Giant intrinsic circular dichroism of prolinol-derived squaraine thin films

Mueller matrix polarimetry reveals chiroptical properties of metal chelates in solutions

Proper characterization of molecular chiroptical properties is vital for organic chemistry and drug development. Nonetheless, narrow spectral ranges and the necessity for specialized equipment often limit traditional methods such as optical rotatory dispersion and electronic circular dichroism. Here, we introduce Mueller matrix polarimetry (MMP) as a more versatile tool for chiroptical analysis, capable of simultaneously capturing circular dichroism and optical rotatory dispersion spectra across ultraviolet to near-infrared wavelengths in a single measurement. We applied MMP to chiral metal complexes of Al, Mn, and Co, commonly used as catalysts in asymmetric syntheses. Using a robust experimental methodology, MMP distinguished enantiomeric forms and provided reliable chiroptical information by leveraging the inherent relationship between circular dichroism and optical rotatory dispersion. We interpreted our findings on the basis of density functional theory simulations, compared them to traditional electronic circular dichroism and absorption spectroscopies, and performed the Kramers-Kronig analysis. The combined approach of chiroptical MMP and ab-initio, for example, reveals delicate near-infrared chiroptical spectra of a neutral cobalt metal complex. Although MMP is more commonly used for solid state, the developed experimental protocol significantly expands its capabilities to solutions. It allows measurements without the need for both enantiomers and offers new insights into molecular chirality with potential applications across traditional and interdisciplinary branches of science and industry.

Angle-Insensitive Broadband Multispectro-Polarimetric Encoding Based on Inverse Design of Mosaic Metasurfaces

Multimodal optical information detection allows precise characterization of light-matter interactions. Current multimodal optical detection systems often encounter challenges such as low spectral resolution, limited polarization sensitivity, or bulky designs that impede seamless integration. Here, a broadband multispectro-polarimetric filter (BMSPF) metasurface array enables simultaneous spectral is proposed and full-Stokes polarimetric imaging in a single snapshot. By combining Metal-Insulator-Metal (MIM) metasurfaces with a mosaic optimization design, angle-robust multispectro-polarimetric encoding under a 30° field-of-view is achieved. The encoded grayscale information is captured via a monochrome image sensor and decoded using a neural network. Operating across the visible-to- near-infrared range (400-1100 nm), the system attains an impressive spectral resolution of 6 nm. This method expands the application scenarios of multispectro-polarimetric imaging and provides a pathway for the manufacturability of metasurface devices.

Disentangling signal contributions in two-dimensional electronic spectroscopy in the pump-probe geometry

Since its introduction almost three decades ago, two-dimensional electronic spectroscopy (2DES) has evolved into a mature and powerful technique to reveal the inner workings of quantum systems with high temporal and spectral resolution. In general, this technique can isolate different contributions to the nonlinear response and provides access to different dynamical quantum pathways of the system evolution. Such isolation of pathways can be achieved in different experimental geometries. In its original, fully noncollinear implementation, directional phase matching allows for such signal isolation, while in the modern commonly employed pump-probe geometry, experimentally challenging phase-cycling schemes are employed. Here, we show how rephasing, non-rephasing, and zero- and double-quantum 2DES signals can be isolated in the pump-probe geometry without a need for phase-cycling. For this, we utilize established causality restrictions of the nonlinear response, allowing us to separate the different contributions in the spectral domain. We demonstrate this using data recorded for a molecular J-aggregate, acting as an effective three-level system. This approach bridges the gap between the capabilities of shaper-based and fully noncollinear 2DES and experimentally simpler implementations, such as those based on birefringent common-path interferometers.

Macroscopically Self-Aligned and Chiralized Carbon Nanotubes: From Filtration to Innovation

Because of their natural 1D structure combined with intricate chiral variations, carbon nanotubes (CNTs) exhibit various exceptional physical properties, such as ultrahigh thermal conductivity and diameter-dependent electrical behavior, ranging from semiconducting to metallic. While CNTs excel individually at the nanoscale, their 1D and chiral nature can be lost on a macroscopic scale when they are randomly assembled. Therefore, the alignment and organization of CNTs in macroscopic structures is crucial for harnessing their full potential. In this Review, we explore recent advancements in understanding CNT alignment mechanisms, improving CNT aligning methods, focusing particularly on the controlled vacuum filtration technique, and demonstrating macroscopically 1D properties of ordered CNT assemblies. We also focus on a recently identified class of CNT architectures, combining CNT alignment and twisting mechanisms to create artificial radial and chiral CNT films at wafer scales. Finally, we summarize recent developments related to aligned and chiral CNT films in optoelectronics, highlighting their distinctive roles in nonlinear optics, thermal emission, and light modulation.

Nanohelix Arrays with Giant Circular Dichroism through Patch-Enthalpy-Driven Self-Confined Self-Assembly of Janus Nanoparticles

Plasmonic nanohelix arrays, exhibiting strong circular dichroism, are among the most promising optical chiral metamaterials. However, achieving chiral plasmonic effects in the visible range remains challenging with current manufacturing techniques, as it requires structures small enough to resonate at visible wavelengths. Herein, we propose a novel strategy for constructing nanohelix arrays through patch-enthalpy-driven self-confined self-assembly of Janus nanoparticles. The hexagonal columnar structures, self-assembled from Janus nanoparticles, create a cylindrical self-confined environment within each column, where patch-enthalpy drives the particles to form helical structures. Numerical simulations reveal that patch-enthalpy induces the sequential formation of helical structures within each column, from multiple helices to double helix and finally to single helix. Additionally, optical property calculations demonstrate that these nanohelix arrays exhibit giant circular dichroism and high g-factors at visible frequencies. Our proposed construction strategy offers a promising route for developing optical chiral metamaterials through patch-enthalpy-driven self-confined self-assembly of Janus nanoparticles.

Enabling solid sample analysis in liquid spectrophotometers with a 3D-printed cuvette

Most commercial systems for ultraviolet-visible (UV-VIS), Fourier-transform infrared, circular dichroism (CD), and fluorescence spectroscopies are designed for measurement of liquid samples. Moreover, adapters enabling the measurement of solid samples are expensive or unavailable for most commercial instruments. Consequently, there is a significant need for solid sample adapters that enable measurement of both liquid and solid samples with a single system. Here, we report two versions of a solid sample adapter cuvette that can be used in most commercial spectroscopy instruments designed for transmission measurement of liquid samples. One version is designed for techniques that do not require changing the sample orientation, and the other allows easy sample rotation. We successfully fabricated these cuvettes by 3D printing with both fused deposition modeling and stereolithography and demonstrated how they enable us to study the optical properties of macroscopic films of aligned carbon nanotubes by performing UV-VIS and CD spectroscopy measurements with the cuvettes. These 3D printed cuvettes and their implementation will help enable a wide range of experiments at a low cost.

Film thickness dependence of nanoscale arrangement of a chiral electron donor in its blends with an achiral electron acceptor

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.

Transforming achiral semiconductors into chiral domains with exceptional circular dichroism

Highly concentrated solutions of asymmetric semiconductor magic-sized clusters (MSCs) of cadmium sulfide, cadmium selenide, and cadmium telluride were directed through a controlled drying meniscus front, resulting in the formation of chiral MSC assemblies. This process aligned their transition dipole moments and produced chiroptic films with exceptionally strong circular dichroism. G-factors reached magnitudes as high as 1.30 for drop-cast films and 1.06 for patterned films, approaching theoretical limits. By controlling the evaporation geometry, various domain shapes and sizes were achieved, with homochiral domains exceeding 6 square millimeters that transition smoothly between left- and right-handed chirality. Our results uncovered fundamental relationships between meniscus deposition processes, the alignment of supramolecular filaments and their MSC constituents, and their connection to emergent chiral properties.

Enhanced circularly polarized luminescence attained via self-assembly of heterochiral as opposed to homochiral dipeptides in water

Circularly polarized luminescence (CPL) is gaining interest across various disciplines, including materials science, pharmaceuticals, and sensing technologies. Organic molecules, due to their ease of synthesis and reduced toxicity, are a focus for achieving high dissymmetry values (g lum) in CPL. Here, we present a low molecular weight molecule (1), a dipeptide (Ala-Phe) covalently linked with tetraphenyl-ethylene (TPE), an Aggregation-Induced Emission luminophore (AIE-gen). Varying the stereochemistry of amino acid chiral centers, we synthesized homochiral 1-(l, l) & 1-(d, d) and heterochiral 1-(l, d) and 1-(d, l). In aqueous media, these molecules exhibit aggregation-induced chirality at the TPE chromophore. Heterochiral systems form sheet-like structures, displaying a bisignate induced circular dichroism signal and a good g lum value for CPL [7.5 (±0.04) × 10-3]. Conversely, homochiral systems adopt fibrillar morphology, exhibiting a monosignate induced circular dichroism signal with a lower dissymmetry value for CPL [1.3 (±0.05) × 10-3]. This study introduces the concept of chiroptical amplification, emphasizing enhanced CPL through heterochiral peptide-induced CPL compared to its homochiral counterpart, with an ON and OFF CPL signal at low and high temperature respectively.

Strain-Amplified Exciton Chirality in Organic-Inorganic Hybrid Materials

Chiral organic-inorganic hybrids combining chirality of organic molecules and semiconducting properties of inorganic frameworks generate chiral excitons without external spin injection, creating the potential for chiroptoelectronics. However, the relationship between molecular chirality and exciton chirality is still unclear. Here we show the strain-amplified exciton chirality in one-dimensional chiral metal halides. Utilizing chirality-induced spin-orbital coupling theory, we quantitatively demonstrate the impact of the strain-engineered molecular assembly of chiral cations on exciton chirality, offering a feasible way to amplify exciton chirality by molecular manipulation.

Impact of Temperature on the Chiroptical Properties of Thin Films of Chiral Thiophene-based Oligomers

According to the theoretical model based on the Mueller matrix approach, the experimental electronic circular dichroism (ECD) for thin films of chiral organic dyes can be expressed as the sum of several contributions, two of which are the most significant: 1) an intrinsic component (CDiso) invariant upon sample orientation, reflecting the molecular and/or supramolecular chirality, due to 3D-chiral nanoscopic structures; 2) a non-reciprocal component (LDLB) which inverts its sign upon sample flipping, which arises from the interaction of linear dichroism and linear birefringence in locally anisotropic domains, expression of 2D-chiral micro/mesoscopic structures. In this work, we followed in parallel through ECD and differential scanning calorimetry (DSC) the temperature evolution of the supramolecular arrangements of thin films of five structurally related chiral thiophene-based oligomers with different LDLB/CDiso ratio. By increasing the temperature, regardless of phase transitions observed by DSC analysis, systems with strong CDiso revealed no changes in the ECD spectrum, while compounds with dominant LDLB contribution underwent a gradual (and reversible) reduction of (apparent) ECD signals. These findings demonstrated that the concomitant occurrence of intrinsic and non-reciprocal components in the ECD spectrum of thin films of chiral organic dyes is strictly correlated with solid-state organizations of different stability.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Controlling the broadband enhanced light chirality with L-shaped dielectric metamaterials

The inherently weak chiroptical responses of natural materials limit their usage for controlling and enhancing chiral light-matter interactions. Recently, several nanostructures with subwavelength scale dimensions were demonstrated, mainly due to the advent of nanofabrication technologies, as a potential alternative to efficiently enhance chirality. However, the intrinsic lossy nature of metals and the inherent narrowband response of dielectric planar thin films or metasurface structures pose severe limitations toward the practical realization of broadband and tailorable chiral systems. Here, we tackle these problems by designing all-dielectric silicon-based L-shaped optical metamaterials based on tilted nanopillars that exhibit broadband and enhanced chiroptical response in transmission operation. We use an emerging bottom-up fabrication approach, named glancing angle deposition, to assemble these dielectric metamaterials on a wafer scale. The reported strong chirality and optical anisotropic properties are controllable in terms of both amplitude and operating frequency by simply varying the shape and dimensions of the nanopillars. The presented nanostructures can be used in a plethora of emerging nanophotonic applications, such as chiral sensors, polarization filters, and spin-locked nanowaveguides.

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.

Plasmon mediated coherent population oscillations in molecular aggregates

The strong coherent coupling of quantum emitters to vacuum fluctuations of the light field offers opportunities for manipulating the optical and transport properties of nanomaterials, with potential applications ranging from ultrasensitive all-optical switching to creating polariton condensates. Often, ubiquitous decoherence processes at ambient conditions limit these couplings to such short time scales that the quantum dynamics of the interacting system remains elusive. Prominent examples are strongly coupled exciton-plasmon systems, which, so far, have mostly been investigated by linear optical spectroscopy. Here, we use ultrafast two-dimensional electronic spectroscopy to probe the quantum dynamics of J-aggregate excitons collectively coupled to the spatially structured plasmonic fields of a gold nanoslit array. We observe rich coherent Rabi oscillation dynamics reflecting a plasmon-driven coherent exciton population transfer over mesoscopic distances at room temperature. This opens up new opportunities to manipulate the coherent transport of matter excitations by coupling to vacuum fields.

Chiral Diketopyrrolopyrrole-Based Conjugated Polymers with Intramolecular Rotation-Isomeric Conformation Asymmetry for Near-Infrared Circularly Polarized Light-Sensing Organic Phototransistors

Recent advances in chiral nanomaterials interacting with circularly polarized (CP) light open new expectations for optoelectronics in various research fields such as quantum- and biology-related technology. To fully utilize the great potential of chiral optoelectronic devices, the development of chiral optoelectronic devices that function in the near-infrared (NIR) region is required. Herein, we demonstrate a NIR-absorbing, chiroptical, low-band-gap polymer semiconductor for high-performance NIR CP light phototransistors. A newly synthesized diketopyrrolopyrrole-based donor-acceptor-type chiral π-conjugated polymer with an asymmetric alkyl side chain exhibits strong chiroptical activity in a wavelength range of 700-1000 nm. We found that the attachment of an enantiomerically pure stereogenic alkyl substituent to the π-conjugated chromophore backbone led to strong chiroptical activity through symmetry breaking of the π-conjugation of the backbone in a molecular rotational motion while maintaining the coplanar backbone conformation for efficient charge transport. The NIR CP light-sensing phototransistors based on a chiral π-conjugated polymer photoactive single channel layer exhibit a high photoresponsivity of 26 A W-1 under NIR CP light irradiation at 920 nm, leading to excellent NIR CP light distinguishability. This study will provide a rationale and strategy for designing chiral π-conjugated polymers for high-performance NIR chiral optoelectronics.

Shortwave Infrared InGaAs Detectors On-Chip Integrated with Subwavelength Polarization Gratings

Shortwave infrared polarization imaging can increase the contrast of the target to the background to improve the detection system's recognition ability. The division of focal plane polarization indium gallium arsenide (InGaAs) focal plane array (FPA) detector is the ideal choice due to the advantages of compact structure, real-time imaging, and high stability. However, because of the mismatch between nanostructures and photosensitive pixels as well as the crosstalk among the different polarization directions, the currently reported extinction ratio (ER) of superpixel-polarization-integrated detectors cannot meet the needs of high-quality imaging. In this paper, a 1024 × 4 InGaAs FPA detector on-chip integrated with a linear polarization grating (LPG) was realized and tested. The detector displayed good performance throughout the 0.9-1.7 um band, and the ERs at 1064 nm, 1310 nm and 1550 nm reached up to 22:1, 29:1 and 46:1, respectively. For the crosstalk investigation, the optical simulation of the grating-integrated InGaAs pixel was carried out, and the limitation of the ER was calculated. The result showed that the scattering of incident light in the InP substrate led to the crosstalk. Moreover, the deviation of the actual grating morphology from the designed structure caused a further reduction in the ER.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies g_abs of up to 2.6 × 10^-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.

Enantiopure J-Aggregate of Quaterrylene Bisimides for Strong Chiroptical NIR-Response

Chiral polycyclic aromatic hydrocarbons can be tailored for next-generation photonic materials by carefully designing their molecular as well as supramolecular architectures. Hence, excitonic coupling can boost the chiroptical response in extended aggregates but is still challenging to achieve by pure self-assembly. Whereas most reports on these potential materials cover the UV and visible spectral range, systems in the near infrared (NIR) are underdeveloped. We report a new quaterrylene bisimide derivative with a conformationally stable twisted π-backbone enabled by the sterical congestion of a fourfold bay-arylation. Rendering the π-subplanes accessible by small imide substituents allows for a slip-stacked chiral arrangement by kinetic self-assembly in low polarity solvents. The well dispersed solid-state aggregate reveals a sharp optical signature of strong J-type excitonic coupling in both absorption (897 nm) and emission (912 nm) far in the NIR region and reaches absorption dissymmetry factors up to 1.1 × 10^-2. The structural elucidation was achieved by atomic force microscopy and single-crystal X-ray analysis which we combined to derive a structural model of a fourfold stranded enantiopure superhelix. We could deduce that the role of phenyl substituents is not only granting stable axial chirality but also guiding the chromophore into a chiral supramolecular arrangement needed for strong excitonic chirality.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Controlling anisotropic properties by manipulating the orientation of chiral small molecules

Chiral π-conjugated molecules bring new functionality to technological applications and represent an exciting, rapidly expanding area of research. Their functional properties, such as the absorption and emission of circularly polarized light or the transport of spin-polarized electrons, are highly anisotropic. As a result, the orientation of chiral molecules critically determines the functionality and efficiency of chiral devices. Here we present a strategy to control the orientation of a small chiral molecule (2,2'-dicyano[6]helicene) by the use of organic and inorganic templating layers. Such templating layers can either force 2,2'-dicyano[6]helicene to adopt a face-on orientation and self-assemble into upright supramolecular columns oriented with their helical axis perpendicular to the substrate, or an edge-on orientation with parallel-lying supramolecular columns. Through such control, we show that low- and high-energy chiroptical responses can be independently 'turned on' or 'turned off'. The templating methodologies described here provide a simple way to engineer orientational control and, by association, anisotropic functional properties of chiral molecular systems for a range of emerging technologies.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm^-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

Synthesis and Characterization of Newly Designed and Highly Solvatochromic Double Squaraine Dye for Sensitive and Selective Recognition towards Cu2

Synthesis and characterization of a novel and zwitterionic double squaraine dye (DSQ) with a unique D-A-A-D structure is being reported. Contrary to the conventional mono and bis-squaraine dyes with D-A-D and D-A-D-A molecular frameworks reported so far, DSQ dye demonstrated strong solvatochromism allowing for the multiple ion sensing using a single probe by judicious selection of the suitable solvent system. The DSQ dye exhibited a large solvatochromic shift of about 200 nm with color changes from the visible to NIR region with metal ion sensitivity. Utilization of a binary solvent consisted of dimethylformamide and acetonitrile (1:99, v/v), highly selective detection of Cu²⁺ ions with the linearity range from 50 μM to 1 nM and a detection limit of 6.5 × 10^-10 M has been successfully demonstrated. Results of the Benesi-Hildebrand and Jobs plot analysis revealed that DSQ and Cu²⁺ ions interact in the 2:1 molecular stoichiometry with appreciably good association constant of 2.32 × 10⁴ M^-1. Considering the allowed limit of Cu²⁺ ions intake by human body as recommended by WHO to be 30 μM, the proposed dye can be conveniently used for the simple and naked eye colorimetric monitoring of the drinking water quality.

Template and Temperature-Controlled Polymorph Formation in Squaraine Thin Films

Controlling the polymorph formation in organic semiconductor thin films by the choice of processing parameters is a key factor for targeted device performance. Small molecular semiconductors such as the prototypical anilino squaraine compound with branched butyl chains as terminal functionalization (SQIB) allow both solution and vapor phase deposition methods. SQIB has been considered for various photovoltaic applications mainly as amorphous isotropic thin films due to its broad absorption within the visible to deep-red spectral range. The two known crystalline polymorphs adopting a monoclinic and orthorhombic crystal phase show characteristic Frenkel excitonic spectral signatures of overall H-type and J-type aggregates, respectively, with additional pronounced Davydov splitting. This gives a recognizable polarized optical response of crystalline thin films suitable for identification of the polymorphs. Both phases emerge with a strongly preferred out-of-plane and rather random in-plane orientation in spin-casted thin films depending on subsequent thermal annealing. By contrast, upon vapor deposition on dielectric and conductive substrates, such as silicon dioxide, potassium chloride, graphene, and gold, the polymorph expression depends basically on the choice of growth substrate. The same pronounced out-of-plane orientation is adopted in all crystalline cases, but with a surface templated in-plane alignment in case of crystalline substrates. Strikingly, the amorphous isotropic thin films obtained by vapor deposition cannot be crystallized by thermal postannealing, which is a key feature for the spin-casted thin films, here monitored by polarized in situ microscopy. Combining X-ray diffraction, atomic force microscopy, ellipsometry, and polarized spectro-microscopy, we identify the processing-dependent evolution of the crystal phases, correlating morphology and molecular orientations within the textured SQIB films.

Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin Films

We observed efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene-type chiral additives based on the dibenzo[c,h]acridine motif. Images obtained from circular dichroism (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction-limited resolution. The CD signal shows a characteristic dependence on the film thickness, which supports a supramolecular origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resolution. A systematic variation of the enantiomeric excess of the chiral additive shows that the "Sergeants and Soldiers" concept and "Majority Rules" are not obeyed.

Chiral Non-Fullerene Acceptor Enriched Bulk Heterojunctions Enable High-Performance Near-Infrared Circularly Polarized Light Detection

Organic photodetectors that can sensitively convert near-infrared (NIR) circularly polarized light (CPL) into modulable electrical signals have promising applications in spectroscopy, imaging, and communications. However, the preparation of chiral NIR organic photodetectors with simultaneously high dissymmetry factor, responsivity, detectivity, and response speed is challenging. Here, direct CPL detectors based on the bulk heterojunctions (BHJs) of chiral BTP-4Cl non-fullerene acceptor with dilute achiral PM6 donor are constructed, which successfully address these issues. The chiral acceptor-enriched BHJs with a donor/acceptor ratio of 1/10 achieve an optimal trade-off between chiroptical properties and optoelectronic performance. The supramolecular chirality from the acceptor aggregates provides the BHJs with a true absorption dissymmetry factor (gabs ) of ±0.02 at 830 nm, the highest value among NIR-sensitive detectors, which endows the photodetector with a photocurrent dissymmetry factor (gsc ) of ±0.03. Impressively, the photodetector demonstrates an external quantum efficiency as high as 60%, a responsivity of 0.4 A W-1 , a detectivity of 3 × 1011 Jones (based on noise current), and a fast response speed on the microsecond scale with the -3 dB bandwidth over 7000 Hz in the NIR region. This study exhibits a promising strategy for building high-performing direct NIR CPL detectors by introducing supramolecular chirality into BHJs.

Best practices in the measurement of circularly polarised photodetectors

Circularly polarised light will revolutionise emerging technologies, including encrypted light-based communications, quantum computing, bioimaging and multi-channel data processing. In order to make use of these remarkable opportunities, high performance photodetectors that can accurately differentiate between left- and right-handed circularly polarised light are desperately needed. Whilst this potential has resulted in considerable research interest in chiral materials and circularly polarised photodetecting devices, their translation into real-world technologies is limited by non-standardised reporting and testing protocols. This mini-review provides an accessible introduction into the working principles of circularly polarised photodetectors and a comprehensive overview of the performance metrics of state-of-the-art devices. We propose a rigorous device characterisation procedure that will allow for standardised evaluation of novel devices, which we hope will accelerate research and investment in this area.

Liquid crystal-templated chiral nanomaterials: from chiral plasmonics to circularly polarized luminescence

Chiral nanomaterials with intrinsic chirality or spatial asymmetry at the nanoscale are currently in the limelight of both fundamental research and diverse important technological applications due to their unprecedented physicochemical characteristics such as intense light-matter interactions, enhanced circular dichroism, and strong circularly polarized luminescence. Herein, we provide a comprehensive overview of the state-of-the-art advances in liquid crystal-templated chiral nanomaterials. The chiroptical properties of chiral nanomaterials are touched, and their fundamental design principles and bottom-up synthesis strategies are discussed. Different chiral functional nanomaterials based on liquid-crystalline soft templates, including chiral plasmonic nanomaterials and chiral luminescent nanomaterials, are systematically introduced, and their underlying mechanisms, properties, and potential applications are emphasized. This review concludes with a perspective on the emerging applications, challenges, and future opportunities of such fascinating chiral nanomaterials. This review can not only deepen our understanding of the fundamentals of soft-matter chirality, but also shine light on the development of advanced chiral functional nanomaterials toward their versatile applications in optics, biology, catalysis, electronics, and beyond.

Chiral carbon dots: synthesis, optical properties, and emerging applications

Carbon dots are luminescent carbonaceous nanoparticles that can be endowed with chiral properties, making them particularly interesting for biomedical applications due to their low cytotoxicity and facile synthesis. In recent years, synthetic efforts leading to chiral carbon dots with other attractive optical properties such as two-photon absorption and circularly polarized light emission have flourished. We start this review by introducing examples of molecular chirality and its origins and providing a summary of chiroptical spectroscopy used for its characterization. Then approaches used to induce chirality in nanomaterials are reviewed. In the main part of this review we focus on chiral carbon dots, introducing their fabrication techniques such as bottom-up and top-down chemical syntheses, their morphology, and optical/chiroptical properties. We then consider emerging applications of chiral carbon dots in sensing, bioimaging, and catalysis, and conclude this review with a summary and future challenges.

Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Two-dimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton-plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule-gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.

Chiral Optoelectronic Functionalities via DNA-Organic Semiconductor Complex

We fabricate the bio-organic field-effect transistor (BOFET) with the DNA-perylene diimide (PDI) complex, which shows unusual chiroptical and electrical functionalities. DNA is used as the chirality-inducing scaffold and the charge-injection layer. The shear-oriented film of the DNA-PDI complex shows how the large-area periodic molecular orientation and the charge transport are related, generating drastically different optoelectronic properties at each DNA/PDI concentration. The resultant BOFET reveals chiral structures with a high charge carrier mobility, photoresponsivity, and photosensitivity, reaching 3.97 cm² V^-1 s^-1, 1.18 A W^-1, and 7.76 × 10³, respectively. Interestingly, the BOFET enables the definitive response under the handedness of circularly polarized light with a high dissymmetry factor of approximately +0.14. This work highlights the natural chirality and anisotropy of DNA material and the electron conductivity of organic semiconducting molecules to be mutually used in significant chiro-optoelectronic functions as an added ability to the traditional OFET.

Polarisation optics for biomedical and clinical applications: a review

Many polarisation techniques have been harnessed for decades in biological and clinical research, each based upon measurement of the vectorial properties of light or the vectorial transformations imposed on light by objects. Various advanced vector measurement/sensing techniques, physical interpretation methods, and approaches to analyse biomedically relevant information have been developed and harnessed. In this review, we focus mainly on summarising methodologies and applications related to tissue polarimetry, with an emphasis on the adoption of the Stokes-Mueller formalism. Several recent breakthroughs, development trends, and potential multimodal uses in conjunction with other techniques are also presented. The primary goal of the review is to give the reader a general overview in the use of vectorial information that can be obtained by polarisation optics for applications in biomedical and clinical research.

Circularly Polarized Photodetectors Based on Chiral Materials: A Review

Circularly polarized light (CPL) plays an important role in many photonic techniques, including tomographic scanning based on circular polarization ellipsometry, optical communication and information of spin, and quantum-based optical calculation and information processing. To fully exploit the functions of CPL in these fields, integrated photoelectric sensors capable of detecting CPL are essential. Photodetectors based on chiral materials can directly detect CPL due to their intrinsic optical activity, without the need to be coupled with polarizers and quarter-wave plates as in conventional photodetectors. This review summarizes the recent research progress in CPL photodetectors based on chiral materials. We first briefly introduce the CPL photodetectors based on different types of chiral materials and their working principles. Finally, current challenges and future opportunities in the development of CPL photodetectors are prospected.

Optical Chirality Determined from Mueller Matrices

Optical chirality, in terms of circular birefringence and circular dichroism, is described by its electromagnetic and magnetoelectric material tensors, and the corresponding optical activity contributes to the Mueller matrix. Here, spectroscopic ellipsometry in the spectral range 210–1690 nm is used to address chiral phenomena by measuring Mueller matrices in transmission. Three approaches to determine chirality parameters are discussed. In the first approach, applicable in the absence of linear polarization effects, circular birefringence and circular dichroism are evaluated directly from elements of a Mueller matrix. In the second method, differential decomposition is employed, which allows for the unique separation of chirality parameters from linear anisotropic parameters as well as from depolarization provided that the sample is homogeneous along the optical path. Finally, electromagnetic modeling using the Tellegen constitutive relations is presented. The last method also allows structural effects to be included. The three methods to quantify optical chirality are demonstrated for selected materials, including sugar solutions, α-quartz, liquid crystals, beetle cuticle, and films of cellulose nanocrystals.

Pathways to increase the dissymmetry in the interaction of chiral light and chiral molecules

The dissymmetric interaction between circularly polarised (CP) light and chiral molecules is central to a range of areas, from spectroscopy and imaging to next-generation photonic devices. However, the selectivity in absorption or emission of left-handed versus right-handed CP light is low for many molecular systems. In this perspective, we assess the magnitude of the measured chiroptical response for a variety of chiral systems, ranging from small molecules to large supramolecular assemblies, and highlight the challenges towards enhancing chiroptical activity. We explain the origins of low CP dissymmetry and showcase recent examples in which molecular design, and the modification of light itself, enable larger responses. Our discussion spans spatial extension of the chiral chromophore, manipulation of transition dipole moments, exploitation of forbidden transitions and creation of macroscopic chiral structures; all of which can increase the dissymmetry. Whilst the specific strategy taken to enhance the dissymmetric interaction will depend on the application of interest, these approaches offer hope for the development and advancement of all research fields that involve interactions of chiral molecules and light.

Polymorphism in Squaraine Dye Aggregates by Self-Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J-Aggregate Nanosheets

A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.

Controlling Ultra-Large Optical Asymmetry in Amorphous Molecular Aggregations

Although ultra-large optical asymmetry appears in crystalline materials, distractions from the mesoscopic ordering often causes inauthenticity in chiropticity. In amorphous materials, however, it remains challenging and elusive to achieve large chiropticity. Herein, we report the quantitative control of chiral amplification, on amorphous supramolecular structures of cholesteryl-linked bis(dipyrrinato)zinc(II), to an exceptionally high level. A proper chiral packing of the building block at several molecular scale considerably contributes to the absorptive dissymmetry factor g_abs , although the system is overall disordered. The intense and tunable aggregation strength renders a variable g_abs value up to +0.10 and +0.31 in the solution and in film state. On this basis, a superior ON-OFF switching of chiropticity is realized under external stimuli. This work establishes a general design principle to control over ultra-large optical asymmetry on a wider scope of chiral materials.

π-Extended perylene diimide double-heterohelicenes as ambipolar organic semiconductors for broadband circularly polarized light detection

Despite great challenges, the development of new molecular structures with multiple and even conflicting characteristics are eagerly pursued for exploring advanced applications. To develop high-performance chiral organic semiconducting molecules, a distorted π-system is required for strong coupling with circularly polarized light (CPL), whereas planar π-stacking systems are necessary for high charge-carrier mobility. To address this dilemma, in this work, we introduce a skeleton merging approach through distortion of a perylene diimide (PDI) core with four fused heteroaromatics to form an ortho-π-extended PDI double-[7]heterohelicene. PDI double helicene inherits a high dissymmetry factor from the helicene skeleton, and the extended π-planar system concurrently maintains a high level of charge transport properties. In addition, ortho-π-extension of the PDI skeleton brings about near-infrared (NIR) light absorption and ambipolar charge transport abilities, endowing the corresponding organic phototransistors with high photoresponsivity of 450 and 120 mA W-1 in p- and n-type modes respectively, along with a high external quantum efficiency (89%) under NIR light irradiations. Remarkably, these multiple characteristics enable high-performance broadband CPL detections up to NIR spectral region with chiral organic semiconductors.

Helicity Control in the Aggregation of Achiral Squaraine Dyes in Solution and Thin Films

Squaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co-assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co-assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin-filtering measurements using mc-AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart.

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.

Surface-Doped Quasi-2D Chiral Organic Single Crystals for Chiroptical Sensing

Chiral organic optoelectronics using circularly polarized light (CPL) as the key element in the photonic signal has recently emerged as a next-generation photonic technology. However, it remains challenging to simultaneously achieve high polarization selectivity and superior optoelectronic performance. Supramolecular two-dimensional (2D) chiral organic single crystals may be good candidates for this purpose due to their defect-free nature, molecular diversity, and morphologies. Here, quasi-2D single crystals of chiral perylene diimides with parallelogram and triangle/hexagon morphologies have been selectively fabricated via self-assembly using different cosolvent systems. These materials exhibit amplified circular dichroism (CD) spectral signals, due to their molecular packing modes and supramolecular chirality. Through molecular surface n-doping using hydrazine, chiral single crystals exhibit electron mobility surpassing 1.0 cm² V^-1 s^-1, which is one of the highest among chiral organic semiconductors, and excellent optoelectronic functions. Theoretical calculations reveal that the radical anions formed by n-doping increase the electron affinity and/or reduce the energy gap, thus facilitating electron transport. More importantly, the doped organic chiral crystals selectively discriminate CPL handedness with a high anisotropy factor of photoresponsivity (∼0.12). These results demonstrate that surface-doped quasi-2D chiral organic single crystals are highly promising for chiral optoelectronics.

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.

Emergent Nonreciprocal Circularly Polarized Emission from an Organic Thin Film

Controlling circularly polarized (CP) emission is key for both fundamental understanding and applications in the field of chiral photonics and electronics. Here, a completely new way to achieve this goal is presented. A luminescent thin film, made from a chiral conjugated phenylene bis-thiophenylpropynone able to self-assemble into ordered structures, emits highly circularly polarized light with opposite handedness from its two opposite faces. Such emergent nonreciprocal behavior in CP emission, so far unprecedented, represents a fundamental advance, opening new opportunities in design, preparation, and applications of CP emitting materials.