Chemiluminescence and bioluminescence are usually multistep processes involving several substrates. The most crucial step of these processes is the chemoexcitation, in which the ground state of the energy-rich substrate or intermediate crosses over to the excited state of the product. It is difficult to unravel the mechanism of chemoexcitation in known chemiluminescent and bioluminescent reactions. We suggest that the surface crossing from the ground state substrate to the excited state product takes place via charge-transfer (CT) surface, and propose to synthesize a number of energy-rich 4n cyclodimers of benzene and 4n mixed cyclodimers of benzene containing appropriate substituents. These compounds, during their thermal retrocycloaddition, (1) may introduce a charge-transfer interaction to affect the surface crossing to the excited product, or (2) they may undergo an intersystem crossing to the triplet state of the product, or (3) they may generate a high energy intermediate which may sensitize the emission of a fluorescer. If we succeed in finding an unimolecular chemiluminescent retrocycloaddition, not only we may achieve a fundamental understanding of chemiluminescence and bioluminescence, but also will have an approach to search for coherent chemiluminescence.
