Porphyrin-Based Donor-Bridge-Acceptor Systems: Intrinsic Excited State Deactivation and Intramolecular Energy/Electron Transfer Reactions
Intramolecular energy and electron transfer reactions have been studied between porphyrin donor and acceptor moieties. The porphyrin units are covalently linked together by one of three different bridging chromophores to constitute the series of D-B-A systems. The D-B-A systems were designed in order to answer questions about the ability of the bridging chromophores to mediate the transfer reactions, i.e., to accelerate the rates of energy and electron transfer. This ability is believed to be related to the electronic structure of the bridge, why both p- and s-type of bridging chromophores have been used. The focus has been on transfer processes with the starting point in the excited triplet state of the donor or acceptor porphyrin.
The excited state intrinsic deactivation of the porphyrin monomers has also been carefully characterized. Primarily, the excited triplet state dynamics was investigated by experimental and theoretical means. The motivation was to rationalize the highly efficient relaxation in terms of conformational distortions on the triplet surface.
In the systems used to study triplet energy transfer, we initially demonstrated the immense difference in mediation capacity between conjugated and non-conjugated bridging chromophores. No energy transfer occurred in the s-bridged system, whereas a pronounced mediation effect was observed in the systems bridged by p-chromophores. The relative mediation efficiencies correlate well with a superexchange-governed transfer mechanism. In the corresponding D-B-A series designed for electron transfer, we observed the same behavior.
From the study on the intrinsic deactivation of the excited triplet state of the porphyrin monomers, we found that the unusually efficient relaxation is a result of the pronounced conformational flexibility on the triplet surface. We established, from experimental and theoretical studies, that the preferred deactivation route involves a passage over an out-of-plane distorted conformation, where the porphyrin macrocycle has adopted a saddle shaped conformation.
The conformational flexibility on the triplet surface was also proved to have interesting implications in the context of triplet energy transfer in the D-B-A systems. The nuclear displacements involved in the triplet relaxation process of the porphyrin donor generates conformations of the macrocycle where the mediation effect of the bridge is drastically enhanced. This implies that the triplet energy transfer process could be controlled by a conformational gating effect.