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

Intramolecular electron transfer (ET) in three photosynthetic model sy stems, oriented in liquid crystals (LCs), was monitored by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectros copy: (1) zinc porphyrin (ZnTPP) linked via an amide spacer to a lumif lavin (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 anisotro pic Liquid crystalline environment makes the ET products detectable ov er a wide range of temperatures, i.e., 210 less than or equal to T les s than or equal to 330 K. Under such experimental conditions the ET ra tes are reduced quite dramatically into the solvent controlled adiabat ic regime. The spectral line shape differences reflect the effect of t he molecular architecture, namely, the relative orientation of the don or-acceptor as well as the spacer moiety. These differences in molecul ar structures are manifested by the TREPR spectra through the magnitud e of the spin-spin coupling (J) and the dipolar interaction (D), thus leading to different electron spin polarization mechanisms.