Chiroptical Properties Research Articles

Rational Design of Crystalline and Enantiomerically Pure Helicenes with Open-Shell Singlet Ground States.

Helicene diradical derivatives have attracted widespread attentions because of their unique magnetic and chiroptoelectronic properties, however, crystalline and enantiomerically pure forms of helicene diradicals are extremely rare. Herein, we describe the rational design and synthesis of o-quinone functionalized helicene diradicals with crystalline enantiomerical purity. Diradical dianion salt Rac-3K and its enantiomers P/M-3K were obtained by reduction of corresponding precursors Rac-3 and P/M-3 with two equivalent potassium graphite in THF in the presence of (di)benzo-18-crown-6. Neutral dioxoborocyclic helicene diradicals (Rac-3B and P/M-3B) were produced by reactions of Rac-3 or P/M-3 with chlorobis(perfluorophenyl)borane (B(C6F5)2Cl. Crystal structures of compounds Rac-3K, Rac-3B and P/M-3K were obtained by single crystal X-ray diffraction. Their open-shell singlet state ground states were confirmed by electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements and theoretical calculations. Their chiroptical properties were investigated by the electronic circular dichroism (ECD) spectroscopy. This work provides the first examples of enantiopure helicene diradical dianions and boron-containing helicene diradicals.

Can Deep Learning Search for Exceptional Chiroptical Properties? The Halogenated [6]Helicene Case

AbstractThe relationship between chemical structure and chiroptical properties is not always clearly understood. Nowadays, efforts to develop new systems with enhanced optical properties follow the trial‐error method. A large number of data would allow us to obtain more robust conclusions and guide research toward molecules with practical applications. In this sense, in this work we predict the chiroptical properties of millions of halogenated [6]helicenes in terms of the rotatory strength (R). We have used DFT calculations to randomly create derivatives including from 1 to 16 halogen atoms, that were then used as a data set to train different deep neural network models. These models allow us to i) predict the Rmax for any halogenated [6]helicene with a very low computational cost, and ii) to understand the physical reasons that favour some substitutions over others. Finally, we synthesized derivatives with higher predicted Rmax obtaining excellent correlation among the values obtained experimentally and the predicted ones.

Rational Design of Crystalline and Enantiomerically Pure Helicenes with Open‐Shell Singlet Ground States

AbstractHelicene diradical derivatives have attracted widespread attentions because of their unique magnetic and chiroptoelectronic properties, however, crystalline and enantiomerically pure forms of helicene diradicals are extremely rare. Herein, we describe the rational design and synthesis of o‐quinone functionalized helicene diradicals with crystalline enantiomerical purity. Diradical dianion salt Rac‐3K and its enantiomers P/M‐3K were obtained by reduction of corresponding precursors Rac‐3 and P/M‐3 with two equivalent potassium graphite in THF in the presence of (di)benzo‐18‐crown‐6. Neutral dioxoborocyclic helicene diradicals (Rac‐3B and P/M‐3B) were produced by reactions of Rac‐3 or P/M‐3 with chlorobis(perfluorophenyl)borane (B(C6F5)2Cl. Crystal structures of compounds Rac‐3K, Rac‐3B and P/M‐3K were obtained by single crystal X‐ray diffraction. Their open‐shell singlet state ground states were confirmed by electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements and theoretical calculations. Their chiroptical properties were investigated by the electronic circular dichroism (ECD) spectroscopy. This work provides the first examples of enantiopure helicene diradical dianions and boron‐containing helicene diradicals.

Chiral optical properties of metasurfaces comprised of chiral media: effects of geometric and material chirality

Chiral optical properties of metasurfaces comprised of chiral media: effects of geometric and material chirality

Direct Three-Dimensional Observation of the Plasmonic Near-Fields of a Nanoparticle with Circular Dichroism.

Characterizing the spatial distribution of the electromagnetic fields of a plasmonic nanoparticle is crucial for exploiting its strong light-matter interaction for optoelectronic and catalytic applications. However, observing the near-fields in three dimensions with a high spatial resolution is still challenging. To realize efficient three-dimensional (3D) nanoscale mapping of the plasmonic fields of nanoparticles with complex shapes, this work established autoencoder-embedded electron energy loss spectroscopy (EELS) tomography. A 432-symmetric chiral gold nanoparticle, a nanoparticle with a high optical dissymmetry factor, was analyzed to relate its geometrical features to its exotic optical properties. Our deep-learning-based feature extraction method discriminated plasmons with different energies in the EEL spectra of the nanoparticle in which signals from multiple plasmons were intermixed; this component was key for acceptable 3D visualization of each plasmonic field separately using EELS tomography. With this methodology, the electric field of the plasmon that induces far-field circular dichroism was observed in 3D. The field linked to this chiroptical property was strong along the swirling edges of the particle, as predicted by a numerical calculation. This study provides insight into the correlation between structural and optical chiralities through direct 3D observation of the plasmonic fields. Furthermore, the strategy of implementing an autoencoder for EELS tomography can be generalized to achieve competent 3D analysis of other features, including the optical properties of the dielectrics and chemical states.

Electropolymerization of a New Diketopyrrollopyrrole Derivative into Inherent Chiral Polymer Films

Electropolymerization is a convenient way to obtain conducting polymers (CPs) directly adhered to an electrode surface. CPs are well-known for their various application fields in photovoltaic cells, chemical sensors, and electronics. By implementing chirality into a CP, the application possibilities will spread further onto chiral sensors or optoelectronics. In this work, we introduce a new inherently chiral polymer based on a macrocyclic 3,4-ethylenedioxythiophene-diketopyrrolopyrrole-3,4-ethylenedioxythiophene triad (EDOT-DPP-EDOT) fused by 1,4-phenylene groups, which was prepared via oxidative electropolymerization directly on the electrode surface. The investigation of the chiroptical properties was performed by circular dichroism spectroscopy in the solid state. The enantiomeric pure polymer films obtained showed dissymmetry factors of up to −2.71 × 10−4, whereby linear dichroism contributions can be widely excluded.

Suppressing Inversion of a Chiral Polycyclic Aromatic Aza Monkey Saddle by Molecular Editing

The synthesis of chiral polycyclic aromatic hydrocarbons with monkey saddle topology are a fascinating class of negatively curved compounds. To make these valuable building blocks for chirality‐assisted synthesis (CAS) approaches, the inversion barriers between the two possible enantiomers need to be high enough to prevent interconversion under CAS conditions. By molecular editing, aza monkey saddles were converted to the corresponding chromene monkey saddles that are configurationally stable even at temperatures of 220°C over a period of 47 days. Their structures were studied by single X‐ray diffraction and enantiomers were separated to study the chiroptical properties. Furthermore, mechanistic assumptions of the formation of the chromene monkey saddles by DFT calculations are discussed.

Supramolecular Control of Helicene Circularly Polarized Luminescence Emitters in Molecular Solids and Bright Nanoparticles

AbstractCircularly polarized luminescence (CPL) from chiral molecules is attracting much attention due to its potential use in optical materials. However, formulation of CPL emitters as molecular solids typically deteriorates photophysical properties in the aggregated state leading to quenching and unpredictable changes in CPL behavior impeding materials development. To circumvent these shortcomings, a supramolecular approach can be used to isolate cationic dyes in a lattice of cyanostar‐anion complexes that suppress aggregation‐caused quenching and which we hypothesize can preserve the synthetically‐crafted chiroptical properties. Herein, we verify that supramolecular assembly of small‐molecule ionic isolation lattices (SMILES) allows translation of molecular ECD and CPL properties to solids. A series of cationic helicenes that display increasing chiroptical response is investigated. Crystal structures of three different packing motifs all show spatial isolation of dyes by the anion complexes. We observe the photophysical and chiroptical properties of all helicenes are seamlessly translated to water soluble nanoparticles by the SMILES method. Also, a DMQA helicene is used as FRET acceptor in SMILES nanoparticles of intensely absorbing rhodamine antennae to generate an 18‐fold boost in CPL brightness. These features offer promise for reliably accessing bright materials with programmable CPL properties.

Encasing Triaryltriazine with a Bulky Chiral Cap: Luminescent Chiral Crystalline Molecular Rotors with Modulation of Solid-State Chiroptical Properties Mediated by Molecular Rotation.

A novel structural motif for luminescent chiral crystalline molecular rotors with chiroptical properties correlated with the rotational motion in crystalline media is presented. This scaffold incorporates bulky chiral caps consisting of a homochiral binaphthyl moiety with a triisopropylsilyl (TIPS) group into triaryltriazine, as confirmed by single-crystal X-ray diffraction (XRD) analysis. Variable-temperature solid-state 2H NMR studies revealed a 4-fold rotation of the phenylenes occurred in the rotor crystal between 263 and 333 K, while a steric rotor analogue shows no rotational motion. Notably, a reduction in the dihedral angle of the binaphthyl moiety upon heating was observed in the chiral rotors, and a corresponding alteration of the circular dichroism (CD) signal was detected in the solid-state, while those of the steric rotors showed no alteration by the temperature change. We propose that the fast rotation of the phenyl rings affects the motion of neighboring isopropyl groups, leading to steric repulsion with the binaphthyl moieties and thereby inducing its conformational change. Furthermore, the chiral rotors exhibited circularly polarized phosphorescence in the solid-state at low temperature, originating from rotational displacement of the phenylene on triphenyltriazine during structural relaxation in the excited state. Meanwhile, the steric rotors showed significant circularly polarized fluorescence induced by the suppressed molecular motion via a sterically hindered lattice environment in the excited state. These results indicate that the bulky chiral cap introduced into the triaryltriazines, acting as a luminescent chiral crystalline molecular rotor, can be a useful scaffold for the modulation of solid-state chiroptical properties via molecular rotational motions.

Synthesis and Characterization of Asymmetric Azatwistarenes with Chiroptical Property.

Three novel azatwistarenes 5a, 8, and 13 have been synthesized via the Povarov reaction and fully characterized. All of the enantiomers were separated using chiral high-performance liquid chromatography, and their optical properties were investigated through circular dichroism and circularly polarized luminescence spectra. In addition, such desired azatwistarenes have a positive response to acid in dichloromethane.

π‐Extension of a Multiple Resonance Core: Double Helical and Heptagon‐Embedded Nanographenes

AbstractMultiple resonance (MR) boron–nitrogen doped polycyclic aromatic hydrocarbons (BN‐PAHs) have shown compelling thermally activated delayed fluorescence (TADF), surpassing those of their hydrocarbon analogues. However, the structural variety of π‐extended BN‐PAHs remains narrow. In this study, we synthesized three double helical BN‐doped nanographenes (BN‐NGs), 2 a–2 c, and three heptagon‐embedded BN‐NGs, 1 a–1 c, by π‐extension of the MR core. During the formation of 2 a, a nanographene with one heptagon (1 a) was obtained, whereas further dehydrocyclization of the [6]helicene units within 2 b and 2 c led to heptagon structures, yielding other two BN‐NGs containing double heptagons (1 b and 1 c). These BN‐NGs (2 a–2 c and 1 a–1 c) showed pronounced redshifts of 100–190 nm compared to the parent MR core, while preserving the TADF characteristics and prolonging the delayed fluorescence lifetime to the millisecond level. Furthermore, the integration of a heptagon ring into 1 a–1 c expanded the conjugation, reduced the oxidation potentials, and yielded a more flexible framework compared to those of 2 a–2 c. The enantiomers of 2 a–2 c, 1 a, and 1 c were resolved and their chiroptical properties were studied. Notably, 1 a and 1 c exhibited increased chiroptical dissymmetry factors.

π-Extension of a Multiple Resonance Core: Double Helical and Heptagon-Embedded Nanographenes.

Multiple resonance (MR) boron-nitrogen doped polycyclic aromatic hydrocarbons (BN-PAHs) have shown compelling thermally activated delayed fluorescence (TADF), surpassing those of their hydrocarbon analogues. However, the structural variety of π-extended BN-PAHs remains narrow. In this study, we synthesized three double helical BN-doped nanographenes (BN-NGs), 2 a-2 c, and three heptagon-embedded BN-NGs, 1 a-1 c, by π-extension of the MR core. During the formation of 2 a, a nanographene with one heptagon (1 a) was obtained, whereas further dehydrocyclization of the [6]helicene units within 2 b and 2 c led to heptagon structures, yielding other two BN-NGs containing double heptagons (1 b and 1 c). These BN-NGs (2 a-2 c and 1 a-1 c) showed pronounced redshifts of 100-190 nm compared to the parent MR core, while preserving the TADF characteristics and prolonging the delayed fluorescence lifetime to the millisecond level. Furthermore, the integration of a heptagon ring into 1 a-1 c expanded the conjugation, reduced the oxidation potentials, and yielded a more flexible framework compared to those of 2 a-2 c. The enantiomers of 2 a-2 c, 1 a, and 1 c were resolved and their chiroptical properties were studied. Notably, 1 a and 1 c exhibited increased chiroptical dissymmetry factors.

Remote chirality transfer in low-dimensional hybrid metal halide semiconductors.

In hybrid metal halide perovskites, chiroptical properties typically arise from structural symmetry breaking by incorporating a chiral A-site organic cation within the structure, which may limit the compositional space. Here we demonstrate highly efficient remote chirality transfer where chirality is imposed on an otherwise achiral hybrid metal halide semiconductor by a proximal chiral molecule that is not interspersed as part of the structure yet leads to large circular dichroism dissymmetry factors (gCD) of up to 10-2. Density functional theory calculations reveal that the transfer of stereochemical information from the chiral proximal molecule to the inorganic framework is mediated by selective interaction with divalent metal cations. Anchoring of the chiral molecule induces a centro-asymmetric distortion, which is discernible up to four inorganic layers into the metal halide lattice. This concept is broadly applicable to low-dimensional hybrid metal halides with various dimensionalities (1D and 2D) allowing independent control of the composition and degree of chirality.

Inside Front Cover: Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge‐Transfer Exciton Coupling

Inside Front Cover: Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge‐Transfer Exciton Coupling

Inside Front Cover: Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge‐Transfer Exciton Coupling

Inside Front Cover: Living Hybrid Exciton Materials: Enhanced Fluorescence and Chiroptical Properties in Living Supramolecular Polymers with Strong Frenkel/Charge‐Transfer Exciton Coupling

Hollow Superhelices Based on Chiral Europium Coordination Polymers.

The construction of helical nanotubes based on chiral coordination polymers (CPs) is an intriguing but challenging task, which is important for the development of functional materials that combine macroscopic chirality with tube-related properties. Here, we selected a chiral europium phosphonate system, e.g., Eu(NO3)3/R-,S-pempH2, and carried out a systematic work. By controlling the hydrothermal reaction conditions such as the pH value of the reaction mixture, the molar ratio and concentration of the reactants, we obtained block-like crystals of R/S-1b, rod-like crystals ofR/S-3r, hollow superhelices of R/S-2hh, and solid superhelices of R/S-4sh. In the latter two cases, the chirality has been successfully transferred and amplificated from the molecular level to the macroscopic level. Interestingly, compounds R/S-2hh and R/S-4sh have the same chemical composition of Eu(R/S-pempH)3×2H2O and show identical PXRD patterns, thus can be considered as the same material except for different morphologies. We further investigatedtheir circularly polarized luminescence (CPL) properties and found that the hollow superhelix of R/S-2hh had a larger dissymmetry factor than the solid superhelix of R/S-4sh. This study not only provides the first example of hollow superhelices of chiral CPs, but also offers the possibility of modulating the chiroptical properties of CPs through morphological control.

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.

Strong Non-Reciprocal Chiroptical Properties in Thin Films of Chiral Alkylthio-Decorated 1,4-Phenylene/Thiophene Dyes.

In the context of chiral π-conjugated materials, the use of enantiopure alkylthio appendages represents a valid alternative to conventional alkoxy groups: sulphur atom is bigger and more electron-rich than oxygen, thus allowing for higher polarizability, greater flexibility, larger bulkiness and lower structural anisotropy. In light of these considerations, here we report two new chiral alkylthio-decorated 1,4-phenylene/thiophene dyes, obtained by simple synthetic strategies involving Pd-catalyzed cross-coupling protocols, looking for strong non-reciprocal chiroptical features in thin films. In particular, for the chiral alkylthio-decorated 1,4-phenylene-bis(thiophenylpropynone) (Thio-PTPO) dye, which proved to be the most promising for our purpose, a detailed investigation in thin films was carried out, involving optical and chiroptical spectroscopies in absorption and emission, as well as optical microscopy techniques.

5]Helicene Based π-Conjugated Macrocycles with Persistent Figure-Eight and Möbius Shapes: Efficient Synthesis, Chiral Resolution and Bright Circularly Polarized Luminescence.

π-Conjugated chiral shape-persistent molecular nanocarbons hold great potential as chiroptical materials, though their synthesis remains a considerable challenge. Here, we present a simple approach using Suzuki coupling of a [5]helicene building block with various aromatic units, enabling the one-pot synthesis of a series of chiral macrocycles with persistent figure-eight and Möbius shapes. Single-crystal structures of 7 compounds were solved, and 22 enantiomers were separated by preparative chiral HPLC. A notable pyrene-bridged figure-eight macrocycle, with its rigid, fully π-conjugated and overcrowded structure, exhibited pure excimer emission and outstanding circularly polarized luminescence (CPL) properties, including a large dissymmetric factor (|glum| = 3.8 × 10-2) and significant CPL brightness (BCPL = 710.5 M-1cm-1). This method provides a versatile synthetic platform for producing various chiral D2-symmetric figure-eight macrocycles and singly or triply twisted Möbius macrocycles with C2 and D3 symmetry, offering tunable chiroptical properties for CPL applications.

Organic Circularly Polarized Room‐Temperature Phosphorescence: Strategies, Applications and Challenges

Organic circularly polarized luminescence (CPL) plays crucial roles in chemistry and biology for the potential in chiral recognition, asymmetric catalysis, 3D displays, and biological probes. The long‐lived luminescence, large Stokes shift, and unique chiroptical properties make organic circularly polarized room‐temperature phosphorescence (CPP) a new research hotspot in recent years. Nevertheless, achieving high‐performance organic CPP is still challenging due to the sensitivity and complexity of integrating triplet excitons and polarization within organic materials. This review summarizes the latest advances in organic CPP, ranging from design strategies and photophysical properties to underlying luminescence mechanisms and potential applications. Specifically, the design strategies for generating CPP are systemically categorized and discussed according to the interactions between chiral units and chromophores. The applications of organic CPP in organic light‐emitting diodes, sensing, chiral recognition, afterglow displays, and information encryption are also illustrated. In addition, we present the current challenges and perspectives on developing organic CPP. We expect this review to provide some instructive design principles to fabricate high‐performance organic CPP materials, offering an in‐depth understanding of the luminescence mechanism and paving the way toward diverse practical applications.