The role of heavy atom effect in regulating multiple-resonance and thermally activated delayed fluorescence features: A theoretical perspective

Abstract

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with high efficiency and narrowband emission characteristics have shown wide applications in organic light-emitting diodes. However, MR-TADF emitters usually exhibit slow reverse intersystem crossing (RISC) rates, the amount and species of efficient MR-TADF emitters are limited, corresponding structure–property relationship needs to be clarified. Herein, based on density functional theory, time-dependent density functional theory and SCS-ADC(2) method, five reported MR-TADF molecules (CzBO, CzBS, CzBSe, Cz-PTZ-BN and BN-Se) are adopted and their photophysical properties are studied by thermal vibration correlation function (TVCF) method, the heavy atom effect on increasing spin–orbit coupling (SOC) and thus accelerating RISC process is elucidated. Based on this strategy with π-conjugated extension of MR unit, four new molecules (wBN-O, wCz-PTZ-BN, wBN-Se and BN-O) are theoretically proposed and their full-width at half-maximum (FWHM) values and excited decay rates are calculated. Results indicate that all studied molecules possess small energy gaps (ΔE_ST) between S₁ and T₁, and they are corresponding well with experimental values. In addition, large SOC constants are determined for designed molecules, remarkable RISC processes are realized and TADF features can be expected. Moreover, the large planarity for studied molecules brings small FWHM value with decreased reorganization energies and restricted geometry changes between S₁ and ground state (S₀). However, the ΔE_ST values for large planarity molecules are increased and insufficient RISC processes are determined. Thus, the relationship between efficient RISC process with large SOC values and narrowband emission with small FWHM values should be carefully balanced by wise molecular design strategy. This work illustrates the key physical parameters in regulating FWHM values and TADF properties, which could pave the way for the development of efficient MR TADF molecules.