Synthesis And Property Of Chiral Luminescent Materials Based On Helicene Derivative

Due to their circularly polarized luminescence,chiral luminescent materials have promising applications in 3D display,information storage,nonlinear optics,spintronic devices and biological probes.Using chiral luminescent materials as guest materials in OLEDs would make these devices possessing circularly polarized luminescence performance,which is a very promising development direction for the realization of thin and efficient 3D display technology.However,the chiral emitting materials reported always show poor device performance with low luminescence asymmetry factors(glum-factors),which can hardly be used in commercial applications.Therefore,it is urgent to improve the glum-factor of chiral circularly polarized luminescent materials and obtain circularly polarized OLEDs with excellent performances and high glum-factors.1.Helicenes showed high anisotropy and strong chiral optical performance due to their inherent spiral chirality and π-conjugate electronic structures.However,traditional helicenes are lack of modification sites and functional groups,which makes them difficult to develop diversified applications and subsequent modifications.Due to the steric hindrance and multiple reaction sites,inortho fused isomers are prone to be produced in the photo-cyclization reaction of the traditional synthesis strategies.In order to solve this problem and improve the selectivity of the cyclization reaction,2-hydroxycarbazole was employed as the precursor to synthesize a novel helicene derivative.By introducing placeholder groups(methoxy group)in the nontarget sites of the photo-cyclization reaction,the reaction becomes high selectivity with good yield,and the helicene skeleton is rich in functional groups(2 methoxy groups and 2 nitrogen heteroatoms)which endows it diverse modification sites and reconstruction potentials.2.Based on the helicene derivative mentioned above,two D-A structure helicene derivatives(9NTN and 9TFP)with electron-withdrawing groups connected to the ring end were synthesized.Electron-withdrawing groups with 2-trifluoromethylpyridine and 2-trifluoromethylpyrimidine were employed,and the racemates of 9NTN and 9TFP were separated by chiral semi-preparative HPLC.The enantiomers obtained can still remain their original configuration without racemization at 320℃.The CD and CPL spectra of P-9TFP and M-9TFP showed glum-factors around their maximum emission peaks of 3.7×10-3 and-3.8×10-3,respectively.P-9TFP and M-9TFP were employed as guest luminescent materials in the circularly polarized OLEDs of ITO/HATCN(4nm)/TAPC(48nm)/P-9TFP or M-9TFP(15wt%):TCTA(8nm)/TCTA(1 nm)/TmPyPB(48nm)/LiF(1 nm)/Al(110 nm).The circularly polarized electroluminescence(CPEL)spectra were measured showing electroluminescence asymmetry factors(gEL-factors)around the maximum emission peaks of 0.8×10-3 and-1.1×10-3,respectively.The device performances including luminance,turn-on voltage,current efficiency and external quantum efficiency(4755 cd m-2 at 10 V,turn-on voltage:3.18 V,maximum current efficiency:5.6 cd A-1,maximum external quantum efficiency:3.0%)are good among materials with the same kind.3.Due to the inherent defects of fluorescent molecules,the internal quantum efficiency of OLEDs with fluorescent molecules as the guest materials cannot exceed 25%,and the external quantum efficiency is commonly below5%,which limits their applications in organic electroluminescence.However,the internal quantum efficiency of thermally activated delayed fluorescence(TADF)molecules can be up to 100%,so we expect to transform 9NTN and 9TFP into TADF molecules by structure modification.For 9NTN and 9TFP,although they have D-A structures,fluorescence lifetime tests indicate that they are not TADF molecules.According to theoretical calculation,it may be that the conjugation of helicene derivative is too big but the electron-withdrawing abilities of electron-withdrawing groups are too small,which would result in large overlap of HOMO and LUMO,and large △Est,which results in no TADF phenomenon.In order to solve this problem,diphenyl sulfone with higher electron-withdrawing ability was used as the electron-withdrawing group to construct the product 9F.Chiral separation and theoretical calculation on 9F were carried on,and their electrochemical properties,ultraviolet-visible absorption spectra,emission spectra,circular dichroism spectra,CPL spectra and low temperature emission spectra were tested.However,the △Est was calculated to be 0.43 eV by low temperature emission spectra,indicating that it is still not a TADF molecule.In combination with related literature and the results of theoretical calculation,the reason of the large AEst may be due to the small torsion angle between the diphenyl sulfone group and the helicene ring,and too big conjugation of the helicene derivative.In the following work,we plan to connect electron-withdrawing groups to the middle of the helicene ring or decrease the conjugation of the helicene derivative,expecting to obtain helicene-like TADF molecules.