Crystal-to-Crystal Photoinduced Reaction of Dinitroanthracene to Anthraquinone

The photochemical reaction of 9,10-dinitroanthracene (DNO2A) to anthraquinone (AQ) + 2NO has been studied by means of lattice phonon Raman spectroscopy in the spectral region 10–150 cm–1. In fact, crystal-to-crystal transformations are best revealed by following changes in the lattice modes, as even small modifications in the crystal structure lead to dramatic changes in symmetry and selection rules of vibrational modes. While analysis of the lattice modes allowed for the study of the physical changes, the chemical transformation was monitored by measuring the intramolecular Raman-active modes of both reactant and product. On the basis of the experimental data it has been possible, at a microscopic level, to infer crucial information on the reaction mechanism by simultaneously detecting molecular (vibrational modes) and crystal structure (lattice phonons) modifications during the reaction. At a macroscopic level we have detected an intriguing relationship between incident photons and mechanical strain, which manifests itself as a striking bending and unfolding of the specimens under irradiation. To clarify the mechanisms underlying the relationship between incoming light and molecular environment, we have extended the study to high pressure up to 2 GPa. It has been found that above 1 GPa the photoreaction becomes inhibited. The solid-state transformation has also been theoretically modeled, thus identifying the reaction pathway along which the DNO2A crystal lattice deforms to finally become the crystal lattice of the AQ product.