| Due to its strong electron absorption and large steric effect,boron atoms have been encouraging many researchers to explore the organic functional materials of triarylboron.When electron acceptor(A)boron substituent and electron donor(D)amino group are present in the system,the compound exhibits strong intramolecular charge transfer(CT)characteristics,and thus has unique optical properties.Triarylboron compounds with CT properties have been widely used as luminescent materials in organic electroluminescent devices,fluoride ions and cyanide ions fluorescence probes,temperature sensors,etc.In 2019,our group synthesized and reported a new class of triarylborane compounds,2,2’-diamino-6,6’-diboryl-1,1’-binaphthyl(dimethylamino substituted BNMe2-BNaph and diphenylamino substituted BNPh2-BNaph).Unlike normal CT-specific molecules,this structure is essentially composed of two D-π-A subunits connected by a single bond.Thus,two possible pathways for fluorescence emission are provided,namely,inter-subunit CT and intra-subunit CT,which correspond to the inactivation of the first excited state(S1)and the second excited state(S2),respectively.In addition,since S1 and S2 are almost degenerate in the Frank-Condon region,the thermal equilibrium between them is possible.Two very challenging properties,temperature sensitive fluorescence dual emission and convertible circularly polarized luminescence(CPL),were found to be achieved simply by selecting different amino groups.Due to the unique optical properties of 1,1’-binaphthyl with two D-π-A units,we hope to conduct in-depth study on the system with two D-π-A units.In this paper,we mainly change the structure of the parent biaryl group to study the influence of the biaryl structure on the optical properties of the system,so we carried out the following two aspects of work:1.Considering that 2,2’-bithienyl is more electron-rich than 1,1’-binaphthyl,we designed two derivatives of 2,2’-bithieyl,3,3’-diamino-5,5’-diboryl-2,2’-bithienyls(dimethylamino substituted BNMe2-BT and diphenylamino substituted BNPh2-BT).The experimental results show that although the two compounds also have the chemical structure of two D-π-A similar to the corresponding binaphthyl compounds,they show completely different stereoscopic and electronic structures and optical properties.Compared with the corresponding binaphthyl compounds,the absorption and emission of bithienyl compounds in cyclohexane showed obvious red shift,which indicates that the emission wavelength can be adjusted effectively by substituting 2,2’-bithienyl for 1,1’-binaphthyl.However,the two compounds had only one fluorescence emission peak at different temperatures and solvents,and did not show the phenomenon of temperature-sensitive fluorescence double emission similar to binaphthyl compounds.The X-ray crystal structure and theoretical calculation show that the two thiophene rings are coplanar in both the ground state(S0)and the first excited state(S1),so the two thiophene derivatives are more like a quadrupolar A-π-D-π-A structure.Thanks to this structure,they have strong two-photon absorption(TPA)in THF,with a two-photon absorption cross section of up to 473 GM and 350 GM,respectively.Moreover,the nanoaggregate solution formed by these two derivatives in the system with 95%water volume fraction(fw)also has good fluorescence quantum yield and two-photon absorption cross section,which may be used in two-photon excited fluorescence(TPEF)microscopy.These results not only show that the selection of biaryl groups has a very important influence on the structure and optical properties of triarylboron compounds with two D-π-A structure,but also provide an important theoretical basis for the design of new TPEF materials and other optical functional materials.2.On the basis of the above work,we further use anthracene instead of naphthalene,and we design two derivatives of 1,1’-binanthracene with two D-π-A structure,2,2’-diamino-1 0,10’-diboroyl-1,1’-binanthracene(dimethylamino substituted BNMe2-BAn and diphenylamino substituted BNPh2-BAn).The absorption and emission of the synthesized dimethyl amino compound BNMe2-BAn also produced a red shift compared with that of BNMe2-BNaph,indicating that the substitution of 1,1’-binaphthalene by 1,1’-binanthracene could effectively regulate the emission wavelength.However,BNMe2-BAn has only one fluorescence emission peak under different polar solvents and different temperatures,and the intensity of emission peak does not change significantly with the change of temperature.In addition,the position of emission peak does not change significantly with the change of solvent polarity.From non-polar cyclohexane to polar tetrahydrofuran,the emission peak is only red shifted by 31 nm,which is very similar to the short-wavelength emission peak of S2 state corresponding to the intra-subunit CT transition of BNMe2-BNaph.X-ray crystal structure and theoretical calculation show that the two anthracene rings of BNMe2-BAn are in s-cis conformation at So and S1,and the anthracene skeleton is highly twisted,which is highly similar to BNMe2-BNaph,and the electronic structure of BNMe2-BAn is also very similar to BNMe2-BNaph.The emission of S1 also corresponds to the inter-subunit CT transition,and we speculate that its fluorescence may be an anti-Kasha rule fluorescence from its S2→S0 deexcitation process.The synthesis of BNPh2-BAn and the optical properties of BNMe2-BAn and BNPh2-BAn need to be further improved. |
