Isomers of B←N‐Fused Dibenzo‐azaacenes: How B←N Affects Opto‐electronic Properties and Device Behaviors?

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.

The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor-donor-acceptor fluorophores

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor-donor-acceptor or donor-acceptor-donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor-acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.

Diagonal and Vertical B ← N Lewis Pair Functionalized Perylenes

Two novel multiple B ← N Lewis pair functionalized perylenes are reported. While OBN-Pery shows a centrosymmetric and planar architecture, PBN-Pery displays an axisymmetric and twist structure. B ← N functionalization in both of them results in a large decrease in the HOMO-LUMO energy gap. PBN-Pery in particular has a low LUMO energy level (-3.00 eV) and red emission at the NIR I region with high fluorescence quantum yield.

Recent Research Progress of Organic Small-Molecule Semiconductors with High Electron Mobilities

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

Butterfly-Shaped Nanographenes with Excellent Second-Order Nonlinear Optical Properties: The Synergy of B/N and Azulene

Azulene, a simple polar polycyclic aromatic hydrocarbon with connected electron donor and acceptor (DA), ignites the hope of designing second-order nonlinear optical (NLO) molecular materials from pure nonpolar carbon nanomaterials. In this work, a butterfly-shaped nanographene (π-DA-π) was designed by incorporating azulene between two coronenes. One more electron in a N atom or one electron fewer in a B atom with respect to a C atom can polarize charge distribution in carbon nanomaterials, and further doping of B and N in the designed butterfly-shaped nanographene changes the system from π-DA-π to D-π-A, leading to strong NLO responses. For example, the largest static first hyperpolarizability even reaches 173.89×10^-30  esu per heavy atom. The synergetic role of B, N and azulene in the nanographene is scrutinized, and such a doping strategy is found to provide an effective means for the design of carbon-based functional materials. The strong second-order NLO responses of these butterfly-shaped carbon-based nanographenes under external fields, for example, sum frequency generation and difference frequency generation, could inspire future experimental exploration.

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.

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.

"Spine Surgery" of Perylene Diimides with Covalent B-N Bonds toward Electron-Deficient BN-Embedded Polycyclic Aromatic Hydrocarbons

BN-embedded polycyclic aromatic hydrocarbons (PAHs) with unique optoelectronic properties are underdeveloped relative to their carbonaceous counterparts due to the lack of suitable and facile synthetic methods. Moreover, the dearth of electron-deficient BN-embedded PAHs further hinders their application in organic electronics. Here we present the first facile synthesis of novel perylene diimide derivatives (B₂N₂-PDIs) featuring n-type B-N covalent bonds. The structures of these compounds are fully confirmed through the detailed characterizations with NMR, MS, and X-ray crystallography. Further investigation shows that the introduction of BN units significantly modifies the photophysical and electronic properties of these B₂N₂-PDIs and is further understood with the aid of theoretical calculations. Compared with the parent perylene diimides (PDIs), B₂N₂-PDIs exhibit deeper highest occupied molecular orbital energy levels, new absorption peaks in the high-energy region, hypsochromic shift of absorption and emission maxima, and decrement of photoluminescent quantum yields. Single-crystal field-effect transistors based on B₂N₂-PDIs showcase an electron mobility up to 0.35 cm² V^-1 s^-1, demonstrating their potential application in optoelectronic materials.

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.

Novel Synthetic Approach to Heteroatom Doped Polycyclic Aromatic Hydrocarbons: Optimizing the Bottom-Up Approach to Atomically Precise Doped Nanographenes

The success of the rational bottom-up approach to nanostructured carbon materials and the discovery of the importance of their doping with heteroatoms puts under the spotlight all synthetic organic approaches to polycyclic aromatic hydrocarbons. The construction of atomically precise heteroatom doped nanographenes has evidenced the importance of controlling its geometry and the position of the doping heteroatoms, since these parameters influence their chemical-physical properties and their applications. The growing interest towards this research topic is testified by the large number of works published in this area, which have transformed a once "fundamental research" into applied research at the cutting edge of technology. This review analyzes the most recent synthetic approaches to this class of compounds.

B←N-Incorporated Dibenzo-azaacenes as n-Type Thermoelectric Materials

Organic thermoelectric materials play a vital role in flexible power generating applications, such as wearable electronics and sensor networks. While there is a wealth of research on p-type organic thermoelectric materials, developments on n-type counterparts as complementary are comparatively limited. Herein, we report a new kind of n-type small-molecule thermoelectric materials based on B←N-incorporated dibenzo-azaacenes 1,2-DBNA-2 and 1,2-DBNA-5. Because of the low-lying lowest unoccupied molecular orbital (LUMO) energy levels, 1,2-DBNA-2 and 1,2-DBNA-5 could be efficiently n-doped, and the rigid and almost planar skeleton could ensure good carrier transfer. When doped with a typical n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), 1,2-DBNA-5 exhibits a moderate conductivity of 0.01 S cm^-1 and a power factor of 0.06 μW m^-1 K^-2 with a Seebeck coefficient of -244.4 μV K^-1 in thermoelectric devices. These results not only demonstrate that B←N-incorporated dibenzo-azaacenes are a novel class of n-type thermoelectric materials but also highlight a new strategy to develop n-type organic thermoelectric materials.