Magnetic Field Dependence of Electron Spin Polarization Generated through Radical-Triplet Interactions Eli Stavitski, Linn Wagnert, and Haim Levanon* Department of Physical Chemistry and Farkas Center for Light-Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: NoVember 5, 2004; In Final Form: December 2, 2004 The magnetic field dependence of electron spin polarization (ESP), generated in free radicals when they encounter photoexcited triplets, was measured experimentally and analyzed theoretically. The time-resolved electron paramagnetic resonance measurements were performed with a microwave setup consisting of low- loss dielectric ring resonators with tunable microwave frequencies and the corresponding magnetic fields. The ESP of the radical was found in the magnetic field range of 170-370 mT, and the results of the calculation based on the numerical solution of the stochastic Liouville equation were found to be in line with the experimental data showing that ESP decreases when the magnetic field increases. 1. Introduction Electron spin polarization (ESP), i.e., the non-Boltzmann population of the electron spin levels, is generated in a variety of photochemical and photophysical reactions. 1,2 The interaction of free radicals with photoexcited triplets in solution resulting in highly spin-polarized radicals 3-7 is one of those processes. The classical mechanisms of chemically induced dynamic electron polarization (CIDEP), i.e., the triplet mechanism and the radical pair mechanism, fail to account for this phenomenon. 8 Thus, two complementary mechanisms contributing to ESP generation were invoked, namely, electron spin polarization transfer (ESPT) and radical-triplet polarization mechanism (RTPM). 8 The former is attributed to the interaction of free radicals with spin-polarized triplets, while in the latter mech- anism polarization of the triplet prior to the encounter is not required. It is evident that the properties of the radical, triplet, and solvent determine the magnitude of the resulting ESP. Specifically, the molecular dimensions and the solvent viscosity, which affect the mutual diffusion of the interacting species, should be taken into account. One should also consider the magnetic properties of the radicals and triplets, such as the line widths of the EPR spectra, the triplet zero-field splitting (ZFS) parameter, and the spin-lattice relaxation (SLR) times of the radicals and the triplets. In a recent study, a comprehensive theoretical treatment of ESPT and RTPM phenomena taking into account all of the above-mentioned parameters was developed. 9 Beyond the basic aspects of ESP generated during the radical-triplet (R-T) encounters, this phenomenon may lead to practical applications in microwave (MW) technology. 10 Such applications are based on the fact that the macroscopic magnetic permeability of the chemical system can be directly related to photoinduced ESP via the change in the real (leading to a phase shift) or imaginary parts (leading to an amplification/attenuation) of permeability providing a platform for MW devices with very low noise characteristics even at room temperature. 10 To approach this goal, the high ESP requirement should be accompanied by the specially designed MW resonators. 11,12 The relevant resonator should possess high filling and quality factors (η and Q, respectively) and be sufficiently small to allow efficient photo- excitation of the chromophore in the sample. In addition to the above-mentioned parameters affecting the ESP magnitude, one should also consider the magnetic field strength. As shown in previous studies, ESP in the R-T encounter is generated mainly in the crossing points of the quartet-doublet energy levels that are formed upon the R-T interaction. As a consequence, the magnetic field should determine the R-T distance at which the level crossing occurs (Figure 1). A qualitative prediction about the effect of the magnetic field on the magnitude of ESP has been reported, 13 showing that the magnitude of ESP is expected to decrease when the magnetic field increases. Preliminary time-resolved electron paramagnetic resonance (TREPR) experiments at the X- (10 GHz) and D-bands (130 GHz) provided confirmation of the theoretical prediction. 14 Nevertheless, it was difficult to analyze the results quantitatively, since we could not compare different types of resonators operating at two MW frequencies. The same type of resonators with a single variable, i.e., the magnetic field strength, must be used to overcome this difficulty and make the comparison on a quantitative basis. In this work, we have employed low-loss dielectric resonators and varied the resonance frequency and corresponding magnetic field by changing the resonator dimensions. Here, we present the experimental results at the X- (∼10 GHz) and S-bands (∼5 GHz) as well as theoretical calculations based on recent extensive theoretical papers. 9,13 2. Experimental Section Chemical System. The free-base etioporphyrin I (Frontier Scientific), the deuterated modification of the trityl radical (Nycomed Innovations, U.S. Patent 6,013,810), and the solvents were used without further purification (Figure 2). The solvent (viscosity ∼30 cP) was prepared by mixing 10% cyclohexanone (to increase the solubility of the trytil radical), 20% chloronaph- thalene, and 70% heavy paraffin oil (Sigma-Aldrich). The porphyrin and radical concentrations were ∼2 mM. The solution was degassed in a 1.2 mm o.d. Pyrex tube by several freeze- pump-thaw cycles and sealed under vacuum. 976 J. Phys. Chem. A 2005, 109, 976-980 10.1021/jp044937o CCC: $30.25 © 2005 American Chemical Society Published on Web 01/25/2005