Tris(pentafluorophenyl)borane: leveraging historical and emerging work to identify alternatives for organic electronic applications

Abstract

Tris(pentafluorophenyl)borane (B(C₆F₅)₃) is a versatile Lewis acid now having played a key role in the development of boron-based Lewis acid chemistry. The borane has found utility in many applications ranging from a co-catalyst for olefin-polymerization to a dopant for organic (semi-)conductors. Highlighted within are historical advances of B(C₆F₅)₃ utilization in the realms of molecular transformations and frustrated Lewis pairs (FLPs). Applications of B(C₆F₅)₃ in various electronic devices is upcoming and presented, as well emerging dopant strategies for organic-based electronic applications are included. This perspective is completed by presenting other known Lewis acids with respective properties relevant for dopant applications in organic electronics.

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1. Introduction

Tris(pentafluorophenyl)borane (B(C₆F₅)₃; Fig. 1) is a popular boron-based Lewis acid that has seen applications in many branches of science and engineering. With a 3-coordinate neutral structure there is an empty p-orbital on boron which can accept lone pairs of electrons, thus enabling reactivity. The Lewis acidity of B(C₆F₅)₃ is influenced by the three electron withdrawing pentafluorophenyl substituents that remove electron density away from boron and make the atom more acidic. The bulky pentafluorophenyl groups sterically encumber the boron center and introduces intermolecular interactions, which renders the compound a solid at room temperature with increased air and moisture stability compared to halide BX₃ (X = F, Cl, Br) counterparts. These properties make B(C₆F₅)₃ a potent yet stable Lewis acid, and therefore has been widely used both academically and industrially.¹ First synthesized in the early 1960's,² B(C₆F₅)₃ received little attention until the early 1990's when the compound was employed as a co-catalyst for the polymerization of olefins.^3–6 Following this, the volume of research and industrial application involving B(C₆F₅)₃ as a catalyst for molecular transformations was exponential. Around this time B(C₆F₅)₃ was also being applied in electronic devices, such as lithium batteries, with reports dating back as early as the 1990's.⁷ Then in the mid 2000's, the discovery of frustrated Lewis pairs (FLPs), based on the combination of B(C₆F₅)₃ with sterically bulky phosphines, re-popularized the chemistry of this borane and led to the first reports of metal-free hydrogenation using hydrogen gas (H₂) and the subsequent sequestration of many other small molecules (e.g. CO₂, N₂O, olefins).^8–11 While the chemistry of B(C₆F₅)₃ for molecular transformations^1,6,12–14 and FLPs^9–11,15 has been extensively reviewed, the use of B(C₆F₅)₃ as a p-type dopant for organic (semi-)conductors with reports of increased stability and efficiency of organic electronic devices is emerging.

Fig. 1 Structural representations of B(C₆F₅)₃. (a) Line drawing of B(C₆F₅)₃ with highlights of F atoms for electron withdrawing effects and F–F interactions, phenyl rings for π–π interactions and providing steric bulk, and B atom as the Lewis acidic site. (b) 3D model of B(C₆F₅)₃ for visual representation of the propeller style conformation.

Within we first highlight established applications of B(C₆F₅)₃ in the realms of organic/organometallic molecular transformations and FLPs. The reported interactions, properties, and reactivity are critical to consider when applying B(C₆F₅)₃ as a p-type dopant in organic electronics, as the role of the Lewis acid has been somewhat elusive and debated, and thus these accounts provide valuable insight. A variety of electronic devices, including batteries, organic transistors, solar cells, etc., are reviewed with relevant dopant mechanisms, emerging applications, and dopant strategies. Lastly, we present alternate borane- and borate-based Lewis acids with the respective properties relevant for dopant applications in organic electronics. Our perspective is that while the utility and scope of B(C₆F₅)₃ is impressive, the variety of materials applied in organic electronics is so vast that alternate Lewis acids need to be considered as B(C₆F₅)₃ will not harbour the optimal properties for every set of materials and device architectures that develop.

2. Molecular transformations using B(C₆F₅)₃

2.1. Overview

The empty p-orbital on the boron of B(C₆F₅)₃ can accept a pair of electrons and thus can activate Lewis basic sites on compounds to promote catalytic reactions & organic/organometallic transformations. Some important reactions & transformations involving B(C₆F₅)₃ include olefin polymerization,^1,5,6 aldol-type Michael reactions,¹⁶ hydrosilylations,^17,18 Piers–Rubinsztajn reactions,^17,19,20 allylstannations,^21–23 and Diels–Alder reactions.^24–27 Unlike other boron/halogen containing Lewis acids (BX₃, X = F, Cl, Br), the pentafluorophenyl groups on B(C₆F₅)₃ increase the sterics about the boron center, embodies B–C bonds that are less prone to hydrolysis, and promotes intermolecular interactions which allows for stable and isolable intermediates/products in a plethora of catalytic reactions. This resulted in mechanistic studies on Lewis acid catalyzed and co-catalysed reactions which were previously not achievable with traditional boron/halogen containing Lewis acids. Furthermore, the high thermal stability of B(C₆F₅)₃ (stable up to 270 °C) is ideal for reactions that require higher temperatures in which traditional metal catalysts would degrade. There are comprehensive reviews on the various organic and organometallic transformations that B(C₆F₅)₃ is capable of catalyzing and/or plays a key role in ref. 13 and 28, therefore, this section merely highlights enlightening studies using B(C₆F₅)₃ in various molecular transformations and FLPs. Reactions involving B(C₆F₅)₃ in the presence of water are also included as B(C₆F₅)₃·H₂O adducts are an attractive p-type dopant for organic electronic materials (vide infra).

2.2. Historical molecular transformations using B(C₆F₅)₃

In 1991, both the groups of Marks and Robinson separately demonstrated highly active and productive olefin polymerization using B(C₆F₅)₃ as a co-catalyst with alkyl group-IV metallocenes, specifically dimethyl zirconocenes (Fig. 2a).^3,29 When paired with metallocenes, the Lewis acidic B(C₆F₅)₃ can abstract an alkyl group and is rendered anionic and the metal center cationic enabling coordination–insertion polymerization. Following this discovery, a plethora of reports using B(C₆F₅)₃ as a co-catalyst for olefin polymerization emerged throughout the 1990's and beyond.^3–6,29

Fig. 2 Critical reactions involving B(C₆F₅)₃ for catalysis & chemical transformations. (a) Olefin polymerization is achieved using Cp₂ZrMe⁺ + BMe(C₆F₅)₃⁻,^3,29 (b) Adol-type and Michael reactions utilizing aldehydes and silyl enol ethers are catalyzed by B(C₆F₅)₃,¹⁶ (c) hydrosilylation of aryl carbonyls are catalyzed by B(C₆F₅)₃,³⁰ (d) allylstannations of ortho-anisaldehydes are favoured over para-anisaldehydes by >20 : 1 in the presence of (B(C₆F₅)₃),³¹ (e) tautomerization of aromatic enols can be done by coordination of the boron on B(C₆F₅)₃ with the oxygen on the enol,³² (f) Piers–Rubinsztajn reactions see siloxane formation using hydrosilanes and alkoxysilanes in the presence of catalytic B(C₆F₅)₃,³³ (g) Friedel–Crafts reactions using 1,2,4-trimethoxy benzene was successfully alkylated with a N-benzyloxycarbonylamino phenyl p-tolylsulfone in the presence of catalytic B(C₆F₅)₃,³⁴ (h) Diels–Alder exo-selective isomers are formed open chain-dienes and α,β-unsaturated enals in the presence of catalytic B(C₆F₅)₃.²⁵

In 1993, B(C₆F₅)₃ was explored as a catalyst for aldol-type and Michael reactions, which were previously achieved with alternate Lewis acids, however, these reactions suffered from poor yields if even trace amounts of water were present.¹⁶ With B(C₆F₅)₃ being somewhat water and air-stable, high yielding aldol-type and Michael reactions were achieved and resulted in the coupling of various silyl enol ethers and aldehydes (Fig. 2b). Hydrosilylation of carbonyls is used for the preparation of silylethers, and typically uses metal catalysts to do so. However, in 1996, Piers and Parks demonstrated the catalytic activity of B(C₆F₅)₃ for the hydrosilylation of aryl carbonyls (Fig. 2c), to which B(C₆F₅)₃ was reported as being comparable with traditional metal catalysts used in terms of selectivity and conversion rates.³⁰ Later, hydrosilylation of imines using B(C₆F₅)₃ was also demonstrated.³⁵ In 1998, Maruoka and coworkers reported allylation and reduction of an ortho-anisaldehyde using B(C₆F₅)₃ and an allyl tributyltin reagent in a competitive reaction with para-anisaldehyde, where the allylation of the ortho-anisaldehyde resulted in a >20 : 1 favour over the para-anisaldehyde (Fig. 2d).³¹ The authors suggested the reaction proceeds via pentacoordinate B(C₆F₅)₃ adducts with the ortho-anisaldehyde, a highly unusual reaction as this requires chelation of a boron-based Lewis acid, something that is notoriously difficult to do. However, Piers and coworkers later elucidated the mechanism: B(C₆F₅)₃ abstracts the allyl group from the tin reagent, producing the activated “SnBu₃⁺” species which then catalyzes the reaction thus suggesting hypercoordination of the borane is not necessary.^21,36

Developed in the 2000's, the Piers–Rubinsztajn reaction is one that involves the synthesis of alkoxysilanes and the respective polymers. It was first noted by Piers and Parks in 1996 that a small amount of silyl ethers were transformed into silyl alkanes in the presence of B(C₆F₅)₃ when attempting to reduce aromatic carbonyl compounds.³⁰ This lead to Rubinsztajn and Cella investigating the formation of siloxane polymers using B(C₆F₅)₃.³³ Shortly after, the Piers–Rubinsztajn reaction was formulated and has since been widely used for the synthesis of polymeric siloxanes (Fig. 2f).^17,19,20 These polymers are composed of strong Si–O bonds, which provides the bulk material with high chemical resistance and thermal stability but at the consequence of requiring harsh conditions for degradation or recycling. However, the depolymerization of polydimethylsiloxane using reagents B(C₆F₅)₃ and dimethyl carbonate under mild conditions was more recently reported.³⁷ This demonstrates the versatility of B(C₆F₅)₃ for both the synthesis and sustainable degradation of organosiloxanes.

Tautomerization of aromatic enols are difficult due to breaking aromaticity, however, due to stable B(C₆F₅)₃-carbonyl adducts, the tautomerization of naphthol with B(C₆F₅)₃ for isolation of the keto isomer was shown to be possible (Fig. 2e).³² Following this, several pyrrole based tautomers were made and put to use in Ziegler–Natta catalysis chemistry as Brønsted acid activators for highly efficient polymerizations.^38–41 Due to the ability to form adducts with carbonyl and imine containing compounds, it was then discovered that B(C₆F₅)₃ can catalyze other important C–C coupling reactions. Indeed, Diels–Alder reactions of 2-cycloalkenones can be catalyzed by B(C₆F₅)₃, however, like other Lewis acids these reactions resulted in high ratios of the endo product.²⁴ Then in 2015, Loh and coworkers demonstrated the use of B(C₆F₅)₃ for exo-selective isomers using open chain-dienes and α,β-unsaturated enals as the dienophiles (Fig. 2h).²⁵ It was hypothesized that due to the bulky nature of B(C₆F₅)₃, sterics cause the favour of the exo-product with ratios ranging from 60 : 40 to 99 : 1, which was later supported by Soós and coworkers in 2018.²⁶ Additionally, B(C₆F₅)₃ is capable of catalyzing Friedel–Crafts reactions, showcasing yet another example of making C–C bond formation more efficient. A reaction of this type was first reported in 2011, where 1,2,4-trimethoxy benzene was successfully alkylated with a N-benzyloxycarbonylamino phenyl p-tolylsulfone in the presence of B(C₆F₅)₃ with high yields (Fig. 2g).³⁴

2.3. Frustrated Lewis pairs involving B(C₆F₅)₃

Frustrated Lewis pairs (FLPs) can be described as compounds that contain Lewis base/acid pairs with sufficient steric hinderance to prevent Lewis adduct formation via dative bonding. The seminal discovery of FLPs (first publication in 2006) provided the next sequence of impactful research involving B(C₆F₅)₃. The serendipitous discovery of FLPs came from a unusual reaction of B(C₆F₅)₃ with the sterically bulky phosphine bismesitylphosphine ((C₆H₂Me₃)₂PH) to form the air & moisture stable white zwitterionic solid, (C₆H₂Me₃)₂PH(C₆F₄)BF(C₆F₅)₂.⁸ Treatment of this compound with Me₂SiHCl resulted in the exchange of a fluorine atom for a hydride at the boron and gave (C₆H₂Me₃)₂PH(C₆F₄)BH(C₆F₅)₂. This compound contained both a proton (on the phosphorus) and hydride (on the boron) which when gently heated (∼100 °C) released H₂ gas and gave the phosphino-borane (C₆H₂Me₃)₂P(C₆F₄)B(C₆F₅)₂ (Fig. 3), which was bright orange in color owing to intramolecular charge transfer characteristics. The process of H₂ liberation was found to be reversible and thus this resulted in the first ever report of metal-free hydrogen activation.⁸ By retaining the Lewis basicity and acidity within the (C₆H₂Me₃)₂P(C₆F₄)B(C₆F₅)₂ molecule, electron density can be donated and accepted simultaneously and allowed for reversible heterolytic cleavage and liberation of H₂ at moderate (25 °C) and high (>100 °C) temperatures, respectively. Upon further investigation of the interaction (or lack thereof) of bulky phosphines (PR₃, R = i-Pr, Cy & PHR′₂, R′ = t-Bu, C₆H₂Me₃-2,4,6) with B(C₆F₅)₃, the term FLP was coined.⁴² Subsequently a diverse and rapid emergence of research involving B(C₆F₅)₃ based FLPs for many reactions, including metal-free hydrogenation, hydroamination, activation of various small molecules, and dehydrogenation has been extensively reviewed.^{9–11,15,43–45} Furthermore, many variations of FLPs have been made, where both the Lewis acid and Lewis base have been exchanged.^10,15 This section aims to highlight select molecular transformations catalyzed by FLPs using B(C₆F₅)₃ as the Lewis acid component.

Fig. 3 The seminal discovery of FLPs; (C₆H₂Me₃)₂PH(C₆F₄)BH(C₆F₅)₂ liberates H₂ when heated (to ∼100 °C) to give the phosphino-borane (C₆H₂Me₃)₂P(C₆F₄)B(C₆F₅)₂, a process which is reversible upon cooling.⁸

Metal-free hydrogenation has positive implications for some important industries, such as the food & beverage and petrochemical industries, where the use of metal catalysts in these industries can be costly. For example, hydrogenation is responsible for the manufacturing of food products such as oils, shortenings, and spreads. Following the discovery of FLPs as a catalyst for metal-free hydrogenation, useful transformations such as the hydrogenation of alkenes/olefins/alkynes (Fig. 4a),^46–49 aromatics,⁵⁰ carbonyls (Fig. 4b),^51–53 imines/nitriles/enamines (Fig. 4c and d),^54–56 silyl enol ethers (Fig. 4f),⁵⁷ and oximes (Fig. 4e)⁵⁸ have been reported using B(C₆F₅)₃ as the Lewis acid component.

Fig. 4 Metal-free hydrogenation using B(C₆F₅)₃ FLPs. (a) B(C₆F₅)₃ FLPs are capable of reducing alkynes to alkenes, and upon further treatment from alkenes to alkanes.^46–49 (b) B(C₆F₅)₃ based FLPs can reduce carbonyls to primary alcohols.^51–53 (c) B(C₆F₅)₃ based FLPs can reduce nitriles into imines, and then into amines.^54,55 (d) B(C₆F₅)₃ based FLPs can reduce enamines into amines.⁵⁶ (e) B(C₆F₅)₃ based FLPs can reduce oximes to hydroxylamines and then amines.⁵⁸ (f) B(C₆F₅)₃ based FLPs can reduce silyl enol ethers into silyl ethers.⁵⁷

Hydroamination reactions are relevant for many synthetic chemists, as they can be employed using amines and alkenes/alkynes intermolecularly or intramolecularly, where the latter results in N-heterocycle formation. Stephan and coworkers demonstrated the first account of using B(C₆F₅)₃ based FLPs for hydroamination reactions using aryl amines and terminal alkynes in an intermolecular fashion to produce an aryl enamine (Fig. 5).⁵⁹ Later, the same authors expanded the scope of B(C₆F₅)₃ based FLPs for hydroamination using aryl amines and terminal alkynes in both an inter- and intramolecular fashion, producing a plethora of compounds including a series of N-heterocycles.⁶⁰ However, instead of forming the imine N-heterocycles, the intramolecular hydroamination reactions were performed under a H₂ atmosphere, resulting in one-pot hydroamination/hydrogenation reactions further demonstrating versatility of B(C₆F₅)₃ based FLPs.

Fig. 5 Hydroamination using B(C₆F₅)₃ as an FLP.⁵⁹ When the combination of B(C₆F₅)₃ and an aryl amine is treated with alkynes it creates a zwitterionic FLP intermediate, and then a 1,3 proton transfer induces the release of an aryl enamine product and regeneration of B(C₆F₅)₃.

Other than activating H₂ for metal-free hydrogenation, capturing and activating alternate small molecules such as CO,^61–63 CO₂,^64–69 N₂O,⁷⁰ NO,⁷¹ and SO₂,⁷² alkenes,⁴³ alkynes,^73–75 and disulphides⁷⁶ using FLPs is highly sought after. We refer the reader to a recent review involving FLPs capturing and converting CO₂.⁶⁹ In brief, Stephan and Erker reported the first use of FLP chemistry for the sequestration of CO₂, where the FLP PtBu₃ + B(C₆F₅)₃ and FLP complex (Me₃C₆H₂)₂PCH₂CH₂B(C₆F₅)₂, can capture and liberate CO₂ reversibly (Fig. 6a).⁶⁴ Later, Kumacheva and coworkers determined the thermodynamics of the capture and liberation of CO₂via FLPs by a novel micro fluidics approach that provided a means for future thermodynamic characterization of FLP CO₂ sequestrations and other reactions of the like.⁷⁷ The first conversion of CO₂ to methanol via FLPs was reported by O’Hare and coworkers, however, thermolysis to produce free methanol resulted in the decomposition of B(C₆F₅)₃.⁶⁵ In 2010, Piers and coworkers reported CO₂ conversion using a 2,2,6,6-tetramethylpiperidine/B(C₆F₅)₃ FLP with triethylsilane for a deoxygenative reduction of CO₂ to CH₄.⁶⁶ Other than CO₂ activation, using the same PtBu₃ & B(C₆F₅)₃ FLP for CO (Fig. 6b),⁶¹ N₂O (Fig. 6c),⁷⁰ and SO₂ (Fig. 6d)⁷² activation has been successful, where all reactions result in complexation of the small molecules with the PtBu₃ & B(C₆F₅)₃ pair by bridging them and producing solids.

Fig. 6 Small molecule activation using B(C₆F₅)₃ FLPs. (a) CO₂ activation & capture using PtBu₃ and B(C₆F₅)₃ at 25 °C results in a PtBu₃CO₂B(C₆F₅)₃ complex, where CO₂ can be liberated at 80 °C under vacuum.⁶⁴ (b) CO activation & capture using PtBu₃ and 2[B(C₆F₅)₃] at room temperature in the presence of H₂ to form a formyl borate derivative, which can be reversed to release CO upon placing under vacuum.⁶¹ (c) PtBu₃, B(C₆F₅)₃, & N₂O form a bridged B(C₆F₅)₃–N₂O–PtBu₃ species in a trans configuration upon combining the reagents and heating at 25 °C.⁷⁰ (d) PtBu₃, B(C₆F₅)₃, & SO₂ react at room temperature to form a bridged zwitterionic B⁻(C₆F₅)₃–SO₂–P⁺tBu₃ adduct.⁷²

2.4. Emerging applications of B(C₆F₅)₃ in the presence of H₂O for molecular transformations

Upon exposure to water, B(C₆F₅)₃ forms oxygen bridged borate adducts and complexes (Fig. 7a) and can decompose into boronic acids and boroxines, especially upon heating.⁷⁸ Therefore, chemists working with B(C₆F₅)₃ typically ensure a moisture free environment for storage and reaction conditions. However, there are emerging applications of adducts involving B(C₆F₅)₃ and water (B(C₆F₅)₃·H₂O; Fig. 7a) which transform B(C₆F₅)₃ from a Lewis acid to a potent Brønsted acid. This results in altered reactivity and applications, including as a p-type dopant for organic electronics (vide infra).⁷⁹ Note, considering the study done by Beljonne and coworkers, upon deprotonation of B(C₆F₅)₃·H₂O, it is likely that the resulting [B(C₆F₅)₃·OH]⁻ anion complexes with free B(C₆F₅)₃ and B(C₆F₅)₃·H₂O to form the energetically favoured complexes [B(C₆F₅)₃·OH·B(C₆F₅)₃]⁻ and [B(C₆F₅)₃·OH·H₂OB(C₆F₅)₃]⁻ (Fig. 7a), respectively.⁷⁹

Fig. 7 B(C₆F₅)₃ in the presence of H₂O for molecular transformations; (a) adducts of B(C₆F₅)₃ with H₂O: B(C₆F₅)₃·H₂O, [B(C₆F₅)₃·OH·B(C₆F₅)₃]⁻, and [B(C₆F₅)₃·OH·H₂O·B(C₆F₅)₃]⁻,⁷⁹ (b) C–F bond substitution of alkanes is catalyzed by B(C₆F₅)₃·H₂O (1 mol%) at room temperature to afford an array of C-, N-, O-, and S-substituted products,⁸⁰ (c) B(C₆F₅)₃ in H₂O affords O–H bond insertion for α-diazoesters to form α-hydroxyesters at moderate temperatures (40 °C),⁸¹ (d) dehydration reactions involving allylation of electron-rich arenes and allyl alcohols using B(C₆F₅)₃ in water,⁸² (e) syntheses of oligosiloxanes using H₂O·B(C₆F₅)₃ as a catalyst for the conversion of hydrosilanes and tethered hydrosilanes to make oligomeric siloxanes,⁸³ (f) regio- and chemo-selective para-alkylation of anilines using H₂O·B(C₆F₅)₃.⁸⁴

Efforts to break C–F bonds under benign conditions have persisted, one example being defluorination functionalization of tertiary aliphatic fluorides mediated by a H₂O·B(C₆F₅)₃ adduct. In 2017, Moran and coworkers reported that 1 mol% of the adduct was capable of catalyzing C–F bond substitution of aliphatic fluorides at room temperature using various heteroatomic nucleophiles to afford a range of S-, O-, N-, and C-substituted products in good yields (Fig. 7b).⁸⁰ The authors previously reported a Friedel–Crafts autocatalytic mechanism, where initiation occurs by abstraction of the fluoride ion by a B(C₆F₅)₃·H₂O adduct (via F–H coordination) resulting in a aliphatic carbocation free to react with a plethora of nucleophiles, and is the proposed mechanism for this work.⁸⁵

Molecular transformations using B(C₆F₅)₃ and water as the solvent is emerging, in 2018 B(C₆F₅)₃ was found to catalyze O–H bond insertion in α-diazoesters to form α-hydroxyesters using B(C₆F₅)₃ in water (Fig. 7c).⁸¹ In 2020, Wang and coworkers reported the allylation of electron-rich arenes and allyl alcohols using B(C₆F₅)₃ in water, and provided a series of indoles coupled to 1,3-diphenylallyl alcohol.⁸² These reactions are proposed to proceed by an FLP of B(C₆F₅)₃ and 2,6-lutidine, which then reacts to abstract the hydroxyl group of 1,3-diphenylallyl alcohol to provide a reactive allyl cation and [B(C₆F₅)₃:OH]⁻ (Fig. 7d). In 2021, Wang and coworkers continued this work by allylation of 1,3-diketones or β-ketone esters with allyl alcohols in water.⁸⁶ The authors propose this proceeds by adduct formation and thus activation of the ketone via adduct formation between the carbonyl and B(C₆F₅)₃, which renders the –CH₂ group bridging the diketone acidic and upon deprotonation forms a reactive carbocation.

While reactions involving B(C₆F₅)₃ and water have been demonstrated, excess water can still be problematic as the Brønsted acidity is expected to decrease.^78,79 This can be mediated by adding controlled amounts of water to a reaction mixture to form B(C₆F₅)₃·H₂O, or by adding the premade adduct directly. This has been demonstrated for the synthesis of oligosiloxanes using B(C₆F₅)₃·H₂O as a catalyst by Leitao and coworkers, who reported the conversion of hydrosilanes and tethered hydrosilanes to make oligomeric siloxanes using controlled mol percentages of B(C₆F₅)₃·H₂O (Fig. 7e).⁸³ Pulis and coworkers reported on the para-alkylation of anilines, where the H₂O·B(C₆F₅)₃ adduct was added to the reaction directly for highly selective para-C-alkylation over the N-alkylation and ortho-C-alkylation pathways (Fig. 7f).⁸⁴ The authors suggest this mechanism proceeds by protonation of the alkyl functional group by B(C₆F₅)₃·H₂O to form a reactive carbocation.

3. Applications of B(C₆F₅)₃ in electronic devices

3.1. Overview

Society relies heavily on electronic devices, with an indication that dependence on such technology is forever increasing. Some examples of on-going research in this field are batteries/energy storage, energy conversion, lighting displays, and sensors. B(C₆F₅)₃ has seen research and application in this field; however, the function of B(C₆F₅)₃ varies significantly, further highlighting the vast scope and utilization of the compound. While the incorporation of boron into organic electronic materials is classic,^87–90 the use of B(C₆F₅)₃ is emerging and has proved to be fruitful. This section serves to highlight various roles of B(C₆F₅)₃ in electronic devices, more specifically for its role in batteries, organic electronics, and emerging applications.

3.2. Incorporating B(C₆F₅)₃ in batteries

3.2.1 Overview. Rechargeable batteries are commonly used in electronic devices such as mobile phones and electric vehicles.^91,92 These applications are currently dominated by lithium-ion batteries (LIBs) owing to their high power density, excellent performances, and long cycling stability.⁹³ However, current LIBs have persistent safety issues, such as overheating which results in fires, which limits the realization of large-scale implementation and further market expansion.⁹⁴ Thermal, electrical, and mechanical stress can trigger thermal runaway reactions in LIBs, resulting from the uncontrollable electrochemical reactions from the decomposition of liquid electrolyte containing Li-salts and flammable organic solvents.^94–96 To address this problem, additives, such as anion receptors, are introduced to facilitate complex formation with anions to prevent ion-pairing of the salt and mitigates the risk of the salt reacting within the electrolyte.⁹⁷ This section highlights early and emerging work involving incorporating B(C₆F₅)₃ (Fig. 8) in batteries.

Fig. 8 The role of B(C₆F₅)₃ in LIBs. (a) B(C₆F₅)₃ as an anion receptor, which serves to aid in dissolution of LiF salts and increase the movement of Li⁺. (b) B(C₆F₅)₃ as a sacrificial additive to LIBs to induce ideal SEI on the anode.

3.2.2 Early applications in lithium ion batteries. In 1998, B(C₆F₅)₃ saw its first application in LIBs as an electrolyte additive, serving as an anion receptor to assist in the dissolution of insoluble LiF salts in organic solvents (Fig. 8a), resulting in high ionic conductivity from higher salt concentrations in electrolyte.⁷ The dissociation of the LiF salt is achieved by scavenging of F-ions by B(C₆F₅)₃ (forming stable B–F bonds), and as well anion migration is hindered due to the low mobility of the large complexes, allowing for improved Li ion movement.^7,98 Following this, several papers reported that addition of B(C₆F₅)₃ improved cycling performance and thermal stability of LIBs due to strong anion coordination along with formation of a passivation film, referred to as solid electrolyte interphase (SEI) on the surface of the electrode (Fig. 8b).^97,99–104 The SEI layer is critical to performance as it serves to prevent surface-reduction of electrolytic solvents,¹⁰⁵ and to protect anodes from Li dendrite growth.¹⁰⁶ However, SEI growth results in consumption of active electrolyte materials, leading to decreases in power density and increases in resistance over time.¹⁰⁷ Even though large concentrations of B(C₆F₅)₃ can benefit from higher ionic conductivity, controlled addition of B(C₆F₅)₃ is important to ensure longevity of the cycling performance of LIBs.¹⁰⁸ In addition to SEI formation, B(C₆F₅)₃ was used as a polymerization initiator for the solvent 1,4-dioxolane to achieve a solid-state polymer electrolyte, which is unlike liquid electrolytes that are prone to leakage which raises safety concerns.⁹⁸ It was also suggested that B(C₆F₅)₃ acts as flame retardant in LIBs due to the fluorine free radical generation from thermal decomposition that acts as a trap for highly reactive radical species such as H˙ and OH˙.⁹⁸ B(C₆F₅)₃ has thus seen success as an electrolyte additive in rechargeable lithium–metal batteries,^98,109–111 metal–gas batteries (e.g., lithium–sulfur hexafluoride batteries),¹¹² and lithium–oxygen batteries.¹¹³

3.2.3 Emerging applications in sodium ion batteries. B(C₆F₅)₃-containing electrolytes for sodium-ion batteries (SIB) or -metal batteries (SMB) is emerging,^114,115 which are attractive alternatives to LIBs due to the natural abundance and low-cost of sodium compared to lithium.¹¹⁶ B(C₆F₅)₃ was recently used as an electrolyte additive in SIBs by Chou and coworkers, who reported that the strong coordination of B(C₆F₅)₃ with ClO₄⁻ anions allowed for the free movement of Na⁺ leading to high conductivity and overall performance.¹¹⁵ This coordination led to an increase of organic solvents in the inner solvation sheath of sodium ions, resulting in fewer free solvents within the electrolyte. Although free solvents partake in SEI formation, controlling the rate is challenging, which can lead to a reduction in battery efficiency over time. Therefore, the decrease in the availability of free solvent molecules promoted preferential oxidation to decompose B(C₆F₅)₃ over free solvents, which led to overall enhanced oxidation stability. This process formed a beneficial NaF-rich solid electrolyte interphase (SEI) on a Na₃V₂(PO₄)₃ cathode, contributing to excellent electrochemical performances at high operating temperature of 60 °C. In another study, the fabrication of (quasi)solid-state electrolyte SMB was achieved with B(C₆F₅)₃ induced polymerization of 1,4-dioxolane solvent, in addition to the control of solvation environment resulting in a great stability with over 1000 cycles at low temperature of −20 °C.¹¹⁴

3.3. B(C₆F₅)₃ as a dopant in organic electronics

3.3.1. Overview. Doping organic materials has been employed since the discovery of conductive polymers in the 1970's,¹¹⁷ where chemical doping enhances conductivity by increasing free electron movement through the introduction of small amounts of electron-rich moieties (n-type doping) or by creating holes with electron-poor moieties (p-type doping). A staple p-type dopant for organic materials is the electron accepting 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ).¹¹⁸ However, application of F₄-TCNQ is limited due its lack of solubility resulting in energy intensive thermal evaporation for processing^119–121 and the need for organic materials with a large highest occupied molecular orbital (HOMO) energy level to allow for efficient integer charge transfer (ICT), which is not always viable or suitable across various types of devices.^122–124 B(C₆F₅)₃ has advantageous properties over F₄TCNQ: (1) superior solubility in organic solvents which enables solution processing, (2) increased thermal, oxygen, and water stability,^1,125,126 and (3) can dope organic materials via multiple mechanisms enabling material use with a wide range of HOMO energy levels.¹²⁷

Early applications using B(C₆F₅)₃ for organic semi-conductor modification was carried out by Bazan and coworkers, where modulated optical properties and electronic energy levels was achieved without synthetic modification (Fig. 9b).^128–130 Oligomers consisting of a dithienosilole (R = C₁₂H₂₅; DTS_C12) core capped with two benzothiadiazole (BT) units were combined with stoichiometric amounts of B(C₆F₅)₃, resulting in Lewis acid–base complexes involving the boron on B(C₆F₅)₃ and the nitrogen of the BT units (Fig. 9a).¹²⁸ This pulled electron density away from the π-system of the BT unit towards the Lewis acid, increasing donor–acceptor charge transfer character of the system, resulting in an overall narrowing of the energy gap and red-shifting of the optical absorption (Fig. 9b). The work was expanded on by complexing B(C₆F₅)₃ with various Lewis basic polymers,¹²⁹ with eventual applications in organic light emitting diodes (OLEDs).¹³⁰

Fig. 9 Bazan and coworkers demonstrate how B(C₆F₅)₃ can tune the electronic gap of benzothiadiazole (BT) & DTSC₁₂ based oligomers.¹²⁸ (a) When added in stoichiometric amounts, B(C₆F₅)₃ forms a complex with the nitrogen on the BT unit of the BT/DTS_C12 based oligomers. (b) Modified from Bazan and coworkers,¹²⁸ increasing the stoichiometric equivalence of B(C₆F₅)₃ (red line is neat B(C₆F₅)₃) in relation to the BT/DTS_C12 oligomers (black line is neat BT/DTSC₁₂), a bathochromic shift is observed for the λ_max of the BT/DTS_C12 spectra.

Nguyen and coworkers first reported on the p-doping effect of B(C₆F₅)₃ on a pyridine-based copolymer for hole-only diode devices in 2014, and observed an enhancement of hole-mobility by two orders of magnitude compared to their pristine counterparts.¹²⁵ Following this, many accounts of doping organics with B(C₆F₅)₃ for use in electronics started to emerge, particularly for solution-processed organic electronic devices. Taking advantage of the unique synergistic effect of B(C₆F₅)₃ doping on modifying optoelectronic properties and solid-state morphologies of organic materials, the application area has broadened to the bulk heterojunctions (BHJs) in organic photovoltaics (OPVs),^131–135 hole transporting layers (HTLs) in perovskite solar cells (PSCs),^136,137 organic thermoelectrics (OTEs),^138–142 organic thin-film transistor (OTFT)-based sensors,^143–147 and organic photodetectors (OPDs).^143,145 This section includes a mini review of B(C₆F₅)₃ as a p-type dopant for the active organic components in organic electronics, proposed mechanisms, emerging applications, and new dopant strategies. For more detailed reviews regarding the effect of doping active organic components, please refer to the reviews by Anthopoulos and coworkers,¹⁴⁸ Koch and coworkers,¹²³ and Baumgartner and coworkers.¹⁴⁹

3.3.2. Doping mechanisms of B(C₆F₅)₃ with organics. Doping mechanisms involving B(C₆F₅)₃ across various organic electronic devices remained elusive for some time. Early work done by Nguyen and coworkers¹²⁵ and Heeney and coworkers¹²⁷ show that B(C₆F₅)₃ doped organic materials do not exhibit typical integer charge transfer (ICT; Fig. 10a) characteristics. ICT requires the HOMO energy level of the hole transport material to be destabilized relative to that of the lowest unoccupied molecular orbital (LUMO) of the p-dopant for efficient charge transfer. However, these authors reported that their materials possessed stabilized HOMO energy levels relative to that of the LUMO energy level of B(C₆F₅)₃, but that upon application doping was indeed occurring and noted unpaired electrons via polaron (p-type carriers) absorbances in the IR region and electron paramagnetic resonance (EPR) signals.^125,127 Nguyen and coworkers suggested that B(C₆F₅)₃ forms adducts with Lewis basic moieties in organic π-conjugated materials, and instead doping via charge transfer complexation (CTC; Fig. 10b) occurs. Heeney and coworkers supported this by reporting frontier-orbital hybridization from B–N bond formation which lead to the strongly electrophilic “pyrazinium” like cations, inducing empty states into the band gap which is similar to CTC doping mechanism suggested by Salzmann and coworkers.¹⁵⁰ Another example is doping poly(3-hexylthiophene) (P3HT) with B(C₆F₅)₃, where P3HT exhibits an estimated HOMO energy level of −5.1 eV and B(C₆F₅)₃ a LUMO energy level of −4.8 eV.¹²⁶ It was in 2020 that B(C₆F₅)₃ was proposed to induce p-doping via Brønsted acid doping (Fig. 10c) upon the formation of the Brønsted acid complex, B(C₆F₅)₃·H₂O, in the presence of water.¹⁵¹ Using DFT calculations, the authors proposed a mechanism which is as follows: (1) the formation of Brønsted acid, B(C₆F₅)₃·H₂O complex, which protonates the polymer backbone, in this case forming a positively charged protonated polymer [PCPDTBT-H]⁺ and anionic [B(C₆F₅)₃·OH]⁻, (2) electron transfer from the neutral polymer to the [PCPDTBT-H]⁺, and (3) formation of a neutral protonated radical species, [PCPDTBT-H]˙, and “p-doped” delocalized cationic radical species, [PCPDTBT]˙⁺, effectively neutralized with the counter ion, [B(C₆F₅)₃·OH]⁻. An additional Brønsted acid doping mechanism was proposed where the B(C₆F₅)₃·H₂O complex will likely to protonate the carbon on the 1 position of the cyclopentadithiophene instead of position 3,⁷⁹ as previously suggested by Nguyen and coworkers.¹⁵¹

Fig. 10 Possible doping mechanisms in organic electronics by B(C₆F₅)₃. (a) Integer charge transfer (ICT) works to abstract an electron from the organic compound, allowing for increased hole transport throughout the material. (b) Charge transfer complex formation (CTC) results in orbital hybridization and an electron transfer into a charge transfer hybrid orbital (c) Brønsted acid doping in organic materials can occur when in the presence of B(C₆F₅)₃ and H₂O, and occurs in three main steps: (i) protonation of a π-conjugated system in the presence of the proposed B(C₆F₅)₃·OH₂·B(C₆F₅)₃ complex (ii) electron transfer from a neighbouring π-conjugated system, and (iii) formation of a neutral protonated radical species, [PCPDTBT-H]˙, and “p-doped” delocalized cationic radical species, [PCPDTBT]˙⁺, balanced with the counter ion, [B(C₆F₅)₃·OH]⁻.¹⁵¹

Later, Beljonne and coworkers suggested that more energetically favorable bridged anion complexes, such as [B(C₆F₅)₃·OH·B(C₆F₅)₃]⁻ and [B(C₆F₅)₃·OH·OH₂·B(C₆F₅)₃]⁻, were more feasible to have formed than the monomeric species [B(C₆F₅)₃·OH]⁻, as per DFT calculations.⁷⁹ In addition, Brønsted acid doping processes are entropically driven by the loss of H₂ gas to counter the highly endergonic process, where H₂ gas was successfully detected by Beverina and coworkers in 2023.¹⁵² This mechanism accounted for the single signal seen in the EPR of these doped systems, as the result is a single spin-carrying species in the form of a radical cation. A follow up study on P3HT doped by B(C₆F₅)₃ was done to include EPR experiments, where a single EPR signature was indeed observed.¹⁵³ While Brønsted acid doping with B(C₆F₅)₃ can occur in the presence of water, if the organic material being doped contains Lewis basic moieties, it should be noted that Brønsted acid and CTC doping compete, however, Brønsted acid doping has been deemed as the more effective doping approach.¹⁴⁰ Therefore, in designing organic systems doped with B(C₆F₅)₃, one must define a target doping mechanism and consider the presence of water and/or Lewis basic moieties. Note, many early studies reviewed within do not elucidate doping mechanisms at play and instead speculate with minimal supporting data.

3.3.3. Organic photovoltaic devices. OPVs offer an appealing approach to harnessing solar energy due to solution processability and a lightweight, flexible design. The mechanism of action involves the absorption of photons by “donor” organic molecules, leading to an excited state which can result in a transfer of an electron to “acceptor” organic molecules (channel I process) or vice versa (channel II process).¹⁵⁴ This results in free charges that generate an electrical current when sandwiched between a cathode and anode. Typically, the donor and acceptor compounds are solution-processed and coated from the same solution, forming an intermixed film with domains of either donor or acceptor compounds, constituting the bulk heterojunction (BHJ) in OPVs (Fig. 11a). Additionally, OPVs tend to incorporate other organic material layers as electron or hole transporting layers, facilitating the movement of charges to the preferred electrodes for efficient current generation.

Fig. 11 Application of B(C₆F₅)₃ in OPVs. (a) Structure of an OPV used to harvest and convert solar radiation into current, B(C₆F₅)₃ can be incorporated into the BHJ for increased charge mobilities and PCEs. (b) In the work done by Chen & coworkers,¹⁵⁵ adduct formation between the boron of B(C₆F₅)₃ with the nitrogen on the pyrazine-based polymer, PBP-Cl, is observed, which was applied to OPVs for enhanced performance.

Both the BHJ and organic-based electron/hole transporting layers can undergo doping to enhance performance. Instances of using B(C₆F₅)₃ to augment OPV device performance have been reported, specifically when utilized in the BHJ. One example is the work of Ma and coworkers, where the authors utilized Lewis basic donor polymers (PCE10) with fullerene (PC₇₁BM) acceptors in the BHJ of OPVs,¹³¹ and then added B(C₆F₅)₃ to induce doping in the donor PCE10 polymers. Not only did B(C₆F₅)₃ dope the PCE10 polymers, albeit mechanisms unknown, highly ordered nanostructures were observed with increased charge transport and thereby device performance for an increase in power conversion efficiency (PCE) from 8.9% to 9.6%. Chen and coworkers designed and synthesized a pyrazine-based donor polymer (PBP-Cl) to induce intermolecular B–N coordination for CTC doping with B(C₆F₅)₃ in BHJ of OPVs (Fig. 11b), where Y6 was used a non-fullerene acceptor.¹⁵⁵ By adding a trace amount of B(C₆F₅)₃ at 0.05 wt% relative to PBP-Cl in the BHJ, increased hole mobilities were observed with PCEs improving from 13.7% to 15.2%. Despite this, morphological alteration in the BHJ due to the presence of dopants can be detrimental, especially when the presence of dopants breaks up intermolecular interactions of the donor and/or acceptor species that are designed to induce favorable orientations that result in efficient charge transport in these types of devices.^155–157 Realizing this, in 2019 Ma and coworkers applied a different approach by employing a planar heterojunction (PHJ), where the donor and acceptor materials are coated individually and subsequentially. In this work, the best device performance was observed when B(C₆F₅)₃ was employed at the donor–acceptor interface, thereby residing between the donor and acceptor layers, and resulted an increase to PCE from 1.20% to 1.39%.¹³² The authors also doped a PCE10/PC₇₁BM BHJ by vapor annealing which resulted in an overall improved photovoltaic performance, marking an increase to PCE from 8.6% to 9.4%.¹³³ Various studies that utilize B(C₆F₅)₃ as a dopant are continuously being published, with many of them putting an efforts into modifying the formation of favorable BHJ morphology.^134,135 Notable is the doping of the electron transporting layers, where employing the H₂O·B(C₆F₅)₃ complex as a dopant for the known cathode interlayer material, (N,N-dimethyl-ammonium N-oxide)propyl perylene diimide (PDINO) resulted in high performance OPVs with a PCE of 17.7%. The authors utilized DFT to show that the complex coordinates to the N-oxide terminal unit of PDINO by the boron core, thus displacing the water moiety.¹⁵⁸ Furthermore, Kelvin probe force microscopy and ultra-violet photoelectron spectroscopy of PDINO·B(C₆F₅)₃ thin films determined the work function of the films were lowered upon increasing levels of the H₂O·B(C₆F₅)₃ complex, enabling a smaller energy mismatch with the Y6 BHJ acceptor material.

3.3.4. Organic thermoelectric devices. OTEs, as depicted in Fig. 12a, are devices that convert heat into electricity, showing promising applications in flexible thermoelectric generators for waste heat recovery and wearable cooling/heating devices.¹⁵⁹ Materials used in OTEs utilize temperature gradients to generate electrical potentials through the diffusion of charge-carriers from the hot to the cold side, known as the Seebeck effect. The efficiency of thermoelectrics is quantified by the dimensionless figure-of-merit, ZT = S²σT/κ, where ZT at a given absolute temperature (T) is directly proportional to electrical conductivity (σ) and Seebeck coefficient (S), and inversely proportional to thermal conductivity (κ). In contrast to inorganic semiconductors, organic semiconducting materials have intrinsically low κ owing to strong electron–phonon coupling and low electrical conductivity.^160,161 Therefore, recent focus has been on enhancing power factor (S²σ) mainly via doping.^162–164

Fig. 12 Application of B(C₆F₅)₃ in OTEs. (a) Structure of an organic thermoelectric device used to convert heat into current. Using B(C₆F₅)₃ to p-dope OTEs results in enhanced conductivity and increases the power factor of the devices. (b) In the work done by Jang & coworkers,¹³⁹ Brønsted acid doping was observed when mixing B(C₆F₅)₃ (in the presence of water) with P3HT, which was used for OTEs and resulted in enhanced conductivity.

Jang and coworkers utilized B(C₆F₅)₃ as a Lewis acid dopant in inert atmosphere and as a Brønsted acid dopant in air to p-dope poly(3-hexylthiophene) (P3HT) through a one-step solution mixing method.¹³⁹ The Brønsted acid doping induced a conformational change in P3HT that resulted in a quinoidal structure, promoting backbone planarity (Fig. 12b). This structural change resulted in improved intra/inter-chain charge transport, reflected by the enhanced σ of 33.0 S cm⁻¹ compared to Lewis acid-doped P3HT films at 0.020 S cm⁻¹. This outcome showcases the doping efficiency and air-stability of B(C₆F₅)₃ when employed as a Brønsted acid dopant. In the case of strong Lewis basic polymers, the competition between Brønsted acid and CTC doping in ambient atmosphere can limit doping efficiency. Consequently, in 2022 Jang and coworkers thermally annealed doped polymer films (PCDTPT) at 120 °C, transitioning from Lewis acid/base adducts (CTC) to Brønsted acid doping. This resulted in an increase in σ by three orders of magnitude reaching power factor of 8.48 μW m⁻¹ K⁻².¹⁴⁰ They suggested that the Lewis acid–base interaction of B(C₆F₅)₃ with PCDTPT can be thermally disrupted to form Brønsted acidic B(C₆F₅)₃·H₂O complexes, and when employed with the weak Lewis basic polymer (PCDTBT), CTC doping was effectively suppressed and Brønsted acid doping increased to achieve a high power factor of 49.6 μW m⁻¹ K⁻².

Despite the impressive increase in performance resulting from B(C₆F₅)₃ by doping via CTC or Brønsted acid doping, B(C₆F₅)₃ (and B(C₆F₅)₃ water complexes) are bulky and can disrupt the solid-state morphology of organic thin films. Cho and coworkers addressed this challenge posed by the steric bulk of B(C₆F₅)₃ dopants,¹⁴² where they employed sequential doping by dissolving B(C₆F₅)₃ in the non-polar aliphatic solvent, hexanes, and then dipped their prepared organic semiconducting films into the solution. This method allowed B(C₆F₅)₃ to diffuse into a semi-crystalline polymer film made up of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) without disrupting the crystallinity of the film. The authors propose that hexanes can intercalate with the side chains of the PBTTT polymer but not the backbone, therefore, this method of subsequential doping resulted in deposition of the B(C₆F₅)₃ dopant in the lamellar spacing of PBTTT. Compared to solution-mixed doping, this method effectively suppressed dopant-induced disorder, resulting in a narrower density of states and reduced charge traps. The outcome was an impressive electrical conductivity (σ) of 230 S cm⁻¹ and power factor of 140 μW m⁻¹ K⁻², ranking among the highest values reported for B(C₆F₅)₃-doped organic polymer films. OTE devices doping using B(C₆F₅)₃ has therefore seen success,¹⁵⁷ including noncontact mode (IR irradiation) OTEs,¹⁶⁵ and self-healable and stretchable OTEs.¹⁶⁶

3.3.5. Organic thin-film transistors. OTFTs are devices composed of a semiconducting channel positioned between two electrodes in the presence of a gate with various possible architectures/configurations. In simplified terms, noting that mechanisms are actually quite complex, when a bias is applied the semiconducting channel activates and generates current. An example is depicted in Fig. 13a, which illustrates a top-contact bottom-gate configuration. Similar to their inorganic counterparts, doping of the semiconducting channel is frequently employed to enhance performance, including improvements in charge generation, charge mobility, and overall conductivity. OTFTs exhibit potential in sensor applications, where analytes or external stimuli can induce electronic and/or morphological changes to the organic semiconducting channel. These changes result in measurable alterations to channel current.¹⁶⁷

Fig. 13 Application of B(C₆F₅)₃ in OTFTs. (a) Structure of a type of OTFTs, specifically a bottom-gate top-contact OTFT. Doping hole transporting materials with B(C₆F₅)₃ in OTFTs results in an improvement in charge mobility and increases the number of charges generated. (b) Modified from Anthopoulos & co-workers,¹⁶⁸ when the mixture of hole transporting materials, C₁₆-IDT-BT & C₈-BTBT, were doped with B(C₆F₅)₃ and applied in OTFTs hole mobilities increased to 11 cm² V⁻¹ s⁻¹.

Early work in this field was done by Heeney and coworkers who were able to achieve 11-fold enhancement of hole-mobility in pyrazine-based copolymer OTFTs.¹²⁷ These devices were highly sensitive to dopant concentrations due to the bulky nature of B(C₆F₅)₃ complex, which disrupts solid-state morphology thereby affecting charge transport properties.^125,127 However, with careful optimization of the stoichiometric amount of B(C₆F₅)₃ added, the organic materials can simultaneously undergo p-doping and induced long-range crystallization resulting in enhanced hole-mobility, which was observed by Anthopoulos and co-workers in 2018.¹⁶⁸ This work explored several systems of organic molecular and polymeric materials for applications in OTFTs, more specifically they compared molecular and polymeric materials to blends of molecular polymeric materials in the presence of B(C₆F₅)₃. The most notable system was that of the polymer C₁₆-IDT-BT blended with the small molecule C₈-BTBT, where upon doping with B(C₆F₅)₃ the resulting OTFT devices reached a maximum of 11 cm² V⁻¹ s⁻¹ (Fig. 13b).

OTFTs are commonly studied for their application as sensors, particularly gas sensors. In 2012, Katz and coworkers fabricated OTFT-based NH₃ gas sensor using Lewis basic, copper phthalocyanine (CuPc) and B(C₆F₅)₃ as an active layer.¹⁴⁴ Upon the exposure to NH₃ gas, which is a stronger Lewis base than CuPc, B(C₆F₅)₃ effectively formed a Lewis acid–base complex with NH₃ resulting in a pronounced changes to the current, achieving a low limit of detection (LOD) of 0.35 ppm. However, it should be noted as per the correction to this work that there is a possibility of B(C₆F₅)₃ forming an adduct with trace water, which can act as a Brønsted acid p-dopant and thereby also affects the observed current in these devices.¹⁶⁹ Another study done in 2020 by Kim and coworkers fabricating solution-processed OTFT-based NH₃ gas sensor using poly(3-hexylthiophene) (P3HT) doped with B(C₆F₅)₃ as an active layer showed enhancement of current response.¹⁴⁷ This OTFT-based NH₃ gas sensor was more selective towards NH₃ compared to acetone, methanol, and dichloromethane, where they attributed to the strong Lewis acid (B(C₆F₅)₃) and Lewis base (NH₃) interaction for CTC type doping.

3.3.6. Organic photodetectors. OPDs are devices capable of detecting light by converting light signals into electrical signals, and harbour several different types of architectures such as phototransistors, photodiodes, and photoconductors.¹⁷⁰ For example, organic phototransistors (OPTRs) are essentially OTFTs that undergoes a change in device on-current upon exposing the organic layer to incident light by means of altering the charge carrier density. B(C₆F₅)₃ has found applications in hole transporting OPTRs (Fig. 14a) owing to its polaron absorption peak in the IR region.^126,171 Typically, OPTRs are limited to visible-light with a few reports of ultraviolet (UV) and near-infrared (NIR), however, Kim and coworkers were the first to report shortwave infrared (SWIR, 1400–3300 nm) OPTRs utilizing a triphenylamine-based polymer (PolyTPD) doped with B(C₆F₅)₃ which the authors claim results in radical formation via ICT (Fig. 14b).¹⁴³ The combination of PolyTPD and B(C₆F₅)₃ allowed for an appropriate doping-induced energy offset between the HOMO energy level of PolyTPD and LUMO energy level of B(C₆F₅)₃, which corresponds to the SWIR wavelength range. As a result, doped PolyTPD:B(C₆F₅)₃ films exhibited a broad optical absorption peak between 1000–3300 nm, allowing for the detection of SWIR, specifically at 1500 nm for 583.4 mA W⁻¹, 2000 nm at 695.4 mA W⁻¹, and 2500 nm at 829.4 mA W⁻¹. This type of radical formation with a Lewis base as a single electron donor and Lewis acid as a single electron acceptor has been probed using FLP chemistry,¹⁷² where the disassociated radicals have been termed frustrated radical pairs. In this work perhaps B(C₆F₅)₃ acts as a radical anion, however, further investigation is needed.

Fig. 14 Application of B(C₆F₅)₃ in OPTRs. (a) Structure of organic phototransistor devices used to detect various wavelengths of light. Addition of B(C₆F₅)₃ in these types of devices can result in SWIR detection and enhanced photocurrent. (b) In the work done by Kim & coworkers,¹⁴³ doping of PolyTPD with B(C₆F₅)₃ to generate radicals was done for applications in OPTRs.

Another example of using B(C₆F₅)₃ as a dopant in OPDs was reported by Someya and Anthopoulos in 2021, where an organic photodiode with a bulk heterojunction consisting of a diketopyrrolopyrrole & thiophene based donor polymer (PMDPP3T) with a fullerene acceptor (PC₆₁BM), and was doped by B(C₆F₅)₃ alongside n-type organic dopants.¹⁴⁵ The authors controlled doping concentration (0.02 wt%) to achieve enhanced photocurrent and NIR detectivity, without increasing dark current. This is notable as doping often increases dark current, which can result in rapid device degradation under ambient conditions.

3.3.7. Emerging applications in organic electronics. While the application of B(C₆F₅)₃ in electronics has been established for doping organics in OPVs, OTEs, and OTFTs, applications of B(C₆F₅)₃ continue to expand in this realm. This section serves to highlight some emerging and innovative approaches to using B(C₆F₅)₃ in electronics, where new methods and types of devices are explored.

In regard to emerging electrochemical device applications, Abe and coworkers reported a new method for enhancing electrochemiluminescence (ECL) by Lewis acid–base pairing complexation of B(C₆F₅)₃ with organic donor–acceptor (D–A) diads. This complexation served to stabilize electrogenerated radicals, preventing the formation of unstable radicals which can lead to unwanted chemical reactions.¹⁷³ In the study, pyridal (py) functionalized benzothienobenzothiophene (BTBT) D–A diads were used as the active component, forming dative B–N bonds with B(C₆F₅)₃. They report that van der Waals interaction between H⋯F of py and –C₆F₅ groups restricted the rotational motion, maintaining a planar configuration during the excited intramolecular charge transfer state. This restriction effectively inhibited non-radiative decay. Additionally, the strong electron-withdrawing coordination of B(C₆F₅)₃ facilitated efficient charge separation. Moreover, single crystals of py-BTBT diads revealed highly ordered 1D columnar π-stacks in the solid state, contributing to efficient singlet exciton delocalization. Therefore, the py-BTBT diads demonstrated improved radical stability with a 156-fold increase in ECL intensity. B(C₆F₅)₃ was reported to not only dope the organic material and induce favorable morphology changes in the solid state, but also acted as an oxygen trap and thus increased the stability of the organic electrochemical transistor (OECT) devices over several cycles.¹⁷⁴

Another promising usage of B(C₆F₅)₃ was to enhance the photophysical properties of organic light-emitting films, which was done by Huang and coworkers in 2023.¹⁷⁵ They employed supramolecular diarylfluorene compounds functionalized with pyridine as a host material and B(C₆F₅)₃ as a guest material to explore B–N coordination and the effect on emission of the host material. By casting this material into films, they observed Förster resonance energy transfer (FRET) from the host material to the coordinated B(C₆F₅)₃ which resulted in longer fluorescence lifetime. In addition, according to DFT, the HOMO and LUMO of the B(C₆F₅)₃-coordinated molecule is highly delocalized, resulting in an intramolecular charge transfer state. Since the intramolecular charge transfer state is highly sensitive to polarity, changing the ratio of host to B(C₆F₅)₃ has led to wide range of fluorescence emission that are highly tunable even in the film. In addition, the blending of coordinated molecule has also led to enhanced PLQY (∼30% to almost 80–90%) and conductivity (two orders of magnitude increase) of the films.

3.3.8. Emerging dopant strategies. While an effective-p-dopant alone, B(C₆F₅)₃ has also been utilized for increasing the performance of other p-dopants. B(C₆F₅)₃ adducts of 7,7,8,8-tetracyanoquinododimethane (TCNQ; Fig. 15) and derivatives have shown remarkably high oxidation power and have thus been of significant interest for doping as first reported by Malischewski and co-workers in 2022.¹⁷⁶ TCNQ·4B(C₆F₅)₃ could not be isolated, and instead was treated with ferrocene or decamethylcobaltacene to give a dianion, or thianthrene or tris(4-bromophenyl)amine to give a radical anion. The instantaneous oxidation of thianthrene gives an estimated reduction potential for TCNQ·4B(C₆F₅)₃ as >0.9 V vs. Fc/Fc⁺. DFT calculations estimate the electron affinity to be 6.04 eV (583 kJ mol⁻¹), closely matching experimental estimates. Koch and co-workers then carried on this work, investigating derivatives based on F₄TCNQ (Fig. 15) and 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F₆TCNNQ).¹⁷⁷ Fluorination proved an effective strategy for increasing electron affinity even further, as electron affinities for F₄TCNQ·4B(C₆F₅)₃ and F₆TCNNQ·4B(C₆F₅)₃ were calculated as 6.85 eV (661 kJ mol⁻¹) and 6.88 eV (664 kJ mol⁻¹), respectively.

Fig. 15 Coordination of B(C₆F₅)₃ with the nitrogen on the –CN functional groups of TCNQ and F₄-TCNQ dramatically increases the EA and oxidation potential of the p-type dopants.¹⁷⁷

Koch and co-workers investigated the utility of TCNQ·4B(C₆F₅)₃ and derivatives as dopants for organic polymers: three p-type organic polymers were evaluated for doping with F₄TCNQ·4B(C₆F₅)₃, poly(3-hexylthiophene) (P3HT), methylated poly(para-phenylene) (MeLPPP), and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT).¹⁷⁷ For both P3HT and MeLPPP, thin film and solution spectroscopic studies showed significant doping with F₄TCNQ·4B(C₆F₅)₃, evident from the appearance of polaron-related absorption. At the same ratio of either B(C₆F₅)₃ or TCNQ, neither polymer showed any evidence of doping. F8BT, a polymer with a very high ionization energy (circa 5.9 eV) was not doped by F₄TCNQ·4B(C₆F₅)₃, but F₆TCNNQ·4B(C₆F₅)₃ did show considerable doping at a 1 : 10 dopant ratio. Further, it was determined that at very low dopant ratios, 1 : 100 or 1 : 50, F₄TCNQ·4B(C₆F₅)₃ dopes P3HT more effectively than F₄TCNQ alone, improving conductivity by a factor of ∼1000 relative to F₄TCNQ-doped P3HT. This work was eloquently expanded upon by Campoy-Quiles and co-workers, who investigated over 20 additional organic semiconductors.¹⁷⁸ Nearly all these semiconductors demonstrated better conductivity when doped with F₄TCNQ·4B(C₆F₅)₃ compared to BCF or F4TCNQ alone, including notable examples Y6 and other NFAs and PM6, suggesting incorporation into the BHJ of OPV devices could be very promising. Lastly, owing to its large molecular volume, F₄TCNQ·4B(C₆F₅)₃ shows improved thermal de-doping performance and consequently improved stability at elevated temperatures.

4. Alternate Lewis acids for doping organics

4.1. Overview & perspective

The application of Lewis acids to enhance electronic device performance by doping requires specific structure–property relationships; upon reviewing molecular transformations and electronics to which B(C₆F₅)₃ is applied several target properties stand out. First, the strength of the Lewis acid is critical for reactivity and performance, and in the case of doping increasing Lewis acidity is hypothesized to enhance p-doping mechanisms such as Brønsted acid doping, adduct formation/CTC doping, and complexation with other dopants. Second, the steric bulk and intermolecular interactions of B(C₆F₅)₃ with organic materials prevents migration in the solid-state as a means of improving stability. Third, B(C₆F₅)₃ is solution processable in a range of organic solvents, which is ideal for devices fabricated by coating/printing methods. Fourth, it is stable and has distinguishable properties that are easily analyzed with common instrumentation, such as NMR spectroscopy, for in depth structural and mechanistic studies. Each aforementioned property plays a key role; for example, while increasing Lewis acidity may lead to better doping efficiencies if a Lewis acid is incompatible with target organic compounds due to a lack of intermolecular interactions it may migrate in processed films and reduce stability and efficiency over time. There are no clear trends for applying Lewis acids as dopants for organic compounds as a whole, as it depends on the materials, device architecture, and role of the Lewis acid. Therefore, this section presents alternates to B(C₆F₅)₃, and when available, relevant properties to encourage a more effective and systematic approach to selecting an appropriate dopant for niche systems.

For notable alternatives to B(C₆F₅)₃ presented within, Lewis acidity based on the Gutmann–Beckett (GB) method, fluoride ion affinity (FIA), and hydride ion affinity (HIA) is provided when available. Note, variability of these values across studies is common, therefore, relative strength to that of B(C₆F₅)₃ for each compound/study is provided when available. The Gutmann–Beckett method involves complexation of a Lewis acid with a phosphine oxide, typically triethylphosphine oxide, and subsequent evaluation of the chemical shifts in ³¹P-NMR spectra. Lewis acidity can be determined via relative correlations of Δδ of P where a higher Δδ indicates a stronger Lewis acid. FIA is a computational method that determines the binding energy or enthalpy change when a fluoride ion (F⁻) binds to a Lewis acid and is typically reported in kJ mol⁻¹. The stronger the Lewis acid, the larger the value, and if a Lewis acid has a FIA greater than that of antimony pentafluoride (SbF₅, 493 kJ mol⁻¹) it is considered to be a Lewis super acid (LSA).^179,180 Similar to FIA is HIA, which computes the binding energy/enthalpy change when a hydride ion (H⁻) binds to a Lewis acid and is reported in kJ mol⁻¹. Electronic characteristics such as calculated LUMO energy levels, formal reductions, and electron affinity are provided when available. Lastly, sterics are considered by including calculated buried volumes (%V_bur) of Lewis acid anions, typically fluoride ion adducts, where the %V_bur is the volume of the ligands that surround a boron center. Note, sterics need to be considered in the context of FLPs when selecting a dopant (for CTC), where using the method of Radius and coworkers¹⁸¹ may define any Lewis acid/base interactions present when doping various organic compounds and compositions. Within are select tri-substituted aryl boranes, carboranyl boranes, and borates and the relative aforementioned properties to be applied for (1) identifying appropriate boron-based Lewis acid for organic electronic applications, (2) to better understand the effect of these types of Lewis acid dopants, and (3) to encourage new dopant and/or active material component designs.

4.2. Tri-substituted aryl boranes & borates

Triphenyl borane (B(C₆H₅)₃; Fig. 16a), being the non-fluorinated analog of B(C₆F₅)₃, exhibits 65%,¹⁸² 78%,¹⁸³ and 74%¹⁸³ relative Lewis acidity (RLA) in relation to B(C₆F₅)₃ using the GB method, FIA, and HIA, respectively, and the LUMO energy level of B(C₆H₅)₃ is destabilized by 1.52 eV¹⁸³ relative to B(C₆F₅)₃ (Table 1). Despite this, B(C₆H₅)₃ has seen many applications in molecular transformations,¹⁸⁴ and is less sterically encumbered than B(C₆F₅)₃ by 5.8% (%V_bur). Increasing the degree of phenyl fluorination, as seen for B(C₆H₃-2,6-F₂)₃, B(C₆H₂-2,4,6-F₃)₃, and B(C₆H-2,3,5,6-F₄)₃ (Fig. 16a), results in RLAs (by GB method) of 82%,¹⁸⁵ 88%,¹⁸⁶ and 98%,¹⁸⁶ respectively, with LUMO energy levels destabilized relative to B(C₆F₅)₃ by 0.91 eV, 0.74 eV, and 0.21 eV, respectively (Table 1). The %V_bur reported for B(C₆H₃-2,6-F₂)₃ and B(C₆H₂-2,4,6-F₃)₃ indicates sterics about the boron center are near identical to that of B(C₆F₅)₃, an expected result when having fluorine substituted 2-positions on all phenyls in these compounds.¹⁸⁷ One method to increase withdrawing behaviour on triaryl borane phenyl groups beyond perfluorination is to install trifluoromethyl groups (–CF₃), as is seen with BAr^F, BTolF, and B(FMes)₃ (Fig. 16a). Indeed, RLAs (by GB method) of BAr^F, BTolF, and B(FMes)₃ are 106%, 108%, and 122%, respectively (Table 1). However, the electron affinity of BAr^F relative to B(C₆F₅)₃ is destabilized by 0.24 eV,¹⁸⁸ while the LUMO energy level of BTolF relative to B(C₆F₅)₃ is stabilized by 0.59 eV.¹⁸⁹ For %V_bur, BAr^F is less encumbered relative to B(C₆F₅)₃ by 5.2%,¹⁸¹ and B(Fmes)₃ is greatly encumbered relative to B(C₆F₅)₃ with an increase to %V_bur of 26.9%.¹⁸¹ Substituting perfluorinated phenyls with perchlorinated phenyls in B(C₆F₅)₃ is seen for B(C₆Cl₅)(C₆F₅)₂, B(C₆Cl₅)₂(C₆F₅), and B(C₆Cl₅)₃ (Fig. 16a). Here, the C₆Cl₅ substituents are more electron withdrawing than C₆F₅ substituents due to the decrease in π-back-donation from Cl relative to that of F, and is apparent by the formal reductions of B(C₆Cl₅)(C₆F₅)₂, B(C₆Cl₅)₂(C₆F₅), and B(C₆Cl₅)₃ being −1.87 V, −1.55 V, and −1.48 V, respectively (Table 1).¹⁹⁰ However, Lewis acidity is both electronic and steric dependent, where using the GB method the RLAs of B(C₆Cl₅)(C₆F₅)₂, B(C₆Cl₅)₂(C₆F₅), and B(C₆Cl₅)₃ are 96%, 93%, and 0%, respectively.¹⁹⁰ While the GB method for B(C₆Cl₅)₃ is 0 due to a lack of complexation with triethylphosphine oxide, a RLA using FIA resulted in 89% relative to that of B(C₆F₅)₃ which indicates B(C₆Cl₅)₃ will complex with small Lewis bases.¹⁹¹ The RLAs are supported when considering %V_bur, where B(C₆Cl₅)(C₆F₅)₂, B(C₆Cl₅)₂(C₆F₅), and B(C₆Cl₅)₃ are encumbered by 4.3%, 7.9%, and 11.3% relative to B(C₆F₅)₃, respectively.¹⁹⁰

Fig. 16 Various tri-substituted aryl boranes & borates. (a) Altering halogenated (fluorine, chlorine) aryl substituents, where increasing the degree of fluorination on phenyl substituents results in increasing Lewis acidity, however, replacing perfluorinated phenyls with perchlorinated phenyls results in a decrease in Lewis acidity. (b) Alternate aryl substituents. (c) Various aryl borates. *RLA^a: relative Lewis acidity (compared to B(C₆F₅)₃) using the Gutmann–Beckett method, RLA^b: relative Lewis acidity (compared to B(C₆F₅)₃) using fluoride ion affinities.

Table 1 Lewis acidity, electronic, and steric factors of tri-substituted aryl boranes & borates

Aryl Borane	Lewis Acidity	RLA	Electronics	Sterics
GB Δδ = Gutmann–Beckett Method ^31P Δδ (ppm), FIA = fluoride ion affinity (kJ mol^−1), HIA = hydride ion affinity (kJ mol^−1), LUMO = calculated LUMO energy (eV), FR = formal reduction potential experimentally found using cyclic voltammetry (V), BV = buried volume of Lewis acids adducts with a fluoride ion [LA–F]^−.a BV is calculated using F-ion adduct cartesian coordinates obtained from associated references and the SambVca 2.1 web application,^199 EA = calculated electron affinity (eV), RLA = relative Lewis acidity compared to B(C6F5)3 (%).b Relative B(C6F5)3 data, NR = not reported or relevant data not available.
B(C6F5)3	Due to variability across studies, relative B(C6F5)3 data for each compound/study is reported
B(C6H5)3	GB Δδ: 19.6 (C6D6), ^b30.2 (C6D6)^182	GB Δδ: 65
	FIA: 354, ^b452^183	FIA: 78	LUMO: −2.02^183	BV: 53.1^181
	HIA: 360, ^b484^183	HIA: 74	LUMO: −3.52^183	BV: 58.9^181
B(C6H3-2,6-F2)3	GB Δδ: 21.2 (CD2Cl2), ^b25.8 (CD2Cl2)^185	GB Δδ: 82	LUMO: −2.39^187	BV: 58.0^187
	FIA: 368, ^b457^187	FIA: 81	LUMO: −3.30^187	^, BV: 59.4^187
B(C6H2-2,4,6-F3)3	GB Δδ: 30.5 (CDCl3), ^b34.8 (CDCl3)^186	GB Δδ: 88	LUMO: −2.56^187	BV: 58.0^187
	FIA: 390, ^b457^187	FIA: 85	LUMO: −3.30^187	^, BV: 59.4^187
B(C6H-2,3,5,6-F4)3	GB Δδ: 34.0 (CDCl3), ^b34.8 (CDCl3)^186	GB Δδ: 98	LUMO: −2.59^192	NR
			LUMO: −2.80^192
BAr^F3	GB Δδ: 28.2 (CD2Cl2), ^b26.6 (CD2Cl2)^193	GB Δδ: 106	EA: −2.79^188	BV: 53.7^181
			EA: −3.03^188 5/22/2025 10:05:00 AM	BV: 58.9^181
BTolF	GB Δδ: 31.9 (C6D6), 29.0 (CD2Cl2), ^b29.5 (C6D6), ^b26.5 (CD2Cl2)^194	GB Δδ: 108 (C6D6) & 109 (CD2Cl2)	LUMO: −4.09^189^bLUMO: −3.50^189	NR
B(FMes)3	GB Δδ = 44.21 (calculated) ^b36.34 (calculated)^195	GB Δδ: 122	NR	BV: 85.8^181
				BV: 58.9^181
B(C6Cl5)(C6F5)2	GB Δδ: 32.5 (CD2Cl2), ^b33.7 (CD2Cl2)^190	GB Δδ: 96	FR: −1.87^190	BV: 63.2^181
			FR: −1.97^190	BV: 58.9^181
B(C6Cl5)2(C6F5)	GB Δδ: 31.2 (CD2Cl2), ^b33.7 (CD2Cl2)^190	GB Δδ: 93	FR: −1.55^190	BV: 66.8^181
			FR: −1.97^190	BV: 58.9^181
B(C6Cl5)3	GB Δδ: 0 (CD2Cl2) ^b33.7 (CD2Cl2)^191	GB Δδ: 0	FR: −1.48^190	BV: 70.2^181
	FIA: 366, ^b413^191	FIA: 89	FR: −1.97^190	BV: 58.9^181
B(C12H8)(C6H5)	GB Δδ: 33.1 (C6D6), ^bNR^196
	FIA: 366, ^b452^183	FIA: 81	LUMO: −2.52^183	BV: 47.3^181
	HIA: 382, ^b484^183	HIA: 79	LUMO: −3.52^183	BV: 58.9^181
B(C12F8)(C6F5)	FIA: 457, ^b452^183	FIA: 101	LUMO: −3.78^183	BV: 52.7^183
	HIA: 491, ^b484^183	HIA: 101	LUMO: −3.52^183	^, BV: 58.1^183
B2(C6F5)2(C6F4)2	FIA: 477, ^b452^197	FIA: 105	LUMO: −4.68^197	NR
	HIA: 514, ^b484^197	HIA: 106	LUMO: −3.93^197
B(C12F9)3	FIA: 431, ^b452^197	FIA: 95	LUMO: −3.95^197	BV: 82.9^181
	HIA: 452, ^b484^197	HIA: 93	LUMO: −3.93^197	BV: 58.9^181
B(C10F7)3	FIA: 483, ^b452^197	FIA: 106	LUMO: −3.91^197	BV: 58.8^181
	HIA: 519, ^b484^197	HIA: 107	LUMO: −3.93^197	BV: 58.9^181
B(OC6H5)3	GB Δδ: 23.0 (C6D6)^b30.2 (C6D6)^182	GB Δδ: 76
	FIA: 350, ^b454^198	FIA: 77	LUMO: −0.39^198	BV: 49.2^198
	HIA: 323, ^b497^198	HIA: 65	LUMO: −3.43^198	^, BV: 59.7^198
B(OC6F5)3	GB Δδ = 34.5 (C6D6) ^b30.2 (C6D6)^182	GB Δδ: 114
	FIA: 431, ^b454^198	FIA: 95	LUMO: −1.27^198	BV: 54.9^181
	HIA: 411, ^b497^198	HIA: 83	LUMO: −3.43^198	BV: 58.9^181
B(OoFC6H4)3	GB Δδ = 23.8 (CDCl3) ^bNR^198
	FIA: 351, ^b454^198	FIA: 77	LUMO: −0.75^198	BV: 50.7^198
	HIA: 339, ^b497^198	HIA: 68	LUMO: −3.43^198	^, BV: 59.7^198
B(OmFC6H4)3	GB Δδ = 22.0 (CDCl3) ^bNR^198
	FIA: 368, ^b454^198	FIA: 81	LUMO: −0.76^198	BV: 43.3^198
	HIA: 340, ^b497^198	HIA: 68	LUMO: −3.43^198	^, BV: 59.7^198
B(OpFC6H4)3	GB Δδ = 14.1 (CDCl3) ^bNR^198
	FIA: 359, ^b454^198	FIA: 79	LUMO: −0.86^198	BV: 43.4
	HIA: 329, ^b497^198	HIA: 66	LUMO: −3.43^198,199	^, BV: 59.7^198

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Altering aryl substituents is another method to influence the Lewis acidity, electronics, and sterics about the boron in tri-substituted boranes. Borafluorene Lewis acids, such as B(C₁₂H₈)(C₆H₅) and B(C₁₂F₈)(C₆F₅) (Fig. 16b), contain antiaromatic five membered borole rings that influence the electronics, sterics, and thereby Lewis acidity relative to B(C₆H₅)₃ and B(C₆F₅)₃, respectively. Comparing B(C₁₂H₈)(C₆H₅) to B(C₆H₅)₃ the RLA (by FIA), LUMO, and %V_bur is 103%,¹⁸³ 0.50 eV stabilized,¹⁸³ and decreased by 5.8%,¹⁸¹ respectively (Table 1). While B(C₆F₅)₃ is a stronger Lewis acid than B(C₁₂H₈)(C₆H₅), it is apparent that borafluorene substitution can slightly increase Lewis acidity while decreasing sterics. This is supported by comparing B(C₁₂F₈)(C₆F₅) to B(C₆F₅)₃ the RLA (by FIA), LUMO, and %V_bur which is 101%, 0.26 eV stabilized, and decreased by 5.4%, respectively (Table 1).¹⁸³ One example of a bridged borane is provided, B₂(C₆F₅)₂(C₆F₄)₂ (Fig. 16b), which bears a six-membered central ring that bridges two C₆F₄ groups via two C₆F₅ substituted boranes. The RLA (by FIA) is 105% and the LUMO enegry level is 0.75 eV stabilized (Table 1).¹⁹⁷ Expanding π-conjugation is another approach to afford alternate tri-substituted boranes, two examples being B(C₁₂F₉)₃ and B(C₁₀F₇)₃ (Fig. 16b). When comparing B(C₁₂F₉)₃ to B(C₆F₅)₃, the RLA (by FIA) is 95%,¹⁹⁷ the LUMO is stabilized by 0.02 eV,¹⁹⁷ and %V_bur is increased by 24%,¹⁸¹ indicating a substantial increase in sterics while maintaining Lewis acidity (Table 1). However, when comparing B(C₁₀F₇)₃ with B(C₆F₅)₃, the RLA (by FIA) is 106%,¹⁹⁷ the LUMO is destabilized by 0.02 eV,¹⁹⁷ and %V_bur is increased by 0.01%, indicating sterics are maintained while slightly increasing Lewis acidity (Table 1).¹⁸¹

Worth noting are select triaryl borates, which differ from boranes by an oxygen atom that links the central boron and carbon atoms of aryl substituents. B(OC₆H₅)₃ (Fig. 16c) compared to B(C₆F₅)₃ reveals a RLA (by FIA) of 77%, a LUMO energy level that is destabilized by 3.04 eV, and %V_bur that indicates 10.5% less encumbrance (Table 1).¹⁹⁸ Further, B(OC₆F₅)₃ (Fig. 16c) compared to B(C₆F₅)₃ reveals a RLA (by FIA) of 95%,¹⁹⁸ a LUMO energy level that is destabilized by 2.16 eV,¹⁹⁸ and is 5.0% less encumbered (Table 1).¹⁸¹ While this suggests that both sterics and Lewis acidity is decreased when comparing borates to boranes, using the GB method to measure the Lewis acidity of B(OC₆F₅)₃ relative to that of B(C₆F₅)₃ affords a RLA of 114%, suggesting the borate counterpart is significantly more Lewis acidic. Furthermore, the RLA (by GB method) for B(C₆H₅)₃ to B(C₆F₅)₃ is 65%, which is lower than the RLA of B(OC₆H₅)₃ to B(C₆F₅)₃ at 77% and suggests that the presence of the bridging oxygen does increase Lewis acidity. Notable for triaryl borates is the variability in properties when considering ortho-, meta-, and para-substitution of fluorine on B(OC₆H₅)₃ affording B(O_oFC₆H₄)₃, B(O_mFC₆H₄)₃, and B(O_pFC₆H₄)₃, respectively (Fig. 16c). In terms of RLA (to B(C₆F₅)₃ by FIA), B(O_oFC₆H₄)₃, B(O_mFC₆H₄)₃, and B(O_pFC₆H₄)₃ afford values of 77%, 81%, and 79%, respectively (Table 1).¹⁹⁸ In terms of LUMO energy levels relative to B(C₆F₅)₃, B(O_oFC₆H₄)₃, B(O_mFC₆H₄)₃, and B(O_pFC₆H₄)₃ afford destabilizations of 2.68 eV, 2.67 eV, and 2.57 eV, respectively.¹⁹⁸ Lastly, the %V_bur compared to that of B(C₆F₅)₃ for B(O_oFC₆H₄)₃, B(O_mFC₆H₄)₃, and B(O_pFC₆H₄)₃ exhibits less encumbrance by 9.0%, 16.4%, and 16.3%, respectively.¹⁹⁸ This suggests that meta-substitution results in the highest Lewis acidity while being the least sterically encumbered.

4.3. Carborane substituted boranes

Carborane substituents on boranes are an emerging strategy for producing Lewis superacids (LSAs) with superior stability to their aryl counterparts. Carboranes are non-classically bonded polyhedral clusters of boron, carbon, and hydrogen atoms (Fig. 17a). Icosahedral closo-carboranes with the formula C₂B₁₀H₁₂ are the most common and can exist in three isomers: 1,2-, 1,7-, and 1,12-, referred to as ortho (_OCb), meta (_MCb), and para (_PCb), respectively (Fig. 17a). All three isomers are very strongly electron withdrawing, and unlike aryl substituents cannot act as π-donors, making them promising candidates for the synthesis of highly Lewis acidic species. Computational studies in the Martin group found that carboranes are superior to aryl groups in general at increasing the Lewis acidity of boranes; for primary, secondary and tertiary boranes it is demonstrated that Lewis acidity follows the trend B_OCb_XH_3−X > B_MCb_XH_3−X > B_PCb_XH_3−X > B(C₆F₅)_XH_3−X > B(C₆H₅)_XH_3−X (X = 1, 2, 3; Fig. 17c).²⁰⁰ Early work with carboranyl substituents focused on perimeter substitution of triaryl boranes (B(Mes)₂(_PPh_OCb) & B(2,6-Me4-_OCb)₃; Fig. 17b) which resulted in increased Lewis acidity.^201,202 Towards applications, Park and coworkers applied these peripheral carboranyl-substituted triaryl boranes as fluoride ion sensors.²⁰³ Considering the increase in Lewis acidity, stability, and emerging work, we preset select directly-bound ortho-carboranyl boranes,^{189,204–220} many of which exceed the pentafluorophenyl analogues in terms of Lewis acidity and performance in chemical reactions.

Fig. 17 Icosahedral closo-carboranes and carboranyl substituted boranes (Cb); (a) ortho-Cb (_OCb), meta-Cb (_MCb), and para-Cb (_PCb), (b) triaryl boranes with peripheral carboranyl substituents B(Mes)₂(_PPh_OCBR) & B(2,6-Me4-_OCbPh)₃, (c) ortho-, meta-, and para-carboranyl tri-substituted boranes B_OCb₃, B_MCb₃, and B_PCb₃, (d) mono-substituted carboranyl boranes: B(Mes₂)(_OCbR), B(2,6-MesPh)_OCb, HB_OCb, and (e) di-substituted carboranyl boranes: HB(_OCbMe)₂, (BR)₂(_OCb)₂, RB(_b,OCb₂).

The use of carborane as a directly-bound functional group towards the synthesis of Lewis acidic boranes has been spearheaded by Martin and coworkers.^{189,200,218,221} Tris(ortho-carboranyl)borane (B_OCb₃, Fig. 17c), analogous to B(C₆F₅)₃, exhibits substantial increases to Lewis acidity when probed by the GB method, FIA, and HIA for RLAs of 115%, 134%, and 129%, respectively (Table 2).²⁰⁰ Indeed, Lewis superacidity was confirmed computationally with an FIA of 605 kJ mol⁻¹.¹⁸⁰ In terms of electronics, relative to B(C₆F₅)₃ the LUMO energy level is stabilized by 0.49 eV. In terms of sterics, relative to B(C₆F₅)₃ B_OCb₃ sees an increase of 18.8% (%V_bur) and will preferentially forms FLPs over adducts.²²¹ Notable is the high thermal stability, with no decomposition when heated to 250 °C.

Table 2 Lewis acidity, electronic, and steric factors of carboranyl boranes

Borane	Lewis acidity	RLA	Electronics	Sterics
GB Δδ = Gutmann–Beckett Method ^31P Δδ (ppm), FIA = fluoride ion affinity (kJ mol^−1), HIA = hydride ion affinity (kJ mol^−1), LUMO = calculated LUMO energy (eV), BV = buried volume of Lewis acids adducts with a fluoride ion [LA–F]^−.a BV is calculated using F–ion adduct cartesian coordinates obtained from associated references and the SambVca 2.1 web application,^199 RLA = relative Lewis acidity compared to B(C6F5)3 (%).b Relative B(C6F5)3 data.c Data is relative to SbF5 rather than B(C6F5), NR = not reported or relevant data not available.
B(C6F5)3	Due to variability across studies, relative B(C6F5)3 data for each compound/study is reported
BoCb3	GB Δδ: 34.1 (C6D6), ^b29.7 (C6D6)^200 FIA: 605, ^b452^200 HIA: 622, ^b484^200	GB Δδ: 115 FIA: 134 HIA: 129	LUMO: −3.99^200^bLUMO: −3.50^200	BV: 71.9^200^bBV: 53.1^200
B(Mes)2(OCbH)	FIA: 556^204	NR	LUMO: −2.13^204	BV: NR
B(Mes)2(OCbPh)	FIA: 533^204	NR	LUMO: −2.11^204	BV: NR
B(2,6-MesPh)OCb	FIA: 466, ^c474^207 HIA: 535, ^b546^207	FIA: 98 HIA: 98	LUMO: NR	BV: NR
HBOCb	FIA: 512, ^c474^207 HIA: 580, ^b546^207	FIA: 108 HIA: 106	LUMO: NR	BV: NR
HB(oCbMe)2	GB Δδ: 35.8 (C6D6), ^b29.7 (C6D6)^200 FIA: 527, ^b452^200 HIA: 540, ^b484^200	GB Δδ: 121 FIA: 117 HIA: 112	LUMO: −3.10^200^bLUMO: −3.50^200	BV: 64.7^200^bBV: 53.1^200
(BH)2oCb2	FIA: 538, ^c474^214 HIA: 615, ^b546^214	FIA: 114 HIA: 113	LUMO: NR	BV: 55.6^214
(BCl)2oCb2	GB Δδ: 36.8 (C6D6), ^b26.6 (C6D6)^214 FIA: 535, ^c474^214 HIA: 601, ^b546^214	GB Δδ: 138 ^cFIA: 113 HIA: 110	LUMO: −1.50^214	BV: 60.6^214
(BBr)2oCb2	GB Δδ: 40.8 (C6D6), ^b26.6 (C6D6)^214 FIA: 537, ^c474^214 HIA: 608, ^b546 ^214	GB Δδ: 153 ^cFIA: 133 HIA: 111	LUMO: −1.57^214	BV: 61.4^214
(BMe)2oCb2	GB Δδ: 32.6 (C6D6), ^b26.6 (C6D6)^214 FIA: 495, ^c474^214 HIA: 562, ^b546^214	GB Δδ: 123 ^cFIA: 104 HIA: 103	LUMO: −1.03^214	BV: 61.5^214
(BPh)2oCb2	GB Δδ: 28.9 (C6D6), ^b26.6 (C6D6)^214 FIA: 523, ^c474^214 HIA: 592, ^b546^214	GB Δδ: 109 ^cFIA: 110 HIA: 108	LUMO: −1.18^214	BV: 66.2^214
(BN3)2oCb2	GB Δδ: 37.8 (C6D6), ^b26.6 (C6D6)^216 FIA: 515, ^c474^216 HIA: 578, ^b546^216	GB Δδ: 142 ^cFIA: 109 HIA: 106	LUMO: −1.33^216	BV: 59.2^216
iPr2NB(b,OCb)	GB Δδ: 6.9 (C6D6)^213 FIA: 414, ^c474^217 HIA: 480, ^b546^217	FIA: 87 HIA:88	LUMO: −2.05^213	BV: 67.7^217
ClB(b,OCb)	GB Δδ: 40.3 (C6D6), ^b29.6 (C6D6)^217 FIA: 560, ^c474^217 HIA: 625, ^b546^217	GB Δδ: 136 ^cFIA: 118 HIA: 114	LUMO: NR	BV: 55.5^217
BrB(b,OCb)	GB Δδ: 41.5 (C6D6), ^b29.6 (C6D6)^217 FIA: 565, ^c474^217 HIA: 635, ^b546^217	GB Δδ: 140 ^cFIA: 119 HIA: 116	LUMO: NR	BV: 56.3^217
PhB(b,OCb)	GB Δδ: 34.3 (C6D6), ^b29.6 (C6D6)^217 FIA: 514, ^c474^217 HIA: 585, ^b546^217	GB Δδ: 116 ^cFIA: 108 HIA: 107	LUMO: −1.17^217	BV: 61.2^217
O CbB(b,OCb)	GB Δδ: 34.9 (C6D6), ^b29.6 (C6D6)^212 FIA: 621, ^c475^212 HIA: 632, ^b484^212	GB Δδ: 118 ^cFIA: 131 HIA: 131	LUMO: −3.94^212^bLUMO: −3.52^212	BV: 67.7^212

---

Two examples of mono-substituted carboranyl boranes include dimesityl(ortho-carboranyl)borane bearing either a H or Ph substituent on the carborane (B(Mes)_2oCbH & B(Mes)_2oCbPh; Fig. 17d). Computational investigation determined both were LSAs with FIAs of 556 kJ mol⁻¹ for B(Mes)_2oCbH and 533 kJ mol⁻¹ for B(Mes)_2oCbPh (Table 2).²⁰⁴ LUMO energy levels were also computed for values of −2.13 eV for B(Mes)_2oCbH and −2.11 eV for B(Mes)_2oCbPh. Notably, both derivatives are water stable when considering aqueous workups in the syntheses, and while long-term experiments revealed hydrolysis on exposure to air over multiple weeks the carboranyl substituent remained bound to central boron. Another example of a mono-substituted carboranyl borane is the borirane, HB_OCb (Fig. 17d). While not a LSA, the FIA for HB_OCb is reported as 512 kJ mol⁻¹ (Table 2), which is higher than typical reports for B(C₆F₅)₃ (∼450–480 kJ mol⁻¹).²⁰⁷ Note, several works have explored the properties and reactivity of these types of boriranes, including N-heterocyclic carbene stabilization.²⁰⁵ For example, 5 and 7 membered ring-expansion products form when treated with aldehydes and ketones, respectively.^{206–208,222,223} Computational investigation found the carboranyl borirane has higher ring strain than borirane, where this combined with the lack of π-delocalisation from the carborane leads to increased Lewis acidity.

Bis(1-methyl ortho-carboranyl)borane (HB(_oCbMe)₂, Fig. 17e) is an example of a di-substituted carboranyl borane and is analogous to Piers’ borane. Lewis superacidity was confirmed computationally with FIA at 527 kJ mol⁻¹ (exceeding SbF₅ at 493 kJ mol⁻¹; Table 2).²⁰⁰ Electronically, the LUMO energy level of HB(_oCbMe)₂ is destabilized by 0.40 eV relative to B(C₆F₅)₃, and in terms of sterics is encumbered by 11.6% relative to B(C₆F₅)₃ (%V_bur).²⁰⁰ Additional examples of di-substituted carboranyl boranes include carboranyl diboranthracene ((BR)_2OCb₂, R = H, Cl, Br, Me, Ph, N₃), though (BH)_2OCb₂ was only observed in situ, and carboranyl borafluorene analogues (RB_(b,O)Cb, R = iPr₂N, Cl, Br, Ph, _OCb; Fig. 17e). For the carboranyl diboranthracene analogues, RLAs (by FIA) compared to B(C₆F₅)₃ are 114%, 113%, 133%, 104%, and 110% for R being H, Cl, Br, Me, Ph, and N₃ (Table 2), respectively, showcasing the high Lewis acidity of these compounds.^214,216 The LUMO energy levels are also reported, albeit no calculations were done for B(C₆F₅)₃ so data cannot be compared, but are −1.50 eV, −1.57 eV, −1.03 eV, −1.18 eV, and −1.33 eV for R being Cl, Br, Me, Ph, and N₃ (Table 2), respectively.^214,216 In terms of sterics, again no %V_bur calculations (or relevant crystal structure data) for B(C₆F₅)₃ was provided so this data cannot be compared, but %V_bur are 55.6%, 60.6%, 61.4%, 61.5%, and 66.2%, and 59.2% for R being H, Cl, Br, Me, Ph, and N₃ (Table 2), respectively.^214,216 Notable is the thermal stability of these compounds, all isolated compounds were stable to at least 235 °C, with (BMe)_2OCb₂ showing the highest melting point of 285 °C. The carboranyl borafluorene analogues exhibit increased Lewis acidity, except for the iPr₂N substituted carborane, with RLAs (by FIA) to that of B(C₆F₅)₃ being 87%, 118%, 119%, 108%, and 131% for R being iPr₂N, Cl, Br, Ph, and _OCb (Table 2), respectively.^212,217 As is observed for classical borafluorenes, fusion of the substituents around the central boron atom results in an increase in Lewis acidity, resulting in an FIA increase of 39 kJ mol⁻¹ for _OCbB(_b,OCb) relative to B_OCb₃. Only select LUMO energy levels are reported for these compounds for −2.05 eV, −1.17 eV, and −3.94 eV energies for the iPr₂N, Ph, and _OCb analogues, respectively.^212,217 Furthermore, relative to B(C₆F₅)₃, the LUMO energy level of _OCbB(_b,OCb) is stabilized by 0.42 eV. Lastly, the buried volumes of this series of carboranyl borafluorenes were 67.7%, 55.5%, 56.3%, 61.2%, and 67.7% for R being iPr₂N, Cl, Br, Ph, and _OCb, respectively.^212,217

5. Conclusion

In summary, historical and emerging applications of B(C₆F₅)₃ has been presented which showcase the Lewis acid to be highly versatile. Compared to its halide counterparts, B(C₆F₅)₃ is unique in its stability and formation of analyzable crystalline structures appropriate for mechanistic studies, which has paved the way for improved chemical designs and applications. This is evident when reviewing the electronics field, where B(C₆F₅)₃ has recently emerged as a popular dopant for the active materials leading to higher performance devices, however, little understanding of the mechanisms and material interactions at play is evident. For example, B(C₆F₅)₃ is hygroscopic and is known to form water adducts, oxygen bridged borate complexes and can decompose into boronic acids and boroxines upon long-term exposure to water, oxygen, and heat. Such varied chemical species are likely to influence the long-term performance of organic electronic devices but have been rarely discussed within this field. Despite this, B(C₆F₅)₃ has shown great promise to date for applications in materials science and is an area that is seemingly ripe for future research. However, an interdisciplinary approach involving both inorganic chemists and materials scientists is needed. To start bridging this gap, we presented select triaryl boranes, triaryl borates, and carboranyl boranes and relevant properties for doping applications in organic electronics. Aluminum, indium, and pentavalent pnictogen (antinomy, phosphorus, bismuth) based Lewis acids were not covered but are notable alternatives. Looking towards future work, collaborations between the materials science and inorganic communities alongside the application of machine learning to identify and screen additional alternatives, in terms of predicted properties and interactions, would be of immense value to the materials science community.

Data availability

There is no original data. All work covered has been published prior.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

GCW acknowledges funding from the NSERC DG program (2019-04392), the Canada Foundation for Innovation, and the University of Calgary. KMW thanks NSERC for a CGS-D scholarship & IEP for a PGS-D scholarship. KMW, MJG, and IEP thank Alberta Student Aid for Alberta Graduate Excellence Scholarships.

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