Chiral Dye Research Articles

Systematic Investigation of Optical and Chiroptical Properties of Chiral π‐Conjugated Oligomers in Solution and Thin Films

AbstractFour families of structurally related chiral π‐conjugated oligomers bearing a hydroquinone (HQ), 1,5‐dihydroxynaphthalene (Naph), benzo[1,2‐b:4,5‐b’]dithiophene‐4,8‐diol (BDT) or 9H‐carbazole (CBZ) central core, for a total of 11 new compounds, have been synthesized by simple and versatile synthetic strategies involving Pd‐catalyzed cross‐coupling protocols: a Suzuki‐Miyaura reaction, a Sonogashira/desilylation/Sonogashira sequence or a Sonogashira/desilylation/acyl Sonogashira sequence. A systematic investigation of the optical and chiroptical features of the final chiral dyes was performed in chloroform solution and on spin‐coated thin films. The ECD investigation was crucial in revealing that, even within the same family of organic dyes, minor variations in the nature and length of the π‐conjugated backbone can result in very different solid‐state arrangements at different hierarchical scales.

Circularly Polarized Luminescence Switching, Chirality Self‐Sorting, and Cell Imaging of Chiral Rhodamine Dyes

AbstractRhodamine dyes, as star molecules of fluorescent dyes, are critical tools in modern analytical and biochemical research. However, switching their chiroptical properties still remains a big challenge because of the absence of a suitable synthesis method to incorporate through conjugation a chiral fragment into the chromophore core of rhodamine. Herein, a simple and straightforward way is utilized to link two rhodamine chromophores by different chiral bridges. The conjugated RRR/SSS birhodamine dyes with one axial chirality, which is constructed directly by two rhodamine chromophores, and two spirocyclic chiralities show the interesting circular dichroism and circularly polarized luminescence switching and lysosome‐targetable properties. On the other hand, the unconjugated RR/SS birhodamine dyes with two chiral carbons do not have such chiroptical switching properties but do have different amazing cellular staining properties. The RR birhodamine dye can stain both lysosomes and mitochondria, but SS birhodamine dye can stain lysosomes only. Moreover, the mechanisms of chiroptical switching and diastereoselectivity are investigated as well, based on single‐crystal structures and density functional theory calculations. Therefore, a new platform is afforded here in designing chiral dyes for chiroptical switching and cell imaging applications.

Liquid Crystal Assembly for Ultra-dissymmetric Circularly Polarized Luminescence and Beyond.

Circularly polarized luminescence (CPL) is attracting much interest because it can carry extensive optical information. CPL shows left- or right-handedness and can be regarded as part of high-level visual perception to supply an extra dimension of information with regard to regular light. A key to meeting the needs for practical applications is to develop the emerging field of ultra-dissymmetric CPL. Chiral liquid crystal (LC) assemblies─otherwise referred to as cholesteric liquid crystals (CLCs)─are essentially organized helical superstructures with a highly ordered one-dimensional orientation, and distinctly superior to regular helical supramolecules. CLCs can achieve a perfect equilibrium of molecular short-range interaction and long-range orientational order, enabling molecule-scale chirality on a helical pitch and observable scale. LC assembly could be an ideal strategy for amplifying chirality, making it accessible to ultra-dissymmetric CPL. Herein, we focused on some basic but important issues regarding CPL: (i) How can CPL be created from chiral dyes? (ii) Is the chirality of luminescent dyes an essential factor for the generation of CPL? That is, can all chiral dyes emit CPL and vice versa? (iii) How can CPL be transferred within intermolecular systems, and what principles of CPL transmission should be followed? Given these queries and our work, in this Perspective we discuss the generation, transmission, and modulation of CPL with chiral LC assembly, aiming to design and build up novel chiroptical materials. Recent applications of CPL-active LC microstructures in three-dimensional displays, circularly polarized lasers, and asymmetric catalysis are also discussed.

Modulating Energy Transfer and Light-Emitting Colors by Chiral Dye Exchange and Achiral-to-Chiral Dye Conversion.

New chiral and achiral dyes were synthesized by using clean, highly efficient Diels-Alder 'click' reactions. We prepared pure endo- and exo- stereoisomers and blue-light-emitting single crystal of an exo-isomer. 1 H NMR spectroscopy and single-crystal X-ray diffraction reveal the precise spatial configurations of endo- and exo-dye stereoisomers. Single-crystal X-ray diffraction results reveal the dynamic nature of Diels-Alder covalent bonds. By reversible formation and cleavage of dynamic covalent bonds, an achiral dye was converted to a chiral dye, where achiral dansyl dye moiety was replaced by chiral moiety. Before achiral-to-chiral dye conversion, the dye system displayed dansyl acceptor green light-emitting color due to intramolecular fluorescence resonance energy transfer, where the blue emission of pyrene donor fluorophore was not observed. After achiral-to-chiral dye conversion, light-emitting colors of dye system change from green to blue, where the pyrene-to-dansyl fluorophore energy transfer was suppressed, and pyrene fluorophore blue emission was restored. Chiral transfer occurs in a chiral dye solid from non-chromophore alky chirality to pyrene chromophore. Thus, the chiral dye solid displayed chiral optical behaviors, i. e., circular dichroism and circularly polarized luminescence.

Blue Axially Chiral Biphenyl Based Thermally Activated Delayed Fluorescence Materials for Efficient Circularly Polarized OLEDs

AbstractThis work presents two axial chiral biphenyl based blue thermally activated delayed fluorescence materials (M‐BPCZ4 and P‐BPCZ4) with four carbazole (CZ) donors and two cyano (CN) acceptors. The rigid CZ‐CN based molecular structures with multiple chromophores result in small gaps among charge transfer states (CTs), and the spin−orbit coupling between CT and locally excited states pushes the reverse intersystem crossing (RISC) process. Besides, this chiral luminescence center strategy with the close distance between chiral units and chiral luminescence region contributes to satisfying circularly polarized luminescence properties with dissymmetry factor |gPL| of 5.0 × 10−3 for (R/S)‐M‐BPCZ4 and 4.7 × 10−3 for (R/S)‐P‐BPCZ4 enantiomers, respectively. Furthermore, the positions of other two CZ units have great effects on the racemization temperatures and circularly polarized electroluminescence properties of the enantiomers. The chiral dyes doped circularly polarized organic light‐emitting diodes compatibly display good performances with external quantum efficiency of 18.3% with slight efficiency roll‐off and high electroluminescence |gEL| of 5.5 × 10−3.

Synthesis of Chiral Carbazole-Based Porphyrins and Bodipys

We have explored the synthesis and functionalization of carbazole-based porphyrins and BODIPYs showing NIR absorption and solid-state fluorescence. Here, we synthesized chiral carbazole-based porphyrins and BODIPYs. The former showed Cotton effect in the NIR region, while the latter showed CPL both in solution and in the solid state. In this presentation, chiroptical properties of this chiral dyes are reported.

Random laser radiation behavior of liquid crystal in photonic crystal fiber carrier

A hollow-core photonic-crystal fiber filled with a mixture of nematic liquid crystal TEB30A, chiral agent S-811 and laser dye PM597 was pumped by a frequency-doubled Nd:YAG laser with a wavelength of 532 nm. The laser emission spectra was measured and the random laser radiation behavior in the photonic-crystal fiber carrier was investigated. When side-pumping was applied to the fiber, the emitted random laser with a wider radiation direction from the side face had a wavelength range of 590−605 nm and an FWMH of 0.3 nm. When end-pumping was employed to the fiber, the emitted random laser from the end face had a wavelength range of 580−605 nm and an FWMH of 0.3 nm. After the sample was heated to the isotropic temperature, the laser emission with both pumping methods was shut down. The experimental results demonstrate that the dye doped liquid crystal mixture in the micropore induce the random laser emission in the photonic-crystal. The change in the mean free path of photon transport and the fluctuation of the dielectric tensor of chiral nematic liquid crystals with temperature are the main factors affecting the laser intensity.

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.

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity

AbstractPreviously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double‐cell magnetic circular dichroism and magnetic ROA experiments with copper‐porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity.

Previously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double‐cell magnetic circular dichroism and magnetic ROA experiments with copper‐porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

Synthesis of Chiral Donor-Acceptor Dyes to Study Electron Transfer Across a Chiral Bridge.

Chiral organic dyes comprising a donor (D), spacer (S), primary acceptor (A1), chiral bridge (B(S)), and final acceptor/radical (A2)/(R) have been synthesized and fully characterized. The goal behind this synthetic pursuit is to study whether the chiral dyes can impart a chiral-induced spin selectivity (CISS) spin-filtering effect during an intramolecular charge-transfer (CT) process. Additionally, appending a stable free radical (SFR) allows the study of how an unpaired spin influences the CT state. The dyes are reported to vary in the position of (A1) with respect to (B(S)). Two series of dyes have been synthesized: one series incorporates (A1) before (B(S)) and the second series places (A1) after (B(S)). In the case where (A1) is before (B(S)), CT is observed between (D) and (A1), but the CT state does not transverse over the chiral bridge to the final (A2)/(R) termini. However, when (A1) is placed after (B(S)), CT over or through the bridge occurs. Computations support the experimental data, which indicate that it is necessary to have (A1) located after (B(S)) to achieve CT over the chiral bridge. Hence, the second set of dyes are optimal candidates to explore intramolecular CISS.

Aggregation-Induced Circularly Polarized Luminescence from Boron Complexes with a Carbazolyl Schiff Base.

A variety of carbazolyl-appended Schiff bases were readily synthesized from 1-formylcarbazoles and aniline derivatives. Boron complexation of the resulting ligands allowed for facile preparation of new carbazole-based BODIPY analogues showing solid-state fluorescence. Furthermore, some dyes were converted into chiral compounds through the Et2 AlCl-mediated incorporation of a binaphthyl unit. The chiral dyes showed aggregation-induced fluorescence and circularly polarized luminescence (CPL) with the ΦF and glum of up to 0.22 and -3.5×10-3 , respectively, in the solid state. The solid-state fluorescence and CPL were well characterized by the crystal packing analyses and DFT calculations.

Visible Light Electromagnetic Interaction of PM567 Chiral Dye for Asymmetric Photocatalysis, a First-Principles Investigation

In asymmetric photocatalytic reactions, it is necessary to study the mechanism of the asymmetric electromagnetic interaction between molecules and light. In this work, we theoretically studied the electromagnetic interactions between the light-induced charge transfer reaction and the chiral reaction of PM567 dye. We found that the chiral responses of molecules in different wavelength ranges were partially due to pyrromethene and binaphthalene. Therefore, the catalytic sites with different chirality also corresponds to the two-part groups. Through quantitative analysis, we found the entire analysis process to be complete and self-consistent.

Synthesis of Chiral Carbazole-Based Porphyrins and BODIPYs

We have explored the synthesis and functionalization of carbazole-based porphyrins and BODIPYs showing NIR absorption and solid-state fluorescence, respectively. Here, we synthesized chiral carbazole-based porphyrins and BODIPYs. The former showed Cotton effect in the NIR region, while the latter showed CPL both in solution and in the solid state. In this presentation, chiroptical properties of these chiral dyes are reported.

Chiral Diketopyrrolopyrrole-Helicene Polymer With Efficient Red Circularly Polarized Luminescence.

Chiral diketopyrrolopyrrole (DPP)-helicene polymers were synthesized to develop efficient red circularly polarized (CP) light emitters. These original chiral dyes display intense electronic circular dichroism (ECD) and CP luminescence (CPL) in the far-red spectral region owing to the presence of excitonic coupling between achiral DPPs within the chiral environment of the polymeric structure. This work affords an interesting example illustrating the potential of π-conjugated helical polymers for chiral optoelectronic applications.

A metal-free blue chromophore derived from plant pigments.

Blue natural pigments are rare, especially among plants. However, flowering species that evolved to attract Hymenoptera pollinators are colored by blue anthocyanin-metal complexes. Plants lacking anthocyanins are pigmented by betalains but are unable to produce blue hues. By extending the π-system of betalains, we designed a photostable and metal-free blue dye named BeetBlue that did not show toxicity to human hepatic and retinal pigment epithelial cells and does not affect zebrafish embryonal development. This chiral dye can be conveniently synthesized from betalamic acid obtained from hydrolyzed red beetroot juice or by enzymatic oxidation of l-dopa. BeetBlue is blue in the solid form and in solution of acidified polar molecular solvents, including water. Its capacity to dye natural matrices makes BeetBlue the prototype of a new class of low-cost bioinspired chromophores suitable for a myriad of applications requiring a blue hue.

Azahelicene‐Fused BODIPY Analogues Showing Circularly Polarized Luminescence

AbstractHelical carbazole‐based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene‐incorporated BF2 dyes were elucidated by x‐ray diffraction analysis. DFT calculations revealed that the π‐conjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene‐fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.

Azahelicene-Fused BODIPY Analogues Showing Circularly Polarized Luminescence.

Helical carbazole-based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene-incorporated BF2 dyes were elucidated by x-ray diffraction analysis. DFT calculations revealed that the π-conjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene-fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.

The amplified circularly polarized luminescence regulated from D-A type AIE-active chiral emitters via liquid crystals system.

Two pairs of D-A type AIE-active chiral dyes (R-/S-1 and R-/S-2) as chiral emitters doped into nematic liquid crystals (5CB, N-LCs) can form chiral nematic liquid crystals (N*-LCs) and regulate the amplified circularly polarized luminescence (CPL) signals. R-/S-2 with stronger D-A structure and more planar conformation could induce higher CPL response (glum = ±0.37) via the supramolecular self-assembly in N*-LCs system.

Chiroptical detection of a model ruthenium dye in water by circularly polarized-electrochemiluminescence.

We demonstrate the possibility to detect selectively the two single enantiomers of a model [Ru(bpy)3]2+-based dye by circularly polarized-electrochemiluminescence (CP-ECL). This new aspect of the ECL emission combines the chiral information intrinsic to CPL methods with an electrogeneration of the excited state. Thus, it opens the possibility to perform ECL-based bioassays or microscopy with efficient chiral dyes.