TEMPERATURE-DEPENDENT AXIAL LIGATION CHANGES AND PHOTOREDUCTION OF IRON PROTOPORPHYRIN-IX DIMETHYL ESTER CHLORIDE AT LOW-TEMPERATURES MONITORED BY RESONANCE RAMAN TECHNIQUE

We report the resonance Raman studies of photoreduced as well as chemically reduced iron-protoporphyrin-IX dimethyl ester chloride [(FePP)-P-III(Cl)] in the presence of 1,2-dimethylimidazole (1,2-Me(2)Im) or 2-methylimidazole (2-MeIm) in dimethyl sulfoxide (DMSO) at different temperatures up to 50 K. The yield of photoreduced (FePP)-P-II(L) (L = 2-MeIm or 1,2-Me(2)Im) is found to increase with decrease in temperature until the DMSO matrix remains soft at the laser focus. From a comparison of the RR marker bands and the spectrum of the chemically reduced complex at low temperatures, the photoreduced species has been identified as a ligand free, four-coordinated, intermediate-spin (FePP)-P-II complex in the soft matrix in the 200-150 K temperature range. The non-photoreducibility of (FePP)-P-III(1,2-Me(2)Im) in the hard DMSO matrix around 50 K provides clear evidence that only short-range electron transfer contributes to photoreduction while long-range quantum mechanical electron tunneling is insignificant in this process.