Thermally responsive multistate fluorescence coupled with uniaxial negative thermal expansion in 1D lead halide hybrids

Lead halide hybrids exhibit excellent optoelectronic properties, particularly in the development of high-performance solar cells and light-emitting diodes (LEDs). Increasing attention is being directed toward their thermal expansion behavior, as temperature-dependent bandgaps are crucial for solar cell and light emitting applications. Here, we report two new isomorphic one-dimensional (1D) lead halide hybrids, [XMePyr][PbX₃] (XMePyr⁺ = 1-(2-haloethyl)-1-methylpyrrolidinium; X = Br (1) or Cl (2)), featuring rare hemidirected PbX₅ (X = Br or Cl) square pyramidal chains, a stereochemically active coordination geometry uncommon in this class of materials. Both compounds undergo isostructural phase transitions at 255 K (1) and 351 K (2), likely driven by the stereochemically active 6s² lone pair electrons of Pb²⁺. Remarkably, they exhibit uniaxial negative thermal expansion (NTE) along the chain direction, arising from transverse vibrations within the chains, representing the first such NTE mechanism identified in 1D lead halide hybrids. Additionally, the NTE is coupled with unique photophysical properties: 1 displays excitation-dependent dual emission, while 2 exhibits negative thermal quenching. Both 1 and 2 show reversible fluorescence switching associated with their phase transitions and NTE behavior. These results deepen our understanding of structure–property correlations in lead halide hybrids and offer insightful guidelines for designing multifunctional optoelectronic materials.