Critical Roles of High- and Low-Frequency Optical Phonons in Photodynamics of Zero-Dimensional Perovskite-like (C 6 H 22 N 4 Cl 3 )SnCl 3 Crystals.

The zero-dimensional perovskites composed of isolated polyhedrons have unique and distinct physical properties compared with three-dimensional perovskites composed of interconnected polyhedrons. Here, we study the photodynamics of the zero-dimensional perovskite-like (C ₆ H ₂₂ N ₄ Cl ₃ )SnCl ₃ single crystals composed of isolated [SnCl ₃ ] ^- tetrahedrons. They exhibit red luminescence with huge Stokes shift (2.49 eV), large spectral broadening (416 meV), and long lifetime (6.9 μs). The experiments in conjunction with the ab initio calculations reveal the special roles of high- and low-frequency phonons in the photodynamics of the (C ₆ H ₂₂ N ₄ Cl ₃ )SnCl ₃ crystals. The resonance between the organic-cation-related high-frequency optical phonons and the singlet-to-triplet state transition induces strong intersystem crossing and resultant spin-forbidden luminescence. The strong electron-tetrahedron-related low-frequency optical-phonon coupling revealed by the low-temperature spectral characterization causes large spectral broadening. The strong lattice relaxation owing to localization of the electronic orbitals along with intersystem crossing accounts for the large Stokes shift.