Knowledge Bank

Crystals of N-(p-chlorobenzylidene)-p-chloroaniline (BAC1) exist as a stable orthorhombic crystal, wherein the benzene rings in the molecule are twisted by $24.8\circ$ in opposite directions, and as a metastable triclinic form where the molecule is planar. The crystal spectra of the two forms permit a direct analysis of the effect of conformation upon the electronic spectrum. Calculations have shown that the twisted form is favored over the planar by 2 kcal/mole. Polarized normal incidence specular reflection spectra taken at $300\circ K$ and their Kramers-Kronig transforms have been obtained for both the triclinic and orthorhombic forms of BAC1. Spectra of the planar form are found to show strong correspondence to the spectra of both stilbene and azobenzene. Integrated intensities are in good agreement with those of the latter systems. Spectra of the twisted form are clearly comparable to those observed for the whole class of benzilidene anilines with intensities in agreement with those for the series. The results for both sets of spectra are correlated with quantum chemical calculations. No excitonic interactions are found.