Chinese Chemical Letters  2016, Vol. 27 Issue (8): 1131-1138   PDF    
Triarylborane π-electron systems with intramolecular charge-transfer transitions
Sun Zuo-Banga, Li Sheng-Yonga, Liu Zhi-Qiangb, Zhao Cui-Huaa     
a School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials Ministry of Education, Shandong University, Jinan 250100, China ;
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
Abstract: The incorporation of B element into p-conjugated system is an efficient strategy to tune the steric and electronic structure and thus optoelectronic properties of π-electron systems. The vacant p orbital on the tricoordinate B center makes it exhibit several electronic and steric features, such as electron-accepting ability through p-π* conjugation, the high Lewis acidity to coordinate with Lewis bases, as well as the steric bulk arising from the aryl substituent on the B center to get enough kinetic stability. As a result, the boryl group is a very unique electron acceptor. When an electron-donating amino group is present, the triarylboranes would display intense intramolecular charge transfer transitions, which lead to interesting optoelectronic properties and great utilities. This short review summarizes the recent progress in π-electron systems, which contain both B and N elements and thus display intramolecular charge-transfer transitions. The triarylboranes are introduced based on their structural features, including the linear π-system with boryl and amino groups at the terminal positions, the lateral borylsubstituted π-system with amino groups at the terminal positions, the biphenyl π-system with an amino and a boryl groups at o,o'-positions, nonconjugated U- and V-shaped π-system, macrocylcic π-system with B and N embedded in the ring, B,N-bridged ladder-type π-system, as well as the polycyclic π-system with B embedded in the center.
Key words: Triarylborane     Electron-acceptor     Electron-donor     Charge-transfer    
1. Introduction

Pioneered by the seminal work of Willams and Kaim [1, 2], tricoordinate B-containing π-electron materials have experienced rapid progress over the past two decades [3].The vacant p orbital on the tricoordinate B center makes it exhibit several electronic and steric features (Fig. 1)[3]. First of all, B center can act as an excellent acceptor via the p-π* conjugation between the vacant p orbital and the π*-orbital of the p-conjugated framework. Kaim et al. demonstrated that trivalent B is isoelectronic to carbocation [2]. Inaddition, trivalent B center is Lewis acidic and easilycomplex with Lewis bases, causing disruption of p-π* conjugation and thus distinct changes in the absorption and emission spectra, which is the basis for their uses in anion sensing. Moreover, bulky ortho-substituted aryls, such as 2, 4, 6-trimethylphenyl (mesityl) and 2, 4, 6-triisopropylphenyl (TIP), are generally required to be introduced on B center to prevent the nucleophilic attack from water in air, providing enough kinetic stability and great steric bulk. As a result, the boryl group is a very unique electron acceptor compared with the general electron- withdrawing groups, such as nitro, cyano groups. In contrast to B, trivalent N is isoelectronic to carboanion and can act as an efficient electron donor due to the presence of lone pair. And thus when an amino group is present, the triarylboranes would display intense intramolecular charge transfer (CT) transitions, which have been exploited extensively in a wide range of applications, such as nonlinearoptics [4, 5], two-photon optics [6, 7], electron transporters and emitters in OLEDs [8-11], as well as fluoride or cyanide anion sensors [12-15]. Initially, the triarylboranes with intramolecular CT were mainly characteristic of linear or star-shaped structures. With the advances in this field, recent efforts have been paid to increasing the electron-accepting ability ofboryl group [16].In addition, many new structures have also been disclosed, such as lateral boryl- substituted π-system withamino at terminal positions [17, 18], o, o'- substituted biphenyl π-system [19], nonconjugated U- or V-shaped π-system [20], macrocyclic π-system with B and N embedded in the ring [21], B, N-bridged ladder-type π-system [22], as well as polycyclic π-system with B embedded in the center [23, 24], which has greatly expanded the applications of this class of materials. Herein, this short review summarizes the recent advances in triarylboranes with intramolecular CT characteristics. For convenience, they will be discussed according to their structural features.

Figure 1. Characteristic features of B element in the triarylborane-based π-system

2. Triarylborane-based π-system with intramolecular CT characteristics 2.1. The linear π-system with boryl and amino groups at the terminal positions

The linear system is one classic type of triarylboranes, which have been investigated most widely and most early. The combination of an electron-accepting boryl group with an electron-donating amino group results in intramolecular CT transition, which generally gives rise to intriguing fluorescence properties with large Stokes shift. The extensive studies on this type π-system have demonstrated their great potential applications, such as nonlinear optics, two photon optics, OLEDs, and fluorescence sensors. Most of the linear organoboranes employ dimesitylboryl, in which two mesityl substituents were introduced on the trivalent B center mainly to provide enough steric protection. Considering that the electron-accepting ability of the boryl group plays an important role in determining the overall performance of the material, T. B. Marder and coworkers have recently devoted great efforts to increasing the electron-accepting ability of boryl group [16]. One strategy is to displace mesityl by a bulky aryl, which has strong electron-withdrawing effect at the same time. Firstly, they introduced two fluoromesityl (FMes, 2, 4, 6- tris(trifluormethyl)phenyl, ) substituents on the B and prepared a series of (FMes)2B-substituted compounds, 1-3 (Fig. 2), which are characteristic of the donor-acceptor structure [16a]. All these compounds show good air-stability, suggesting enough steric protection from FMes. Compared with the Mes2B substituted analogue 4, compound 3 displays a larger quinoidal distortion, as determined by X-ray crystallography. In addition, the emission of 3 is much red-shifted and more solvent dependent. The fluorescence of 3 shifts from yellow (λmax = 563 nm) in hexane to red (λmax = 743 nm) in THF while the emission of 4 only changes from blue (λmax = 410nm) in hexane to blue-green (λmax = 497 nm) in CH3CN. Moreover, the reduction potential of 3 is much lower than 4. All these results suggest (FMes)2B is a much stronger acceptor.

In addition to (FMes)2B, the same group also modified the (Mes)2B through substitution of the methyl substituents para to the B atom by electron-withdrawing perfluorophenyl and 3, 5- bis(trifluoromethyl)phenyl substituents, to produce (2, 6-Me-4- C6F5-C6H2)2 ((Pfp)2B) and (2, 6-Me2-4-(3, 5-(CF3)2-C6H3)-C6H2)2B ((Tfp)2B) as new acceptors [16b]. The comprehensive crystallographic, photophysical, electrochemical and theoretical studies on a series of donor-bithienyl-acceptor compounds 5-8 (Fig. 2), which contain triphenylamine as a donor and different boryl as an acceptor, established that the electron-accepting ability increases in the order of (Mes)2B < (Pfp)2B ≈ (Tfp)2B << (FMes)2B. The compound 8 with the strongest acceptor (FMes)2B suffered from the strongly quenched emission in solution while the other analogues show efficient green to red (Φf = 0.80-1.00) emission, depending on solvent polarity. In MeCN, intense near infrared (NIR) emission could be achieved for derivatives (9, 10) containing the moderately strong acceptors (Pfp)2B and (Tfp)2B and a stronger donor 1, 1, 7, 7-tetramethylijuloidine donor (Φf = 0.27-0.48). This represents the first example of efficient NIR emission from triarylborane compounds, making them particularly attractive for practical applications.

Figure 2. The representative organoboranes of linear π-system with boryl and amino groups at the terminal positions.

2.2. Lateral boryl-substituted π-system with amino groups at the terminal positions

When the boryl group is introduced at the terminal position of a linear π-system, it is mainly its electronic effect that has great influences on electronic structure and thus properties. Yamaguchi and coworkers have presented a new molecular design, in which the bulky Mes2B groups were introduced on the side position of electron-donating p-framework (Fig. 3a). In addition to the electron-accepting ability, the steric bulk of lateral boryl also plays an important role in tuning the property via control of the steric structure of molecules [17a]. It was envisioned that the electron-accepting ability of boryl would lead to intramolecular CT transition and large Stokes shift. The large Stokes shift would accordingly decrease the spectral overlap between absorption and emission and thus suppress the energy transfer of Foster mechanism in the solid state. In addition, the steric effect of lateral boryl group would cause twisted main chain structure, which can separate the molecules from each other and prevent the energy transfer of Dexter mechanism in the solid state. Accordingly this system was supposed to be highly emissive not only in solution, but also in the solid state [17a]. The utility of this molecular design was first demonstrated by a Mes2B-substituted phenyleneethynylene derivative 11 (Fig. 3b). The X-crystal structure confirmed its significantly twisted main chain, in which the dihedral angle between the central and terminal benzene rings is 47.5°. Intense fluorescence was observed in either benzene solution (λF = 559 nm, Φf = 0.98) or spin-coated film (λF = 562 nm, Φf = 0.90). In contrast, the analogues 12 and 13, which contain the bulky neutral triisopropylsily (TIPS) or electron-accepting cyano groups instead of Mes2B, only show intense fluorescence in benzene solution (λf = 389 nm, Φf = 0.92 for 12; λf = 465 nm, Φf = 0.98 for 13). The emission is remarkably quenched in the solid state(λf = 395nm, Φf = 0.39for 12; λF = 489nm, Φf = 0.29 for 13), suggesting both the steric bulk and electronic effect oflateral boryl groups are essential to suppress fluorescence quenching in the solid state.

Figure 3. (a) Schematic representation of lateral boryl-substituted π-system. Reprinted with permission from ref. [17a]. Copyright (2006) American Chemical Society; (b) structures of lateral boryl-substituted phenyleneethynylene 11 and related analogues 12 and 13.

Based on this molecular design principle, the intense solid-state emission covering the whole visible range from blue (477 nm) to deep red (660 nm) can be achieved by attaching various electron- donating groups to the core unit of 3-boryl-2, 2'-bithiophene (Fig. 4) (17d). In addition, this molecular design concept can also be extended to polymers [17b]. A series of poly(aryleneethynylene)s with bis(4-hexyl-2, 6-dimethylphenyl)boryl (HDMP)2B at side chains show intense fluorescence emissions both in solution and in solid-state films (Φf = 0.36-0.62) (Fig. 5). Although a large number of fluorophores are known to be highly fluorescent in dilute solution, most of them suffer from the aggregation-induced quenching effect and thus are weakly emissive or even nonemissive in the solid state. However, fluorescent materials are often used in the solid state, for example, in organic optoelectronic devices, such as OLEDs and organic solid-state lasers. Therefore, it is an important and challenging issue to obtain highly emissive organic material with intense fluorescence even in the solid state [25]. The introduction ofbulky electron-accepting boryl groups at the lateral positions of electron-donating π-electron framework would possibly provide an efficient molecular design for highly emissive organic solids.

Figure 4. (a) Molecular design approach; (b) structures; and (c) pictures for fluorescence of compounds containing 3-boryl-2, 2'-bithiophene as a core unit. Reprinted with permission from ref. [17d]. Copyright (2007), Wiley-VCH.

Figure 5. Structures oflateral boryl-substituted poly(aryleneethynylene)s. Reprinted with permission from ref. [17b]. Copyright (2007) American Chemical Society.

2.3. Biphenyl π-system with an amino and a boryl groups at o, o'-positions

Very recently, our group have disclosed a new organoborane π-system, in which an amino and a boryl groups are introduced at lateral o, o'-positions of biphenyl skeleton [19]. Owing to the steric and electronic effect of lateral groups, this system was envisioned to display highly twisted main chain structure and intramolecular CT transition, which are two important features for intense solid state emission [17]. Moreover, the conformation of biphenyl skeleton might be highly dependent on the steric bulk of lateral groups. The boryl and amino groups might be arranged at the same side or on the two opposite sides of biphenyl axis due to the free rotation of single bond between two phenyl rings of biphenyl framework (Fig. 6a). The change of conformation would likely cause great changes of photophysical properties. To verify our points, we first designed and synthesized biphenyl derivative 14, which contains a Mes2B and a dimethylamino (Me2N) groups [19a]. Just as expected, the biphenyl unit is highly twisted with torsion angle up to 70.7°. It was interesting to find that despite the remarkable steric congestion between Mes2B and Me2N, they are still arranged at the same side of biphenyl axis with a close B - N distance (3.59 Å), suggesting possible direct electronic interaction between B and N centers. With regard to the photophysical properties, apart from intense solid state emission (Φf = 0.47 in cyclohexane; 0.86 in spin-coated film), one notable feature is that the absorption band corresponding to the first excited state transition is hardly distinguished due to the highly twisted main chain structure, which leads to the very poor HOMO-LUMO overlap and makes the first excited state almost prohibited. In addition, considering the highly twisted main chain structure and very close B…N, the charge transfer takes place most likely through space rather than through bond. Most notably, the emission of this compound is particularly long (λF = 521nm in cyclohexane; 523 nm in spin-coated film). Compared with its normal linear regioisomer 15, the emission is red shifted by 102 nm and 53 nm for cylcohexane and spin-coated film, respectively (Fig. 6c). For this o, o'-substituted biphenyl system, it was found that the conformation is completely different when Me2N was displaced by a more bulky dibenzylamino (Bn2N) [19b], In compound 16, boryl and amino groups are not located at the same side, but on the two opposite sides instead. At the same time, the biphenyl skeleton turns more coplanar with a torsion angle of 50°. With these changes in structure, no obvious changes were observed in absorption. Only the intramolecular CT absorption band turns more obvious. However, the fluorescence is significantly blue shifted (△λF = 71 nm in cyclohexane; 62 nm in spin-coated film). The remarkable hypsochromism in fluorescence should be ascribed to conformation change as the result of steric effect difference between Me2N and Bn2N since these two amino groups has very close electron-donating ability, which was confirmed by the almost same absorption and emission spectra for the linear p, p'-substituted biphenyl analogues 15 and 17. The theoretical calculations denoted that the unique structure of 14 with close B … N and direct B … N electronic interaction is helpful to stabilize the highest singly occupied orbital (H-SOMO) in the excited state. And as a consequence, the emission properties are more easily tunable by choosing different amino groups for o, o'-substituted biphenyl π-system.

Figure 6. (a) Schematic representation of o,o'-substituted biphenyls. Reprinted with permission from ref. [19b]. Copyright (2011) American Chemical Society. (b) structures; and (c) absorption and fluorescence spectra (in cyclohexane) of o,o'- substituted biphenyls 14, 16 and related regioisomers 15, 16.

Figure 7. Structures of compounds containing (2-Mes2B-2'-Me2N)biphenyl core unit.

Promoted by the fascinating properties of compound 14, we were interested in the further modification this skeleton to tune its photophysical properties and explore the potential applications. The para-position of Me2N is highly reactive towards electrophilic substitution, and thus it was possible to introduce various electron- withdrawing or donating substituents by fomylation or iodination followed by cross-coupling reaction to produce a series of derivatives 18 (Fig. 7) [19d]. It was found the introduction of electron-withdrawing substituents would facilitate a HOMO →LUMO CT transition (18a-c). In contrast, the intramolecular CT transition is significantly prohibited when electron-donating substituents are incorporated (18d-f). Notably, the HOMO→LUMO CT transition mainly consists of the transition from the electron- donating amino group to an electron acceptor other than boryl when a strong electron acceptor such as dicyanovinyl group is present (18c). In addition, the dicyanovinyl-substituted compound 18c displays sensing abilities to discriminate F- and CN- ions. In THF solution, the F ions first bind to the B center, then attack the α- carbon atom of the dicyanovinly group, whereas the CN-anion acts on the electron-accepting centers in the reverse sequence. As a result, the absorption and emission change in different manners upon addition of F-and CN-ions. Moreover, a highly selective ratiometric bifunctional fluorescence probe 18 g for Hg2+ and F-could also be obtained viafunctionalization of (2-Mes2B-2'- Me2N)biphenyl core unit with a dithioacetal substituent [19e]. Compound 18g displays intense intramolecular CT fluorescence, even for its nano-aggregates in water. The Hg2+-promoted deprotection of the dithioacetal group generates 18a, leading to an obvious blue shift of fluorescence. And complexation of F-with the tri-coordinate B center causes significant blue shift in emission. Interestingly, the ratiometric fluorescence sensing of Hg2+ is feasible in aqueous medium consisting of almost pure water.

2.4. Nonconjugated U- and V-shaped π-system

In addition to our o, o'-substituted biphenyl system, Wang and coworkers have also disclosed a through-space CT U-shaped p- system 19 (Fig. 8), in which the donor (N(1-naph)Ph or NPA) and the Mes2B acceptor are linked to two separate biphenyl groups and further connected by naphthalene unit [20a]. In this molecule, the donor and acceptor groups have a nonplanar arrangement. The two biphenyl units are approximately orthogonal to the naphthalene ring. As a result, the intramolecular CT from amino to boryl takes place most likely through space. The through-space CT emission can be switched off by the addition of F-, which in turn activates the fluorescence from the donor chromophore. Consequently, binding of F shifts the emission from 504 nm to a shorter wavelength (λF = 453 nm) with dramatic enhancement of emission intensity. Meanwhile, the emission color changes vividly from green to blue. Hence, compound 19 can be used as a "turn-on" fluorescent sensor for F-. And the U-shaped molecule 20 acts in a similar manner. In contrast, an analogue of 19, which only contain two acceptor Mes2B groups, displays blue-shifted emission with considerably higher quantum yield. And the emission is simply quenched upon addition of F-. Probably due to the steric congestion, these two U-shaped molecules have much lower the binding constants with fluoride (K =1.4 × 104 and 4.0 × 104 L mol-1, respectively in CH2Cl2) than the normal triarylboranes.

Figure 8. (a) Structure and (b) operating principle of "turn on" fluorescent sensing for F-of U-shaped π-system. Reprinted with permission from ref. [20a]. Copyright (2006), Wiley-VCH.

The same group also investigated the V-shaped molecules 21 and 22 (Fig. 9a), in which the donor and acceptor are separated by a flexible tetrahedral silane skeleton [20c]. Similar to the U-shaped molecules 19 and 20, compounds 21 and 22 also exhibit through- space charge-transfer emission. However, the CT emission was observed only as a weak shoulder band to a much stronger π-π* transition for 22 while π-π* transition as a shoulder band to the CT emission for 21. This difference in the relative intensity of CT emission and π-π* transition arises from the different donor- acceptor distances. The theoretical calculations suggest the amino- boryl distance of 21 (10.1 Å) is much shorter than 22 (13.9 Å). Thus the increase of donor-acceptor distance causes weakening of CT fluorescence and strengthening of π-π* emission. In the presence of F- , the emissions of both 21 and 22 are greatly intensified with the fluorescence color changing from green to blue. It was notable that these two V-shaped molecules exhibit stronger binding capacity to F-than the U-shaped compounds due to the released steric congestion. As a consequence, the V-shaped molecules also can be utilized as excellent "turn-on" fluorescent sensors for F-, the operating principle of which is illustrated in Fig. 9b. These results supported that the "turn-on" fluorescence sensing of F can be achieved viacontrol of the donor-acceptor geometries in tricoordinate organoboranes.

Figure 9. (a) Structure and (b) operating principle of "turn on" fluorescent sensing for F- of V-shaped π-system. Reprinted with permission from ref. [20c]. Copyright (2007), Wiley-VCH.

2.5. Macrocylic π-system with B and N embedded in the ring

Very different from acyclic π-system, macrocycles are quite unique because of their ability to act as hosts for guest molecules and great tendency to form well-defined porous supermolecular structures in the solid state [26]. jakle's group has been recently interested in the macrocylic π-system featuring highly electron- deficient organoboranes as an integral part of the ring system (Fig. 10) [21]. jakle and coworkers first disclosed a boracycophane 23, which contain six Lewis acidic B centers [21a]. Its HOMO is localized on the six fluorence moieties while The LUMO shows delocalization of all six empty p orbitals on B with the organic π-system, suggesting efficient π-electron delocalization through B atom. Although the lowest energy absorption is symmetry- forbidden (f= 0.000), this macrocycle still displays very intense blue fluorescence (Φf = 0.98 in CH2Cl2). One unique effect of this macrocyclic structure is the high affinity for electron-rich substrates, such as F- and CN-. Anion binding can result in amplified fluorescence quenching with formation of a highly charged hexaborate species. Subsequently, same group reported the first example of ambipolar p-conjugated macrocycle 24, in which both B and N are embedded in the ring [21b]. The X-ray single crystal structural analysis confirmed electron-donating N and electron-accepting B sites are alternating in the highly symmetric ring system. Due to the cancellation of the transition dipole moments, the lowest excited state transition S0 → S1 is also symmetry forbidden. The experimentally observed longest absorption bands at 420 and 390 nm are assigned higher excited state transitions. The fluorescence is located 460 nm with a high quantum yield (Φf = 0.76 in CH2Cl2). A pronounced red shift of emission with increasing solvent polarity suggests efficient mutual interactions between B and N, which would lead to intense intramolecular CT and thus a more polarized excited state than the ground state. The electrochemical property investigations revealed macrocyclic compound 24 exhibits both reduction and oxidation potentials, denoting possible applications as ambipolar semiconductor materials. In addition, this compound can also be used as fluorescent sensor for CN-. The fluorescence still arises from emissive CT states in the presence of low level of CN-, which is in stark contrast to the only B-containing macrocycle 23. More recently, the same group also synthesized the ambipolar macro- cyles 25 and 26 to demonstrate the versatile design principles for facile access to unstrained conjugated organoborane macrocycles [21c]. These two compounds are also promising optical and electronic materials, evidenced by the photophysical property, electrochemical property and anion sensing measurements.

Figure 10. Structures of macrocyclic π-system with B and N embedded in the ring.

2.6. B, N-bridged ladder-type π-system

The ladder-type π-systems with fully fused polycyclic conjugated skeletons are attractive because of their rigid structure, leading to more efficient electron delocalization and strong intermolecular π-π interactions and thus highly desirable properties like intense fluorescence and high carrier mobility [27]. Pioneered by Yamaguchi's work on B-bridged biphenyl (dibenzoborole) [12a], we have recently described a new class of ladder type π-system [Fig. 11], B, N-bridged p-terpheyl 27, in which both electron-donating N and electron-accepting B are introduced as bridging atoms [22a]. In cyclohexane, the anti-type B, N-bridged 27 shows a weak shoulder band at 430 nm (log ε = 3.28) and a yellowish green fluorescence at 529 nm with a moderate quantum yield (Φf = 0.21). It was noted that the Stokes shift is much larger than the general ladder-type π-electron system. Another noteworthy feature of 27 is the significantly long fluorescence lifetime (τ = 82.5 ns), which suggests very slow nonradiatve and radiative decay processes (kr = 2.5 × 106s-1; knr = 9.6 × 106s-1). The theoretical calculations suggest the lowest excited state transition, which essentially consists of the transition from the HOMO spreading over the entire p-terphenyl framework to the LUMO located on the dibenzoborle moiety, is almost forbidden (f = 0.003). This is well consistent with the slow radiative decay process. The detailed comparison of photophysical properties of 27 with its analogues 28 and 29, which lack bridging N or B atom, showed that the bridging B atom exhibits a more significant effect on the tuning of photophysical properties than the bridging N atom. The introduction of bridging B atom greatly lowers the LUMO energy level and thus leads to pronounced red shift of absorption and emission. In contrast, the HOMO and LUMO energy levels are elevated with almost the same extent once bridging N atom is introduced. As a consequence, no obvious shifts in absorption and emission spectra are observed. More recently, we also synthesized B, S-bridged p-terphenyl 30 with changing of electron-donating atom from N to S [22b]. The displacement of N by S has no much influence on the absorption and emission spectra. It was interesting to find that the fluorescence efficiency of 30 is more than doubled compared with 27 (Φf = 0.55). Similarly, its fluorescence lifetime is also unusually long (τ = 78.7 ns). Moreover, the bulky TIP substituent on B center is effective to prevent intermolecular interaction in the solid state, as evidenced by the similarity of absorption and emission spectra in solution and solid state. So the B, S-bridged p-terphenyl 30 can retain intense fluorescence in the solid state (Φf = 0.44 for the spin-coated film). Considering the unique fluorescence properties, including long lifetime, high fluorescence efficiency in both solution and solid state, the B, S-bridged p-terphenyl 30 might be useful for OLEDs or bioimaging.

Figure 11. Structures of B, N/S-bridged p-terphenyls and related analogues.

2.7. Polycyclic π-system with B embedded in the center

In the design of triarylborane-based π-electron systems, one of the most important issues to overcome is the intrinsic instability towards moisture due to the presence of vacant p orbital and thus high Lewis acidity of B center. The most common adopted strategy is to introduce one or two bulky aryl substitutents to sterically protect the Lewis acidic B atom [3]. Alternatively, Yamaguchi and coworkers have recently adopted the structural constraint as a new strategy to achieve kinetic stability of triarylboranes, which is first demonstrated by the synthesis of a triphenylborane 31 and 9, 10- diphenyl-9,10-dihydro-9,10-diboraanthracene 32 [23a]. In these two molecules, the B center is locked in a planar fashion and thus kinetically stable against pyramidalization and nuclephilic attack from Lewis basis. As a result, they are both stable against air and water and could be purified by silica gel column chromatography. The validity of this strategy enabled the successful preparation of B-doped nanographene 33 [23b] and 34 [23c], in which the B atom is embedded in a fully fused polycyclic π-conjugated skeleton. The p orbital of B atom makes great contributions to the frontier orbital. And these two compounds show broad absorption over the entire visible region and fluorescence in the near-IR region (Fig. 12).

Figure 12. Structures of polycyclic molecules with B embedded in the center.

Utilizing this strategy, Hatakeyama and coworkers have very recently prepared two polycyclic organboranes 35 and 36 (Fig. 13a), which also contain two electron-donating N atoms [24]. Due to the opposite resonance effect of B and N, the HOMO and LUMO can be significantly separated in these two compounds, minimizing energy gap (△EST) between singlet and triplet excited state. Herein, these two compounds show the thermally activated delayed fluorescence (TADF) property. In addition, the structural relaxation in the excited state is greatly suppressed owing to the highly rigid structure, giving rise to the very sharp emission and small Stokes shift. This is an important improvement compared with the general TADF emitters [28], which likely display large Stokes shift and broad emission due to large structural relaxation (Fig. 13b). The OLEDs employing 35 exhibited a pure blue emission at 459 nm with a full width at half maximum (FWHM) of 28 nm, CIE coordinates of (0.13, 0.09), and a high EQE of 13.5%. The performances of OLEDs using 36 were further improved. The emission is located at 467 nm with an FWHM of 28 nm, CIE coordinates of (0.12, 0.13), and a high EQE of 20.2%. These results also support the great utility of the "structural constraint" strategy for the creation of useful organoborane materials.

Figure 13. (a) Structures and (b) design principle of polycyclic organoboranes used for TADF emitters. Reprinted with permission from ref. [24]. Copyright (2016),Wiley-VCH.

3. Outlook

In summary, it has been well demonstrated that triarylboranes characteristic of intramolecular CT are promising materials. The electron-accepting ability of boryl and degree of CT can be increased by introducing bulky electron-withdrawing aryl substituent. In addition, fascinating properties and great utilities can be achieved with careful control of molecular geometry. Despite the remarkable progress in this field, there is still much to be explored with regard to the structural diversity, structural-property relationship as well as new applications. We believe that fascinating functional organoborane π-electron materials will be further developed on the basis of electronic and structural features of B element.

Acknowledgment We are grateful to the National Natural Science Foundation of China (Nos. 21072117, 21272141, 21572120) for financial support.
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