Electron spin dynamics of triplet and doublet molecules in room temperature ionic liquids studied by a time-resolved EPR method

A time-resolved (TR-) EPR method was applied to measure the triplet spectra of porphyrins such as ZnTPP (TPP, tetraphenylporphyrin) and H4TPP2+ in room temperature ionic liquids (RTIL). The spectra were successfully obtained at 298 K with clear canonical peaks of the triplet sublevels. Magneto-photo selection experiments were carried out for ZnTPP in NR4BF4 (NR4: N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium) and H4TPP2+ in BmimPF6 (Bmim: 1-butyl-3-methylimidazolium). In these systems, the intensity ratio of the canonical peaks depends on the orientation between the laser polarization and the external magnetic field at the delay time on the order of submicro-seconds. This experimental finding suggests that there are very slow tumbling motions of ZnTPP and H4TPP2+ in RTILs. Electron spin polarization created in the triplet–doublet system is also successfully observed in RTILs by the TR-EPR method. Based on the analysis of the time evolution curves of both the triplet and doublet molecules, spin polarization mechanisms in the RTILs were discussed.