Open Access

Show Abstract ▼

Abstract: The ability to synthesise lemniscular molecules to allow for the study and application of their chiroptical properties is a notable technical challenge. Herein, we report the design and synthesis of enantiomers of a [5]helicenoid derived molecular lemniscate, in which two homochiral helicenes are linked via the formation of two azine motifs. We demonstrate that these molecules, and their helicenoid constituents, are also excellent chiral dopants that induce dissymmetry in the ground and excited states of the achiral emissive polymer F8BT, leading to high CPL activity. The ability to control the handedness of the helicenoid dopants via enantiopure synthesis affords control of the sign of CP emission. This manipulation of circularly polarised light is of great interest for optoelectronic technologies.

Show Abstract ▼

Abstract: INTRODUCTION: The creation of mesoscale chiroptic materials from nanoscale achiral building blocks has been previously realized through exciton coupling in organic supramolecular assemblies and with plasmons in inorganic metal nanoparticle assemblies, but extending excitonic chirality to inorganic semiconducting systems has remained elusive. Among the many challenges to achieving this goal is the need for degenerate excited electronic states, which are difficult to obtain in nanocrystals with nonzero size distributions, and the requirement for aligned transition dipoles, an unlikely characteristic in spherical nanocrystals that lack a distinct axis. This goal is consequential, as introducing chirality into the band structure of semiconductors enables simultaneous control over light, spin, and charge, key capabilities for driving innovations in next-generation photonic, optoelectronic, and spintronic technologies. RATIONALE: We hypothesized that magic-sized nanoclusters could overcome these obstacles to form an exciton-coupled chiroptic inorganic assembly. Magic-sized nanoclusters are atomically and electronically identical, enabling greater wavefunction overlap and coupling between neighboring nanoclusters. And they need not be spherical, allowing for anisotropic arrangements of their transition dipoles through dipole-dipole interactions during solution processing. Additionally, the bandgap, composition, and size of semiconducting magic-sized clusters can be rationally synthesized to tailor the chiroptic responses over a range of wavelengths. RESULTS: Following our hypothesis, we found that three different species of nanoclusters [cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe)] could be assembled through meniscus-guided evaporative processing into films with strong circular dichroic (CD) responses. Films made from CdS achieved dissymmetry factors, the CD figure of merit, as large as 1.30 for unpatterned, drop-cast films and 1.06 for patterned assemblies, values that approach the theoretical maximum. CD maps collected with Mueller matrix polarimetry showed that controlling the evaporative processing geometry enabled the creation of millimeter-scale homochiral domains that continuously transition between left- and right-handed, with their spatial organization adjustable through processing parameters. The mechanism behind these organized assemblies is elucidated through linear dichroism (LD), small-angle x-ray scattering (SAXS), and in situ microscopy, which reveal that the meniscus guides highly concentrated fibrous solutions, aligning their transition dipoles within fibrous bands as they deposit on a substrate. These bands are subsequently twisted by fluid flows, generating homochiral domains. CONCLUSION: We report on a method to form chiral films of three different inorganic semiconductor nanocrystals, with near-limit dissymmetry factors and large homochiral domains. Our results for the three different semiconducting nanocluster systems demonstrate the generality of the method, suggesting the applicability to other nanocluster species or colloidal nanoplatelets. Our experiments uncovered the key mechanisms for achieving these chiroptic films by the meniscus-guided deposition process, including the alignment of transition dipoles of the constituents and the fluid flows responsible for twisting the fibers, and make a connection to emergent chiral properties. Harnessing methods to form chiral films from achiral, solution-processable semiconductors offers an opportunity in the design and fabrication of complex chiroptical metamaterials in ways that are both scalable and versatile. Beyond nanocluster films, this study provides valuable insights into the complexities of hierarchical assembly found in nature and offers a pathway to extend these principles to other chiral molecules and nanomaterials for engineering sophisticated, twisted structures.

Open Access

Show Abstract ▼

Abstract: The nanoscale chiral arrangement in a bicomponent organic material system comprising donor and acceptor small molecules is shown to depend on the thickness of a film that is responsive to chiral light in an optoelectronic device. In this bulk heterojunction, a previously unreported chiral bis(diketopyrrolopyrrole) derivative was combined with an achiral non-fullerene acceptor. The optical activity of the chiral compound is dramatically different in the pure material and the composite, showing how the electron acceptor influences the donor’s arrangement compared with the pure molecule. Mueller matrix polarimetric imaging shows the authenticity of this effect and the homogeneity of short range chiral orientations between the molecules, as well as more heterogeneous short and longer range arrangements in the films observed in linear dichroic and birefringent effects. The two-dimensional circular dichroism (CD) maps and spectra show the uniformity of the short range supramolecular interactions both in spun-cast films on quartz and blade-coated films on photovoltaic device substrates, where evidence for the chiral arrangement is uniquely provided by the synchrotron CD measurements. The external quantum efficiency of the devices depends upon the handedness of the light used to excite them and the film thickness, that influences the supramolecular arrangement and organization in the film, and determines the selectivity for left or right circularly polarised light. The difference in external quantum efficiency of the photovoltaic devices between the two handedness’ of light correlates with the apparent differential absorbance (g-factor) of the films.

Open Access

Show Abstract ▼

Abstract: A new technology to write and read covert information in authentication labels is described. This technology uses the phenomenon of photo-induced chirality in Ge2Sb2Te5 thin films to encode the left- or right-circular or linear polarization of the laser beam used to write the label. The written polarization can be revealed by a simple reading device, which is demonstrated to provide the same qualitative information as reading based on cyclotron circular dichroism spectroscopy and imaging. The suggested method, while based on existing manufacturing approaches, offers a balance between technological complexity for writing and simplicity for reading, and may be advantageous as a new authentication technology.

Open Access

Show Abstract ▼

Abstract: Organic light-emitting diodes (OLEDs) that are able to emit high levels of circularly polarized (CP) light hold significant promise in numerous future technologies. Such devices require chiral emissive materials to enable CP electroluminescence. However, the vast majority of current OLED emitter classes, including the state-of-the-art triplet-harvesting Thermally Activated Delayed Fluorescence (TADF) materials, produce very low levels of CP electroluminescence. Here we showcase a host-guest strategy that allows for energy transfer between a chiral polymer host and a representative chiral TADF emitter. Such a mechanism results in large amplification of the circular polarization of the emitter. As such, this study presents a promising avenue to further boost the performance of CP-OLED devices, enabling their further development and eventual commercialization.