Design And Synthesis Of Blue Fluorescent Materials With Narrow Full-width At Half-Maximum Spectra And Their Photoelectric Property

At present,OLED display is developing in the direction of high color purity and high resolution.In the current technology,the method of improving the color purity of electroluminescence is mainly through color filter or constructing a special optical microcavity structure,but these methods usually reduce the external quantum efficiency of the device and increase energy loss.Therefore,designing and developing organic light-emitting materials with intrinsic high-color purity light emission is a fundamental way to improve the color purity and obtian high resolution of OLED devices in the future.However,organic fluorescence molecules inherently have larger excited-state electrons and nuclear vibrational relaxation and stronger vibration coupling,resulting in their emission bands generally being wider.Therefore,the development of organic light-emitting materials with narrow-band emission has always been a challenge.This paper hopes to systematically study the factors that affect the full-width at half-maximum?FWHM?of fluorescence spectra through four types of molecular structure systems,mainly from the overlapping of molecular transition orbitals,an intramolecular hydrogen bond structure and rigid conjugate structure that inhibits molecular vibration,and moderate stiffness and flexible molecular structure dominated by a small recombination energy and low-frequency vibration mode.The main tasks are as follows:?1?The effect of molecular transition orbital overlap on the FWHM of spectra.In this part of the work,two D???A type deep-blue fluorescent materials DFPBI and TFPBI with arylamine as electron donor,imidazole derivatives as electron acceptor,and9,9-dimethylfluorene as?-conjugated bridge were designed and synthesized.We focus on the analysis of the effect of molecular transition orbital distributions on spectra broadening.According to the analysis of the frontier orbital distribution,it is found that the HOMO/LUMO of the two molecules are mainly concentrated in the central core of the molecule,while the distribution on the peripheral benzene ring is less.In addition,its excited state transition is mainly concentrated on the?-conjugated bridge in the middle of the molecule,which has a large orbital overlap,so both materials exhibit deep blue emission with small spectral FWHM,and the fluorescence quantum efficiency is greater than 60%.After device optimization,the EL spectrum FWHM of the device based on DFPBI is only 40 nm,and the CIE coordinate is?0.156,0.056?.?2?The effect of intramolecular hydrogen bond on the FWHM of spectra.Based on the classic deep-blue emitting material TPA-PPI,the target molecule TAP-PPI-OH was designed and synthesized by introducing a hydroxyl group?-OH?into its molecular structure.Similar to TPA-PPI,the lowest excited state of TPA-PPI-OH is a special hybrid local and charge transfer excited?HLCT?state,which enables the molecule to have high fluorescence quantum efficiency in both solution and thin films.The single crystal analysis of TPA-PPI-OH shows that there are multiple intramolecular and intermolecular hydrogen bond interactions in the crystal,and this hydrogen bond effect is beneficial to suppress the structural vibration of the ground state and excited state of the molecule.Therefore,the fluorescence spectrum of TPA-PPI-OH has a smaller FWHM?in films and low-polarity solvents?than TPA-PPI.The non-doped electroluminescent device based on TPA-PPI-OH as the emitting layer has good performance,its highest external quantum efficiency is 7.37%,and the FWHM of the electroluminescence spectrum is 58 nm?71 nm for TPA-PPI?.Moreover,the efficiency roll-off of the device is small,and the external quantum efficiencies at brightness of 100 cd m^-2 and 1000 cd m^-2 are 7.37%and 5.48%,respectively.?3?The effect of rigid conjugate structure on the FWHM of spectra.In this part of the work,we designed and synthesized a new kind construction moiety of luminescent material,dimethyl benzo[a]acridine,with rigid conjugated structure.Connect the benzene ring at the N position of this group to prevent the?-?close packing between the molecules,and the fluorine atom?-F?,cyano?-CN?and phenylcyano?-PCN?with different electron withdrawing capacity are replaced at No.4 position.Four fluorescent molecules ACPH,ACPF,ACPCN and ACPPCN are obtained,respectively.Theoretical calculations show that dimethyl benzo[a]acridine exhibits a nearly planar molecular configuration.Tests of photophysical properties found that as the CT state increases,the broadening of the molecular fluorescence spectrum increases.Therefore,the special rigid conjugated structure gives ACPH a minimum spectral FWHM of 40 and 46 nm in solution and film,respectively.When evaluating the spectral width in the form of energy difference,whether in solutions or in films,the energy difference of ACPCN is the smallest,indicating that the CT state of appropriate intensity is beneficial to the narrowing of spectrum.Finally,we used ACPCN and ACPPCN fluorescence materials to fabricate non-doped and doped devices.The maximum EQEs of all devices are more than 5%,and the FWHM of the EL spectrum of the ACPCN-based doped device is the smallest of 41 nm/0.26 e V,with the standard blue CIE coordinates of?0.15,0.05?.The planar multiple ring conjugated rigid structure of dimethyl benzo[a]acridine effectively suppresses the vibration of the molecule under ground and excited states.However,the vibration coupling of this rigid structure is large,so in order to continue to reduce the broadening of the spectrum,further adjustment and optimization of the molecular structure are required.?4?The effect of conjugated structure with moderate stiffness and flexibility on the FWHM of spectra.Based on non-planar QAO groups,four symmetric fluorescent molecules QA-PF,QA-PCN,QA-PMO and QA-PCZ were synthesized by linking groups with different electronic properties in the axis-symmetrical directions of QAO.Through theoretical calculations and analysis of photophysical properties,we found that the rigid QAO makes the structural deformation of these materials in the excited state smaller,making the molecular reorganization energy small.The partial flexibility in the molecular structure allows the torsional vibration of QAO and the rotational vibration between QAO and peripheral substituents,rather than the intense stretching vibration of molecular bonds.Therefore,these materials exhibit very narrow-band spectra in both solutions and doped devices.In addition,the S₁ state of these materials exhibits the HLCT characteristics,and the energy gaps between the S₁ state and the T₁ state are small,which is conducive to the improvement of photoluminescence efficiency and the occurrence of the TADF process.Therefore,doped devices based on these materials exhibit high external quantum efficiency of 15?17.5%and pure blue emission of EL spectrum FWHMs?30 nm.