Solid-state effect on luminescent properties of thermally activated delayed fluorescence molecule with aggregation induced emission: A theoretical perspective

Solid-state effect plays an important role in defining the nature of excited states for thermally activated delayed fluorescence (TADF) molecules and further affects their luminescence properties. Theoretical investigation of photophysical properties with explicit consideration of intermolecular interactions in solid phase, is highly desired. In this work, the luminescent properties of new TADF molecule SBF-BP-DMAC with aggregation induced emission (AIE) feature are theoretically studied both in solution and solid phase. Solvent environment effect in Tetrahydrofuran (THF) is simulated by polarizable continuum model (PCM) and solid-state effect is considered by the combined quantum mechanics and molecular mechanics (QM/MM) method. By combing thermal vibration correlation function (TVCF) theory with first principles calculation, excited state energy consumption process is investigated. Our results show that the calculated prompt fluorescence efficiency, delayed fluorescence efficiency and total fluorescence efficiency in THF is 3.0%, 0.4‰ and 3.0% respectively, and corresponding value increases to 14.4%, 31.5% and 45.9% for molecule in solid phase, this verifies the AIE feature. To detect the inner mechanisms, the geometrical structures, Huang-Rhys (HR) factors and reorganization energies as well as excited state transition properties are analyzed. Decreased HR factor and reorganization energy are found in solid phase, this is caused by the restricted torsion motion of DMAC unit in rigid environment. Thus, non-radiative energy consumption process is suppressed and enhanced fluorescence efficiency is found in the solid phase. Moreover, the smaller energy gap between S1 and T1 in the solid state than that in THF, is more conducive for reverse intersystem crossing process and further improves the efficiency. This work provides reasonable explanation for the experimental measurements and reveals the inner perspectives for AIE and TADF mechanisms, which is advantageous to develop new non-doped OLEDs with advanced feature.