Circularly Polarized Luminescence Signals Research Articles

Circularly Polarized Luminescence (CPL) from Pyrene‐Appended Cyclohexanediamides and Photoirradiation‐Tuned CPL Inversion

AbstractCircularly polarized luminescence (CPL) has been achieved in various chiral molecules and self‐assembled systems, but it remains a challenge to modulate CPL signals. To address this issue, a pair of enantiomeric pyrene‐appended cyclohexanediamide derivativesS,S/R,R‐CHPA with a V‐shaped conformation was designed and their CPL generation and photo‐response were investigated. Owing to the peculiar spatial location of the two pyrene segments in each molecule, CHPA emitted strong circularly polarized excimer emission (CPEE) with a |glum|value up to 1.49×10−2. Due to the presence of the vinylene groups, the optical properties could be regulated by UV light. It was found that under 365 nm irradiation, the absorption and emission bands of CHPA exhibited a pronounced hypochromatic shift, and the bluish white fluorescence gradually turned into dark blue. More intriguingly, these spectral changes were accompanied by a chirality inversion both in the ground and excited states as demonstrated by CD and CPL spectra, which could be attributed to the photoinducedE–Zisomerization of the vinylene groups. This work provides an extraordinary example in which the CPL signal of a single organic molecule could be efficiently inverted.

Induced and Inversed Circularly Polarized Luminescence of Achiral Thioflavin T Assembled on Peptide Fibril

AbstractChiroptical inversion of amyloid fibrils is a novel phenomenon and is of fundamental importance; however, the underlying structural basis remains poorly understood. Here, the co‐assembly of Thioflavin T (ThT) with T1 amyloid fibril and the induced supramolecular chirality is investigated by induced circular dichroism (ICD) and circularly polarized luminescence (CPL), followed by direct morphological helicity observation of the fibril by an atomic force microscope (AFM). ThT exhibits negative ICD and CPL when assembled on the left‐handed T1 fibril. Interestingly, when ThT dynamically interacts with the T1 fibril, the left‐handed fibril partially converts into right‐handed, accompanied with the inversion of CD and CPL signals. These results indicate that the morphological helicity of template fibril cannot be arbitrarily distinguished by the sign of chiroptical spectra of the dye/peptide assemblies.

Circularly polarized luminescent 4, 4′-bicarbazole scaffold for facile construction of chiroptical probes

Small molecules with circularly polarized luminescence (CPL) are especially valuable in solution-based applications, like chiroptical probes for effective sensing. However, such application has not been well explored due in part to hard introduction of appropriate triggerable fluorophores. Herein, we report an easily post-modifiable CPL-active scaffold (4) and further propose a feasible strategy to construct chiroptical probes for sensing hypochlorous acid. Helically chiral 4 with a 4, 4′-bicarbazole scaffold exhibits a large Stokes shift up to 142 nm and glum value of 0.9 × 10−3. Through facile construction of phenothiazine moiety on 4, red-emitting compound 5 was obtained in high yield (85.3%), showing distinct optical and chiroptical responses towards hypochlorous acid with good liner relationships and CPL sign blue-shifting of 120 nm in vitro. The probe is also applied to detect endogenous hypochlorous acid in living macrophages and can serve as a CPL sensor. Taking 5 as an application example, we envision that CPL-enabling scaffold 4 could find broad use in the design of more chiroptical probes.

Phthalocyanine-Triggered Helical Dipeptide Nanotubes with Intense Circularly Polarized Luminescence.

Nanotubes with circularly polarized luminescence (CPL) are attracting much attention due to many potential applications, such as chiroptical materials, displays, and sensing. However, it remains a challenge to change the assemblies of ordinarily molecular building blocks into CPL supramolecular nanotubes. Herein, the regulation of quite common dipeptide (Fmoc-FF) assemblies into unprecedented helical nanotubes exhibiting intense CPL is reported by simply doping a few phthalocyanine (octakis(carboxyl)phthalocyaninato zinc complex (Pc)) molecules. Interestingly, altering the Fmoc-FF/Pc molar ratios over a wide range cannot change the nanotubes structures according to transmission electron microscopy (TEM) and atomic force microscope (AFM) measurements. Although molecular dynamics simulations suggest that the noncovalent interactions between Fmoc-FF and Pc are quite weak, few Pc molecules can still change the secondary structures of a large number of Fmoc-FF assemblies, which hierarchically form helical supramolecular nanotubes with long-range ordered molecular packing, leading to intense CPL signals with large luminescence dissymmetry factor (glum = 0.04). Consequently, the chiral reorganization of Fmoc-FF assemblies is dependent on the coassembly between Pc molecule and Fmoc-FF supramolecular architectures. These results open the possibility for the fine-tuning of helix and supramolecular nanotubes with CPL properties by using a small number of cofactors.

Modulation of Chirality and Intensity of Circularly Polarized Luminescence Emitting from Cholesteric Liquid Crystals Triggered by Photoresponsive Molecular Motor

AbstractA chiral‐switchable device for circularly polarized luminescence (CPL) is fabricated based on dynamic superstructure of cholesteric liquid crystals (N*‐LC) doped with light‐driven molecular motor (MM), which achieves simultaneous modulation of chirality and intensity of CPL. Functional MM, designed with the modification of alkyl chains on the rotor, allows for advanced chemical isomerization to be driven with light. The selective reflection band due to the N*‐LC is shifted upon UV irradiation so that the reflection band moves toward the emissive band of the luminescence dyes, yielding CPL with opposite handedness and high dissymmetry factor values. The geometric changes of the motor during the rotary steps finally cause a remarkable reversible handedness inversion of the N*‐LC, which also leads switchable chirality of CPL. The dynamic N*‐LC cell bearing the light‐controlled selective reflection is useful for generating CPL from fluorescent materials and for allowing light chirality‐switching in CPL signals, which presents new possibilities for optoelectronic and photonics applications.

Chiral Self‐Assembled Film from Semiconductor Nanorods with Ultra‐Strong Circularly Polarized Luminescence

AbstractChiroptical nanomaterials have generated significant levels of interest for generating strong circularly polarized luminescence (CPL) signals. We used the Langmuir–Schaeffer technique to generate the continuous and compact assembly of CdSe/CdS chiral film. We assembled achiral CdSe/CdS nanorods by controlling the number of layers and angles between different layers. This allowed us to tailor chiroptical properties to achieve high CPL signals. The chiral film was symmetrical and had the highest circular dichroism (CD) response and CPL signals with ten layers (RH (right‐handed)‐/LH (left‐handed)‐5 + 5 layers) and a 45° inter‐angle. Specifically, RH‐5+5 of the chiral film exhibited 1431 mdeg of CD activity and strong CPL signals with a dissymmetry factor (glum) of 0.0997. The helical stacked crystal plates with linear birefringence resulted in strong circular birefringence, as determined by the Reusch model. Electromagnetic simulations indicated that such remarkable optical activity was attributed to the birefringence and dichroism of the well‐aligned CdSe/CdS nanorod layers in the chiral films. Under right/left circular polarized (RCP/LCP) light excitation, the well aligned semiconductor nanorods exhibited differences in the coupling efficiencies to RCP and LCP light. Our CdSe/CdS chiral films, which exhibit ultra‐strong CPL activity, will provide a novel strategy for the fabrication of chiroptical devices.

Luminescent Chiral Exciplexes with Sky-Blue and Green Circularly Polarized-Thermally Activated Delayed Fluorescence.

Luminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with glum up to 7×10-3 , one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone (glum ∼7×10-4 ). This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge-transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor-acceptor model. Interestingly, blue, green-yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi-color CP-TADF emitters.

Syntheses and photophysical properties of axially chiral thiazolothiazoles: Multi-stimuli-responsive fluorescence and circularly polarized luminescence

The synthesis, characterization, single crystal X-ray diffraction structures and photophysical and chiroptical properties of a novel series of binaphthol-substituted thiazolothiazoles are reported. The introduction of two binaphthol groups can help to achieve not only axial chirality but also helical molecule arrangements. Their charge contributions, transition processes (π−π* transition and/or donor-acceptor charge transfer) and excited-state intramolecular proton transfer properties are highly depend on alkyl-protecting groups at naphthol units. They exhibit good thermal and anti-photobleaching stabilities and strong blue fluorescence with quantum yields up to 0.40 in solution and polymer film and can be potentially used in the circularly polarized luminescence (CPL), ion sensing, vapochromism (HCl and NH3 vapor; pH-triggered chiroptical switch with CPL sign reversal) and thermochromism (fluorescence at 298 K and phosphorescence at 77 K) applications. Therefore, the present work affords a new platform in designing multi-stimuli-responsive chiral organic dyes.

Circularly polarized luminescence based on cholesterol-tetraphenylethylene-perylene liquid crystal

Perylene derivative with circularly polarized luminescence (CPL) at aggregated state was seldom reported due to the strong ACQ (aggregation-caused quench) effect at aggregation. In this work, a novel cholesterol-tetraphenylethylene-perylene derivative (TPE-P) was designed and synthesized in moderate yield. It exhibited liquid crystalline behavior with orderly hexagonal columnar mesophase and good fluorescence emission at long wavelength (600-700 nm) not only in solution but also at aggregated states based on the AIE (aggregation-induced emission)-FRET (fluorescence resonance energy transfer) effect between tetraphenylethylene unit and perylene moiety. Moreover, the circular dichroism (CD) and CPL studies suggested the effective chiral transfer from cholesterol unit to tetraphenylethylene unit and perylene skeleton due to the spiral liquid crystalline self-assembly. The CD and CPL signals showed the order of THF < THF-hexane < solid film < meosphase, indicating that the higher spiral orderly degree resulted in the stronger chiral transfer. The largest |glum| value for mesophase excited at 320 nm was as high as 1.5×10−2 based on the combining effect of AIE-FRET and chiral transfer. This research not only reported a novel CPL perylene derivative at aggregated state, but also confirmed that the combination of AIE-FRET effect and chiral transfer of liquid crytalline phase was an effective method to construct normal dye with excellent CPL property in aggregated state.

Cover Feature: A Pyrene‐Modified Serinol Nucleic Acid Nanostructure Converts the Chirality of Threoninol Nucleic Acids into Circularly Polarized Luminescence Signals (Chem. Eur. J. 59/2021)

Creating chiroptical signals: A circularly polarized luminescent (CPL) probe composed of serinol nucleic acids forms a one-dimensional nanostructure and expresses no CPL signals. However, helicity is induced when a chiral nucleic acid is added. As a result, the probe can switch CPL signals depending on the chirality of added nucleic acids. These findings will provide a new platform to convert the chiral information of biomolecules into chiroptical signals. More information can be found in the Communication by H. Kashida, H. Asanuma et al. (DOI: 10.1002/chem.202102333).

Tunable Circularly Polarized Luminescence from Single Crystal and Powder of the Simplest Tetraphenylethylene Helicate.

Tetraphenylethylene and its derivatives are a class of aggregation-induced emission (AIE) compounds that are most extensively and successfully studied. It has been found that the simplest TPE is easy to crystallize into homochiral M crystals or P crystals. However, no research on circularly polarized luminescence (CPL) of TPE solid is documented. In this paper, we report that TPE can grow into big and nonefflorescent single crystals in single helical conformation and has large birefringence that is comparative with commercially available products. The TPE single crystals can emit strong CPL with a very high glum value up to 0.53. Moreover, the sense and magnitude of CPL signals can be willfully tuned by simple rotation of the single crystal due to anisotropy of the crystals. This simple and promising CPL photonic material integrates emission, chirality, and birefringence together in one single crystal without needing an additional chiral dopant or conjugate polymer that can produce linearly polarized light. After being ground into fine powder and pressed as KBr pellets, the obtained nanocrystals of TPE also emit strong CPL light. Exceptionally, by mixing other achiral luminescent dyes together with TPE powder in KBr pellets, induced CPL signals were obtained, which could give full-color CPL emission. Although there were no interactions between TPE and the dyes in the pellets, induced CPL signals were realized through radiative energy transfer, providing a very simple method for the tuning of CPL emission.

Double Heterohelicenes Composed of Benzo[b]- and Dibenzo[b,i]phenoxazine: A Comprehensive Comparison of Their Electronic and Chiroptical Properties.

Heterohelicenes are potential materials in molecular electronics and optics because of their inherent chirality and various electronic properties originating from the introduced heteroatoms. In this work, we comprehensively investigated two kinds of double NO-hetero[5]helicenes composed of 12H-benzo[b]phenoxazine (BPO) and 13H-dibenzo[b,i]phenoxazine (DBPO). These helicenes exhibit good electron-donor properties reflecting the electron-rich character of their monomers and were demonstrated to work as p-type semiconductors. The enantiomers of these helicenes show the largest class of dissymmetry factors for circularly polarized luminescence (CPL) (|gCPL| > 10-2) among helicenes reported to date. Interestingly, the signs of CPL are opposite for BPO and DBPO double helicenes of the same helicity. The origin of the large gCPL values and the inversion of the CPL sign was addressed by analysis of the transition electronic dipole moments and transition magnetic dipole moments based on TD-DFT calculations.

Amplified circularly polarized luminescence promoted by hierarchical self-assembly involving Pt···Pt interactions

The design and construction of highly effective circularly polarized luminescence (CPL)-active materials has aroused considerable attention due to their widespread applications in sensors, optical devices, and asymmetric synthesis. However, the exploration of novel CPL-active materials with high luminescence dissymmetry factor (glum) values is still a challenge. Herein, we describe a new approach for the preparation of supramolecular metallacycles with amplified CPL promoted by hierarchical self-assembly involving Pt···Pt interactions. Notably, the resultant metallacycles exhibited strong CPL signals with high glum values, while their corresponding precursors were CPL silent. The CPL amplification mechanism was comprehensively validated by ultraviolet-visible absorption, emission spectroscopy, nuclear magnetic resonance spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, coarse-grained molecular dynamics simulations, and time-dependent density functional theory calculations. This work thus provides the first example of preparing highly effective CPL-active materials based on hierarchical self-assembly involving Pt···Pt interactions.

Iodization-enhanced fluorescence and circularly polarized luminescence for dual-readout probe design

Circularly polarized luminescence (CPL) is a preferential emission from chiral systems. Introducing CPL to traditional small fluorescence probes will greatly enhance selectivity and sensitivity due to avoiding the background fluorescence. Small-molecule probes are practically useful for bioimaging and sensing but always suffer from weak CPL properties. To optimize chiral perturbation, the study of structure-CPL correlation is an urgent necessity. Here, we investigated the influence of 3, 3′-substituent of BINOL on optical properties and found that iodization could significantly enhance the fluorescence quantum yield (ΦF) and dissymmetry factor (|glum|). Based on this finding, we presented a strategy to design dual-readout probes emiting both robust total fluorescence and CPL upon activation. As an application example, we first designed a pair of thiols probes, R/S-P1, with the fluorescence readout showing good linear response to cysteine, and CPL signal displaying significant enhancement with |glum| from undetectable value to 0.7 × 10−3.

Self-Assembly and Circularly Polarized Luminescence from Achiral Pyrene-Adamantane Conjugates by Selective Inclusion with Cyclodextrins.

The interaction between guest chromophores or lumiphores with host chiral cavity and their induced chirality is an important topic in supramolecular chemistry. Kodaka and Harata proposed a rule to explain the induced circular dichroism of the guest chromophores by host cyclodextrins. However, it remains unknown how a circularly polarized luminescence (CPL) signal will change when the lumiphores interacted with cyclodextrins in different modes. Here, we designed an achiral pyrene-adamantane conjugated guest molecule, N-(pyren-1-yl)adamantane-1-carboxamide (ACNP), and investigated its interactions with α/β/γ-cyclodextrins (CDs) and its induced CPL. Depending on the size match of the pyrene, adamantine with different cyclodextrins, distinct performance was observed. While α-CD could not induce a CPL signal of ACNP, β-CD could induce CPL in two modes, through adamantyl or direct pyrenyl induction, which could produce a CPL signal with opposite signs. γ-CD could always induce a negative CPL signal. Therefore, a rule of induced CPL of lumiphores by cyclodextrins can be proposed.

Optically Active CdSe/CdS Nanoplatelets Exhibiting Both Circular Dichroism and Circularly Polarized Luminescence

AbstractLigand‐induced chirality in colloidal semiconductor nanocrystals attracts attention because of their tunable chiroptical properties. Here, the induced chirality and circularly polarized luminescence (CPL) are investigated as a function of the CdS shell growth in a range of 2D CdSe/CdS nanoplatelets (NPLs) capped with chiral ligands. Five samples of CdSe/CdS NPLs are synthesized by a one‐pot approach to vary the island‐like shell on a four‐monolayer (4 ML) CdSe NPLs core, which effectively reduces the interfacial strain energy. The successful preparation of L‐/D‐Cysteine‐capped CdSe/CdS NPLs with both tunable circular dichroism (CD) and CPL behaviors and a maximum anisotropic luminance factor (glum) of 5.29 × 10−4 is described. The induced chiroptical response shows a direct relationship with the formation of island‐like shell in the first and second stages and shows a clear signal evolution. In the third stage with a full coating shell, the CD and CPL signals are inversely proportional to the CdS shell thickness. The island‐like shell gives birth to the CPL signal, while the formation of full coating shell decreases the induced chirality. Such chiral and emissive NPLs provide an ideal platform for the rational design of semiconductor nanocrystals with chiroptical properties in areas of biomedicine, polarizers, and new generation of display devices.

Full‐Color and White Circularly Polarized Luminescence Promoted by Liquid Crystal Self‐Assembly Containing Chiral Naphthalimide Dyes

AbstractChiral emissive nematic liquid crystals (N*‐LCs) can greatly amplify the circularly polarized luminescence (CPL) signals promoted by a highly regular spiral self‐assembly in the LCs system. Naphthalimide (NI) derivatives have been assigned to be one of the most excellent chromophores for fluorescent materials. In this paper, a novel strategy is first developed to regulate full‐color CPL behavior of N*‐LCs through different chiral NI‐based dyes. Chiral NI‐based dyes are doped into nematic liquid crystals (E7, N‐LCs) to form N*‐LCs as CPL‐active liquid crystal materials. Most interestingly, this kind of N*‐LCs could emit standard white CPL signals (CIE coordinates: (0.31, 0.33)) by choosing the blue‐red match of two chiral NI‐based dyes at the appropriate ratio.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Propeller Configuration Flipping of the Trivalent Boron-Inducing Substituent Dependence of the Circularly Polarized Luminescence Sign in Triarylborane-Based [7]Helicenes.

We here disclose two triarylborane-based [7]helicenes, which contain a dimesitylboryl or a 2-(dimesitylboryl)phenyl at position 9 of the [7]helicene skeleton. The change in the peripheral substituent from dimesitylboryl to 2-(dimesitylboryl)phenyl induced doubling of |glum| and sign inversion of the circularly polarized luminescence (CPL). The substituent dependence of the CPL sign is reasonably explained by the propeller configuration flipping of boron, which has a significant influence on the chiroptical properties.

Self-Assembly of Peptide Hierarchical Helical Arrays with Sequence-Encoded Circularly Polarized Luminescence.

Self-assembled peptide materials with sequence-encoded properties have attracted great interest. Despite their intrinsic chirality, the generation of circularly polarized luminescence (CPL) based on the self-assembly of simple peptides has been rarely reported. Here, we report the self-assembly of peptides into hierarchical helical arrays (HHAs) with controlled supramolecular handedness. The HHAs can emit full-color CPL signals after the incorporation of various achiral fluorescent molecules, and the glum value is 40 times higher than that of the CPL signal from the solutions. By simply changing the amino acid sequence of the peptides, CPL signals with opposite handedness can be generated within the HHAs. The peptide HHAs can provide hydrophobic pockets to accommodate the fluorescent molecules with helical arrangement through strong aromatic stacking interactions, which are responsible for the CPL signals. This work provides a pathway to construct highly ordered chiral materials, which have broad applications in the chiroptical field.