ENERGETICS OF ORGANIC SOLID-STATE REACTIONS - ELECTRONIC-STRUCTURE AND PHOTOREACTION MECHANISM OF THE 2,5-DISTYRYLPYRAZINE OLIGOMER MOLECULAR-CRYSTAL

An investigation of the 2,5-distyrylpyrazine (DSP) oligomer crystal electronic structure is presented in order to more completely characterize the oligomer photoreaction leading to the formation of the crystal of the macromolecule. The electronic transitions observed in the oligomer crystal include an n* <-- n transition at 30 250 cm(-1) and a pi* <-- pi transition at 34 000 cm(-1) which are both due to the styrylpyrazine moiety in the oligomer. Higher energy transitions include those from the pyrazine groups in the oligomer backbone and the phenyl side groups. In solution these transitions occur at lower energies, indicating a change in molecular conformation of the oligomer from that in the crystal. The extensive excitonic interaction of the monomer crystal is absent in the oligomer, causing it to behave as an oriented gas. While the pi* <-- n transition that was crucial to understanding the monomer reactivity is still active in the oligomer crystal, its role in the photophysics of the oligomer crystal is changed. In the oligomer crystal, the pi* <-- n transition no longer serves as an exciton-phonon trap for the pi* <-- pi exciton but rather leads directly to the formation of product.