Multiple-boron–nitrogen (multi-BN) doped π-conjugated systems for optoelectronics

Abstract

Boron–nitrogen-doped π-conjugated systems have been regarded as a class of organic materials with exceptional electronic and optical properties, which makes them promising in optoelectronic devices. Due to the isoelectronic relationship between the carbon–carbon (CC) unit and the boron–nitrogen (BN) unit, the location, number and orientation of BN units in π-conjugated systems bearing more than one BN unit are believed to have significant impacts on their electronic properties and intermolecular interactions. In addition, the coordination pattern of boron atoms in the conjugated systems could drastically tune their optoelectronic properties. This structural uniqueness makes these multiple-boron–nitrogen (multi-BN) doped π-conjugated systems exhibit great potential for application in organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs). This review covers recent advances in multiple boron–nitrogen doped π-conjugated systems including their synthetic strategies and applications in optoelectronic devices. We will rationalize the relationship between optoelectronic properties and the location, number and orientation of BN dopants as well as the coordination number of boron atoms, which could enlighten the design and synthesis of multi-BN-doped optoelectronic materials with better performances.

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Xiaobin Chen

Xiaobin Chen received his bachelor's degree from the School of Materials Science and Engineering, Luoyang Institute of Science and Technology, in 2020. He is currently pursuing his master's degree at the School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, under the supervision of Prof. Deng-Tao Yang. His research interest is in MR-TADF materials.

Dehui Tan

Dehui Tan received his bachelor's degree from the School of Chemistry and Chemical Engineering, Hefei University of Technology, in 2020. He is currently studying for his master's degree at the School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, under the supervision of Prof. Deng-Tao Yang. His research interest is in the synthesis of boron-based chiral CPL molecules.

Deng-Tao Yang

Deng-Tao Yang has been a professor in the School of Chemistry and Chemical Engineering at Northwestern Polytechnical University since September 2020. He received his BS in Chemistry and BEng in Computer Science from Lanzhou University in 2010. After he obtained MS in Organic Chemistry from the same university in 2013, he moved to Canada and joined Prof. Suning Wang group at Queen's University pursuing his PhD degree (2013–2017). He continued his academic career at MIT as a postdoc associate (advisor: Karthish Manthiram) from 2018 to 2020. His current research interests focus on boron-based organic functional materials.

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1. Introduction

Tunable properties and flexibility have made organic π-conjugated systems attractive targets for applications in organic light emitting diodes (OLEDs),^1,2 organic photovoltaics (OPVs),^3,4 and organic field-effect transistors (OFETs).^5–7 Besides optimizing the optoelectronic device structures, searching for new organic molecules with advanced performance is also the other aspect of pursuing higher performances of devices. The optoelectronic properties of organic molecules often can be significantly altered by subtle changes in their molecular structures.^8–10 Doping heteroatoms into organic π-conjugated molecules has attracted increasing attention because it not only can increase the structural diversity of the parent organic π-conjugated systems, but can also endow them with novel physical and chemical properties.^11,12 Boron as a dopant has received exceptional attention due to its empty p_z orbital; however, the instability of boron-doped organic molecules usually limits their practical applications as organic functional materials.^13,14 In addition to increasing the stability of boron-doped molecules by either introducing bulky substituents on boron atoms or doping boron into constraint structures, replacing the carbon–carbon (CC) unit in organic π-conjugated molecules with the isoelectronic counterpart-boron–nitrogen (BN) unit has been shown to be an effective strategy to improve their stability.¹⁵ The introduction of the BN unit can produce novel organic optoelectronic materials with similar structures to their all-carbon analogues but distinct electronic and optical properties due to the dipolar nature of the BN unit.^16–19 In the past decades, plenty of BN-doped polycyclic arenes have been reported, and their applications have been applied in various areas.^18,19 BN-doped polycyclic aromatic hydrocarbons (PAHs) have been found to exhibit better charge transport performance in OFETs with better stability.^20–22 Discrete boron and nitrogen atoms in PAHs can induce multi-resonant HOMO–LUMO separation, giving rising to thermally activated delayed fluorescence (TADF) with narrow-band emission.²³ Organoboron compounds doped with the BN unit with tetracoordinated boron atoms display excellent electron-injecting and -transporting properties because of their rather low LUMO energy levels.^24–26

From a molecular engineering point of view, the number, location, and orientation of BN units could adjust the overall dipolar nature of BN-doped molecules, resulting in distinct electronic and optical properties. For example, BN-doped PAHs with different BN locations could display entirely different optoelectronic properties.^27–32 In addition, the unique coordination pattern of boron atoms could further enrich the structural diversity of BN-doped molecules. Mono-BN-doped organic π-conjugated systems have been recently summarized by several reviews.^{16–19,33–36} However, although there are increasing reports on multi-BN-doped organic π-conjugated molecules, a review summarizing how to utilize different doping patterns (different numbers, locations and orientations of multiple BN units) to design multi-BN-doped organic π-conjugated molecules and how the doping pattern affects their properties and applications is still highly desired. In this review, we focus on molecular structures bearing two and more BN units and rationalize the relationship between optoelectronic properties and BN units’ doping patterns. Of course, we will also pay attention to their synthetic methodologies since they are still the bottle-neck for constructing the designed structures, which limits the applications of BN-doped molecules in optoelectronics. In order to systematically investigate the effect of BN-dopants on properties, we will compare the differences of electronic and optical properties if the same PAHs with multiple BN units have more than one BN-doped analogue; otherwise, we will compare multi-BN-doped molecules with either mono-BN-doped versions or all-carbon analogues depending on their availability. The review will be divided into three sections. Firstly, we will summarize multi-BN-doped π-conjugated systems with bonded boron and nitrogen atoms, which have been widely used in various areas including OFETs. Secondly, multi-BN-doped π-conjugated systems with discrete boron and nitrogen atoms will be discussed, and 1,4-BN-doped PAHs with multi-resonant HOMO–LUMO separation effects will be the focus of attention. Thirdly, we will discuss multi-BN-doped π-conjugated systems bearing tetracoordinate boron atoms, which usually have low LUMOs and could be used in charge transport devices. The molecules with one BN unit will only be discussed when needed to compare with their multi-BN doped analogues. BN-doped polymers for optoelectronics are out of scope of this review.

2. Multi-BN-doped π-conjugated systems with bonded boron and nitrogen atoms

BN-doped benzenes with bonded boron and nitrogen atoms are also named 1,2-azaborines. In order to distinguish different BN-doped locations in PAHs, we simply divide them into three categories (Fig. 1): BN center-doped PAHs, in which the BN unit is in the center of PAHs; BN edge-doped PAHs, in which the BN unit is at the edge of PAHs; and BN partially-edge-doped PAHs, in which either the boron atom or the nitrogen atom is located at the edge of PAHs.

Fig. 1 The terminology of different locations of the BN unit in this review.

2.1 BN-PAHs

As one of the representative PAHs with three fused phenyl rings, phenanthrene 1 doped with the BN unit has been widely studied,^37–41 especially mono-BN-doped phenanthrenes (2–7) (Fig. 2).^42–50 However, multi-BN-doped phenanthrenes have been excluded until recently. The di-BN edge-doped phenanthrene 8 was obtained through two-fold Stille cross coupling followed by borylative cyclization (Fig. 2).⁵¹ Compared to its mono-BN-doped analogue 7, compound 8 showed a blue-shifted absorption spectrum and a red-shifted emission spectrum, resulting in a larger Stokes shift. The polarity of solvents had a negligible impact on its absorption and emission. The number of BN units has been found to have a critical impact on the bromination reaction for further functionalization.

Fig. 2 Phenanthrene and BN-doped phenanthrenes and the synthesis of 8.

Embedding multiple BN units into anthracene 9 has gained more attention than its isomer phenanthrene. There are different isomers of the bis-BN-doped anthracene (10–12),^52–54 as shown in Fig. 3. Compounds 10 and 11 were synthesized through a double borylative cyclization reaction (Fig. 3a). A gold-catalyzed cyclization of the diamino-functionalized precursor 12a was exploited to generate compound 12 (Fig. 3b).⁵⁴ Due to the lack of the photophysical properties of 10, we will only discuss the photophysical properties of 11 and 12. Compared to anthracene 9, both the mono-BN-doped anthracene and 11 showed a strong second electronic transition. With respect to the emission, 11 had an almost doubled quantum yield compared with 9 and the mono-BN-doped anthracene. The well-known cycloaddition chemistry of 9 did not happen to the mono-BN-doped anthracene and 11. In addition to the fact that compound 12 exhibited ∼0.2 eV higher HOMO and LUMO energy levels, it had quite similar absorption, emission and quantum yields to its mono-BN doped analogue, which indicates that the number of BN units does not affect the photophysical properties in this specific doping pattern. Compared to 11 that has the same number of BN units but different doping patterns, both the absorption and emission of 12 showed obvious red-shift, which indicates that the doping pattern of 12 enables better conjugation of BN units in the anthracene.

Fig. 3 Examples of bis-BN-doped anthracenes.

Very recently, exploiting 11 as the core structure, Wang and coworkers synthesized tetraphenyl bis-BN-doped anthracene 11-TP and diphenyl bis-BN-doped anthracene 11-DP from a perspective of molecular engineering (Fig. 3c).⁵⁵ The substituents on the boron atoms were found to have a significant impact on the charge transport properties. The unique herringbone packing mode of 11-DP resulted in anisotropic charge transport with the highest hole mobility of up to 1.3 cm² V⁻¹ s⁻¹ in OFETs. In addition, organic phototransistors (OPTs) based on 11-DP single crystals exhibited excellent photodetector performance. In addition to constructing brand-new molecules, this work points out another direction to discover optoelectronic molecules with excellent performance.

Attention has been paid to BN-pyrenes due to their rigid conjugated structures and high fluorescence.^52,56–58 In addition to mono-BN-doped pyrenes, bis-BN-doped pyrenes have also been synthesized. Dewar and coworkers reported the synthesis of bis-BN-pyrene 14 while they discovered the synthesis of bis-BN-doped anthracene 10 using double borylative cyclization (Fig. 4).⁵² Similar to 10, the photophysical properties of 14 have not been reported. Wang and coworkers exploited the photoelimination reaction they developed earlier to achieve the synthesis of another bis-BN-doped pyrene, 15.⁵⁹ This photoelimination would be applied to construct more diverse BN-PAHs if the problem of low reaction quantum yields could be tackled, since these BN-PAHs are hardly accessed through other borylation reactions. Compared to 14, compound 15 had a much smaller HOMO–LUMO energy gap with an increased HOMO level and a decreased LUMO level. Compared to BN-phenanthrene 5 (Fig. 2), the maxima of the absorption and fluorescence of 15 were red-shifted by ∼70 nm. Due to the instability of 15, Wang et al. proposed the in situ exciton-driven elimination (EDE) strategy, in which the stable precursor compound 15a was fabricated into an OLED device rather than the unstable 15 and then the double elimination was driven by excitons to in situ generate the fluorescent bis-BN-doped 15 in the OLED device. Other BN-heterocycle precursors could also undergo EDE to generate the corresponding fully conjugated BN-arenes.

Fig. 4 Bis-BN-doped pyrenes.

Isoelectronic to pyrene, ullazines are of great interest in dye-sensitized solar cells (DSCs).^60–63 Doping the BN unit into ullazines still lacks investigation; only one kind of bis-BN-doped ullazine 17 has been reported (Fig. 5).⁶⁴ Three different electrophilic borylation approaches have been developed to introduce a variety of functional groups on boron atoms. The HOMO energy level was significantly lowered to −5.78 eV after two BN units were doped into ullazines, which is in agreement with the higher stability of 17 toward air and moisture. The fact that the doped BN units could not conjugate well with the other part of the molecule results in a larger HOMO–LUMO energy gap and less aromaticity of BN-rings. The absorption and emission spectra of bis-BN-doped ullazine 17 showed hypsochromic shift relative to the corresponding all-carbon ullazine analogues. The larger Nucleus-Independent Chemical Shifts (NICS) of 17 indicate that bis-BN-doped ullazine 17 is less aromatic than ullazine 16.

Fig. 5 Synthesis of bis-BN-doped ullazine and the numbers in blue represent the NICS.

Perylene derivatives are attractive segments of organic optoelectronic molecules. Recently, the BN unit doping patterns of perylene have been systematically investigated by the Wagner group and the Wang group (Fig. 6).^32,65 For the synthesis of the four bis-BN-doped perylenes 19–22, Wagner and coworkers once again used a gold-catalyzed cyclization reaction to generate 19 (Fig. 6a),⁶⁵ and Wang and coworkers developed an impressive approach to produce three bis-BN-perylenes 20–22 using BN-naphthalene as the building block (Fig. 6b).³² Compared to the parent perylene 18, bis-BN-perylene 19 had a much larger HOMO–LUMO energy gap with an increased LUMO level and a decreased HOMO level; however, molecules 20 and 21 had similar HOMO levels, LUMO levels and HOMO–LUMO energy gaps to 18, while 22 had a slightly smaller HOMO–LUMO energy gap with a similar HOMO level and a decreased LUMO level. The emission wavelength of 19 was quite close to that of 18, while molecules 20–22 exhibited much longer emission wavelengths than 18. The quantum yields of all these four bis-BN-doped perylenes were much lower than that of perylene. Doping BN units into perylene did not obviously change the aromaticity of bis-BN-doped perylenes. This is a good example showing that different BN orientations of BN-perylenes could impact the solid-state packing mode, aromaticity and photophysical properties.

Fig. 6 Bis-BN-doped perylenes.

The other reason why perylene derivatives have been extensively studied is that they can be extended to a new ubiquitous class of molecules – perylene diimides (PDIs) which have high chemical stability with high electron affinities and are regarded as one of the best n-type classes of semiconductors.^66–70 Wang and coworkers continued using BN-naphthalene as the building block and exploiting a similar strategy which was used in the synthesis of bis-BN-doped perylenes to construct bis-BN-doped PDIs 24 (Fig. 7).⁷¹ (BN)₂-PDIs 24 displayed similar quartet but hypsochromically shifted (about 20 nm) absorption in the range of 400–500 nm. Similar to BN-perylenes, the strong absorption of 24 was observed in the 300–375 nm window, which did not appear in the absorption of PDI 23. Compared to the green emission of 23, the emission of (BN)₂-PDIs 24 was also green but blue-shifted about 20 nm. Relative to BN-perylenes, the quantum yield of 24 was significantly decreased compared to PDI 23. Embedding BN units into PDI could further result in a lower LUMO level. The substituents on nitrogen atoms had a significant impact on the solid-state packing modes (Fig. 7a–f). The hydrogen bonding between the C=O and hydrogen atoms of the neighboring molecules existed in the crystal structures of 24 with different substituents. The bulkiness of 2,6-diisopropylphenyl groups made 24-DIPP hardly form effective π–π packing, while the less bulky functional groups in 24-CH and 24-EH led to cofacial packing and slipped-cofacial packing, respectively. Due to the preferred condensed crystal packing, (BN)₂-PDIs 24 with 2-ethylhexyl groups on nitrogen atoms showed the highest electron mobilities up to 0.35 cm² V⁻¹ s⁻¹. This once again indicates that introduction of BN units into popular organic semiconducting molecules is an effective strategy to enrich the scope of semiconducting materials.

Fig. 7 Synthesis of (BN)₂-PDIs, and crystal structures and packing motifs of 24-DIPP (a and b), 24-CH (c and d), and 24-EH (e and f). Crystal structures of (a–f) reprinted with permission from ref. 71. Copyright 2022 American Chemical Society.

Corannulene derivatives are an attractive class of bowl molecules in a variety of applications.^72–85 The synthesis of no matter the carbonaceous corannulene itself or heteroatom-doped corannulene derivatives is quite challenging. Hatakeyama et al. reported an efficient four-step synthesis of (BN)₂-embedded corannulene 26 from commercially available starting material 4,7-dibromobenzo[c][1,2,5]thiadiazole.⁸⁶ Compound 26 was obtained on a multigram scale in 25% total yield of four steps, which is beneficial to providing enough material for optoelectronic applications. According to the single crystal X-ray diffraction analysis, the bowl depth of 26 was 0.15 Å (Fig. 8), which was smaller than that of corannulene (0.87 Å). NICS(0) calculations showed that the aromaticity of the five-membered ring in the center of corannulene switched from antiaromatic (8.6) to nonaromatic (−2.3) after BN units were doped. The HOMO–LUMO energy gap of 26 was smaller than that of corannulene 25 accompanied by an increased HOMO level. The higher oscillator strength (0.1762) was attributed to asymmetrization by tetrabenzo-annelation and polarization parallel to the molecular plane induced by the two BN units. Due to the introduction of BN units, (BN)₂-corannulene 26 displayed strong blue fluorescence at λ_max = 424 nm with a quantum yield of 69%, which is almost ten times higher than that of corannulene. The first OLED based on the BN-corannulene 26 was fabricated with an external quantum efficiency of 2.61% at 1000 cd m⁻². The wavelength of the electroluminescence of 26 indicated a notable red-shift from 424 nm to 467 nm, which was probably caused by the π–π interaction with the host material and optical interference in the OLED device.

Fig. 8 Synthesis of (BN)₂-embedded corannulene. The inset shows the bowl depth of the crystal structure of 26. The crystal structure of 26 reprinted with permission from ref. 86. Copyright 2018 American Chemical Society.

Coronene is also known as superbenzene, which contains seven peri-fused benzene rings, and an appealing building block of organic optoelectronic materials.⁸⁷ Multiple-BN unit-doped coronene and its derivatives have been paid more and more attention. Very recently, Liu, Yu and coworkers reported the synthesis of bis-BN-doped coronene 28 with two syn-paralleled BN units using a double BBr₃-indued Scholl reaction.⁸⁸ The symmetry changed from D_6h for coronene to m_v for BN-coronene 28 (Fig. 9a). Based on the single crystal X-ray diffraction analysis, 28 adopted a slightly distorted saddle-like structure which is probably caused by the bulky mesityl groups. Introduction of two BN units notably narrowed the HOMO–LUMO band gap with a higher HOMO level and a lower LUMO level compared to those of coronene. Compared to its carbonaceous analogue, 28 had significantly red-shifted absorption bands and blue-shifted emission bands. The intensities of both absorption and emission as well as the fluorescence quantum yield of 28 were remarkably enhanced.

Fig. 9 Multi-BN-doped coronene and its derivatives. Crystal structures of 28 reprinted with permission from ref. 88. Copyright 2022 American Chemical Society. Crystal structures of 29 reprinted with permission from ref. 89. Copyright 2015 American Chemical Society. Crystal structures of 30b reprinted with permission from ref. 21. Copyright 2014 American Chemical Society.

The Zhang group and the Pei group independently discovered the synthesis of (BN)₃-doped coronene 29 (Fig. 9b).^89,90 The backbone of 29 demonstrated a fully planar geometry. The longer B=N double bond and shorter C–N single bonds were ascribed to the significant π-electron delocalization in the crystal structure of 29. The HOMO–LUMO band gap was further enlarged with an increased LUMO level and a decreased HOMO level. The absorption and emission spectra were notably blue-shifted, while the quantum yield of 29 was considerably higher than that of 27. The stability of 29 toward moisture was studied by Zhang et al.; 29 underwent the deprotection of boron atoms with the formation of borinic acids. On comparing the bis-BN-doped coronene 28 to tri-BN-doped coronene 29, compound 28 had a smaller HOMO–LUMO energy level and smaller Stokes shift than its all-carbon analogue, while compound 29 had a wider HOMO–LUMO band gap and larger Stokes shift than its carbonaceous coronene.

As continued interest in BN-PAHs with thiophene moieties, Pei et al. disclosed the synthesis of bis-BN-doped π-conjugated systems with four thiophenes (30 in Fig. 9c).²¹ A rare phenomenon for PAHs which contained two different conformers (saddle-shaped conformation and alternately up−down conformation) in the single crystal was observed (Fig. 9c). The two conformers were self-assembled into a sandwich structure and further formed a π-stacking column. Doping BN units did not change the HOMO–LUMO band gap too much with a 0.1 eV lower HOMO level and an unchanged LUMO level. The packed microribbons were investigated to be a good charge transport material with superior photoconductivity.

Coronene diimides (CDIs, 31 in Fig. 10) have blue-shifted absorption maxima and lower quantum yields compared to PDIs.^91,92 Hissler, Staubitz et al. discovered a synthetic route for using 1,7-dibromo-PDIs as starting materials to achieve bis-BN-doped CDIs 32.⁹³ Doping two BN units at the edge of the CDIs resulted in large bathochromically shifted absorption with higher molar extinction coefficients, narrow Stokes shifts and significantly higher quantum yields compared to its carbonaceous CDIs. The different substituents on nitrogen atoms did not impact the optical properties at the single molecule level, but substantially influence optical properties and performance in OLEDs and OFETs in the solid state.

Fig. 10 Multi-BN-doped CDI, HBC and their derivatives.

As the further conjugation extension of coronene, hexa-peri-hexabenzocoronene (HBC, 33 in Fig. 10) has been mainly studied in supramolecular electronics.^94–103 The isolation of the first BN center-doped HBC 34 was achieved as a minor product by Bettinger and coworkers via pyrolysis of the borazino precursor at 550 °C, accompanying the major product tetraazatetraborocine derivative 37 (Fig. 11a).¹⁰⁴ Scanning tunneling microscopy (STM) was exploited to confirm that (BN)₃-HBC 34 lay on a Au(111) surface in a 2D pattern. The thermolytic process highly depended on the temperature. If the temperature of thermolysis was decreased to 400–410 °C, only borylated product 36 as well as a small amount of 37 was obtained. The poor solubility of BN-HBC 34 limited further structural and optoelectronic studies in solutions. Bonifazi et al. documented the first rational synthesis of (BN)₃-HBC 35, which started from the formation of a (BN)₃-core and fused six C–C bonds through intramolecular Friedel–Crafts-type reactions (Fig. 11b).¹⁰⁵ 2,5-Dimethylphenyl groups were used to improve the solubility. In this example, almost all the photophysical properties dramatically changed after BN units were introduced. The maxima of absorption, fluorescence and phosphorescence in solutions were ∼80 nm blue-shifted compared to those of its all-carbon analogue. The optical gap of BN-HBC 35 was ca. 0.53 eV larger than that of its carbonaceous structure. The fluorescence decays of 35 were slightly shorter than those of its all-carbon analogue both in solution and in the solid state, while the lifetime (4.0 s) of phosphorescence of 35 at 77 K in solution was 5-fold longer compared to the 2,5-dimethylphenyl group substituted HBC.

Fig. 11 Synthesis of (BN)₃-HBCs.

While more and more attention has been paid to construction of BN-PAHs, researchers are also focusing on these PAHs which themselves have not been synthesized. Recently, the tetrabenzo-fused pyrene, which is also named ixene 38, and its BN partially edge-doped derivative 39 were synthesized by Lee, Shin, Park and coworkers (Fig. 12).¹⁰⁶ For the synthesis of 38 and 39, a double annulation reaction was exploited, and then an intramolecular C–H bond arylation with aryl chloride was used to achieve the fusion of the two final phenyl rings. The crystal structure of 39 revealed a planar pyrene core, while the four peripheral phenyl rings were distorted, which was consistent with STM studies. The lengths of the BN bonds in 39 were shorter than those previously reported BN edge-doped-PAHs, which emphasizes the importance of BN units’ locations. With respect to the optoelectronic properties, due to the smaller HOMO–LUMO gap, both the absorption and emission spectra of BN-ixene 39 displayed ca. 20 nm red-shift with a doubled quantum yield compared to those of ixene 38, while cyclic voltammetry indicated that the smaller HOMO–LUMO band gap resulted from the lower LUMO of 39.

Fig. 12 Synthesis of ixene and its bis-BN-doped derivative.

Doping two BN units into the helical dibenzo[a,m]rubicene (DBR) was achieved by Shang and Nakamura et al. via the construction of diphenylindolocarbazole 41a followed by double electrophilic borylation (Fig. 13).¹⁰⁷ The six-membered rings with the BN units in 41 changed from aromatic to non-aromatic, while the aromaticity of the five-membered rings with the BN units in 41 trended in the opposite direction. Compared to DBR 40, BN-doped DBR 41 exhibited a larger optical gap with blue-shifted absorption and emission bands and a much smaller Stokes shift. The photoluminescence quantum yield of 41 was substantially increased up to 88%. In addition, BN-DBR 41 showed rich color tunability induced by coordination with the fluoride anion.

Fig. 13 (BN)₂-doped dibenzo[a,m]rubicene.

2.2 BN-doped ladder-type acenes

Ladder-type acenes usually refer to PAHs made up of linearly fused aromatic rings.^97,108,109 This class of molecules usually features rigid planar backbones, unique photophysical properties and self-assembly behaviors, as well as attractive charge transport properties.^{5,33,110–114}

Dibenzo[a,e]pentalene (DBP, 42) is a ladder-type molecule consisting of two antiaromatic five-membered rings and two phenyl rings (Fig. 14).^115,116 The investigation of the effect of BN units’ substitution on the aromaticity and photophysical properties of BN-DBPs 43 and 44 was reported by Feng, Müllen and coworkers.²⁹ The synthesis of BN-DBP 43 started with the formation of 2,2′-dibromohydrazobenzene from reduction of 1-bromo-2-nitrobenzene, following a ^tBuLi-assisted double borylation reaction (Fig. 14b). 2-Bromophenylboronic acid was exploited to construct the first BN five-membered ring, then the ^tBuLi-assisted double borylation reaction was once again utilized to achieve BN-DBP 44. As expected, the skeletons of 43 and 44 were planar in the crystal structures. With respect to aromaticity, NICS(1) study indicated that doping BN units could make the two phenyl rings of both 43 and 44 have higher aromaticity, while the pentalene core of BN-DBP 43 changed from antiaromatic to almost nonaromatic and the BNB 5-membered ring in 44 was still antiaromatic. Different doping patterns also affected the frontier molecular orbitals with higher LUMOs and lower HOMOs. The nonnegligible role of the orientation of multiple BN units in π-conjugated molecules was stood out by the totally different emission properties, in which the center-symmetric BN-DBP 43 exhibited fluorescence at λ_em = 403 with a quantum yield of 18%, while the unsymmetric 44 was non-emissive. Meanwhile, Cui and coworkers independently reported the BN-benzopentalene 46, which showed a larger HOMO–LUMO band gap than that of its carbonaceous benzopentalene 45 (Fig. 14c).¹¹⁷ In 2017, Rautiainen and Piers et al. developed zirconocene-based methods for the preparation of BN-indenes 48, in which the aromaticity of the five- and six-membered rings shifted to nonaromatic compared with that of 47.¹¹⁸

Fig. 14 (BN)₂-Dibenzo[a,e]pentalenes. Crystal structures of 43 and 44 reprinted with permission from ref. 29. Copyright 2015 American Chemical Society.

Peripheral benzenoid rings of tetrabenzopentacene (TBP) have been introduced to increase the stability. Although the stability of TBP indeed showed improved stability compared to pentacene, it is still prone to decompose. Wang, Pei and coworkers reported BN-doped TBP 50 with improved stability compared to the carbonaceous analogue 49 (Fig. 15a).¹¹⁹ Bis-BN-doped 50 exhibited exceptional stability toward air, moisture, light and heat, which could easily cause the decomposition of large acenes. The high stability was ascribed to the decreased HOMO energy level after two BN units were introduced into TBP. With respect to the synthesis of 50, the precursor compound 50b was synthesized from several steps of C–C bond cross coupling reactions and deprotection reactions, borylative cyclization was utilized to access the precursor 50a, and the two end rings of product 50 were finally sealed by a UV-light-mediated radical cyclization reaction. The high stability of the bis-BN-doped TBP 50 enabled achieving p-type OFETs with mobilities of up to 0.33 cm² V⁻¹ s⁻¹.

Fig. 15 (a) Tetrabenzopentacene (TBP) and bis-BN-doped TBP, and (b) dibenzo[a,o]picene (DBP) and bis-BN-doped DBP.

As a U-shape ladder-type acene, dibenzo[a,o]picene (DBP, 51) has intriguingly predicted structural and electronic properties (Fig. 15b).^120,121 Although the synthesis of DBP has not been achieved, Piers et al. reported the synthesis of bis-BN-doped DBP via the methodology developed before,^{48,57,122–126} where the diethynylbenzene 52a reacted with a slightly excess of the boracycle 52b, followed by the elimination of TMSCl and cycloisomerization of the pendant alkynes.¹²⁵ The boracylce 52b could be used in other types of BN partially edge-doped PAHs and BN center-doped PAHs. NICS(1) calculations indicated that all the rings in BN-DBP 52 were aromatic.

The BN-doped ladder-type molecules linked through a central benzene ring with different BN orientations have been extensively studied (Fig. 16). Zhang, Feng and coworkers successively studied the synthesis and optoelectronic applications of ladder molecules 53 and 54 that have different terminal aromatic rings and BN orientations.^127,128 Compared to 53, both the absorption and emission bands of 54 showed about 30 nm blue-shift with improved quantum yields. Compound 53 had been demonstrated as host materials in blue OLEDs. As the isomers of 53 and 54, compound 55 and 56 were angular with syn-located BN units.¹²⁹ The different orientations of ladder molecules 53 and 55 as well as 54 and 56 had a negligible impact on the maximum wavelengths of absorption and emission, but on switching the bilateral aromatic rings from thiophene to benzene in 55 and 56, the absorption and emission were blue shifted ca. 30 nm. Due to their rigid structures, 53–56 showed very small Stokes shifts. Compared to 53 and 55, compound 57 displayed blue-shifted absorption and emission.¹³⁰ The benzothiadiazole segment in 58 significantly moved the absorption and emission toward longer wavelengths with a very large Stokes shift (95 nm).¹³⁰ Although compounds 59 and 60 had different orientations from 55 and 56, the photophysical properties did not show noteworthy changes.¹³¹

Fig. 16 Representative examples of bis-BN-doped ladder-type molecules linked with a benzene.

Gryko et al. reported a simple synthetic approach to ladder-type diazabenzoindoles 61 and bis-BN-benzoindoles 62 (Fig. 17).¹³² Different from the NN unit doped 61, BN-heteroacene 62 exhibited a large blue-shift of absorption with a high molar absorption coefficient. In addition, 62 was highly fluorescent with a quantum yield of 70%. The non-emissive properties of 61 were likely caused by the nonradiative decay because of the diazine scaffold. Changing the central aromatic core from pyrolopyrrole in 62 to thienothiophene in 63 did not substantially change the photophysical properties; however, tailoring the orientation of the central thienothiophene made the absorption and emission bands of 64ca. 20 nm blue-shifted compared to 63.¹²⁸ Two azaborine-thiophene ladder-type heteroacenes 65 and 66 with a central thieno core were reported by the Perepichka group (Fig. 18).¹³³ Both 65 and 66 showed highly rigid structures and deep-blue fluorescence. The existence of the linker between two nitrogen atoms did not notably shift the photophysical properties. Changing the bilateral aromatic rings from thiophene to benzene substantially moved the absorption and emission bands of 67 toward shorter wavelengths. Manipulating the BN units’ arrangement from tail-to-tail to head-to-head did not have a noteworthy effect on the photophysical properties of 67 and 68.³¹ Thiophene has been widely used as segments of BN-doped PAHs. This is mainly due to the high reactivity of C–H bonds in thiophene, which makes electrophilic borylation easier.

Fig. 17 Representative examples of bis-BN-doped ladder-type molecules linked with heteroaromatic rings.

Fig. 18 Representative examples of bis-BN-doped ladder-type molecules linked with a thiophene.

Chen and coworkers reported a series of BN-functionalized benzotrithiophene-based azaborines with different numbers of BN units prepared via electrophilic borylation (Fig. 19).¹³⁴ Surprisingly, the patterns of the absorption spectra of these three compounds were nearly identical with different molar absorption coefficients. The maximum wavelength of fluorescence was only 5 nm red-shifted with the increase of BN units. Compounds 69–71 showed red-shifted absorption and emission upon fluoride titration.

Fig. 19 BN-functionalized benzotrithiophene-based azaborines.

PAHs bridged with the BN unit were investigated as charge transport materials by Hatakeyama, Seki and Nakamura in 2011 (Fig. 20).¹³⁵ Compared to the superior hole mobility of 72a, they only reported the synthesis of bis-BN-doped PAHs 72 without investigation of charge transport properties, which was believed to be also suitable for organic electronics. Xing, Zhu and Cui reported a transition-metal catalyzed synthesis of the ladder-type acene 73 with central oxepin cores (Fig. 20).¹³⁶ This type of ladder molecule exhibited dually-fluorescent emissions. The tunable emission colors of 73 could be achieved by manipulating the ratio of solvents, the concentration of solution, or temperature. Integrating highly dipolar azulene into BN-PAHs was also studied (Fig. 20).¹³⁷ Compared to mono-BN-doped molecule 74a, the bis-BN-doped ladder molecule 74 showed red-shifted absorption with a higher molar absorption coefficient. Both compounds 74a and 74 had very low quantum yields of fluorescence, while the fluorescence of 74a and 74 originated from the non-lowest excited states. Compared to their carbonaceous analogues, introducing BN units could lower the HOMO level and keep the LUMO unchanged.

Fig. 20 Selective examples of other types of BN-doped ladder-type acenes.

2.3 NBN-doped PAHs

Introduction of NBN units into PAHs was initially exploited to increase the stability of PAHs and nanographenes through BN edge doping.^138–140 Now the NBN unit is not only doped at the edge of PAHs, but also embedded into the center of PAHs. There are several examples of mono-NBN-doped PAHs, which will not be discussed if they do not have multi-BN-doped analogues.^138–145

Feng et al. discovered the synthesis of a series of mono-NBN and bis-NBN edge-doped PAHs with a central phenalene (Fig. 21).¹⁴⁶ The NBN unit significantly enhanced the stability of NBN-PAHs compared to their all-carbon analogues. The chemical oxidation of 76 resulted in a dimer of 76 by Cu(OTf)₂. The bis-NBN-doped PAH 77 showed largely red-shifted absorption and emission due to the larger conjugation with a significantly enhanced quantum yield compared to mono-NBN-doped PAH 76. As continued interest in NBN-PAHs, the same group recently demonstrated the introduction of two NBN units into bis-tetracene 78 and peri-tetracene 80 (Fig. 22).^147–149 The bis-NBN-doped bis-tetracence 79 was obtained by electrophilic borylation of the tetraamine substrate using highly Lewis acidic BI₃. The further fusion of 79 using traditional in-solution cyclization did not succeed in accessing BN-peri-tetracene 81, which was achieved by surface-assisted intramolecular cyclodehydrogenation. The structure of 81 was further characterized by high resolution mass spectrometry (HRMS) and noncontact atomic force microscopy (nc-AFM). The excellent stability of BN-bis-tetracence 79 and BN-peri-tetracene 81 was ascribed to the doubled energy gap compared with their carbonaceous bis-tetracene 78 and peri-tetracene 80. For the NBN-PAHs with two NBN units, the differences between bis-NBN-doped PAH 77 and bis-BN-doped-peri-tetracene 81 were mainly attributed to the fact that 81 had a smaller optical gap. In addition, helical 79 underwent stepwise chemical oxidation to generate the corresponding radical cation and dication.

Fig. 21 NBN edge-doped PAHs with a phenalene core.

Fig. 22 NBN-doped bis-tetracene and peri-tetracene.

The first (NBN)₃-[4]triangulene 82 was reported by the Miao group through a threefold electrophilic borylation reaction (Fig. 23).¹⁵⁰ The crystal structure of 82 showed that it was slightly bent with dihedral angles of about 5.7° between peripheral benzenoid rings. NICS(1) studies showed that the benzenoid rings at the periphery and centers of 82 exhibited aromaticity, while BN-benzenoid rings showed non-aromaticity. The unprotected NBN units provided a dual hydrogen-bond donor, which could form a 2D network with a twofold dual-hydrogen bond acceptor (TDHBA) like 2,7-di(tert-butyl)-pyrene-4,5,9,10-tetraone (Fig. 23). In addition, the hydrogen-bonding between 82 and some hydrogen acceptors could result in a change in fluorescence. Recently, multiple NBN center-doped triangulene was theoretically investigated.¹⁵¹

Fig. 23 (NBN)₃-[4]triangulene 82 and the proposed hydrogen-bonded 2D network of 82 and TDHBA.

New methodologies to construct BN-PAHs are always highly desired. Electrophilic borylation cyclization is one of the most used methods to build BN-rings. The methodologies for forming BN-rings after introducing BN units are quite limited. Recently, Ma and Zhao et al. developed a palladium-catalyzed Larock-type cyclization of NBN-pentagons and hexagons with acetylenes (Fig. 24);¹⁵² this methodology was also successfully exploited to construct large ladder-type acene 83 with two NBN units. The twisted geometry of 83 could undergo photoinduced structural planarization (PISP) with changes in color and quantum yield in solutions once the concentrations were changed.

Fig. 24 Construction of the bis-NBN-ladder-type acene.

2.4 BN-doped oligomers

Oligomers, molecules consisting of a few similar or identical repeating units, have been extensively studied. Due to the difficulty of introducing multiple boron atoms at once, the research of oligomers with multiple BN units is relatively scarce.¹⁵³

Based on an extensive study on the selective functionalization of 1,2-azaborines, Liu and Jäkle collaborated the first regioregular synthesis of azaborine oligomers 85 and 86 as well as the corresponding azaborine polymer 87 (Fig. 25).¹⁵⁴ The crystal structure of dimer 85 exhibited a nearly coplanar syn arrangement of the heterocycles, which was partially contributed by N–H⋯π interactions. The lowest-energy absorption peaks of 84 (λ_abs = 277 nm), 85 (λ_abs = 334 nm), 86 (λ_abs = 383 nm) and 87 (λ_abs = 457 nm) exhibited a substantial bathochromic shift as the conjugation of the backbone was extended. The molar absorption coefficients of dimer 85 and trimer 86 were two-fold and three-fold that of monomer 84, respectively. However, the polymer 87 had a lower molar absorption coefficient than the monomer 84. The maximum of emission moved toward the long wavelength region from the oligomer 85 (λ_em = 411 nm) and 86 (λ_em = 491 nm), to polymer 87 (λ_em = 600 nm), though the monomer 84 was not fluorescent.

Fig. 25 1,2-Azaborine oligomers.

Recently, Zhou and coworkers reported a series of BN-doped PAHs with peripheral thiophenes 88–91 (Fig. 26).¹⁵⁵ The electron-rich properties of the thiophene moiety are ascribed to the successful embedding of multiple BN units via electrophilic borylation. The B–N bond length (1.4555 Å) in the crystal structure of monomer 88 was between the B–N single bond (1.58 Å) and the localized B=N double bond (1.40 Å), which indicates that the BN bond was delocalized due the effective conjugation extension through the BN bond. Although both the absorption and emission bands of these compounds 88–91 displayed red-shift as the conjugation was extended, the red-shifts among 89–91 were negligible. The optical band gaps of these molecules also decreased following the order of 88 to 91. The aromaticity and intramolecular charge transfer of these four BN-PAHs were significantly impacted by chemical oxidation.

Fig. 26 BN-PAH oligomers with peripheral thiophenes.

In order to investigate the influence of the BN/CC isosterism on the cation–π binding ability, Liu et al. exploited the BN-doped indole as the building block to synthesize the oligomer 93 (Fig. 27).¹⁵⁶ Nuclear magnetic resonance (NMR) monitored titrations with cations Li⁺ and K⁺ showed that embedding BN units into the indole only marginally decreased the cation–π binding ability compared to its carbonaceous triindole 92.

Fig. 27 Oligomer of BN-indole and its cation–π binding ability. Reproduced from ref. 156 with permission from The Royal Society of Chemistry.

BN-aromatic rings are often achieved by either electrophilic borylation or transition-metal catalyzed cyclization. Breaking aromatic rings is regarded as a hardly accessible approach. Recently, Yorimitsu and coworkers developed a brand-new strategy for constructing BN-naphthalene derivatives via a lithium-mediated reductive ring-opening reaction of indole derivatives followed by a boron insertion reaction (Fig. 28).¹⁵⁷ A variety of BN-naphthalene derivatives were obtained under the developed conditions. Even oligomers 94, 95 and 97 could be accessed using the same conditions. The dimer 95 further underwent transition-metal catalyzed cyclization, forming the bis-BN-doped ladder-type acene 96. Compound 96 had the absorption maximum at λ_abs = 387 nm and emission maxima at λ_em = 400 and 419 nm with a quantum yield of 19%. The photophysical properties of the remaining BN-PAHs obtained through this approach were not reported, but this new synthetic strategy provides an alternative scenario for construction of BN-PAHs.

Fig. 28 Synthesis of BN-PAH oligomers via lithium-mediated boron insertion.

3. Multi-BN-doped π-conjugated systems with discrete boron and nitrogen atoms

BN-doped benzenes with discrete boron and nitrogen atoms are also named 1,3-azaborines and 1,4-azaborines. Due to the scarcity of 1,3-azaborine derivatives, this section will focus on 1,4-azaborines. Studying 1,4-azaborines has a long history;^158–160 however, the renaissance in the study of 1,4-BN-PAHs was ignited until Hatakeyama and coworkers introduced multi-resonant thermally activated delayed fluorescence (MR-TADF) molecules in 2016.²³

3.1 1,4-BN-PAHs without MR-TADF

In 2006, Kawashima and coworkers explored the incorporation of anthracene, pentacene and heptacene with the aim of providing emitting materials for OLEDs (Fig. 29).²⁷ The 1,4-BN-anthracene 98 and the ladder-type molecules 99–101 were synthesized in high yields through lithium–bromide exchange of the corresponding precursors followed by addition of MesB(OMe)₂. The crystal structure of para-type compound 99 exhibited a nearly planar bis-BN-doped pentacene core. No obvious intermolecular interactions in the crystalline state of 99 were observed likely due to the bulky mesityl groups on the boron atoms. Compared to the 1,4-BN-anthracene 98, bathochromically-shifted absorption and fluorescence maxima were observed for para-type BN-pentacene 99 and tri-BN-heptacene 101 by extending the π-conjugation using ladder-type frameworks. However, the absorption and emission bands of meta-type BN-pentacene 100 were only slightly red-shifted compared with those of 98, probably due to the weak π-conjugation between the two fused azaborine units in the syn-parallel orientation mode. The distinct absorption and emission spectra of para-type BN-pentacene 99 and meta-type BN-pentacene 100 indicate that even incorporating BN units with different orientations in the same carbonaceous PAHs could result in dissimilar photophysical properties.

Fig. 29 BN-doped ladder-type acenes with multiple 1,4-BN units.

A variety of thieno-fused ladder-type 1,4-BN-acenes have been reported by Mitsudo and Suga et al. using a Friedel–Crafts-type C–H borylation to introduce boron atoms (Fig. 30).¹⁶¹ Compared to the 1,4-BN-acenes in Fig. 28, the thieno-fused molecules 102 and 103 only exhibited weak fluorescence (Φ < 0.1), indicating that fusing benzo and thieno rings with 1,4-azaborines could cause substantial changes in photophysical properties. Not surprisingly, the ladder-type thieno-acene 103 showed a red-shifted absorption onset compared to the dithieno-1,4-azaborine 102.

Fig. 30 Dithieno-fused 1,4-azaborine derivatives.

Atom-specifically substituting graphene nanoribbons (GNRs) could be achieved by so-called “bottom-up” synthesis.^162–165 Starting from BN-doped anthracene derivative 104b, Kawai, Hatakeyama and Foster et al. developed an on-surface chemical synthesis strategy involving halide–halide coupling cyclodehydrogenation reactions (Fig. 31), which was used to access the series BN-GNR 104 bearing one BN unit in its repeating unit, and the alternative BN-GNR 105 that had two BN units in its repeating unit.¹⁶⁶ The elemental differences in the GNRs were resolved by AFM with a CO-functionalized tip.

Fig. 31 Multi-BN-doped graphene nanoribbons.

Very recently, Ma and Feng et al. reported a modular cascade synthetic strategy to construct structurally constrained BN-PAHs (Fig. 32).¹⁶⁷ The tandem process consisted of three steps: borylative 6-endo-dig cyclization, 1,4-boron migration, and subsequent two-fold electrophilic borylation. This methodology was further verified by the general substrate scope. The bis-BN-doped PAHs 107 and 108 were also obtained by this domino process. The single crystal X-ray diffraction analysis of 107 confirmed the double [4]helicene structure. The solid-state structure of 107 had a π-stacked trimeric sandwich structure with a 2D lamellar π-stacking. The bis-BN-nanographene 101 had red-shifted absorption and emission bands compared with mono-BN-doped PAH 100, due to its increased π-conjugation structure. The difference of photophysical properties between anti-parallel bis-BN-nanographene 107 and syn-parallel bis-BN-nanographene 108 is likely due to the opposite orientation of two BN units. All these compounds had very small Stokes shift due to their constrained structures.

Fig. 32 The cascade synthetic approach toward BN-PAHs.

3.2 1,4-BN-PAHs with MR-TADF

Due to the opposite resonance effect of the boron atoms and nitrogen atoms, the resonance effect could be enhanced by para-substitution between the boron atoms and the nitrogen atoms, resulting in substantially separated HOMO and LUMO levels, which is known as multiresonance HOMO–LUMO separation (Fig. 33). In 2016, Hatakeyama and coworkers exploited this strategy to design BN-PAHs DABNA 109 and 110 that exhibited ultrapure blue fluorescence (Fig. 33).²³ Structurally, the introduction of substituents on 110 improved the oscillator strength without changing the localization of molecular orbitals. Both 109 and 110 exhibited excellent quantum yields and small singlet-triplet energy gaps (ΔE_st = 0.20 eV). Compared to 109, the device based on 110 exhibited better electroluminescent performance with an EQE of 20.2%, a current efficiency of 21.1 cd A⁻¹, a power efficiency of 15.1 lm W⁻¹, a narrow FWHM of 28 nm and Commission Internationale de l’Eclairage (CIE) coordinates of (0.12, 0.13). The improved performance of 109 might be ascribed to the higher radiative rate (k_F = 14.1 × 10⁷ s⁻¹) and reverse intersystem crossing decay rate (k_RISC = 14.8 × 10³ s⁻¹). The serious efficiency roll-off was due to the small k_RISC and charge carrier imbalance. This problem was significantly improved by designing doubled DABNA PAH 111.¹⁶⁸ The OLED device using 111 as the emitter exhibited narrowband (FWHM = 18 nm) deep-blue emission at 469 nm, indicating that the extended conjugation of PAHs based on multiresonance HOMO–LUMO separation does not result in bathochromic shift of emission. The k_F and k_RISC of compound 111 were remarkably increased to 2.0 × 10⁸ s⁻¹ and 2.0 × 10⁵ s⁻¹. The device based on 111 had record-breaking EQEs of 34.4% at the maximum and 26.0% at 1000 cd m⁻². The bis-BN-PAH 112, a nitrogen atom replaced with an oxygen atom compared to 111, showed hypsochromically shifted emission due to oxygen atom incorporation prompted restricted π-conjugation of the HOMO.¹⁶⁹ Further expanded heterohelicene 113 with tripled DABNA was achieved by a one-shot triple borylation.¹⁷⁰ The OLED device based on (BN)₃-doped helicene 113 showed narrowband red-shifted emission (λ_em = 480 nm) compared to 111. Although the k_RISC (4.4 × 10⁵ s⁻¹) of compound 113 was double that of 111, the serious efficiency roll-off was still observed probably due to the charge carrier imbalance.

Fig. 33 1,4-BN-PAHs with MR-TADF by Hatakeyama.

Developing efficient electrophilic borylation reactions plays a critical role in construction of BN-doped PAHs since these reactions are highly dependent on the types of substrates and boron reactants, temperature, and even additives. Hatakeyama et al. discovered one-shot multiple borylation using highly Lewis acidic boron reagent BI₃ to synthesize a series of BN-nanographenes with different numbers of BN units (Fig. 34).¹⁷¹ In the presence of 12 eq. of BI₃, quadruple borylation reaction was proceeded under reflux conditions affording four-boron atom-doped compound 114 and a small amount of 115. The tri-BN-doped nanographene 115 was obtained in a much higher yield if less BI₃ was added in the presence of additive Ph₃B at higher temperature. The same conditions used to access 115 were exploited to achieve bis-BN-doped PAH 116 in a higher yield at relatively lower temperature. The crystal structure of 115 confirmed its helical structure. The absorption maximum (396 nm) of 114 was noteworthily blue-shifted compared to those of 114 (438 nm) and 116 (440 nm) due to the forbidden S₀–S₁ and S₀–S₂ transitions and high oscillator strengths of S₀–S₃ and S₀–S₄. All these three BN-nangraphenes emitted blue fluorescence with moderate to good quantum yields and full-widths at half-maximum (FWHM) between 32 and 38 nm. The phosphorescence maxima of 114–116 at 77 K were very close to their fluorescence maxima, indicating that they are promising for TADF-based OLEDs. Compound 116 was selected to demonstrate the potential in OLEDs. The fabricated OLED device showed exceptional performance with a maximum EQE of 18.3%.

Fig. 34 One-shot multiple borylation toward multi-BN-nanographenes. The crystal structure of 115 reprinted with permission from ref. 171. Copyright 2018 American Chemical Society.

The Hatakeyama group and Wang group independently reported the synthesis of BNB-doped PAHs 117 (Fig. 35).^172,173 This class of BNB-doped molecules was found to exhibit efficient MR-TADF, featuring small ΔE_st, narrow emission bands (FWHM = ∼33 nm), comparable k_RISC values to that of NBN-doped PAH 109, and high photoluminescence quantum yields up to 89%. With an extended π-skeleton with oxygen atoms and bulky substituents, compound 118 exhibited solution-processable pure green MR-TADF.¹⁷⁴ Similar to the other OLED based MR-TADF emitters, the device of 118 presented attractive performances with CIE coordinates of (0.12, 0.63) and EQEs of 21.8% at the maximum and 17.4% at 1000 cd m⁻². The solution-processability of 118 could substantially reduce the cost of the device fabrication. Agou and Yasuda et al. documented a ternary-doped MR-TADF system (119) with multiple boron, nitrogen and sulfur atoms (Fig. 35).¹⁷⁵ In addition to being an electron-rich moiety for inducing the MR effect with the nitrogen atoms, the heavy-atom effect of the phenothiaborin moiety has an important impact on enhancing the spin-flipping RISC. The OLED device emitted narrowband sky-blue light with a balance high k_RISC (1.9 × 10⁶ s⁻¹) and high k_F (9.0 × 10⁷ s⁻¹), a high maximum EQE of 21.0%, and suppressed efficiency roll-off.

Fig. 35 BNB-doped PAHs for MR-TADF.

Zysman-Colman et al. reported a linear ladder-type BN-doped heptacene, 120, in which three boron atoms were introduced once through electrophilic borylation.¹⁷⁶ Structurally, BN-heptacene 120 is isomeric to compound 121, which shares the same (BN)₃-heptacene core with 101, which was reported by Kawashima and coworkers.²⁷ Compared to anti-paralleled BN-heptacene 121, the syn-paralleled BN-heptacene 120 was demonstrated to have higher S₁ energy and deep-blue emission with CIE coordinates of (0.17, 0.01) (Fig. 36). Opposite to 121, compound 120 exhibited MR-TADF properties at ambient temperature, which are very rare for linear 1,4-BN-doped PAHs. The distinct difference of singlet and triplet energies as well as the ΔE_st once again indicates that the orientation of BN units could significantly impact the optoelectronic properties of BN-PAHs.

Fig. 36 (BN)₃-doped heptacenes.

As an attractive building block for optoelectronic materials, incorporating the carbazole moiety into BN-doped PAHs has gained more and more attention. Especially, the carbazole-based DABNA 109 analogues have been extensively studied.^177–182 Recently, Hatakeyama et al. reported a series of carbazole-based DABNA analogues which have the capability of being highly efficient MR-TADF materials (Fig. 37).¹⁸³ The electrophilic borylation has two potential reaction sites: the ortho position of the carbazolyl group and the ortho position of the diarylamino group. The DFT calculations indicated that the HOMO was mainly localized at the ortho position of the carbazolyl group, resulting in compound 122 instead of the compound of borylation at the ortho position of the diarylamino group reported by Huang.¹⁷⁷ This showcases that theoretical calculation could be helpful to determine the selectivity of reaction reactive sites. All these carbazole-based DABNA molecules had excellent photoluminescence quantum yields with very small ΔE_ST. Compared to mono-NBN-doped PAHs 122 and 124, incorporation of the second boron atom red-shifted the emissions and widened the FWHMs of di-NBN-doped PAHs 123 and 125. The bis-NBN-doped PAH 123 with two ^tbutyl groups replaced with the phenyl group in OLEDs displayed green emission at 497 nm with a FWHM of 29 nm and CIE coordinates (0.12, 0.57).

Fig. 37 Carbazole-based multi-BN-doped PAHs developed by Hatakeyama.

In order to achieve full-color luminophores, the Yasuda group discovered that strategic implementation of electron-deficient boron atoms and electron-rich carbazole units provided a variety of BN-PAHs with efficient TADF properties (Fig. 38).¹⁷⁹ The carbazole-based DABNA 126 was independently synthesized and investigated in OLEDs by Duan and Wang.^178,182 The donor strength of the carbazole unit in bis-NBN-doped DABNA analogue 127 was reduced by incorporating the second boron atom at the para-position of the carbazole, while the acceptor strength via B–π–N conjugation was decreased by the fused third carbazole moiety. This resulted in a wider HOMO–LUMO energy gap and blue-shifted emission of 127. Inversely, the acceptor and donor strengths were substantially increased by the two para-doped boron atoms and two nitrogen atoms in 128, featuring notable bathochromic shifts of the absorption and emission spectra. The HOMOs of 129 and 130 extended to the extra meta-positioned carbazoles of the boron atom, which can strengthen the intramolecular charge-transfer (ICT) with lower S₁ and T₁ energies accompanied by red-shifted emissions. Based on the family of MR-TADF PAHs 125–129, full-color and narrowband OLEDs were obtained with excellent maximum EQEs up to 31.8%.

Fig. 38 Carbazole-based multi-BN-doped PAHs with full-color MR-TADF developed by Yasuda.

The fact that non-radiative transitions will dramatically increase with decreasing energy gap, also known as energy gap law,^184,185 has significantly limited the discovery of deep-red and near-infrared (NIR) emitters with high efficiency. Duan and coworkers reported distorted (NBN)₂-doped helical molecules 131 and 132 bearing four carbazole units and two boron atoms (Fig. 39).¹⁸¹ Both 131 and 132 had exceptionally high photoluminescence quantum yields of 100%. The introduction of ^tbutyl groups on carbazole moieties slightly decreased the energy gap with an unchanged FWHM of 38 nm, as well as a significantly slower k_RISC. Not surprisingly, significant efficiency roll-off was observed since the low k_RISC was at the order of 10⁴ s⁻¹. Almost at the same time, the Wang group independently demonstrated the same molecules with different application in chiroptical materials.¹⁸⁰ Molecule 131 exhibited a record-high absorption dissymmetry factor (g_abs) of up to 0.033 in the visible spectral range and red to near-infrared circularly polarized luminescence (CPL). The HOMO–LUMO separation induced by para-positioned boron atoms and nitrogen atoms is related to the high g_abs.

Fig. 39 Carbazole-based multi-BN-doped helicenes in different applications developed by Wang and Duan. Reprinted with permission from ref. 180. Copyright 2021 American Chemical Society.

4. Multi-BN-doped π-conjugated systems bearing tetracoordinate boron atoms

Multiple BN-doped π-conjugated systems bearing tetracoordinate boron atoms have been extensively studied, especially boron-dipyrromethene (BODIPY) and its derivatives,^186–188 as well as tetracoordinate boron-doped small molecules and polymers and their corresponding applications in OLEDs¹⁸⁹ and OPVs.^190–192 However, we will only focus on two types of PAHs with four-coordinated boron atoms in this section (Fig. 40): (1) π-conjugated systems consisting of 5-membered BN-chelate units and (2) π-conjugated systems consisting of 6-membered BN-chelate units.

Fig. 40 Two types of four-coordinated boron-doped PAHs.

4.1 π-Conjugated systems consisting of 5-membered BN-chelate units

In 2006, Yamaguchi and coworkers exploited the (3-boryl-2-thienyl)-2-thiazole 133 as the building block to design a series of regioisomeric bis-BN-doped π-conjugated systems (Fig. 41).¹⁹³ The different orientations of BN units in the dimers 134–136 resulted in blue-shifted absorption and emission bands from 134 (λ_abs = 443 nm, λ_em = 492 nm) to 136 (λ_abs = 414 nm, λ_em = 472 nm). Compared to the thienylthiazole backbone, incorporation of BN units not only rendered the π-conjugated framework in a plane, but also lowered the LUMO level. The time-of-flight (TOF) carrier-mobility measurement on a vacuum-deposited film of 134 exhibited a fairly high electron mobility (m) of 1.5 × 10⁻⁴ cm² V⁻¹ s⁻¹.

Fig. 41 A series of regioisomeric dimers of 5-membered BN-chelate units by Yamaguchi.

The N,C-chelate boron compounds was the first class of chelate boron compounds to display reversible photochromism, which was discovered by Wang et al. in 2008.¹⁵ The solution of compound 137 could rapidly lose its strong blue fluorescence and change color from colorless to deep blue (Fig. 42) upon irradiation at 365 nm, which could be thermally reversible. Wang and coworkers continued to extend this photochromic N,C-chelate boron systems to dimers 138 linked with a non-conjugated silane and 139 linked with a conjugated 1,4-diethynylbenzene, even oligomer 140 with six N,C-chelate boron units.^194,195 Although these dimers and oligomers of 137 still underwent photochromism, only one boron unit underwent photoisomerization. The photochromism of the remaining boron units in 138–140 was quenched by the intramolecular energy transfer from the non-isomerized boron unit to the dark isomer unit featuring a low energy absorption band.

Fig. 42 The multi-BN-doped π-conjugated systems consisting of 5-membered BN-chelate units by Wang.

Intramolecular Lewis acid–base coordination accompanied by the introduction of BN units into ladder-type conjugated molecules usually plays a critical role in the rigid and coplanar structures of these tetracoordinate boron-doped ladder-type molecules as well as the electronic structures. Bis-BN-doped ladder acenes 141–144 with an indacene core with different bilateral units exhibited broad visible to near-infrared (NIR) light absorption (Fig. 43) due to the significantly decreased LUMO level compared to the backbone ligands.^196,197 The surrounding substituents were found to have an essential impact on the absorption maxima of 143 (R = ^tBu) and 144 (R = N(4-^tBuPh)₂). Replacing the halide substituents on boron atoms with aromatic substituents (e.g. tolyl, 2,4,6-F₃C₆H₂) of 143 and 144 resulted in lower energy absorption significantly blue-shifted. Using compound 143 with Cl replaced with 2,4,6-F₃C₆H₂, proof-of concept organic solar cells (OSCs) were fabricated with power conversion efficiencies of 2%.

Fig. 43 Bis-BN-doped ladder acenes with an indacene core.

A series of bis-BN-doped ladder-type pyrrolo[3,2-b]pyrroles were synthesized, featuring large molar extinction coefficients and orange to deep red fluorescence with quantum yields up to 90% (Fig. 44).¹⁹⁸ These dyes exhibited superb photostability and high post-functionalization capability; a variety of R groups in compounds 145 and 146 were introduced with tunable photophysical properties. Large two-photon absorption cross sections were observed in these dyes.

Fig. 44 Bis-BN-doped ladder acenes fused with pyrrolo[3,2-b]pyrrole.

The introduction of BN units could not only induce unique photophysical properties and intermolecular interactions, but also make the synthesis of BN-doped molecules simpler compared to the synthesis of their all-carbon analogues because a B–N dative bond usually can be much more easily formed than a C–C bond. For example, carbon-bridged oligophenylenevinylenes ((COPV)_n, 147) have been used in molecular wire and solid-state lasers; however, the synthesis of COPV oligomers with long wavelengths of emission is very challenging.^199–201 Replacing the CC unit with the BN unit could result in feasible synthesis and longer wavelengths of emission. Recently, Shang, Nakamura and coworkers reported a class of linear bis-BN-doped π-conjugated molecules through four-step synthesis in high yields (Fig. 45).²⁰² The absorption and emission bands of 148 were red-shifted about 40–50 nm compared to its carbonaceous analogue ((COPV)₂, 147). The extension of π-conjugation in 149further moved the absorption and emission spectra toward the long wavelength direction. Incorporation of BN units still kept the structures as rigid as the all-carbon analogue, so the quantum yields of the bis-BN-doped ladder-type molecules 148 and 149 were close to unity, which was similar to that of (COPV)₂. The LUMO levels of 148 and 149 were significantly decreased compared to (COPV)₂, while the HOMO levels of 148 and 149 remained almost unchanged. COPVs and their two BN-doped congeners 148 and 149 showed similarly high photostability. The molecules 148 and 149 served as lipophilic fluorescent dyes for live cell imaging with unchanged sky-blue and yellow fluorescence in 25 min under irradiation, respectively, which is superior to commercially available BODIPY-based dyes.

Fig. 45 Bis-BN-(COPV)₂.

Recently, helicenes have attracted increasing attention as chiroptical dyes due to their enhanced circular dichroism (CD) and circularly polarized luminescence (CPL) properties compared with other chiral organic molecules.^203,204 However, helicenes usually show low fluorescence quantum yields due to the low transition probability of the first excited state S₁.^205,206 Introducing boron atoms into helicenes has proven to be an effective way for increasing quantum yields.^145,180 Nowak-Król et al. discovered a modular synthesis of mono-BN-doped azabora[7]helicenes 150 and bis-BN-doped azabora[9]helicenes 151via an electrophilic borylation followed by a substitution reaction with aluminum reagents (Fig. 46).²⁰⁷ In contrast to the high configurational stability of 150, the bis-BN-doped helicenes 151 showed the interconversion of four confirmers via three transition states. The interconversion of 151 was observed at room temperature, which made the resolution of the stereoisomers impossible. Azabora[7]helicenes 150 exhibited strong chiroptical response with large dissymmetry factors of up to 1.12 × 10⁻² and noteworthily higher fluorescence quantum yields of 18–24% than those of [6]carbohelicenes. The azabora[9]helicenes 151 showed brighter emission with quantum yields of 43–47%.

Fig. 46 Bis-BN-doped organoboron helicenes.

4.2 π-Conjugated systems consisting of 6-membered BN-chelate units

Doping four-coordinated boron atoms into π-conjugated systems could markedly decrease the LUMO level and increase the electron affinity, which is also true for π-conjugated systems consisting of 6-membered BN-chelate units.^192,208,209 In the subsection, we will focus on molecules with exceptionally low LUMO levels in different applications.

In addition to the lowered LUMO level, the introduction of BN units could also endow tetracoordinate boron-doped π-conjugated systems with remarkable redox activities. Fang and coworkers designed two cruciform ladder-type molecules 152 and 153 (Fig. 47) with the B ← N coordination-promoted delocalization and hyperconjugation.²¹⁰ These two molecules featured two reversible oxidation peaks and two reversible oxidation peaks, corresponding to five stable redox states, as shown at the bottom of Fig. 47. The gain of electrons and loss of electrons of compound 153 were accompanied by the interconversion of different forms of its central core between a benzenoid character and two quinonoid characters. Both 152 and 153 showed reversible multicolor electrochromism. The molecule 153 with phenyl groups on the boron atoms displayed red-shifted absorption and emission bands (λ_abs = 768 nm, λ_em = 812 nm) compared to that of 152 with fluoride on the boron atoms (λ_abs = 695 nm, λ_em = 720 nm). Low fluorescence quantum yields of 152 (Φ_fl = 2.8%) and 153 (Φ_fl = 1.2%) were measured due to their small energy band gaps. Very recently, collaborating with Zhu and Al-Hashimi, Fang and coworkers documented a quadruply BN-doped polycyclic π-system with 23 fused rings (154 in Fig. 47).²¹¹ This large ladder-type molecule 154 exhibited intensive absorption in the NIR region with a decreased LUMO level of –3.82 eV.

Fig. 47 Multi-BN-doped ladder-type conjugated molecules based on the indolo[3,2-b]carbazole.

Liu and coworkers reported a tetracoordinate boron-doped pyrene with two strong electron-withdrawing groups (Fig. 48a).²¹² Due to the delocalized LUMO at −3.93 eV spread over the entire molecule and the localized HOMO at the BN-pyrene core, compound 155 exhibited a wide absorption spectrum in the visible range with two absorption maxima at λ_max = 502 nm and 771 nm. OSCs based on 155 were reported to have a power conversion efficiency of 7.06%. A similar doping strategy has been used in the synthesis of tri-NBN-doped triazatrinaphthylene (Fig. 48b).²¹³ Both 156 and 157 exhibited a disc-shaped and slightly deformed skeleton. The molecule 157 with phenyl groups on the boron atoms showed ca. 30 nm red-shift in absorption and ca. 80 nm red-shift in emission compared to compound 156 with fluorides on the boron atoms. The low quantum yields of 156 (Φ_fl = 2.46%) and 157 (Φ_fl = 1.91%) were probably due to the deformed structure-enhanced nonradiative decay. The substituents on the boron atoms have been found to have a substantial impact on the electrochemical behaviors.

Fig. 48 (a) Doubly NBN-doped pyrene and (b) triply NBN-doped triazatrinaphthylene with tetracoordinate boron atoms.

Liu and coworkers systematically investigated how B ← N coordination affects the optoelectronic properties and device behaviors (Fig. 49).^24–26 When two NBN units were doped into the same conjugated system, the different locations of NBN units had notable effects on the frontier molecular orbitals, particularly on the LUMO level. The 1,4-NBN-positioned molecule 160 had the lowest LUMO up to −4.01 eV among molecules 158–160. The molecule 160 and 1,2-NBN-positioned molecule 158 showed electron transporting properties with mobilities of 0.06 and 0.12 cm² V⁻¹ s⁻¹, respectively. The 1,3-NBN-doped molecule 159 had the highest LUMO among these three molecules with ambipolar electron/hole transporting behaviors (μ_h = 4.2 × 10⁻³ cm² V⁻¹ s⁻¹, μ_e = 0.07 cm² V⁻¹ s⁻¹). Compounds 158 and 159 showed strong absorption spectra with maxima near 700 nm and emitted NIR fluorescence, while the regioisomer 160 exhibited further red-shifted absorption at λ_max = 808 nm and emitted no fluorescence. These differences in optoelectronic properties again indicate that the location and orientation of BN units should be fully taken into account. The PAHs 161–163 with four NBN units further lowered the LUMO levels. The disc-type molecule 161 displayed a well-resolved absorption spectrum with two major peaks at 569 and 618 nm, as well as an intense NIR fluorescence spectrum with a small Stokes shift and an exceptional quantum yield of up to 50%. Compared to compound 162, the molecule 163 with two extra bilateral benzo rings exhibited the lowest LUMO level of up to −4.58 eV among boron-doped π-conjugated molecules. Compared to the average electron mobilities less than 10⁻² cm² V⁻¹ s⁻¹, the electron mobility of 163 in OFETs was two orders of magnitude higher up to 1.60 cm² V⁻¹ s⁻¹. This indicates that pushing the number limit of BN units in PAHs could help achieve certain properties that could not be accessed by other types of molecules.

Fig. 49 Multiple NBN-doped PAHs with low LUMO levels and high electron affinity.

The introduction of tetracoordinate boron into conjugated molecules can not only lead to functional materials with properties mentioned above like photochromism, NIR-absorption/emission, and significantly lowered LUMO levels, but also enhance the singlet O₂ sensitivity. In 2017, Jäkle et al. reported a class of tetracoordinated boron-functionalized anthracenes (Fig. 50).²¹⁴ Compared to the all-carbon analogues, the doubly BN-fused anthracenes 164 and 165 showed dramatically decreased LUMO levels with slightly increased HOMO levels. The intensively red colored compounds 164 and 165 were found to rapidly react with oxygen upon photoirradiation with the formation of colorless endoperoxides 164 + O2 and 165 + O2, respectively. Unlike the reactions of other diarylanthracenes with oxygen which usually require an external photosensitizer, the BN-functionalized anthracenes 164 and 165 underwent the O₂ sensitization through a self-sensitizing process. This O₂ sensitization process could be thermally reversed back. The high reactivity of 164 and 165 toward O₂ was mainly ascribed to the strong absorption in the visible range, small ΔE_st, and the release of steric strain. Further studies indicated that the position of methyl groups on pyridyl rings sterically and electronically affected the self-sensitized reactivity with singlet O₂. Compounds 165, 165-3Me, 165-4Me and 165-5Me showed different reaction rates toward O₂ sensitization as well as the thermal release reaction of O₂.²¹⁵

Fig. 50 Doubly BN-fused anthracenes for singlet O₂ sensitization.

The introduction of BN units at the zigzag edges of PAHs is regarded as an effective strategy to improve their chemical stabilities. This is particularly useful to stabilize large acenes (longer than pentacene) which are usually unstable in the carbonaceous form. Recently, Ma, Liu and Feng et al. documented the synthesis of a class of BN-doped large acenes (Fig. 51).²¹⁶ All the modified acenes exhibited high stability. Unlike previous examples in which the BN units affect the LUMO level, the bis-BN-doped acenes 167–169 showed higher HOMO levels than the mono-BN-doped PAH 166 with nearly unchanged LUMO levels. The fusing units in compounds 167–169 also had a notable impact on the HOMO level; that is, the larger the fusing unit is, the higher HOMO level the BN-doped acene has. With respect to emission, the emission maxima moved toward the long wavelength following the order of 166, 167, and 168. The quantum yields decreased by following the same trend. The pyrene-fused acene 169 was non-emissive due to the forbidden transition from the lowest-lying excited state to the ground state.

Fig. 51 Large acene derivatives doped with two BN units.

5. Conclusions and outlook

In this Review, we have summarized the major contributions to recent progress in the synthesis of multi-BN doped π-conjugated systems as well as their optoelectronic properties. The influence of the location, number and orientation of BN dopants as well as the boron coordination number on structures and optoelectronic properties has been compared if applicable.

Although 1,2-BN-doped PAHs consist of the largest multi-BN-doped π-conjugated systems, only a few of them have been used in optoelectronic devices like OFETs. Therefore, in addition to constructing novel structures, searching for new applications of 1,2-BN-doped PAHs is highly demanded, especially with the aid of a molecular engineering strategy to modify the existing molecules. 1,4-BN-doped PAHs have exhibited promising in OLEDs; however, the roll-off problem of OLEDs based on 1,4-BN-doped PAHs with MR-TADF has limited their practical applications. Tetracoordinate boron-doped PAHs have showed many different applications like photochromism, OFETs and O₂ sensitization, but the PAHs with the four-coordinated boron atoms doped into the center of PAHs rather than edges are still quite limited. Certainly, developing new methodologies for introducing boron atoms into π-conjugated systems is fundamentally important.

At last, as the models for studying defected graphene, π-conjugated systems containing heptagonal rings have attracted increasing attention due to their dynamic behaviors, electronic properties, aromaticity and solid-state packing. Doping heptagon-containing π-conjugated systems with the BN unit not only makes π-conjugated systems with the boron-doped heptagonal ring have structural similarity to their all-carbon analogues, but could also significantly change their electronic properties and intermolecular interactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (22001211) and Northwestern Polytechnical University for financial support. This project is also supported by the Fundamental Research Funds for the Central Universities.

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Footnote

† X. C. and D. T. contributed equally to this work. First authorship was determined by coin toss.

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