A Novel Magnetic Field Effect observed in the Photochemical Reactions of Phosphorous Compounds

采用纳秒激光光解方法研究了三芳基膦在非粘性均匀溶液中的光化学反应。发现三芳基膦从三重态激发态分解为二磷基和芳基自由基。在外加磁场作用下，二芳基膦基自由基的产率降低。小于0.1T的磁场影响不大。产量的降低主要发生在0.1 ~ 5T。在此之上，几乎没有观察到影响。观察到的磁场依赖性不能用传统的机制如超精细耦合、DELTAg和弛豫机制来解释，这些机制描述了自由基对的磁场效应。提出的自由基对配合物的斯坦纳三重态机制部分解释了观测、磁场依赖和重原子效应。正构烷烃溶剂对溶剂粘度的期望依赖性大于观测值，2-丙醇和环己烷溶液的结果不符合粘度关系。我们发现，将三重态激发分子作为关键中间体而不是自由基对配合物，可以避免粘度依赖性的差异。其他不一致之处也被这个假设消除了。因此，三芳基磷化氢的磁场效应可以用一种新的机制来解释，这种机制不是源于自由基对，而是源于磷化氢分子的三态激发态的磁场依赖失活。

The photochemical reactions of triarylphosphines in the non-viscous homogeneous solutions were investigated by the nanosecond laser photolysis. Triarylphosphines were found to decompose into diaryphosphinyl and aryl radicals from their triplet excited states. The yields of diarylphosphinyl radicals reduced in the presence of the external magnetic fields. The magnetic field less than 0.1T has little effect. Most of the reduction of the yiled appeared from 0.1 to 5T.Above it, little effect was observed. The observed magnetic field dependence could not be interpreted by the conventional mechanisms such as, hyperfine coupling, DELTAg, and relaxation mechanisms which describe the magnetic field effects of radical pairs. The Steiner's triplet mechanism proposed for radical pair complexes partially interprets the observation, the magnetic field dependence and the heavy atom effects. The expected solvent viscosity dependence among n-alkane solvents is larger than the observation and the results in 2-propanol and cyclohexane solutions did not obey the viscosity relationship.We found that the discrepancy of the viscosity dependence can be avoided by regarding the triplet excited molecule to be a key intermediate instead of the radical pair complex. Other inconsistencies are also removed by this assumption. Consequently, the magnetic field effects of triarylphosphines are interpreted by a novel mechanism which is originated not from the radical pairs but from the magnetic field dependent deactivation of triplet excited state of the phosphine molecules.