Remarkable Increase of Fluorescence Quantum Efficiency by Cyano Substitution on an ESIPT Molecule 2-(2-Hydroxyphenyl)benzothiazole: A Highly Photoluminescent Liquid Crystal Dopant

Fluorescent molecules with excited-state intramolecular proton transfer (ESIPT) character allow the efficient solid-state luminescence with large Stokes shift that is important for various applications, such as organic electronics, photonics, and bio-imaging fields. However, the lower fluorescence quantum yields (Phi(FL)) in the solution or viscous media, due to their structural relaxations in the excited state to reach the S-0/S-1 conical intersection, shackle further applications of ESIPT-active luminophores. Here we report that the introduction of a cyano group (-CN) into the phenyl group of 2-(2-hydroxyphenyl)benzothiazole (HBT), a representative ESIPT compound, remarkably increase its fluorescence quantum yield (Phi(FL)) from 0.01 (without -CN) to 0.49 (with -CN) in CH2Cl2, without disturbing its high Phi(FL) (=0.52) in the solid state. The large increase of the solution-state Phi(FL) of the cyano-substituted HBT (CN-HBT) is remarkable, comparing with our previously reported Phi(FL) values of 0.05 (with 4-pentylphenyl), 0.07 (with 1-hexynyl), and 0.15 (with 4-pentylphenylethynyl). Of interest, the newly-synthesized compound, CN-HBT, is miscible in a conventional room-temperature nematic liquid crystal (LC), 4-pentyl-40 -cyano biphenyl (5CB), up to 1 wt% (similar to 1 mol%), and exhibits a large Phi(FL) of 0.57 in the viscous LC medium. A similar Phi(FL) value of Phi (FL) = 0.53 was also recorded in another room-temperature LC, trans-4-(4-pentylcyclohexyl)benzonitrile (PCH5), with a doping ratio of 0.5 wt% (similar to 0.5 mol%). These 5CB/CN-HBT and PCH5/CN-HBT mixtures serve as light-emitting room-temperature LCs, and show anisotropic fluorescence with the dichroic ratio of 3.1 upon polarized excitation, as well as electric field response of luminescence intensity changes.

Automated summary

This study demonstrates that introducing a cyano group can significantly enhance the fluorescence quantum yield of ESIPT fluorophores, leading to improved performance in room-temperature liquid crystals.