It has been reported that electronic excitation of diphenylcyclopropenone (DPCP) into the second excited state (S-2) results in an adiabatic ring-opening process within 200 fs to give an excited product with a lifetime of ca. 8 ps in S-2. Knowing that energy transfer in crystals may occur within 1-2 ps, we recognized that the conditions could be right for a quantum chain process with the excited-state product sensitizing a neighboring ground-state reactant to set up a domino effect. In agreement with that, we discovered that excitation or macroscopic crystals result in a remarkably efficient reaction to give diphenylacetylene as a polycrystalline powder by a process that releases a great deal of mechanical energy. By taking advantage of nanocrystalline suspensions and using dicumyl ketone as an internal actinometer and after accounting for differences in adsorbance at the irradiation wavelength, we were able to establish that the quantum yield of the reaction for DPCP was Phi(DPCP) = 3.30 +/- 0.35, which is well above unity and consistent with a remarkable quantum chain process.