Design And Synthesis Of High-Efficiency Near-Infrared Radicals And Their Application On OLEDs

Near-infrared light refers to electromagnetic waves with a wavelength range of 700-2500 nm.Compared to visible light with a wavelength range of 400-700 nm,near-infrared light has the characteristics of strong penetration,invisibility to the naked eye and minimal damage to biological tissues.Both the ground state and excited state of radical luminescent material are doublet,avoiding the problem of utilizing triplet states of traditional fluorescent molecules,and theoretically achieving 100% utilization of excitons,which has attracted widespread attention once published.However,compared with other material systems,research on radical near-infrared luminescent materials and their OLED is relatively lagging behind,mainly due to two reasons.First,in recent years,although there have been many reports of applying luminescent radical materials to OLEDs,the electroluminescence of radical OLEDs mainly focuses on red and deep red light.Secondly,according to the energy gap rule,the narrower the energy gap in the near-infrared region,the non-radiative transition rate of the molecule increases exponentially,resulting in a decrease in the fluorescence efficiency or even no luminescence.Therefore,designing and synthesizing an efficient near-infrared luminescent radical material system has significant research significance.Therefore,this paper focuses on the following work:1.To explore how to achieve efficient near-infrared(NIR)emitting free radical molecules,we designed efficient NIR free radical molecules from the perspective of molecular configuration.We chose strong donor 9,9-dimethyl-9,10-dihydro-acridine(DMAC)as the donor group and tri-(2,4,6)-trityl radical(TTM)as the acceptor group,and designed and synthesized two NIR emitting free radical molecules TTM-PDMAC and TTM-3PDMAC with the same molecular formula but different molecular configurations.Characterization of the basic photophysical properties of the two molecules revealed that TTM-3PDMAC had a more red-shifted emission and higher emission efficiency.Furthermore,the OLED fabricated with TTM-3PDMAC as the emitting material had a peak emission wavelength of 830 nm and an EQE of 3.1%,almost reaching the highest value of non-metallic emitting materials.In-depth research found that the smaller dihedral angle between the donor and acceptor in TTM-3PDMAC was the reason for the above results.This study provides a method to design efficient NIR emitting free radicals.2.We first attempted to use metal complexes as the main component to prepare near-infrared(NIR)radical OLEDs.Through device characterization,we found that compared to 4,4’-bis(9-carbazole)biphenyl(CBP)as the host,NIR radical devices with metal complexes as the host not only exhibited redshift in electroluminescence but also increased the device’s current density and radiance.Through a series of device characterizations,we believe that metal complexes have a stronger carrier transport ability,which leads to an increase in the overall current density of the device,and thus an enhancement in radiance.3.We investigated the role of thiophene as a conjugated connecting group in designing and synthesizing near-infrared emitting radicals for the first time.Firstly,we designed and synthesized a new radical,TTM-2PTI,with thiophene-fused [3,2-b]indole(PTI)as the donor,and explored its series of photophysical properties.TTM-2PTI has an emission peak wavelength of 830 nm in toluene solution,which is 143 nm redshifted compared to the radical TTM-3PCz with a benzene-fused conjugated group in toluene solution.This indicates that thiophene as a conjugated connecting group can promote the redshift of radical emitting materials.However,the emission efficiency of TTM-2PTI is low,and the decreased rigidity of the material due to the narrowing of the energy gap and rotational deactivation are important reasons.Then,based on the thiophene conjugated connecting group,we further increased the electron-donating ability of the donor group and synthesized the radical TTM-DPS.TTM-DPS has an emission peak wavelength of up to 900 nm in toluene.Meanwhile,the electroluminescence peak wavelength of the OLED device made with TTM-DPS as the emitting material reached 950 nm.The study indicates that using thiophene as the donor and the conjugated connecting group can effectively promote the redshift of the radical emission and provide new ideas and methods for designing and synthesizing near-infrared radical molecules.