Excited-state intramolecular proton transfer on 2-(2′-hydroxy-4′-R-phenyl)benzothiazole nanoparticles and fluorescence wavelength depending on substituent and temperature

다국어 초록 (Multilingual Abstract)

<P>The fluorescence emission properties of 2-(2′-hydroxy-4′-R-phenyl)benzothiazole (HBT-R) nanoparticles with different substituents (R = –COOH, –H, –CH<SUB>3</SUB>, –OH, and –OCH<SUB>3</SUB>) were investigated using spectroscopic and theoretical methods. HBT-Rs displayed dual enol and keto (excited-state intramolecular proton transfer (ESIPT)) emissions in nonpolar solvents. The spectral change of their ESIPT emissions was affected differently by the electron donating (or withdrawing) power of the substituents; a bathochromic shift for the electron donating group and a hypsochromic shift in electron withdrawing group. In addition, the changes in energy levels calculated by the <I>ab initio</I> method were consistent with the spectral shifts of HBT-R in solution. We prepared aggregated HBT-R nanoparticles using a simple reprecipitation process in tetrahydrofuran–water solvents. The ESIPT emission of aggregated HBT-R nanoparticles was strongly enhanced (over 45 times) compared to those of monomer HBT-Rs in toluene, as markedly shifted ESIPT emissions are observed at longer wavelength without any quenching by self-absorption. Aggregated HBT-R nanoparticles showed longer lifetimes than those of monomer molecules. The temperature effect on the aqueous dispersion of the aggregated HBT-R nanoparticles was also explored. It shows a fluorescent ratiometric change in a range of temperature from 7 to 65 °C. A mechanism of a temperature-dependent equilibrium between the nanoparticles and the solvated enols is proposed for the emission color change.</P> <P>Graphic Abstract</P><P>The ESIPT emission properties of HBT-R nanoparticles can be controlled by the different substituents and temperature without any quenching by self-absorption.
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