Fatigue Property of Oxidized Photochromic Dithienylethene Derivative for Permanent Optical Recording

Abstract

Photochromic diarylethenes, such as 1,2-bis(2-methyl-1benzothiophene-3-yl)perfluorocyclopentene (BTF6) and 1,2-bis(2,5-dimethylthien-3-yl)perfluorocyclopentene (DMTF6), have been extensively investigated in recent years in order to develop materials for molecular photonic devices such as optical memory and switch. In the design of photochromic materials, thermal stability and fatigue resistant are important features to be considered. The thiophene analogues undergo photochromic ring closure efficiently but the fatigue property is generally low, resulting irreversible photochromism. If the photochromism is in an irreversible manner it could be applied in the permanent optical recording such as write once read many (WORM) memory. This motivates us to examine the effect of oxidation in the photophysical properties of diarylethenes with thiophene unit. As the thiophene analogues, we chose DMTF6 and its oxidized analogue, 1,2-bis(2,5-dimethylthien-1,1-dioxide-3-yl)perfluorocyclopentene (DMTFO4). Herein we report the synthesis and characterization of the photochromic properties including the fatigue property of DMTFO4. DMTF6 was prepared according to the procedures described in the literature. DMTFO4 was prepared from the oxidation of DMTF6 using 3-chloroperbenzoic acid (mCPBA) in 85% yield. The structure of DMTF6 and DMTFO4 was characterized using H-NMR and HRMS. Figure 1 shows the change of absorption spectra of DMTF6 (A) and DMTFO4 (B), as a function of time in ethyl acetate solution (1 × 10−5 M) at room temperature upon 312 nm irradiation using hand-held lamp. Upon UV illumination, the open-ring isomer of DMTF6 (o-DMTF6) and DMTFO4 (o-DMTFO4) undergoes photo-cyclization reaction showing new absorption bands at 480 nm and 360 nm for c-DMTF6 and c-DMTFO4, respectively. Upon visible illumination, the c-DMTF6 is completely returned back to the corresponding open-ring isomer (Figure 1C), whereas the absorption spectrum of c-DMTFO4 slightly changes and doesn’t match with corresponding o-DMTFO4 as can be seen in Figure 1D. This result implies that an irreversible side reaction might be occurred from c-DMTFO4 under UV irradiation. To elucidate the origin of the spectral changes in absorption spectra of DMTFO4 upon UV exposure, we examined H-NMR change under UV light irradiation for 20 hours. Figure 2 shows H-NMR of the thiophene ring protons of