Photoinduced Dynamics of (CH₃NH₃)₄Cu₂Br₆ Thin Films Indicating Efficient Triplet Photoluminescence

Hybrid organic-inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful preparation of (CH₃NH₃)₄Cu₂Br₆ thin films with a zero-dimensional molecular salt structure featuring "isolated" [Cu₂Br₆]^4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114 μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption experiments combined with DFT/TDDFT calculations suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addition, time scales for structural relaxation and cooling processes are extracted from a global kinetic analysis of the transient spectra. Calculations for the isolated [Cu₂Br₆]^4- anion and the (CH₃NH₃)₄Cu₂Br₆ crystal suggest a strong impact of the crystal environment on the structure of the anion.