Retro-Diels–Alder Reaction on Surface: Generating Energy-Prohibited Structures in Bulk Film Condition through Surface-Adsorbing Neutralization Effect

Retro-Diels–Alder (rDA) reactions on surfaces were few reported in comparison with the fruitful results of rDA in liquids, gas phase, or bulk films. On the other hand, the important effects of inert substrates on surface reactions failed to be recognized clearly in current research either, although the influences of active metal substrates on varied surface reactions have been researched widely. Herein, the stepwise rDA reactions of tetrabicyclo[2.2.2]octadienoporphyrin on graphite surface were investigated through scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Besides the final product, several other types of topographies were revealed at submolecular level in the STM micrographs, including the bright rod (R), bright triangle (T), and mini triangle (mini-T). Combining STM results with DFT data allows the reasonable intermediates and pathways in each step of surface rDA to be definitively recognized. The structures corresponding to the R, T, and mini-T topography were identified, respectively, according to the features of molecular orbital. On surface, the energy of rDA intermediates is codecided by the number of benzo groups on the nonprotonated-pyrrole lobes and the number of surface-adsorbed ethene groups whereas in bulk it depends solely on the number of benzo groups on the nonprotonated-pyrrole lobes. Thus, many energy-forbidden structures in bulk rDA can be formed reasonably in surface rDA by lowering their energy through surface adsorption of ethene groups, which is rationalized as the surface-adsorbing neutralization effect. This effect enriched the reasonable intermediates and pathways of surface rDA greatly.