Molecular modeling of Congo Red analogues containing terphenyl and quarterphenyl moieties

The molecular structures of Congo Red and its terphenyl and quarterphenyl a nalogues were optimized by applying AM1 and PM3 semiempirical methods to pa rtially optimized starting structures. It was necessary to carry out repeti tive sequences consisting of manual adjustments to the input structures fol lowed by optimization, in order to locate minima in each structure. In addi tion, the conformational space associated with rotations about the central phenyl rings was explored. AM1 predicted non-planar biphenyl, terphenyl, an d quarterphenyl structures, whereas PM3 predicted planar structures. Both m ethods were in agreement with experimental data, in that the differences in the energy of planar and non-planar structures were small. PISYSTEM predic ted that increasing the number of central phenyl rings in Congo Red would h ave a hypsochromic and hyperchromic effect on the absorption maximum. (C) 2 000 Elsevier Science Ltd. All rights reserved.