Stabilities of bis(thienyl)ethenes in polymethyl methacrylate (PMMA) coatings as absorbance modulation layers for nanoscale imaging

For absorption modulation layers for nanoscale imaging, a series of photochromic 1,2-bis(thienyl)ethenes (BTEs) was prepared, spectroscopically characterized in solution, and fatigue resistancies in PMMA films were investigated. The BTEs examined possess hydrogen or methyl groups in the β-position (C4/C4′) of the thiophene wings. The non-symmetrically substituted BTEs studied are substituted at the C5/C5′-positions of the thiophenes with combinations of pyridin-3-yl, pyridin-4-yl, 3-PhOMe, 4-PhOMe, 3-PhOH, and 4-PhOH groups. The substitution pattern of the symmetric BTEs studied was varied over a broad range (Ph, 4-tol, 4-PhOMe, 4-PhOH, pyridin-4-yl, 5-Me-thiophen-2-yl, 6-MeO-naphthalen-2-yl). In addition, a benzothiophene derivative was examined. It was shown that the stabilities in PMMA films cannot be derived from results in solution. Thus, the β-methylated BTEs proved to be less stable in PMMA films than their non-methylated counterparts. Best fatigue resistancies were obtained when a BTE with cross-conjugated 3,3-connectivity of a pyridin-3-yl at C5 and a 3-methoxyphenyl group at C5′ without methyl groups at C4/C4′ was applied. Substitution by 4-anisyl groups at C5/C5′ gave the best fatigue resistancies of the series of symmetrically substituted BTEs. The benzothiophene derivative proved to be less stable in PMMA films than its thiophene analog.