Highly Fluorescent Liquid Crystals from Excited-State Intramolecular Proton Transfer Molecules

Fluorescence via excited-state intramolecular proton transfer (ESIPT) provides strong light emission with a large Stokes shift and environment-sensitive unique spectral patterns. Particular systems including 2-(2-hydroxyphenyl)benzothiazole (HBT) serve as efficient solid-state emitters with the ESIPT mechanism and aggregation-induced emission enhancement (AIEE) property, but have not been used for liquid crystalline (LC) materials. Here, rod-shaped fluorescent LCs with ESIPT characters are newly developed based on the HBT motif. The design of the targeted molecules is in line with a simple design principle: a molecule with an alkyl tail, rigid ring, and active HBT core. The LC C6Ph-HBT is highly luminescent in the solid state with an absolute fluorescence quantum yield (phi(FL)) up to 0.39 and exhibits anisotropic fluorescence in its nematic LC phase. C6Ph-HBT is miscible up to 6 wt% with a conventional room-temperature (r.t.) nematic LC 4-cyano-4 '-pentylbiphenyl (5CB), allowing homogeneous fluorescent r.t. LCs. The LC mixture 5CB/C6Ph-HBT shows phi(FL) of 0.26 even in the nonaggregated state, and responds to an electric field with dynamic modulation of not only the optical transparency but also polarized fluorescence. The interplay of highly efficient fluorescence from ESIPT cores and their visible transparency is encouraging for future optical applications of the present molecular systems.