ESR and ENDOR study of the photooxidation of magnesium and zinc tetrakis(4-sulfonatophenyl)porphyrins

Journal Article · Wed Apr 15 00:00:00 EDT 1987 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00242a010· OSTI ID:6629385

van Willigen, H; Ebersole, M H

An ESR and ENDOR study has been made of the photoinduced electron transfer from magnesium and zinc tetrakis(4-sulfonatophenyl)porphyrins (Mg- and Zn TPPS) to K/sub 3/Fe(CN)/sub 6/ in a H/sub 2/O-Me/sub 2/SO glass. ESR signals from the porphyrin photoexcited triplet state and ..pi..-cation radical were used to monitor the yield of the photooxidation reaction and the kinetics of growth and decay of the redox ions. At low temperature, electron-transfer products are formed very slowly. However, a slight softening of the glassy matrix causes a dramatic increase in the rate of the establishment of a photostationary equilibrium between reactants and redox ion products. In a soft glass, virtual quantitative photooxidation of the porphyrin can be attained within seconds. The electron-transfer reaction is completely reversible. A comparison of the rigid matrix ENDOR spectra from chemically oxidized ZnTPP (or MgTPP) and the photochemically generated doublet radical confirms that the doublet radical ESR signal is due to the ..pi..-cation radical of Zn(or Mg)TPPS. ENDOR spectra provide evidence that under the conditions of the experiments, molecular motion is restricted severely. It is concluded that free radicals are formed as a result of long-range electron transfer. The viscosity effect is attributed to the fact that during the excited-state lifetime of the donor molecules, donor-acceptor pairs can attain a configuration favoring electron transfer. Solvent reorganization around the solvent-separated redox pairs stabilizes the electron-transfer products. Kinetic data show that the back reaction takes place before the redox ions have a chance to diffuse apart.

• Research Organization: Univ. of Massachusetts, Boston
• DOE Contract Number: FG02-84ER13242
• OSTI ID: 6629385
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 109:8; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
140505 -- Solar Energy Conversion-- Photochemical
Photobiological
& Thermochemical Conversion-- (1980-)
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
ALKALINE EARTH METAL COMPOUNDS
CARBOXYLIC ACIDS
CATIONS
CHARGED PARTICLES
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISTRY
DATA
DOUBLE RESONANCE METHODS
ELECTRON SPIN RESONANCE
ELECTRON TRANSFER
ENERGY LEVELS
EXCITED STATES
EXPERIMENTAL DATA
GLASS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
INFORMATION
IONS
KINETICS
MAGNESIUM COMPOUNDS
MAGNETIC RESONANCE
NUMERICAL DATA
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANOMETALLIC COMPOUNDS
OXIDATION
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
PORPHYRINS
RADICALS
REACTION KINETICS
REDOX REACTIONS
RESONANCE
ZINC COMPOUNDS