Synthesis and photophysical properties of methyl 2-hydroxy-4-(5-R-thiophen-2-yl)benzoate: Quantum yields and excited-state proton transfer (R=CH3O and CN)

Abstract

Methyl 2-hydroxy-4-(5-methoxythiophen-2-yl)benzoate (MHmoTB) and methyl 2-hydroxy-4-(5-cyanothiophen-2-yl)benzoate (MHncTB) have been synthesized and their photophysical properties are investigated in various solvents. Introducing methoxy and cyano groups to 5 position of the thiophenyl moiety results in very unique characters in the luminescence properties depending on the substituted group. Compared with the unsubstituted MHTB, the methoxy group as electron donator enhances the quantum yield of the deep blue luminescence more than 9 times in methylene chloride, while do not present an apparent solvatochromic effect on the luminescence spectral feature. In contrast, the cyano group as electron withdrawer enhances the quantum yield slightly. However, the UV excitation (<360 nm) of MHncTB produces not only deep blue emission but also orange emission in dimethylformamide and dimethyl sulfoxide. The new low-energy emission is attributed to an excited-state proton transfer from the solute to the solvent. Quantum mechanical calculations on the structural geometries and the electronic structures have been conducted to reveal ESPT of MHncTB by using the time-dependent density functional theory.