Liquid Crystalline Fluorene-2,1,3-Benzothiadiazole Oligomers with Amplified Spontaneous Emission.

Conjugated fluorene-based molecules are a powerful class of materials for optoelectronic applications, known for their outstanding photoluminescence quantum yields and easily tunable optical properties. While conjugated polymers like poly(fluorene-co-benzothiadiazole) have been extensively studied, their performance is often hindered by product inhomogeneity and morphological constraints. By contrast, well-defined oligofluorenes offer precise molecular structures and better morphological control, making them an attractive alternative to conjugated polymers. Among them, benzothiadiazole (BT)-centered oligofluorenes exhibit strong yellow emission with remarkably high photoluminescence quantum yields, yet their morphological properties remain largely unexplored. In this study, a straightforward synthesis of BT-cored pentafluorenes is reported, where the alkyl side chains are systematically varied to investigate their impact on the morphology. These pentamers demonstrate amplified spontaneous emission (ASE) with thresholds as low as 1.64 µJ cm⁻. By fine-tuning the alkyl chains, crystalline, amorphous, and liquid crystalline morphologies are achieved, while maintaining consistent optical properties, paving the way for defined materials in advanced optoelectronic applications.