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.