Halogen-driven emission switching in zero-dimensional indium halides: from efficient blue to yellow emission

Abstract

Blue light, as a primary color, is indispensable for lighting and display technologies. However, the development of high-performance blue-emitting materials lags behind that of their green and red counterparts, posing a critical bottleneck in optoelectronics. Herein, we report the synthesis and characterization of two isostructural 0D indium halides, namely, [Epy]₂[InCl₅(py)] (1, Epy⁺ = 1-ethylpyridinium cation; py = pyridinium) and its bromide analogue [Epy]₂[InBr₅(py)] (2). 1 exhibits bright blue photoluminescence (PL) centered at 410 nm under 350 nm excitation, with a photoluminescence quantum yield (PLQY) of 28.9%, ranking among the highest for single-component blue-emitting indium-based OIHMs. Strikingly, isostructural 2 shows only weak yellow emission (544 nm) under the same conditions. Structural and spectroscopic analyses combined with density functional theory (DFT) calculations reveal that their distinct PL behaviors stem from radiative transitions at different energy levels, namely the single-state (S₁) of the organic cations and the corresponding self-trapped exciton (STE) state, which are modulated by the halide anions and supramolecular interactions. In 1, stronger π–π stacking interactions and a higher activation energy (E_a) synergistically suppress non-radiative decay pathways, leading to highly efficient blue emission (from the S₁ state). In 2, the larger inorganic distortion promotes the orange emission arising from the STE state. A prototype white light-emitting diode (WLED) fabricated using 1 demonstrates its potential for solid-state lighting. This work not only presents a high-efficiency blue emitter but also provides fundamental insights into the halogen-dependent luminescence mechanism in 0D hybrid halides.