Intramolecular Electron and Energy Transfer within a Bisporphyrin in a Low-Temperature Glass

• Published in: Journal of physical chemistry (1952) Vol. 98; no. 19; pp. 4982 - 4989
• Main Authors: Harriman, Anthony, Heitz, Valerie, Ebersole, Marie, van Willigen, Hans
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01-05-1994
• Subjects: 400102 - Chemical & Spectral Procedures; 400201 - Chemical & Physicochemical Properties; 400500 - Photochemistry; ALCOHOLS; CARBOXYLIC ACIDS; CHEMICAL REACTIONS; COMPILED DATA; COMPLEXES; DATA; DECOMPOSITION; DIELECTRIC PROPERTIES; ELECTRICAL PROPERTIES; ELECTRON SPIN RESONANCE; ELECTRON TRANSFER; ENERGY TRANSFER; ETHANOL; FLUORESCENCE; FREEZING; GLASS; GOLD COMPLEXES; HETEROCYCLIC ACIDS; HETEROCYCLIC COMPOUNDS; HYDROXY COMPOUNDS; INFORMATION; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; LASERS; LUMINESCENCE; MAGNETIC RESONANCE; MULTIPLETS; NUMERICAL DATA; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; ORGANOMETALLIC COMPOUNDS; PHOTOCHEMICAL REACTIONS; PHOTOLYSIS; PHYSICAL PROPERTIES; PORPHYRINS; RADICALS; RESONANCE; TRANSITION ELEMENT COMPLEXES; TRIPLETS; ZINC COMPLEXES
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/j100070a007

The photophysical properties of an oblique bisporphyrin, comprising zinc(II) and gold (III) porphyrins separated by a 2,7-diphenyl-1,10-phenanthroline spacer moiety, have been measured in a low-temperature ethanol glass. Comparison with the properties recorded earlier in fluid, polar solution indicates that the energy of the intermediate charge-transfer state (CTS), formed by electron transfer from zinc porphyrin to appended gold porphyrin, is increased by 0.75 eV upon freezing. This is due to a substantial decrease in the static dielectric constant of the solvent which occurs upon moving from a fluid solution to a frozen glass. There is a corresponding decrease in the solvent reorganization energy upon freezing, and, at 77 K, both the reorganization energy and the reaction exergonicity are essentially zero. The rate of formation of the CTS at 77 K is much slower than that predicted for a superexchange mechanism unless there is a change in the molecular architecture upon freezing. Formation of an intermediate CTS has been confirmed by low-temperature EPR studies. The gold porphyrin excited triplet state is unable to abstract an electron from the appended zinc porphyrin, and, instead, quantitative intramolecular triplet energy transfer takes place. 51 refs., 11 figs., 3 tabs.