Cubane and Triptycene as Scaffolds in the Synthesis of Porphyrin Arrays

Abstract

The primary aim of this research was to synthesise multichromophoric arrays that are linked through rigid isolating units with the capacity to arrange the chromophores in a linear and fixed orientation. The electronically isolated multichromophoric systems could then ultimately be tested in electron transfer studies for their applicability as photosynthesis mimics. Initially, 1,4-diethynylcubane was employed as the rigid isolating scaffold and one or two porphyrins were reacted with it in order to obtain the coupled product(s). Pd-catalysed Sonogashira cross-coupling reactions were utilised to try and achieve these bisporphyrin complexes. After extensive optimisation attempts, of both copper and copper-free Sonogashira reaction conditions, the desired alkynylcubane-linked bisporphyrin was detected, but unfortunately not isolated. The instability of the alkynylcubane-linked systems prevented efficient purification and inhibited further reactions, and while the alkynylcubane-linked porphyrin dimer was identified, its insolubility prevented further analysis. Overall, several new alkynylcubane-linked monoporphyrin compounds were synthesised. Following on from 1,4-diethynylcubane, 9,10-diethynyltriptycene was investigated for its uses as a rigid isolating unit. Initially, Sonogashira cross-coupling conditions that were developed from the previous work with cubane, were utilised with various porphyrins and BODIPYs and worked seamlessly. Although there are previous examples of triptycene porphyrin complexes, this was the first example of a linear porphyrin dimer that was connected to triptycene through the bridgehead carbons. Symmetric and unsymmetric examples of these complexes were obtained. In a slightly different vein, a series of porphyrin-cubane/bicyclo[1.1.1]pentane-porphyrin arrays were synthesised via amide coupling reactions. The utilisation of semi-rigid amide bonds to attach the porphyrin skeletons to a rigid scaffold introduced a controlled conformational flexibility into porphyrin dyad/s. This allowed significant modulation of the photophysical properties in the porphyrin dyad/s through the coordination of transition metal(II) ions. These reversible photophysical changes suggest that the dimeric systems are acting like switchable porphyrin tweezers. Lastly on a different note, a cationic dimer with a phenylene linker and an anionic porphyrin dimer with a conjugated butadiene linker were synthesised. The central aim of this work was to introduce water-soluble moieties into porphyrin dimers as there are very limited examples in the literature of porphyrin dimers with application in photodynamic therapy. It was hoped that by introducing cation/anionic moieties to a dimer that these water-soluble moieties would allow it to be suitable for biological application. The porphyrin dimer was successfully obtained, but tests to see its efficiency as a photodynamic therapy agent are still underway.