Theoretical Investigation And Molecular Design Of Highly Efficient Fluorescence Organic Light Emitting Materials

Considerable research efforts have been devoted to the development of organic light-emitting materials motivated by their potentially commercialized applications in the field of flat-panel displays and solid lighting.In comparison to traditional fluorescent and coordinate bond-based phosphorescent materials,thermally activated delayed fluorescence（TADF）and hybridized local and charge transfer（HLCT）emissions have become the research hot spot for highly efficient organic light emitting materials because they can achieve high internal quantum efficiency of nearly 100%by maximally converting of electrically generated triplet excitons to singlet excitons.To date,high efficiency and stable TADF materials have been widely reported,but theoretical insight on the luminescence mechanism is not in-depth and the efficiency roll-off at high current densities still seriously limits the practical application.Therefore,it is of utmost importance to study the transition characters of excited-states and understand the luminescence mechanism through theoretical calculation to maximize the reverse intersystem crossing（RISC）rate.Additionally,both TADF and HLCT emitters are promising and efficient electroluminescent materials,but the basic emission mechanisms of TADF and HLCT materials are clearly distinct and both emitters are designed independently due to their distinct excited-state properties.We comprehensively considered the intrinsic structure-property relationships between TADF and HLCT materials,and proposed a simple design strategy to construct structurally similar TADF and HLCT materials.For donor-acceptor（D-A）type molecules,the main difference between TADF and HLCT materials lies in the proportion of charge transfer（CT）excitation in the singlet excited states.To achieve this goal,structurally similar D-A type molecules with fixed acceptors（or donors）will be considered and TADF and HLCT emissions could be realized by modulating the percentage of CT excitation in the singlet excited state using tunable donors（or acceptors）with different electron-donating（-withdrawing）ability.We aim to reveal the intrinsic structure-property relationships between TADF and HLCT materials at the microscopic level.Therefore,our work mainly investigates the effect of excited-state properties on the luminescence process of organic small molecules by quantum chemical calculation,including four research contents:molecular design and property prediction of di-boron derived TADF emitters,importance of spin-triplet excited-state character on the RISC process of the spiro-based TADF emitters,realization of switching between TADF and HLCT emissions through modulation of the intramolecular CT character with the fixed acceptor units and the changeable donor fragments,and rational designs of structurally similar TADF and HLCT emitters with benzo-or naphtho-carbazole units as electron donors.The structure-property relationships of organic molecules are systematically explored.It provides theoretical guidelines for designing and screening highly efficient electroluminescent materials.The main research contents are as follows:（1）A class of TADF molecules with electron-deficient diboron-derived units have been investigated by means of quantum chemical calculation methods to provide theoretical insights into the differences in luminescence efficiency of di-boron derived TADF emitters.Through the fixation of 9,10-diboraanthracene（DBA）with its adjacent benzene rings and the extension ofπ-conjugation of DBA unit could effectively regulate the electron-withdrawing ability of di-boron acceptor units,thus realizing the full-color emission in the visible light range.The TADF mechanism is revealed through the analyze of RISC and radiative process.The calculated results indicate that the RISC process of the emitter may be promoted by the nonadiabatic coupling effect or the dynamic change of molecular conformation activated at room temperature.The structure-property relationships of diboron-derived compounds were established to screen the highly efficient diboron-containing TADF materials.（2）We perform a systematic quantum chemical calculation to elucidate the effect of spin-triplet excited-state character on the RISC process of the spiro-based TADF emitters through the tuning of electron-donating abilities of donors and electron-withdrawing abilities of acceptor groups.Our calculations indicate that the RISC rates of the spiro-based TADF molecules are mainly affected by the parameters of reorganization energy(λ_RISC)and spin-orbit coupling（SOC）constant,which are closely related with the excited-state properties.The singlet excited states show similar CT characters,and the lowest triplet excited states exhibit five distinct transition properties:LE_A,CT+LE_A,CT,CT+LE_D,LE_D.This study clarifies the effect of different excited-states characters onλ_RISC and SOC,and provides a deeper understanding of the relationship between RISC process and excited state properties,aiming to the optimization of TADF performance.（3）We propose a rational molecular design strategy for realization of switching between TADF and HLCT emissions in structurally similar D-A type molecules with a theoretical investigation,which are constructed with the same acceptor units（benzothiadiazole（BZ）or 9-borafluorene（BF））and donor fragments with the changeable electron-donating abilities（triphenylamine（TPA）and phenoxazine（POZ））.Such switching between HLCT and TADF emissions could be attributable to modulation of the weighting of CT excitation in their singlet excited states.The properties of excited states were analyzed under the framework of state hybridization theory to reveal the intrinsic structure-property relationships between HLCT and TADF molecules.This study provides a rational design strategy for design both HLCT and TADF molecules together.（4）A simple design strategy for the construction of structurally similar D-A type molecules to realize both HLCT and TADF emissions is proposed based on density functional theory（DFT）,which are assembled with the same electron donors（benzo-or naphtho-carbazole）and acceptors with tunable electron-withdrawing abilities(benzonitrile（BN）and benzene-1,2,3,4,5-pentacarbonitrile（BPN）.Such switching of two types of emissions could be realized by modulation of the intramolecular CT character between donor and acceptor units.In the framework of state hybridization theory,the excited state characteristics are analyzed to reveal the intrinsic structure-property relationships for the donor-based HLCT and TADF molecules.This work provides theoretical guidances for the design and screen of highly efficient electroluminescent materials.