Photochromism of some thienobenzopyrans

The molecular structures of Congo Red and its terphenyl and quarterphenyl analogues were optimized by applying AM1 and PM3 semiempirical methods to partially optimized starting structures. It was necessary to carry out repetitive sequences consisting of manual adjustments to the input structures followed by optimization, in order to locate minima in each structure. In addition, the conformational space associated with rotations about the central phenyl rings was explored. AM1 predicted non-planar biphenyl, terphenyl, and quarterphenyl structures, whereas PM3 predicted planar structures. Both methods were in agreement with experimental data, in that the differences in the energy of planar and non-planar structures were small. PISYSTEM predicted that increasing the number of central phenyl rings in Congo Red would have a hypsochromic and hyperchromic effect on the absorption maximum.