(Hetero)arene-fused boroles: a broad spectrum of applications

(Hetero)arene-fused boroles are a class of compounds containing a 5-membered boron diene-ring. Based on their molecular framework, the (hetero)arene-fused boroles can be considered as boron-doped polycyclic antiaromatic hydrocarbons and are thus of great interest. Due to the vacant pz orbital on the 3-coordinate boron atom, the antiaromaticity and strain of the 5-membered borole ring, (hetero)arene-fused boroles possess strong electron accepting abilities and Lewis acidity. By functionalization, they can be tuned to optimize different properties for specific applications. Herein, we summarize synthetic methodologies, different strategies for their functionalization, and applications of (hetero)arene-fused boroles.


Introduction
Three-coordinate boranes have been studied intensely for applications such as anion sensors, 1-3 nonlinear optical materials (NLOs), 4-14 live cell imaging, [15][16][17][18] sensing of DNA, RNA and proteins, 19,20 etc. [21][22][23][24][25][26][27][28][29][30][31][32] Among them, boroles are distinct, being 5membered unsaturated 4p-electron heterocycles containing a 3coordinate boron center. Interest in boroles originates from their being isoelectronic with the cyclopentadiene cation (Cp + ) which, in terms of Hückel's rule, [33][34][35][36] is antiaromatic and thus highly reactive. The isolation of "free" Cp + has not been achieved. Cp + has a triplet electronic ground state, which was conrmed by ESR spectroscopic measurements of the pentaphenylcyclopentadienyl cation at low temperature. 37 The C 2v symmetry of a borole is lower than that of Cp + (D 5h , Fig. 1), which results in a splitting of the previously degenerate half-lled molecular orbitals. The orbital with a nodal plane passing through the boron atom ("as" in Fig. 1) is lowered in energy and occupied, which leads to a singlet ground state and diamagnetic character of boroles, in contrast to the biradical character of Cp + . The small HOMO-LUMO gap in boroles leads to their intense color. with ortho-tri-uoromethylated aryls and their application for thermally activated delayed uorescence as well as their inuence on borauorenes. He is currently working as a postdoctoral researcher in the same group. Fig. 1 Molecule orbitals of the cyclopentadiene cation (Cp + ) and borole; "as" and "s" denote the antisymmetric and symmetric orbitals, respectively, with respect to the mirror plane perpendicular to the molecule. He was elected to the Bavarian Academy of Sciences, Fellowship of the American Association for the Advancement of Science (AAAS), and Fellowship of the European Academy of Sciences (EurASc) and received an honorary doctorate from the University of Rennes 1, France. He has held Visiting/Honorary/Distinguished Professorships in the UK, France, Hong Kong, China, Japan, India, and Australia, and served on 10 journal editorial/advisory boards. His research interests include synthesis, structure, bonding, reactivity, homogeneous catalysis, luminescence, non-linear optics, bioimaging, liquid crystals, and crystal engineering.
The rst dibenzo-fused borole, namely the parent 9-bora-uorene (Bf), was reported in the early 1960s by Köster and Benedikt,55 but only at the start of this century, has the chemistry of Bfs started to attract increasing interest. (Hetero)arenefused boroles have been reviewed very briey either as part of reviews on "free" boroles 47,49,51,54 or on boron-doped polycyclic aromatic hydrocarbons (PAHs) 56,57 or in the context of subvalent boranes. 58 In this review, we address the synthesis, properties and applications of (hetero)arene-fused boroles in detail, our primary focuses are: (i) the optoelectronic behavior of the compounds; and (ii) their classication based on structure property relationships. New developments in the use of the exosubstituents on boron are also discussed. The structures of all compounds with Arabic numerals are shown in Scheme 25 at the end of the paper for the reader's convenience.
The rst approach relies on readily available 9-X-9-borauorenes (XBf, X ¼ Cl or Br or I). Four synthetic approaches to XBf derivatives have been developed. In 1985, Nöth and co-workers applied a boron-mercury exchange reaction (Scheme 2, path a). 59 Analogous to the preparation of PhB, 60 a boron-tin exchange reaction can also be utilized for the synthesis of XBfs (Scheme 2, path b). 61,62 To avoid using these highly toxic metals, a boron-silicon exchange reaction was developed (Scheme 2, path c). 63 ClBf can be obtained directly from reaction of 2,2 0 -dilithiobiphenyl with BCl 3 in an aliphatic solvent (Scheme 2, path d). 64 The advantage of this approach is that once intermediate XBf is obtained, it is useful for subsequent derivatization.
For the one-step assembly of 9-aryl-9-borauorenes, a borontin exchange reaction was utilized. Instead of using BX 3 (X ¼ Cl or Br or I), PhBCl 2 was used directly (Scheme 3, path e). 65 The drawback of this methodology is that only sterically relatively unencumbered groups (e.g., phenyl and pentauorophenyl) can be used; 61 e.g., MesBCl 2 (Mes ¼ mesityl group) is unsuitable for this methodology. By changing the boron source from PhBCl 2 to dimethoxy aryl borates, and subsequent reaction with dilithiobiphenyl, 9-aryl-9-borauorenes can be synthesized (Scheme 3, path f). 66,67 There are two advantages to this approach: (1) dimethoxy aryl borates can tolerate coordinating solvents and are much more stable than their corresponding aryldihaloboranes, which makes the work up much easier; and (2) bulkier aryl groups (e.g., 2,4,6-triisopropylphenyl (Tip)) can be used. The third approach which can assemble 9-aryl-9-borauorenes in one step was reported by Marder and co-workers, 45,68 in which more stable aryltriuoroborate salts were used as the boron source (Scheme 3, path g). [69][70][71][72][73][74] The downside of this approach is the comparably low yields. Besides these three widely applied approaches, which can assemble 9-aryl-9-borauorenes in one step, the Wehmschulte group synthesized two unsymmetric 9-borauorenes (2a and 2b, Scheme 3, path h) in one step by using H 2 ClB$SMe 2 as the boron source. 75,76 The formation of the borole ring takes place via a facile intramolecular C-H activation process, but the requirement of bulky terphenyl precursors limits its further application.
The third approach is a stepwise substitution reaction at boron, which was rst reported in 1963 by Köster and Benedikt (Scheme 4). 55 Thus, 9-alkyl-9-borauorenes can be synthesized from B,B-dialkyl-2-biphenylborane intermediates by thermal dissociation of one alkyl-group at 180-200 C. The compound 9phenyl-9-borauorene (PhBf) can also be synthesized from B,Bdiphenyl-2-biphenylborane in the same way, but the temperature needs to be increased to 280-300 C. The harsh conditions in this approach limit its further application. In 2011, the Yamaguchi group applied boronic esters as the boron source for the synthesis of heteroarene-fused boroles. 77 Using the dithiophene-fused borole (3a) as an example, the boronic ester was introduced at the bithiophene via boron-lithium exchange in the rst step, then the protecting group at the boron was introduced with a Grignard reagent. Finally, an intramolecular cyclization reaction completed the synthesis of 3a. More recently, Urban and co-workers applied a similar strategy, but used an intramolecular cyclization in the second step, obtaining 9-methoxy-9-borauorene (MeOBf), 78 a potential intermediate for synthesizing other 9-substituted-9-borauorenes. In 2012, the Piers group used a reductive route with a haloborane (Precursor-4, obtained by three stepwise boron-metal exchange reactions) while attempting to synthesize the diborole 4 (Scheme 4, bottom). Instead, they initially obtained an isomer of 4 (Isomer-4) which, under irradiation with UV light (254 nm), isomerized to the desired diborole 4. 79 Subsequently, the same group reported another, more efficient, thermal route to the diborole 4 from Isomer-4. 80

Stability of 9-borafluorenes
The advantage of incorporating a boron atom into a 5membered diene-ring is the enhancement of electron accepting ability and Lewis acidity, but at the same time, stability is sacriced. By fusing two phenyl rings onto a borole, the stability is greatly enhanced. The stability of the resulting 9-bora-uorenes depends largely on the exo-substituent at boron. In this section, we compare the stability of different 9-borauorenes.
When the exo-substituent is not a bulky aryl group, 9-bora-uorenes remain highly reactive (Scheme 5). In fact, HBf in an unsymmetric dimer in solution and, aer some time, it forms oligomers via a ring-opening mechanism even in dry and deoxygenated C 6 D 6 . 55,81-84 ClBf 85-87 and i Pr 2 NBf both show more than 50% decomposition within 1 hour in solution when exposed to the atmosphere. 88 t BuOBf is much more stable and exhibits less than 10% decomposition aer 1 hour in CDCl 3 in air. This can be attributed to the steric demand of the tert-butyl group as the less bulky derivative MeOBf 59 is as sensitive to air/ moisture as i Pr 2 NBf and ClBf. 88 An aryl group as the exo-substituent at 9-borauorene increases the stability drastically (Scheme 6). PhBf and MesBf decompose only slowly in the air. 54 PhBf nonetheless retains high reactivity, e.g., azides can insert into one of the B-C bonds of PhBf to generate 9,10-B,N-phenanthrenes. 62,89 The reaction of PhBf with 1,2-dipolar substrates leads to the formation of the corresponding ring expansion adducts. 90,91 By employing Tip or F Mes as the protecting group at the boron atom of 9-bora-uorenes, their stability is greatly enhanced, and both derivatives can be puried by column chromatography, even in air. 66,88 Stability tests indicate that TipBf shows ca. 15% decomposition in solution in air aer 24 hours and F MesBf shows only ca. 5% decomposition under the same conditions. Considering that the triuoromethyl group is less bulky than the isopropyl group, 92 this observation initially seems counterintuitive. In fact, the higher stability of F MesBf is likely due to the stabilizing interaction of the vacant p z -orbital of boron by lone pairs of the uorine atoms of the two ortho-CF 3 groups of the exo-F Mes. This s-donation was conrmed by the short B/F distances (2.682(6) and 2.577(5)Å) observed in the solid state (Fig. 2), which are much shorter than the sum of the van der Waals radii for boron and uorine (3.39Å). 93 The s-donation from the uorine atom(s) of ortho-CF 3 group(s) to the vacant p z -orbital of boron was also observed in other boranes [94][95][96][97][98][99][100][101][102] and boroles. 45,68 The 2,4,6-tri-tert-butylphenyl (Mes*) group is the bulkiest substituent among these protecting groups and, thus, provides the most stable 9-borauorenes. 103 Compared to TipBf, which still exhibits reactivity towards the small F À anion and can be applied as a F À sensor, Mes*Bf is inert to F À . This demonstrates the superior stability of Mes*Bf.
To gain a deeper understanding of the relation between the structure and the stability, selected bond parameters of PhBf, 104 MesBf, 105 and F MesBf 88 derived from single crystal X-ray studies are listed in Table 1  Apparently, the bulkier the exo-aryl group is, the larger the torsion angle becomes, resulting in more efficient kinetic protection.

9-Borafluorenes with a fluorinated backbone
Inspired by the wide application of B(C 6 F 5 ) 3 , 106-108 and the fact that 9-borauorenes are more Lewis acidic than their corresponding boranes, Piers and co-workers synthesized a series of 9-bora-uorenes with uorinated backbones (Scheme 7). Compounds 5a and 5b are pale yellow or orange solids which exhibit lowest energy absorption maxima at 398 nm and 440 nm, respectively, in hexane. 61 To explore the effect of two uorinated 9-borauorene centers in a molecular framework on the Lewis acidity, 5c was synthesized. 109 In 5c, two uorinated 9-borauorenes are situated ortho to one another, forming a chelating bidentate Lewis acid. Compound 5c is a deep orange solid with a lowest energy absorption maximum at 425 nm (3 ¼ 590 M À1 cm À1 ) in hexane, comparable to that of 5b, indicating that the two chromophores of the uorinated 9-borauorenes are not coupled. To the best of our knowledge, 5d is the only example of mono-aryl fused borole. 110 Compound 5d is a red solid with its lowest energy absorption maximum at 465 nm (3 ¼ 900 M À1 cm À1 in toluene), and is readily soluble in most solvents. 111 Compared with the corresponding uorinated peruoroboranes (MeB(C 6 F 5 ) 2 , B(C 6 F 5 ) 3 and C 6 F 4 -1,2-[B(C 6 F 5 ) 2 ] 2 ), 109,112-114 uorinated 9-borauorenes show stronger Lewis acidities, as demonstrated by Lewis base competition reactions, the Childs method, 115 and semiempirical MNDO calculations. 116 Apparently, compared with the corresponding uorinated triarylboranes, the loss of two uorine atoms is compensated by the antiaromaticity and strain of the 5-membered borole ring. Weak Lewis bases (LBs), e.g., THF and CH 3 CN, both bind to these four uorinated 9-borauorenes. Aer introduction of a Lewis base, the orange solutions of 5a and 5c, or lime green solution of 5b, become colorless, 117 and the red solution of 5d turns pale yellow. 111 This color change is due to the interruption of p p (B)-p* conjugation upon coordination of the Lewis base to the boron center, which results in a higher LUMO energy. 66 In a CH 3 CN competition reaction between 5b and B(C 6 F 5 ) 3 $CH 3 CN at 25 C, an equilibrium constant of ca. 1.3 was found indicating a preference for formation of 5b$CH 3 CN vs. B(C 6 F 5 ) 3 $CH 3 CN. 117 In another competition experiment, with the bulkier THF as the base (in a ratio of 1 : 1 : 1 for 5b, B(C 6 F 5 ) 3 and THF in d 8 -toluene), only the 5b$THF adduct was observed by NMR spectroscopy. Applying the Childs method, 5a and 5b have a relative Lewis acidity value of 0.58 AE 0.02 and 0.70 AE 0.02, respectively, which is only slightly higher than that of the corresponding MeB(C 6 F 5 ) 2 (0.56 AE 0.02) and B(C 6 F 5 ) 3 (0.68 AE 0.02) obtained by Piers. 61 For the smaller Lewis base CH 3 CN, 5b and B(C 6 F 5 ) 3 show comparable Lewis acidities, but for the larger Lewis base THF, 5b shows a much stronger Lewis acidity than B(C 6 F 5 ) 3 . Based on these results, the authors concluded that the Scheme 7 9-Borafluorenes with a fluorinated backbone. relative Lewis acidities of 5b and B(C 6 F 5 ) 3 are determined by steric factors, rather than the antiaromaticity of 5b. Addition of Cp 2 Zr(CH 3 ) 2 to 5a or 5b, leads to Me À abstraction, and the corresponding ion pairs are formed rapidly. 61 Both of them are remarkably more stable than their corresponding borane ion pairs in toluene (the ion pairs formed from MeB(C 6 F 5 ) 2 and Cp 2 Zr(CH 3 ) 2 can exchange a C 6 F 5 group from MeB(C 6 F 5 ) 2 with a methyl group from Cp 2 Zr(CH 3 ) 2 , resulting in Me 2 B(C 6 F 5 ) and Cp 2 Zr(CH 3 )(C 6 F 5 ) under similar conditions). 118 The combination 5a/Cp 2 Zr(CH 3 ) 2 and 5b/Cp 2 Zr(CH 3 ) 2 are more active and stable than their corresponding borane/Cp 2 Zr(CH 3 ) 2 ion pairs as activators for olen polymerization. To investigate further the coordination chemistry of 5a and 5b, [Cp*Al] 4 was used. 65 Surprisingly, only the thermally robust h 1 Lewis acidbase adduct was observed. Thus, the fragment of Cp*Al behaves only as a Lewis base rather than as a two-electron reducing agent. The reaction of Cp*Al with the less Lewis acidic PhBf also provides the h 1 Lewis acid-base adduct. Alternative routes to h 5 9-borauorene aluminum complexes via reaction of PhBfLi 2 with Cp*AlCl 2 (THF) were also unsuccessful. The reaction of 5b with L tBu ScR 2 (L tBu ¼ ((Ar)NC( t Bu)CHC(R)N( t Bu)), Ar ¼ 2,6-i Pr-C 6 H 3 ) produced the corresponding contact ion pairs, the structures of which were thoroughly investigated both in solution and the solid state. 119 Ortho-phenylene-bridged diboranes are interesting compounds and can be applied as co-initiators for olen polymerizations. 120 Depending on the binding position of the Lewis base, ortho-phenylene-bridged diboranes (and diborole 5c) can adopt inner or outer facial coordination modes (Scheme 8). [112][113][114]121 By adding a neutral Lewis base, e.g., CH 3 CN or THF, coordination to the less sterically encumbered outer face of 5c was observed exclusively. 109 This is in contrast to the corresponding diborane C 6 F 4 -1,2-[B(C 6 F 5 ) 2 ] 2 , to which CH 3 CN coordinates to the inner face. The authors suggested that this is likely a result of less strain in the outer coordination mode caused by pyramidalization of the boron center in 5c; it may also be a result of the more rigid adjacent borauorene's steric interaction which prevents the inner coordination mode in 5c. The reaction of 5c with PhCMe 2 X (X ¼ Cl, OMe or N 3 ) gives thermally stable and isolable ion pairs which feature a weakly coordinating anion (WCA). The application of these ion pairs as initiators for isobutene polymerization were studied and the results show that the combination of C 6 F 4 -1,2-[B(C 6 F 5 ) 2 ] 2 with PhCMe 2 X is more suitable than 5c with PhCMe 2 X. 122 While peruoropentaphenylborole reacts rapidly and irreversibly with dihydrogen (H 2 ), 123,124 5b is inert to H 2 under various conditions. Therefore, 5d, which is a structural hybrid of 5b and peruoropentaphenylborole, was designed and synthesized. 110 The reaction between 5d and H 2 was investigated experimentally and by theoretical calculations. Compound 5d reacts reversibly with H 2 , but side reactions occur resulting in only limited turnover numbers of this metal-free H 2 activation reaction. Compound 5d has a comparable Lewis acidity to that of peruoropentaphenylborole but exhibits a much better solubility than per-uoropentaphenylborole in non-coordinating solvents. Due to the better solubility of 5d, a low temperature experiment between 5d and Et 3 SiH was possible. 111 The borole-silane complex formation in d 8 -toluene was studied by variable-temperature NMR spectroscopy. The trends of the Si-H coupling constant and the infrared stretching frequency of the Si-H bond as a function of temperature, and the molecular structure of the complex determined by X-ray diffraction (Fig. 3), clearly prove that an interaction exists between the boron atom and the silicon atom through the Si-H bond. These direct observations thus conrmed the previously proposed mechanism, i.e., that peruoroarylboranes catalyze the hydrosilylation of C]C, C]N and C]O bonds via borane activation of the Si-H bond, not via a classical Lewis acid/base adduct process. 125

Donor-acceptor 9-borafluorenes
The 9-borauorenes exhibit a weakly allowed lowest energy absorption which extends into the visible region. This absorption was attributed to the low-lying LUMO which originates from the p p -p* conjugation through the vacant p z orbital of boron. 126 By incorporating electron donating group(s) or electron withdrawing group(s) at different positions, the photophysical properties can be modied (Scheme 9). In this section, the 9-borauorenes are classied according to their functional groups at different positions.  Pioneered by Yamaguchi and co-workers in 2002, three functionalized TipBfs (6a-6c) with donors (methoxy and amine groups or methoxy and thiophene groups) at the m-and ppositions were reported (Scheme 10). 66 Compared with the nonfunctionalized TipBf, both the absorption and emission of these functionalized TipBfs are red shied and the quantum yields decrease ( Table 2). The red shied absorption and emission of donor-functionalized TipBfs were attributed to intramolecular charge transfer (ICT). Addition of F À (or coordinating solvents) leads to a blue shi of both the absorption and emission of these functionalized TipBfs and, the quantum yields dramatically increase to ca. 0.5-0.9. Thus, these functionalized TipBfs can be applied as F À sensor. In contrast to tri(9-anthryl)borane, which loses its uorescence properties aer coordination with F À and was labeled as a "turn off" sensors, 1 due to the increase of the emission intensity aer adding F À , these functionalized 9-borauorenes were termed uorescence "turn on" sensors.
Six years later, the same group synthesized another two 9-Mes*-borauorenes (7a and 7b) with donors (amine or thiophene groups) at the m-positions (Scheme 10). 103 Compared with the non-functionalized Mes*Bf, the molar extinction coefficients are much higher and a red shi was observed in both absorption and emission. Compared withtheir corresponding 9-Tip-9-borauorenes, the uorescence quantum yields of 9-Mes*-9-borauorenes are higher, which is most likely due to the restricted rotation of the bulky Mes* group. The bulky Mes* group also leads to enhanced stability of these 9-Mes*-9-borauorenes, which paves the way for their application as accepting units in organic (opto)electronics.
In 2016, Rupar and co-workers synthesized 6d with two methoxy donors at the p-positions (Scheme 10). The lowest energy absorption and emission peak of 6d appear in the same range as that of TipBf, and the quantum yields are also the same. The similar photophysical properties of these two compounds may be due to the weak donating ability of the methoxy groups. 88 Encouraged by the wide application of carbazole as a donating group and 9-borauorene as an accepting group, the Zhao group synthesized three ladder-type B,N-bridged p-terphenyls, with indole fused at the p-, m-positions (8a and 8b) or o-, m-positions (8c) on one side of the borauorene (Scheme 11). 127 Later, the same group replaced the indole with benzothiophene and reported another two ladder-type B,S-bridged pterphenyls (9a and 9b). 128 In these ladder-type boroles, the products of fusing at the p-, m-positions (8a, 8b and 9a), are tolerant to air and moisture, but fusing at the o-, m-positions (8c and 9b) leads to products that show slow decomposition in dilute solution in air (no specic solvent is mentioned), which is probably caused by steric congestion. Both absorption and emission show negligible solvatochromism for these laddertype boroles which indicates only a small polarity change between the ground state and the excited state. No difference of absorption and emission was observed between N-methyl 8a and N-phenyl 8b, which may be due to the large torsion angle between the phenyl group and pyrrole. Compared to 8a, 8c shows a red shi in both absorption and emission which was attributed to the lower LUMO energy as evidenced by computational studies. Compared with carbazole-fused 9-bora-uorenes, the benzothiophene fused 9-borauorenes show only slight hypochromism of both absorption and emission but double the quantum yields. In addition to the broad use of 9-borauorenes as F À sensors, 9a was applied as a Hg 2+ sensor because of its high affinity for Hg 2+ due to the S atom. Additionally, these ladder-type boroles exhibit considerable potential for application as bipolar electron-transporting materials.
In contrast to the incorporation of donating groups at the 9-borauorene core, more recently, Marder and co-workers reported compound 1a with four inductively withdrawing CF 3 groups attached to the o-and p-positions at the biphenyl main core (Scheme 12). 68 To investigate the effect of substitution at the p 0 -position in 1a, 1b with a CF 3 electron withdrawing group at the p 0 -position and 1c with an NMe 2 electron donating group at the p 0 -position were also synthesized. Although examples appear in patents, 130 1c is the only example of a 3-coordinate 9-borauorene which incorporated a donor at the exo-aryl to have been reported in a paper. 68 Due to the strong electron withdrawing ability of the four CF 3 groups at the 9-borauorene core, the electron accepting ability of boron was greatly enhanced, as evidenced by cyclic voltammetry (see below in the Electrochemistry section). With the extra protection of two CF 3 groups at the o-positions, 1a and 1b are more stable than F MesBf. Surprisingly, although 1c has a strong donor at the p 0position, its absorption appears in the same region as those of 1a and 1b (this could also be caused by the weak absorption of 1c in the lower energy region), but the emission shows a large red shi (1a: l em ¼ 510 nm; 1b: l em ¼ 521 nm; 1c: l em ¼ 627 nm in hexane). Both 1a (s F ¼ 151 ns) and 1b (s F ¼ 224 ns) exhibit very long uorescence lifetimes in hexane, but behave differently; 1c exhibits two radiative processes (s p ¼ 9.2 ns and s d ¼ 1.6 ms), the latter resulting from thermally activated delayed uorescence (TADF). Compound 1c is the rst example of a borauorene to exhibit TADF, but the rather low quantum yield (F F ¼ 0.03 in hexane) limits its further application. In contrast to the low quantum yield of 1c in hexane, 1a (F F ¼ 0.30) and 1b (F F ¼ 0.37) exhibit relatively high quantum yields. Theoretical studies indicate that the LUMO of 1c is located on the biphenyl core with a large contribution from the boron atom, whereas the HOMO is located on the exo-aryl moiety. Thus, the HOMO to LUMO transition is an ICT process with a small overlap coefficient (L) which also ts the requirement for TADF.

Heteroarene-fused boroles
In 2011, the Yamaguchi group fused electron-rich thiophene(s) onto boroles by stepwise substitution reactions and synthesized 3a-3c (Scheme 13). 77 Surprisingly, these three Tip-protected thiophene-fused boroles are air-and moisture-sensitive. Considering that TipBf is stable enough to be puried in air, this instability is in opposition to an expectation that applying electron-rich thiophene would decrease the Lewis acidity of boron to form more stable compounds. The antiaromaticity of the borole rings was evaluated by DFT calculations of the nucleus-independent chemical shis (NICS) values ( Table 3).
The NICS(1) ZZ (ppm) values increase in the order TipBf < TipB < 3b < 3a < 3c. Thus, the biphenyl-fused borole, TipBf exhibits less antiaromatic character whereas the antiaromaticity of the electron-rich thiophene-fused boroles is enhanced, and is even higher than that of the non-fused "free" borole (1-Tip-1borole, TipB), as suggested by the NICS(1) ZZ values. This result is also opposite to the conventional understanding that fusing electron-rich aromatic arenes decreases the antiaromaticity of the 5-membered borole ring. 131,132 To study what governs the antiaromaticity and Lewis acidity of heteroarene-fused boroles, another three heteroarene-fused boroles, 3d-3f were synthesized (Scheme 13). NICS calculations were Scheme 12 9-Borafluorenes with four CF 3 groups at the biphenyl core.

Scheme 13
Electron-rich heteroarene-fused boroles. conducted with the geometries derived from the crystal structures of these heteroarene-fused boroles (Fig. 4). The conclusion reached was that the smaller extent of bond alternation in the 5-membered borole ring in heteroarene-fused boroles is responsible for the high degree of antiaromaticity. Theoretical and experimental studies suggest that the LUMO energy of these heteroarene-fused boroles are relevant to the antiaromaticity, which also linearly correlates with their Lewis acidities. 105 In contrast to the Yamaguchi group's fused boroles with electron-rich heteroarene(s), more recently, Marder and coworkers 129 switched to the electron-poor pyridine to synthesize phenylpyridyl-fused boroles (Scheme 14). Using 4-phenylpyridine to prepare a fused borole, [10a] 4 was obtained as a white solid and adopts a unique coordination mode, forming a tetramer with a central cavity in both the solid state (X-ray diffraction,  136 and sterically hindered dibenzoborole$pyridine (1.638(3)Å); 75 however, in contrast to both pentaphenylborole$2,6-lutidine and the sterically hindered dibenzoborole$pyridine which dissociate in solution at room temperature, [10a] 4 persists as a tetramer in C 6 D 6 even at 50 C ( 1 H DOSY). By switching 4-phenylpyridine to 2-phenylpyridine, 10b was prepared and isolated as a light yellow solid. The boron center of 10b is 3-coordinate in solution but 4-coordinate in the solid state, as evident from solid-state 11 B{H} RSHE/MAS NMR measurements. The difference is ascribed to the steric protection of the pyridine nitrogen by the attached phenyl group at the 2-position.
Due to the inherent electron withdrawing properties of pyridine, the electron accepting ability of 10b is enhanced (see Electrochemical section below). The lowest energy absorption maximum of [10a] 4 appears at 322 nm in hexane, which is blue shied compared to those of 10b (375 nm) and other 3-coordinate 9-borauorenes. Compared with other Tip-protected 9-borauorenes, 10b exhibits a relatively high quantum yield (0.34 in hexane) in solution and shows an interesting dual uorescence property. Two lifetimes are observed at the same emission wavelength of 520 nm. The authors suggested that the dual uorescence in solution is caused by an equilibrium between the free 3-coordinate 10b and a weak intermolecular coordination adduct of 10b. This hypothesis was further supported by lifetime measurements at different concentrations, low temperature excitation spectra, low temperature 1 H NMR spectra and lifetime measurements upon addition of DMAP to a solution of 10b to simulate the 4-coordinate 10b species. Thus, this dual uorescence is different from dual uorescence induced by B-N dissociation in the excited state. 137 Interestingly, the ratios of the relative percentage of the two lifetimes shows a linear relationship with temperature.

Intramolecular dative bond in 9borafluorenes
Instead of using bulky Tip or Mes* as the protecting group at boron, Chujo and co-workers used the Mamx ligand (Mamx ¼ 2,4-di-tert-butyl-6-[(dimethylamino)methyl]phenyl) as the steric protecting group at boron in 9-borauorenes (Scheme 15). 138 The X-ray crystal structure of MamxBf indicates that the nitrogen atom coordinates to the boron atom with a B-N bond length of 1.712Å. The 11 B NMR spectrum shows a peak at 5.96 ppm, which is in the typical range for 4-coordinate boron. With the double protection of steric hindrance and nitrogen atom coordination to boron, MamxBf is stable to moisture and can be puried in air. The lowest energy absorption of MamxBf is at ca. 280 nm and it exhibits a weak emission at ca. 360 nm. Interestingly, by addition of B(C 6 F 5 ) 3 to a benzene solution of MamxBf, phosphorescence (s p ¼ 8.95 ms (69%)) with a peak at 597 nm was observed at room temperature, which the authors suggest is caused by triplet exciplexes. Theoretical analysis for the excited state of MamxBf suggests that this robust B-N coordination in the ground state is cleaved in the S 1 state. This B-N bond cleavage in the excited state is also suggested to be responsible for the weak emission of MamxBf.
By incorporating electron withdrawing or electron donating groups at the biphenyl core, the energy of 9-Mamx-9-borauorenes are modulated and emission from bond-cleavage-induced intramolecular charge transfer (BICT) was realized. Compound 11a has two electron withdrawing triuoromethylphenyl groups at the biphenyl core and it shows a single emission with a peak at 373 nm, which is similar to that of MamxBf. Changing the electron withdrawing triuoromethylphenyl groups to electron-neutral phenyl groups or electron donating methoxyphenyl groups in 11b and 11c, respectively, result in similar shaped dual emissions (ca. 330 nm and ca. 520 nm, respectively), which is in contrast to the single emission of 11a and MamxBf (Scheme 15). The authors concluded that by incorporation of electron donating groups at the biphenyl core, the boron atom exhibits a more negative charge, and a BICT process thus occurs which results in dual emission. Theoretical calculations further support the BICT transition. The short wavelength emission was assigned to a locally-excited (LE) emission from a p-p* transition and the long wavelength emission was assigned to the BICT transition. The BICT emission is highly sensitive to the solvent viscosity and, thus, 11c can be applied as a ratiometric sensor. 139 By exchanging the strongly donating dimethylamine group with the weakly donating methoxy group, Rupar and co-workers synthesized 12a (Scheme 16) 140 which is a colorless powder that is air-stable in the solid state and solution. The absorption maximum appears at 284 nm and the emission maximum appears at 536 nm with a long lifetime (s F ¼ 122 ns) in CH 2 Cl 2 . This is an extraordinary large Stokes shi (16 500 cm À1 ) for a small molecule; in fact, it is the largest Stoke shis ever reported. 141 This large Stokes shi is caused by the photodissociation of the B-O dative bond in the excited state, which is further supported by theoretical studies. By changing the methoxy groups to methylthio groups or tert-butoxy groups, 12b and 12c were synthesized, respectively, which show nearly identical structural and optical properties to that of 12a. By incorporation of two bithiophene groups as donors at the biphenyl core of 12a, the photophysical properties change signicantly. The lowest energy absorption of 12d red shis to 408 nm and the compound exhibits dual emission with peaks at 446 (s F ¼ 0.5 ns) and 639 nm (s F ¼ 4.38 ns). DFT calculations indicate that two stable structures are present in the excited state: in one, the B-O bond remains intact (4-coordinate excited state) and in the other one, the B-O bond dissociates (3-coordinate excited state). The shorter wavelength emission exhibits the shorter lifetime and is assigned to the emission from the 4coordinate S 1 state. The long wavelength emission exhibits the longer lifetime and is assigned to the 3-coordinate S 1 state.
The Gabbaï group synthesized a diborane (13) with a BMes 2 group and a 9-borauorene group at the 1-and 8-positions of a naphthalene, respectively (Scheme 17). 142 Interestingly, an interaction occurs between the boron atom of the 9-bora-uorene and one of the Mes groups. This interaction was conrmed by a short B-C distance (2.730(3)Å) between the boron atom of 9-borauorene and the carbon atom of the Mes group which is connected to boron. Due to this interaction, the boron atom of the 9-borauorene is slightly pyramidalized. By changing the BMes 2 group to a diisopropylphosphino group, Bourissou and co-workers synthesized the naphthyl-protected 9-borauorene (14) 143 which is only stable under an inert atmosphere, but is much more stable than 9-(2-diisopropylphosphinophenyl)-9-borauorene. 144,145 The 11 B NMR signal appears at À8.5 ppm, conrming the presence of a P-B dative bond. The short P-B distance (2.011(2)Å) and the signicant pyramidalization ( P C-B-C ¼ 338.45 (5) ) of the boron conrmed the strong P / B interaction. This strong P / B interaction, even with fairly bulky substituents on the phosphine, indicates the exibility of the system. In addition to the above mentioned intramolecular dative bonds in 9-borauorenes, another interesting topic, namely intramolecular B-B dative bonds (one and two electron s-bonds) in 9-borauorenes which can be formed by one or two electron reductions, is discussed in the "Chemical reduction of fused boroles" section of this review (see below).

9-Borafluorene-based main chain polymers
By incorporating 3-coordinate boron atoms into the main chain of conjugated polymer systems, the p-systems are extended compared to the corresponding monomers, leading to different optical properties. 22 It could be envisaged that incorporation of more electron-decient 9-borauorenes into polymers will lead to interesting properties. 146 In 2008, Scherf and co-workers reported a co-polymer incorporating polyuorenes and 9-bora-uorenes in the main chain, and applied it as an anion sensor (Scheme 18). 147 Interestingly, the para-cyanophenyl group surprisingly stabilizes the 9-borauorene, supposedly providing good environmental stability. In contrast to the bulky Tip or F Mes groups, para-cyanophenyl is an "unprotected" phenyl group. Unfortunately, changing the para-cyanophenyl group to other "unprotected" phenyl groups provided unstable 9-bora-uorenes. Recently, Rupar and co-workers tried to reinvestigate this compound, but although different techniques were applied, they could not reproduce the reported results. 88 The Rupar group adopted Yamamoto or Stille coupling reactions to synthesize two 9-Tip-9-borauorenes based polymers (Scheme 19). 148 Compared with their monomeric precursor, a red shi of the absorption and emission was observed which can be ascribed to the extended conjugation in the polymers. Quantum yields of 0.50 (16a) and 0.28 (16b) in solution were obtained. 16a and 16b have a much smaller optical bandgap (calculated from the onset of lowest energy absorption) than polyuorenes or polycarbazoles, which is mainly attributed to the lower LUMO energies of borauorenes, and their lower LUMO energies were conrmed by measurements of their electrochemical reduction potentials and further supported by calculations. In solution, 16a and 16b are suitable F À sensors. In a lm, 16a can also be applied as a sensor for gaseous NH 3 .

Electrochemistry
In this section, selected fused boroles are chosen for comparison of their electron accepting abilities as determined by electrochemical measurements. Although many fused boroles can be reduced twice, here only the rst reduction potentials are compared. The reduction potentials for all known aryl group protected fused boroles range from À1.1 to À2.5 V (Table 4). PhBf 117 exhibits a rst reversible reduction potential at À2.16 V, which is in the same range as TipBf (À2.11 V (ref. 77) and À2.31 V (ref. 88) were obtained by two different groups) and Mes*Bf (À2.28 V). 103 Aer incorporation of donating groups (methoxy, amino, or thiophene) on the core of 9-borauorene, the rst reduction potentials of 6d, 88 7a 103 and 7b 103 do not change signicantly, indicating that the donating groups have only a small effect on the electron accepting ability of boron in 9-borauorenes.
By employing the electron withdrawing F Mes group as the exo-aryl group on a Bf, the electron accepting ability of F MesBf was enhanced and the reduction potential shis to À1.93 V. 88 The rst reduction potential of phenylpyridyl-fused borole 10b 129 is comparable to that of F MesBf, which suggests that the effect of fusing a pyridyl group onto boroles on their reducibility is comparable to that of the exo-F Mes group in 9-borauorenes. Benzothiophene-fused borole 3c 77 exhibits a rst reversible reduction potential of À1.72 V, which is less negative than the electron withdrawing group-functionalized F MesBf and 10b. The strong electron accepting property of 3c is attributed to its enhanced antiaromaticity. Diborole 4 (ref. 80) and the biphenyl-linked diborole 18 (Scheme 21) 85,150 exhibit much less negative half reduction potentials of À1.51 and À1.49 V, respectively. The strong electron accepting ability of 4 and 18 is due to the two boron centers being linked to a pconjugated unit. This enhancement is also observed in triarylboranes with two or more boron centers. 100,151-153 By incorporation of four additional CF 3 groups at the biphenyl core, the rst half reduction potentials of 1a and 1b shi to À1.21 and À1.13 V, respectively. 68 Surprisingly, although 1a and 1b exhibit extraordinarily low reduction potentials, both are stable in air.
Attempts to use noncoordinating solvents (CH 2 Cl 2 and a,a,a-triuorotoluene) for the cyclic voltammetry study of per-uorinated-PhBf, 5b were unsuccessful, so THF was employed. 117 An irreversible process with a reduction potential at À2.42 V vs. Fc/Fc + was observed. Such a negative reduction potential, which is even more negative than that of PhBf, was not expected. The authors suggested that the reduction process actually takes place at the uorinated aromatic framework, not at the boron center due to the formation of 5b$THF in THF. i Pr 2 NBf has the most negative reversible half reduction potential of À2.95 V. 88 The weak electron accepting ability of i Pr 2 NBf is caused by the electron-rich nitrogen atom p-backbonding to the vacant p z -orbital of boron.

Chemical reduction of fused boroles
Boroles readily accept one electron to form a radical anion or two successive electrons to give a dianion. The chemical reduction of non-fused "free" boroles and some 9-borauorenes was reviewed by the Braunschweig group in 2011 (ref. 48) and 2016. 49 Here we focus only on fused boroles.
One electron reduction of 9-borauorene leads to a radical anion. In 2008, the Yamaguchi group reduced Mes*Bf with potassium in THF and the EPR signal of the reduced Mes*Bf exhibits an eleven-line signal (g ¼ 2.002). 103 According to the simulation of the spectrum, the spin density on boron is estimated to be 0.21, indicating delocalization over the biphenylene unit of the 9-bora-uorene. A similar reaction was also carried out with 7b and a spin density of 0.18 on boron was estimated by simulation. The lower spin density on boron in 7b suggested that it is delocalized over the bithiophene skeleton. The Piers group synthesized the ladder-type diborole 4 which exhibits a rst reversible reduction at À1.51 V. Such a small negative reduction potential makes it possible to perform a one electron reduction with bis(pentamethylcyclopentadienyl)cobalt(II) (E 00 (Cp* 2 Co) ¼ À1.9 V). 154 Isolated [4][CoCp* 2 ] is a deep blue solid (Scheme 21). 80 The C1-C1 0 distance in [4][CoCp* 2 ] of 1.410(3)Å is signicantly longer than that of its neutral form for which d(C1-C1 0 ) ¼ 1.367(5)Å while d(B1-C1) of [4][CoCp* 2 ] with 1.524(3)Å is signicantly shorter compared to 1.571(4)Å for 4. A detailed inspection of the structure combined with a theoretical analysis shows that there is still a high degree of delocalization of the unpaired electron throughout the whole p system.
In 2014, Wagner and co-workers linked two 9-borauorenes by a biphenyl (18) and carried out the one electron reduction with lithium naphthalenide in toluene (Scheme 21), 150 obtaining [18]Li$(THF) 4 Â 0.5C 10 H 8 as black crystals. Single crystal Xray diffraction shows that the distance between the two boron centers of [18]Li$(THF) 4 Â 0.5C 10 H 8 is 2.265(4)Å, which is 0.655 A shorter than that in its precursor 18 (2.920(6)Å) (Fig. 6), 84 4 Â THF, the authors concluded that there is a strong contribution from p z orbital on each boron to the SOMO. This is similar to the situation in [18]c À , but the hyperne coupling to protons in [19]c À suggests that the spin density is delocalized over the p-systems.
The tetrakis-CF 3 -functionalized F MesBf, 1b synthesized by Marder and co-workers exhibits a rst half-reduction potential at À1.13 V, and thus can be reduced by Cp 2 Co (E 00 (Cp 2 Co) ¼ À1.3 V). 154 [1b][Cp 2 Co] was obtained as a dark purple solid with a complex EPR signal centered at g iso ¼ 2.004 in THF, consisting of hyperne splitting to boron (a( 11 B) ¼ 3.3 G), the uorine atoms (a( 19 F) ¼ 11.3 and 6.0 G) from the CF 3 groups at the 9-borauorene core, and the hydrogen atoms (a( 1 H) ¼ 6.1 and 2.9 G) at the 9-borauorene core. The relatively large proton and uorine hyper-ne couplings, in contrast to the relatively small boron hyperne coupling, indicates that the spin density is delocalized signicantly over the biphenyl core of the 9-borauorene. Compared to the crystal structure of neutral 1b, changes in bond lengths of [1b] [Cp 2 Co] were mainly observed on the 9-borauorene core, indicating delocalization of the unpaired electron on the 9-borauorene core, with no contribution from the exo-aryl F Mes group. 68 The discovery and study of 9-borauorenyl dianions were reported earlier than that of the radical anions of 9-borauorenes. In 1996, during an investigation of the reduction of sterically encumbered arylboron dihalides, the Power group isolated a bislithium-9-borauorenyl complex [ 2 are also solvated by diethyl ether and are situated almost symmetrically above and below of the 5-membered borole core. The 11 B NMR (d 8 -THF) chemical shi of [PhBf]Li 2 shows a sharp peak at 6.3 ppm. By treatment of diborole 4 with potassium naphthalenide (2 eq.) in THF, [4]K 2 was isolated as a red solid with a 11 B NMR (C 6 D 6 ) chemical shi of 32.1 ppm. Its Xray crystal structure indicates that the two potassium atoms are situated above and below the center of the dibenzo-fused-diborole core in a centrosymmetric arrangement. 80 Similarly to diborole 4, compound 19 possesses two 9-borauorene moieties and thus can easily accept two electrons. 158 neutral form 19). The spectroscopic and structural parameters clearly suggest the presence of a covalent two center-two electron (2c-2e À ) B-B bond in [19]Li 2 , which was further supported by theoretical studies.
The Wagner group also investigated the redox chemistry of 9-H-9-borauorene, 21 (Scheme 22). 159,160 Upon reduction, adducts arising from extensive structural rearrangement were   [Na$THF], the authors draw the conclusion that the two added electrons are delocalized over the 9-borauorene core, rather than being localized at the p z -orbital of boron. [21] 2À is the rst example of a hydride ligand-stabilized 9-borauorene anion. Due to the easy abstraction of the hydride, [21] 2À is a surrogate of a nucleophilic 9-borauorene anion. The reaction of [21] 2À with MeI and Et 3 SiCl further proves that [21] 2À can be applied as a 9-borauorene anion. Similarly, reaction of B-B dianions with MeI led to the formation of 9-methyl-borauorenes. 161,162 At the same time, radical reactivity of [21] 2À was found, e.g., by the reaction of [21] 2À with Me 3 SnCl or (bromomethyl)cyclopropane.

Three-coordinate borafluorenium cations
Instead of adding electrons to Bfs in a reduction process, another interesting topic is extracting an anion from 3-coordinate Bf to generate a 3-coordinate borauorenium cation. In 1985, Nöth and co-workers applied GaCl 3 and AlCl 3 as a Cl À acceptor for the pyridine adduct of ClBf and the acridine adduct of ClBf (Scheme 23). 163 Scheme 24 Carbene-stabilized borafluorenium cations.  6 ], the solution became colorless. When this solution was heated to 40 C, the red color was recovered. Aer cooling, this solution became colorless again. These phenomena further support the hypothesis of an intermolecular O/B interaction at low temperature.

Conclusions and outlook
This review begins with the fundamental synthetic strategies for preparing (hetero)arene-fused boroles and the stability of different 9-substituent-9-borauorenes, and then discusses functionalized (hetero)arene-fused boroles which can be applied as Lewis acids, activators of H 2 , uorescent materials, electron accepting units, etc. For functionalized (hetero)arenefused boroles, the chemistry of reported 9-borauorenes is classied, and a guide for the design of novel (hetero)arenefused boroles to achieve different properties is provided.
Compared to the corresponding triarylboranes, (hetero) arene-fused boroles exhibit an enhanced electron accepting ability, which is attributed to the antiaromaticity and strain of the 5-membered borole ring. Triarylboranes have found wide application, e.g., as acceptors in TADF materials. The exo-aryl group of 9-aryl-9-borauorene adopts a twisted arrangement with respect to the 9-borauorene core and thus, by functionalization, may also generate good candidates for TADF materials. Surprisingly, thus far, only one such example was reported outside of patents. More studies on the functionalization of the exo-aryl moiety will be of particular interest.
Compared to non-fused "free" boroles, arene-fused boroles exhibit higher stability and potential for functionalization. Depending on the fused aryl groups, enhanced electron accepting ability and enhanced antiaromaticity, even greater than that of non-fused "free" boroles, unique coordination modes, and dual uorescence can be realized. Heteroarenefused boroles are interesting compounds which require further study, (e.g., other electron-rich or -poor heteroarenefused boroles), as they have many potential applications.

Conflicts of interest
There are no conicts to declare.