Excited State Chirality Dynamics Revealed by Using Time‐Resolved Circularly Polarized Luminescence Spectroscopy

Supramolecular Nanohelix Fabricated by Pillararene-Based Host–Guest System for Chirality Amplification, Transfer, and Circularly Polarized Luminescence in Water

Elucidating the underlying mechanism of effective chirality and energy transfer processes observed in biological assemblies has cross-disciplinary significance, and it is of special interest in the fields of chemistry and biology due to the pivotal role of chirality in life. Challenges in the field include how to achieve real-time monitoring of the chirality and energy transfer dynamics simultaneously, as well as how to distinguish whether these processes take place in the ground or excited state. Herein, we achieve the first attempt at real-time observation of the concerted ultrafast dynamics between the Förster resonance energy transfer (FRET) and the generation of circularly polarized luminescence (CPL) in the excited state in near-infrared CPL supramolecular nanofibers (SNFs) by using femtosecond time-resolved circularly polarized luminescence (fs-TRCPL) spectroscopy. Our findings reveal a cooperative interplay between FRET and CPL emission, unfolding over time scales from several to hundreds of picoseconds. Notably, we identify that the pivotal mechanism leading to a 0.045 glum value in SNFs is the difference in the FRET rates between left- and right-handed circularly polarized emission channels, which is a reason beyond the well-known relationship of the electronic and magnetic dipoles. Our results not only shed light on the understanding of the chirality transfer mechanism in the excited states but also pave the road for the development of novel CPL materials in the future.

Circularly polarized luminescence (CPL) spectroscopy measures the difference in luminescence intensity between left- and right-circularly polarized light, and is often used to analyze the structure of chiral molecules in their excited state. Recently, it has found an increasing range of applications in the analysis of molecules that emit circularly polarized light and can be employed in 3D displays. Thus, the number of articles focusing on CPL spectroscopy has increased dramatically. However, since the luminescence dissymmetry factor (glum) for organic compounds is generally <|0.01|, CPL spectrometers must offer high sensitivity and produce spectra that are artifact-free for chiral molecules. Until now, the principal targets of CPL measurements have been solution samples. However, for practical device applications, it is also necessary to be able to measure the CPL spectra of solid-state samples. In addition, since electronic devices often operate at high temperatures, it is important to evaluate the thermal dependence of the CPL characteristics. Moreover, in the measurement of solid-state samples, the degree of anisotropy of the samples must be evaluated, because a large degree of anisotropy can cause artifacts. Therefore, we describe methods to evaluate the degree of anisotropy of solid-state samples and their high-temperature applications.

Objects are chiral when they cannot be superimposed with their mirror image. Materials can emit chiral light with an excess of right- or left-handed circular polarization. This circularly polarized luminescence (CPL) is key to promising future applications, such as highly efficient displays, holography, sensing, enantiospecific discrimination, synthesis of drugs, quantum computing, and cryptography. Here, a practical guide to CPL spectroscopy is provided. First, the fundamentals of the technique are laid out and a detailed account of recent experimental advances to achieve highly sensitive and accurate measurements is given, including all corrections required to obtain reliable results. Then the most common artifacts and pitfalls are discussed, especially for the study of thin films, for example, based on molecules, polymers, or halide perovskites, as opposed to dilute solutions of emitters. To facilitate the adoption by others, custom operating software is made publicly available, equipping the reader with the tools needed for successful and accurate CPL determination.

Chiral functional materials with circularly polarized luminescence (CPL) have risen rapidly in recent years because of their fascinating characteristics and potential applications in various research fields. CPL refers to the differential spontaneous emission of left (L)- and right (R)-handed circularly polarized light upon photon or electron excitation. Generally, an outstanding CPL-active material needs to possess a high luminescence dissymmetry factor (glum) (defined as 2(IL - IR)/(IL + IR) where I is the emission intensity), which is between -2 and +2. Although the exciting development in CPL-active materials was achieved, the modulation of CPL signs is still a challenge. For small organic systems, a relatively small glum value, one of the key parameters of CPL, limits their practical applications. Searching for efficient approaches for amplifying glum is important. Therefore, over the past decades, besides optimizing the structure of small molecules, many other strategies to obtain efficient CPL-active materials have been developed. For instance, self-assembly has been well demonstrated as an effective approach to amplify the supramolecular chirality as well as the glum values. On the other hand, chiral liquid crystals (CLCs), which are capable of selective reflection of left- and right-handed circularly polarized light, also to serve as a host matrix for endowing guest emitters with CPL activity and high glum values. However, self-assembly focuses on modulating the conformation and spatial arrangement of chiral emitters. And the CPL of a luminophore-doped CLC matrix depends on the helix pitch and band gap positions. Lately, novel photophysical approaches to modulate CPL signs have gradually emerged.In this Account, we discuss the recent progress of excited-state-regulation involved CPL-active materials. The emergence, amplification, and inversion of CPL can be adjusted through regulation of the excited state of chiral emitters. For example, Förster resonance energy transfer (FRET) can amplify the glum values of chiral energy acceptors in chiral supramolecular assemblies. By combining the concepts of photon upconversion and CPL, high-energy upconverted circularly polarized emission was achieved under excitation of low-energy light, accompanied by an amplified glum. In addition, the organic systems with unpaired electrons, i.e., charge transfer (CT) system and open-shell π-radical, show favorable CPL properties, which can be flexibly tuned with an applied magnetic field. It should be noted that these photophysical process are associated with the excited state of chiral emitters. So far, while the main focus is on the regulation of the molecular and supramolecular nanostructures, direct regulation of the excited state of the chiral system will serve as a new platform to understand and regulate the CPL activity and will be helpful to develop smart chiroptical materials.

Circularly polarized luminescence spectroscopy reveals low-energy excited states and dynamic localization of vibronic transitions in CP43

Circularly polarized luminescence (CPL) spectroscopy provides information on the excited-state chirality of a lumiphore analogous but complementary to information regarding the ground-state chirality derived from circular dichroism. The sensitivity of CPL spectra to molecular conformation makes this technique uniquely suited for the study of biomolecular structure, as extensively demonstrated in earlier studies. Unfortunately, the CPL spectra of many biomolecules often contain significantly overlapping contributions from emitting species either because multiple lumiphores are present (e.g., tryptophan residues in a protein) or because multiple conformations of the biomolecule simultaneously exist, each with a unique CPL spectrum. Increased resolution between individual contributions to the CPL may be achieved by time-resolving this signal, thus taking advantage of the fact that, as a rule, each of the emitting species also has a characteristic decay time associated with its electronically excited state. In addition, the time resolution provides information regarding dynamics associated with the different chiral states of the system. The present study describes an instrument for the determination of time-resolved CPL (TR-CPL) with subnanosecond resolution and its application to several chiral systems. The technique was first demonstrated on a model system with a strong time-dependent CPL signal. Subsequently, the circularly polarized component in the fluorescence of reduced nicotinamide adenine dinucleotide (NADH) bound to liver alcohol dehydrogenase was time-resolved. The CPL of NADH in the binary enzyme-coenzyme complex is time-dependent, reflecting structural differences around the reduced nicotinamide possibly due to a dynamic restructuring. In contrast, the CPL of the coenzyme in the ternary complex formed with enzyme and the substrate analog isobutyramide is essentially time-independent, likely reflecting a more rigid binding domain. Since the linear polarization of the fluorescence of the two complexes did not show any local flexibility of the NADH chromophore, the excited-state conformational rearrangement of the binary complex indicates a subtle change in its interactions with group(s) in direct contact with it.

The interaction between Tb(III) and several α-hydroxycarboxylic acids has been studied by means of circularly polarized luminescence (CPL) spectroscopy. Complexes having the general formula Tb(DPA)(L) in ratios of 1:2:1 and 1:1:1 were studied (where DPA signifies pyridine-2,6-dicarboxylate) were studied in an attempt to use CPL spectroscopy as a probe of the solution chemistry existing between the Tb/DPA complexes and the chiral hydroxycarboxylic acids. The chiral L ligands used were L-hydroxy-isocaproic acid, L-arginnic acid, L-hydroxyglutaric acid, and D-isocitric acid. All of the ligands were found to bind Tb(III) in a bidentate fashion, except for isocitrate which appeared to be terdentate. Equilibrium quotients for the interaction of the Tb/DPA complexes were calculated from the CPL data.

Chirality transfer, induction, and circularly polarized luminescence (CPL) using supramolecular hosts, such as macrocycles and cages, have been explored for wide-ranging applications in chiral reco...

Materials that exhibit circularly polarized light characteristics are finding an increasing range of applications, such as liquid crystal display backlights, three-dimensional displays, holographic displays, plant growth control illumination, security systems for optical communications, and printing. In this chapter, we will describe the basic principles of circularly polarized luminescence (CPL) spectroscopy, the instruments used to measure CPL signals, including optical systems and signal acquisition methods in commercially available CPL instruments, and also methods for calibrating such equipment. Example measurements are presented for camphor and camphorquinone, which are long-established CPL samples, in addition to lanthanoid complexes that are promising CPL luminescent materials, biopolymers that are highly sensitive to circularly polarized light, and solid samples that have been attracting attention in recent years.

Bilayer graphene (BLG), which changes its electronic properties significantly depending on the interplanar distance and rotation angle, is a hot research topic. We successfully synthesized discrete BLG models by bridging two coronenes with a 1,8‐naphthalene linkage (C2). Owing to the large rotational energy, the syn‐ and anti‐atropisomers of C2 could be isolated. X‐ray crystallographic analysis revealed that the syn‐form (C2S) is an AB stacking type and the anti‐form (C2A) is a chiral twisted bilayer graphene (TBG) type. Therefore, the anti‐form of C2 was optically resolved and characterized by circular dichroism (CD) and circularly polarized luminescence (CPL) spectroscopy. The Cotton effect was observed in the CD spectra at the longest absorption wavelength, and the corresponding bisignate CPL spectra were observed. The dissymmetry factors |glum| of C2A were found to be 2.0×10−3 at 435 nm and 1.2×10−3 at 450 nm.

The luminescence and circularly polarized luminescence (CPL) spectra of M(I)[Eu((+)-hfbc)(4)] show a similar behavior to the exciton CD in the intraligand π-π* transitions when the alkali metal ions and solvents are manipulated. There is a difference in susceptibility in solvation toward the alkali metal ions but not toward the Eu(III) ion, as in the case of axially symmetric DOTA-type compounds. The remarkable CPL in the 4f-4f transitions provide much more information on the stereospecific formation of chiral Eu(III) complexes, since CPL spectroscopy is limited to luminescent species and reflects selectively toward helicity of the local structural environment around the lanthanide(III). While in comparison, exciton CD reveals the chiral structural information from the helical arrangement of the four bladed chelates. Of special importance, the observation of the highest CPL activities measured to date for lanthanide(III)-containing compounds (i.e., Eu and Sm) in solution supports the theory that the chirality of lanthanide(III) in the excited state corresponds to that in the ground state, which was derived from the exciton CD.

Circularly Polarised Luminescence (CPL) spectroscopy often deals with the generation of circularly polarised photons from macroscopically isotropic systems, such as liquid or solid solutions. The magnitude of CPL transitions depends on inherent specific properties of chiral chromophores/luminophores, which is connected to the strength and mutual intra-molecular orientation of the molecular electronic and magnetic transition dipoles. The present work aims to theoretically investigate the influence of orientation and dynamics on CPL emission for luminophores dissolved in microscopic and macroscopic anisotropic systems. Commonly studied systems of this kind are (liquid) crystals, lipid membranes, or model membranes. Typically, they exhibit uniaxial symmetry, and the luminophores considered may undergo fast, intermediate, or negligible reorientations on the timescale of luminescence emission. CPL from anisotropic and isotropic systems have been compared and the obtainable molecular information is described and discussed. Macroscopically isotropic systems, which are simultaneously microscopic anisotropic, are here exemplified by lipid vesicles. Taken together, a fundamental theoretical treatment is presented with connection to various experimental conditions. In brief, different experimental setups and aspects for monitoring the CPL emission are described, as well as commented on.

The source of optical artifacts in circularly polarized luminescence is examined by application of the method of Mueller matrices. Expressions for the circularly polarized luminescence intensity are developed that illustrate that the source of the artifact in most cases is due to the small inherent birefringence in the photoelastic modulator employed in the experimental measurement. Experimental results are presented that are consistent with this conclusion.

Chirality Transfer from Chiral Mesoporous Silica to Perovskite CsPbBr ₃ Nanocrystals: The Role of Chiral Confinement