazine and further confirm the degeneracy of S1 and T1.

Structure and Chiroptical Properties of Anthra[1,2-a]anthracene-1-yl Dimers as New Biaryls

Dimers of anthra[1,2-a]anthracene-1-yl units and its mesityl derivative were synthesized by Ni(0)-mediated coupling of the corresponding chloro derivatives as new biaryls. The X-ray analysis and DFT calculations revealed that two polycyclic aromatic units with nonplanar deformations took a twisted conformation about the single bond as a chiral axis. Enantiomers of the nonsubstituted compound were resolved by chiral HPLC, and the enantiopure samples showed intense Cotton effects at 321 nm in the circular dichroism (CD) spectra and emission bands at 449 nm in the circularly polarized luminescence (CPL) spectra with dissymmetry factor of |glum| 3.6×10-3. The absolute stereochemistry of this biaryl was determined by the theoretical calculation of CD spectrum by the time-dependent DFT method. The barrier to enantiomerization was determined to be 108 kJ mol-1 at 298 K. The dynamic process proceeded via a stepwise mechanism involving the helical inversion of each aromatic unit and the rotation about the biaryl axis as analyzed by the DFT calculations.

Longitudinal Extension of Double π-Helix Enables Near-Infrared Amplified Dissymmetry and Chiroptical Response

Near-infrared (NIR) circularly polarized light absorbing or emitting holds great promise for highly sensitive and precise bioimaging, biosensing, and photodetectors. Aiming at designing NIR chiral molecular systems with amplified dissymmetry and robust chiroptical response, herein, we present a series of double π-helical dimers with longitudinally extended π-entwined substructures via Ullmann or Yamamoto homocoupling reactions. Circular dichroism (CD) spectra revealed an approximate linear bathochromic shift with the rising number of naphthalene subunits, indicating a red to NIR chiroptical response. Particularly, the terrylene diimide-entwined dimers exhibited the strongest CD intensities, with the maximal |Δε| reaching up to 393 M-1 cm-1 at 666 nm for th-TDI[2]; and a record-high chiroptical response (|ΔΔε|) between the neutral and dianionic species of 520 M-1 cm-1 at 833 nm for th-TDI[2]Cl was achieved upon further reduction to its dianionic state. Time-dependent density functional theory (TDDFT) calculations suggested that the pronounced intensification of the CD spectra originated from a simultaneous enhancement of both electric (μ) and magnetic (m) transition dipole moments, ultimately leading to an overall increase in the rotatory strength (R). Notably, the circularly polarized luminescence (CPL) brightness (BCPL) reached 77 M-1 cm-1 for th-TDI[2]Cl, among the highest values reported for NIR-CPL emitters. Furthermore, all chiral dianions exhibited excellent air stability under ambient conditions with half-life times of up to 10 days in N-methylpyrrolidone (NMP), which is significant for future biological applications and chiroptic switches.

Doubly Spiro-Conjugated Chiral Carbocycles Exhibiting SOMO-HOMO Inversion in Persistent Radical Cations

Persistent chiral organic open-shell systems have captured growing interest due to their potential applications in organic spintronic and optoelectronic devices. Nevertheless, the integration of configurationally stable chirality into an organic open-shell system continues to pose challenges in molecular design. The π-extended skeleton incorporated in spiro-conjugated carbocycles can provide robust chiroptical properties and a significant stabilization of the excited and ionic radical states. However, this approach has been relatively less explored in the design of persistent organic open-shell systems. We report here the (S,S)-, (R,R)-, and meso-isomers of doubly spiro-conjugated carbocycles featuring flat and rigid carbon-bridged para-phenylenevinylene (CPV) of different conjugation lengths connected by two spiro-carbon centers, which we denote D-spiro-CPV for its quasi-dimeric structure. Our synthetic method based on a double lithiation cyclization approach enables facile production of D-spiro-CPV. D-spiro-CPVs exhibit circularly polarized luminescence (CPL) with high fluorescence quantum yields (ΦFL) resulting in a high CPL brightness of 21 M-1 cm-1 and also exhibit high thermal and photostability. The monoradical cation of D-spiro-CPV absorbing near-infrared light is notably persistent, exhibiting a half-life of 570 h under ambient conditions due to doubly spiro-conjugative stabilization. Theoretical and electrochemical studies indicate the radical cation of D-spiro-CPVs presents a non-Aufbau electron filling, exhibiting inversion of the energy level of the singly occupied molecular orbital (SOMO) and the highest (doubly) occupied molecular orbitals with the SOMO level even below the HOMO-1 level (double SHI effect). Our discoveries provide valuable insights into non-Aufbau molecules and the development of configurationally stable, optically active persistent radicals.

Enhancing Chiroptical Responses in Helical Nanographenes via Geometric Engineering of Double [7]Helicenes

Helical nanographenes with high quantum yields and strong chiroptical responses are pivotal for developing circularly polarized luminescence (CPL) materials. Here, we present the successful synthesis of novel π-extended double [7]helicenes (ED7Hs) where two helicene units are fused at the meta- or para-position of the middle benzene ring, respectively, as the structural isomers of the reported ortho-fused ED7H. The structural geometry of these ED7Hs is clearly characterized by single-crystal X-ray analysis. Notably, this class of ED7Hs exhibits bright luminescence with high quantum yields exceeding 40 %. Through geometric regulation of two embedded [7]helicene units from ortho-, meta- to para-position, these ED7Hs display exceptional amplification in chiroptical responses. This enhancement is evident in a remarkable approximate fivefold increase in the absorbance and luminescence dissymmetry factors (gabs and glum), respectively, along with a boosted CPL brightness up to 176 M-1 cm-1, surpassing the performance of most helicene-based chiral NGs. Furthermore, DFT calculations elucidate that the geometric adjustment of two [7]helicene units allows the precise alignment of electric and magnetic transition dipole moments, leading to the observed enhancement of their chiroptical responses. This study offers an effective strategy for magnifying the CPL performance in chiral NGs, promoting their expanded application as CPL emitters.

Recent Advances of Chiral Isolated and Small Organic Molecules: Structure and Properties for Circularly Polarized Luminescence

This paper explores recent advancements in the field of circularly polarized luminescence (CPL) exhibited by small and isolated organic molecules. The development and application of small CPL molecule are systematically reviewed through eight different chiral skeleton sections. Investigating the intricate interplay between molecular structure and CPL properties, the paper aims at providing and enlighting novel strategies for CPL-based applications.

Functionalized Phenanthrene Imide-Based Polymers for n-Type Organic Thin-Film Transistors

High-performing n-type polymers are crucial for the advance of organic electronics field, however strong electron-deficient building blocks with optimized physicochemical properties for constructing them are still limited. The imide-functionalized polycyclic aromatic hydrocarbons (PAHs) with extended π-conjugated framework, high electron deficiency and good solubility serve as promising candidates for developing high-performance n-type polymers. Among the PAHs, phenanthrene (PhA) features a well-delocalized aromatic π-system with multiple modifiable active sites . However, the PhA-based imides are seldom studied, mainly attributed to the synthetic challenge. Herein, we report two functionalized PhAs, CPOI and CPCNI, by simultaneously incorporating imide with carbonyl or dicyanomethylene onto PhA. Notably, the dicyanomethylene-modified CPCNI exhibits a well stabilized LUMO energy level (-3.84 eV), attributed to the synergetic inductive effect from imide and cyano groups. Subsequently, based on CPOI and CPCNI, two polymers PCPOI-Tz and PCPCNI-Tz were developed. Applied to organic thin-film transistors, owing to the strong electron-deficiency of CPCNI, polymer PCPCNI-Tz shows an improved electron mobility and largely decreased threshold voltage compared with PCPOI-Tz. This work affords two structurally novel electron-deficient building blocks and highlights the effectiveness of dual functionalization of PhAs with strong electron-withdrawing groups for devising n-type polymers.

Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State

We report a neutral high-spin diradical of chiral C2-symmetric bis[5]diazahelicene with ΔEST ≈ 0.4 kcal mol-1, as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) (MM)- and (PP)-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C2-symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is ΔG‡ = 43 ± 0.01 kcal mol-1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, ΔG‡ ≥ 26 kcal mol-1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of ΔG‡ = 31 kcal mol-1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor |g| = |Δε|/ε = 0.036 at 376 nm, |g| ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol-1.

Synthesis of Chiral Pyrene-Based 1,4-Dithiins

The 1,4-dithiin motif is known for its reversible redox properties to generate radical cations and diradical dications and thus is interesting for organic electronic applications. However, examples where this motif is embedded into chiral larger fused aromatic compounds are very rare. Here we describe the syntheses of several structurally related pyrene fused dithiins and their spectroscopic investigations with a focus on tuning circular dichroism, with respect to the g values, depending on their connectivity.

Circularly polarized luminescence and high photoluminescence quantum yields from rigid 5,10-dihydroindeno[2,1-a]indene and 2,2'-dialkoxy-1,1'-binaphthyl conjugates and copolymers

5,5,10,10-Tetramethyl-5,10-dihydroindeno[2,1-a]indene (COPV1(Me)) was installed into either the 3,3'- or 6,6'-positions of chiral 2,2'-dioctyloxy-1,1'-binaphthyl to afford 2 : 1 conjugates (monomeric compounds) and 1 : 1 copolymers. These compounds showed high photoluminescence quantum yields of >0.95 whilst also exhibiting circular dichroism (CD) and circularly polarized luminescence (CPL). The dissymmetry factors of CPL (gCPL) for the 3,3'- and 6,6'-monomeric compounds in THF were 6.6 × 10-4 and 3.3 × 10-4, respectively. The 3,3'-isomer has a higher g value than the 6,6'-isomer, which was attributed to the difference in the extent of π-conjugation and the angle between electronic and magnetic transition moments. The gCPL values of the 3,3'-linked and 6,6'-linked copolymers were 1.1 × 10-3 and 6.8 × 10-4, respectively. The structural rigidity of the COPV units is beneficial to achieve relatively high g values whilst maintaining a photoluminescence quantum yield that is close to unity by using a single type of fluorophore.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

Highly Luminescent Chiral Double π-Helical Nanoribbons

Unveiling the mechanism behind chirality propagation and dissymmetry amplification at the molecular level is of significance for the development of chiral systems with comprehensively outstanding chiroptical performances. Herein, we have presented a straightforward Cu-mediated Ullmann homocoupling approach to synthesize perylene diimide-entwined double π-helical nanoribbons encompassing dimer, trimer, and tetramer while producing homochiral or heterochiral linking of chiral centers. A significant dissymmetry amplification was achieved, with absorption dissymmetry factors (|gabs|) increasing from 0.009 to 0.017 and further to 0.019, and luminescence dissymmetry factors (|glum|) rising from 0.007 to 0.013 and eventually to 0.015 for homochiral double π-helical oligomers. The disparity of magnetic transition dipole moment (m) densities in homochiral and heterochiral tetramers by time-dependent density functional theory calculations confirmed that homochiral oligomerization can maximize the total m, which is favorable for achieving ever-increasing g factors. Notably, these double π-helices exhibited exceptional photoluminescence quantum yields (ΦPL) ranging from 83 to 95%. The circularly polarized luminescence brightness (BCPL) eventually reached a remarkable 575 M-1 cm-1 for the homochiral tetramer, which is among the highest values reported for chiral small molecules. This kind of linearly extended double π-helices offers a platform for a comprehensive understanding of the mechanism behind chirality propagation and dissymmetry amplification.

B-N Covalent Bond Embedded Double Hetero-[n]helicenes for Pure Red Narrowband Circularly Polarized Electroluminescence with High Efficiency and Stability

Chiral B/N embedded multi-resonance (MR) emitters open a new paradigm of circularly polarized (CP) organic light-emitting diodes (OLEDs) owing to their unique narrowband spectra. However, pure-red CP-MR emitters and devices remain exclusive in literature. Herein, by introducing a B-N covalent bond to lower the electron-withdrawing ability of the para-positioned B-π-B motif, the first pair of pure-red double hetero-[n]helicenes (n = 6 and 7) CP-MR emitter peaking 617 nm with a small full-width at half-maximum of 38 nm and a high photoluminescence quantum yield of ≈100% in toluene is developed. The intense mirror-image CP light produced by the enantiomers is characterized by high photoluminescence dissymmetry factors (gPL ) of +1.40/-1.41 × 10-3 from their stable helicenes configuration. The corresponding devices using these enantiomers afford impressive CP electroluminescence dissymmetry factors (gEL ) of +1.91/-1.77 × 10-3 , maximum external quantum efficiencies of 36.6%/34.4% and Commission Internationale de I'Éclairage coordinates of (0.67, 0.33), exactly satisfying the red-color requirement specified by National Television Standards Committee (NTSC) standard. Notably a remarkable long LT95 (operational time to 95% of the initial luminance) of ≈400 h at an initial brightness of 10,000 cd m-2 is also observed for the same device, representing the most stable CP-OLED up to date.

Transforming Hetera-Buckybowls into Chiral Conjugated Polycycles Incorporating Epoxycyclooctadiene: a Two-Step Approach

Chiral π-conjugated polycycles have garnered increasing attention due to versatile applications in optoelectronic materials and biological sciences. In this study, we report the synthesis of chiral π-conjugated polycycles incorporating a chiral epoxycyclooctadiene moiety. Our synthetic strategy capitalizes on the novel reactions of hetera-buckybowl triselenasumanene (TSS) and is achieved in two-step manner. Firstly, the TSS is regio-selectively transformed into its ortho-quinone form. Subsequently, the nucleophilic addition reactions of TSS ortho-quinone by phenylethynides are metal ion-dependent. When utilizing (phenylethynyl)magnesium bromide as the nucleophile, two phenylethynyls are furnished onto the edged benzene ring of TSS. When the nucleophile is (phenylethynyl)lithium, a cascade of nucleophilic addition, intermolecular electron-transfer, ring-opening, and tetradehydro-Diels-Alder (TDDA) reactions occur sequentially in one-pot, ultimately affording chiral π-conjugated polycycles featuring the epoxycyclooctadiene moiety as an integral part of their backbones. This work represents a step forward in the synthesis of chiral π-conjugated polycycles using TSS as synthon.

Molecular Engineering of Rylene Diimides via Sila-Annulation Toward High-Mobility Organic Semiconductors

The continuous innovation of captivating new organic semiconducting materials remains pivotal in the development of high-performance organic electronic devices. Herein, a molecular engineering by combining sila-annulation with the vertical extension of rylene diimides (RDIs) toward high-mobility organic semiconductors is presented. The unilateral and bilateral sila-annulated quaterrylene diimides (Si-QDI and 2Si-QDI) are designed and synthesized. In particular, the symmetrical bilateral 2Si-QDI exhibits a compact, 1D slipped π-π stacking arrangement through the synergistic combination of a sizable π-conjugated core and intercalating alkyl chains. Combining the appreciable elevated HOMO levels and reduced energy gaps, the single-crystalline organic field-effect transistors (SC-OFETs) based on 2Si-QDI demonstrate exceptional ambipolar transport characteristics with an impressive hole mobility of 3.0 cm2 V-1 s-1 and an electron mobility of 0.03 cm2 V-1 s-1 , representing the best ampibolar SC-OFETs based on RDIs. Detailed theoretical calculations rationalize that the larger transfer integral along the π-π stacking direction is responsible for the achievement of the superior charge transport. This study showcases the remarkable potential of sila-annulation in optimizing carrier transport performances of polycyclic aromatic hydrocarbons (PAHs).

Molecular Carbons: How Far Can We Go?

The creation and development of carbon nanomaterials promoted material science significantly. Bottom-up synthesis has emerged as an efficient strategy to synthesize atomically precise carbon nanomaterials, namely, molecular carbons, with various sizes and topologies. Different from the properties of the feasibly obtained mixture of carbon nanomaterials, numerous properties of single-component molecular carbons have been discovered owing to their well-defined structures as well as potential applications in various fields. This Perspective introduces recent advances in molecular carbons derived from fullerene, graphene, carbon nanotube, carbyne, graphyne, and Schwarzite carbon acquired with different synthesis strategies. By selecting a variety of representative examples, we elaborate on the relationship between molecular carbons and carbon nanomaterials. We hope these multiple points of view presented may facilitate further advancement in this field.

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Study on Asymmetric Vibrational Coherent Magnetic Transitions and Origin of Fluorescence in Symmetric Structures

In this work, the physical mechanisms of three highly efficient circularly polarized luminescent materials are introduced. The UV-vis spectra are plotted; the transition properties of their electrons at the excited states are investigated using a combination of the transition density matrix (TDM) and the charge difference density (CDD); combining the distribution of electron clouds, the essence of charge transfer excitation in three structures is explained. The resonance Raman spectrum of the three structures at the S1 and S2 excited states are calculated. The M, M-4 and M, M-5 structures are found to produce novel chirality by electronic circular dichroism (ECD) spectrum, and the reasons for the chirality of the M, M-4 and M, M-5 structures are discussed by analyzing the density of transition electric/magnetic dipole moments (TEDM/TMDMs) in different orientations. Finally, the Raman optical activity (ROA) of M, M-4, and M, M-5 are calculated, and the spectra are plotted. This study will provide guidance for the application of carbon-based nanomaterials in organic electronic devices, solar cells, and optoelectronics.

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.

Bis(naphthobipyrrolyl)methene-derived hexapyrrolic BODIPY as a single-molecule helicate with near-infrared emission

Helically twisted bis(naphthobipyrrolyl)methene-derived open-chain hexapyrroles have been synthesized as HCl salts and the corresponding BODIPY. Their solid-state structures elucidated by single-crystal X-ray diffraction analysis clearly showed the presence of intramolecular hydrogen bonds, which were concluded to play a pivotal role in stabilizing the twisted conformation. Both molecules were observed to be NIR active, with the BODIPY moiety emission extending beyond 800 nm.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Bowl-Shaped Bispyrrole-Fused Perylene-diimide and Its Anions

Incorporating two pyrrole subunits at the bay positions of perylene-diimide has been a long-pursued goal since 2009, but it has not been achieved due to high strain. Herein, via one step Buchwald-Hartwig reaction, PDI-2N was successfully generated with a bowl depth of 1.52 Å. Though with electron-rich pyrrole embedding, PDI-2N's radical anion and dianion were facilely prepared and were investigated both experimentally and theoretically. Moreover, PDI-2N crystallized in different manners under distinct conditions, and it formed tubular crystals with infinite two-directional columnar stacking under DMF conditions. This finding develops a dream bowl-shaped PDI derivative that holds great promise in organoelectronics.

Double [4]Helicene-like Naphthobisbenzothiophene Diimides and Their Thienyl-S,S-dioxidized Derivatives with Attractive Solid-State Fluorescence and High Electron Affinity

A series of double [4]helicene-like naphthobisbenzothiophene diimides and their thienyl-S,S-dioxidized derivatives are synthesized via MoCl₅-catalyzed cyclization and m-CPBA-mediated oxidation reactions. The functional five-membered ring diimides show a helicene-like geometry, strong solid-state fluorescence, and deep LUMO of -4.37 eV.

Helical β-isoindigo-Based Chromophores with B-O-B Bridge: Facile Synthesis and Tunable Near-Infrared Circularly Polarized Luminescence

It is essential to create organic compounds that exhibit circularly polarized luminescence (CPL) in the near-infrared (NIR) range. Helicene-type emitters possess appealing chiroptical features, however, such NIR molecules are scarce due to a paucity of synthetic strategies. Herein, we developed a series of helical β-isoindigo-based B-O-B bridged aza-BODIPY analogs that were synthesized conveniently. The reaction of diimino-β-isoindigo with a heteroaromatic amine produced a restricted ligand cavity, which triggered off the generation of a B-O-B bridge. The B-O-B bridge led to distorted conformations that satisfy the helical requirements, resulting in excellent spectroscopic and chiroptical properties. Tunable CPL with the highest luminescence dissymmetry factor (g_lum ) of 1.3×10^-3 and a CPL brightness (B_CPL =11.5 M^-1  cm^-1 ) in the NIR region was achieved. This synthetic approach is expected to offer a new opportunity to chiral chemistry and increase flexibility for chiroptical tuning.

Helical Synthetic Nanographenes with Atomic Precision

ConspectusUnderstanding and harnessing the properties of nanoscale molecular entities are considered as new frontiers in basic chemistry. In this regard, synthetic nanographene with atomic precision has attracted much attention recently. For instance, taking advantage of the marvelous bonding capability of carbon, flat, curved, ribbon-type, or cone-shaped nanographenes have been prepared in highly controllable and elegant manner, allowing one to explore fascinating molecular architectures with intriguing optical, electrochemical, and magnetic characteristics. This stands in stark contrast to other carbon-rich nanomaterials, such as graphite oxides or carbon quantum dots, which preclude thorough investigations because of complicate structural defects. Undoubtedly, synthetic nanographene contributes strongly to modern aromatic chemistry and represents a vibrant field that may deliver transforming functional materials crucial for optoelectronics, nanotechnologies, and biomedicine.Nonetheless, in many cases, synthesis and characterization of nanographene compounds are highly demanding. Low solubility, high molecular strain, undesired selectivity, as well as incomplete or excessive C-C bond formation are common impediments, that require formidable efforts to control the molecular geometry, to modulate the edge structure, to achieve accurate doping, or to push the upper size boundary. These endeavors are indispensable for establishing structure-property relationships, and lay down foundation for exploring synthetic nanographenes at a high level of sophistications.In this Account, we summarize our contributions to this field by presenting a series of helical synthetic nanographenes, such as hexapole [7]helicene (H7H), nitrogen-doped H7H, hexapole [9]helicene (H9H), superhelicene, and supertwistacene. This kind of giant synthetic nanographene reaches the size domain of carbon quantum dots, albeit has precise atomic structure. It provides a unique platform to study aromatic chemistry and chirality at the nanoscale. We discuss synthetic methods and point out, in particular, the strengths and pitfalls of Scholl oxidation, which are expected to be valuable for making synthetic nanographenes in general. In addition, we illustrate their exciting electrochemical and photophysical performance, which include, but are not limited to, reversible multielectron redox chemistry, record high panchromatic absorption, impressive photothermal behavior, and extremely strong Cotton effect. These unusual characteristics are convincingly traced back to their three-dimensional conjugated architectures, highlighting the critical roles of π-electron delocalization, heteroatom-doping, substitution, and molecular symmetry in determining nanographenes' properties and functions. Lastly, we put forward our understanding on the challenges and opportunities that lies ahead and hope this Account will inspire ever more ambitious achievements from this attractive area of research.

Dimerized Nitrogen-Annulated Perylene Synthesized from 1,6-Diazecine as Chiral Emitter

In this work, nitrogen-annulated perylene (NP) was dimerized into one framework connected by two nitrogen atoms, generating the target molecule of DNP-DA. Owing to the substructure of 1,6-diazecine ten-membered ring, DNP-DA illustrates helical chirality with moderate dissymmetry factor, elevated molecular levels, expanded conjugation and supramolecular interactions with acceptors etc. Notably, DNP-DA represents a limited example of nitrogen-perylene based CPL emitter with g_lum around 6×10^-3 . Intrigued by the facile fabrication via a simple amination-cross coupling sequence and other above advancing features, this work demonstrates the potential generality of utilizing 1,6-diazecine as a chiral unit to build CPL-active materials.

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 .

Remote Control of Dynamic Twistacene Chirality

We report a reliable way to manipulate the dynamic, axial chirality in perylene diimide (PDI)-based twistacenes. Specifically, we reveal how chiral substituents on the imide position induce the helicity in a series of PDI-based twistacenes. We demonstrate that this remote chirality is able to control the helicity of flexible [4]helicene subunits by UV-vis, CD spectroscopy, X-ray crystallography, and TDDFT calculations. Furthermore, we have discovered that both the chiral substituent and the solvent each has a strong impact on the sign and intensity of the CD signals, highlighting the control of the dynamic helicity in this flexible system. DFT calculations suggest that the steric interaction of the chiral substituents is the important factor in how well a particular group is at inducing a preferred helicity.

Molecular Carbon Imides

The creation and development of new forms of nanocarbons have fundamentally transformed the scientific landscape in the past three decades. As new members of the nanocarbon family with accurate size, shape, and edge structure, molecular carbon imides (MCIs) have shown unexpected and unique properties. Particularly, the imide functionalization strategy has endowed these rylene-based molecular carbons with fascinating characteristics involving flexible syntheses, tailor-made structures, diverse properties, excellent processability, and good stability. This Perspective elaborates molecular design evolution to functional landscapes, and illustrative examples are given, including a promising library of multi-size and multi-dimensional MCIs with rigidly conjugated π-architectures, ranging from 1D nanoribbon imides and 2D nanographene imides to cross-dimensional MCIs. Although researchers have achieved substantial progress in using MCIs as functional components for exploration of charge transport, photoelectric conversion, and chiral luminescence performances, they are far from unleashing their full potential. Developing highly efficient and regioselective coupling/ring-closure reactions involving the formation of multiple C-C bonds and the annulation of electron-deficient aromatic units is crucial. Prediction by theory with the help of machine learning and artificial intelligence research along with reliable nanotechnology characterization will give an impetus to the blossom of related fields. Future investigations will also have to advance toward─or even focus on─the emerging potential functions, especially in the fields of chiral electronics and spin electronics, which are expected to open new avenues.