Determining the Recombination Efficiency of Asymmetric Radical Pairs
Radicals generated in solution have a propensity to recombine, either re-forming the parent molecule or generating new bond with other reactive species. In a general photochemical radical initiation pathway, a parent molecule (such as a Mo-Mo dimer) can homolyze to produce a radical pair (Mo•, •Mo). The kinetic behavior of this radical pair in solution has implications in radical polymerizations as a source of termination and initiation. The radicals generated are held together initially by a “cage” of solvent molecules that prevents diffusion, increasing the recombination rate. Femtosecond spectroscopy and steady state experiments have been used to determine the rate of primary and secondary recombination event for symmetric systems; however current models are incomplete, being unable to account for asymmetric radical pairs. By using the observed recombination efficiencies of asymmetric radical pairs, a new model can be determined that predicts the effects of size and mass of a single radical side (Mo• or •Mo) on the efficiency of both primary and secondary recombination.
Incorporating Photosensitive Organometallics into Non-Linear Polymers Using an Asymmetric Dimer
Kinetic investigations of light degradation can be effectively conducted on photochemically reactive polymers. By varying the location of photoactive sites or the frequency of their occurrence within a polymer, insight into structural effects on polymer degradation can be determined. The photoactive molybdenum dimer (η5-C5H5)2Mo2(CO)6 is an ideal candidate for a photoactive site as it contains a well characterized Mo-Mo bond that homolyzes under visible light. The two molybdenum radicals produced can subsequently be trapped with halides. The degradation of the molybdenum dimer can be followed by the carbonyl stretches using IR spectroscopy. Although having one degradation pathway simplifies analysis, the molybdenum dimer functionalization is difficult when producing non-linear polymers. This is because non-linear polymers require an asymmetric dimer to act as a building block in their synthesis. Manipulations with protecting groups as well as different synthetic schemes have allowed for the construction of non-linear polymers containing photosensitive bonds.