Structural modulations: unraveling the impact of benzothiazole positions on the optical properties of carbazole–cyanostilbenes

Abstract

This study explores the influence of positional substitution of benzothiazole on the optical and solid-state properties of carbazole–cyanostilbene derivatives, focusing on two key positional isomers: 2-BTCZCS and 5-BTCZCS. These isomers exhibit remarkable aggregation-induced emission (AIE) with solid-state quantum yields reaching up to 24%. Despite anticipated variations in molecular geometry and electronic interactions due to positional substitution, both isomers display remarkably similar photophysical behaviors, underscoring the robustness and versatility of the carbazole–cyanostilbene framework. The isomers exhibit notable mechanofluorochromic (MFC) behavior, characterized by a subtle blue shift of 12 nm in emission upon mechanical grinding. This shift highlights their sensitivity to external mechanical stimuli. Powder X-ray diffraction (PXRD) analysis confirms that the grinding process does not disrupt the crystallinity of the materials, preserving their ordered structure. Furthermore, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) reveal excellent thermal stability, emphasizing their durability and suitability for practical applications. Morphological stability and aggregation behavior were further validated using scanning electron microscopy (SEM) and dynamic light scattering (DLS). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations validate the experimental findings, providing deeper insight into their electronic structures and excited-state properties. This work highlights the minimal impact of benzothiazole substitution positions on the optical properties of these materials, emphasizing their reliability and potential for high-performance optoelectronic applications. These findings open new avenues for the design of stable and predictable materials tailored for next-generation devices.