PHOTOINDUCED INTRAENSEMBLE ELECTRON-TRANSFER IN A BASE-PAIRED PORPHYRIN-QUINONE SYSTEM - TIME-RESOLVED EPR SPECTROSCOPY

Recent optical data suggest that porphyrin-guanine (PG) interacts in aprotic solvents with quinone-cytosine (QC) to form a 1 : 1 donor-acceptor preorganized base-paired supramolecule (PG-QC). To confirm such significant findings, for this never class of photosynthetic model system, we report here on an independent study by time-resolved electron paramagnetic resonance spectroscopy, combined with selective light excitation. The assemblies (BG-QC) of different porphyrins (zinc-porphyrin, Zn, and free base-porphyrin, H-2) were oriented in liquid crystals of different properties, and the results confirm unambiguously the formation of photoinduced charge-separated states, i.e., ZnG(.+)...QC(.-) and H(2)G(.+)...QC(.-), With lifetimes of a few microseconds. The unique spin-polarized EPR spectra, which depend on the temperature and solvent, allow the determination of whether the genesis of the electron transfer route is the photoexcited singlet or triplet states of the porphyrin-guanine subunits.