Reconfigured Spin-Flip Process Enables Efficient and Persistent Triplet Excitons in Organic-Inorganic Metal Halides

Triplet excitons, driven by spin-flip processes, play a crucial role in enabling efficient room-temperature phosphorescence across various applications. However, attaining a significant accumulation of long-lived excitons is impeded by the simultaneous influence of nonradiative and radiative decay pathways alongside intersystem crossing efficiencies. Here, we introduce a solvent intercalation approach that leverages the triplet exciton processes in a family of zero-dimensional organic-inorganic halides, A₂ZnBr₄ (A = organic phosphonium cations). By intercalating phosphorescence inactive molecules into these halides, their spin-flip processes can be reconfigured. This leads to significantly amplified intersystem crossing but attenuated radiative and nonradiative transitions, which give rise to 16- and 6-fold increases in lifetime and quantum yield, respectively. Our single crystal X-ray diffraction, transient absorption, and theoretical calculation results reveal that such dramatic improvement is attributed to the unique spatial effect on both electrons and holes induced by the intercalated molecules. The consequently reduced orbital degeneracy increases the number of spin-allowed channels, promoting intersystem crossing, while the synergistically enhanced electron localization diminishes the triplet exciton decay, leading to high efficiency and enduring phosphorescence. Our findings offer a new pathway for manipulating the spin-flip process to boost the emission of triplet excitons, with potential applications in designing a wide spectrum of phosphorescent materials.