Photochromism of Diarylethenes Linked by Hydrogen Bonds in the Single‐Crystalline Phase

Photochromic diarylethenes, which bear carboxyl groups at the ortho, meta, or para positions of both terminal phenyl groups, have been synthesized. The diarylethenes adopt linear chain structures as a result of hydrogen bonding in the crystalline phase, and the crystals exhibit photochromic properties. The absorption maximum of the photogenerated closed-ring isomer of the para-substituted derivative shows an 80 nm bathochromic shift in comparison with that of the ortho-substituted derivative. The maximum of the closed-ring isomer of the meta-substituted derivative is located in between those of the para- and ortho-substituted derivatives. The shifts can be attributed to the differences in conformation among the derivatives, arising from the restrictions imposed by the hydrogen-bonded chains.