Substitution effect on luminescent property of thermally activated delayed fluorescence molecule with aggregation induced emission: A QM/MM study

Pure organic molecules with blue emission have attracted much attention due to its important application in organic light emitting diodes (OLEDs), especially for which with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties. Theoretical study to reveal the inner luminescent mechanisms can promote its development. In this work, four kinds of molecules with perfluorobiphenyl (PFBP) unit as acceptor, non-substituted and tert-butyl substituted 9,9-dimethyl-9,10-dihydro-acridine (DMAC) unit as donors, are selected and their photophysical properties are studied in detail. The surrounding environment effects in toluene and solid phase are taken into consideration by the polarized continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. Results show that geometric changes between the first singlet excited state (S-1) and ground state (S-0) are restricted in solid phase with decreased root-mean squared displacement (RMSD). Moreover, the Huang-Rhys factors and reorganization energies we calculated are all decreased in solid phase, which indicates that the non-radiative energy consumption process of S1 is hindered by enhanced intermolecular interactions in rigid environment, and it brings aggregation induced emission phenomenon. Furthermore, the substitution effect of tert-butyl in donor unit can efficiently decrease the energy gap and increase the spin-orbit coupling (SOC) constant, further promotes the intersystem crossing (ISC) and reverse intersystem crossing (RISC) rates. Meanwhile, molecules with donor-acceptor-donor (D-AD) configuration have more efficient luminous performance than D-A type molecules due to the enhanced ISC and RISC processes. Thus, tert-butyl substituted D-A-D type molecules have outstanding TADF features. Our investigations provide a theoretical perspective for AIE and TADF mechanisms and propose a design strategy for efficient TADF molecules, which could promote the development of OLEDs. (c) 2019 Elsevier B.V. All rights reserved.