Excited State Properties of a Thermally Activated Delayed Fluorescence Molecule in Solid Phase Studied by Quantum Mechanics/Molecular Mechanics Method

摘要

Excited state properties of a thermally activated delayed fluorescence molecule (4-(10H-phenoxazin-10-yl)phenyl)(dibenzo-[b,d]thiophen-2-yl)methanone (DBT-BZ-PXZ) are theoretically studied in liquid (tetrahydrofuran (THF)) and solid phases, respectively. Solvent environment in THF is considered by polarizable continuum model (PCM) and the molecule in solid phase is investigated by a combined quantum mechanics and molecular mechanics (QM/MM) method. Results show that the geometrical changes between ground state (S-0) and lowest singlet excited state (S-1) are hindered in solid phase by the restricted intramolecular rotation (RIR) and restricted intramolecular vibration (RIV) effects, which brings smaller values of Huang-Rhys (HR) factors and reorganization energies compared to those in liquid phase. Thus, nonradiative energy consumptions are suppressed and enhanced fluorescent efficiency is found in solid phase. The calculated prompt fluorescence efficiency (Phi(prompt)) and delayed fluorescence efficiency (Phi(TADF)) in solid phase are 14.2% and 31.2% respectively, which demonstrates the aggregation induced emission (ME) mechanism for DBT-BZ-PXZ. Moreover, temperature dependence of the reverse intersystem crossing (RISC) rate is theoretically illustrated. Furthermore, a hybridized local and charge transfer (HLCT) property of the lowest triplet excited state (T-1) is found. This transition feature brings a large spin-orbit coupling (SOC) constant and a small energy gap (Delta Est) between S-1 and T-1, which facilitates the RISC process. Our calculations give reasonable explanation for the previous experimental measurements and provide underlying perspectives for nonradiative assumptions of excited state energy.