A study of electron spin polarization transfer in a system of photo-excited triplet phthalocyanine and a nitroxide radical

Electron spin polarizations were studied fbr a system of excited triplet (T -1) phthalocyanine (MTNPc; M = H-2, Zn) and a nitroxide radical (TEMPO) in toluene solution by means of time-resolved EPR (TREPR) spectroscopy. TREPR spectra of the T-1 state and the radical (R) were observed simultaneously. Spin polarizations of both signals were found to consist of two components with different decay rates. From analyses of the polarizations and the deca y curves, it was found that the triplet polarizations are generated from an isotropic S-1-->T-1 intersystem crossings and thermal populations. The init ial and late polarizations on TEMPO ware interpreted in terms-of electron s pin polarization transfer (ESPT) from T-1 and a radical-triplet pair mechan ism (RTPM) with a quartet precursor under J < 0 (J; exchange coupling param eter), respectively. The ESPT rate was determined and found to be dependent on temperature and the central atom(s) of MTNPc. In the H2TNPc system, the ESPT rate is much slower than the diffusion rate and axial ligation of TEM PO remarkably accelerates the ESPT rate in the ZnTNPc system.