Dual-state emission properties of 2-(2-carboalkoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazoles and its Zn(II) and Cd(II) complexes

Abstract

Dual-state emission is an uncommon, but very attractive property of organic or complex compounds designed for various applications, including chemical sensors and probes for biological objects. This property is in demand for molecular systems with a fluorescent signal function, the functioning of which involves a wide range of changes in the aggregate state due to temperature, pressure, the nature of solvents, etc. In order to find moleculas with dual-state emission properties, fluorescent Zn(II) and Cd(II) complexes based on 2-(2-carboisopropoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazole, as well as a Cd(II) complex based on 2-(2-carboethoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazole, have been synthesized. The study of the fluorescent properties of 2-(2-carbomethoxy(ethoxy/isopropoxy)-3,4-dichloro-6-hydroxyphenyl)benzoxazole and its Zn(II) and Cd(II) complexes in the crystalline state shows that these compounds have dual-state emission properties. The fluorescence quantum yields of the studied 2-(hydroxyphenyl)benzoxazoles were increased by 4–5 times at the transition from solutions to the crystalline state and reached record values of 0.8–0.95. Unlike solutions, the fluorescence efficiency of metal complexes in the solid state decreases by a factor of about 2, resulting in values of 0.02–0.12. It has been established that H- and J-aggregates are formed in the solid-state samples of complexes, and the reason for the decrease in the fluorescence efficiency of complexes in the solid-phase is aggregation-caused quenching. It has been shown that an increase in the volume of alkyl substituents in the carboxyl moiety of 2-(2-carboalkoxy-3,4-dichloro-6-hydroxyphenyl)benzoxazole leads to a progressive deformation of their planar conformation in the metal complexes, which is accompanied by a decrease in the fluorescence efficiency of the complexes in the crystalline state.