Highly selective fluorescent chemosensors for the detection of Hg2+ based on photochromic diarylethenes with a terminal terpyridine unit

Two new diarylethenes with a terpyridine unit were synthesized and their photochromic and fluorescent properties were investigated. The diarylethenes exhibited favorable photochromism by UV/vis irradiation. Compared to the one with a vinyl-linked terpyridine unit, the one with a simple benzene linker had a smaller molar absorption coefficient and absorption maximum, and larger cyclization/cycloreversion quantum yields. Hg2+ coordination with the diarylethenes resulted in remarkable photochromism and fluorescence changes. The fluorescence intensity of the diarylethene with a benzene linker was enhanced significantly by 2.5-fold and its emission peak exhibited a notable red-shift from 420 to 463 nm with an evident color change from dark blue to bright cyan. In contrast, the one with a vinyl linker showed a dramatic increment of fluorescence intensity by 35-fold with a color change from darkness to bright green. Both of the diarylethenes were highly selective toward the recognition of Hg2+ without interference from other metal ions.