Circularly Polarized Luminescence from Helically Chiral N,N,O,O-Boron-Chelated Dipyrromethenes

Tetradentate carbene-anilido boron complexes: highly fluorescent dyes with larger Stokes shifts than BODIPY analogues

A new class of carbene-anilido boron complexes have been designed and synthesized. The complexes show intense fluorescence with large Stokes shift because of their charge-transfer excited states, different from typical BODIPY dyes. By using a chiral 1,1'-bi(2-naphthol) ligand, dyes exhibiting circularly polarized luminescence can also be facilely developed.

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.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

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.

Single Molecular Persistent Room-Temperature Phosphorescence and Circularly Polarized Luminescence from Binaphthol-Decorated Optically Innocent Cyclotriphosphazenes

Circularly polarized luminescence (CPL) features of BINOL-decorated cyclotriphosphazenes (CPs) are reported for the first time. The luminescence dissymmetry factor (glum ) of these compounds in chloroform solutions and polymethyl methacrylate (PMMA) thin films with wt 1 % doping concentrations are found to be 1.0×10-3 , and 2.9×10-3 , respectively. However, no CPL signal is observed for the pristine solids. The enantiomers (CP-(R)/CP-(S)) show ultraviolet photoluminescence (~350-360 nm) in solution and the solid state. These compounds show ~10 times larger absolute photoluminescence quantum yield (PLQY) than the simple BINOLs in the solutions state. In the solid state, CP-(R) shows larger PLQY than binaphthol-(R); in contrast, the S enantiomer shows lower PLQY than binaphthol-(S); this indicates that the isomer-dependent solid-state packing of these compounds plays a crucial role in controlling the PL. Thin films with more than 1 % doping concentration and pristine solids of these compounds do not show persistent room-temperature phosphorescence (pRTP) due to concentration-caused quenching. However, thin films with wt 1 % of these chiral emitters exhibit pRTP characteristics with a ~159-343 ms lifetime under vacuum. Theoretical calculations reveal that the cyclophosphazene acts as an optically innocent dendritic core, and the optical features of these compounds are dictated by the pendent BINOL chromophore.

Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ_Δ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE_S1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ_Δ is increased to 40%. The improved ISC yield is rationalized by the presence of a T₂ state, localized on the anthryl unit, with energy close to that of the S₁ state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T_z sublevel of the T₁ state overpopulated. The small zero-field splitting D parameter (-1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S₁/T_n energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.

Helical β-isoindigo-Based Chromophores with B-O-B Bridge: Facile Synthesis and Tunable Near-Infrared Circularly Polarized Luminescence

It is essential to create organic compounds that exhibit circularly polarized luminescence (CPL) in the near-infrared (NIR) range. Helicene-type emitters possess appealing chiroptical features, however, such NIR molecules are scarce due to a paucity of synthetic strategies. Herein, we developed a series of helical β-isoindigo-based B-O-B bridged aza-BODIPY analogs that were synthesized conveniently. The reaction of diimino-β-isoindigo with a heteroaromatic amine produced a restricted ligand cavity, which triggered off the generation of a B-O-B bridge. The B-O-B bridge led to distorted conformations that satisfy the helical requirements, resulting in excellent spectroscopic and chiroptical properties. Tunable CPL with the highest luminescence dissymmetry factor (g_lum ) of 1.3×10^-3 and a CPL brightness (B_CPL =11.5 M^-1  cm^-1 ) in the NIR region was achieved. This synthetic approach is expected to offer a new opportunity to chiral chemistry and increase flexibility for chiroptical tuning.

(BO)2 -Doped Tetrathia[7]helicene: A Configurationally Stable Blue Emitter

Helicenes combine two central themes in chemistry: extended π-conjugation and chirality. Hetero-atom doping preserves both characteristics and allows modulation of the electronic structure of a helicene. Herein, we report the (BO)2 -doped tetrathia[7]helicene 1, which was prepared from 2-methoxy-3,3'-bithiophene in four steps. 1 is formally derived by substituting two (Mes)B-O moieties in place of (H)C=C(H) fragments in two benzene rings of the parent tetrathia[7]helicene. X-ray crystallography revealed a dihedral angle of 50.26(9)° between the two terminal thiophene rings. The (P)-/(M)-1 enantiomers were separated by chiral HPLC and are configurationally stable at room temperature. The experimentally determined enantiomerization barrier of 27.4±0.1 kcal mol-1 is lower than that of tetrathia[7]helicene (39.4±0.1 kcal mol-1 ). The circular dichroism spectra of (P)- and (M)-1 show a perfect mirror-image relationship. 1 is a blue emitter (λem =411 nm) with a photoluminescence quantum efficiency of ΦPL =6 % (cf. tetrathia[7]helicene: λem ≈405 nm, ΦPL =5 %).

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.

Highly Fluorescent Bipyrrole-Based Tetra-BF2 Flag-Hinge Chromophores: Achieving Multicolor and Circularly Polarized Luminescence

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF₂ ) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S₁ excited state. As the rotational motion was suppressed around the 2,2'-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF₂ complexes can potentially realize novel photofunctional materials.

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.

Circularly polarized luminescence from helical N,O-boron-chelated dipyrromethene (BODIPY) derivatives

We report N,O-boron-chelated dipyrromethene derivatives exhibiting circularly polarized luminescence (CPL) in the red/near-infrared region, both in solution and the aggregated state. The CPL is originated from the helical chirality through intramolecular substitution of fluorine by an alkenolic substituent. The self-assembly of the fluorophores significantly enhances the |g_lum| values from 10^-4 to 10^-2.

Synthesis and photophysical properties of nitrated aza-BODIPYs

A series of nitrated aza-BODIPYs on the 2- and 6-positions were regioselectively synthesized and their photophysical properties were examined.

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...

3D Boranil Complexes with Aggregation-Amplified Circularly Polarized Luminescence

The development of small organic CPL-active molecules with large luminescent dissymmetry factors is highly demanded due to their promising applications in chiroptical devices and sensors. This work describes the design and synthesis of a new family of CPL-active BF₂ complexes, (R_p)/(S_p)-3a-3e, which were constructed by fusing a N̂O-chelated BF₂ complex with [2.2]paracyclophane. These complexes display aggregation-amplified CPL with moderate dissymmetry factors values and moderate quantum yields both in solution and in the solid state. In addition, these photophysical properties were rationalized via X-ray diffraction and TD-DFT calculations.

Circularly polarized luminescence of Eu(III) complexes with chiral 1,1'-bi-2-naphtol-derived bisphosphate ligands

The interest for lanthanide circularly polarized luminescence (CPL) has been quickly growing for 10 years. However, very few of these studies have involved correlation between the dissymmetry factor (g_lum ) and the chemical modifications in a series of chiral ligands. Four polymeric compounds of Eu(III) were prepared by using a series of binaphtyl derivatives for which the size of the π system as well as the number of stereogenic elements (i.e., the binaphtyl moiety) are modulated. The resulting {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) have been characterized by powder X-ray diffraction by comparison with the X-ray structures on single crystal of the Dy(III) analogs. In solution, the structure of the complexes is deeply modified and becomes monomeric. The nature of the ligand induces change in the shape of the CPL spectra in CH₂ Cl₂ solution. Furthermore, a large |g_lum | = 0.12 of the magnetic-dipole transition for the [Eu(hfac)₃ ((S,S,S)/(R,R,R)-L² )] complex involving the ligand with three stereogenic elements and an extended 𝜋 system has been measured. This report also shows CPL measurements in solid state for the series of {[Eu(hfac)₃ ((S)/(R)-L^x )]}_n (x = 1 and 3) and {[Eu(hfac)₃ ((S,S,S)/(R,R,R)-L^x )]}_n (x = 2 and 4) polymers.

Synthesis and Reactivity of 3,5-Diiodo-BODIPYs via a Concerted, Double Aromatic Finkelstein Reaction

3,5-Diiodo-8-aryl-BODIPYs are prepared from the corresponding 3,5-dichloro- and 3,5-dibromo- congeners via a double aromatic Finkelstein reaction with NaI in refluxing propionitrile, in yields from 51 to 100%. Rate enhancement is observed for 3,5-dibromo-BODIPYs (ArBr > ArCl) suggesting a concerted S_NAr mechanism for this transformation. Thus formed 3,5-diiodo-BODIPYs are examined as coupling partners in Migita-Kosugi-Stille, Mizoroki-Heck, Sonogashira, and Suzuki-Miyaura reactions, providing routes toward complex BODIPY architectures.

Insight into the Influence of the Chiral Molecular Symmetry on the Chiroptics of Fluorescent BINOL-Based Boron Chelates

The prominent influence of the molecular symmetry, as defined by the symmetry point group, on the chiroptical behavior (electronic circular dichroism and, especially, circularly polarized luminescence) of simple fluorescent boron chelates (BODIPY and related BOPHY analogues) is studied and discussed. It is shown that increasing the dye symmetry by means of the D₃ chiral symmetry group is a workable design option to enhance the level of differential emission of right- and left-circularly polarized light in BODIPY dyes and related emitters, and that the influence of the level of symmetry is stronger than the influence of the higher number of chiral moieties perturbing the acting achiral chromophore.

Planar Chiral B-N Heteroarenes Based on [2.2]Paracyclophane as Circularly Polarized Luminescence Emitters

Planar chiral boron-nitrogen heteroarenes based on [2.2]paracyclophane were successfully synthesized in a few steps as a new family of circularly polarized luminescence emitters. It represents the first case of boron-nitrogen heteroarenes with planar chirality. Those compounds have been demonstrated to exhibit strong circularly polarized luminescence signals and high quantum yields, in both solution and doped film (with g_lum up to 5.0 × 10^-3 and Φ_solution up to 73%).

Twist to Boost: Circumventing Quantum Yield and Dissymmetry Factor Trade-Off in Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) enables promising applications in asymmetric photonics. However, the performances of CPL molecules do not yet meet the requirements of these applications. The shortcoming originates from the trade-off in CPL between the photoluminescence quantum yield (PLQY) and the photoluminescence dissymmetry factor (g_PL). In this study, we developed a molecular strategy to circumvent this trade-off. Our approach takes advantage of the strong propensity of [Pt(N^C^N)Cl], where the N^C^N ligand is 1-(2-oxazoline)-3-(2-pyridyl)phenylate, to form face-to-face stacks. We introduced chiral substituents, including (S)-methyl, (R)- and (S)-isopropyl, and (S)-indanyl groups, into the ligand framework. This asymmetric control induces torsional displacements that give homohelical stacks of the Pt(II) complexes. X-ray single-crystal structure analyses for the (S)-isopropyl Pt(II) complex reveal the formation of a homohelical dimer with a Pt···Pt distance of 3.48 Å, which is less than the sum of the van der Waals radii of Pt. This helical stack elicits the metal-metal-to-ligand charge-transfer (MMLCT) transition that exhibits strong chiroptical activity due to the electric transition moment making an acute angle to the magnetic transition moment. The PLQY and g_PL values of the MMLCT phosphorescence emission of the (S)-isopropyl Pt(II) complex are 0.49 and 8.4 × 10^-4, which are improved by factors of ca. 6 and 4, respectively, relative to the values of the unimolecular emission (PLQY, 0.078; g_PL, 2.4 × 10^-4). Our photophysical measurements for the systematically controlled Pt(II) complexes reveal that the CPL amplifications depend on the chiral substituent. Our investigations also indicate that excimers are not responsible for the enhanced chiroptical activity. To demonstrate the effectiveness of our approach, organic electroluminescence devices were fabricated. The MMLCT emission devices were found to exhibit simultaneous enhancements in the external quantum efficiency (EQE, 9.7%) and the electroluminescence dissymmetry factor (g_EL, 1.2 × 10^-4) over the unimolecular emission devices (EQE, 5.8%; g_EL, 0.3 × 10^-4). These results demonstrate the usefulness of using the chiroptically active MMLCT emission for achieving an amplified CPL.

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.

Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φfl ) of 18-24 % in solution, green or yellow solid-state emission (Φfl up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12×10-2 . Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φfl of up to 47 % in CH2 Cl2 and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that of H1. Single-crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.

Planar chiral boron difluoride complexes showing circularly polarized luminescence

A novel family of boron difluoride complexes based on [2.2]paracyclophane have been designed and facilely synthesized.

Isopinocampheyl-based C-BODIPYs: a model strategy to construct cost-effective boron-chelate emitters of circularly polarized light

Use of building blocks coming from Nature (Chiral Pool) as a new strategy towards cheap and efficient CPL emitters based on boron chelates.

3,5-Anthryl-Bodipy dyad/triad: Preparation, effect of F-B-F induced conformation restriction on the photophysical properties, and application in triplet-triplet-annihilation upconversion

We prepared a series of compact Bodipy-anthryl electron donor/acceptor triads and dyads by attaching anthryl moieties at the 3-,5-positions of the Bodipy core, with a novel conformation restriction approach, to study the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The conformation restrictions are imposed by the BF₂ unit of Bodipy without invoking the previously reported method with 1,7-dimethyl or 1,3-dimethyl groups. Our new approach shows a few advantages, including the stronger electron accepting ability of the methyl-free Bodipy core (reduction potential anodically shifted by +0.3 V vs the methylated Bodipy), red-shifted absorption (by 21 nm), and longer triplet state lifetime (372 µs vs 126 µs). The effects of the different mutual orientations of the electron donor and acceptor on ultraviolet-visible absorption, fluorescence, triplet state quantum yields, and lifetimes were studied. Triads with orthogonal geometries show higher singlet oxygen quantum yields (Φ_Δ = 37%) than those with more coplanar geometries. Since the non-radiative decay for the S₁ state is significant in the parent Bodipy chromophore (Φ_F = 6.0%), we propose that in dyads/triads, the charge separation and recombination-induced ISC outcompete the non-radiative decay to the ground state, which is new in the study of SOCT-ISC. Density functional theory computation indicated a shallow torsion potential energy curve as compared to the meso-anthryl-Bodipy dyad analog, which may contribute a low triplet state quantum yield of the new dyads/triads. Triplet-triplet annihilation upconversion was performed with the electron donor/acceptor dyads as the triplet photosensitizer, with an upconversion quantum yield of 12.3%.

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

Theoretical Investigation of the Circularly Polarized Luminescence of a Chiral Boron Dipyrromethene (BODIPY) Dye

Over the last decade, molecules capable of emitting circularly polarized light have attracted growing attention for potential technological and biological applications. The efficiency of such devices depend on multiple parameters, in particular the magnitude and wavelength of the peak of emitted light, and also on the dissymmetry factor for chiral applications. In light of these considerations, molecular systems with tunable optical properties, preferably in the visible spectral region, are particularly appealing. This is the case of boron dipyrromethene (BODIPY) dyes, which exhibit large molecular absorption coefficients, have high fluorescence yields, are very stable, both thermally and photochemically, and can be easily functionalized. The latter property has been extensively exploited in the literature to produce chromophores with a wide range of optical properties. Nevertheless, only a few chiral BODIPYs have been synthetized and investigated so far. Using a recently reported axially chiral BODIPY derivative where an axially chiral BINOL unit has been attached to the chromophore unit, we present a comprehensive computational protocol to predict and interpret the one-photon absorption and emission spectra, together with their chiroptical counterparts. From the physico-chemical properties of this molecule, it will be possible to understand the origin of the circularly polarized luminescence better, thus helping to fine-tune the properties of interest. The sensitivity of such processes require accurate results, which can be achieved through a proper account of the vibrational structure in optical spectra. Methodologies to compute vibrationally-resolved electronic spectra can now be applied on relatively large chromophores, such as BODIPYs, but require more extensive computational protocols. For this reason, particular attention is paid in the description of the different steps of the protocol, and the potential pitfalls. Finally, we show how, by means of appropriate tools and approaches, data from intermediate steps of the simulation of the final spectra can be used to obtain further insights into the properties of the molecular system under investigation and the origin of the visible bands.

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.

A new chiral boron-dipyrromethene (BODIPY)-based fluorescent probe: molecular docking, DFT, antibacterial and antioxidant approaches

A new chiral BODIPY-based fluorescent compound, 5-bromo-4,4-difluoro-3(S)-1-phenylethyl)amino) BODIPY, 4 was synthesized for biomedical applications. Optical, antimicrobial, antioxidant properties of the compound 4 are investigated. The partition coefficient suggested that the compound 4 has the potential to be developed as an active antibacterial and antioxidant candidate. In this context, antibacterial assay was carried out for compound 4 against various bacterial strains, revealing maximum inhibition zone (24 ± 2.19 mm) in Escherichia coli. Moreover, results of antioxidant activity of compound 4 revealed IC₅₀ values to be greater than ascorbic acid. Molecular docking has given brief insight about the binding of the compound 4, suggesting that it has a strong potential to inhibit bacterial target enzymes viz., DNA gyrase, enzymes in the type II fatty acid synthesis and Ddl (d-alanine: d-alanine ligase) in peptidoglycan synthesis. The molecular geometry and electrostatic potential of compound 4, was established by DFT (Density Functional Theory) calculations.AbbreviationsBBBblood‒brain barrierBDEbond dissociation energyBODIPYboron-dipyrromethaneDdlD-alanine:D-alanine ligaseDDQ2,3-dichloro-5,6-dicyano-1,4-benzoquinoneDFTdensity functional theoryDNAdeoxyribonucleic acidDPPH1,1‒diphenyl‒2‒picrylhydrazylNBSN-bromo succinimideROSreactive oxygen speciesUV-visultraviolet-visibleFMOfrontier molecular orbitalsHOMOhighest occupied molecular orbitalLUMOlowest unoccupied molecular orbitalCommunicated by Ramaswamy H. Sarma.

Triarylborane-Based Helical Donor-Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

The synthesis and characterization of 10-(dimesitylboryl)-N,N-di-p-tolylbenzo[c]phenanthren-4-amine (3-B(Mes)₂ -[4]helix-9-N(p-Tol)₂ 1) and 13-(dimesitylboryl)-N,N-di-p-tolyldibenzo[c,g]phenanthren-8-amine (3-B(Mes)₂ -[5]helix-12-N(p-Tol)₂ 2) are reported herein. Their electrochemical and photophysical properties have been studied experimentally and theoretically. The donor and acceptor-substituted helicene derivatives exhibit moderate fluorescence quantum yields in THF (Φ_f =0.48 and 0.61 for 1 and 2, respectively), which are higher than unsubstituted ones (Φ_f =0.18 for [4]helicene; Φ_f <0.05 for [n]helicenes (n≥5)). In the solid state, the Φ_f values are higher (Φ_f =1.00 and 0.55 for 1 and 2, respectively) than those in solution, most likely due to the restrictions of molecular motions. The S₁ ←S₀ transitions of 1 and 2 are predominately HOMO→LUMO transitions. Upon excitation with UV light, the interplanar angle between the two terminal aryl rings of the [5]helix core of 2 decreases (S₁ state compared with S₀ state), which is similar to placing a spring under an external force.

A series of novel BODIPY-fluorene copolymers: Synthesis, characterization, optical-electronic and nonlinear optical properties

A series of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) with different connection sites and conjugation length and fluorene alternating conjugated polymers have been synthesized via palladium-catalyzed Suzuki coupling reaction. The polymers were fully characterized by ¹H NMR spectroscopy and gel permeation chromatography (GPC), and their properties were measured by thermogravimetric analysis (TGA), UV-vis absorption spectroscopy, cyclic voltammetry (CV), photoluminescence (PL) spectroscopy, the Z-scan technique, and theoretical calculation using density-functional theory (DFT) method. The results indicate that the connection sites and conjugation length of BODIPY have great effect on the optical-electronic and nonlinear optical properties of the conjugated BODIPY- fluorene copolymers although they don't affect the polymerization process. The absorption and emission peaks of P1 exhibit red shifts compared to those of P2 and P3. The polymers show good nonlinear optical properties that are closely related to the connection sites of the BODIPY pendants. P1 exhibits relatively richer nonlinear optical (NLO) properties than P2 and P3. The third order nonlinear optical coefficient (χ⁽³⁾) of P1 is 3.57 × 10^-11 esu, which is ~4 times larger than those of P2 and P3.

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.