Tuning of Energy and Electron Transfer Rates in Donor-Bridge-Acceptor Systems

Sammanfattning: The intramolecular processes of Donor-Bridge-Acceptor (D-B-A) systems have been investigated by spectroscopic methods. The systems consist of porphyrin donors and acceptors linked by oligo-p-phenyleneethynylene bridges. Two series of bridges have been used: one where the energy splitting between the singlet excited states of the donor and bridge (?EDB) varies at constant length and one where the bridge length and ?EDB vary. By changing the metalation state of the acceptor porphyrin different intramolecular processes have been studied separately. It was shown that the rate of energy and electron transfer has a strong distance dependence and a ?EDB dependence. It was concluded that the??EDB dependence is important in D-B-A systems with small ?EDB and/or large variation of ?EDB within a set of bridges. Further, it was shown that the attenuation factor, ?, is a system specific parameter and not only bridge specific as generally claimed. In a D-B-A system with small ?EDB it was found that a sequential transfer mechanism can be turned on and off by changing the temperature and/or solvent polarity. At high temperatures in polar solvents direct electron transfer and sequential electron transfer act in parallel but at low temperature or in non-polar solvents the direct process dominates. Finally, it was demonstrated that by changing the spin of a paramagnetic acceptor the main deactivation process changes. Electron transfer dominates in the low spin configuration whereas energy transfer and enhanced intersystem crossing are the main deactivation processes in the high spin configuration.