MOLECULAR ARCHITECTURE AND ENVIRONMENTAL-EFFECTS IN INTRAMOLECULAR ELECTRON-TRANSFER - AN ELECTRON-PARAMAGNETIC-RESONANCE STUDY

Intramolecular electron transfer (ET) in three photosynthetic model systems, oriented in liquid crystals (LCs), was monitored by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy: (1) zinc porphyrin (ZnTPP) linked via an amide spacer to a lumiflavin (PaF); (2) ZnTPP linked to a benzoquinone via a phenyl spacer in the para (p-PpQ); and (3) in the meta (m-PpQ) positions. The anisotropic Liquid crystalline environment makes the ET products detectable over a wide range of temperatures, i.e., 210 less than or equal to T less than or equal to 330 K. Under such experimental conditions the ET rates are reduced quite dramatically into the solvent controlled adiabatic regime. The spectral line shape differences reflect the effect of the molecular architecture, namely, the relative orientation of the donor-acceptor as well as the spacer moiety. These 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.