Application of 2-pyridyl-substituted hemithioindigo as a molecular switch in hydrogen-bonded porphyrins.

When the photochromism of 2-(3'-pyridylmethylene)-7-ethylbenzo[b]thiophen-3(2H)-ones (4) was investigated, high thermal stability of the E isomer of 4, 4(E) and good repeatability of the photoinduced E,Z-isomerization were found. Association constants of the 1:1 complexations of 4(Z) and 4(E) with the ureidoporphyrin 1 and with the pentafluorobenzamidoporphyrin 2 were evaluated. We found that 1 captures 4(E) preferentially to 4(Z) and, reversely, 2 prefers 4(Z) to 4(E). On the basis of these differences in the binding ability, we concluded that the repeatable movement of the hemithioindigo, so-called the hemithioindigo shuttle, between two kinds of porphyrins was controlled by the photoirradiation. These movements were applied to create a molecular switch for changes in the quinone distribution between two kinds of porphyrins.