Preparation of novel polymerizable diarylethene derivatives and their photochromic polymer films
Two new easily polymerizable dithienylethenes have been synthesized, analyzed and tested in polymerization. 1,2-Bis(5-ethylenyl-2-methylthien-3-yl)cyclopentene (DT1) and 1,2-bis(5-(2-methylprop-2-enoyl)-2-methylthien-3-yl)cyclopentene (DT2) (Figure) were obtained by incorporating polymerizable functional groups (ethyleneyl and methacryloyl) at the 5 and 5’ positions of the thiophene rings in a diarylethene structure. DT1 and DT2 displayed reversible color changes. Initially colorless DT1 and DT2 isomers turned to colored isomers by irradiating, respectively, with UV and visible light and the color disappeared upon removing the radiation source. The colorless open isomer of DT1 shows a maximum absorption at 269 nm which, upon irradiation with UV light, converts to the red colored, closed isomer. The spectral shift upon photoisomerization is about 248 nm and an isosbestic point is observed at 314 nm. The open ring isomer of colorless DT2 (λmax= 273 nm) turns in to yellow with a new peak appears at 373 nm upon irradiation at 300 nm. The isosbestic point of DT2 occurs at 264 nm indicate open-to-closed isomer conversion. Thermal stability, fatigue resistance and polymerizability of each were analyzed. The synthesized dithienylethenes are thermochromic. Irradiated DT1 dithienylethene derivatives were stable for more than one month while irradiated DT2 derivative was only stable for 24 h.
Organic photochromes need to be incorporated into a polymer matrix in order to make an efficient, durable optoelectronic device. Both derivatives can be easily incorporated into polymer matrixes either by photo-curing or radical polymerization. Polymeric materials incorporating DT derivatives also undergo reversible color change upon irradiation with UV and visible light, respectively. Photopolymerization yields a polymer film containing the closed dithienylethene moiety that can be converted to its open form following irradiation with visible light. Free radical polymerization, on the other hand, results in a polymer with the dithienylethene in the open isomer, which can be converted to its closed isomer upon irradiation with UV light. The photochromic films were applied in an erasable, recording photonic mode.
Keywords: Dithienylethenes / Photochromic / Thermal stability / Fatigue resistance
Acknowledgement: K. M. T. D. Gunasekara is thankful to the McMaster Endowment for providing a fellowship.