In nature, the antenna chlorophylls in photosynthetic bacteria are arranged as non-covalent macrorings in a spatially defined manner to absorb and transfer solar energy efficiently to the reaction centers. To mimic this, porphyrin assemblies with rigid, well defined geometries are of interest. In the field of biomimetic porphyrins, the strategy for synthetic chemists has become one of designing novel porphyrin scaffolds that exhibit similar arrangements as those found in nature, for use as model compounds in electron transfer studies. For photochemical systems such as these the correct geometry and spatial arrangement of the molecular building block is mandatory, yet this aspect is often neglected in modern tetrapyrrole chemistry. Here we report on the synthesis of tailored multiporphyrin arrays with defined structures and regiochemical arrangement for use in electron transfer studies. This requires the availability of appropriate scaffold molecules and linker units. While many of these are available, there is a notable absence of rigid, non-conjugating units with defined geometry and we have undertaken to develop synthetic chemistry for the use of cubane and triptycene units in this context.

The lack of research into aliphatically linked, multiporphyrin systems can be explained by the inherent flexibility of standard sp³ linker units. Cubane, with a virtually inflexible carbon skeleton, overcomes this problem, theoretically allowing it to be used as an isolator in bridged porphyrin systems for electron transfer studies. Furthermore, unlike most linker systems used in organic chemistry, cubane is a saturated hydrocarbon, and thus will also serve as an electronic isolator, preventing through bond communication between substituents, a key goal necessary for electron transfer studies to investigate through space versus through bond ET processes. In addition, the rigid three-dimensional framework of triptycene together with its symmetry, shape persistency and ability to project up to six functional groups in a spatially defined 120° orientation makes it an especially attractive scaffold in modern synthesis. The key goal here is the synthesis of a multichromophoric arrays based on the triptycene scaffold to serve as a key model compound both in light harvesting and energy transfer studies, as well as antenna cores for optical drug delivery systems.