Electron Spin Polarization after Photolysis of AIBN in Solution: Initial Spatial Radical Separation

Transient 2-cyano-2-propyl radicals are generated by laser flash photolysis of 2,2‘-azobisisobutyronitrile (AIBN) in solvents of various viscosities. Their spin magnetization is followed by time-resolved EPR spectroscopy and analyzed in terms of modified Bloch equations accounting for chemical reaction, Heisenberg spin exchange, and rise and decay of chemically induced electron polarization (CIDEP). The results are: (i) The cross-section for spin exchange exceeds that for radical termination by a factor of 1.8. (ii) No geminate net polarization is found, implying that eventually formed primary diazenyl radicals decay on the picosecond time scale. (iii) The ratio of F-pair and geminate pair polarization increases with viscosity from | P F / P G | = 1 at η = 0.6 cP to | P F / P G | = 4.7 at η = 18 cP. According to the diffusion theory of CIDEP, this means that the initial spatial distance r i of the radicals escaping the geminate cage is independent of the viscosity. This finding is at variance with the cage effect, which is predicted correctly by classical Langevin models assuming a decrease of r i with increasing viscosity. It is tentatively concluded that, at least for AIBN, the successful simulation of the η-dependent cage effect is only fortuitous and conceptually wrong.