A Multifunctional Organic Crystal Featuring Large Thermal Expansion and Tunable Emission for Light-Emitting Diode Applications

Abstract

Developing multifunctional organic fluorophores that dynamically respond to external stimuli is a significant challenge in materials chemistry. Here, we present a green fluorescent protein chromophore (GFPc) analogue with highly tunable photophysical properties strictly dictated by its macroscopic crystal habit. By simply altering the crystallization solvent, the crystal morphology and fluorescent emission change, even though the underlying molecular packing remains identical. Furthermore, this material exhibits extreme anisotropic thermal expansion. Heating from 25 to 125 °C triggers a large thermal expansion (αb = 264.95 ± 8.36 × 10–6 K–1) along the b-axis, resulting in a reversible solid–solid phase transformation. Upon further heating beyond the phase transition temperature, the thermal expansion induces severe internal lattice strain, causing needle-like microcrystals to form on the parent bulk crystal’s surface. Moreover, its shallow HOMO level (−5.34 eV), semiconducting nature, and high hole mobility make it a suitable hole-transport layer in LEDs. Incorporated into PbS quantum-dot near-infrared LEDs, it delivered ∼5% external quantum efficiency, highlighting strong potential for optoelectronic applications.