Charge-Transfer Emitting Triarylborane Ï-Electron Systems

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Charge-Transfer Emitting Triarylborane π‑Electron Systems Sheng-Yong Li, Zuo-Bang Sun, and Cui-Hua Zhao* School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Jinan 250100, People’s Republic of China ABSTRACT: Triarylboranes have attracted significantly increasing research interest as a remarkable class of photoelectronic π-electron materials. Because of the presence of vacant p orbital on the B center, the boryl group is a very unique electron acceptor that exhibits not only electron-accepting ability through p−π* conjugation but also high Lewis acidity to coordinate with Lewis bases and steric bulk arising from the aryl substituent on the B center to get enough kinetic stability. Thus, the incorporation of a trivalent B element into π-conjugated systems is an efficient strategy to tune the electronic and stereo structures and thus the photoelectronic properties of π-electron systems. When an electron-donating group, such as amino, is present, triarylboranes would likely display intramolecular charge-transfer transitions. These kinds of molecules are often highly emissive. In addition, the geometry of the molecules has a great impact on the emission properties. In this Forum Article, we herein describe our recent progress on the charge-transfer emitting triarylborane π-electron systems with novel geometries, which include the lateral borylsubstituted π-system with amino groups at the terminal positions, the o,o′-substituted biaryl π-system with boryl and amino groups at the o,o′-positions, a triarylborane-based BODIPY system, and a B,N/S-bridged ladder-type π-system. We mainly put the emphasis on the molecular design concept, structure−property relationships, intriguing emission properties and great applications of the corresponding triarylborane π-systems.

1. INTRODUCTION With the rapid progress in organic photoelectronic functional materials, triarylborane π-electron materials have gained significantly increasing research interest.1−10 The notable structural characteristic of trivalent B is that it contains one vacant 2p orbital, which makes it exhibit unique electronic and stereo features (Figure 1).9 First, the trivalent B center has

sufficient stability of the trivalent B center, and the corresponding boryl group is denoted as dimesitylboryl (Mes2B). Another often used aryl group is 2,4,6-triisopropylphenyl (TIP), and only one TIP is enough because of its higher steric bulk.15−17 More recently, Marder et al. have explored a new ortho-substituted aryl, 2,4,6-tris(trifluoromethyl)phenyl (FMes), which can provide not only a dramatic enhancement of stability but also a significantly improved acceptor ability with respect to Mes.18−22 Moreover, tricoordinate B is Lewis acidic and easily forms complexes with Lewis bases, causing disruption of the extended π-conjugation and thus distinct changes in the absorption and emission properties (Figure 1c). Furthermore, the bulky substituents on the B center usually allow nucleophilic attack from only small anions, such as fluoride and cyanide, which is the basis for the utility of triarylboranes as selective fluoride and cyanide sensors.16,23−30 As a result, the boryl group is a very unique electron acceptor compared with the general electron-withdrawing groups, such as nitro and cyano. It has been well demonstrated that the introduction of tricoordinate B into π-conjugated systems is an efficient strategy to tune the electronic and stereo structures and thus the photoelectronic properties of the corresponding πsystems, enabling its great use in a wide range of fields, such as nonlinear optics,31−41 two-photon optics,42−50 electron transporters and emitters in organic light-emitting diodes

Figure 1. Characteristic features of trivalent B in triarylboranes.

strong electron-accepting ability as a result of the p−π* conjugation between the vacant p orbital of B and the π* orbital of the π-conjugated framework (Figure 1a).11,12 Kaim et al. demonstrated that trivalent B is isoelectronic to a carbocation based on the electrochemical properties of πsystems containing trivalent B.13,14 In addition, bulky orthosubstituted aryls are generally required to be introduced on the B center to provide kinetic protection and thus sufficient stability under ambient conditions (Figure 1b).12 Consequently, boryl might be a very bulky substituent. The most widely utilized aryl group is 2,4,6-trimethylphenyl (mesityl, Mes).1−10 In general, two Mes groups are necessary to get © 2017 American Chemical Society

Special Issue: Advances in Main-Group Inorganic Chemistry Received: November 28, 2016 Published: February 6, 2017 8705

DOI: 10.1021/acs.inorgchem.6b02847 Inorg. Chem. 2017, 56, 8705−8717

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Inorganic Chemistry (OLEDs),51−59 hosts in phosphorescent OLEDs,60−62 and highly selective fluoride and cyanide sensors.16,23−30 Because of the electron-accepting ability of trivalent B, triarylboranes would possess large electronic dipoles in the excited state,12 when an electron-donating group, such as amino, is present. The large electronic dipole would promote intramolecular donor−acceptor charge-transfer (CT) and thus the corresponding interesting emission properties, such as a high fluorescence efficiency, a red shift in emission, a large Stokes shift, and significant fluorescence solvatochromism. Initially, triarylboranes with intramolecular CT are characteristic of linear or star-shaped geometries. With the advances in this field, many new structures have been disclosed, such as a nonconjugated U- or V-shaped π-system,29,30 a macrocyclic πsystem with B and N embedded in the ring,63−65 a polycyclic πsystem with B embedded in the center,66−69 and a 9,10diboraanthracene-based π-system.70−76 It was revealed that the geometry of the molecules has a great impact on the emission properties. In this context, we have been interested in the development of triarylborane π-electron systems with novel geometric structures to achieve intriguing emission properties and thus to realize great applications. Herein, in this Forum Article, we present our recent progress on the CT-emitting triarylborane π-electron systems, which include the lateral boryl-substituted π-system with amino groups at the terminal positions,77−80 the o,o′-substituted biaryl π-system with boryl and amino groups at the o,o′-positions,60,81−86 a triarylboranebased BODIPY (the dipirromethene−BF2 complex) system,87 and a B,N/S-bridged ladder-type π-system.88,89 We here put the emphasis on the molecular design concept, structure−property relationships, intriguing emission properties and great applications of the corresponding triarylborane π-systems. The detailed preparation methods for the compounds discussed in this overview can be found in the original publications.

Figure 2. (a) Schematic representation of the molecular design concept for a lateral boryl-substituted π-system. (b) Structures of 1a and the related analogues 2 and 3.

tion of a spiro skeleton,101−103 and taking advantage of aggregation-induced emission.90,104 On the other hand, the energy migration of the Förster mechanism goes through Coulombic resonance interactions and is effective over longrange distances. Therefore, simple steric protection is not effective in preventing this process. However, the rate constant of energy migration via the Förster mechanism is highly influenced by the degree of absorption−emission spectral overlap, which is generally reflected by the Stokes shift. A large Stokes shift normally means weak absorption−emission spectral overlap. Thus, it would be possible to suppress this process via an increase of the Stokes shift. Considering the steric effect and electron-accepting electronic effect of the boryl group, we envisioned that the introduction of the boryl group at the side position of the π-conjugated framework, which contain electron-donating groups at the terminal positions, would influence the emission properties in two ways (Figure 2a). First, the steric effect of the lateral groups would cause significant twisting of the main-chain framework. The steric bulk of the lateral boryl groups and the highly twisted mainchain framework would lead to well-separation of the molecules. Second, the electronic effect of the lateral boryl group would lead to the intramolecular CT transition and thus a large Stokes shift. As a consequence, both energy migration processes of the Dexter and Förster mechanisms could be suppressed effectively, and this system would exhibit intense fluorescence even in the solid state. To verify this molecular design concept, we first designed and prepared lateral Mes2B-substituted phenyleneethylnylene 1a (Figure 2b).77 Single-crystal structure analysis of 1a confirmed its highly twisted main-chain structure, in which the dihedral angle between the central and terminal benzene rings is 47.5°. In addition, the large fluorescence solvatochromism from 536 nm in cyclohexane to 601 nm in tetrahydrofuran (THF) and 627 nm in methanol suggests a highly polarized excited-state structure as a result of the intense intramolecular CT transition, which is consistent with its large Stokes shift (Δν = 4491 cm−1 in cyclohexane). It was interesting to find that no obvious red shift was observed for its absorption spectra from solution to the spin-coated film and the fluorescence spectrum of the spin-coated film is close to that in benzene (λem

2. LATERAL BORYL-SUBSTITUTED π-SYSTEM WITH AMINO GROUPS AT THE TERMINAL POSITIONS Our research about CT-emitting triarylboranes initially started with the lateral boryl-substituted π-system, which contains electron-donating amino groups at the terminal positions (Figure 2a).77−80 This molecular design was essentially motivated by the difficulty in obtaining highly emissive organic materials that exhibit a high fluorescence efficiency even in the solid state. The efficient solid-state emission of organic materials is essential for various photoelectronic applications, such as OLEDs, organic solid-state lasers, and organic fluorescent sensors.90−92 However, most organic fluorophores show intense fluorescence only in dilute solution and display weak fluorescence or even no fluorescence in the solid state because of the severe aggregation-caused quenching effect as a result of certain intermolecular interactions, such as aggregate or excimer formation and energy migration in the solid state.93,94 To obtain solid-state emissive fluorophores, one problem that has to be overcome is to suppress the fluorescence quenching processes in the solid state, which include nonradiative energy migration processes of the Dexter and Förster mechanisms.95 The nonradiative energy migration of the Dexter mechanism proceeds via electron exchange interactions. Thus, this process could be effectively suppressed through steric protection to separate molecules from each other, which is the basis for the generally adopted strategies to achieve solid-state emission, such as bulky or dendritic substituent protection,96−98 cross-dipole stacking,99,100 utiliza8706


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Inorganic Chemistry = 562), verifying that this structure is effective in suppressing the intermolecular interactions in the solid state whether in the ground state or in the excited state. Most notably, this compound displays intense fluorescence either in solution (ΦF = 0.98 in cyclohexane) or in the solid state (ΦF = 0.90 in the spin-coated film), as was expected. To elucidate the effects of lateral Mes2B groups, we compared the photophysical properties of 1a with those of its two phenyleneethynylene analogues, 2 and 3 (Figure 2b), which contain the bulky neutral triisopropysilyl or electron-accepting cyano groups instead of Mes2B. In contrast to 1a, these two compounds only show intense fluorescence in a benzene solution (λem = 430 nm and ΦF = 0.92 for 2; λem = 520 nm and ΦF = 0.98 for 3). The fluorescence efficiency decreases significantly in the solid state (λem = 434 nm and ΦF = 0.39 for 2; λem = 575 nm and ΦF = 0.29 for 3). These facts suggest that both the steric and electronic effects of lateral boryl groups are essential to suppress fluorescence quenching in the solid state. The intense solid-state fluorescence is a general characteristic for the lateral boryl-substituted π-system. Intense fluorescence was also observed for spin-coated films of other compounds (Figure 3), such as phenyleneethynylene 1b (ΦF = 0.85) and

Figure 4. Structures and solid-state fluorescence (excited at 365 nm) of lateral Mes2B-substituted p-quaterphenyls 4.

the spin-coated film), which is one of the highly desired properties for the organic emissive materials. This molecular design is applicable to obtaining not only solid-state emissive small molecules but also polymers.80 We have synthesized a series of poly(aryleneethynylene)s containing diarylboryl groups as the side chains (Figure 5). To improve

Figure 5. Structures and solid-state fluorescence (excited at 365 nm) of lateral (HDEMP)2B-substituted poly(aryleneethynylene)s 5.

solubility, a new diarylboryl group, bis(4-hexyl-2,6dimethylphenyl)boryl (HDEMP)2B, in which a hexyl group was attached to the aryl groups, was utilized instead of most widely used Mes2B. All the obtained polymers showed intense fluorescence emission not only in solution but also in the solid state. For instance, the fluorescence quantum yields of polymer 5 in benzene and in film are 0.85 and 0.63, respectively. Moreover, both the absorption and fluorescence spectra of 5 in thin film are almost identical to those in benzene, confirming the effectiveness of the introduction of diarylboryl groups at the side chains for preventing interactions between the conjugated backbones. Through these results, it has been well demonstrated that the introduction of bulky electron-accepting boryl group at the side position of electron-donating framework is an efficient strategy to obtain organic solid-state emissive materials.

Figure 3. Structures and solid-state fluorescence (excited at 365 nm) of lateral Mes2B-substituted phenyleneethynylenes 1a and 1b and phenylenevinylene 1c.

phenylenevinylene 1c (ΦF = 0.73). In addition, the emission is widely tunable from green for 1b (λem = 504 nm) to reddish orange for 1c (λem = 596 nm) by choosing the appropriate terminal amino groups or π-conjugated framework. Utilizing this molecular design, it was also possible to obtain solid-state blue emitters via the introduction of only one Mes2B group at the side position of the p-quaterphenyl framework, which contains two electron-donating amino groups at the terminal benzene rings (Figure 4).79 Unlike the phenyleneethynylene and phenylenevinylene frameworks, the conjugation of p-quaterphenyl is very limited, which was expected to prevent the emission from red shifting. As a result, in addition to the high solid-state fluorescence efficiency, the fluorescence is located in the ideal blue region for both the lateral boryl-substituted p-quaterphenyl 4a and 4b (λem = 473 nm and ΦF = 0.99 for 4a; λem = 447 nm and ΦF = 0.83 for 4b in

3. O,O′-SUBSTITUTED BIARYL π-SYSTEM WITH BORYL AND AMINO GROUPS AT THE O,O′-POSITIONS Inspired by the results about the lateral boryl-substituted πsystem, we have recently disclosed another new CT-emitting triarylborane π-system, in which boryl and amino groups are introduced at the lateral o,o′-positions of a biaryl skeleton.60,81−86 Owing to the steric and electronic effects of the lateral groups, it was envisioned that this system would exhibit significant steric bulk and a large Stokes shift, which are two important factors to suppress nonradiative energy migration processes in the solid state. Thus, this system was expected to be solid-state emissive. Moreover, the biaryl skeleton might be 8707


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close absorption and fluorescence spectra in a cyclohexane solution and in the spin-coated film. Similar to the lateral borylsubstituted triarylboranes, this compound shows very intense fluorescence in the solid state (ΦF = 0.47 in cyclohexane and 0.86 in the spin-coated film). In addition to the intense solidstate emission, two other features are notable for the photophysical properties of 6a. One is the particularly large Stokes shift (Δλ = 215 nm; Δν = 13485 cm−1 in cyclohexane), which is partially ascribed to the invisibility of the absorption band corresponding to the first excited-state transition due to the highly twisted main-chain biphenyl skeleton structure and thus very poor overlap between the highest occupied molecular orbital (HOMO) localized on the (dimethylamino)phenyl unit and the lowest unoccupied molecular orbital (LUMO) located on the (dimesitylboryl)phenyl moiety. The longest wavelength band with appreciable intensity at ca. 300 nm essentially consists of the transition from HOMO−1 to LUMO. Another notable feature for the photophysical properties of 6a is the significantly long emission wavelength for the biphenyl unit with such a limited conjugation length (λem = 521 nm in cyclohexane and 523 nm in the spin-coated film). Compared with its normal linear regioisomer 7a, the emission is redshifted by 102 and 53 nm for cyclohexane and the spin-coated film, respectively (Figure 8). In addition, considering the highly

flexible as a result of the free rotation of the single bond between two aryl rings. Thus the biaryl skeleton would display different conformations, in which the boryl and amino groups might be arranged at the same side or on the two opposite sides of the biaryl axis (Figure 6a). The change in the biaryl conformation would likely lead to great changes of the photophysical properties.

Figure 6. (a) Schematic representation of the o,o′-substituted biaryl πsystem. (b) Structures of o,o′-substituted biphenyls 6a−6c and related regioisomers 7a−7c.

As the model compound, we first designed and synthesized the biphenyl derivative 6a (Figure 6b), which contains a Mes2B and a dimethylamino (Me2N) group at the o,o′-positions.81 Just as expected, the biphenyl skeleton is highly twisted and the torsion angle is up to 70.7°. Interestingly, despite the remarkable steric congestion between the Mes2B and Me2N groups, they are still located at the same side of the biphenyl axis with a very short B···N distance of 3.59 Å, which is within the range of the sum of the van der Waals radii of the B and N atoms (3.74 Å).105 The short B···N distance suggests a possible direct electronic interaction between the B and N centers (Figure 7a). As a result of the steric bulk of the lateral groups and the highly twisted biphenyl skeleton, the intermolecular interactions are well suppressed, which is evidenced by the very

Figure 8. Absorption and fluorescence spectra of o,o′-substituted biphenyls 6a and 6b and related regioisomers 7a and 7b (in cyclohexane).

twisted main-chain structure and very close B···N distance, CT of 6a takes place most likely through space rather than through bond. The first example of a through-space intramolecular CTemitting triarylborane π system was reported by Wang and coworkers.29,30 For this o,o′-substituted biphenyl system, it was revealed that the conformation is highly dependent on the steric bulk of the amino group.83 When Me2N was replaced by a more bulky dibenzylamino (Bn2N) in compound 6b, the boryl and amino groups are not arranged at the same side but on two opposite sides instead (Figure 7b). At the same time, the biphenyl skeleton turns more coplanar with a torsion angle of 50°. With these changes in the structure, the intramolecular CT absorption band becomes a little more obvious (Figure 8). Although changes in the absorption are trivial, the fluorescence is significantly blue-shifted (Δλem = 71 nm in cyclohexane and 62 nm in the spin-coated film). The remarkable hypsochromism in fluorescence should be ascribed to conformational change as a result of steric effect differences between Me2N and Bn2N because these two amino groups have similar electrondonating ability, which was confirmed by almost the same

Figure 7. Single-crystal X-ray structures and solid-state fluorescence (excited at 365 nm) of (a) 6a and (b) 6b. 8708


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Inorganic Chemistry absorption and emission spectra for the linear p,p′-substituted biphenyl analogues 7a and 7b. Theoretical calculations denoted that, in the excited state, 6a has a much smaller dihedral angle between BC3 and the Mes2B-bonded phenyl ring of the biphenyl unit (17.0°) and a much shorter B−C bond between the B and C atoms of the Mes2B-bonded phenyl ring of the biphenyl unit (1.528 Å) than compound 6b (26.9 and 1.546 Å for 6b, respectively), which might suggest more efficient conjugation between the vacant p orbital and the Mes2Bbonded phenyl ring of the biphenyl unit for 6a. As a result, the unique structure of 6a with a close B···N distance and direct B···N electronic interaction is helpful in stabilizing the highest singly occupied molecular orbital (H-SOMO) in the excited state (Figure 9). As for the solid-state fluorescence, molecule 6b

Figure 10. (a) Two views of the crystal structures of 6c. (b) Absorption, fluorescence (in cyclohexane), and phosphorescence spectra (77 K, in 2-MeTHF) of 6c and 7c.

quite unexpected. The o,o′-substituted 6c shows phosphorescence at a much shorter wavelength than 7c, suggesting that 6c has a much higher triplet energy (ET). The higher ET of 6c can be ascribed to its almost orthogonal conformation of the biphenyl skeleton, which would lead to well separation of HOMO and LUMO and thus a small energy gap between the singlet and triplet states (ΔEST). The ET of 6c is determined to be 2.57 eV, which is close to that of the typical blue phosphorescent emitter FIrpic (ET = 2.62 eV). The high triplet energy of 6c enables its great utility as a host material in blue and green phosphorescent OLEDs. The phosphorescent OLEDs using 6c as a host material can achieve excellent performance, with maximum external quantum efficiencies (EQEs), current efficiencies (CEs), and power efficiencies (PEs) of 15.3% and 22.2%, 34.5 and 84.2 cd A−1, and 31.4 and 76.6 lm W−1 for blue and green phosphorescent OLEDs, respectively. Moreover, the green phosphorescent OLED has an extremely low efficiency roll-off. The EQE remains almost unchanged at 1000 cd m−2 and decreases by only 2.6% even at 10000 cd m−2. On the contrary, the blue and green phosphorescent OLEDs hosted by 7c display significantly poorer performances. Considering the very limited examples of triarylboranes as hosts for phosphorescent OLEDs,60−62 the current results might provide some important bases for the molecular design of triarylborane-based host materials. Inspired by the fascinating properties of 6a, we were interested in the further modification of this skeleton to tune its photophysical properties and explore potential applications.84 Utilizing the high reactivity of the para position of Me2N toward electrophilic substitution, we prepared a series of derivatives 8 (Figure 11), in which various electron-withdrawing or -donating substituents were introduced by electrophilic formylation or iodination, followed by a cross-coupling reaction. Detailed investigations on the photophysical proper-

Figure 9. Pictorial drawings and a plot of the Kohn−Sham energy levels for frontier orbitals of (a) the DFT-optimized S0 state and (b) the time-dependent-DFT-optimized S1 state for 6a and 6b (red, positive; green, negative; surface isovalue, 0.015), calculated with the B3LYP function [basis sets: 6-31G(d) for the H, B, and C atoms; 631G+(d) for the N atom]. H atoms are omitted for clarity.

is also very emissive, although its fluorescence efficiency is lower than that of 6a to some extent (ΦF = 0.35 for 6b in the spin-coated film). Encouraged by the significant influence of the amino group on the emission properties of o,o′-substituted biphenyl, we also fully explored the diphenylamino (Ph2N)-substituted biphenyl 6c.60 In this compound, two phenyl rings of the biphenyl framework are almost perpendicular to each other with a torsion angle up to 88.2° (Figure 10a). It was noted that the boryl and amino groups are slightly twisted to two opposite sides of the biphenyl axis with a B···N distance of 4.35 Å. The long B···N distance of 6c suggests that there should be no direct electronic interaction between the boryl and amino groups. Again, with the changes in the structure, the changes in the absorption are negligible. The absorption of 6c is very close to that of 6b, in terms of the absorption maxima position and intensity. However, the fluorescence of 6c is further blueshifted compared with that of 6b (Δλem = 21 nm in cyclohexane). For the p,p′-substituted biphenyls, the changes of both absorption and emission are very small. As a consequence, the fluorescence spectra of 6c and its p,p′substituted regioisomer 7c are very close (Figure 10b). Notably, the phosphorescence spectra at low temperature are 8709


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intense intramolecular CT fluorescence, even for its nanoaggregates in water. The Hg2+-promoted deprotection of the dithioacetal group can generate 8a, which would lead to an obvious blue shift of fluorescence. The binding of F− with the tricoordinate B center causes a much more remarkable blue shift in emission. Interestingly, ratiometric fluorescence sensing of Hg2+ is feasible in an aqueous medium consisting of almost pure water (Figure 13). Despite the large number of triarylboranes capable of fluoride sensing, those that can behave as bifunctional sensors are very rare.109−111

Figure 11. Structures of triarylboranes containing the (2-Mes2B-2′Me2N)biphenyl core unit.

ties showed that the introduction of electron-withdrawing substituents would facilitate a HOMO → LUMO CT transition (8a−8c). On the contrary, the intramolecular CT transition is significantly prohibited when electron-donating substituents are incorporated (8d−8f). Notably, when a dicyanovinyl group is present, the HOMO → LUMO CT transition mainly consists of the transition from the electron-donating amino group to the electron-accepting dicyanovinyl other than boryl (8c), indicating that the electron-accepting ability of dicyanovinyl is stronger than that of Mes2B. In addition, there are two electron-accepting sites, the trivalent B center and the α-C atom of the dicyanovinyl group, in molecule 8c. In THF solution, the F− ions would first bind to the B center and then attack the α-C atom of the dicyanovinyl group, whereas the CN− anion acts on the electron-accepting centers in the reverse sequence (Figure 12a). Consequently, the absorption and

Figure 13. Sensing mechanism and photographs of fluorescence (excited at 365 nm) of 8g upon the addition of TBACN and TBAF.

As for the o,o′-substituted biphenyl 6a, another notable feature in the structure is that there exists π−π interaction between the Me2N-bonded phenyl ring and the phenyl ring of one adjacent Mes group. Probably because of the existence of this π−π interaction, the signals of the Mes groups in 1H NMR are very broad, which suggests that the motions of the Mes groups are suppressed to some extent. Inspired by these results, we next investigated the 1,1′-binaphthyl derivative 9 (Figure 14), in which one Mes2N and one Mes2B are introduced at the

Figure 12. (a) Sensing mechanism of 8c as a bifunctional probe to discriminate F− and CN− ions. Photographs of the color change in (b) absorption and (c) fluorescence (excited at 365 nm) of 8c upon the addition of TBACN and TBAF.

Figure 14. Chemical and X-ray crystal structures of 2,2′-substituted 1,1′-binaphthyl 9 and related compound 10.

emission change in different ways upon the addition of F− and CN− ions (Figure 12b,c), demonstrating the great sensing ability of 8c to discriminate fluoride and cyanide anions, which is an important and advantageous feature relative to known triarylborane sensors. Recently, Thilagar et al. also reported the dicyanovinyl-containing triarylboranes for discrimination between the two interfering anions, F− and CN−.106−108 Moreover, a highly selective ratiometric bifunctional fluorescence probe 8g for Hg2+ and F− could also be obtained via functionalization of the (2-Mes2B-2′-Me2N)biphenyl core unit with a dithioacetal substituent.85 Compound 8g displays

2,2′-positions of the 1,1′-binaphthyl skeleton.86 It was expected that replacement of the biphenyl by a more conjugated chiral 1,1′-binaphthyl framework would have two effects. First, the more extended conjugation of naphthyl can give rise to stronger π−π interactions between the Me2N-bonded naphthyl ring and the phenyl ring of the adjacent Mes group, and thus the motions of the Mes groups can be suppressed more efficiently. In addition, the chirality of the binaphthyl might make the configurations of the B center have different stabilities. As a consequence, the configuration of the B center 8710


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Inorganic Chemistry would be highly dependent on the configuration of the binaphthyl skeleton. Thus, it is possible to realize the chirality transfer from the axially chiral binaphthyl to the propeller chiral trivalent B center. The propeller chirality of the trivalent center is another fascinating structural feature of triarylboranes, for which it is very useful to obtain some intriguing chiral photophysical properties, such as circular dichromism (CD) and circularly polarized luminescence (CPL). However, the utility of the propeller chirality has not gained enough attention because of the free rotation of the B−C bond as a result of the low transition barrier between the P and M forms.112 To achieve the propeller chirality of triarylboranes, one strategy is to introduce two long rigid substituents on aryls at the ortho positions to the B atom.113 In this context, it is of great interest to pursue a new strategy to stabilize the configuration of a propeller-shaped B center. The effects of the replacement of the biphenyl by binaphthyl were evidenced by single-crystal X-ray structures, 1H NMR spectra, and theoretical calculations. The X-ray crystal structure analysis of 9 revealed that the Me2N-bonded naphthyl ring P1 and the phenyl ring P2 of the adjacent Mes group are almost parallel and very close to each other (Figure 14). The interplane angle and centroid−centroid distance are only 9.4° and 3.42 Å, respectively, which are much smaller than 27.3° and 3.87 Å in the biphenyl derivative 6a. These results suggest that there exists a very strong π−π-stacking interaction between P1 and P2. In 1H NMR, triarylboranes containing Mes2B generally show very sharp signals, with a singlet peak corresponding to the phenylic protons of the Mes groups. In contrast, the singlet signal of four phenylic protons of two Mes groups is resolved into three broad peaks. Among them, two protons overlap at 6.80 ppm, which is similar to the general Mes2B, whereas another two protons are located at 5.58 and 5.98 ppm, which are significantly upfield-shifted. The two upfield singlet peaks are ascribed to two phenylic protons of P2, while the signal at 6.80 ppm is assignable to the phenylic protons of the phenyl ring P3 of another Mes group. The upfield shift of the phenylic protons of P2 suggests a great shielding effect of the ring current from P1 and P2. Complete resolution of the signals for two phenylic protons of P2 clearly denotes that the rotation of B−C(P2) should be totally suppressed. On the basis of the variable-temperature 1H NMR spectra of 9, the rotation barriers for the B−C(P2) and B−C(P3) bonds were estimated to be about 67.3 and 62.1 kJ mol−1, respectively. More notably, only one pair of enantiomers crystallized from the racemic compound 9, and they have the (S,M) and (S,P) configurations. The geometry optimizations for one pair of diastereomers of 9 (Figure 15), which have (S,M) and (S,P) configurations, demonstrated that the (S,M) isomer is more stable than its diasteromeric (S,P) isomer. The total energy is about 11.4 kJ mol−1 different. The lower stability of the (S,P) isomer is likely ascribed to its absence of π−π interaction between P1 and P2. In the optimized structure of the (S, P) isomer, the phenyl ring P2 greatly deviates from the top of the naphthyl ring P1 and the centroid−centroid distance is elongated to 5.08 Å, which is much longer than the π−π-stacking interaction range (Figure 15). Most notably, even though the phenylic protons of the Mes groups become coalesced with increased temperature, the 1 H NMR spectra can restore to the original ones after cooling, indicating the selective formation of one pair of more stable enantiomers. Thus, the propeller-shaped chirality of the tricoordinate B center can be successfully induced from the axial chirality of the binaphthyl moiety in compound 9. In

Figure 15. Optimized structures for one pair of diastereomers of 9 with (S,M) and (S,P) configurations.

contrast to 9, the Mes groups in an analogue 10, in which Mes2B is separated from 1,1′-binaphthyl by a p-phenylene spacer, are able to rotate freely, as evidenced by a very sharp singlet corresponding to four phenylic protons of two Mes groups. The calculated total energies are only 1.5 kJ mol−1 different between one pair of diastereomers of 10. The detailed comparisons confirmed the essential role of π−π interaction between P1 and P2 for the successful chirality relay in compound 9. Through supercritical fluid chromatography with chiral columns, the binaphthyl derivative 9 could be resolved into optically pure forms, which exhibit clear mirror images on the CD and CPL spectra. The luminescence dissymmetry factor (glum) is 2.8 × 10−3, which is comparable to the general organic CPL molecules. Unfortunately, this compound displays very weak emission either in solution or the solid state (ΦF = 0.072 in cyclohexane and 0.065 in powder form). Further structural optimization is necessary to achieve promising chiral photophysical properties. Considering the very limited example of triarylboranes exhibiting stable propeller chirality, the current successful chirality relay in 9 should provide an effective strategy to obtaining the propeller chirality of triarylboranes.

4. TRIARYLBORANE-BASED BODIPY π-SYSTEM The results about the above two systems demonstrated that the combined steric bulk of the nonplanar main chain and the Mes2B groups is effective in suppressing the intermolecular interactions in the solid state. We were herein curious about whether the boryl group alone is also bulky enough to suppress the intermolecular interactions in the solid state when the main chain is a rigid planar framework. To examine this point, we chose BODIPY as a core unit. BODIPY has been known as an extraordinary class of fluorophores because of its unusual and excellent properties, such as high fluorescent quantum yield, large molar extinction coefficient, and sharp emission spectra.114 However, most BODIPY dyes hardly fluoresce in the solid state because of the high planarity, which would cause strong intermolecular interactions and thus severe fluorescence quenching in the solid state.115 We thus designed triarylboranebased BODIPY 11 (Figure 16), in which two Mes2B groups were introduced into the BODIPY core unit through the phenyleneethynylene linker.87 In contrast to the remarkable contribution of the boryl group to the LUMO for the general triarylboranes, the boryl groups in compound 11 almost have no contribution to both the HOMO and LUMO. The LUMO of 11 is mainly localized on the central BODIPY moiety, which 8711


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photophysical properties, such as intense fluorescence. In addition, the flat structures are suitable to form strong intermolecular π−π interactions, which are favorable for the intriguing electronic property of high carrier mobility. Pioneered by Yamaguchi’s work on B-bridged biphenyl (dibenzoborole),16,117 we were interested in a new ladder πsystem, in which both an electron-accepting B atom and an electron-donating heteroatom, such as N and S, are introduced as bridging atoms (Figure 18a).88,89 Initially, we envisioned that

Figure 16. Structures of triarylborane-based BODIPY 11 and the related analogue 12.

probably suggests a stronger electron-accepting ability of the BODIPY core than the trivalent boryl group. Because of the negligible effect of the boryl groups on the electronic structures in 11, this compound displays almost the same absorption and emission properties as its analogue 12, which lacks two Mes2B groups. However, their photophysical properties are completely different in the solid state (Figure 17). Upon going from THF

Figure 18. (a) Schematic presentation and structures of B,N/Sbridged p-terphenyls 13 and 16. (b) Structures of related compounds 14 and 15.

this system might have high fluorescence efficiency because of its rigid planar structure. In addition, the electronic effect of B and N would induce an intramolecular CT transition and thus red-shifted absorption and emission. Moreover, the bulky substituent on the B center would suppress intermolecular interaction in the solid state and thus can retain intense fluorescence even in the solid state. As the model compound, we first designed and synthesized this B,N-bridged p-terphenyl, 13.88 In cyclohexane, this compound shows a very weak shoulder band at 430 nm (log ε = 3.28) in the absorption spectrum (Figure 19). The fluorescence is located at 529 nm Figure 17. UV/vis absorption and emission spectra of 11 and 12 in the spin-coated films. The inset shows the fluorescence (excited at 365 nm) of 11 in a THF solution and powder form.

solution to the spin-coated film, the triarylborane-based BODIPY 11 only displays minor red shifts (Δλabs = 8 nm; Δλem = 16 nm). In contrast, the absorption and emission spectra of 12 are significantly red-shifted by 48 and 44 nm, respectively. It is most noteworthy that the fluorescence intensity of 11 is about 10 times stronger than that of 12 in the spin-coated film state. Thus, the introduction of the bulky Mes2B group is very effective at enhancing the solid-state emission efficiency of BODIPY dyes. In addition, it is interesting to note that the emission band of 11 in the spincoated film belongs to the red-light region (λem = 627 nm). The current molecular design may also provide an efficient strategy to achieving solid-state red emissive materials, which is of great interest for realizing the full color display of OLEDs.

Figure 19. (a) Absorption and (b) Fluorescence spectra of B,N/Sbridged p-terphenyls 13 and 16 and related compounds 14 and 15. The inset shows the fluorescence photographs (excited at 365 nm) of 16 in a THF solution and powder form.

with only a moderate quantum yield (ΦF = 0.21). It was noted that the Stokes shift is close to 100 nm, much larger than the general ladder-type system. Another noteworthy feature for the photophysical properties of 13 is the significantly long fluorescence lifetime (τ = 82.5 ns), which suggests very slow nonradiative and radiative decay processes (kr = 2.5 × 106 s−1; knr = 9.6 × 106 s−1). Moreover, quite different from the general intramolecular CT π-system, no obvious solvent effect on

5. B,N/S-BRIDGED LADDER-TYPE π-SYSTEM The ladder-type π-systems with fully fused polycyclic conjugated skeletons are very attractive owing to their rigid and flat structures.116 The rigid and flat structures would lead to efficient electron delocalization and suppress the nonradiative process of the excited state, which can provide desirable 8712


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Inorganic Chemistry fluorescence was observed for this compound, suggesting similar polarity between the ground and excited states. The theoretical calculations suggest that 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 dibenzoborole moiety, is almost forbidden (f = 0.003). The effects of bridging B and N atoms were illustrated through a detailed comparison of the photophysical properties of 13 and its analogues 14 and 15 (Figure 18b), which lack bridging N and B atoms, respectively. The absorption and emission spectra of 13 and 14 are very close. In contrast, both the absorption and emission of 13 are significantly red-shifted once a bridging B atom is introduced compared with molecule 15 (Figure 19). The density functional theory (DFT) calculations suggest that the introduction of a bridging B atom greatly lowers the LUMO energy level but not the HOMO, while the HOMO and LUMO energy levels are elevated by almost the same extent once a bridging N atom is introduced. These results might suggest that the effect of the bridging B atom is greater than that of the bridging N atom for tuning of the photophysical properties. More recently, we also synthesized the B,S-bridged pterphenyl 16 by changing the electron-donating atom from N to S.89 Interestingly, it was found that when the bridging N atom was replaced by an S atom, although the absorption and emission spectra were not very different, the fluorescence efficiency was increased more than two times (ΦF = 0.55 in cyclohexane). This is very different from the general tendency that the S-containing compounds display fluorescence properties inferior to those of the corresponding N-containing analogues. Similar to the B,N-bridged p-terphenyl 13, the B,S-bridged p-terphenyl 16 also shows a very long lifetime (τ = 78.7 ns). The increase of the fluorescence efficiency arises from acceleration of the radiative decay process and deceleration of the nonradiative decay process. Moreover, the bulky TIP substituent on the B center is effective in preventing intermolecular interactions in the solid state, as evidenced by the similarity of the absorption and emission spectra in solution and the solid state. As a consequence, compound 16 can retain intense fluorescence in the solid state (ΦF = 0.44 for the spincoated film and 0.72 for the powder form). Considering the unique fluorescence properties, including long lifetime and high fluorescence efficiency in both solution and the solid state, the B,S-bridged p-terphenyl 16 might be useful for OLEDs or bioimaging.

facilely by choosing different amino groups and to realize the utility of triarylboranes as hosts for phosphorescent OLEDs. It is also possible to obtain bifunctional probes through the introduction of another ion-responsive group into the o,o′substituted biphenyl core unit. Moreover, the propeller chirality of a trivalent B center can be induced from the axial chirality of the binaphthyl moiety for the o,o′-substituted 1,1′-binaphthyl. In addition, the introduction of Mes2B into the rigid planar BODIPY core is also effective in suppressing severe fluorescence quenching of BODIPY in the solid state and to obtain red solid emitters. For the ladder π-system, a B,N/Sbridged p-terphenyl, which contains electron-accepting B and electron-donating N/S as bridging atoms, also exhibits a large Stokes shift and a particularly long fluorescence lifetime apart from high fluorescence efficiency in the solid state. We expect that our results will provide some important bases for the further design of new functional triarylboranes with fascinating properties and applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Cui-Hua Zhao: 0000-0002-4077-5324 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank all of the former members in Zhao’s group, as well as external collaborators, for their valuable contribution to the work described in this overview. The National Natural Science Foundation of China (Grants 21272141, 21572120, and 21072117) is also greatly acknowledged for financial support.

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REFERENCES

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6. CONCLUSION Over the past 2 decades, significant progress has been made in the triarylborane π-electron materials, especially in the triarylboranes with intramolecular CT characteristics. In this Forum Article, we introduced several novel CT-emitting triarylborane π-electron systems, which were developed in our group. Fascinating properties and great utilities can be achieved with careful control of the molecular geometry. Thus, it is possible to obtain organic emissive solids with fluorescence efficiency close to unity by the introduction of a bulky electronaccepting boryl group at the side positions of a π-electron framework, which contains electron-donating groups at the terminal positions. When boryl and amino groups are introduced at the o,o′-positions of the biphenyl π-electron framework, it is possible not only to prepare organic solid-state emissive materials but also to tune the emission wavelength 8713


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