| Abstract
| - Phosphinoyl radicals were produced in benzene solution by photolysis of three acylphosphine oxidephotoinitiators, diphenyl-2,4,6-trimethylbenzoyl phosphine oxide (I), bis(2,6-dimethoxybenzoyl)-(2,4,4-trimethylpentyl) phosphine oxide (II), and bis(2,4,6-trimethylbenzoyl) phenylphospine oxide (III). Thechemically induced dynamic electron polarization (CIDEP) of the radicals was measured by time-resolvedelectron paramagnetic resonance spectroscopy at different microwave frequencies/magnetic fields, in S- (2.8GHz, 0.1 T), X- (9.7 GHz, 0.34 T), Q- (34.8 GHz, 1.2 T), and W-bands (95 GHz, 3.4 T). The CIDEP wasfound to be due to a triplet mechanism (TM) superimposed by a radical pair mechanism comprising ST0 aswell as ST- mixing. Contributions of the different CIDEP mechanisms were separated, and the dependenceof the TM polarization on microwave frequency was determined. It agrees well with the numerical solutionof the relevant stochastic Liouville equation, which proves the TM theory quantitatively. The applicability ofprevious approximate analytical formulas for the TM polarization is discussed. Parameters of the excitedtriplet state of III were estimated from the dependence of the TM polarization on microwave frequency.They are zero-field splitting constant 0.169 cm-1 ≤ DZFS ≤ 0.195 cm-1, lifetime 40 ps ≤ τT ≤ 200 ps, andinitial population of its Tz spin sublevel 0.92 ≤ wz ≤ 1.
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