Ultrafast Spectroscopic Observation of a Quantum Chain Reaction: The Photodecarbonylation of Nanocrystalline Diphenylcyclopropenone

Nanocrystalline diphenylcyclopropenone (DPCP) undergoes a photodecarbonylation reaction to form diphenylacetylene (DPA) with a quantum yield of ∼3, presumably the result of a quantum chain reaction (QCR). We have performed an ultrafast spectroscopic investigation of this process, and found direct evidence that, following photoexcitation of solid DPCP, DPA product molecules appear in a sequential manner over a ∼30 ps time scale. We also see spectroscopic evidence for an excited state of DPCP that is optically inaccessible from the ground state; this state can be populated via short-range energy transfer from excited DPA product molecules, providing a mechanistic explanation of the sequential re-excitation and decarbonylation of DPCP and constituting a direct observation of a QCR.