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

Polycationic Open-Shell Cyclophanes: Synthesis of Electron-Rich Chiral Macrocycles, and Redox-Dependent Electronic States

π-Conjugated chiral nanorings with intriguing electronic structures and chiroptical properties have attracted considerable interests in synthetic chemistry and materials science. We present the design principles to access new chiral macrocycles (1 and 2) that are essentially built on the key components of main-group electron-donating carbazolyl moieties or the π-expanded aza[7]helicenes. Both macrocycles show the unique molecular conformations with a (quasi) figure-of-eight topology as a result of the conjugation patterns of 2,2',7,7'-spirobifluorenyl in 1 and triarylamine-coupled aza[7]helicene-based building blocks in 2. This electronic nature of redox-active, carbazole-rich backbones enabled these macrocycles to be readily oxidized chemically and electrochemically, leading to the sequential production of a series of positively charged polycationic open-shell cyclophanes. Their redox-dependent electronic states of the resulting multispin polyradicals have been characterized by VT-ESR, UV/Vis-NIR absorption and spectroelectrochemical measurements. The singlet (ΔES-T=-1.29 kcal mol-1) and a nearly degenerate singlet-triplet ground state (ΔES-T(calcd)=-0.15 kcal mol-1 and ΔES-T(exp)=0.01 kcal mol-1) were proved for diradical dications 12+2⋅ and 22+2⋅, respectively. Our work provides an experimental proof for the construction of electron-donating new chiral nanorings, and more importantly for highly charged polyradicals with potential applications in chirospintronics and organic conductors.

Simultaneous discovery of chiral and achiral dyes: elucidating the optical functions of helical and flag-hinged boron tetradentate complexes

The construction of novel complexes can lead to the manifestation of unexpected structures and properties, thereby making chemical exploration in experiments a potential source for novel discoveries. In this study, by reacting 6,6'-dihydrazineyl-2,2'-bipyridine with acyl chlorides and subsequently coordinating with boron trifluoride, two different boron-tetradentate ligand complexes were simultaneously generated. One of these complexes exhibited a unique structure in which tetra-BF2 moieties coordinated to all four coordination sites of the ligand molecule, forming a flag-hinged structure around the bipyridine part. The second complex featured a helical structure formed by the hybridization of two BF2 and one B-O-B moieties, representing a highly unusual form of the complex. The structures of these two boron complexes were consistently observed when various substituted acyl chlorides were employed. Furthermore, it was found that enhancing electron-donor properties could induce a redshift in emissions. Utilizing the dimethylamino group as the proton receptor promoted a yellow-to-blue fluorescence switch in the tetra-BF2 complex and an OFF/ON fluorescence in the B-O-B bridged complex upon protonation. The helical chirality observed in the latter complex resulted in stable (P)/(M)-enantiomers after optical resolution. This complex exhibited circular dichroism with a |gabs| of up to 1.2 × 10-2 and circularly polarized luminescence with a |glum| on the order of 10-3 in solution and polymer film.

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.

Axially-chiral boramidine for detailed (chir)optical studies

The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence (Φ up to 94%), and ca. |10-3| gabs and glum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin-orbit coupling between the lowest singlet excited state and an upper triplet state.

Catalytic asymmetric construction of helicenes via transformation of biaryls

This review showcases a systematic overview of the available tools for the catalytic asymmetric transformation of biaryl substrates toward the construction of challenging enantioenriched helicenes and the conceptual aspects associated with each type of transformation. Depending on the properties of the biaryl and the nature of the process, several methodologies have been developed, including olefin metathesis, hydroarylation of alkynes, C-X (X = C, O, N) coupling, and C-H functionalization. Pioneering studies and an array of representative reactions are discussed to underscore the potential of these synthetic protocols.

Pyrrole βC-B-N Fused Porphyrins: Molecular Structures and Opto-Electrochemical Studies

Herein, we report the design, synthesis, structure, and electrochemical study of doubly βC-B-N fused Ni(II) porphyrins (1-trans, 1-cis, 2-trans, and 2-cis). These compounds have been synthesized from A2B2 type dipyridyl Ni(II) porphyrins (Ar=Ph for 1 a; Ar=C6F5 for 2 a) via Lewis base-directed electrophilic aromatic borylation reactions. The solution state structures of these compounds have been established using 1H NMR, 11B NMR, 1H-1H COSY, 1H-13C HSQC, and 19F-13C HSQC NMR techniques. Single crystal X-ray analysis have revealed that 1-trans, 1-cis, and 2-trans adopt ruffled conformations, with alternate meso-carbons on the opposite sides of the mean porphyrin plane. The Soret bands in the absorption spectra of the B-N fused molecules are ~40 nm redshifted compared to unfused Ni(II) porphyrin precursors. The B-N fusion have diminished the redox potential of fused porphyrins. Although 1-trans and 1-cis, show four oxidation processes, 2-trans and 2-cis show only three oxidation processes. DFT studies have revealed that the tetrahedral geometry of the boron has induced a twist in the π-conjugation, which destabilizes the HOMO and stabilizes the LUMO in 1-trans, 1-cis, 2-trans, and 2-cis.

π-Extended Diaza[7]helicenes with Dual Negatively Curved Heptagons: Extensive Synthesis and Spontaneous Resolution into Strippable Homochiral Lamellae with Helical Symmetry

We report a unique category of π-extended diaza[7]helicenes with double negative curvatures. This is achieved by two-fold regioselective heptagonal cyclization of the oligoarylene-carbazole precursors through either intramolecular C-H arylation or Scholl reaction. The fusion of two heptagonal rings in the helical skeleton dramatically increases the intramolecular strain and forces the two terminal carbazole moieties to stack in a compressed fashion. The presence of the deformable negatively curved heptagonal rings endows the resulting diaza[7]helicenes with dynamic chiral skeletons, aggregation-induced emission feature and relatively low racemization barrier of ca. 25.6 kcal mol-1 . Further π-extension on the carbazole moieties subsequently leads to a more sophisticated C2 -symmetric homochiral triple helicene. Notably, these π-extended diaza[7]helicenes show structure-dependent stacking upon crystallization, switching from heterochiral packing to intra-layer homochiral stacking. Interestingly, the C2 -symmetric triple helicene molecules spontaneously resolve into a homochiral lamellar structure with 31 helix symmetry. Upon ultrasonication in a nonsolvent, the crystals can be readily exfoliated into large-area ultrathin nanosheets with height of ca. 4.4 nm corresponding to two layers of stacked triple helicene molecules and relatively thicker nanosheets constituted by even-numbered molecular lamellae. Moreover, regular hexagonal thin platelets with size larger than 30 μm can be readily fabricated by flash aggregation.

A highly fluorescent bora[6]helicene exhibiting circularly polarized light emission

Heteroatom-doped helicenes have attracted great research interest due to their inherent chirality enabling fascinating new applications. Herein we present our successful synthesis of 19c-boratribenzo[gh,jk,mn][6]helicene, the hitherto longest and first configurationally stable pristine bora[n]helicene. It displays intense orange fluorescence and circularly polarized light (CPL) emission with a high quantum yield of up to 84%. X-ray single crystal analysis reveals a highly twisted, helical shape and intriguing intermolecular stacking. Complexation with a size-complemental aza[4]helicene yielded an unprecedented hetero-chiral π-π-stacked helicene dimer.

B-N Covalent Bond Embedded Double Hetero-[n]helicenes for Pure Red Narrowband Circularly Polarized Electroluminescence with High Efficiency and Stability

Chiral B/N embedded multi-resonance (MR) emitters open a new paradigm of circularly polarized (CP) organic light-emitting diodes (OLEDs) owing to their unique narrowband spectra. However, pure-red CP-MR emitters and devices remain exclusive in literature. Herein, by introducing a B-N covalent bond to lower the electron-withdrawing ability of the para-positioned B-π-B motif, the first pair of pure-red double hetero-[n]helicenes (n = 6 and 7) CP-MR emitter peaking 617 nm with a small full-width at half-maximum of 38 nm and a high photoluminescence quantum yield of ≈100% in toluene is developed. The intense mirror-image CP light produced by the enantiomers is characterized by high photoluminescence dissymmetry factors (gPL ) of +1.40/-1.41 × 10-3 from their stable helicenes configuration. The corresponding devices using these enantiomers afford impressive CP electroluminescence dissymmetry factors (gEL ) of +1.91/-1.77 × 10-3 , maximum external quantum efficiencies of 36.6%/34.4% and Commission Internationale de I'Éclairage coordinates of (0.67, 0.33), exactly satisfying the red-color requirement specified by National Television Standards Committee (NTSC) standard. Notably a remarkable long LT95 (operational time to 95% of the initial luminance) of ≈400 h at an initial brightness of 10,000 cd m-2 is also observed for the same device, representing the most stable CP-OLED up to date.

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.

Triple B←N Lewis Pair-Functionalized Triazatruxenes with Large Stokes Shifts

A novel class of multiple B←N Lewis pair-functionalized polycyclic aromatic hydrocarbons with different BR2 groups (R = Cl or Et) directly attached at positions 1, 6, and 11 of triazatruxene was synthesized. The triazatruxene backbone of 4 displays a bowl shape, and its molecular skeleton shows a highly twisted propeller-like structure with C3 symmetry. The introduction of B←N Lewis pairs not only results in a large decrease in the HOMO-LUMO gap but also lowers the LUMO to -3.00 eV. Both compounds show excellent stability with large Stokes shifts of ≤8234 cm-1 and solvatochromic emission in solvents of different polarities.

Boosting quantum yields and circularly polarized luminescence of penta- and hexahelicenes by doping with two BN-groups

The incorporation of boron-nitrogen (BN) units into polycyclic aromatic hydrocarbons (PAHs) as an isoelectronic replacement of two carbon atoms can significantly improve their optical properties, while the geometries are mostly retained. We report the first non-π-extended penta- and hexahelicenes comprising two aromatic 1,2-azaborinine rings. Comparing them with their all-carbon analogs regarding structural, spectral and (chir)optical properties allowed us to quantify the impact of the heteroatoms. In particular, BN-hexahelicene BN[6] exhibited a crystal structure congruent with its analog CC[6], but displayed a fivefold higher fluorescence quantum yield (φfl = 0.17) and an outstanding luminescence dissymmetry factor (|glum| = 1.33 × 10-2). Such an unusual magnification of both properties at the same time makes BN-helicenes suitable candidates as circularly polarized luminescence emitters for applications in materials science.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

Design and Synthesis of BODIPY-Hetero[5]helicenes as Heavy-Atom-Free Triplet Photosensitizers for Photodynamic Therapy of Cancer

Designing heavy-atom-free triplet photosensitizers (PSs) is a challenge for the efficient photodynamic therapy (PDT) of cancer. Helicenes are twisted polycyclic aromatic hydrocarbons (PAHs) with an efficient intersystem crossing (ISC) that is proportional to their twisting angle. But their difficult syntheses and weak absorption profile in the visible spectral region restrict their use as heavy-atom-free triplet PSs for PDT. On the other hand, boron-containing PAHs, BODIPYs are highly recognized for their outstanding optical properties. However, planar BODIPY dyes has low ISC and thus they are not very effective as PDT agents. We have designed and synthesized fused compounds containing both BODIPY and hetero[5]helicene structures to develop red-shifted chromophores with efficient ISC. One of the pyrrole units of the BODIPY core was also replaced by a thiazole unit to further enhance the triplet conversion. All the fused compounds have helical structure, and their twisting angles are also increased by substitutions at the boron centre. The helical structures of the BODIPY-hetero[5]helicenes were confirmed by X-ray crystallography and DFT structure optimization. The designed BODIPY-hetero[5]helicenes showed superior optical properties and high ISC with respect to [5]helicene. Interestingly their ISC efficiencies increase proportionally with their twisting angles. This is the first report on the relationship between the twisting angle and the ISC efficiency in twisted BODIPY-based compounds. Theoretical calculations showed that energy gap of the S1 and T1 states decreases in BODIPY-hetero[5]helicene as compared to planar BODIPY. This enhances the ISC rate in BODIPY-hetero[5]helicene, which is responsible for their high generation of singlet oxygen. Finally, their potential applications as PDT agents were investigated, and one BODIPY-hetero[5]helicene showed efficient cancer cell killing upon photo-exposure. This new design strategy will be very useful for the future development of heavy-atom-free PDT agents.

Chiral Luminescent Aza[7]helicenes Functionalized with a Triarylborane Acceptor and Near-Infrared-Emissive Doublet-State Radicals

This paper presents new chiral luminescent molecules (N7-BMes2 and N7-TTM) using configurationally stable aza[7]helicene (1) as a universal heteroatom-doped chiral scaffold. The respective reactions of electron-donating 1 with a triarylborane acceptor via palladium-catalyzed Buchwald-Hartwig C-N coupling and with the open-shell doublet-state TTM radical via nucleophilic aromatic substitution (SN2Ar) resulted not only in tunable emissions from blue to the NIR domain but also in significantly enhanced emission quantum efficiency up to Φ = 50%.

X-ray and Optical Circular Dichroism as Local and Global Ultrafast Chiral Probes of [12]Helicene Racemization

Chirality is a fundamental molecular property that plays a crucial role in biophysics and drug design. Optical circular dichroism (OCD) is a well-established chiral spectroscopic probe in the UV-visible regime. Chirality is most commonly associated with a localized chiral center. However, some compounds such as helicenes (Figure 1) are chiral due to their screwlike global structure. In these highly conjugated systems, some electric and magnetic allowed transitions are distributed across the entire molecule, and OCD thus probes the global molecular chirality. Recent advances in X-ray sources, in particular the control of their polarization and spatial profiles, have enabled X-ray circular dichroism (XCD), which, in contrast to OCD, can exploit the localized and element-specific nature of X-ray electronic transitions. XCD therefore is more sensitive to local structures, and the chirality probed with it can be referred to as local. During the racemization of helicene, between opposite helical structures, the screw handedness can flip locally, making the molecule globally achiral while retaining a local handedness. Here, we use the racemization mechanism of [12]helicene as a model to demonstrate the capabilities of OCD and XCD as time-dependent probes for global and local chiralities, respectively. Our simulations demonstrate that XCD provides an excellent spectroscopic probe for the time-dependent local chirality of molecules.

Impact of Truncation on Optoelectronic Properties of Azaborole Helicenes

Herein, we report configurationally stable singly-truncated (ST) and structurally flexible doubly-truncated (DT) helically chiral compounds derived from azabora[7]helicenes by a hypothetical removal of a single or two C=C double bonds. The singly-truncated constitutional isomers were synthesized from either benzoisoquinoline (BIQ) or phenantherene building blocks and the corresponding biaryls in excellent yields to give azabora[5]helicenes with a pendant phenyl ring at a sterically hindered position. These systems highlight the electronic impact of the nitrogen donor substitution position. The compounds with a disrupted BIQ moiety (STN) possess remarkable photoluminescence quantum yields of up to 0.53 in the solid state and a blue emission in solution with dissymmetry factors of up to ca. 3×10-3 . Upon cooling to 79 K all compounds exhibit phosphorescence with lifetimes of up to ca. 0.5 s. A methyl complex of azabora[7]helicene showing excellent configurational stability was used as a chiral inducer embedded in an emissive polymer (F8BT) to produce circularly polarized organic light emitting diodes with an electroluminescence dissymmetry factor gEL of up to 0.54.

Strong Chiroptical Effects in the Absorption and Emission of Macrocycles Based on the 2,5-Diaminoterephthalate Minimal Fluorophore

Chiral fluorescent macrocycles consisting of two to four units of dimethyl 2,5-diaminoterephthalate can be readily synthesized in a one-pot manner from inexpensive building blocks. Depending on the concentration, either a paracyclophane-like dimer with closely stacked benzene rings or a triangular trimer is the main product of the reaction. The macrocycles exhibit fluorescence in solution as well as in the solid state with maxima that are red-shifted with decreasing size of the macrocyclic ring and are observed at wavelengths from 590 (tetramer in solution) to 700 nm (dimer in the solid state). Chirality dictates the differential absorption and emission of circularly polarized light by these molecules. The ECD and CPL effects are particularly strong for the trimer, which is characterized by relatively large dissymmetry factors gabs =±2.8×10-3 at 531 nm and glum =±2.3×10-3 at 580 nm in n-hexane, being at the same time highly luminescent (Φfl =13.7 %). Despite the small chromophore, the circularly polarized brightness BCPL of 2.3 dm3  mol-1  cm-1 is comparable to values reported for other classes of established CPL emitters in the visible region, such as expanded helicenes or larger π-conjugated systems.

Synthesis and Excited-State Properties of Donor-Acceptor Azahelical Coumarins

A set of three donor-acceptor azahelical coumarins (DA-AHCs), namely, H-AHC, Me-AHC, and Ph-AHC, were rationally designed and synthesized, and their excited-state properties were comprehensively investigated. All three DA-AHCs are shown to display very high fluorosolvatochromic shifts as a result of significant intramolecular charge transfer in their excited states. The para-quinoidal forms of the latter apparently contribute predominantly to large dipole moments in their excited states. By virtue of the fact that these helical systems structurally incorporate a highly fluorescent coumarin dye, they exhibit high quantum yields in both solution and solid states. Indeed, their emission behaviors in the crystalline media are shown to be remarkably correlated with their respective crystal packings. Incisive analyses demonstrate (i) strengthening of hydrogen bonding in the excited state promotes quenching (H-AHC), (ii) efficient crystal packing promotes high emission (Me-AHC) by precluding deactivations via vibrational motions, and (iii) loose crystal packing contributes to excited-state deactivation to account for low quantum yields of emission (Ph-AHC).

Synthesis of π-Conjugated Chiral Organoborane Macrocycles with Blue to Near-Infrared Emissions and the Diradical Character of Cations

Highly emissive π-conjugated macrocycles with tunable circularly polarized luminescence (CPL) have sparked theoretical and synthetic interests in recent years. Herein, we report a synthetic approach to obtain new chiral organoborane macrocycles (CMC1, CMC2, and CMC3) that are built on the structurally chiral [5]helicenes and highly luminescent triarylborane/amine moieties embedded into the cyclic systems. These rarely accessible B/N-doped main-group chiral macrocycles show a unique topology dependence of the optoelectronic and chiroptical properties. CMC1 and CMC2 show a higher luminescence dissymmetry factor (g_lum) together with an enhanced CPL brightness (B_CPL) as compared with CMC3. Electronic effects were also tuned and resulted in bathochromic shifts of their emission and CPL responses from blue for CMC1 to the near-infrared (NIR) region for CMC3. Furthermore, chemical oxidations of the N donor sites in CMC1 gave rise to a highly stable radical cation (CMC1^·+SbF₆^-) and diradical dication species (CMC1^2·2+2SbF₆^-) that serve as a rare example of a positively charged open-shell chiral macrocycle.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Efficient Narrowband Circularly Polarized Light Emitters Based on 1,4-B,N-embedded Rigid Donor-Acceptor Helicenes

Narrow-band emission is essential for applicable circularly polarized luminescence (CPL) active materials in ultrahigh-definition CP-OLEDs. One of the most promising classes of CPL active molecules, helicenes, however, typically exhibit broad emission with a large Stokes shift. We present, herein, a design strategy capitalizing on intramolecular donor-acceptor interactions between nitrogen and boron atoms to address this issue. 1,4-B,N-embedded configurationally stable single- and double helicenes were synthesized straightforwardly. Both helicenes show unprecedentedly narrow fluorescence and CPL bands (full width at half maximum between 17-28 nm, 0.07-0.13 eV) along with high fluorescence quantum yields (72-85 %). Quantum chemical calculations revealed that the relative localization of the natural transition orbitals, mainly on the rigid core of the molecule, and small values of root-mean-square displacements between S₀ and S₁ state geometries, contribute to the narrower emission.

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 (λ_em =411 nm) with a photoluminescence quantum efficiency of Φ_PL =6 % (cf. tetrathia[7]helicene: λ_em ≈405 nm, Φ_PL =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.

[5]-Helistatins: Tubulin-Binding Helicenes with Antimitotic Activity

Helicenes are high interest synthetic targets with unique conjugated helical structures that have found important technological applications. Despite this interest, helicenes have had limited impact in chemical biology. Herein, we disclose a first-in-class antimitotic helicene, helistatin 1 (HA-1), where the helicene scaffold acts as a structural mimic of colchicine, a known antimitotic drug. The synthesis proceeds via sequential Pd-catalyzed coupling reactions and a π-Lewis acid cycloisomerization mediated by PtCl₂. HA-1 was found to block microtubule polymerization in both cell-free and live cell assays. Not only does this demonstrate the feasibility of using helicenes as bioactive scaffolds against protein targets, but also suggests wider potential for the use of helicenes as isosteres of biaryls or cis-stilbenes-themselves common drug and natural product scaffolds. Overall, this study further supports future opportunities for helicenes for a range of chemical biological applications.

Enhanced Optical Properties of Azaborole Helicenes by Lateral and Helical Extension

The synthesis and characterization of laterally extended azabora[5]-, -[6]- and -[7]helicenes, assembled from N-heteroaromatic and dibenzo[g,p]chrysene building blocks is described. Formally, the π-conjugated systems of the pristine azaborole helicenes were enlarged with a phenanthrene unit leading to compounds with large Stokes shifts, significantly enhanced luminescence quantum yields (Φ) and dissymmetry factors (g_lum ). The beneficial effect on optical properties was also observed for helical elongation. The combined contributions of lateral and helical extensions resulted in a compound showing green emission with Φ of 0.31 and |g_lum | of 2.2×10^-3 , highest within the series of π-extended azaborahelicenes and superior to emission intensity and chiroptical response of its non-extended congener. This study shows that helical and lateral extensions of π-conjugated systems are viable strategies to improve features of azaborole helicenes. In addition, single crystal X-ray analysis of configurationally stable [6]- and -[7]helicenes was used to provide insight into their packing arrangements.

Near-Infrared-Absorbing B-N Lewis Pair-Functionalized Anthracenes: Electronic Structure Tuning, Conformational Isomerism, and Applications in Photothermal Cancer Therapy

B-N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (x-isomer) or a zig-zag conformation (z-isomer). In the solid state, the z-isomer is prevalent according to an X-ray crystal structure of a C₆F₅-substituted derivative (4-Pf), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B-N fusion results in a large decrease of the HOMO-LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. 4-Pf in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG₂₀₀₀, 4-Pf nanoparticles (4-Pf-NPs) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. 4-Pf-NPs are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B-N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.

Applying the B/N Lewis Pair Approach to Access Fusion-Expanded Binaphthyl-Based Chiral Analogues

We herein describe the synthesis of two axially chiral systems (HBN and BBN) by the incorporation of B centers into binaphthyl derivatives (HPy and BPy). Heteroatom-doped chiral polycyclic aromatic hydrocarbons were thus formed by fusion of the azaboroles to binaphthyls with the formation of B-N dative bonds. The resulting B-N Lewis pairs that serve as attractive fluorophores enabled modulation of the chiroptical properties both in solution and in the solid state.

Divergent Synthesis of Double Heterohelicenes as Strong Chiral Double Hydrogen-Bonding Donors

We developed a very efficient and expandable divergent approach initiated by a direct electrophilic borylation at phenyl rings to synthesize a series of double heterohelicenes. Their π-extended structures with pristine zigzag nitrogen (N)-boron (B)-nitrogen (N) edges offer them substantial physical properties and strong double hydrogen-bond donating capability. The isolated (P,P) and (M,M) enantiomers exhibit circularly polarized luminescence in response to hydrogen-bonding interactions.

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.

Azulene-Embedded [n]Helicenes (n=5, 6 and 7)

Azulene is a non-benzenoid aromatic building block with unique chemical structure and physicochemical properties. By using the "bottom-up" synthetic strategy, we synthesized three azulene-embedded [n]helicenes ([n]AzHs, n=5, 6 and 7), in which one terminal azulene subunit was fused with n-2 benzene rings. P- and M-enantiomers were observed in the packing diagrams of [5]-, and [6]AzHs. However, P- and M-[7]AzHs could be isolated by recrystallization of the racemic mixture. These [n]AzHs were endowed with new properties through the azulene moiety such as low-lying first electric state (S₁ ), small optical energy gap and anti-Kasha emission. [6]-, and [7]AzHs exhibit strong chiroptical responses with high absorption dissymmetry factor (g_abs ) maxima of about 0.02, which is among the highest |g_abs | values of helicenes in the visible range. These azulene-embedded [n]helicenes contribute to the non-benzenoid helicene library and allow the structure-property relationships to be better understood.

Synthesis, photophysical, electrochemical, and non-linear optical properties of triaryl pyrazole based B-N coordinated boron compounds

We report here a set of triaryl pyrazole based B-N coordinated boron compounds ( 11 - 17 ) synthesized by electrophilic aromatic borylation strategy. All the pyrazole boron compounds were thoroughly characterized using multinuclear NMR spectroscopy, LCMS, and single crystal X-ray diffraction analysis (for 12 - 17 ). The photoluminescence measurements of 11 - 17 revealed that the emission peak maxima were tuned based on the substitution on Nphenyl. The photophysical and electrochemical properties were further supported by theoretical calculations. Z-scan based investigations at 515 nm pump wavelength showed that B-N coordination led to enhancement of nonlinear absorption (two-photon absorption (TPA)) in these compounds if an electron deficient moiety is attached. It has also been observed that an appropriate choice of moiety allows to optimally maneuver the molecular polarizability of the π-system and consequently, assists in controlling the third-order nonlinear optical response.

Recent advances in the synthesis of luminescent tetra-coordinated boron compounds

Tetra-coordinated boron compounds offer a plethora of luminescent materials. Different chelation around the boron center (O,O-, N,C-, N,O-, and N,N-) has been explored to tune the electronic and photophysical properties of tetra-coordinated boron compounds. A number of fascinating molecules with interesting properties such as aggregation induced emission, mechanochromism and tunable emission by changing the solvent polarity were realised. Owing to their rich and unique properties, some of the molecules have shown applications in making optoelectronic devices, probes and so on. This perspective provides an overview of the recent developments of tetra-coordinated boron compounds and their potential applications.

Large Acene Derivatives with B-N Lewis Pair Doping: Synthesis, Characterization, and Application

Here, we report the synthesis of a novel class of B-N Lewis pair (LP_B/N)-doped large acene derivatives (1a-1d) from the well-designed phenanthridine-based precursors. The resultant LP_B/N-doped benzo-tetracene (1a), dibenzo-heptacene (1b), dibenzo-octacene (1c), and V-shaped tribenzo-nonacene (1d) are thoroughly characterized by X-ray crystallography, cyclic voltammetry, UV-vis absorption, and fluorescence spectroscopies together with DFT calculations. As a proof of concept, a 1a-based organic light-emitting diode device is fabricated to demonstrate the promising application in organic optoelectronics.

Enhanced N-directed electrophilic C-H borylation generates BN-[5]- and [6]helicenes with improved photophysical properties

Helicenes are chiral polycyclic aromatic hydrocarbons (PAHs) of significant interest, e.g. in supramolecular chemistry, materials science and asymmetric catalysis. Herein an enhanced N-directed electrophilic C-H borylation methodology has been developed that provides access to azaborine containing helicenes (BN-helicenes). This borylation process proceeds via protonation of an aminoborane with bistriflimidic acid. DFT calculations reveal the borenium cation formed by protonation to be more electrophilic than the product derived from aminoborane activation with BBr3. The synthesised helicenes include BN-analogues of archetypal all carbon [5]- and [6]helicenes. The replacement of a CC with a BN unit (that has a longer bond) on the outer helix increases the strain in the BN congeners and the racemization half-life for a BN-[5]helicene relative to the all carbon [5]helicene. BN incorporation also increases the fluorescence efficiency of the helicenes, a direct effect of BN incorporation altering the distribution of the key frontier orbitals across the helical backbone relative to carbo-helicenes.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

Conformationally flexible heterohelicenes as stimuli-controlled soft molecular springs

Structurally engineered molecules which can behave as stimuli-controlled mechanical nanomachines such as molecular shuttles, rotors, ratchets, and springs are important in several research areas, including molecular robotics, actuation, sensing, cargo transportation, etc. Helicenes, by virtue of their unique screw-type structures, were proposed as functional models for molecular springs; however, experimental realization has remained an elusive and unmet task until now, because of the lack of appropriate helicene molecules consisting of backbone-decorated dynamic architectures. Aiming to explore this unearthed direction, we present herein a novel class of modular flexible heterohelicenes with a stimuli (acid/base and light)-responsive core and peripheral modules. By applying pH (at core-embedded free imidazole sites) and light (at backbone-tethered dithienylethene units) stimuli, we demonstrate that these flexible heterohelicenes exhibit spring-like movement, with the reversible contraction/extension of the helical pitch. The uniquely functionalized structure of these molecules played a critical role in bestowing such capability, as revealed by crystallographic, spectroscopic and computational data. Careful assessment disclosed that the protonation/deprotonation-induced reversible generation and delocalization of positive charge throughout the π-conjugated helical rim switch the operative interactions between the π clouds of the terminal overlapping arene rings of the helicenes between repulsive and attractive, leading to extension/contraction of the helical pitch. On the other hand, in the case of the light stimulus, it was analyzed that the light-induced ring-closure of the photoactive dithienylethene units created a geometric distortion causing the helicenic wings to bend outward from the helicene rim, which resulted in extension of the helical pitch. The photo-assisted (or thermal) reverse ring-opening reaction converted the system to its original conformation, thus enabling the helicene molecule to display spring-like reversible extension/contraction motion. The new insights on the reversible dynamic features of this class of heterohelicenes under the influence of external stress would guide crucial design principles of helicene-based molecular springs for potential applications.

Sub-expanded flexible heterohelicenes were configured through a modular synthetic approach to experimentally demonstrate their capability of stimuli-controlled soft molecular spring-like behavior.

Bis-borylated arylisoquinoline-derived dyes with a central aromatic core: towards efficient fluorescent singlet-oxygen photosensitizers

Polycyclic aromatic hydrocarbon chromophores that show an ideal bipartition between fluorescence and singlet oxygen production have been developed.

One-Shot Synthesis of Expanded Heterohelicene Exhibiting Narrowband Thermally Activated Delayed Fluorescence

An expanded heterohelicene consisting of three BN₂-embedded [4]helicene subunits (V-DABNA-Mes) has been synthesized by one-shot triple borylation. The key to success is the excessive use of boron tribromide in an autoclave. Based on the multiple resonance effect of three boron and six nitrogen atoms, V-DABNA-Mes exhibited a narrowband sky-blue thermally activated delayed fluorescence with a full width at half-maximum of 16 nm. The resonating π-extension minimized the singlet-triplet energy gap and enabled rapid reverse intersystem crossing with a rate constant of 4.4 × 10⁵ s^-1. The solution-processed organic light-emitting diode device, employed as an emitter, exhibited a narrowband emission at 480 nm with a high external quantum efficiency of 22.9%.

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500-600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520-670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ _2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800-900 nm with the maximum σ _2PA ∼50-250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. -5.35 eV making them relatively electron rich in spite of the presence of two B^--N⁺ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

Linear Extension of Anthracene via B←N Lewis Pair Formation: Effects on Optoelectronic Properties and Singlet O2 Sensitization

The functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine-tune the structural features and optoelectronic properties, to suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N-directed electrophilic borylation of 2,6-di(pyrid-2-yl)anthracene offers access to linearly extended acene derivatives Py-BR (R=Et, Ph, C₆ F₅ ). In comparison to indeno-fused 9,10-diphenylanthracene, the formal "BN for CC" replacement in Py-BR selectively lowers the LUMO, resulting in a much reduced HOMO-LUMO gap. An even more extended conjugated system with seven six-membered rings in a row (Qu-BEt) is obtained by borylation of 2,6-di(quinolin-8-yl)anthracene. Fluorinated Py-BPf shows particularly advantageous properties, including relatively lower-lying HOMO and LUMO levels, strong yellow-green fluorescence, and effective singlet oxygen sensitization, while resisting self-sensitized conversion to its endoperoxide.

Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C-C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.