Pitch and Handedness of the Cholesteric Order in Films of a Chiral Alternating Fluorene Copolymer

From individuals to families: design and application of self-similar chiral nanomaterials

Establishing an intimate relationship between similar individuals is the beginning of self-extension. Various self-similar chiral nanomaterials can be designed using an individual-to-family approach, accomplishing self-extension. This self-similarity facilitates chiral communication, transmission, and amplification of synthons. We focus on describing the marriage of discrete cages to develop self-similar extended frameworks. The advantages of utilizing cage-based frameworks for chiral recognition, enantioseparation, chiral catalysis and sensing are highlighted. To further promote self-extension, fractal chiral nanomaterials with self-similar and iterated architectures have attracted tremendous attention. The beauty of a fractal family tree lies in its ability to capture the complexity and interconnectedness of a family's lineage. As a type of fractal material, nanoflowers possess an overarching importance in chiral amplification due to their large surface-to-volume ratio. This review summarizes the design and application of state-of-the-art self-similar chiral nanomaterials including cage-based extended frameworks, fractal nanomaterials, and nanoflowers. We hope this formation process from individuals to families will inherit and broaden this great chirality.

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.

Helical polymers for dissymmetric circularly polarized light imaging

Control of the spin angular momentum (SAM) carried in a photon provides a technologically attractive element for next-generation quantum networks and spintronics1-5. However, the weak optical activity and inhomogeneity of thin films from chiral molecular crystals result in high noise and uncertainty in SAM detection. Brittleness of thin molecular crystals represents a further problem for device integration and practical realization of chiroptical quantum devices6-10. Despite considerable successes with highly dissymmetric optical materials based on chiral nanostructures11-13, the problem of integration of nanochiral materials with optical device platforms remains acute14-16. Here we report a simple yet powerful method to fabricate chiroptical flexible layers via supramolecular helical ordering of conjugated polymer chains. Their multiscale chirality and optical activity can be varied across the broad spectral range by chiral templating with volatile enantiomers. After template removal, chromophores remain stacked in one-dimensional helical nanofibrils producing a homogeneous chiroptical layer with drastically enhanced polarization-dependent absorbance, leading to well-resolved detection and visualization of SAM. This study provides a direct path to scalable realization of on-chip detection of the spin degree of freedom of photons necessary for encoded quantum information processing and high-resolution polarization imaging.

Helical donor-acceptor platinum complexes displaying dual luminescence and near-infrared circularly polarized luminescence

A series of chiral platina[5]helicenes displaying dual luminescence, i.e., fluorescence between 450 and 600 nm and red/NIR phosphorescence between 700 and 900 nm, has been synthesised, characterised and studied by first-principle calculations. This unusual behavior has been attributed to limited electronic interactions between the d orbitals of the metal and the π-orbitals of the organic ligand on the excited-state. Accordingly, the electron richness of the donor group on the helical ligand does not affect the energy of the phosphorescence process but does play a role on its efficiency. Interestingly, near-infrared circularly polarized luminescence can be obtained for the three complexes with dissymmetry factors up to 3 × 10^-3 at 750 nm.

Robust Angular Anisotropy of Circularly Polarized Luminescence from a Single Twisted-Bipolar Polymeric Microsphere

It has long been surmised that the circular polarization of luminescence (CPL) emitted by a chiral molecule or a molecular assembly should vary with the direction in which the photon is emitted. Despite its potential utility, this anisotropic CPL has not yet been demonstrated at the level of single molecules or supramolecular assemblies. Here we show that conjugated polymers bearing chiral side chains self-assemble into solid microspheres with a twisted bipolar interior, which are formed via liquid-liquid phase separation and subsequent condensation into a cholesteric lyotropic liquid crystalline mesophase. The resultant microspheres, when dispersed in methanol, exhibit CPL with a g_lum value as high as 0.23. The microspheres are mechanically robust enough to be handled with a microneedle under ambient conditions, allowing comprehensive examination of the angular anisotropy of CPL. The single microsphere is found to exhibit distinct angularly anisotropic birefringence and CPL with g_lum up to ∼0.5 in the equatorial plane, which is 2.5-fold greater than that along the polar axis. Such optically anisotropic solid materials are important for the application to next-generation microlight-emitting and visualizing devices as well as for fundamental optics studies of chiral light-matter interaction.

Hierarchical communication of chirality for aromatic oligoamide sequences

The communication of chirality at a molecular and supramolecular level is the fundamental feature capable of transmitting and amplifying chirality information. Yet, the limitation of one-step communication mode in many artificial systems has precluded the ability of further processing the chirality information. Here, we report the chirality communication of aromatic oligoamide sequences within the interpenetrated helicate architecture in a hierarchical manner, specifically, the communication is manipulated by three sequential steps: (i) coordination, (ii) concentration, and (iii) ion stimulus. Such approach enables the information to be implemented progressively and reversibly to different levels. Furthermore, the chiral information on the side chains can be accumulated and transferred to the helical backbones of the sequences, resulting in that one of ten possible diastereoisomers of the interpenetrated helicate is finally selected. The circular dichroism experiments with a mixture of chiral and achiral ligands demonstrate a cooperative behavior of these communications, leading to amplification of chiral information.

Communication of chirality at a molecular level is the fundamental for transmitting chirality information but one-step communication modes in many artificial systems limits further processing the chirality information. Here, the authors report chirality communication of aromatic oligoamide sequences within interpenetrated helicate architecture in a hierarchical manner.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π-Conjugated Polymers by Light

Control over main-chain motion of chiral π-conjugated polymers can lead to unexpected new functionalities. Here, it is shown that by combining photoswitchable azobenzene units in conjugation with chiral fluorene comonomers and appropriate plasticizers, the polymer organization and chiroptical properties of these alternating copolymers steered by light and its state of polarization can be dynamically controlled. The configuration of the stereogenic centers in the side chains of the fluorene units determines the handedness of the cholesteric organization in thermally annealed films, indicating cooperative behavior. The polymer alignment and helicity of the supramolecular arrangement can be switched by irradiating with linearly and circularly polarized light, respectively. Intriguingly, when switching the handedness of thermally induced cholesteric organizations by illuminating with circularly polarized light that is opposite to the handedness of the cholesteric phases, a nematic-like intermediate state is observed during helix interconversion. By the sequence of irradiation with left and right circularly polarized light followed by thermal annealing, an asymmetric motion, reminiscent of that seen in molecular motors is observed. These findings suggest that functional conjugated polymers can exhibit emergent properties at mesoscopic scale.

Natural optical activity as the origin of the large chiroptical properties in π-conjugated polymer thin films

Polymer thin films that emit and absorb circularly polarised light have been demonstrated with the promise of achieving important technological advances; from efficient, high-performance displays, to 3D imaging and all-organic spintronic devices. However, the origin of the large chiroptical effects in such films has, until now, remained elusive. We investigate the emergence of such phenomena in achiral polymers blended with a chiral small-molecule additive (1-aza[6]helicene) and intrinsically chiral-sidechain polymers using a combination of spectroscopic methods and structural probes. We show that – under conditions relevant for device fabrication – the large chiroptical effects are caused by magneto-electric coupling (natural optical activity), not structural chirality as previously assumed, and may occur because of local order in a cylinder blue phase-type organisation. This disruptive mechanistic insight into chiral polymer thin films will offer new approaches towards chiroptical materials development after almost three decades of research in this area.

Polymer thin films that emit and absorb circularly polarised light are promising in achieving important technological advances, but the origin of the large chiroptical effects in such films has remained elusive. Here the authors demonstrate that in non-aligned polymer thin films, large chiroptical effects are caused by magneto-electric coupling, not structural chirality as previously assumed.

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.

Simulation of Circular Dichroism by Chromophores Coupled with Selective Reflection by Cholesteric Stacks

The Good-Karali theory is extended to simulate composite circular dichroism (CD) through a cholesteric stack by incorporating chromophore's selective absorption (SA) and cholesteric stack's selective reflection (SR). Based on the independently evaluated anisotropic refractive indices and absorption coefficients, helical sense and pitch length, and film thickness, the theory is capable of describing transmission, reflection, and absorption through spin-cast cholesteric glassy liquid crystal films of nonafluorene. The resulting composite CD spectra agree quite well with experimental observations. The theory informs that SA plays a dominant role over SR in the composite CD. Specifically, the right-handed stack of the chromophore with its absorption dipole aligned with the local director in the cholesteric stack preferentially absorbs the left-handed over the right-handed circularly polarized light. The algebraic sign of the predicted composite CD flips by reversing the cholesteric host film's handedness without altering other parameter values. The established theory and computation constitute a solid foundation for optimizing circular polarizers by exploring the readily accessible parameter space targeting various potential applications.

Circular Intensity Differential Scattering Reveals the Internal Structure of Polymer Fibrils

The optical and electronic properties of π-conjugated polymers in organic electronic devices depend on their intra- and interchain interactions, dictated by the internal arrangement of the polymer chains in an amorphous or semicrystalline aggregated state. Here, we discuss the utility of circular intensity differential scattering (CIDS) of circularly polarized light as a sensitive probe to identify the internal arrangement of the polymer chains in helical polymer aggregates. We advance existing theoretical models to utilize the CIDS response and extract structural properties such as the size, orientation, and periodicity of a polymer aggregate. As an example, we analyze the CIDS signatures of helically assembled fibrillar aggregates of a chiral polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzothiadiazole)] (PFBT) in solution and reveal that PFBT fibrils incorporate at least five intertwined polymer chains. We anticipate our approach can be extended more generally to investigate the internal arrangement of supramolecular assemblies of a wide range of fibrillar aggregates of π-conjugated polymers.

Coherent acoustic phonon dynamics in chiral copolymers

Coherent phonon oscillations in the UV-Vis transient absorption and circular dichroism response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical determination of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s^-1, respectively. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extracted from a Fourier analysis of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient circular dichroism (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack.

Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer

We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet-visible-near-infrared (UV-vis-NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV-vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV-vis region.

Modulating ICT emission: a new strategy to manipulate the CPL sign in chiral emitters

A new strategy to manipulate the circularly polarized luminescence (CPL) handedness in chiral emitters, based on modulating the population of an emissive ICT state, is proposed. Such a strategy is particularly interesting for conformationally rigid and non-aggregating chiral organic emitters, opening up new perspectives for the development of CPL applications based on organic molecules.

Chiral sculptured thin films for circular polarization of mid-wavelength infrared light

Being an assembly of identical upright helices, a chiral sculptured thin film (CSTF) exhibits the circular Bragg phenomenon and can therefore be used as a circular polarization filter in a spectral regime called the circular Bragg regime. This has been already demonstrated in the near-infrared and short-wavelength infrared regimes. If two CSTFs are fabricated in identical conditions to differ only in the helical pitch, and if both are made of a material whose bulk refractive index is constant in a wide enough spectral regime, then the center wavelengths of the circular Bragg regimes of the two CSTFs must be in the same ratio as their helical pitches by virtue of the scale invariance of the frequency-domain Maxwell postulates. This theoretical result was confirmed by measuring the linear transmittance spectra of two zinc-selenide CSTFs with helical pitches in the ratio 1:7.97. The center wavelengths were found to be in the ratio 1:7.1, and the deviation from the ratio of helical pitches is explainable at least in part because the bulk refractive index of zinc selenide decreases a little with wavelength. We concluded that CSTFs can be fabricated to function as circular polarization filters in the mid-wavelength infrared regime.

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.