Energy and Electron Transfer in Porphyrin-Based Donor--Bridge--Acceptor Systems

Abstract: The photophysical properties of supramolecular donor--bridge--acceptor systems have been investigated by spectroscopic and quantum mechanical methods. The systems consist of porphyrin donor and acceptor moieties linked by bridging chromophores of varying excitation energy, and are shown to be geometrically well-defined in terms of length and rotational conformation. In order to focus on one photophysical process at a time, several series, each consisting of a specific donor/acceptor pair and four different bridges, have been studied. Special emphasis is put on how the bridges affect singlet energy and electron transfer.

In the series where only excitation energy transfer is possible, it was shown that the bridging chromophore can mediate energy transfer, and that this effect is separable from the Förster mechanism. The thermally activated mediation is specific to each bridge, and is inversely proportional to the energy gap between the lowest singlet excited states of donor and bridge. Theoretical modeling of the mediation describes the difference between the bridging chromophores correctly.

The series containing a paramagnetic acceptor porphyrin show significantly enhanced S₁ -> T₁ intersystem crossing in the donor moiety. This, together with bridge-mediated energy transfer, accounts for the observed quenching of donor fluorescence, and no electron transfer could be observed. However, for one of the systems, stepwise electron transfer occurred in highly polar solvents. This process is controllable by selecting specific combinations of the donor auxiliary ligand and solvent polarity.

In the series designed to study direct electron transfer, the .pi.-conjugated bridging chromophores were found to promote electron transfer, whereas systems with a non-conjugated bridge showed only energy transfer. The solvent dependent electron transfer rate showed the electronic coupling to be inversely proportional to the energy gap between donor and bridge lowest singlet excited state, just as was found for the energy transfer mediation. Furthermore, quantum mechanically calculated electronic couplings were found to be highly sensitive to molecular conformation, and agree qualitatively with the measured couplings.