Circularly polarized luminescence of helicenes: A data-informed insight

Double [8]Helicene Featuring a Dibenzo[g,p]chrysene Core: Synthesis and Chiroptical Response

Double [8]helicene 1, featuring a dibenzo[g,p]chrysene core, was synthesized via the Scholl reaction, and its structure was unambiguously confirmed by single-crystal X-ray diffraction analysis of its dicationic salt [1-Cl]2+·(SbCl6-)2. The compound exhibits red fluorescence with an emission maximum at 618 nm (λem) and a quantum yield of 16.2%, highlighting its potential in optoelectronic applications. Furthermore, circular dichroism (CD) and circularly polarized luminescence (CPL) measurements reveal notable chiroptical activity, with absorption and emission dissymmetry factors of |gabs| = 5.11 × 10-3 and |glum| = 7.1 × 10-4, respectively.

π-Extended Heli(aminoborane)s with Highly Bright Circularly Polarized Luminescence and Narrowband Emission

Helical molecular carbons (HMCs) possess high absorption/luminescence dissymmetry factors (gabs/glum) and significant luminescence quantum yield (Φlum), resulting in a high circularly polarized luminescence (CPL) brightness (BCPL), which is essential for the development of CPL materials for practical applications. Herein, we designed and synthesized a series of boron-nitrogen (BN)-doped HMCs, named π-extended heli(aminoborane)s (E[10]HAB-A, E[10]HAB-B and E[10]HAB-C), consisting of laterally π-extended [10]helicene skeleton with alternating N and B atoms at the inner rim. The aromaticity, electronic structures, and photophysical properties of E[10]HAB-A/B/C were systematically investigated through experiments and theoretical calculations. E[10]HAB-A/B/C displayed remarkable photophysical properties, including high molar extinction coefficient and bright narrowband emission. The isolated enantiomers of E[10]HAB-A/B/C exhibited intense circular dichroism (CD) and CPL, in which E[10]HAB-A shows gabs and glum values up to 0.024 and 0.017, simultaneously with high Φlum of 82 % and a narrow full width at half maximum of 16 nm. Accordingly, E[10]HAB-A exhibits a BCPL as high as 583 M-1 cm-1, which is the largest value among the reported BN-doped HMCs. Our study indicates that inner rim BN-doping and π-extension are effective strategies to achieve high Φlum and balanced glum values in HMCs.

Two-photon Excitation of Bright Diaza[4]Helicenes for Isotropic and Circularly Polarized Emission

Helicenes are chiral organic dyes that are attracting growing attention due to the high tunability of their (chir)optical and electronic properties. In this work, a series of functionalized cationic diaza[4]helicenes based on dimethoxyquinacridinium (DMQA) scaffolds are presented. By merging branched N-alkyl side chains and triple para-functionalization with OMe groups, structures combine improved chiroptical responses (5x increase) and strong fluorescence quantum yields (Φf ≈ 70% in acetonitrile). An overall improved efficiency of the emission of circularly polarized light with BCPL values reaching 3.4 M-1 cm-1 is obtained. Additionally, two-photon excitation (2PE) studies were performed, showing good values of cross section (CS) at 810 nm. Interestingly, 2PE circularly polarized luminescence (CPL) spectra were acquired for the most performant derivatives (N-isopropyl and N-cyclohexyl); this type of measurement being usually challenging for organic molecular species. Finally, the viability of these compounds in single-photon (1PE) and 2PE microscopy is also shown.

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.

Chiral light-matter interactions in solution-processable semiconductors

Chirality is a fundamental property widely observed in nature, arising in objects without a proper rotation axis, therefore existing as forms with distinct handedness. This characteristic can profoundly impact the properties of materials and can enable new functionality, especially for spin-optoelectronics. Chirality enables asymmetric light and spin interactions in materials, with widespread potential applications ranging from energy-efficient displays, holography, imaging, and spin-selective and enantio-selective chemistry to quantum information technologies. This Review focuses on the emerging material class of solution-processable chiral semiconductors, a broad material class comprising organic, inorganic and hybrid materials. These exciting materials offer the opportunity to design desirable light-matter interactions based on symmetry rules, potentially enabling the simultaneous control of light, charge and spin. We briefly discuss the various types of solution-processible chiral semiconductors, including small molecules, polymers, supramolecular self-assemblies and halide perovskites. We then examine the interplay between chirality and spin in these materials, the various mechanisms of chiral light-matter interactions, and techniques utilized to characterize them. We conclude with current and future applications of chiral semiconductors that take advantage of their chiral light-matter interactions.

Ground and Excited State Aromaticity in Azulene-Based Helicenes

Electron delocalization is studied in the ground singlet and first excited triplet states of azulene-containing helicenes. After showing that the compounds we study can be synthesized, we show that they exhibit a charge separation in the ground state, which does not appear in their triplet excited state. Then, magnetically induced properties (IMS3D and ACID) and electron density decomposition methods (EDDB) are used to rationalize aromaticity in these systems. For azulene-based helicenes larger than a critical size, that is, for more than six fused cycles, unexpected aromatic delocalization circuits appear. This feature is understood via the decomposition of the wavefunction on sets of carefully chosen local electronic structures and fragment orbital diagrams.

Recent development of azahelicenes showing circularly polarized luminescence

Recently, a variety of circularly polarized luminescence (CPL) dyes have been developed as next-generation chiroptical materials. Helicenes, ortho-fused aromatics, have been recognized as some of the most promising CPL dyes. Although typical carbohelicenes show CPL, weak fluorescence is often emitted in the blue region. In contrast, heteroatom-embedded helicenes (heterohelicenes) can show intense fluorescence and CPL in the visible region because heteroatoms alter the electronic states of helicene frameworks. Among various heterohelicenes, nitrogen-embedded helicenes (azahelicenes) have unique features such as facile functionalization and sensitive responses to acid/base or metal ions. Furthermore, polycyclic aromatic hydrocarbons (PAHs) containing azaborine units have been recognized as excellent luminescent materials, and the helical derivatives, B,N-embedded helicenes, have been rapidly growing recently. In this feature article, we review and summarize the synthesis and chiroptical properties of azahelicenes, which are classified into imine-type and amine-type azahelicenes and B,N-embedded helicenes. CPL switching systems of azahelicenes are also reviewed.

Helicene-Fluorescein Hybrids: A Reversible Base-Triggered (Chir)optical Switch with Sign Inversion of Circularly Polarized Luminescence

Novel circularly-polarized-luminescence (CPL) materials were designed and synthesized by integrating the intrinsic chirality of helicene with the acid/base-responsive properties of fluorescein. The synthesized helicene-fluorescein hybrids exhibit reversible switching between blue fluorescence under neutral conditions and red fluorescence under basic conditions. Furthermore, these hybrid compounds demonstrate unique chiroptical switching behavior with a rare base-triggered CPL sign inversion, where (P)-isomers show a negative CPL signal around 450 nm under neutral conditions and a positive signal around 620 nm under basic conditions, with three-fold enhanced dissymmetry factors (|glum|) under basic conditions. The structural basis for this switching was elucidated through TD-DFT calculations, revealing distinct angles between magnetic and electric transition dipole moments in closed versus open forms. Live-cell imaging experiments using HeLa cells revealed that the hybrid compounds exhibit intracellular red emission with minimal cytotoxicity, thus promising potential as chiral fluorescent probes for biological applications.

Selective and Divergent Synthesis of Naphthalene- and Phenanthrene-Fused Azahelicenes by Turning Rearrangement On or Off

The Scholl reaction has been used to synthesize a variety of polycyclic aromatic hydrocarbons, where 1,2-aryl shifts have sometimes occurred to yield unique rearrangement products. However, such 1,2-aryl shifts are often uncontrollable, and the selective and divergent synthesis with or without rearrangement is desired. Here, we achieved the control of the rearrangement in the Scholl reaction of carbazoles by changing the N-substituents. The Scholl reaction of 3,6-bis{2-(2-naphthyl)phenyl}carbazoles and 3,6-bis{2-(9-phenanthrenyl)phenyl}carbazoles with an N-benzyl group gave multiple azahelicenes via double rearrangement, while those with an N-benzoyl group gave aza[9]helicene and quadruple [4]helicene in the former and latter cases, respectively. The reaction mechanisms on the divergent reaction pathways were investigated by DFT calculations, which well supported the experimental results.

Chiral helical scaffolds: Unlocking their potential in biomolecular interactions and biomedical applications

In nature, various molecules possess spiral geometry. Such helical structures are even prevalent within the human body, represented classically by DNA and three-dimensional (secondary structure) protein folding. In this review, we chose helicenes and helicene-like structures -synthetically accessible carbon-rich molecules- as a compelling example of helically chiral scaffolds. Helicene chemistry, traditionally anchored in materials science, has been a subject of increasing interest in the biomedical field due to the unique optical and chiral properties of these helical structures. This review explores the diverse applications of helicenes in biomedicine, focusing on their role in cell imaging, protective coatings for implants, drug delivery systems, biosensors, and drug discovery. We discuss the unique properties of helicenes and helicene-like structures, highlighting their ability to form complex interactions with various biomolecules and their potential in the development of candidates for therapeutic agents. Recent advances in helicene derivatives with enhanced circularly polarized luminescence and other photochemical properties are also reviewed, underlining their utility in precise bio-imaging and diagnostic techniques. The review consolidates the current literature and emphasizes the growing importance of helicenes in bridging chemistry, materials science, and biology for innovative technological and biomedical applications.

B,N-Embedded Helical Nanographenes Showing an Ion-Triggered Chiroptical Switching Function

Intramolecular oxidative aromatic coupling of 3,6-bis(m-terphenyl-2'-yl)carbazole provided a bis(m-terphenyl)-fused carbazole, while that of 3,6-bis(m-terphenyl-2'-yl)-1,8-diphenylcarbazole afforded a bis(quaterphenyl)-fused carbazole. Borylation of the latter furnished a B,N-embedded helical nanographene binding a fluoride anion via a structural change from the three-coordinate boron to the four-coordinate boron. The anionic charge derived from the fluoride anion is stabilized over the expanded π-framework, which leads to the high binding constant (Ka) of 1×105 M-1. The four-coordinate boron species was converted back to the parent three-coordinate boron species with Ag+, and the chiroptical switch between the three-coordinate boron and four-coordinate boron species has been achieved via the ion recognition with the change in the color and glum values.

Readily Accessible, Versatile, and Adaptive Biaxially Chiral Chromophores

Precisely controlling quantum states is relevant in next-generation quantum computing, encryption, and sensing. Chiral organic chromophores host unique light-matter interactions, which allow them to manipulate the quantized circular polarization of photons. Axially chiral organic scaffolds, such as helicenes or twisted acenes, are powerful motifs in chiral light manipulation. However, these systems usually require complex syntheses and small-scale (10 mg) enantiomer separations, typically complicating systematic investigations of their structure-property relationships. We report here the straightforward synthesis of both enantiomers (R/S) of 10 different axially chiral chromophores. This protocol relies on a readily accessible, enantiomerically pure, and axially chiral contorting element, benzodinaptho[1,4]dioxicine-2,3-diamine (DODA), that we synthesize in two steps with high purity and good yield at gram scale. Subsequent derivation of DODA transfers the chirality from one axis to twist the dominant chromophore around a second, orthogonal axis. Using this biaxial contortion design element, we produce 10 enantiopure biaxial chromophores, without the need for chromatographic separations, and no observable compromise to chiroptical integrity. These chromophores exhibit broadband single-handed absorption without Cotton effects from 265 to 485 nm, indicating chiral excitonic character that forms between the DODA and twisted core chromophore. This platform is responsive to solvent polarity in the excited state, displaying >50 nm bathochromic shifts in the photoluminescence spectra. In addition, this scaffold intensely interacts with changes in pH, which allows us to ultimately access monosignate circular dichroism absorption over a 300 nm range.

Vibronic Effects Analysis of the Substituent Effect on the Spectral Properties of the EMI and CPL of Three [7]Helicene Derivatives

The substituent effect has a significant influence on the optical properties of spectral shape, width, and wavelength, and the intensities of the maximum peaks of emission (EMI) and circularly polarized luminescence (CPL). In this work, we conducted a systematic theoretical study to investigate how substituents alter the optical response in the EMI and CPL spectra of three [7]helicene derivatives at the vibronic level. To incorporate the vibronic effect, a state-of-the-art time-dependent (TD) method was used to achieve the fully converged spectra. In the meantime, a time-independent (TI) approach also provided a way to show the progression of the spectra, serving as a complementary strategy and creating reliable documentation for the experiment. The experimental spectra of EMI and CPL are nicely reproduced, which validates the reliability of the Adiabatic Hessian (AH) model in simulating experimental data. This allowed us to analyze in detail the effect of substituents, particularly on the optical responses. The introduction of cyano and methoxy groups is highlighted, as they altered the transition dipole moments and led to a 1000-fold increase in the intensity of EMI and CPL. Moreover, substituents can also rationally alter the spectral shape of EMI and CPL by affecting the responsible normal modes. The unique CN stretching and the MeO rotation in the substituted [7]helicene are highlighted as key factors contributing to the different behaviors of EMI and CPL. This sheds light on the design and synthesis of helicenes that can serve as ideal full-color EMI and CPL emitters.

Enantiomerically Pure Helical Bilayer Nanographenes: A Straightforward Chemical Approach

The semiconductor properties of nanosized graphene fragments, known as molecular nanographenes, position them as exceptional candidates for next-generation optoelectronics. In addition to their remarkable optical and electronic features, chiral nanographenes exhibit high dissymmetry factors in circular dichroism and circularly polarized luminescence measurements. However, the synthesis of enantiomerically pure nanographenes remains a significant challenge. Typically, these materials are synthesized in their racemic form, followed by separation of the enantiomers using high-performance liquid chromatography (HPLC). While effective, this method often requires expensive instrumentation, extensive optimization of separation conditions, and typically yields analytical quantities of the desired samples. An alternative approach is the enantioselective synthesis of chiral molecular nanographenes; however, to date, only two examples have been documented in the literature. In this work, we present a straightforward chemical method for the chiral resolution of helical bilayer nanographenes. This approach enables the effective and scalable preparation of enantiomerically pure nanographenes while avoiding the need for HPLC. The incorporation of a BINOL core into the polyarene precursor facilitates the separation of diastereomers through esterification with enantiomerically pure camphorsulfonyl chloride. Following the separation of the diastereomers by standard chromatographic column, the hydrolysis of the camphorsulfonyl group yields enantiomerically pure nanographene precursors. The subsequent graphitization, achieved through the Scholl reaction, occurs in an enantiospecific manner and with the concomitant formation of a furan ring and a heterohelicene moiety. The absolute configurations of the final enantiomers, P-oxa[9]HBNG and M-oxa[9]HBNG, have been determined using X-ray diffraction. Additionally, electrochemical, photophysical, and chiroptical properties have been thoroughly evaluated.

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

Circularly 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.

Benzo-Extended Heli(aminoborane)s: Inner Rim BN-Doped Helical Molecular Carbons with Remarkable Chiroptical Properties

Atomically precise synthesis of three-dimensional boron-nitrogen (BN)-based helical structures constitutes an undeveloped field with challenges in synthetic chemistry. Herein, we synthesized and comprehensively characterized a new class of helical molecular carbons, named benzo-extended [n]heli(aminoborane)s ([n]HABs), in which the helical structures consisted of n = 8 and n = 10 ortho-condensed conjugated rings with alternating BN atoms at the inner rims. X-ray crystallographic analysis, photophysical studies, and density functional theory calculations revealed the unique characteristics of this novel [n]HAB system. Owing to the high enantiomerization energy barriers, the optical resolution of [8]HAB and [10]HAB was achieved with chiral high-performance liquid chromatography. The isolated enantiomers of [10]HAB exhibited record absorption and luminescence dissymmetry factors (|gabs|=0.061; |glum|=0.048), and boosted CPL brightness up to 292 M-1 cm-1, surpassing most helicene derivatives, demonstrating that the introduction of BN atoms into the inner positions of helicenes can increase both the |gabs| and |glum| values.

Dinuclear enantiopure Ln3+ complexes with (S-) and (R-) 2-phenylbutyrate ligands. Luminescence, CPL and magnetic properties

The reaction of Ln(NO3)2·6H2O (Ln = Nd, Sm, Eu, Tb, Dy, Tm and Yb) with the respective enantiopure (R)-(-)-2-phenylbutyric or (S)-(+)-2-phenylbutyric acid (R/S-2-HPhBut) and 4,7-diphenyl-1,10-phenanthroline (Bphen) allows the isolation of chiral dinuclear compounds of the formula [Ln2(μ-R/S-2-PhBut)4(R/S-2PhBut)2(Bphen)2] where Ln = Nd3+ (R/S-Nd-a), Sm3+ (R/S-Sm-a), Eu3+ (R/S-Eu-a), Tb3+ (R/S-Tb-a and R/S-Tb-b), Dy3+ (R/S-Dy-a and R/S-Dy-b), Tm3+ (R/S-Tm-b) and Yb3+ (R/S-Yb-b). Single crystal X-ray diffraction was performed for compounds S-Eu-a and S-Tm-b. Powder crystal X-ray diffraction was performed for all complexes. From the crystallographic data two different structural motifs were found which are referred to as structure type a and structure type b. In structure type a, the Ln3+ atoms are bridged through four R or S-2-PhBut ligands with two different kinds of coordination modes whereas in structure type b the two Ln3+ atoms are bridged through four R or S-2-PhBut ligands showing only one kind of coordination mode. For those lanthanide ions exhibiting both structure types, Tb3+ and Dy3+, a difference in the luminescence and magnetism behavior is observed. All compounds (except R/S-Tm-b) exhibit sensitized luminescence, notably the Eu3+ and Tb3+ analogues. Circular Dichroism (CD) and Circular Polarized Luminescence (CPL) in the solid state and in 1 mM dichloromethane (DCM) solutions are reported, leading to improved chiroptical properties for the DCM solutions. The asymmetry factor (glum) in 1 mM DCM is ±0.02 (+ for R-Eu-a) for the magnetically allowed transition 5D0 → 7F1 and ±0.03 (+ for R-Tb-a and R-Tb-b) for the 5D4 → 7F5 transition. Magnetic properties of all compounds were studied and the Dy3+ compound with the structural motif b (R-Dy-b) shows Single Molecular Magnet (SMM) behavior under a 0 T magnetic field. However, R-Dy-a is a field-induced SMM.

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.

Optical Property Control by the Interligand Charge Transfer Excited State in Brominated Homoleptic and Heteroleptic Aluminum Dinuclear Triple-Stranded Helicates

The utilization of aluminum, an abundant and inexpensive element, for the synthesis of novel functional complexes is extremely important, but the design and control of photofunctionality are still unexplored. In this study, we focused on our previously developed dinuclear triple-stranded helicates incorporating two aluminum ions (ALPHY) to synthesize both homoleptic and heteroleptic complexes with bromine atoms at the 3-position of the pyrrole moiety in the Schiff base ligands. The brominated Schiff base ligands were reacted with AlCl3 to synthesize homoleptic complexes, while different ligands were mixed to prepare heteroleptic complexes. Single-crystal X-ray structural analysis revealed the structures of these novel complexes. We found that increasing the degree of bromination resulted in a tunable emission color, shifting progressively from 550 (yellow) to 566 nm (orange). Optical resolution of the complexes facilitated the observation of mirror-image circular dichroism and circularly polarized luminescence. Furthermore, employing ultrafast spectroscopy techniques, we have elucidated that the optical properties are governed by the interligand charge transfer (ILCT) among the three ligands. The formation of heteroleptic complexes induces the ILCT state even in nonpolar environments, thereby accelerating nonradiative decay and intersystem crossing. These findings mark significant advancements in photofunctional materials based on multinuclear complexes.

Hierarchical chiral MOFs with the induced chirality of AIE ligands exhibiting non-reciprocal CPL

Two pairs of chiral MOFs with hierarchical chiral structures were constructed through assembly of achiral AIE-type multidentate linkers and chiral camphoric acid. Non-reciprocal circularly polarized luminescence (CPL) can be observed on the macroscopic due to the coexistence of optical anisotropic and chiroptical nature. This study provides a new perspective to recognize and construct chiral crystalline materials.

Endowing Metal-Organic Coordination Materials with Chiroptical Activity by a Chiral Anion Strategy

Recently, chiral metal-organic coordination materials have emerged as promising candidates for a wide range of applications in chiroptoelectronics, chiral catalysis, and information encryption, etc. Notably, the chiroptical effect of coordination chromophores makes them appealing for applications such as photodetectors, OLEDs, 3D displays, and bioimaging. The direct synthesis of chiral coordination materials using chiral organic ligands or complexes with metal-centered chirality is very often tedious and costly. In the case of ionic coordination materials, the combination of chiral anions with cationic, achiral coordination compounds through noncovalent interactions may endow molecular materials with desirable chiroptical properties. The use of such a simple chiral strategy has been proven effective in inducing promising circular dichroism and/or circularly polarized luminescence signals. This concept article mainly delves into the latest advances in exploring the efficacy of such a chiral anion strategy for transforming achiral coordination materials into chromophores with superb photo- or electro-chiroptical properties. In particular, ionic small-molecular metal complexes, metal clusters, coordination supramolecular assemblies, and metal-organic frameworks containing chiral anions are discussed. A perspective on the future opportunities on the preparation of chiroptical materials with the chiral anion strategy is also presented.

Crystallization-Induced Chirality Transfer in Conformationally Flexible Azahelicene Au(I) Complexes with Circularly Polarized Luminescence Activation

Flexible and twisted annulated π-systems exhibit numerous unique and desirable features, owing to their ability to display chirality. However, preventing their racemization due to the dynamic nature of their chirality remains a challenge. One promising approach to stabilize homochirality in such systems is chirality transfer from a chiral auxiliary to a moiety displaying dynamic chirality. Herein, we introduce a new approach for dynamic chirality stabilization in conformationally flexible azahelicene species via crystallization-induced intermolecular chirality transfer in Au(I) complexes featuring azahelicene (dibenzo[c,g]carbazole and benzo[c]carbazole) and enantio-pure chiral N-heterocyclic carbene (NHC) ligands with a complementary tailored shape. Crystallization of these azahelicene Au(I) complexes not only suppresses the dynamic chirality of the dibenzocarbazole species but also stabilizes their homochirality through the intermolecular conjunction between the chiral NHC and dibenzocarbazole ligands. In the Au(I) benzocarbazole complexes, the intermolecular conjunction and chirality transfer in the crystals induce chirality in the initially achiral benzocarbazole ligand. Furthermore, the crystallization of the studied complexes activates their circularly polarized luminescence (CPL) properties, which were suppressed in solution. Importantly, chirality transfer leads to significant CPL enhancement; the complexes that feature chirality transfer within the crystal structure exhibit luminescence dissymmetry factors 5 to 10 times higher than those of the complexes without chirality transfer.

Linker-dependent control of the chiroptical properties of polymethylene-vaulted trans-bis[(β-iminomethyl)naphthoxy]platinum(II) complexes

The effects of polymethylene bridges on the chiroptical properties of trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms were examined both experimentally and theoretically using newly designed planar chiral Pt analogues (1) having three-dimensional superstructures. A series of optically pure polymethylene-vaulted Pt complexes (R)- and (S)-1 were synthesized and characterized with regard to the chiroptical behaviour of the trans-bis[(β-iminomethyl)naphthoxy]platinum(II) platforms. These complexes were found to exhibit structure-dependent chiroptical characteristics in solution, such that the absolute values of specific rotation, the circular dichroism dissymmetry factor (gabs) and the circularly polarized luminescence dissymmetry factor (glum) all increased upon shortening the polymethylene bridges. Density functional theory and time dependent density functional theory calculations were used to analyse vaulted and non-vaulted complexes, which demonstrated that the present linker-dependent chiroptical properties resulted from constraint-induced changes in the square planar Pt coordination centres rather than from chiral distortion along the coordination platforms.

Impact of helical elongation of symmetric oxa[n]helicenes on their structural, photophysical, and chiroptical characteristics

The adjustment of the main helical scaffold in helicenes is a fundamental strategy for modulating their optical features, thereby enhancing their potential for diverse applications. This work explores the influence of helical elongation (n = 5-9) on the structural, photophysical, and chiroptical features of symmetric oxa[n]helicenes. Crystal structure analyses revealed structural variations with helical extension, impacting torsion angles, helical pitch, and packing arrangements. Through theoretical investigations using density functional theory (DFT) calculations, the impact of helical extension on aromaticity, planarity distortion, and heightened chiral stability were discussed. Photophysical features were studied through spectrophotometric analysis, with insights gained through time-dependent DFT (TD-DFT) calculations. Following optical resolution via chiral high-performance liquid chromatography (HPLC), the chiroptical properties of both enantiomers of oxa[7]helicene and oxa[9]helicene were investigated. A slight variation in the main helical scaffold of oxa[n]helicenes from [7] to [9] induced an approximately three-fold increase in dissymmetry factors with the biggest values of|glum| of oxa[9]helicene (2.2 × 10-3) compared to|glum|of oxa[7]helicene (0.8 × 10-3), findings discussed and supported by TD-DFT calculations.

Axially-chiral boramidine for detailed (chir)optical studies

The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence (Φ up to 94%), and ca. |10-3| gabs and glum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin-orbit coupling between the lowest singlet excited state and an upper triplet state.

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.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

A Perspective on the Simulation of Electronic Circular Dichroism and Circularly Polarized Luminescence Spectra in Chiral Solid Materials

Chiral materials have shown tremendous potential for many technological applications, such as optoelectronics, sensing, magnetism, information technology, and imaging. Characterization of these materials is mostly based on chiroptical spectroscopies, such as electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). These experimental measurements would greatly benefit from theoretical simulations for interpretation of the spectra as well as predictions on new materials. While ECD and CPL simulations are well established for molecular systems, they are not for materials. In this Perspective, we describe the theoretical quantities necessary to simulate ECD and CPL spectra in oriented systems. Then, we discuss the approximate strategies currently used to perform these calculations, what computational machinery is already available to develop more general approaches, and some of the open challenges for the simulation of ECD and CPL spectra in solid materials. When methods that are as reliable and computationally efficient as those for molecules are developed, these simulations will provide invaluable insight and guidance for the rational design of optically active materials.

Circularly Polarized Luminescence from Schiff-base [4]Helicene Boron Complexes

Boron complexes with Schiff-base [4]helicene ligands were synthesized. These complexes were characterized by NMR spectroscopy and their helical molecular structures were unequivocally established by X-ray diffraction (XRD) analysis. The helical boron complexes exhibited efficient photoluminescence under UV irradiation, and the circularly polarized luminescence (CPL) properties were investigated for optically pure samples. Density functional theory (DFT) calculations were conducted to further understand their photophysical properties including chiroptical responses.

Modulation of circularly polarized luminescence by swelling of microgels functionalized with enantiopure [Ru(bpy)3]2+ luminophores

Chemoresponsive microgels functionalized with enantiomeric Δ- or Λ-[Ru(bpy)3]2+ showed tunable chiroptical properties upon swelling and shrinking. The tuning is triggered by a modulation of the local mobility of [Ru(bpy)3]2+ upon addition of fructose, controlling interactions and distances between [Ru(bpy)3]2+ and phenylboronic acid.

Synthesis of π-Conjugated Chiral Aza/Boracyclophanes with a meta and para Substitution

We herein describe the synthesis of a new class of axially chiral aza/boracyclophanes (BDN1, BXN1, BDB1 and BXB1) using binaphthyls as chiral building blocks and the main-group (B/N) chemistry with tunable electronic effects. All macrocycles substituted with triarylamine donors or triarylborane acceptors are strongly luminescent. These macrocycles showed two distinct meta and para π-conjugation pathways, leading to the formation of quasi figure-of-eight and square-shaped conformations. Interestingly, comparison of such structural models revealed that the former type of macrocycles BXN1 and BXB1 gave higher racemization barriers relative to the other ones. The results reported here may provide a new approach to engineer the optical stability of π-conjugated chiral macrocycles by controlling π-substitution patterns. The ring constraints induced by macrocyclization were also demonstrated to contribute to the configurational persistence as compared with the open-chain analogues p-BTT and m-BTT.

Conformational, Structural, and Chiroptical Properties of an Overcrowded Triply Fused Carbo[7]helicene

Recently, the synthesis of the racemate of an overcrowded triply fused carbo[7]helicene of formula C66H36 with three carbo[7]helicenes fused within a central six-membered ring was described. This molecule was found to embed an extremely contorted central six-membered ring and two negative curvatures. We report herein the resolution of the corresponding enantiomers and their conformational, structural, photophysical, and chiroptical properties. The racemization of the triply fused carbo[7]helicene was determined to proceed at a rate of krac = 8.06 × 10-4 s-1 at 175 °C in ortho-dichlorobenzene, corresponding to a barrier to enantiomerization ΔGenant‡ = 140.4 kJ·mol-1, a value significantly lower than for pristine carbo[7]helicene. Interestingly, the crystalline structures of the racemic and enantiopure materials show some differences regarding the molecular geometry, with an increased negative curvature in the latter cases. This unusual curved delocalized π-conjugated system afforded notably green fluorescence at room temperature and far-red phosphorescence at low temperature. Finally, electronic circular dichroism and circularly polarized luminescence responses of the enantiopure compounds have been measured and showed very close absorption and emission dissymmetry factors, gabs and glum, respectively, of ca. 2.6 × 10-3, indicating a similar chiral rigid geometry for both ground and excited states.

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.

Aqueous Circularly Polarized Luminescence Induced by Homopolypeptide Self-Assembly

Remarkable advances have been achieved in solution self-assembly of polypeptides from the perspective of nanostructures, mechanisms, and applications. Despite the intrinsic chirality of polypeptides, the promising generation of aqueous circularly polarized luminescence (CPL) based on their self-assembly has been rarely reported due to the weak fluorescence of most polypeptides and the indeterminate self-assembly mechanism. Here, we propose a facile strategy for achieving aqueous CPL based on the self-assembly of simple homopolypeptides modified with a terminal group featuring both twisted intramolecular charge transfer and aggregation-induced emission properties. A morphology-dependent CPL can be observed under different self-assembly conditions by altering the solvents. A nanotoroid-dispersed aqueous solution with detectable CPL can be obtained by using tetrahydrofuran as a good solvent for the self-assembly, which is attributed to the involvement of the terminal group in the chiral environment formed by the homopolypeptide chains. However, such a chiral packing mode cannot be realized in nanorods self-assembled from dioxane, resulting in an inactive CPL phenomenon. Furthermore, CPL signals can be greatly amplified by co-assembly of homopolypeptides with the achiral small molecule derived from the terminal group. This work not only provides a pathway to construct aqueous CPL-active homopolypeptide nanomaterials but also reveals a potential mechanism in the self-assembly for chiral production, transfer, and amplification in polypeptide-based nanostructures.

Porphyrin- and Bodipy-helicene conjugates: syntheses, separation of enantiomers and chiroptical properties

The synthesis of several new compounds containing a chromophore and a helicenic moiety is reported. The preparation, characterisation and some physico-chemical studies are detailed. In particular, the two enantiomers of several chiral molecules of this type were separated by chiral HPLC (both analytically and in a preparative way) and their racemisation rates were determined for short-lived species. Electronic circular dichroism (ECD) and circular polarised luminescence (CPL) measurements were performed for the compounds with a very long racemisation half-life. Chiral porphyrins and Bodipys both gave ECD and CPL responses over a large area of the visible spectrum.

Acid/Base-Triggered Photophysical and Chiroptical Switching in a Series of Helicenoid Compounds

A series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes.

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.

Strong Chiroptical Effects in the Absorption and Emission of Macrocycles Based on the 2,5-Diaminoterephthalate Minimal Fluorophore

Chiral fluorescent macrocycles consisting of two to four units of dimethyl 2,5-diaminoterephthalate can be readily synthesized in a one-pot manner from inexpensive building blocks. Depending on the concentration, either a paracyclophane-like dimer with closely stacked benzene rings or a triangular trimer is the main product of the reaction. The macrocycles exhibit fluorescence in solution as well as in the solid state with maxima that are red-shifted with decreasing size of the macrocyclic ring and are observed at wavelengths from 590 (tetramer in solution) to 700 nm (dimer in the solid state). Chirality dictates the differential absorption and emission of circularly polarized light by these molecules. The ECD and CPL effects are particularly strong for the trimer, which is characterized by relatively large dissymmetry factors gabs =±2.8×10-3 at 531 nm and glum =±2.3×10-3 at 580 nm in n-hexane, being at the same time highly luminescent (Φfl =13.7 %). Despite the small chromophore, the circularly polarized brightness BCPL of 2.3 dm3  mol-1  cm-1 is comparable to values reported for other classes of established CPL emitters in the visible region, such as expanded helicenes or larger π-conjugated systems.