Photoinduced Dynamics of [N(C3H7)4]2[Cu4Br6] Thin Films with Dual Self-Trapped Exciton Emission and Negative Thermal Quenching

Abstract

Organic-inorganic halocuprates(I) form a promising class of light-emitting materials with high photoluminescence (PL) quantum yield. However, the understanding of their emission properties and the PL mechanism is still limited. Here, we investigate thin films of bis(tetrapropylammonium) hexa-μ-bromo-tetrahedro-tetracuprate(I), [N(C₃H₇)₄]₂[Cu₄Br₆], which has a zero-dimensional (0D) molecular salt structure containing [Cu₄Br₆]²⁻ ions. The compound shows a bright orange PL, consisting of two bands, with a quantum yield of about 95 % at room temperature. An analysis of the temperature-dependent width of the two emission bands provides large Huang-Rhys factors of 81 and 33, which are assigned to two self-trapped exciton states (denoted as STE1 and STE2) with different excited-state structures of the anion. For both STE bands, a decrease of the lifetime from 82 to 32 μs over the temperature range 80–323 K is accompanied by an increase of the PL band integral, indicating an unusual negative thermal quenching process. The microsecond lifetimes are consistent with a phosphorescence process. Broadband transient absorption experiments from the femto- to the microsecond regime provide a time constant for S₁→T₁ intersystem crossing (ISC) step of 490 ps and time scales for the cooling processes in S₁ and T₁.