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

Abstract

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.

Acknowledgements

The authors thank to Professor T. Kitazume and his group members (Tokyo Institute of Technology) for providing us RTIL samples and for stimulating discussion. EPR spectra were measured by one of the on-campus cooperative research facilities of the Tokyo Institute of Technology, ‘a pulsed EPR system’. The present work is partly funded by the Grant-in-Aid for Scientific Research (No. 15550005 and 17073006) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.