A perpendicular phenyl-induced exceedingly efficient solid-state excited state intramolecular proton transfer fluorophore based on 2-(2-hydroxyphenyl)benzothiazole

• HBT derivatives bearing styryl and 2,2-diphenylvinyl, respectively, were synthesized.
• In solution state Φf of HBT-s-Ph was larger due to stronger intramolecular H-bond.
• In crystal state Φf of HBT-d-Ph 78% was larger due to weak intermolecular interaction.
• The results were verified through crystal analysis and DSC.
• A perpendicular phenyl induced exceedingly efficient solid-state ESIPT fluorophore.

Fluorescent solid-state organic materials have been intensively researched in recent years. In this study, two 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives: phenylethylene-modified HBT (HBT-s-Ph) and diphenylethylene-modified HBT (HBT-d-Ph), were synthesized by facile Suzuki cross-coupling reactions. In toluene solutions, the fluorescence quantum yield (Φf) of HBT-s-Ph is slightly larger than that of HBT-d-Ph, because of the presence of a stronger intramolecular H-bond in HBT-s-Ph than in HBT-d-Ph. In crystalline forms, both of their Φf has substantial improvement, and the Φf of HBT-d-Ph reaches 78%, which is twice that of HBT-s-Ph. The crystal structure revealed that the striking emission enhancement of HBT-d-Ph was attributable to its distinct molecular packing model and the resultant relatively weak intermolecular interactions induced by the single phenyl group cis to the HBT moiety.

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