Toward the Photoinduced Reactivity of 1,5-Diphenylpenta-1,4-diyn-3-one (DPD): Real-Time Investigations by Magnetic Resonance

Abstract

The reactivity of 1,5-diphenylpenta-1,4-diyn-3-one (DPD) upon photolysis in acetonitrile and propan-2-ol was followed by continuous-wave time-resolved EPR (TR-EPR) and 1H 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 an 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 parent BP, where the steric hindrance of the ortho phenyl protons causes a twisted geometry, DPD and ODPD adopt an essentially planar minimum geometry 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 derivatives to act as initiators for two-photon-induced radical polymerizations.