Abstract
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.
Highlights
The light-emitting nematic liquid crystal (LC) phase at room temperature is important for polarizer-less display applications with bright and low-power characteristics [1,2,3,4].it is quite challenging, by using a single new compound, to obtain nematicLC phases with a single molecule in a wide temperature range covering room temperature.The mixing strategy, a characteristic feature of LC materials, is a solution to overcome this challenge, which is utilized for the preparation of functional room-temperature LCs—-functional molecules are doped in a known room-temperature LCs
Effect on the photoluminescence quantum yield is available with a cyano group in addition
We turned our attention to cyano-substituted HBT (CN-HBT) to investigate whether the positive “triple bond effect on the photoluminescence quantum yield is available with a cyano group in addi tion to an alkynyl one
Summary
The light-emitting nematic liquid crystal (LC) phase at room temperature is important for polarizer-less display applications with bright and low-power characteristics [1,2,3,4].it is quite challenging, by using a single new compound, to obtain nematicLC phases with a single molecule in a wide temperature range covering room temperature.The mixing strategy, a characteristic feature of LC materials, is a solution to overcome this challenge, which is utilized for the preparation of functional room-temperature LCs—-functional molecules are doped in a known room-temperature LCs. The light-emitting nematic liquid crystal (LC) phase at room temperature is important for polarizer-less display applications with bright and low-power characteristics [1,2,3,4]. It is quite challenging, by using a single new compound, to obtain nematic. -functional molecules are doped in a known room-temperature LCs. These doping systems are called as host-guest LCs, where functional molecules (=dopant) are “guest” and room temperature LCs are “host”. Guest LCs, for the application of new display systems [5,6,7]. Light-emitting nematic LCs are accessible by such host–guest
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