Magnetic field effects on the lifetimes of triplet biradicals photogenerated from zinc(II) tetraphenylporphyrin-viologen chain-linked compounds: Dependence on spacer chain length and environment

The effects of magnetic fields on the lifetimes of chain-linked triplet biradicals ((3)BRs), generated by photoinduced electron transfer from zinc(II) tetraphenylporphyrin (ZnP) to appended viologen (V2+), were investigated in homogeneous solutions of acetonitrile-water and reversed micellar solutions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) by means of laser flash photolysis. It was found that the decay rate constants (k(obs)) of the (3)BRs increased significantly in the absence of magnetic fields as the number of carbon atoms (n) in the spacer chain between the ZnP and V2+ moieties increased, and then reached a constant value above n = 8. When the magnetic field was increased, the k(obs) values de creased. A sharp decrease in a low magnetic field was observed, followed by a gradual decrease as the magnetic field was increased, eventually reaching an asymptotic value under the high magnetic field (B > 100 mT) for the compounds with the longer spacer chains. In AOT/2,2.4-trimethylpentane/water reversed micellar solutions, the kobs values were smaller than those in MeCN-H2O homogeneous solutions; this trend became increasingly prominent the longer the spae er chain length. Therefore, the kobs values showed the opposite spacer chain length dependence in comparison with those in homogeneous solutions at B = 470 mT. These results were interpreted in terms of the variation of tripler-singlet energy separation with the interradical distance, Zeeman splitting of the triplet sublevels, electron-nuclear hyperfine coupling, and spin relaxation mechanisms.