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.