Enhancing reverse intersystem crossing and horizontal dipole orientation for near-infrared thermally activated delayed fluorescence molecules by donor engineering strategy

Abstract

Improving the internal quantum efficiency and optimizing the horizontal dipole orientation in near-infrared thermally activated delayed fluorescence (TADF) molecules are highly desired outcomes. Herein, the excited-state properties of three reported TADF molecules (DPDBP-TPA, 2CNDPDBP-TPA and 4CNDPDBP-TPA) are theoretically investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method. The results demonstrate that the introduction of cyano groups enhances the charge transfer features of the molecules, leading to small energy gaps (ΔE_ST). Additionally, this adjustment effectively optimizes the energy levels and transition characteristics of the excited states, and also significantly accelerates the reverse intersystem crossing (RISC) process. Based on this point, five new near-infrared TADF molecules are theoretically designed using a donor engineering strategy. Their crystal structures are predicted with a lower energy and higher packing density, and the photophysical properties of the designed molecules both in toluene and solid phase are studied in detail. The designed molecules not only possess increased molecular lengths but also ensure quasi-planar conformations. Moreover, intramolecular hydrogen bonds enhance the rigidity of the molecules. Collectively, these factors assist in enhancing the horizontal dipole orientation. Notably, the significantly reduced ΔE_ST in the solid phase lead to rapid RISC processes and enhance TADF properties. Thus, five new near-infrared TADF molecules are verified and inner relationships between molecular structures and photophysical properties are detected, which could pave the way for the development of novel efficient near-infrared TADF molecules.