Synthesis of Covalently Linked Diporphyrins: Model Compounds for Intramolecular Energy and Electron Transfer
Three series of donor-bridge-acceptor compounds have been synthesized to probe the influence of various bridge chromophores on energy and electron transfer. The donor-bridge-acceptor compounds in each series comprise two different porphyrins, acting as donor and acceptor, covalently linked by bridge chromophores. The bridge chromophores are 9,10-bis(phenylethynyl)anthracene and 1,4-bis(phenylethynyl) substituted benzene, naphthalene, and bicyclo[2.2.2]octane.
The first series consists of zinc/free base diporphyrins, which were assembled using a building block approach. The starting materials were porphyrins and bridge building blocks functionalized with ethyne or iodide. Each bridge chromophore was formed as the building blocks were covalently linked using a palladium-catalyzed coupling reaction (Sonogashira reaction).
The second series was prepared by converting the zinc/free base diporphyrins to their zinc/iron(III) analogs. The free base moiety was metallated to the iron(III) chloride porphyrin using FeCl2. Despite the paramagnetic nature of the iron(III) porphyrins, these mixed metal diporphyrins and iron porphyrin monomers could be characterized using 1H NMR. The zinc/iron(III) diporphyrins were converted to the corresponding free base/iron(III) diporphyrins by acid treatment. Finally, zinc/gold(III) diporphyrins were synthesized using a building block synthesis as well as from a bis(free base porphyrin) dimer by a sequence of gold insertion followed by zinc insertion.
Steady state and time resolved spectroscopic measurements have shown that singlet energy transfer from the zinc porphyrin to the free base porphyrin in these systems is strongly dependent on the bridge chromophore. Similar results were obtained in a study of triplet energy transfer. In contrast to previous work on similar systems, we did not detect electron transfer in the diporphyrins with iron porphyrins as electron acceptors. Preliminary investigations of the zinc/gold(III) diporphyrins indicate that these systems display bridge dependent electron transfer.