Time-Resolved Electron Paramagnetic Resonance Spectroscopy — Principles and Applications

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The complexity of natural photosynthesis has prompted intensive studies of simpler model systems that might reproduce essential features of the in vivo apparatus. This is the so-called mechanistic approach, intended to solve some of the pertinent problems still open in primary photosynthesis. In this survey we concentrate on the application of time-resolved electron paramagnetic resonance (TREPR) Spectroscopy in studying photoinduced processes in model systems. The model systems consist of expanded π-electron systems as donors, and of covalently as well as non-covalently linked (electrostatically bound) donor-acceptor systems. The required details of the electronic structure and interactions can be determined by advanced TREPR (and related spectroscopies described in this book) which combines high spectral resolution and time resolution in the 10−100 ns domain. One of the most important features in TREPR is the electron spin polarization (ESP) effects associated with the reaction products of photoexcited donor-acceptor systems. Therefore, by controlling the environmental conditions, such as solvent polarity, viscosity, temperature, and medium anisotropy, the ESP pattern of such reactions is quite unambiguous. For example, the different line shape patterns reflect the variation of the molecular architecture, namely the relative orientation of the donor-acceptor and the nature of the spacer. The differences in molecular structures are manifested by the TREPR spectra through the magnitude of the spin-spin coupling (J) and the dipolar interaction (D), thus leading to different electron spin polarization mechanisms. In that respect, TREPR has a clear advantage over optical spectroscopy, which lacks adequate energy resolution. It is shown that TREPR applied to donor-acceptor systems, in isotropic and anisotropic liquid crystal (LC) media, allows a better understanding of the role of the microenvironment in long-range ET, a subject directly related to the in vivo protein-chromophore interaction.