Elaborating the influence of substituent on energy gap and spin-orbit coupling of singlet-triplet excited states of novel organic light-emitting anthraquinone compounds in solution

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In this work, we presented a theoretical investigation of single-triplet excited state transition process by the energy gap (ΔEST) and spin-orbit coupling (SOC) for a series of novel designed anthraquinone compounds in solution. The molecular conformation, steady-state spectra and molecular orbitals were performed by quantum chemistry calculations to investigate the substituent effects on the ΔEST and SOC and single-triplet state transition process. The molecular conformation gradually change from the planar of the ground state to torsion, which reduced the orbital overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It was further demonstrated that the ΔEST and SOC remarkably depend on substituents with different conjugation and the presence of benzene in the donor units in solution. The change of ΔEST from 1.80 eV to 0.70 eV and SOC from 0.495 cm−1 to 5.127 cm−1. Furthermore, it was confirmed that the presence of benzene ring and the enhancement of conjugation in the donor substituent can reduce the ΔEST and enhance the SOC in different anthraquinone compounds. This work paves a way for an understanding of the single-triplet excited state transition process controlled by ΔEST and SOC characters of these kinds of anthraquinone compounds in solution.