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 boryl-substituted 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.

A triarylborane-based biphenyl, 4,4′-dibromo-2-dimesitylboryl-2′-(N,N-dimethylamino)biphenyl (Br2-BN-BPh), exhibits very unique temperature-dependent fluorescence behavior. Although Br2-BN-BPh is weakly emissive at room temperature, its fluorescence increases abruptly at a specific high temperature (68 °C in MOE, 88 °C in toluene). In addition, the fluorescence still remains very strong even after cooling to room temperature, providing the possibility of the fluorescence detection of a specific high temperature.

We report herein a novel triarylborane-based biphenyl derivative, o,o′-NPh2, which contains dimesitylboryl and diphenylamino groups at the lateral o,o′-positions of the biphenyl skeleton. This triarylborane-based biphenyl exhibits a high triplet energy (ET = 2.57 eV), which is much higher than its normal linear regioisomer, p,p′-NPh2 (ET = 2.28 eV), although their fluorescence spectra and thus singlet energies are very close. The high ET of o,o′-NPh2 is probably ascribed to its almost orthogonal conformation of the biphenyl skeleton, which would likely lead to good separation of the HOMO and LUMO and thus a small energy gap between the singlet and triplet excited states (ΔEST). The high ET of o,o′-NPh2 has allowed incorporation of it as a host material in blue and green phosphorescent OLEDs. The devices hosted by o,o′-NPh2 can achieve high performances with maximum external quantum efficiencies (EQEs), current efficiencies (CEs) and power efficiencies (PEs) of 15.3% and 22.2%, 34.5 cd A−1 and 84.2 cd A−1, 31.4 lm W−1 and 76.6 lm W−1 for blue and green phosphorescent OLEDs, respectively. Moreover, the green phosphorescent OLED has 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.

In this work, a series of nanoparticles were prepared assembled by a highly emissive solid-state organoboron-based stilbene (OBS) and PS-PEG-COOH via regulating the ratio of these two compounds using a co-precipitation method. The resultant OBS nanoparticles (OBSNs) were homogeneously dispersed in water media with average sizes of 36–82 nm and exhibited large Stokes shift, high fluorescence quantum yield, excellent photostability and low cytotoxicity. These organic nanoparticles could be internalized by MCF-7 cells and could accumulate in the cytoplasm outside the organelles. In addition, it is found that the nanoparticles with least negative charge and moderate size are most easily internalized by cells. Furthermore, the internalized nanoparticles could be retained inside the live cells for a long time and could be uniformly separated into two cells after the cell division process, permitting long-term cellular imaging applications.

The incorporation of B element into π-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 boryl-substituted 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.The incorporation of the electron-accepting B element with a vacant p orbital and the electron-donating N element with a lone pair into the π-conjugated system is an efficient strategy to construct organic optoelectronic materials with intense intramolecular charge-transfer transitions and thus intriguing properties. This short review summarizes the recent progress in this field according to the structural features of π-systems.

Two benzothieno[b]-fused BODIPYs, BT-BODIPY and BBT-BODIPY, in which one parent BODIPY core is fused with one and two benzothieno rings, respectively, were synthesized from BODIPYs substituted with 2-(methylsulfinyl)phenyl at the β-position. The first H2SO4-induced cyclization and deborylation afforded benzothieno[b]-fused dipyrrin derivatives, which can easily complex with BF3·OEt2 to form the desired benzothieno[b]-fused BODIPYs. It was revealed that the fusion of the benzothieno ring is more effective at extending conjugation than simple attachment of the 2-(methylthio)phenyl substituent, which presumably results from conformational restriction. Compared with the corresponding unstrained SPh-BODIPY and BSPh-BODIPY, which contain one and two 2-(methylthio)phenyl groups at the β-position, BT-BODIPY and BBT-BODIPY display red shifted absorption, increased absorptivity, and fluorescence efficiency. Furthermore, the ring fusion is also helpful to increase stability of the formed cation in BBT-BODIPY. Thus, BBT-BODIPY exhibits very intriguing properties, such as intense absorption and emission in the red region, very sharp emission spectra, and reversible oxidation and reduction potentials.

A triarylborane derivative BN-S, which contains a Hg2+-responsive dithioacetal group and a F−-responsive boryl group, has been designed and synthesized via the functionalization of 2-dimesitylboryl-2′-(N,N-dimethylamino)biphenyl core skeleton with a dithioacetal substituent. This compound displays intense intramolecular charge transfer fluorescence, even for its nano-aggregates in water. The Hg2+-promoted deprotection of the dithioacetal group and complexation of F− with the tri-coordinate boron center cause hypochromism of fluorescence to different extents. And thus BN-S behaves as a promising ratiometric bifunctional fluorescence probe to detect Hg2+ and F− simultaneously. In addition, the detection of Hg2+ is performable in aqueous medium using its nano-aggregates.

A series of organoboron-based biphenyls o,o′-NMe2, o,p′-NMe2, p,p′-NMe2, which contain an electron-donating NMe2 and an electron-accepting BMes2 groups at o,o′-, o,p′-, p,p′-positions of biphenyl skeleton, respectively, as well as o,o′-NBn2, which contains more bulky NBn2 rather than NMe2, were fully characterized to explore the effect of structural modification on the intramolecular charge-transfer emissions. In addition to significant effect of substitution position on photophysical properties, remarkable influence of conformation was also observed for o,o′-substituted compounds. The emission is substantially blue-shifted as conformation changes from the location of NMe2 and BMes2 at same side of biphenyl axis with a close B···N distance, and thus direct B···N electronic interaction in o,o′-NMe2, to the location of NBn2 and BMes2 on two opposite sides in o,o′-NBn2. And o,o′-NMe2 exhibits the longest emission wavelength, but the shortest absorption wavelength, and thus largest Stokes shift among these four organoboron-based biphenyls in both solution and solid state. The theoretical calculations demonstrated that the unique structure of o,o′-NMe2, in which boryl and amino located at the same side of biphenyl axis with close B···N distance and direct B···N electronic interaction, is helpful to stabilize the lowest singly occupied orbital in the exited state.

The efficient synthesis has been described to achieve a new class of ladder π-conjugated molecules, B,N-bridged p-terphenyls. The bridging B atom exhibits a more significant effect than the bridging N atom on photophysical properties.

A new family of p-quaterphenyls 1–6 laterally substituted with a bulky electron-accepting dimesitylboryl group has been designed and synthesized. These compounds were characterized by X-ray crystallography, UV–vis and fluorescence spectroscopy, and DFT calculations as well as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and cyclic voltammetry (CV). X-ray single-crystal analysis revealed that the p-quaterphenyl main chain framework exhibits a twisted structure due to the steric effect of the lateral boryl group, and the intermolecular interactions are effectively suppressed in the solid state. Despite the significantly twisted main-chain structure, these molecules still display efficient intramolecular charge-transfer emissions with large Stokes shifts. An intriguing finding is that all these molecules show bright fluorescence with good to excellent quantum yields in the blue region in the solid state. In addition, the two representative p-quaterphenyls 3 and 4 containing both the electron-accepting boryl group and the electron-donating carbazolyl (3) or diphenylamino group (4) possess high thermal stability and good oxidation–reduction reversibility, which together with their excellent solid-state fluorescence efficiency make them promising bipolar transporting blue emitters.

Intense solid-state emissions with good to excellent quantum yields were achieved through the introduction of bulky electron-donating diphenylamino groups at the side positions of an electron-accepting para-terphenyl framework.

We disclose two novel BODIPY dyes, which contain the bulky substituent, [(4-dimesitylboryl)phenyl]ethynyl at 2- and 2,6-positions. The steric bulkiness of the boryl group is effective to suppress the intermolecular interaction in the solid state and thus these two compounds display intense fluorescence not only in solution but also in the solid state. In addition, the BODIPY dyes display sensitive fluorescence responses to fluoride and cyanide anions through the complexation with the boron center of the boryl group and the subsequent decomposition of the BODIPY core, illustrating their potential uses for the fluorescence sensing of fluoride and cyanide ions.

We disclosed a novel cruciform tri-coordinate organoboron compound, 2′,5′-bis{[(4-dimesitylboryl)phenyl]ethynyl}-1′,4′-bis[(4-N,N-diphenylamino)phenyl]-[1,1′:4′,1′]terphenyls, which displays a characteristic intramolecular charge transfer transition and is highly emissive both in solutions and solid state. The complexation with fluoride ions induces a large blue shift in fluorescence, enabling ratiometric fluorescence sensing of fluoride. In addition, its prompt response to fluoride ions was also observed even in the solid state.

We report herein a novel triarylborane-based biphenyl derivative, o,o′-NPh2, which contains dimesitylboryl and diphenylamino groups at the lateral o,o′-positions of the biphenyl skeleton. This triarylborane-based biphenyl exhibits a high triplet energy (ET = 2.57 eV), which is much higher than its normal linear regioisomer, p,p′-NPh2 (ET = 2.28 eV), although their fluorescence spectra and thus singlet energies are very close. The high ET of o,o′-NPh2 is probably ascribed to its almost orthogonal conformation of the biphenyl skeleton, which would likely lead to good separation of the HOMO and LUMO and thus a small energy gap between the singlet and triplet excited states (ΔEST). The high ET of o,o′-NPh2 has allowed incorporation of it as a host material in blue and green phosphorescent OLEDs. The devices hosted by o,o′-NPh2 can achieve high performances with maximum external quantum efficiencies (EQEs), current efficiencies (CEs) and power efficiencies (PEs) of 15.3% and 22.2%, 34.5 cd A−1 and 84.2 cd A−1, 31.4 lm W−1 and 76.6 lm W−1 for blue and green phosphorescent OLEDs, respectively. Moreover, the green phosphorescent OLED has 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.

A triarylborane-based biphenyl, 4,4′-dibromo-2-dimesitylboryl-2′-(N,N-dimethylamino)biphenyl (Br2-BN-BPh), exhibits very unique temperature-dependent fluorescence behavior. Although Br2-BN-BPh is weakly emissive at room temperature, its fluorescence increases abruptly at a specific high temperature (68 °C in MOE, 88 °C in toluene). In addition, the fluorescence still remains very strong even after cooling to room temperature, providing the possibility of the fluorescence detection of a specific high temperature.

The efficient synthesis has been described to achieve a new class of ladder π-conjugated molecules, B,N-bridged p-terphenyls. The bridging B atom exhibits a more significant effect than the bridging N atom on photophysical properties.

Intense solid-state emissions with good to excellent quantum yields were achieved through the introduction of bulky electron-donating diphenylamino groups at the side positions of an electron-accepting para-terphenyl framework.

In this work, a series of nanoparticles were prepared assembled by a highly emissive solid-state organoboron-based stilbene (OBS) and PS-PEG-COOH via regulating the ratio of these two compounds using a co-precipitation method. The resultant OBS nanoparticles (OBSNs) were homogeneously dispersed in water media with average sizes of 36–82 nm and exhibited large Stokes shift, high fluorescence quantum yield, excellent photostability and low cytotoxicity. These organic nanoparticles could be internalized by MCF-7 cells and could accumulate in the cytoplasm outside the organelles. In addition, it is found that the nanoparticles with least negative charge and moderate size are most easily internalized by cells. Furthermore, the internalized nanoparticles could be retained inside the live cells for a long time and could be uniformly separated into two cells after the cell division process, permitting long-term cellular imaging applications.

A triarylborane derivative BN-S, which contains a Hg2+-responsive dithioacetal group and a F−-responsive boryl group, has been designed and synthesized via the functionalization of 2-dimesitylboryl-2′-(N,N-dimethylamino)biphenyl core skeleton with a dithioacetal substituent. This compound displays intense intramolecular charge transfer fluorescence, even for its nano-aggregates in water. The Hg2+-promoted deprotection of the dithioacetal group and complexation of F− with the tri-coordinate boron center cause hypochromism of fluorescence to different extents. And thus BN-S behaves as a promising ratiometric bifunctional fluorescence probe to detect Hg2+ and F− simultaneously. In addition, the detection of Hg2+ is performable in aqueous medium using its nano-aggregates.

We disclose two novel BODIPY dyes, which contain the bulky substituent, [(4-dimesitylboryl)phenyl]ethynyl at 2- and 2,6-positions. The steric bulkiness of the boryl group is effective to suppress the intermolecular interaction in the solid state and thus these two compounds display intense fluorescence not only in solution but also in the solid state. In addition, the BODIPY dyes display sensitive fluorescence responses to fluoride and cyanide anions through the complexation with the boron center of the boryl group and the subsequent decomposition of the BODIPY core, illustrating their potential uses for the fluorescence sensing of fluoride and cyanide ions.