Synthesis and Properties of Photochromic Dithienylethene Compounds with Triphenylamine Units

Abstract

Combining dithienylethene photochromic unit with triphenylamine unit, three new photochromic dithienylethene compounds 1a~1c were successfully synthesized using 5-chloro-2-methyl-3-acetylthiophene and diphenylaminobenzalde- hydes as starting materials through Aldol condensation reaction, Michael addition reaction and McMurry coupling reaction, the structures of which were characterized by MS, 1 H NMR and 13 C NMR. Different from majority of previous studies that introduce different functional groups through thiophene rings, we bridged the triphenylamine unit with fluorescence property to the cyclopentene at β-position via C—C single bond, which will maintain the activation of photochromism and prevent other functional groups affecting the dithienylethene backbone. Their photochromic and fluorescence properties in solutions and polymethylmethacrylate films were investigated by UV-Vis and fluorescence spectra, respectively. The colorless solu- tions and films of 1a~1c switch to yellow upon irradiation with 302 nm UV light and reach the photostationary state fast, while bleaching of the colored solutions happen upon irradiation with 450 nm blue light. When 1a~1c are excited by UV light, the maximum fluorescence wavelengths of 1a~1c increase along with the enhancement of the abilities of electron donating of substituent R, and 1a~1c emit strong fluorescence in n-hexane, toluene and ethyl acetate, while weak in acetoni- trile and none in chloroform. Moreover, upon irradiation at 302 nm, Forster resonance energy transfer occurs between triphenylamine unit and dithienylcyclopentene unit of 1a~1c. Thus, the fluorescence intensities of 1a~1c decrease along with the prolonging of irradiation time, which indicates the potential application of these compounds as fluorescence mo- lecular switches. Keywords dithienylethene; triphenylamine; photochromic; fluorescence molecular switch; Forster resonance energy trans- fer