Circularly Polarized Luminescence Spectra Research Articles

Temperature-dependent NIR-CPL spectra of chiral Yb(III) complexes.

Chiral, enantiopure Yb(III) complexes exhibit circularly polarized luminescence (CPL) in the near infrared (NIR) wavelength region. This CPL is quantified by the dissymmetry factor (glum). The excited state 2F5/2 consists of six mJ' states degenerated in three Stark levels, due to the crystal-field splitting (CFS), which are populated in accordance with the Boltzmann distribution. Consequently, room temperature CPL spectra are the sum of various - either positive or negative - contributions, that are practically impossible to quantify. To address this issue, an advanced setup enabling CPL measurements over a broad temperature range (300 to 4 K) has been developed. The interrelation of CFS, glum and temperature was explored using a pair of enantiopure Yb(III) complexes, highlighting the individual contribution of each crystal-field sublevel to the overall CPL spectrum, as anticipated by simulations performed in the framework of multireference wave-functions. Hence, the CPL spectra of chiral lanthanide complexes were found to be indeed strongly temperature-dependent, as is the glum dissymmetry factor, as a consequence of the variation in thermal sublevel population.

Ligand Chirality Transfer from Solution State to the Crystalline Self-Assemblies in Circularly Polarized Luminescence (CPL) Active Lanthanide Systems.

The synthesis of a family of chiral and enantiomerically pure pyridyl-diamide (pda) ligands that upon complexation with europium [Eu(CF3SO3)3] result in chiral complexes with metal centered luminescence is reported; the sets of enantiomers giving rise to both circular dichroism (CD) and circularly polarized luminescence (CPL) signatures. The solid-state structures of these chiral metallosupramolecular systems are determined using X-ray diffraction showing that the ligand chirality is transferred from solution to the solid state. This optically favorable helical packing arrangement is confirmed by recording the CPL spectra from the crystalline assembly by using steady state and enantioselective differential chiral contrast (EDCC) CPL Laser Scanning Confocal Microscopy (CPL-LSCM) where the two enantiomers can be clearly distinguished.

Significance of Vibronic Coupling that Shapes Circularly Polarized Luminescence of Double Helicenes

AbstractThe circularly polarized luminescence (CPL) spectra of S‐ and X‐shaped double helicenes exhibit distinct vibrational structures and overall shape variations. In this study, we conducted an in‐depth investigation into the vibronic effects influencing the CPL spectra of two double helicenes, namely DPC and DNH. Employing state‐of‐the‐art computations utilizing the FC‐HT1|VH model at the CAM−B3LYP/def2‐TZVP level, we unveiled the paramount impact of Franck–Condon (FC), Herzberg‐Teller (HT), and Duschinsky effects on their chiroptical responses. Our research underscores the pivotal role of structural deformations associated with the S1‐to‐S0 electronic transition in molding CPL spectra and wavelength‐dependent dissymmetry (g) factor values, as well as the significance of HT effects in shaping and enhancing CPL responses. This extensive investigation not only advances our comprehension of the vibronic characteristics in configurationally distinct double helicenes but also offers valuable insights for the design of chiral molecules featuring controllable or finely‐tunable CPL responses.

Significance of Vibronic Coupling that Shapes Circularly Polarized Luminescence of Double Helicenes.

The circularly polarized luminescence (CPL) spectra of S- and X-shaped double helicenes exhibit distinct vibrational structures and overall shape variations. In this study, we conducted an in-depth investigation into the vibronic effects influencing the CPL spectra of two double helicenes, namely DPC and DNH. Employing state-of-the-art computations utilizing the FC-HT1|VH model at the CAM-B3LYP/def2-TZVP level, we unveiled the paramount impact of Franck-Condon (FC), Herzberg-Teller (HT), and Duschinsky effects on their chiroptical responses. Our research underscores the pivotal role of structural deformations associated with the S1-to-S0 electronic transition in molding CPL spectra and wavelength-dependent dissymmetry (g) factor values, as well as the significance of HT effects in shaping and enhancing CPL responses. This extensive investigation not only advances our comprehension of the vibronic characteristics in configurationally distinct double helicenes but also offers valuable insights for the design of chiral molecules featuring controllable or finely-tunable CPL responses.

A Perspective on the Simulation of Electronic Circular Dichroism and Circularly Polarized Luminescence Spectra in Chiral Solid Materials.

Chiral materials have shown tremendous potential for many technological applications, such as optoelectronics, sensing, magnetism, information technology, and imaging. Characterization of these materials is mostly based on chiroptical spectroscopies, such as electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). These experimental measurements would greatly benefit from theoretical simulations for interpretation of the spectra as well as predictions on new materials. While ECD and CPL simulations are well established for molecular systems, they are not for materials. In this Perspective, we describe the theoretical quantities necessary to simulate ECD and CPL spectra in oriented systems. Then, we discuss the approximate strategies currently used to perform these calculations, what computational machinery is already available to develop more general approaches, and some of the open challenges for the simulation of ECD and CPL spectra in solid materials. When methods that are as reliable and computationally efficient as those for molecules are developed, these simulations will provide invaluable insight and guidance for the rational design of optically active materials.

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.

Vibrationally resolved deep–red circularly polarised luminescence spectra of C70 derivative through Gaussian curvature analysis of ground and excited states

Over the last years, the interaction of fullerene with circularly polarized light has attracted growing attention for potential electronic and optical applications. However, in literature there is only one example of fullerene derivative capable of emitting circularly polarized light, showing an active circularly polarized luminescence (CPL) signal in the deep-red visible region. This unique luminophore offered us the possibility to study the connection between the topological features of C70 spheroid and its chiral emission properties. In light of these considerations, we proposed a theoretical protocol that combines three different step: (1) The Ball Pivoting Algorithm for C70 surface reconstruction. (2) The discrete gaussian curvature analysis in the ground (S0) and excited states (S1). (3) The computation of the vibrationally-resolved CPL spectrum. The first step allowed us to extract useful information that linked the topological shape of C70 to the sp2 carbon network chemistry. The DFT benchmark in the second step guided us in grasping the best functional for the C70 curvature simulation in the ground state, spotlighting how B97D3 excellently succeed for this task. The curvature investigation in the first excited state showed that (for all the twenty exchange–correlation functional tested) the C70 fragment is more curved in S1 than in S0. The final step collected the topological information from the previous sections to provide a detailed overview of the theoretical factors (such as the quantum formalism, the potential energy surface description and the transition dipole moment approximation) impacting on the C70 vibrationally resolved CPL spectrum. We found that the adiabatic hessian model coupled with the Franck-Condon Herzberg-Teller approximation computed at PW6B95D3/6-311G(d,p) level provides excellent results in emulating the band-shape and position of the experimental CPL spectrum.

High solid-state CPL active materials based on chiral BINOL-dicyanodistyrylbenzene

Herein, we synthesized a pair of aggregation-induced emission (AIE) chiral fluorescence materials (R/S-BIN-Cyn enantiomers). The enantiomers were constructed by connecting a typical AIE molecule to a chiral 1,1′-bi-2-naphthol (BINOL) with ether linkages. The obtained enantiomers exhibited excellent AIE properties and unique state circularly polarized luminescence (CPL) behavior. Specifically, the enantiomers were CPL-active in the solid film with a high glum value of −1.1 × 10−1/+1.2 × 10−1. The CPL and circular dichroism (CD) spectra showed that chirality transfer was achieved from the chiral BINOL to the achiral AIE molecule. The present study is expected to provide a new perspective for preparing chiral luminescent devices with solid-state CPL active.

Theoretical and experimental analysis of circularly polarized luminescence spectrophotometers for artifact-free measurements using a single CCD camera

Circularly polarized luminescence (CPL) is a fast growing research field as a complementary chiroptical spectroscopy alternative to the conventional circular dichroism or in the quest of devices producing circularly polarized light for different applications. Because chiroptical signals are generally lower than 0.1%, conventional chiral spectroscopies rely on polarization time modulation requiring step-by-step wavelength scanning and a long acquisition time. High throughput controls motivated the development of CPL spectrophotometers using cameras as detectors and space polarization splitting. However, CPL measurements imposes careful precautions to minimize the numerous artifacts arising from experimental imperfections. Some previous work used complex calibration procedure to this end. Here we present a rigorous Mueller analysis of an instrument based on polarizations space splitting. We show that by using one camera and combining spatial and temporal separation through two switchable circular polarization encoding arms we can record accurate CPL spectra without the need of any calibration. The measurements robustness and their fast acquisition times are exemplified on different chiral emitters.

Circularly Polarized Luminescent Bis(Boron Difluoride) Complexes with Chiral Schiff‐base Ligands

AbstractThe Front Cover image is provided by the Ikeshita and Tsuno group at Nihon University (Japan), together with the Imai group at Kindai University (Japan). The image depicts the three chiral bis(boron difluoride) complexes whose circularly polarized luminescence (CPL) properties were studied in this work. The three complexes reflect off the sea surface, reminiscent of the mirror image CPL spectra obtained in the study, and similar to the reflection of the three islands in the background image. Read the full text of the Research Article at 10.1002/cptc.202200318.

BINOL-like atropisomeric chiral nanographene.

Interest in making chiral polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) has greatly increased recently. To date, a majority of chiral nanocarbons have been designed based on helical chirality. Here, we describe a novel atropisomeric chiral oxa-NG 1 by the selective dimerization of naphthalene-containing, hexa-peri-hexabenzocoronene (HBC)-based PAH 6. The photophysical properties of the oxa-NG 1 and monomer 6 were investigated, including UV-vis absorption (λ max = 358 nm for 1 and 6), fluorescence emission (λ em = 475 nm for 1 and 6), fluorescence decay (15 vs. 16 ns), and fluorescence quantum yield, and it was found that the photophysical properties of the monomer are nearly maintained in the NG dimer due to its perpendicular conformation. Single-crystal X-ray diffraction analysis shows that both enantiomers cocrystallize in a single crystal, and the racemic mixture can be resolved by chiral high-performance liquid chromatography (HPLC). The circular dichroism (CD) spectra and circularly polarized luminescence (CPL) of the enantiomers of 1-S and 1-R were studied and the CD and CPL spectra exhibited opposite Cotton effects and fluorescence signals. Density functional theory (DFT) calculations and HPLC-based thermal isomerization results showed that the racemic barrier is as high as 35 kcal mol-1, suggesting a rigid chiral nanographene structure. Meanwhile, in vitro studies indicated that oxa-NG 1 is an efficient photosensitizer for white-light-induced singlet oxygen generation.

Circularly polarized luminescence of natural products lycorine and narciclasine: role of excited-state intramolecular proton-transfer and test of pH sensitivity.

Circularly polarized luminescence (CPL) is increasingly gaining interest not only for its applicative potentialities but also for providing an understanding of the excited state properties of chiral molecules. However, applications of CPL are mainly in the field of materials science: special organic molecules and polymers, metal (lanthanide) complexes, and organic dyes are actively and intensely studied. So far natural compounds have not been investigated much. We fill the gap here by measuring circular dichroism (CD) and CPL of lycorine and narciclasine, the most abundant known alkaloid and isocarbostyril from Amaryllidaceae, which exhibit a large spectrum of biological activities and are promising anticancer compounds. Dual fluorescence detection in narciclasine led us to unveil an occurring excited-state intramolecular proton transfer (ESIPT) process, this mechanism well accounts for the Stokes shift and CPL spectra observed in narciclasine. The same molecule is interesting also as a pH chiroptical switch. Both in absorption and emission, lycorine and narciclasine are also studied computationally via density functional theory (DFT) calculations further shedding light on their properties.

Structure and Circularly Polarized Luminescence of Copper(I) Complexes Bearing Bipyridine and Chiral Diphosphine Ligands

Copper(I) (Cu(I)) complexes bearing chiral phosphine and diimine ligands were prepared, and their chiroptical properties studied. The complex with a twisted diimine ligand and a diphosphine ligand showed moderate-intensity emission and circularly polarized luminescence (CPL) with considerable dissymmetry factor, gem. The signs and intensities in the CPL spectra were found to reflect the dihedral angles of the two pyridine rings in the diimine ligand.

Linear Axially Chiral Conjugated Polymers Exhibiting Ultralong Low‐Temperature Phosphorescence and Intense Circularly Polarized Luminescence

AbstractUltralong organic phosphorescence (UOP) and circularly polarized luminescence (CPL), as two important branches in the field of organic optical functional materials have broad prospects. In this work, a pair of novel conjugated polymers with low‐temperature UOP and intense CPL properties are constructed by incorporating rigid axially chiral chromophores into a polymeric backbone. The twisted and rigid biphenyl skeleton not only effectively reduces the energy gap between the S1 and T to greatly facilitate the intersystem crossing process, but also suppress the non‐radiative decay from T1 to S0. As a result, a strong and ultralong afterglow of up to 33 s is achieved in 2‐MeTHF at 77 K. Moreover, the repeating rigid biphenyl skeleton with its helix‐like form also promotes the chiroptical activity of the polymers, and intense signals are shown in the low‐temperature CPL spectra with absolute glum values of 9.7×10−3 and 5.8×10−3 at the fluorescence and phosphorescence emission peaks, respectively. This work represents the first low‐temperature materials with UOP and CPL properties, which provides a new guideline and perspective for the development of persistent luminescence materials.

Linear Axially Chiral Conjugated Polymers Exhibiting Ultralong Low-Temperature Phosphorescence and Intense Circularly Polarized Luminescence.

Ultralong organic phosphorescence (UOP) and circularly polarized luminescence (CPL), as two important branches in the field of organic optical functional materials have broad prospects. In this work, a pair of novel conjugated polymers with low-temperature UOP and intense CPL properties are constructed by incorporating rigid axially chiral chromophores into a polymeric backbone. The twisted and rigid biphenyl skeleton not only effectively reduces the energy gap between the S1 and T to greatly facilitate the intersystem crossing process, but also suppress the non-radiative decay from T1 to S0 . As a result, a strong and ultralong afterglow of up to 33 s is achieved in 2-MeTHF at 77 K. Moreover, the repeating rigid biphenyl skeleton with its helix-like form also promotes the chiroptical activity of the polymers, and intense signals are shown in the low-temperature CPL spectra with absolute glum values of 9.7×10-3 and 5.8×10-3 at the fluorescence and phosphorescence emission peaks, respectively. This work represents the first low-temperature materials with UOP and CPL properties, which provides a new guideline and perspective for the development of persistent luminescence materials.

Chiral‐at‐Cage Carboranes for Circularly Polarized Luminescence and Aggregation‐Induced Electrochemiluminescence

AbstractHerein, we report the structures of chiral‐at‐cage carborane derivatives bearing carbazole chromophores that emit circularly polarized luminescence (CPL) and aggregation‐induced electrochemiluminescence (AIECL). By adjusting the substituent positions on the carborane derivatives, two chiral luminescent molecules, Cb1 and Cb2, with different properties were obtained. The photoluminescence dissymmetry factors |gPL| of both (R/S)‐Cb1 and (R/S)‐Cb2 enantiomers in neat films were as high as 6.24×10−3 and 7.38×10−3, respectively. Cb1 showed a deep blue emission peak at 434 nm in n‐pentane. Interestingly, distinct fluorescence and CPL spectra were observed in solvents of different polarities due to the twisted intramolecular charge transfer effect, suggesting its potential use in solvent recognition. Meanwhile, Cb2 exhibited good AIECL property, excellent ECL stability and could be used for determining dopamine concentrations, suggesting its potential applications in biology and diagnosis.

Chiral-at-Cage Carboranes for Circularly Polarized Luminescence and Aggregation-Induced Electrochemiluminescence.

Herein, we report the structures of chiral-at-cage carborane derivatives bearing carbazole chromophores that emit circularly polarized luminescence (CPL) and aggregation-induced electrochemiluminescence (AIECL). By adjusting the substituent positions on the carborane derivatives, two chiral luminescent molecules, Cb1 and Cb2, with different properties were obtained. The photoluminescence dissymmetry factors |gPL | of both (R/S)-Cb1 and (R/S)-Cb2 enantiomers in neat films were as high as 6.24×10-3 and 7.38×10-3 , respectively. Cb1 showed a deep blue emission peak at 434 nm in n-pentane. Interestingly, distinct fluorescence and CPL spectra were observed in solvents of different polarities due to the twisted intramolecular charge transfer effect, suggesting its potential use in solvent recognition. Meanwhile, Cb2 exhibited good AIECL property, excellent ECL stability and could be used for determining dopamine concentrations, suggesting its potential applications in biology and diagnosis.

Molecular sensors producing circularly polarized luminescence responses

Circularly polarized luminescence (CPL) refers to spontaneous emission with the oscillation axis of the emission's electric dipole rotating either right handedly or left handedly along the propagation axis. CPL provides a unique principle in luminescence visualization of target analytes: both the direction and magnitude of light polarization are sensitive to an asymmetric environment around a luminophore, which enables chirality sensing. In addition, CPL spectra are usually less complex than electric circular dichroism spectra because they emerge only from the lowest excited state. To exploit these benefits, researchers have vigorously developed CPL probes. This minireview provides the basic principles for creating molecular CPL activity, including asymmetric exciton coupling and helical intramolecular charge transfer. Emphasis is placed on molecular design. Analyte interactions can perturb the magnitude, reverse the polarization direction, or shift the peak wavelength of CPL, thereby enabling tractable strategies for creating CPL sensors. This minireview outlines selected examples of CPL sensors for physical stimuli, including changes in external magnetic fields, solvent polarity, and temperature. Probes capable of detecting chemical species, such as protons, metal ions, anions, amino acids, nucleosides and DNA, reactive oxygen species, and humidity, are also highlighted. These examples demonstrate the unique sensing utility of CPL sensors. We hope that this minireview will stimulate future research interest toward developing advanced CPL sensors with applications in bioimaging.

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

A Versatile Tetraphenylethene Derivative Bearing Excitation Wavelength Dependent Emission, Multistate Mechanochromism, Reversible Photochromism, and Circularly Polarized Luminescence and Its Applications in Multimodal Anticounterfeiting

Tunable Multicolor Circularly Polarized Luminescence via Co-assembly of One Chiral Electron Acceptor with Various Donors

Tunable Multicolor Circularly Polarized Luminescence via Co-assembly of One Chiral Electron Acceptor with Various Donors