Exploring charge and energy transport in donor-bridge-acceptor systems is an important research field which is essential for the fundamental knowledge necessary to develop future applications. These studies help creating valuable knowledge to respond to today's challenges to develop functionalized molecular systems for artificial photosynthesis, photovoltaics or molecular scale electronics. This tutorial review focuses on photo-induced charge/energy transfer in covalently linked donor-bridge-acceptor (D-B-A) systems. Of utmost importance in such systems is to understand how to control signal transmission, i.e. how fast electrons or excitation energy could be transferred between the donor and acceptor and the role played by the bridge (the "molecular wire"). After a brief description of the electron and energy transfer theory, we aim to give a simple yet accurate picture of the complex role played by the bridge to sustain donor-acceptor electronic communication. Special emphasis is put on understanding bridge energetics and conformational dynamics effects on the distance dependence of the donor-acceptor electronic coupling and transfer rates. Several examples of donor-bridge-acceptor systems from the literature are described as a support to the discussion. Finally, porphyrin-based molecular wires are introduced, and the relationship between their electronic structure and photophysical properties is outlined. In strongly conjugated porphyrin systems, limitations of the existing electron transfer theory to interpret the distance dependence of the transfer rates are also discussed.

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Conjugated zinc porphyrin oligomers of various lengths are shown to form well-defined planar aggregates at low temperatures. The aggregation occurs over a narrow temperature interval (170–150 K) and is accompanied by dramatic changes in the electronic absorption and emission spectra. Similar changes are found in J-aggregates in which the transition dipole moments of aggregated chromophores couple to form a new and intense transition in the absorption spectrum, red shifted from the monomeric chromophore band. For the present porphyrin oligomers, the dramatic absorption changes are not associated with the formation of large aggregates, but rather with the dimerization accompanied by planarization of the oligomers. Free oligomers have a broad distribution of porphyrin–porphyrin dihedral angles and show a broad and unstructured absorption spectrum. As the oligomers stack to form aggregates, they planarize and the width of the conformational distribution is reduced to include virtually only the planar conformers, resulting in the observed change of the absorption spectrum. No experimental evidence for the formation of large aggregates was found, while a small aggregate, probably only dimer, is supported by the minor changes of the fluorescence rate constant upon aggregation and the fact that pyridine has no significant effect on the formation of this aggregate, which otherwise is very effective for inhibiting aggregation of zinc porphyrin oligomers. Compared to most porphyrin aggregates, which show broad absorption spectra and quenched fluorescence, these aggregates give sharp absorption and emission spectra with little change in the fluorescence quantum yield. Similar aggregates were also observed for oligomers substituted with both a fullerene electron acceptor and a ferrocene donor. The results presented here will be potentially useful as tools to understand how electron transfer and delocalization processes are influenced by molecular order/disorder transitions.

A series of highly fluorescent conjugated anthracene dendrimers having monomeric emission profile, but still exhibiting fast exciton depolarisation, are here presented.