Dual Fluorescence and Aggregation-Induced Emission in Cyclohexadienone Spiro Molecules: Mechanism and Applications

Most planar organic fluorescent dyes suffer from aggregation-caused quenching (ACQ) due to strong intermolecular π-π interactions, severely limiting their utility in practical applications. To address this, we developed a strategy based on cyclohexadienone spiro molecules that exhibit aggregation-induced emission (AIE) and dual fluorescence properties. Using spiro-PT-OMeTAD as a model, we demonstrated that its dual emission peaks arise from a locally excited (LE) state and a twisted intramolecular charge transfer (TICT) state. Structural analysis revealed that the cyclohexadienone spiro architecture is critical to this performance: the electron-withdrawing carbonyl group facilitates charge separation (promoting TICT), while the asymmetric spiro geometry provides steric hindrance that prevents close stacking and ACQ. The universality of this mechanism was confirmed by synthesizing a series of derivatives (spiro-PT-BA, spiro-PT-PA, and spiro-PT-MA), all of which displayed similar dual fluorescence. Finally, leveraging these optical characteristics, we successfully used spiro-PT-BA as a polarity-sensitive probe to detect water content in tetrahydrofuran (THF) and as a visual temperature sensor.