Porphyrin Dimers and Arrays

Current applications of porphyrins in medicine and optics, such as photodynamic therapy or nonlinear absorbers, increasingly require the use of far-red absorbing dyes. Modifications of the porphyrin structure to accommodate these conditions can be achieved by extending the conjugation of the porphyrin pi-system, which causes a bathochromic shift in the absorption spectrum. Thus, conjugated porphyrin oligomers have found widespread use. However, past synthetic strategies have mainly targeted symmetric porphyrin dimers, trimers, and oligomers, which limit the practical use of such chromophores. Here, a series of symmetric and unsymmetric dimeric and trimeric porphyrin systems, which are connected by conjugated linkers, namely alkyne and phenylacetylene, were synthesized by palladium-catalyzed C-C coupling reactions. Adopting two approaches, firstly, a series of new unsymmetric dimers was synthesized by the incorporation of all substituents on the monomeric components prior to coupling. The second was the synthesis of new symmetric dimers and trimers with free meso positions enabling further chemistry to be carried out. The majority of these conjugated arrays exhibited a bathochromic shift in their UV/Vis absorption, in particular the alkyne-linked arrays showed absorption greater than 720 nm. The mass spectra of phenylacetylene and diphenylbutadiene-linked zinc arrays exhibited detachment of zinc from the porphyrin core. These unusual results are both linker and metal dependent, usually only seen for more labile metals.