ENERGETICS OF ORGANIC SOLID-STATE REACTIONS - THE TOPOCHEMICAL PRINCIPLE AND THE MECHANISM OF THE OLIGOMERIZATION OF THE 2,5-DISTYRYLPYRAZINE MOLECULAR-CRYSTAL

The 30-fold difference in reactivity of the isomorphous crystals of 2,5-distyrylpyrazine (DSP) and 1,4-bis-(3-pyridyl-2-vinyl)benzene (P2VB), two isoelectronic and nearly structurally identical molecules, has remained an important problem in organic solid-state chemistry. This disparity of oligomerization rates in the solid state is further accentuated by the fact they have similar rates of reaction in solution. The differences in reactivity of these molecular crystals cannot be explained from topochemical considerations. To more completely understand the discrepancy of reactivity of these archetypical photochemical solid-state reactions, the oligomerization of DSP has been reexamined. The polarized, low-temperature reflection spectra were obtained for the DSP monomer crystal and two excited states are found to be involved in the oligomerization. These two states have exciton branches in the same frequency region. The most prominent is a pi* <-- pi transition which forms a delocalized exciton. The second, less evident one, is an pi* <-- n transition which is shown to be a localized exciton that couples with the lattice phonons. This exciton-phonon coupling generates a lattice distortion which provides a trap for the delocalized exciton to initiate the photoreaction. The exciton trapping observed in DSP is not available to P2VB. This mechanism demonstrates the importance of considering the collective nature of solid-state properties in the investigation of organic solid-state reactions and resolves a long-standing question in organic solid-state chemistry.