Planar 2-(2-hydroxyphenyl)benzothiazole-based dyes functionalized via triple bonds as exceedingly efficient solid-state fluorophores

Two crystalline 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives, phenylethynyl-modified HBT (HBT-H) and 4-methylphenylethynyl-modified HBT (HBT-Me), show large Stokes shifts and exceedingly high fluorescence quantum yields (Φf) despite the lack of sterically bulky groups or propeller-shaped moieties in their extended π-conjugated planar structures. Furthermore, although the intramolecular H-bonding in HBT-Me is weaker than that in HBT-H, the Φf of crystalline HBT-Me (91.19%) is higher than that of crystalline HBT-H (75.46%). X-ray diffraction analysis demonstrates that HBT-Me and HBT-H crystals form J- and weak H-aggregates, respectively.