Manipulating Chiroptical Activities in 0D Chiral Hybrid Manganese Bromides by Solvent Molecular Engineering

0D hybrid metal halides (0D HMHs) with fully isolated inorganic units provide an ideal platform for studying the correlations between chiroptical activities and crystal structures at atomic levels. Here, through the incorporation of different solvent molecules, a series of 0D chiral manganese bromides (RR/SS-C20H28N2)3MnBr8·2X (X = C2H5OH, CH3OH, or H2O) are synthesized to elucidate their chiroptical properties. They show negligible circular dichroism signals of Mn absorptions due to C2v-symmetric [MnBr4]2- tetrahedra. However, they display distinct circularly polarized luminescence (CPL) signals with continuously increased luminescence asymmetry factors (glum) from 10-4 (X = C2H5OH) to 10-3 (X = H2O). The increased glum value is structurally revealed to originate from the enhancement of [MnBr4]2- tetrahedral bond-angle distortions, due to the presence of different solvent molecules. Furthermore, (RR/SS-C20H28N2)MnBr4·H2O enantiomers with larger bond-angle distortions of [MnBr4]2- tetrahedra are synthesized based on hydrobromic acid-induced structural transformation of (RR/SS-C20H28N2)3MnBr8·2H2O enantiomers. Therefore, such (RR/SS-C20H28N2)MnBr4·H2O enantiomers exhibit enhanced CPL signals with |glum| up to 1.23 × 10-2. This work provides unique insight into enhancing chiroptical activities in 0D HMH systems.

Plasticizer-Induced Enhancement of Mesoscale Dissymmetry in Thin Films of Chiral Polymers with Variable Chain Length

Conjugated polymers with chiral side chains are of interest in areas including chiral photonics, optoelectronics, and chemical and biological sensing. However, the low dissymmetry factors of most neat polymer thin films have limited their practical application. Here, a robust method to increase the absorption dissymmetry factor in a poly-fluorene-thiophene (PF8TS series) system is demonstrated by varying molecular weight and introducing an achiral plasticizer, polyethylene mono alcohol (PEM-OH). Extending chain length within the optimal range and adding this long-chain alcohol significantly enhance the chiroptical properties of spin-coated and annealed thin films. Mueller matrix spectroscopic ellipsometry (MMSE) analysis shows good agreement with the steady-state transmission measurements confirming a strong chiral response (circular dichroism (CD) and circular birefringence (CB)), ruling out linear dichroism, birefringence, and specific reflection effects. Solid-state NMR studies of annealed hybrid chiral polymer systems show enhancement of signals associated with aromatic π-stacked backbone and the ordered side-chain conformations. Further studies using Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), atomic force microscopy (AFM), and polarized optical microscopy (POM) indicate that PEM-OH facilitates mesoscopic crystal domain ordering upon annealing. This provides new insights into routes for tuning optical activity in conjugated polymers.

Chiral polymer hosts for circularly polarized electroluminescence devices

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications. The fabrication of a CP PLED requires a polymer host that provides the appropriate chiral environment around the emitting dopant. However, chemical strategies for the design of chiral polymer hosts remain underdeveloped. We have developed new polymer hosts for CP PLED applications. These polymers were prepared through a free-radical polymerization of 3-vinylcarbazole with a chiral N-alkyl unit. This chiral unit forces the carbazole repeat units to form mutually helical half-sandwich conformers with preferred (P)-helical sense along the polymer main chain. Electronic circular dichroism measurements demonstrate the occurrence of chirality transfer from chiral monomers to achiral monomers during chain growth. The (P)-helical-sense-enriched polymer interacts diastereoselectively with an enantiomeric pair of new phosphorescent (R)- and (S)-dopants. The magnitude of the Kuhn dissymmetry factor (g_abs) for the (P)-helically-enriched polymer film doped with the (R)-dopant was found to be one order of magnitude higher than that of the film doped with the (S)-dopant. Photoluminescence dissymmetry factors (g_PL) of the order of 10⁻³ were recorded for the doped films, but the magnitude of diastereomeric enhancement decreased to that of g_abs. The chiral polymer host permits faster energy transfer to the phosphorescent dopants than the achiral polymer host. Our photophysical and morphological investigations indicate that the acceleration in the chiral polymer host is due to its longer Förster radius and improved compatibility with the dopants. Finally, multilayer CP PLEDs were fabricated and evaluated. Devices based on the chiral polymer host with the (R)- and (S)-dopants exhibit electroluminescence dissymmetry factors (g_EL) of 1.09 × 10⁻⁴ and −1.02 × 10⁻⁴ at a wavelength of 540 nm, respectively. Although challenges remain in the development of polymer hosts for CP PLEDs, our research demonstrates that chiroptical performances can be amplified by using chiral polymer hosts.

Polymer electroluminescence devices producing circularly polarized luminescence (CP PLEDs) have valuable photonic applications.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Inter- and intramolecular excimer circularly polarised luminescence of planar chiral paracyclophane-pyrene luminophores

Two types of planar chiral [2,2]paracyclophane-pyrene luminophores (1 and 2) with different binding positions of the fluorescent pyrene units were synthesised. (R)/(S)-1 with 1-pyrene units exhibited green intermolecular excimer circularly polarised luminescence (CPL) at 530 nm in the KBr-pellet, but exhibited no CPL signal in dilute CHCl3 solution. In contrast, (R)/(S)-2 with 2-pyrene units exhibited a blue intramolecular excimer CPL at 450 nm in CHCl3 solution. This is the first example of using the binding position of pyrene and the external environment to tune the type (inter- or intramolecular) and chiroptical sign of excimer CPL.

Solid-state AIEnh-circularly polarised luminescence of chiral perylene diimide fluorophores

Chiral N,N′-bis(1-phenylethyl)perylene-3,4,9,10-tetracarboxylic acid diimide (BPP) exhibits solid-state aggregation-induced enhanced circularly polarized luminescence (AIEnh-CPL) in KBr, PMMA, and myo-IPU matrices..

Control over chiroptical and electronic properties of poly(9,9′-bifluorenylidene) bearing chiral side chains

The introduction of chiral side chains at peripheral positions of the 9,9′-bifluorenylidene moiety causes an efficient bias to its dynamic molecular helicity, which helps control the chiroptical and electronic properties of resultant polymers.

Amplifying Chiroptical Properties of Conjugated Polymer Thin-Film Using an Achiral Additive

Chiral conjugated polymers bearing enantiopure side chains offer the possibility to harness the effect of chirality in organic electronic devices. However, its use is hampered by the low degree of circular polarization in absorption (g_abs) in most of the conjugated polymer thin-films studied. Here we demonstrate a versatile method to significantly increase the g_abs by using a few weight percentages of a commercially available achiral long-chain alcohol as an additive. This additive enhances the chiroptical properties in both absorption and emission by ca. 5-10 times in the thin-films. We envisage that the alcohol additive acts as a plasticizer which enhances the long-range chiral liquid crystalline ordering of the polymer chains, thereby amplifying the chiroptical properties in the thin-film. The application of this methodology to various conjugated polymers has been demonstrated.

Ambidextrous Chirality Transfer Capability from Cellulose Tris(phenylcarbamate) to Nonhelical Chainlike Luminophores: Achiral Solvent-Driven Helix-Helix Transition of Oligo- and Polyfluorenes Revealed by Sign Inversion of Circularly Polarized Luminescence and Circular Dichroism Spectra

We investigated whether helicity and/or chirality of cellulose tris(phenylcarbamate) (CTPC) can transfer to noncharged, nonhelical oligo- and polyfluorenes when CTPC was employed as a solution processable homochiral platform of a D-glucose-skeletal polymer. Noticeably, CTPC revealed the solvent-driven, ambidextrous intermolecular helicity/chirality transfer capability to these fluorenes. The chiroptical inversion characteristics of circularly polarized luminescence (CPL) and the corresponding CD spectra were realized by solely choosing a proper achiral solvent and/or achiral cosolvent. When the solution of PF6 and CTPC in tetrahydrofuran (THF) was cast on a quartz substrate, the dissymmetry ratio of CPL (g_CPL) from the polymer film showed g_CPL = +2.1 × 10^-3 at 429 nm. Conversely, when dichloromethane (DCM) was used as the solvent, the CPL sign was inverted to g_CPL = -2.4 × 10^-3 at 429 nm. The dissymmetry ratio of Cotton CD band (g_CD) from the THF solution was g_CD = +3.2 × 10^-3 at 392 nm; conversely, from the DCM, the CD sign inverted to g_CD = -0.8 × 10^-3 at 371 nm. The sign and magnitude of the g_CD values were interpreted to a London dispersion term (δ_d) of Hansen solubility parameter (δ) of the casting solvents rather than a dipole-dipole interaction term (δ_p) and a hydrogen bonding interaction term (δ_h) of the δ values and dielectric constant (ε). Analysis of solvent-driven changes in FTIR spectra, wide-angle X-ray diffraction profiles, and differential scanning calorimetry diagrams indicated that solvent driven on-off switching of multiple hydrogen bonds due to three urethane groups of CTPC play the key for the inversion. Intermolecular CH/π and π-π interactions among phenyl rings and alkyl groups were assumed to be crucial for helicity/chirality transfer capability based on molecular mechanics and molecular dynamics simulations of PF6-CTPC hybrids. These chiroptical inversion characteristics arose from solvent-driven order-disorder transition characteristics of the CTPC helix rather than a helix-helix transition of CTPC itself.

Synthesis, chiroptical properties, and self-assembled nanoparticles of chiral conjugated polymers based on optically stable helical aromatic esters

By Suzuki coupling reaction, three pairs of chiral conjugated polymers with optically stable helical aromatic ester subunits as the main-chain were designed and synthesized. Polymers (+)-P-P1 and (-)-M-P1, (+)-P-P2 and (-)-M-P2 showed strong fluorescence emission, strong mirror image CD and circularly polarized luminescence (CPL) signals in THF. For polymers (+)-P-P3 and (-)-M-P3, containing the tetraphenylethene (TPE) moiety, they not only showed obvious aggregation induced enhancement emission (AIEE), but also exhibited mirror image CD signals and aggregation-induced enhancement CPL signals in THF-water mixtures. Moreover, (+)-P-P3 and (-)-M-P3 could also form chiral nanoparticles by solvent evaporation induced self-assembly. Interestingly, it was further found that the size of the nanoparticles could be controlled by the changing of THF/water ratio, and their CPL properties were also shown.

Control of circularly polarized luminescence from a boron ketoiminate-based π-conjugated polymer via conformational locks

Circularly polarized luminescence from a boron ketoiminate-based π-conjugated polymer was successfully realized at the unimolecular level via conformational locks that blocked the intramolecular rotations.

Stimuli-responsive circularly polarized luminescence from an achiral perylenyl dyad

Stimuli-responsive circularly polarized luminescence (CPL) was successfully achieved through fine-tuning the conformation of a perylenyl dyad by using external stimuli.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Stapled helical o-OPE foldamers as new circularly polarized luminescence emitters based on carbophilic interactions with Ag(i)-sensitivity

ortho-Oligo(phenylene)ethynylenes (o-OPEs) stapled with enantiopure 2,3-dihydroxybutane diethers have highly intense circular dichroism (CD) spectra and excellent circular polarized luminescence (CPL) responses (glum values up to 1.1 × 10-2), which are consistent with homochiral helically folded structures. In the presence of Ag(i), a change in the CPL emission is observed, representing the first example of CPL active small organic molecular emitters, which can be modulated by carbophilic interactions in a reversible manner.

New fluorene-based chiral copolymers with unusually high optical activity in pristine and annealed thin films

Polyfluorene (PF)-based chiral alternating copolymers, i.e., PFPh and PFTh, were successfully synthesized to obtain high circular dichroism (CD) in pristine thin films.

Pyrene magic: chiroptical enciphering and deciphering 1,3-dioxolane bearing two wirepullings to drive two remote pyrenes

2,2-Dimethyl-1,3-dioxolane connected to two pyrene moieties through flexible wires in chloroform exhibited cryptochirality in the ground state, as proven by the lack of detectable circular dichroism signals.

A single-step emulsion approach to prepare fluorescent nanoscale coordination polymers for bioimaging

A facile and convenient microemulsion method is demonstrated to prepare fluorescent nanoscale coordination polymers. And the nanoscale coordination polymers exhibited bright blue fluorescence and good biocompatibility, thus giving them the ability for bioimaging.