Theoretical Study Of The Luminescence Mechanism Of Non-conventional Organic Light-emitting Systems

With the rapid development of science and technology,organic light-emitting materials are at-tracting the attention of researchers and enterprises because of their applications in the fields of display,sensing,bio-imaging,etc.Conventional organic fluorescent molecules have the dis-advantage of short lifetime and low internal quantum efficiency.Conventional phosphorescent molecules have the disadvantages of being difficult to stabilize at room temperature and con-taining heavy metals.Moreover,the conventional organic systems with planarπ-conjugated structures are prone to fluorescence quenching in aggregated environments,which limits their practical application.Therefore,the development of organic light-emitting materials with rela-tively high efficiency is very necessary.In organic room-temperature phosphorescent materials,long-lived phosphorescence at room temperature can be achieved due to sufficiently large spin-orbit coupling to break the spin-forbidden between single and triplet states.Organic radical molecules avoid the problem of using triplet excitons and therefore may achieve high external quantum efficiency,which have been applied in organic light-emitting diodes.Aggregation-induced luminescence system may improve the efficiency by suppressing non-radiative reces-sion in the aggregated environment.Based on the background,this paper will study the mech-anism of above three non-traditional organic light-emitting materials through computational simulation,and provides some guidance for experimental and other theoretical groups.The research contents are as follows:（1）The phosphorescence mechanism of S-BF2 organic molecule is studied theoretically,its phosphorescence comes from the T₂state that violates Kasha’s rule.The phosphorescence of（E）-3-（（（4-nitrophenyl）imino）methyl）-2H-thiochroman-4-olate-BF2 compound（S-BF2）at 575nm was assigned to the first triplet state（T₁）in previous experimental and theoretical studies.Whereas in this study it has been reassigned to the second triplet state（T₂）by the density matrix renormalization group（DMRG）method combined with the multi-configurational pair density functional theory（MCPDFT）to approach the limit of theoretical accuracy,being contrary to the previous conclusion.The calculated radiative and non-radiative rate constants support the breakdown of Kasha’s rule further.Based on the revised phosphorescence mechanism,we have purposefully designed some novel compounds in theory to enhance the phosphorescence efficiency from T₂by replacing substitute groups in S-BF2.Overall,both S-BF2 and newly de-signed high-efficiency molecules exhibit anti-Kasha T₂phosphorescence instead of the conven-tional T₁emission.This work provides a useful guidance for future design of high-efficiency green-emitting phosphors.（2）The high quantum efficiency of TTM-3NCz radical has been studied.A new organic light-emitting diode（OLED）based radical（3-substituted-9-（naphthalen-2-yl）-9H-carbazole）bis（2,4,6-trichlorophenyl）-methyl（TTM-3NCz）with 27%external quantum efficiency（EQE）was synthesized and reported while the reasons for its high EQE remain unclear,comparing with（4-N-carbazolyl-2,6-dichlorophenyl）bis（2,4,6-trichlorophenyl）-methyl（TTM-l Cz）.We com-prehensively and thoroughly investigate the origins of perfect EQE through first principles calculations and theoretical analysis.The calculation results indicate that photoluminescence quantum efficiency（PLQE）of TTM-3NCz is 44%,being twice that of TTM-1Cz（20%）due to fast radiation rate(2.31×10⁷s^-1)and slow non-radiation rate(2.95×10⁷s^-1).The re-sult is consistent with the experiment value of 49%.In addition,TTM-3NCz exhibits fast and balanced electron-hole transmission rates than TTM-1Cz,leading to a large carrier recombina-tion.Moreover,the rigid molecule presents a high out-coupling efficiency due to the”rod-like”structure.Our results confirm that high PLQE,efficient electron-hole recombination as well as the enhanced out-coupling efficiency are key parameters to control high EQE of TTM-3NCz.（3）The decay mechanism of MN10 and MN12 systems in solution is investigated based on conical cross model.2-（（E）-（（9H-fluoren-2-yl）methylene）amino）-3-aminomaleonitrile（M-N10）and 2-amino-3-（（E）-（4-（diphenylamino）benzylidene）amino）maleonitrile（MN12）were reported experimentally.The MN10 system does not emit light in solution,but glows green in a concentrated environment.The MN12 system emits weak light in solution and orange-red light in aggregates.In this study,theoretical calculations were performed to provide a com-prehensive understanding of the luminescence mechanism of MN10 and MN12 systems.The experimental absorption peaks of both systems are from the S₀→S₁excitation.Among the six isomers of MN10,MN10a is the dominant configuration for the absorption spectrum,whereas among the four isomers of MN12,MN12a contributes the most to the spectrum.The overall oscillation intensity of the MN10 system is larger than that of the MN12,so that the radiative rate of MN10 is twice that of MN12.The conical intersection points（CI）between S₀and S₁of the two systems have been optimized by CASSCF（8,8）to study the non-radiative paths.The S₁state of MN10 may quickly relax through a very low energy barrier to the S₀potential energy surface,leading to no emission in solution.While the S₁state of MN12 overcomes a relatively high energy barrier before decaying to the S₀state,causing relatively weak green light.The structural transformation of MN10 and MN12 from S₁to CI is mainly caused by the torsion of several important dihedral angles,indicating that these dihedral angles are the main factors of the non-radiative decay of the systems.Therefore,it can be predicted that these dihedral angles will be limited in an aggregate state,making the systems less prone to decline from S₁to S₀.