Photonic Amplification by a Singlet-State Quantum Chain Reaction in the Photodecarbonylation of Crystalline Diarylcyclopropenones

The photochemical decarbonylation of diphenylcyclopropenone (DPCP) to diphenylacetylene (DPA) proceeds with remarkable efficiency both in solution and in the crystalline solid state. It had been previously shown that excitation to the second electronic excited state (S-2) of DPCP in solution proceeds within ca. 200 fs by an adiabatic ring-opening pathway to yield the S-2 state of DPA, which has a lifetime of ca. 8 ps before undergoing internal conversion to S-1(Takeuchi, S.; Tahara, T. J. Chem. Phys. 2004, 120, 4768). More recently, we showed that reactions by excitation to S-2 in crystalline solids proceed by a quantum chain process where the excited photoproducts transfer energy to neighboring molecules of unreacted starting material, which are able to propagate the chain. Quantum yields in crystalline suspensions revealed values of Phi(DPCP) = 3.3 +/- 0.3. To explore the generality of this reaction, and recognizing its potential as a photonic amplification system, we have synthesized nine crystalline diarylcyclopropenone derivatives with phenyl, biphenyl, naphthyl, and anthryl substituents. To quantity the efficiency of the quantum chain in the crystalline state, we determined the quantum yields of reaction for all of these compounds both in solution and in nanocrystalline suspensions. While the quantum yields of decarbonylation in solution vary from Phi = 0.0 to 1.0, seven of the nine new structures display quantum yields of reaction in the solid that are above 1. The chemical amplification that results from efficient energy transfer in the solid state, analyzed in terms of the quantum yields determined in the solid state and in solution (Phi(cryst)/Phi(soln)), reveals quantum chain amplification factors that range from 3.2 to 11.0. The remarkable mechanical response of the solid-to-solid reaction previously documented with macroscopic crystals, where large single-crystalline specimens turn into fine powders, was investigated at the nanometer scale. Experiments with dry crystals of DPCP analyzed by atomic force microscopy showed the formation of DPA in the form of isolated crystalline specimens ca. 35 nm in size.