The reactivity of 1,5-diphenylpenta-1,4-diyn-3-one (DPD) upon photolysis in acetonitrile and propan-2-ol wsa followed by continuous-wave time-resolved EPR (TR-EPR) and H-1 CIDNP (chemically induced dynamic nuclear polarization) spectroscopy It was Found that radical polymerizations using DPD can only be accomplished if an efficient hydrogen donor, such as propan-2-ol, is present, reflecting in analogous reaction pathway as the related benzophenone (BP). The same behavior was also established for ODPD, the bis(p-methoxy) derivative of DPD In contrast to patent BP. where the steric hindrance of this ortho phenyl protons causes a twisted geometry. DPD and ODPD adopt,an essentially planar mimimum geometric Which is also conserved in the corresponding radicals of type C as revealed by density functional theory calculations This planarity is possibly one important factor For the considerable efficiency of DPD and its dematives to act as initiators for two-photon-induced radical polymerizations