Strategy to modulate the singlet-triplet energy gap for spiro-based thermally activated delayed fluorescence molecules

The energy gap between singlet and triplet states plays an important role in determining the reverse intersystem crossing (RISC) process for thermally activated delayed fluorescence (TADF) molecules, ingenious strategy to modulate the singlet-triplet energy gap is highly desired. In this work, the photophysical properties of eleven TADF molecules are theoretically investigated, which are composed of 9,10-dihydro-10-phenylacridine as an electron donor and various electron acceptor units based on spiro donor-acceptor structure as well as adopting the indene-carbazole unit as the bridging body. Two new proposed calculation methods are applied. The method of overlapping integral and Delta Est, is used to screen efficient TADF molecules, and the optimal Hartree-Fock method is adopted to investigate the excited state properties for promising TADF molecules. Results show that different substituents in the indene-carbazole unit are distinctive in modulating the singlet-triplet energy gaps and the emission colors. Moreover, two promising TADF molecules with purple and blue emission are detected and the performances of emitting molecules in host matrix are theoretically investigated. Remarkable spectral overlap between host emission and guest absorption can be found and efficient energy transfer from host to guest is expected. Furthermore, the charge transfer properties are revealed based on Marcus equation, excellent hole and electron transfer properties are detected, which are comparable with these experimental detected molecules. Our molecular design strategy could provide a rational approach to regulate the emission and energy transfer as well as charge transfer properties, two promising TADF molecules with purple and blue emission are theoretically proposed.