A Study of Electron Spin Polarization Transfer in a System of Photo-Excited Triplet Phthalocyanine and a Nitroxide Radical

Abstract

Electron spin polarizations were studied for a system of excited triplet (T1) phthalocyanine (MTNPc; M = H2, Zn) and a nitroxide radical (TEMPO) in toluene solution by means of time-resolved EPR (TREPR) spectroscopy. TREPR spectra of the T1 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 decay curves, it was found that the triplet polarizations are generated from anisotropic S1→T1 intersystem crossings and thermal populations. The initial and late polarizations on TEMPO were interpreted in terms of electron spin polarization transfer (ESPT) from T1 and a radical-triplet pair mechanism (RTPM) with a quartet precursor under J < 0 (J; exchange coupling parameter), 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 TEMPO remarkably accelerates the ESPT rate in the ZnTNPc system.

Electron spin polarization in an excited triplet-radical pair system: Generation and decay of the state

“…These parameters agree well with the experimentally determined lifetime of 70 ns [30] of the I(nD~~/z ) spin state. When kr~ I ranges from 1.5.10 6 to 2.5" 107 S -1 , k D and /ce are variable within the intervals (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). 106 and (7-10).…”

“…Hoijtink [1], Murrell [2], Robinson [3], and Chiu [4] have interpreted this mechanism in terms of exchange interactions in encounter complexes of molecular triplets and radicals. The "third-spin" effects on the spin evolution in radical pairs [5][6][7] and chemically induced dynamic electron polarization (CIDEP) in radicals escaped from encounters with molecular triplets [8][9][10][11][12] have rekindled this interest. The radical-triplet pair mechanism (RTPM) [8,13] and the electron spin polarization transfer (ESPT) [14, t5] have been suggested to account for the latter.…”

Developments of magnetic-resonance-related spin chemistry in Japan

“…Yamauchi's group [66,67] presented the convincing evidence that ESPT really takes place in the systems involving metalloporphy¡ ZnTPP and MgTPP, which exhibit different polarization characteristics. They have made quantitative analyses of the dynamics and extended such studies to phthalocyanine [68].…”

Investigation of Liquid‐Phase Inhomogeneity on the Nanometer Scale Using Spin‐Polarized Paramagnetic Probes

“…As in the case of other mechanisms of CIDEP such as the triplet mechanism (TM) [34], the electron spin polarization transfer (ESPT) [35,36], and the radical triplet pair mechanism (RTPM) [37], the observed EPR signals, usually acquired in a time-resolved experiment, belong to "free" radicals and are characterized by the same line widths and resonance frequencies as if the radicals were in the Boltzmann equilibrium state (nonpolarized). But the molecular mobility information potentially extractable from these parameters can be obtained using the nitroxide spin probe method in a more efficient and less expensive way.…”