Circularly Polarized Luminescence Properties Research Articles

Switching of Circularly Polarized Luminescence via Dynamic Axial Chirality Control of Chiral Bis(Boron Difluoride) Complexes with Salen Ligands

AbstractThe intensity and handedness of circularly polarized luminescence (CPL) were successfully controlled by dynamic molecular motion in solutions. Bis(boron difluoride) complexes with chiral salen ligands were synthesized and their photophysical properties were investigated. Although these complexes showed rapid molecular rotation about the C−N bond axis in solution at room temperature, two conformers assigned as atropisomers were observed in the NMR spectra at low temperature. Furthermore, the equilibrium of these atropisomers was found to change depending on the external environment, such as the solvent and temperature, allowing precise control of the intensity and handedness of CPL without luminescence color shifts. Theoretical calculations based on density functional theory (DFT) revealed that intramolecular chiral exciton coupling is the key to changes in CPL properties.

A Multiple Circularly Polarized Luminescence Regulation Platform Driven by Acid, Base, and Primary Amines

AbstractFunctional materials that exhibit circularly polarized luminescence (CPL) have shown immense potential in applications such as 3D optical displays and advanced information encryption. However, facile and efficient regulation of the multiple emission properties in chiral systems has remained a challenge due to the scarcity of suitable materials and the complexity of the systems. Herein, a multiple CPL regulation platform is fabricated by the helical co‐assembly of an aldehyde‐functionalized fluorane dye (Rhodol‐CHO, 6′‐(diethylamino)‐3′‐hydroxy‐3‐oxo‐3H‐spiro[isobenzofuran‐1,9′‐xanthene]‐4′‐carbaldehyde) and chiral liquid crystals, which can efficiently combine multiple stimuli‐responsive characteristics and high optical activity. By introducing acid, base, and compounds containing primary amino (─NH2) groups, the CPL property of the system can be easily modified. The obtained CPL systems exhibit apparent asymmetry, with glum values reaching 0.54–0.75). Further, these CPL modules are successfully applied in multi‐mode information encryption, highlighting their practical utility. Therefore, this study presents a novel strategy for developing materials with diverse CPL behaviors based on a simple‐designed platform, advancing further applications of CPL materials.

Recent Advances of Chiral Isolated and Small Organic Molecules: Structure and Properties for Circularly Polarized Luminescence.

This paper explores recent advancements in the field of circularly polarized luminescence (CPL) exhibited by small and isolated organic molecules. The development and application of small CPL molecule are systematically reviewed through eight different chiral skeleton sections. Investigating the intricate interplay between molecular structure and CPL properties, the paper aims at providing and enlighting novel strategies for CPL-based applications.

Construction of Circular Polarized Luminescence Molecules for Intense Near Infrared OLEDs

AbstractThe exploration of organic near‐infrared (NIR) luminescence, along with circularly polarized luminescence (CPL), is of vital importance due to its significance in medical and optoelectronic applications. Integrating a chiral unit into a D–A molecular structure in this research realized emission peaks at 744 nm, exhibiting a dissymmetric factor of −1.67 × 10−3. The introduced chiral TPA derivative not only imparts aggregation‐induced emission properties and CPL capabilities but also induces a redshift in the emission spectrum. Consequently, the fabricated NIR organic light‐emitting devices (OLEDs), achieve maximum electroluminescent peaks at 748 nm in nondoped devices with the external quantum efficiency of 13.7% in doped devices. This pioneering research provides inspiration for the design of NIR molecules possessing CPL properties in the future, particularly in promoting the application of intense NIR‐OLEDs.

Color-tunable circularly polarized luminescence from liquid crystalline polymer networks

Chiroptical materials with circularly polarized luminescence (CPL) activities have attracted increasing interest because of their promising applications in optoelectronics, biosensing and imaging. Nevertheless, the construction of CPL-active materials with large luminescence dissymmetry (glum) factors and high emission efficiency is still challenging. Herein, a series of liquid crystalline polymer network films have been prepared by photopolymerization of cholesteric liquid-crystalline mixtures doped with aggregation-induced emission-active luminogens, TPE-TA and BDABFN. Through adjustment of the mass ratio of the two luminogens, the resultant films exhibit multicolour CPL from blue to red to white with absolute glum values up to 0.71 and fluorescence quantum yields up to 44 %. This work provides a strategy for fabricating polymeric film materials with color-tunable CPL properties and high fluorescence efficiency.

Synthesis and Chiroptical Properties of Radially Extended Carbazole with Chiral [2.2]Paracyclophane Core

Abstract[2.2]Paracyclophanes ([2.2]PCs) with substituents at specific positions possesses planar chirality and stands as a promising building unit for the development of chiroptical materials associated with polarized light. However, one of the easily accessible starting materials, bromo‐substituted [2.2]PC, shows low reactivity, and effective methods for π‐extension of planar chiral [2.2]PC are limited. Herein, an effective π‐extension method has established for chiral [2.2]PC with four substitutions from 4,7,12,15‐tetrabromo[2.2]PC. This has led to tetrakis(trimethylsilylphenyl)‐[2.2]PC that serves as a versatile building resource for radial‐type chiral π‐conjugated compounds with a [2.2]PC core. In this study, synthesizing compounds are further succeeded with the introduction of carbazole (Cbz4‐[2.2]PC and tBuCbz4‐[2.2]PC) and investigated their chiroptical properties and chiral induction ability in achiral π‐conjugated polymers. A thin film of [Poly[9,9‐dioctylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)]] (F8BT) with 3wt% Cbz4‐[2.2]PC prepared by spin‐coating exhibits circularly polarized luminescence (CPL) properties with a high dissymmetry factor (|glum| = 0.01), which is much higher than that of Cbz4‐[2.2]PC in CH2Cl2 solution. By contrast, the doped film of F8BT with 3wt% tBuCbz4‐[2.2]PC exhibited only weak CPL properties. In addition, these chiral compounds show facile redox properties owing to the presence of carbazole units. Furthermore, Cbz4‐[2.2]PC undergoes electropolymerization to produce an insoluble thin film on the electrode.

Modulation of luminescence properties of circularly polarized thermally activated delayed fluorescence molecules with axial chirality by donor engineering.

Multifunctional thermally activated delayed fluorescence (TADF) materials are currently a trending research subject for luminescence layer materials of organic light-emitting diodes (OLEDs). Among these, circularly polarized thermally activated delayed fluorescence (CP-TADF) materials have the advantage of being able to directly achieve highly efficient circularly polarized luminescence (CPL). The simultaneous integration of outstanding luminescence efficiency and excellent luminescence asymmetry factor (glum) is a major constraint for the development of CP-TADF materials. Therefore, on the basis of first-principles calculations in conjunction with the thermal vibration correlation function (TVCF) method, we study CP-TADF molecules with different donors to explore the feasibility of using the donor substitution strategy for optimizing the CPL and TADF properties. The results indicate that molecules with the phenothiazine (PTZ) unit as the donor possess small energy difference, a great spin-orbit coupling constant and a rapid reverse intersystem crossing rate, which endow them with remarkable TADF features. Meanwhile, compared with the reported molecules, the three designed molecules exhibit better CPL properties with higher glum values. Effective molecular design strategies by donor engineering to modulate the CPL and TADF properties are theoretically proposed. Our findings reveal the relationship between molecular structures and luminescence properties of CP-TADF molecules and further provide theoretical design strategies for optimizing the CPL and TADF properties.

Construction of an Au12Cd2 nanocluster with circularly polarized luminescence by a metal- and ligand-exchange strategy.

Excellent luminescence properties and unique chiral structures enable nanoclusters to be a novel class of circularly polarized luminescence (CPL) materials, and their precise structures facilitate the clarification of structure-activity relationships. However, efficiently preparing nanoclusters with CPL properties is still a great challenge. In this work, the luminescent properties as well as the molecular symmetry were simultaneously manipulated to transform the centrosymmetric Au14Cd1 into a chiral Au12Cd2 nanocluster, which has CPL properties. In detail, Cd doping and chiral-ligand exchange were performed simultaneously on the Au14Cd1 nanocluster to realize its photoluminescence enhancement and chiral-framework construction by increasing the alloying degree which is defined as deep-alloying and chiral ligand induction at the same time, resulting in the formation of an Au12Cd2 nanocluster with CPL properties. Further investigations revealed an increased alloying degree in the structure-maintained M6 kernel of Au12Cd2, which results in a 15-fold enhancement in quantum yield.

Cooperative supramolecular polymerization of styrylpyrenes for color-dependent circularly polarized luminescence and photocycloaddition

Developing facile and efficient methods to obtain circularly polarized luminescence (CPL) materials with a large luminescence dissymmetry factor (glum) and fluorescence quantum yield (ΦY) is attractive but still challenging. Herein, supramolecular polymerization of styrylpyrenes (R/S-PEB) is utilized to attain this aim, which can self-assemble into helical nanoribbons. Benefiting from the dominant CH-π interactions between the chromophores, the supramolecular solution of S-PEB shows remarkable blue-color CPL property (glum: 0.011, ΦY: 69%). From supramolecular solution to gel, the emission color (blue to yellow-green) and handedness of CPL (glum: –0.011 to +0.005) are concurrently manipulated, while the corresponding supramolecular chirality maintains unchanged, representing the rare example of color-dependent CPL materials. Thanks to the supramolecular confine effect, the [2 + 2] cycloaddition reaction rate of the supramolecular solution is 10.5 times higher than that of the monomeric solution. In contrast, no cycloaddition reaction occurs for the gel and assembled solid samples. Our findings provide a vision for fabricating multi-modal and high-performance CPL-active materials, paving the way for the development of advanced photo-responsive chiral systems.

Chiral benzo[ghi]perylene substituted [n]helicenes with strong blue fluorescent emission and circularly polarized luminescence

Chiral organic fluorescent molecules with circularly polarized luminescence (CPL) properties have attracted much attention for their potential applications in chiral optical and electronic devices. Benzo [ghi]perylene is one of the classical polycyclic aromatic hydrocarbons (PAHs) with good fluorescent properties and has many applications in the fields including optoelectronic devices and fluorescence probes. In this work, chiral benzo [ghi]perylene substituted [n]helicenes were directly obtained from chiral BINOL. The introduction of benzo [ghi]perylene structure greatly improved the fluorescent efficiency of the pristine helicene molecules. These benzo [ghi]perylene substituted [n]helicenes show strong blue fluorescent emission in organic solutions with good fluorescent quantum yields. These molecules could also exhibit moderate fluorescent emission in the solid-states. Based on the DFT calculations results, the HOMOs and LUMOs of these molecules are mainly contributed by benzo [ghi]perylene, which forms an orthogonal geometry with helicene moieties. The enantiomers of these molecules show opposite signals in CD and CPL spectra. The gabs and glum values could be increased with elongation of helicene structure. Compounds with [7]helicene moiety show relatively large gabs and glum values, which could reach up to 3.6 × 10−3 and 1.0 × 10−3, respectively. This work has provided a new strategy to develop chiral organic emitters through integration of benzo [ghi]perylene and helicene structures, which exhibit efficient fluorescent emission and intense CPL properties.

Optically Active A‐Shaped Cyclic Molecules Based on Planar Chiral [2.2]Paracyclophanes Emitting Bright Circularly Polarized Luminescence with High Anisotropy Factors

AbstractOptically active A‐shaped cyclic molecules are synthesized by incorporating a planar chiral [2.2]paracyclophane unit to form a helical structure. These molecules exhibit large molar absorption coefficients, moderate photoluminescence (PL) quantum efficiencies, and high circularly polarized luminescence (CPL) anisotropy factors, resulting in high CPL brightness values (BCPL values). According to the time‐dependent density functional theory (TD‐DFT) theoretical studies, extended π‐conjugated moieties act as light‐harvesting units, and emissions occur from the helical skeletons. The TD‐DFT calculations reproduce the experimentally determined absorption, PL, circular dichroism, and CPL properties of the A‐shaped cyclic molecules.

Synergistically Boosting the Circularly Polarized Luminescence of Functionalized Pillar[5]arenes by Polymerization and Aggregation.

Supramolecular polymers based on chiral macrocycles have attracted increasing attention in the field of circularly polarized luminescence (CPL) owing to their unique properties. However, the construction of macrocyclic supramolecular polymers with highly efficient CPL properties in aggregate states still remains challenging. Herein, w e constructed a class of macrocycle-based coordination polymers by combining the planar chiral properties of pillar[5]arene with the excellent fluorescence properties of aggregation-induced emission luminogens. The formation of polymers enhances both the fluorescence and chiral properties, resulting in chiral supramolecular polymers with remarkable CPL properties. Increasing the aggregation degree of the polymers can further improve their CPL properties, as evidenced by a 21-fold increase in the dissymmetry factor and an over 25-fold increase in the fluorescence quantum yield in the aggregate state compared to the solution state. Such a synergistic effect of polymerization- and aggregation-enhanced CPL can be explained by the restriction of intramolecular motions and aggregation-induced conformation confinement. This work provides a promising method for developing highly efficient CPL supramolecular polymers.

Inducing Circularly Polarized Luminescence by Confined Synthesis of Ultrasmall Chiral Carbon Nanodot Arrays in Pyrene-Based MOF Thin Film.

Combining the features of the host-guest system and chirality is an efficient strategy to achieve circularly polarized luminescence (CPL). Herein, well-defined chiral carbon nanodot (chirCND) arrays were confined-synthesized by low-temperature calcination of a chiral amino acid loaded metal-organic framework (MOF) to induce high CPL. An achiral porous pyrene-based MOF NU-1000 thin film as the host template was prepared by a liquid-phase epitaxial layer-by-layer fashion, and chiral amino acids as the carbon sources could be confined in the porous MOF and carbonized to homogeneous and ultrasmall chirCND arrays, resulting in a chirCNDs@NU-1000 thin film (l-CNDsx@NU-1000; x = l-cysteine (cys), l-serine, l-histidine, l-glutamic acid, and l-pyroglutamic acid). The results show the pristine chirCNDs by directly carbonizing chiral amino acids hardly endow them with a CPL property. By contrast, benefiting from the arrayed confinement and coordination interaction between chirCNDs and NU-1000, the chirality transfer on the excited state of chirCNDs@NU-1000 is enabled, leading to strong CPL performance (a high luminescence dissymmetry factor glum of l-CNDscys@NU-1000 thin film reached 1.74 × 10-2). This study of chirCNDs encapsulated in fluorescent MOF thin films provides a strategy for developing uniform chiral carbon nanoarrays and offers chiral host-guest thin-film materials for optical applications.

Non-Classical Circularly Polarized Luminescence Control of Peptide Luminophore Based on Precise Chiral Space Control

Light that rotates in a circular spiral when viewed from the front is known as circularly polarized luminescence (CPL), and can be divided into two types, namely, left- and right-rotating light. To emit both left- and right-rotating CPLs, two types of optically active luminophores, namely, enantiomer D- and L-bodies, are generally required. This mini-review mainly discusses our latest study on CPL properties via the control of the pyrene ring as the luminescent unit incorporated into chiral peptides. In this study, optically active peptide–pyrene organoluminescent materials that emit CPL were synthesized by combining a peptide as a frame and two pyrene rings as a luminescent unit. By adjusting the interpyrene distance, external conditions, and absolute chiral configuration (D- or L-configuration), the chiral spatial configuration of the luminescent pyrene ring was precisely controlled. Consequently, the direction of CPL rotation from pyrenylalanine-containing peptides with the same configuration was successfully controlled.

Acceptor engineering for modulating circularly polarized luminescence and thermally activated delayed fluorescence properties

As the new type of organic photoelectric device, circularly polarized thermally activated delayed fluorescence (CP-TADF) organic light-emitting diodes (OLEDs) with the advantages of high exciton utilization and full color saturation show wide applications in 3D display, encrypted information storage and quantum computing. However, due to the conflict between high luminescence efficiency and large luminescence asymmetry factor (glum), the species of efficient CP-TADF molecules are limited. Herein, theoretical studies are performed by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods coupled with thermal vibration correlation function, the effects of different acceptors on circularly polarized luminescence (CPL) and TADF properties are investigated, inner mechanisms between basic molecular structures and luminescence properties are revealed. By calculating intersystem crossing rate, reverse intersystem crossing rate, radiative decay rate and non-radiative decay rate, we determine that the SPHCN molecule exhibited dominate TADF properties. Additionally, the electronic circular dichroism (ECD) spectrum and glum values are quantitatively calculated to measure the CPL properties. Based on our findings, we conclude that lengthening the acceptor unit is a feasible strategy for improving the luminescence properties of CP-TADF molecules. Our study provides a novel approach for understanding the molecular luminescence mechanism of CP-TADF, which could promote the development of CP-OLEDs.

Circularly polarized luminescence switch based on a photochromic europium(III) helicate derived coordination polymer

Chiroptical switches based on circularly polarized luminescence (CPL) have shown the promising applications in advanced information technologies. Herein, a pair of lanthanide coordination polymer enantiomers [Eu2(LR)3(BTFPO)2]n and [Eu2(LS)3(BTFPO)2]n with light-regulated CPL property are designed, which are assembled by a chiral binuclear triple-stranded Eu3+ helicates [Eu2(LR/S)3] coordinated with two photochromic triphenylphosphine oxides (BTFPO). Upon the alternative UV and 526 nm light irradiation, the complexes show the reversible photochromism, PL and CPL responses. Notably, the luminescence dissymmetry factor, glum of 5D0→7F1 (591 nm) transition shows an obvious increase from 0.19 to 0.29 before and after 275 nm light irradiation. Additionally, the emission from Eu3+ center is not completely quenched in closed-ring state due to the low photocyclization (Фo-c) quantum yield of the polymer. The partial maintenance of emissive intensity is of essential importance for the monitor of CPL signal. More importantly, the CPL photo-switching property of the complexes in solid hybrid film is maintained, and still displays the enhanced CPL emission in photostationary state. Further, the potential applications of the doping film in logic gate and anti-counterfeiting were investigated.

Helical Polymer Working as a Chirality Amplifier to Generate and Modulate Multicolor Circularly Polarized Luminescence in Small Molecular Fluorophore/Polymer Composite Films.

In-depth studies of chirality and circularly polarized luminescence (CPL) have become indispensable in the process of learning human nature. Small molecules with CPL activity are one of the research hotspots. However, the CPL properties of such materials are generally not satisfying. Here, we synthesized a series of chiral small molecular fluorophores that cannot demonstrate CPL emission themselves. By introducing an optically inactive helical polymer, chirality transfer and chirality amplification efficiently occur, thereby generating intense CPL emission. Through combining different chiralized fluorophores, multicolor CPL-active films with emission wavelength centered at 463, 525, and 556 nm were fabricated, with the maximum luminescence dissymmetry factor (glum) being up to -0.028. Then, benefiting from the strong CPL emission and appropriate energy donor-acceptor system, we further established a circularly polarized fluorescence-energy transfer (CPF-ET) strategy in which the CPL-active films work as a donor emitting circularly polarized fluorescence to excite an achiral fluorophore (Nile red) as the acceptor, producing red CPL with glum of up to -0.011 at around 605 nm.

Stepwise Solution-Interfacial Nanoarchitectonics for Assembled Film with Full-Color and White-Light Circularly Polarized Luminescence.

The fabrication of chiral thin films with tunable circularly polarized luminescence (CPL) colors is important in developing chiroptical materials but remains challenging due to the lack of assembly-initiated chiral film formation methodology. Here, by adopting a combined solution aggregation and interfacial assembly strategy, we report the fabrication of chiral film materials with full-color and white-light CPL. A biquinoline glutamic acid ester (abbreviated as BQGE) shows a typical aggregation-induced emission property with blue CPL after solution aggregation. Subsequent interfacial assembly of these solution aggregates on a solid substrate leads to the formation of a CPL active film consisting of nanobelt structures. Since the BQGE molecule has a coordination site, the CPL emission of an individual BQGE film can be extended from blue to green emission upon coordination with a zinc ion, accompanied by morphology transition from nanobelts to nanofibers. Further extension to red-color CPL is successfully achieved by coassembly with an achiral acceptor dye. Interestingly, the proper combination of coordination ratio and acceptor loading ratio provides bright white-light CPL emission from the BQGE/Zn2+/PDA triad composite film. This work provides a new approach to fabricating chiroptical film materials with controlled microscopic morphology and tunable CPL properties.

Layer-by-Layer Grafting Dye on Chiral MOF Thin Films for Circularly Polarized Luminescence

Metal-organic frameworks with chiral feature (chirMOFs) are attracting great attention on circularly polarized luminescence (CPL). However, developing new efficient strategy to achieve or improve CPL properties is an urgent task. Herein, new chiral MOF thin films prepared by liquid-phase epitaxial layer by layer (lbl) growth method (SUR-chirMOF) are composed of D- or L-camphorate (D/Lcam) and aminopyrazine (Pr-NH2) by using liquid phase epitaxial layer by layer (lbl) method. The resulted Zn2(D/Lcam)2Pr-NH2 SURchirMOF shows strong chirality and luminescence but weak CPL emission at 390 nm. After lbl modifying a dye molecule FluoresceinIsothiocyanate (FITC), the chiroptical Zn2(D/Lcam)2Pr-NH-FITC SURchirMOFs with ~7 times CPL signal improvement and ~3 times glum value amplification are obtained. This work provides a new strategy to develop chiral MOF thin films for CPL improvement using lbl grafting approach.

Arene-Perfluoroarene Force Driven Sublimation-Removable Chiral Coassemblies.

Multiple constituent coassembly is an emerging strategy to manipulate supramolecular chirality and chiroptical properties such as circularly polarized luminescence (CPL). However, the second or third constituent could not be removed from pristine self-assembly. Here we developed a constitute-removable chiral coassembly using sublimation that could realize coassembly with tunable supramolecular chirality, luminescence and CPL properties. Octafluoronapthalene (OFN) with small sublimation enthalpy formed coassemblies with perylene-conjugated peptoids via arene-perfluoroarene (AP) interaction that induced the emergence of macroscopic chirality and hypochromic luminescence from yellow to green. Coassembly with OFN accelerated one-dimensional growth and induced the emergence of macroscopic chirality and CPL. Despite the stability at ambient conditions, vacuum-treatment triggered fast sublimation of OFN, which behaved as a sacrificial template. Physical removal of OFN retained the helical nanoarchitectures as well as the basic features of Cotton effects and CPL activities. X-ray diffraction suggested the back-fill consolidation occurred on the molecular voids by OFN removal that slightly varied the templated molecular arrangements. Sublimation of perfluorinated building units is green and efficient and non-destructive, which is potentially applicable in constructing template-directed chiroptical materials and devices.