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 (ΦFL) in the solution or viscous media, due to their structural relaxations in the excited state to reach the S0/S1 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 (ΦFL) from 0.01 (without -CN) to 0.49 (with -CN) in CH2Cl2, without disturbing its high ΦFL (=0.52) in the solid state. The large increase of the solution-state ΦFL of the cyano-substituted HBT (CN-HBT) is remarkable, comparing with our previously reported ΦFL 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-4′-cyano biphenyl (5CB), up to 1 wt% (~1 mol%), and exhibits a large ΦFL of 0.57 in the viscous LC medium. A similar ΦFL value of ΦFL = 0.53 was also recorded in another room-temperature LC, trans-4-(4-pentylcyclohexyl)benzonitrile (PCH5), with a doping ratio of 0.5 wt% (~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.
Theoretical and Experimental Investigations of Large Stokes Shift Fluorophores Based on a Quinoline Scaffold
