Design,Synthesis And Properties Of Optical Functional Organobaranes

Organic optical functional molecules have the advantages of facile synthesis,wellcontrolled structures,solution processing,excellent luminescence properties and charge transporting capability,and thus exhibit great potential applications in organic lightemitting displays,solar cells and optoelectronic transmission,some of which have been successfully industrialized and appear in our lives.The recent development of technologies has put forward the demands on high performance and multi-functionality of organic optical molecules.Therefore,organic optical materials with multiple optical properties represent an important and frontier field.The boron atom has a characteristic vacant p-orbital,and thus tricoordinate organoboranes possess intriguing electron deficiency and Lewis acidity,which can make them coordinate with Lewis bases to produce tetracoordinate organoboranes.Organoboranes display fascinating optical and electronic properties,such as lightemitting properties,near-infrared absorption and electron transport,and have been employed as the key material systems in organic light-emitting diodes and organic solar cells.Herein,we focus on organoborane optical functional materials and disclose a few tetracoordinate organoboranes with chiral structures and push-pull electronic structures,and tricoordinate organoboranes with open-shell structures.The intriguing optical properties,such as circularly polarized luminescence(CPL),aggregation-induced emission(AIE),thermally activated delayed fluorescence(TADF),near-infrared(NIR)absorption and photothermal conversion have been partially integrated and explored.The main research contents are as follows:1.Two chiral tetracoordinate organoboranes(1 and 2)that feature two axially chiral binaphthyl(BINOL)groups and one double B←N bridged bipyridine(BNBP)were synthesized via an Et2AlCl-mediated reaction.The chiral structures of the thus-obtained BNBP derivatives were studied by X-ray diffraction analysis in detail.The BINOL and BNBP moieties are linked together with the formation of two O-B-O-containing seven membered rings,which induce two adjacent naphthalene units to form a helicene,resulting in a tetracoordinate organoboranes with two chiral centers.Importantly,they possess high fluorescence quantum yields(46%)and moderately high luminescence dissymmetric factors(±2.0 × 10-3)in both solution and solid states,thus demonstrating their efficient circularly polarized luminescence performance.2.A series of D-A-D-type organoborane emitters(3-5)with BNBP as the key electron-accepting unit were synthesized.We fully investigated their single-crystal and electronic structures,as well as photophysical and electrochemical properties.Notably,by varying the electron-donating groups from diphenylamine(DPA)and carbazole(CZ)to 9,9-dimethyl-9,10-dihydroacridine(DMAC),the torsion angles between the electron-accepting and electron-donating groups are significantly enlarged,thus affecting the luminescent properties of these three molecules.The aggregation-induced emission characteristic was unexpectedly observed for 5.Furthermore,it exhibits the intriguing TADF property,as determined by the slight overlapping between HOMO and LUMO and small singlet-triplet energy gaps(ΔES-T=0.26 eV).As a result,5 containing the BNBP and DMAC groups displays the AIE and TADF effects.3.Based on the above two design strategies,we report a series of D-A-D-type chiral tetracoordinated organoboranes(6-8)via constructing push-pull electronic structures and incorporating the BINOL groups into BNBP.The X-ray diffraction single-crystal analysis shows that the BINOL groups are linked to the boron atoms of BNBP to form two helicenes,thereby inducing the chiroptical properties of these three molecules.By varying the electron-donating groups from DPA and CZ to DMAC,their emission colors are modulated from yellow to red.The luminescence dissymmetric factors of 6-8 all exceed 1.50×10-3,thus indicative of their good circularly polarized luminescence performance.As a result,tetracoordinate organoborane 8 is an interesting molecule that features the CPL and AIE properties.4.Two quinoidal π-extended tricoordinate organoboranes(10 and 12)with polycyclic structures were synthesized.They can be regarded as a fusion of two tri coordinate organoborane moieties with one antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core.Variable-temperature 1H NMR and variabletemperature electron paramagnetic resonance prove the closed-shell and open-shell structures of 10 and 12,respectively,owing to the larger quinoidal conjugation in 12.As determined by the theoretical calculation,12 has the diradical character of 0.579 and singlet-triplet energy gap of-5.62 kcal mol-1.Compared with the unoxidized precursors 9 and 11 that exhibit the light absorption and fluorescence in the visible region,10 and 12 have two absorption bands in the visible and near-infrared regions,which are significantly contributed by the boron-containing moieties and quinoidal πskeletons.We also studied the photothermal conversion characteristics of 10 and 12.A broader absorption spectrum that covers the range of 230-1150 nm was observed for 12,thus indicative of its stronger light absorption capacity.Under irradiation with a mimic solar source(1.0 kW m-2),the maximum stable temperature of 12 reaches around 67℃,the photothermal conversion efficiency is 13.26%and the efficiency for solar-driven water evaporation is 88.89%,revealing the potential application of openshell tricoordinate organoboranes in solar-driven water evaporation.