Energy Transfer Studied Using Spin Polarized Free Radicals. Effect of Substrate Structure and a Comparison Between Micellar Confinement and Free Solution

Singlet and triplet energy transfer from a series of sensitizers to three diazo compounds has been investigated in solution and in micelles by time resolved electron paramagnetic resonance (TREPR) of the ensuing free radicals. The singlet lifetime of the sensitizer determines whether the energy transfer occurs from the singlet or the triplet manifold. A radical pair is created after energy transfer by loss of N2 from the diazo acceptor. The phase of the chemically induced dynamic electron polarization (CIDEP) in the TREPR spectrum describes the multiplicity of the radical pair precursor, and therefore the spectrum carries with it information about the energy transfer mechanism. This principle is established by way of a direct, quantitative relationship between the singlet precursor CIDEP intensity and the singlet lifetime. As the size of the diazo compound is increased, energy transfer from the singlet state is suppressed. Micellar confinement of the radical pair also decreases singlet energy transfer rates significantly.