Fully fused boron-doped polycyclic aromatic hydrocarbons: their synthesis, structure–property relationships, and self-assembly behavior in aqueous media

Carbon-sulfur bond elongation as the promoting reaction coordinate in the efficient sub-nanosecond intersystem crossing in thianaphthene derivatives

Thiophene derivatives have become integral to OLEDs, photovoltaics, and photodynamic therapy research. A deeper understanding of their excited state dynamics and electronic relaxation mechanisms is expected to provide important physical insights of direct relevance for these applications. In this study, thianaphthene (TN), 2-methylbenzothiophene (2MBT), and 3-methylbenzothiophene (3MBT) are investigated using femtosecond broadband transient absorption and steady-state spectroscopy techniques along with time-dependent density functional calculations in cyclohexane and acetonitrile. The photophysical properties and electronic relaxation mechanisms of these derivatives are elucidated. Small fluorescence quantum yields ranging from 0.4 to 1.1% are measured. It is demonstrated that excitation of TN at 290 nm leads primarily to intersystem crossing to the triplet manifold with a lifetime of 400 ± 15 ps in either solvent, whereas four- to twofold shorter intersystem crossing lifetimes are measured for 2MBT and 3MBT depending on whether cyclohexane or acetonitrile is used. Linear interpolation of internal coordinates evidence that elongation of the S-C bonds enables ultrafast intersystem crossing in these thiophene derivatives involving singlet and triplet states with ππ* and πσ* characters. Excitation at 266 nm results in an additional 5 ± 1 ps lifetime, which is assigned to intramolecular vibrational relaxation dynamics occurring in the excited singlet state.

Narrowband Blue Circularly Polarized Luminescence Emitter Based on BN-Doped Benzo[6]helicene with Stimuli-Responsive Properties

Boron-doped helicenes, known for their unique electronic and photophysical properties, are of great interest for numerous applications. This research introduces two new azabora[6]helicenes, H[6]BN1 and H[6]BN2, synthesized through an efficient method. These molecules have boron and nitrogen atoms in opposing positions, enhancing their distinctive attributes. Both helicenes show excellent emission properties, with H[6]BN1 and H[6]BN2 exhibiting narrowband blue fluorescence and circularly polarized luminescence (CPL), achieving glum values of 4~5×10-4 which is beneficial for chiroptical applications. The addition of a donor group, 3, 6-di-tert-butyl-9H-carbazole, in H[6]BN2 improves luminescence, likely due to enhanced molecular orbital overlap and electron delocalization. H[6]BN1's needle-like single crystals exhibit mechanochromism, changing luminescent color from yellow to green under mechanical stress, which is promising for stimulus-responsive materials. In conclusion, this study presents a novel class of BN[6]helicenes with superior chiroptical properties. Their combination of electronic features and mechanochromism makes them ideal for advanced chiroptical materials, expanding the potential of helicene-based compounds and offering new directions for the synthesis of molecules with specific chiroptical characteristics.

π-Extended 9b-Boraphenalenes: Synthesis, Structure, and Physical Properties

Boraphenalenes, compounds in which one carbon atom in the phenalenyl skeleton is replaced with a boron atom, have attracted attention for their solid-state and electronic structures; however, the construction of boraphenalene skeletons remains challenging because of the lack of suitable methods. Through this study, we showed that the tandem borylative cyclization of C3-symmetric dehydrobenzo[12]annulenes produces a new class of fully fused boron-atom-embedded polycyclic hydrocarbons possessing a 9b-boraphenalene skeleton. The obtained compounds exhibited high electron-accepting characteristics, and their two-step redox process was reversible in the reductive region, involving interconversion of 9b-boraphenalene between Hückel aromaticity and antiaromaticity. Notably, the benzo[b]fluorene-fused derivative exhibited a stepwise single-crystal-to-single-crystal (SCSC) phase transition triggered by thermal annealing. Intermolecular electron coupling calculation of the crystal structures suggested a significant improvement of charge transporting ability associated with the SCSC phase transition. Moreover, adequate photoconductivity was observed for the single crystals before and after the SCSC phase transition through flash photolysis-time-resolved microwave conductivity.

In Silico Screening and Experimental Verification of Near-Infrared-Emissive Two-Boron-Doped Polycyclic Aromatic Hydrocarbons

Embedding two boron atoms into a polycyclic aromatic hydrocarbon (PAH) leads to the formation of a neutral analogue that is isoelectronic to the corresponding dicationic PAH skeleton, which can significantly alter its electronic structure. Based on this concept, we explore herein the identification of near-infrared (NIR)-emissive PAHs with the aid of an in silico screening method. Using perylene as the PAH scaffold, we embedded two boron atoms and fused two thiophene rings to it. Based on this design concept, all possible structures (ca. 2500 entities) were generated using a comprehensive structure generator. Time-dependent DFT calculations were conducted on all these structures, and promising candidates were extracted based on the vertical excitation energy, transition dipole moment, and atomization energy per bond. One of the extracted dithieno-diboraperylene candidates was synthesized and indeed exhibited emission at 724 nm with a quantum yield of 0.40 in toluene, demonstrating the validity of this screening method. This modification was further applied to other PAHs, and a series of thienobora-modified PAHs was synthesized.

Carborane-arene fused boracyclic analogues of polycyclic aromatic hydrocarbons accessed by intramolecular borylation

Arenes are 2D aromatics while dicarbadodecaborane clusters are branded as 3D aromatic molecules. In this work we prepare molecules that feature fused 2D/3D aromatic systems that represent boron-doped analogues of polycyclic aromatic hydrocarbons. The electron withdrawing nature of the ortho-carborane substituent enables swift arene borylation on boron bromide or hydride precursors to furnish five- and six-membered boracycles in conjugation with the arene. The mechanism was modeled by DFT computations implying a concerted transition state and analyzing the photophysical properties revealed high quantum yields in the six-membered systems.

BN-Phenanthrene- and BN-Pyrene-Based Fluorescent Uridine Analogues

Two unprecedented fluorescent nucleosides that feature BN-doped polycyclic aromatic hydrocarbons are presented. One of them, having a BN-modified phenanthrene moiety incorporated, shows blue fluorescence but suffers from poor stability under aqueous conditions. The other nucleoside comprises an internally BN-doped pyrene as the chromophore. It shows green fluorescence in various solvents and is stable under aqueous and alkaline conditions.

Synthesis, Structure, and Properties of a Nitrogen-Boron-Nitrogen-Embedded Polycyclic π-System Containing a Pleiaheptalene Framework

A novel polycyclic π-system (1) featuring both a pleiaheptalene framework (a three-fused heptagon system) and nitrogen-boron-nitrogen (NBN) unit was constructed by electrophilic borylation. A combined experimental and computational study demonstrated that 1 has a highly twisted π-backbone with approximate C2 symmetry, which can undergo conformational isomerization at room temperature in contrast to pleiaheptalene. It was also found that 1 can bind the fluoride ion in the solution, which induces changes in the absorption and emission spectra.

A Combination of B- and N-Doped π-Systems Enabling Systematic Tuning of Electronic Structures and Properties

Doping heteroatoms into polycyclic aromatic hydrocarbons (PAHs) may alter their structures and thereby physical properties. This study reports the construction of B/N-codoped PAHs via combining the B- and N-doped π-systems. Two π-extended B/N-codoped PAHs were synthesized through the Mallory photoreaction. Both feature a C48 BN2 π-skeleton, which is assembled by linearly fusing three substructures including B-doped and sp2 -hybridized N-doped π-moieties and one pyrene unit. In comparison to the pristine B-doped analog, their intramolecular charge transfer (ICT) states are distinctly modulated by the fused N-doped π-system and the further incorporated cyano group, leading to their tunable optical properties, as revealed by detailed theoretical and experimental analysis. Furthermore, these three molecules have sufficient Lewis acidity and can coordinate with Lewis base to form Lewis acid-base adducts, and notably, such intermolecular complexation can further dynamically modulate their ICT transitions and thereby photophysical properties, such as producing blue, green and red fluorescence.

Diradicaloid Boron-Doped Molecular Carbons Achieved by Pentagon-Fusion

Molecular carbons (MCs) are molecular cutouts of carbon materials. Doping with heteroatoms and constructing open-shell structures are two powerful approaches to achieve unexpected and unique properties of MCs. Herein, we disclose a new strategy to design open-shell boron-doped MCs (BMCs), namely by pentagon-fusion of an organoborane π-system. We synthesized two diradicaloid BMC molecules that feature C24 B and C38 B π-skeletons containing a pentagonal ring. A thorough investigation reveals that such pentagon-fusion not only leads to their local antiaromaticity, but also incorporates an internal quinoidal substructure and thereby induces open-shell singlet diradical states. Moreover, their fully fused structures enable efficient π conjugation, which is expanded over the whole frameworks. Consequently, some intriguing physical properties are achieved, such as narrow energy gaps, very broad light absorptions, and superior photothermal capability, along with excellent photostability. Notably, the solid of the C38 B molecule exhibits absorption that covers the range of 300-1200 nm and an efficiency of 93.5 % for solar-driven water evaporation, thus demonstrating the potential of diradicaloid BMCs as high-performance organic photothermal materials.

B2,N4-Doped Heptacenes: Ambipolar Charge-Transfer Compounds with Deep LUMO Levels

The B2,N4-doped heptacene H4 in which two N,N'-dihydrophenazine units are linked by two BMes bridges (Mes = mesityl) was synthesized via fourfold Buchwald-Hartwig coupling between 2,3,6,7-tetrachloro-9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene and o-phenylenediamine (tBuXPhos-Pd-G3, DBU/NaOTf, 2-MeTHF, 50 °C, 16 h). Upon exposure to ambient air, H4 is oxidized to its N,N'-dihydro form H2; further oxidation with MnO2 furnishes the di(phenazine) derivative H0. Stirring under a blanket of H2 in the presence of Pd/C hydrogenates H0 back to H2 and ultimately H4. Yellow-colored H0 is a remarkably good electron acceptor with a LUMO-energy level of -3.9 eV; upon irradiation with a 405 nm LED in the presence of THF or 1,4-cyclohexadiene, H0 accepts two H atoms to furnish H2. One-electron reduction of H0 yields the isolable radical-anion salt Li[H0] (lithium naphthalenide, THF, -30 °C to rt). The ambipolar compounds H2 and H4 possess a navy blue and deep purple color, respectively, due to charge-transfer interactions from the electron-rich N,N'-dihydrophenazine donor(s) to the electron-accepting B2C4 core.

Vicinal Vinylation of Boron-Doped Acenes via Heck Coupling

Silylated and halogenated benzenes 1,2-(Me3Si)2-4,5-X2-C6H2 [X = Br (3), I (4)] are versatile synthetic building blocks. 3 was prepared from known 1,2-(Me3Si)2-4,5-Cl2-C6H2 via C-Cl borylation/bromodeboronation; CuI-catalyzed Br/I exchange on 3 affords 4. 3 or 4 and BBr3 yield 9,10-dibromo-9,10-dihydro-9,10-diboraanthracenes (DBAs) 7 or 8. The B centers were protected with mesityl (Mes; 9, 10) or 2,4,6-tris(trifluoromethyl)phenyl (FMes; 11, 12) groups. Heck coupling of 9 and styrene/2,3,4,5,6-pentafluorostyrene furnishes the two tetravinyl-substituted green/blue emitters 13/14.

Photodissociative Modules that Control Dual-Emission Properties in Donor-π-Acceptor Organoborane Fluorophores

Donor-π-acceptor fluorophores that consist of an electron-donating amino group and an electron-accepting triarylborane moiety generally exhibit substantial solvatochromism in their fluorescence while retaining high fluorescence quantum yields even in polar media. Herein, we report a new family of this compound class, which bears ortho-P(=X)R2 -substituted phenyl groups (X=O or S) as a photodissociative module. The P=X moiety that intramolecularly coordinates to the boron atom undergoes dissociation in the excited state, giving rise to dual emission from the corresponding tetra- and tricoordinate boron species. The susceptibility of the systems to photodissociation depends on the coordination ability of the P=O and P=S moieties, whereby the latter facilitates dissociation. The intensity ratios of the dual emission bands are sensitive to environmental parameters, including temperature, solution polarity, and the viscosity of the medium. Moreover, precise tuning of the P(=X)R2 group and the electron-donating amino moiety led to single-molecule white emission in solution.

Flow-Chemistry-Enabled Synthesis of 5-Diethylboryl-2,3'-bipyridine and Its Self-Assembly Dynamics

5-Diethylboryl-2,3'-bipyridine (1), which is inaccessible by conventional batch methods, was synthesized by using a flow microreactor. Compound 1 was obtained as an equilibrium mixture of a cyclic trimer and a cyclic tetramer in solution, the latter of which was crystallized in benzene by vapor diffusion of hexane at 7 °C. The dynamic nature of this system was confirmed by solvent- and concentration-dependent experiments. Notably, the dynamics was verified by using flow NMR spectroscopy, which revealed that the time required to reach equilibrium was influenced by the solvent ratio (<18 s, 24-28 s, and 34-42 s in 2 : 1, 1 : 1, and 1 : 2 mixtures of [D₆ ]acetone and C₆ D₆ , respectively). Compound 1 and 3-[4'-(diethylboryl)phenyl]pyridine (2) exhibited different self-assembly behavior in solution and crystals. Density functional theory calculations suggested that this difference was largely due to enhanced planarity between two consecutive aromatic rings.

Fusion of Aza- and Oxadiborepins with Furans in a Reversible Ring-Opening Process Furnishes Versatile Building Blocks for Extended π-Conjugated Materials

A modular synthesis of both difurooxa- and difuroazadiborepins from a common precursor is demonstrated. Starting from 2,2'-bifuran, after protection of the positions 5 and 5' with bulky silyl groups, formation of the novel polycycles proceeds through opening of the furan rings to a dialkyne and subsequent re-cyclization in the borylation step. The resulting bifuran-fused diborepins show pronounced stability, highly planar tricyclic structures, and intense blue light emission. Deprotection and transformation into dibrominated building blocks that can be incorporated into π-extended materials can be performed in one step. Detailed DFT calculations provide information about the aromaticity of the constituent rings of this polycycle.

Double boron-embedded multiresonant thermally activated delayed fluorescent materials for organic light-emitting diodes

The subclass of multi resonant thermally activated delayed fluorescent emitters (MR-TADF) containing boron atoms has garnered significant attention in the field of organic light emitting diode (OLED) research. Among boron-based MR-TADF emitters, double boron-embedded MR-TADF (DB-MR-TADF) emitters show excellent electroluminescence performances with high photoluminescence quantum yields, narrow band emission, and beneficially small singlet-triplet energy levels in all the full-color gamut regions. This article reviews recent progress in DB-MR-TADF emitters, with particular attention to molecular design concepts, synthetic routes, optoelectronic properties, and OLED performance, giving future prospects for real-world 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.