Tailoring the π–π stacking interaction among organic cations in hybrid metal halide crystals towards tunable light emission

Abstract

As an important interaction among organic cations containing aromatic rings, the π–π stacking interaction is a crucial factor determining the crystal structure of organic–inorganic hybrid metal halides (OIMHs). However, the relationship between π–π interaction and optical properties of luminescent OIMHs is yet to be studied. (C₁₀H₁₀N₂)Cd_1−xZn_xCl₄ (23-Cd_1−xZn_xCl₄) crystals with 0D structures are synthesized by using optically active 2,3′-bipyridine as the organic ligand. When x ≤ 0.2, there is no π–π interaction among 2,3′-bipyridinium cations in 23-Cd_1−xZn_xCl₄ crystals. When x > 0.2, π–π interaction among organic cations occurs and enhances as the Zn²⁺ content increases. The tailoring of the π–π interaction endows 23-Cd_1−xZn_xCl₄ with tunable light emitting properties, resulting in a broad band emission involving blue and orange species. The blue emission dominates when there is no π–π interaction, while the orange emission enhances and finally becomes dominant as the strength of π–π interaction increases. The π–π interaction promotes the energy transfer from the higher energy valley to the lower one of the S₁ state of the organic cation, resulting in the enhancement of orange emission. Notably, the 23-ZnCl crystal exhibits a photo-luminescence quantum yield (PLQY) of 32%. This is the highest reported value to date among the Zn-based OIMHs, which have optical emission originating from the organic component. The mechanism of π–π stacking induced tunable light emission revealed in our work provides new guidance for the design of luminescent OIMHs.