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

利用纳秒激光光解技术研究了三芳基膦在非粘性均相溶液中的光化学反应。发现三芳基膦从三重激发态分解为二芳基膦基和芳基自由基。外磁场的存在降低了二芳基氧膦基自由基的产率。小于0.1T的磁场影响不大。产率的下降主要出现在0.1 ~ 5 T之间，高于5 T则影响不大。所观察到的磁场依赖性不能被解释的传统机制，如超精细耦合，Δ g，和弛豫机制，描述了自由基对的磁场效应。自由基对络合物的Steiner三重态机理部分解释了观测结果、磁场依赖性和重原子效应。正构烷烃溶剂的粘度依赖性比实验值大，而异丙醇和环己烷溶液的粘度依赖性不符合实验值，我们发现将三重激发态分子视为关键中间体而不是自由基对络合物可以避免粘度依赖性的差异.这个假设也消除了其他不一致之处。因此，三芳基膦的磁场效应可以用一种新的机理来解释，该机理不是由自由基对引起的，而是由膦分子三重激发态的磁场依赖性失活引起的.

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.