Circularly Polarised Luminescence from Helically Chiral “Confused” N ,N ,O ,C -Boron-Chelated Dipyrromethenes (BODIPYs)

Recent advances in the development of enantiopure BODIPYs and some related enantiomeric compounds

During the process of developing smart chiroptical luminophores, small chiral organic dyes have emerged as candidates of utmost importance. In this regard, the chiral variants of boron dipyrromethene (BODIPY) serve as suitable molecules owing to their excellent photophysical properties such as high fluorescence quantum yields, narrow emission bandwidths with high peak intensities, high photo and chemical stability, and higher molar extinction coefficients. Thus, the last decade observed an influx of research from various research groups for the induction of chirality in originally achiral BODIPY. Among these, the generation of chiral centers at various positions in BODIPY favored the synthetic accessibility towards this particular chiral pool, which in turn is found to be applicable in various areas like photodynamic therapy, bio-imaging, dye-sensitized solar cells, optoelectronics, fluorescent indicators, dye lasers, and chiral sensing. This review summarizes these various aspects of creating stereogenic centers at various positions, like α, β, meso, or at boron, in BODIPYs.

Enantioselective Addition of 1,3,5,7-Tetramethyl-BODIPYs to Isatins by Bifunctional Quinine-Based Squaramides

This work describes the development of the first enantioselective addition reaction between 1,3,5,7-tetramethyl-BODIPYs and isatin derivatives. The reaction utilizes bifunctional quinine/squaramide organocatalysts and affords nine novel chiral BODIPY dyes under mild conditions, with enantioselectivities reaching up to 60%. The synthesized BODIPY-oxindoles exhibit high fluorescence emissions, consistent with their parent BODIPYs, and display tunable colors. A representative example demonstrates a remarkably high quantum yield of 0.78 compared to fluorescein. Notably, the newly created carbon-stereocenter on the isatin skeleton induces detectable asymmetry in the electronically decoupled BODIPY chromophore. This is confirmed by the presence of Cotton effects in the visible region of the electronic circular dichroism (ECD) spectra. Density Functional Theory calculations suggested that the model oxindole 3aa adopts an (R) absolute stereochemical configuration, unveiling key interactions between the catalyst and substrates.

Multicomponent synthesis of stereogenic-at-boron fluorophores (BOSPYR) from boronic acids, salicylaldehydes, and 2-formylpyrrole hydrazones

This work describes one-step syntheses of various stereogenic-at-boron fluorochromes (BOSPYR) via multicomponent reactions involving readily accessible boronic acids, salicylaldehydes, and 2-formylpyrrole hydrazones. The dyes absorb and emit in the visible region of the electromagnetic radiation, and are characterized by large Stokes shifts (2850-4930 cm-1) with weak fluorescence emissions (Φfl: 1.5-9.1%). Notably, the dimmed fluorescence of BOSPYRs recovers upon transition to viscous media (21-fold for 1a). The representative compound 1a exhibits clear Cotton effects with dissymmetry factors of ca. |gabs| ∼ 1.9 × 10-3 in the visible region, indicating efficient asymmetry induction to the chromophore. The X-ray molecular structure of 1a shows that the chromophore deviates from planarity by 17.2°, which may contribute significantly to the inherent chirality of the fluorophore. A computational examination of excited states by time-dependent density functional theory (TD-DFT) identifies the emission mechanism as arising from a locally-excited (LE) state.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Circularly Polarized Organic Light-Emitting Diodes Based on Chiral Hole Transport Enantiomers

The circularly polarized organic light-emitting diodes (CP-OLEDs) demonstrate promising application in 3D display due to the direct generation of circularly polarized electroluminescence (CPEL). But the chiral luminescence materials face challenges as intricated synthetic route, enantiomeric separation, etc. Herein, fresh CP-OLEDs are designed based on chiral hole transport material instead of chiral emitters. A pair of hole transport enantiomers (R/S-NPACZ) exhibit intense dissymmetry factors (|gPL|) about 5.0 × 10-3. With R/S-NPACZ as hole transport layers, CP-OLEDs are fabricated employing six achiral phosphorescence and thermally activated delayed fluorescence (TADF) materials with different wavelengths, in consistence with the generated CPEL spectra. The CP-OLEDs based on achiral red, green, and blue iridium(III) complexes exhibit external quantum efficiencies (EQEs) of 14.9%, 30.7%, and 14.1% with |gEL| factors of 8.8 × 10-4, 2.3 × 10-3, and 2.0 × 10-3, respectively. Moreover, the devices using achiral blue, blueish-green, and green TADF materials display EQEs of 24.1%, 17.9%, and 25.4% with |gEL| factors of 1.0 × 10-3, 3.6 × 10-3, and 2.2 × 10-3, respectively. As far as known, it is the first example of CP-OLEDs based on chiral hole transport materials, which act as the organic circularly polarizers and have potential to generate CPEL from achiral luminescence materials.

Azaborahelicene fluorophores derived from four-coordinate N,C-boron chelates: synthesis, photophysical and chiroptical properties

A series of six azaborahelicenes with varying electron-donor substitution at the 4-position of the aryl residue (i.e., naphthyl) or with variable π-extension of the aryl residue (thianthrenyl, anthryl, pyrenyl) was prepared with an efficient and flexible synthetic protocol. These different types of functionalization afforded notably pronounced intramolecular charge-transfer (ICT) character for the dyes with the strongest electron donor substitution (NMe2) or easiest to oxidize aryl residues, as evidenced by photophysical investigations. These effects also impact the corresponding chiroptical properties of the separated M- and P-enantiomers, which notably display circularly polarized luminescence (CPL) with dissymmetry factors in the order of magnitude of 10-4 to 10-3. Theoretical calculations confirm the optical spectroscopy data and are in agreement with the proposed involvement of ICT processes.

Red-Colored Circularly Polarized Luminescence from a Benzo-Fused BODIPY-BINOL Complex

Red emission with sharp bandwidth and high quantum yield is a desired characteristic for organic chromophores in optoelectronic, spintronic, and biomedical applications. Here, we observe circularly polarized luminescence (CPL) with these characteristics from a benzo-fused BODIPY-BINOL complex (1). Using time-resolved optical spectroscopy, electrochemistry, and density functional theory calculations, we showed that the emissive excited state of 1 does not have a charge-transfer (CT) character, unlike that of the regular BODIPY counterpart (2). The rigidity and the lack of CT character make this class of molecules an appealing platform for CPL-active molecules in the red spectral region, with ample room for improvement in the dissymmetry factor and brightness.

Recent advances in the construction of tetracoordinate boron compounds

Tetracoordinate boron compounds are a highly important class of molecules, which are the key intermediates in many organoboron-related chemical transformations and have unique luminescence properties. However, the synthesis of tetracoordinate boron compounds has never been reviewed. In this highlight, we summarize recent progress on the construction of racemic and chiral tetracoordinate borons, and hope to provide ideas for the assembly of them in more efficient ways, especially for the construction of boron-stereogenic compounds.

Synthesis and Stereodynamic and Emission Properties of Dissymmetric Bis-Aryl Carbazole Boranes and Identification of a CPL-Active B-C Atropisomeric Compound

We synthesized bis-aryl carbazole borane derivatives having emissive properties and axial chirality. The resolution of a thermally stable atropisomeric pair (compound 1b), due to a B-C chiral axis, was achieved by chiral stationary-phase high-performance liquid chromatography (CSP-HPLC). Complete photophysical properties of all compounds were measured and simulated by time-dependent density functional theory (TD-DFT) calculations of the complete fluorescence cycle, and circularly polarized luminescence spectra were obtained for the atropisomers of compound 1b, whose absolute configuration was derived using a TD-DFT simulation of the electronic circular dichroism (ECD) spectra.

Multi-colour circularly polarized luminescence properties of chiral Schiff-base boron difluoride complexes

A series of chiral Schiff-base boron difluoride complexes was synthesized and their photophysical properties were examined. These complexes showed multi-colour (blue, yellow and red) photoluminescence in solution and in the solid state with good emission quantum yield (Φ) depending on the π-systems of the ligands. The chiral complexes exhibited circularly polarized luminescence (CPL) with an absolute luminescence dissymmetry factor (g_lum) of up to the 1.3 × 10^-3 in solution and 1.9 × 10^-2 in the drop-cast film state. Density functional theory (DFT) and time-dependent (TD) DFT calculations were conducted to further understand the photophysical properties.

A Boron Dipyrromethene Chiral-at-Boron and Carbon with a Bent Geometry: Synthesis, Resolution and Chiroptical Properties

We report a boron dipyrromethene chiral-at-boron and carbon (B*C*-BODIPY) accessible through a two-pot, one-step synthesis—an interrupted Knoevenagel condensation. The ECD spectra of chiral HPLC resolved enantiomers show clear Cotton effects...

A Convenient Approach to meso-Uracil–4,4-Difluoro-4-bora-3a, 4a-diaza-s-indacene Derivatives

An effective and convenient protocol for the synthesis of 1-substituted uracil-6-carbaldehyde derivatives has been developed. A three-step sequence permits the preparation of uracil-6-carbaldehydes with various substituents at the N-1 in large quantities by using low-cost precursors. The aldehyde-functionalized uracils served as useful precursors for the preparation of meso-(1-substituted 6-uracil)-derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY). In this way, regioselectively functionalized BODIPYs with a direct connection to a nucleobase were prepared in yields of 30–45%. MALDI-TOF mass spectrometry, NMR, UV/vis absorption, and steady-state and time-­resolved fluorescence spectroscopies were used to characterize the structures and the spectroscopic/photophysical properties of the resultant dyes.

Synthesis, Structure and Photophysical Properties of a New Class of Inherently Chiral Boron(III) Chelates-The tert-Leucine Complexes

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O-salicylaldehyde-based derivative shows pronounced intramolecular charge-transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2-methylamino-benzaldehyde-derived N,N,O-chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature-dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)-based chelates, this derivative undergoes intersystem crossing to form a meta-stable triplet-excited state. X-ray crystal structures are reported for both compounds, along with simulated ECD spectra.

Studying the reactivity of alkyl substituted BODIPYs: first enantioselective addition of BODIPY to MBH carbonates

The first enantioselective addition of alkyl BODIPYs to Morita-Baylis-Hillman (MBH) carbonates is reported. This is the first reported enantioselective methodology using the methylene position of BODIPYs as a nucleophile. The reaction is efficiently catalyzed by cinchona alkaloids, achieving high enantioselectivities and total diastereoselectivity. The use of cinchona alkaloid pseudo enantiomers (chinine/cinchonine) allows us to obtain both pairs of enantiomers in similar yields and enantioselectivities, a common issue in this type of reaction. The photophysical study of these dyes (absorption and fluorescence) has been performed in order to determine their parameters and explore future possible application in bioimaging. In addition, electronic circular dichroism (ECD) studies supported by time-dependent density functional theory (TD-DFT) calculations were also performed.

Two-photon excitable boron complex based on tridentate imidazo[1,5-a]pyridine ligand for heavy-atom-free mitochondria-targeted photodynamic therapy

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand. The boron complex was found to have potential applications as not only a chiroptical material but also a heavy-atom-free mitochondria-targeted photosensitizer for cancer treatment.

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand.

Circularly polarized lanthanide luminescence for advanced security inks

Authenticating products and documents with security inks is vital to global commerce, security and health. Lanthanide complexes are widely used in luminescent security inks owing to their unique and robust photophysical properties. Lanthanide complexes can also be engineered to undergo circularly polarized luminescence (CPL), which encodes chiral molecular fingerprints in luminescence spectra that cannot be decoded by conventional optical measurements. However, chiral CPL signals have not yet been exploited as an extra security layer in advanced security inks. This Review introduces CPL and related concepts that are necessary to appreciate the challenges and potential of lanthanide-based, CPL-active security inks. We describe recent advances in CPL analysis and read-out technologies that have expedited CPL-active security ink applications. Further, we provide a systematic meta-analysis of strongly CPL-active Euⁱⁱⁱ, Tbⁱⁱⁱ, Smⁱⁱⁱ, Ybⁱⁱⁱ, Cmⁱⁱⁱ, Dyⁱⁱⁱ and Crⁱⁱⁱ complexes, discussing the suitability of their photophysical properties and highlighting promising candidates. We conclude by providing key recommendations for the development and advancement of the field.

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 Φ_F and g_lum 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.

Synthesis and properties of chiral fluorescent helicene-BODIPY conjugates

A series of chiral fluorescent helicene-BODIPY conjugates was prepared by the regioselective formylation of aza[4]helicene precursors and then an efficient one-pot two-step BODIPY synthesis (13 examples, 28-82%). Fused conjugates exhibit absorption and fluorescence properties (ΦF 30-45%) in the red visible domain, and a CPL signature could be measured at 605 nm (glum ±5 × 10-4). Photophysical and electronic properties were investigated and rationalized through first principles.

Synthesis and Functions of Oligomeric and Multidentate Dipyrrin Derivatives and their Complexes

The dipyrrin–metal complexes and especially the boron complex 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) have recently attracted considerable attention because of their interesting properties and possible applications. We have developed two unique and useful ways to extend versatility and usefulness of the dipyrrin complexes. The first one is the linear and macrocyclic oligomerization of the BODIPY units. These arrangements of the B–F moieties of the oligomerized BODIPY units provide sophisticated functions, such as unique recognition ability toward cationic guest, associated with changes in the photophysical properties by utilizing unprecedented interactions between the B–F and a cationic species. The second one is introduction of additional ligating moieties into the dipyrrin skeleton. The multidentate N2Ox dipyrrin ligands thus obtained form a variety of complexes with 13 and 14 group elements, which are difficult to synthesize using the original N2 dipyrrin derivatives. Interestingly, these unique complexes exhibit novel structures, properties, and functions such as guest recognition, stimuli-responsive structural conversion, switching of the optical properties, excellent stability of the neutral radicals, etc. We believe that these multifunctional dipyrrin complexes will advance the basic chemistry of the dipyrrin complexes and develop their applications in the materials and medicinal chemistry fields.

1 Introduction

2 Linear Oligomers of Boron–Dipyrrin Complexes

3 Cyclic Oligomers of Boron–Dipyrrin Complexes

4 A Cyclic Oligomer of Zinc–Dipyrrin Complexes

5 Group 13 Element Complexes of N2Ox Dipyrrins

6 Chiral N2 and N2Ox Dipyrrin Complexes

7 Group 14 Element Complexes of N2O2 Dipyrrins

8 Other N2O2 Dipyrrin Complexes with Unique Properties and Functions

9 Conclusion

Rapid time-resolved Circular Polarization Luminescence (CPL) emission spectroscopy

Circular polarisation luminescence (CPL) emission spectroscopy is a powerful tool for probing the fundamental chiroptical features of optically emissive chiral molecular systems. However, uptake of CPL spectroscopy has been impeded by the limitations of conventional scanning monochromator (SM) CPL spectrometers, which are costly to acquire and maintain, and typically require tens of minutes to acquire a typical CPL spectrum. Here, we demonstrate a design of CPL spectrometer which uses rapid readout solid state (SS) spectrometer detectors and a dual channel optical layout to acquire CPL spectra in as little as 10 milliseconds. We validate and demonstrate equivalent CPL measurement by measuring CPL spectra of two reference europium(III) complexes. Further, we demonstrate time-gated CPL acquisition, enabling long-lived CPL luminescence to be distinguished from short-lived emission of other fluorescent species. We anticipate that SS-CPL spectrometers will enable flexible, rapid, and relatively low-cost CPL spectroscopy for diverse applications.

Circular polarization luminescence (CPL) spectroscopy is a tool to study chiroptical systems, but the measurement process is generally very slow. The authors introduce a CPL technique with much faster acquisition, demonstrating meaningful time-dependent measurements and enabling new applications.

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.

Synthesis and Chiroptical Properties of Chiral Carbazole-Based BODIPYs

A series of carbazole-based boron dipyrromethenes (BODIPYs) 2 a-g bearing binaphthyl units have been synthesized by the Et₂ AlCl-mediated reaction of the corresponding BODIPY difluorides 1 a-g with 1,1'-binaphthalene-2,2'-diol. Substituents such as halogen, nitrile, and amino groups were tolerated under the reaction conditions, and the reaction of the phenylethynyl-substituted 1 h gave (R,R)-3 h bearing two binaphthyl units. The chiroptical properties of these dyes with different substituents were investigated by UV/Vis, CD, fluorescence, and circularly polarized luminescence (CPL) spectroscopy. The CD spectra showed Cotton effects in the absorption region of the BODIPY moieties. In addition, they showed CPL both in solution and in the solid state. Interestingly, several dyes recorded higher g_lum values in the solid state, probably due to intermolecular interactions. Because (R,R)-3 h recorded relatively low g_lum values, the diastereomer (R,S)-3 h was prepared. The (R,S) diastereomer showed intense CPL, which suggests a synergetic effect of the two binaphthyl groups. Finally, chiral carbazole-based BODIPY dimers have been synthesized for the first time and their chiroptical properties were investigated. They showed redshifted fluorescence and CPL, which reached the near-IR (NIR) region in the solid state.

Chiral Octahydro-Binaphthol Compound-Based Thermally Activated Delayed Fluorescence Materials for Circularly Polarized Electroluminescence with Superior EQE of 32.6% and Extremely Low Efficiency Roll-Off

Circularly polarized organic light-emitting diodes (CP-OLEDs) are particularly favorable for the direct generation of CP light, and they demonstrate a promising application in 3D display. However, up to now, such CP devices have suffered from low brightness, insufficient efficiency, and serious efficiency roll-off. In this study, a pair of octahydro-binaphthol (OBN)-based chiral emitting enantiomers, (R/S)-OBN-Cz, are developed by ingeniously merging a chiral source and a luminophore skeleton. These chirality-acceptor-donor (C-A-D)-type and rod-like compounds concurrently generate thermally activated delayed fluorescence with a small ΔE_ST of 0.037 eV, as well as a high photoluminescence quantum yield of 92% and intense circularly polarized photoluminescence with dissymmetry factors (|g_PL |) of ≈2.0 × 10^-3 in thin films. The CP-OLEDs based on (R/S)-OBN-Cz enantiomers not only display obvious circularly polarized electroluminescence signals with a |g_EL | of ≈2.0 × 10^-3 , but also exhibit superior efficiencies with maximum external quantum efficiency (EQE_max ) up to 32.6% and extremely low efficiency roll-off with an EQE of 30.6% at 5000 cd m^-2 , which are the best performances among the reported CP devices to date.

Configurationally Stable Platinahelicene Enantiomers for Efficient Circularly Polarized Phosphorescent Organic Light-Emitting Diodes

Chiral materials with circularly polarized luminescence (CPL) are potentially applicable for 3D displays. In this study, by decorating the pyridinyl-helicene ligands with -CF₃ and -F groups, the platinahelicene enantiomers featured superior configurational stability, as well as high sublimation yield (>90 %) and clear CPPL properties, with dissymmetry factors (|g_PL |) of approximately 3.7×10^-3 in solution and about 4.1×10^-3 in doped film. The evaporated circularly polarized phosphorescent organic light-emitting diodes (CP-PhOLEDs) with two enantiomers as emitters exhibited symmetric CPEL signals with |g_EL | of (1.1-1.6)×10^-3 and decent device performances, achieving a maximum brightness of 11 590 cd m^-2 , a maximum external quantum efficiency up to 18.81 %, which are the highest values among the reported devices based on chiral phosphorescent Pt^II complexes. To suppress the effect of reverse CPEL signal from the cathode reflection, the further implementation of semitransparent aluminum/silver cathode successfully boosts up the |g_EL | by over three times to 5.1×10^-3 .

Synthesis of chiral carbazole-based BODIPYs showing circularly polarized luminescence

Chiral carbazole-based BODIPYs with a binaphthyl unit were synthesized via an Al-mediated reaction. Et2AlCl was found to be a convenient reagent for the reaction to give the chiral BODIPYs in high yields. It has been shown for the first time that these chiral carbazole-based BODIPYs show circularly polarized luminescence (CPL) both in solution and in the solid state.

Circular dichroism and circularly polarised luminescence of bipyrenyl oligopeptides, with piperidines added in the peptide chains

Upon combining chiral peptides (the most basic chiral source) with pyrene moieties, we found that chiral oligopeptides bearing two-pendant pyrenyl units exhibited circularly polarised luminescence (CPL) originating from intramolecular excimers at 450-490 nm in various solvents, and the sign of their CPL signals depended on the type of solvent employed. The CPL and circular dichroism signs and intensities could be tuned by the introduction of a piperidine unit into the chiral peptide chain; thus, the obtained structure could be considered a practical Lock ON-OFF system for oligopeptide luminophores.

Modulating ICT emission: a new strategy to manipulate the CPL sign in chiral emitters

A new strategy to manipulate the circularly polarized luminescence (CPL) handedness in chiral emitters, based on modulating the population of an emissive ICT state, is proposed. Such a strategy is particularly interesting for conformationally rigid and non-aggregating chiral organic emitters, opening up new perspectives for the development of CPL applications based on organic molecules.

Advances in helicene derivatives with circularly polarized luminescence

This review summarizes the recent research progress of helicenes and their derivatives with circularly polarized luminescence properties.

Solvatochromism, acidochromism and aggregation-induced emission of propeller-shaped spiroborates

Propeller-shaped pyridyl-enolato-catecholate/-salicyl spiroborates (Sborepy1–6) were synthesized.