Improving the electronic communication between a porphyrin core and peripheral substituents: towards enhanced solar energy conversion

The conversion of solar energy into a usable form via photocatalysis and photovoltaics is a continually growing area of research due to the ever-growing need to shift away from fossil fuel-derived energy. Porphyrins have become a focal point for the design of new dyes and photocatalysts due to their optical properties, high stability, scope of functionalisation and cheapness. This work synthesised a series of meso-substituted porphyrins with furan and thiophene based groups in an attempt to extend the conjugation of the porphyrin system. The synthesis of trans-5,15-di(methyl furan-2-carboxylate)10,20-diphenylporphyrin (26a) and trans-5,15-di(methylthiophene-2-carboxylate)10,20-diphenylporphyrin (27a) followed a Suzuki-Miyaura cross-coupling methodology. To get to this point, a modified Lindsey 2+2 condensation reaction, followed by di-bromination of a trans-5,15-diphenyl porphyrin was completed. The modified condensation reaction was the foundation of the synthetic pathway and resulted in exceptional yields of >40 %. Initially a palladium catalysed Miyaura-borylation of the furan and thiophene rings were undertaken but following complications in purification, an alternative C-H activated borylation reaction was undertaken using an [Ir(COD)(OMe)]2 catalyst. This resulted in the successful synthesis of methyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2-furancarboxylate (35) and methyl 5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2-thiophene-carboxylate (36), with yields of 51 % and 56 % respectively. Finally, the furan and thiophene-substituted porphyrins were synthesised in reasonable yields. These porphyrins were characterised and studied by UV-Vis spectroscopy and emission studdies. Both techniques gave evidence of increased extended conjugation in the free-base porphyrins compared to porphyrins with phenyl substituents. Supporting density functional theory (DFT) calculations confirmed that the red shift in the spectral data of the porphyrins containing the different 5-membered rings were due to a decrease in the dihedral angles along the porphyrin plane. Future work aims to tether the porphyrins to a dimolybdenum paddlewheel complex, so that the extent of the conjugation can be fully studied.