Fluorene Oligomers Featuring a Central 2,1,3-Benzothiadiazole Unit with High Photoluminescence Quantum Yield and Amplified Sponta-neous Emission

Fluorene-based conjugated materials are pivotal to optoelectronic applications, due to their tunable optical properties and high pho-toluminescence quantum yields. While conjugated polymers like poly(fluorene-co-benzothiadiazole) have been extensively studied, their performance is often limited by product inhomogeneity and morphological constraints. Oligofluorenes, with their well-defined structures and controlled morphologies, offer a promising alternative but have been underexplored in sub-stoichiometric fluorene-benzothiadiazole systems. Here, we report the synthesis and characterization of a novel series of sub-stoichiometric fluorene-BT oligomers featuring a central BT unit flanked by dioctylfluorene arms of varying repeat lengths, ranging from trimers to heptamers. These oligomers exhibit superior optical properties compared to conventional F8BT polymers, including enhanced quantum yields and amplified spontaneous emission thresholds down to 1.5 µJ cm⁻². The optical performance is analyzed in relation to oligomer length and fluorene-to-BT ratio, providing insights into the relationship between molecular design and photophysical behavior. Our findings highlight the potential of these defect-free, tunable green-emitting oligomers for next-generation light-emitting diodes and low-threshold lasers, paving the way for further advancements in photonic and optoelectronic materials.