An Organic Crystalline Ternary Pseudo‐Heterojunction with Contrasting Luminescent and Mechanical Properties Prepared by Thermotropic Phase Transition

Abstract Advancements in crystal engineering have enabled the precise tuning of the physical properties of organic crystals, thereby enhancing their potential in advanced optoelectronics. Here, we design an organic compound which crystallizes into distinct polymorphs with contrasting luminescent and mechanical behaviors. Controlled growth conditions yield flexible (Cry‐ O : orange fluorescence) and brittle (Cry‐ Y : yellow fluorescence) crystals which exhibit unique fluorescence shifts and reversible spectral changes between room and low temperatures, alongside unidirectional phase transitions upon thermal stimulation. The optical and mechanical differences between the polymorphs O and Y stem from their distinct molecular packing arrangements and intermolecular interactions. Notably, Cry‐ O undergoes sequential phase transitions upon heating to 98 and 132 °C, enabling the creation of a ternary “pseudo‐heterojunction” structure, facilitating the integration of multiple functional phases into a single material. These findings not only enhance the understanding of structure–property relationships of organic materials but also demonstrate the potential of polymorphism‐driven property tuning for optoelectronic devices, thermal management systems, and optical sensors.