Theoretical exploration of the bromine substitution effect and hydrostatic pressure responsive mechanism for room temperature phosphorescence

Abstract

Stimulus-responsive organic room temperature phosphorescence (RTP) materials with long lifetimes, high efficiencies and tunable emission properties have broad applications. However, the amounts and species of efficient RTP materials are far from meeting the requirements and the inner stimulus-responsive mechanisms are unclear. Therefore, developing efficient stimulus-responsive RTP materials is highly desired and the relationship between the molecular structures and luminescent properties of RTP materials needs to be clarified. Based on this point, the influences of different substitution sites of Br on the luminescent properties of RTP molecules are studied by the combined quantum mechanics and molecular mechanics (QM/MM) coupled with thermal vibration correlation function (TVCF) theory. Moreover, the hydrostatic pressure effect on the efficiencies and lifetimes is explored and the inner mechanism is illustrated. The results show that, for the exciton conversion process, the o-substitution molecule possesses the largest spin–orbit coupling (SOC) value (〈S₁|Ĥ_so|T₁〉) in the intersystem crossing (ISC) process and this is conducive to the accumulation of triplet excitons. However, for the energy consumption process, the large SOC value (〈S₀|Ĥ_so|T₁〉) for the p-substitution molecule brings a fast non-radiative decay rate, and the small SOC value for the m-substitution molecule generates a decreased non-radiative decay rate which is helpful for realizing long lifetime emission. Keeping with this perspective, the conflict between high exciton utilization and long RTP emission needs to be balanced rather than enhancing the SOC effect by simply adding heavy atoms in RTP systems. Through regulating the molecular stacking modes by the hydrostatic pressure effect, the inner stimulus-responsive mechanism is revealed. The data of 〈S₁|Ĥ_so|T₁〉 in the ISC process remain almost unchanged, while 〈S₀|Ĥ_so|T₁〉 values and transition dipole moments are sensitive to the hydrostatic pressure. Under 1 GPa, the RTP molecule achieves a maximum efficiency (81.17%) and long lifetime (2.72 ms) with the smallest SOC and decreased non-radiative decay rate. To our knowledge, this is the first time that the hydrostatic pressure responsive mechanism for RTP molecules is revealed from a theoretical perspective, and the relationships between molecular structures and luminescent properties are detected. Our work could facilitate the development of high performance RTP molecules and expand their applications in multilevel information encryption.