Triplet Excitation Energy Transfer in Donor-Bridge-Acceptor Systems: Experimental and Theoretical Investigations

Doctoral thesis, 2007

This thesis is a contribution to the understanding of bridge mediation of electron exchange processes in Donor-Bridge-Acceptor (D-B-A) systems. The field of research is relevant for the development and optimization of dye-sensitized solar cells, artificial photosynthesis, and the possible future development of molecular electronic devices. First an investigation of porphyrins, a group of molecules commonly used as donors and acceptors in D-B-A systems, is presented. Then an experimental and theoretical investigation of the influence of several system parameters on triplet excitation energy transfer (TEET) ensues. Finally, the work aims for future applications with the characterization of self-assembled monolayers (SAMs) of OPE-structures, commonly used as bridges in D-B-A systems, on gold. The main focus of this thesis is to investigate what governs the bridge mediation of TEET in D-B-A systems. The investigation starts with an experimental study of TEET in a series of D-B-A systems, where a zinc(II) porphyrin acts as the donor and the corresponding free-base porphyrin acts as the acceptor. The donor and acceptor chromophores are separated by oligo-p-phenyleneethynylene bridge units where the number of phenyleneethynylene groups varies between 2 and 5. The experiments are supported by a large number of theoretical calculations, leading to a model for the conformational dependence of the electronic coupling that reproduces the experimentally determined temperature dependence of the bridge mediation. In a parallel, purely theoretical, investigation, the bridge mediation of the electronic coupling for TEET is found to depend on the energy gap between relevant states, EDB, and the donor-acceptor separation, RDA, in accordance with the McConnell model. The applicability of this model to systems with conjugated bridges is discussed and examples of systems where the model does not apply are identified. For these systems an alternative model, based on electron tunneling through a square barrier, first derived by Gamow, is found to qualitatively describe the data. A more elaborate version of this model is found to accurately describe both the dependence of donor-acceptor separation and donor-bridge energy gap on the electronic coupling for experimental data from literature. The advantage of this model is that it will potentially allow for the use of molecular properties of the individual building blocks, available from experiments and/or calculations, to predict the properties of D-B-A systems.