Structure, property and mechanism study of fluorenone-based AIE dyes

• Three carbonyl substituted 2,7-diphenylfluorenone derivatives were synthesized.
• They exhibited prominent aggregation-induced emission (AIE) properties.
• Their powder showed a 160 nm red-shifted emission compared to their THF solution.
• The crystal structures revealed the bathochromic emission originated from the static excimers.
• A model was designed to prove the AIE was impossibly caused by restricted intramolecular rotation.

Aggregation-induced emission (AIE) has attracted considerable interest in recent years. An understanding of the underlying mechanisms for the AIE phenomena is also of great importance. Here, we synthesized four fluorenone derivatives, which all show typical AIE properties. Their enhanced solid-state luminescence showed a 160 nm red-shift (from 380 to 540 nm) in comparison to their luminescence in dilute tetrahydrofuran (THF) solutions. The single-crystal structure and photophysical properties revealed that the bathochromic luminescence is due to the formation of static excimers. To further demonstrate the AIE mechanism of static excimers, rather than restricted intramolecular rotation (RIR), we designed a diphenylfluorenone-diboronic acid adduct, which can react with d-glucose to create structurally rigid oligomers, resulting in the restriction of intramolecular rotations. The spectral change in the d-glucose titration of the adduct indicated that the bathochromic AIE of these fluorenone compounds cannot possibly be caused by RIR and conformational planarization.

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