Porphyrins remain promising chromophores for the development of organic semiconducting materials that possess electronic conductivitites which rival traditional inorganic semiconductors. Successful application of conjugated porphyrins would impact a wide range of opto-electronic applications due to their low molecular weight and physical versatility. Zinc porphyrin oligomers (PZnn) elaborated along the long molecular axis and bridged by acetylene units feature uniquely strong electronic interactions, allowing significant modulation of ground and excited state properties. These meso-substituted porphyrin arrays give rise to broad B-band transitions and dramatic frontier molecular orbital stabilization. To harness the intense electronic coupling of PZnn and introduce charge transfer character, alternating electron rich (donor) and electron withdrawing (acceptor) zinc porphyrin arrays have been developed. These arrays must feature photogenerated excitons which readily dissociate into free carriers. Crucial to that ability are the singlet and triplet delocalization lengths in perfluoroalkyl-substituted acceptor zinc porphyrins and their electron rich analogs. Notably, porphyrins with non pi-conjugating and strong sigma-withdrawing perfluoroalkyl groups substituted in the meso positions feature stabilization of the frontier molecular orbitals by 0.67 eV when compared to (tetraphenyl)PZn as determined by potentiometric measurements. While studies of acceptor and alternating donor-acceptor porphyrin arrays have shown that the Sn excited state is highly delocalized, the T1 state is more spatially confined. The triplet exciton is unique for its prolonged lifetime and for its emissive application in OLEDs and OPVs. Thus, understanding spatial organization of the triplet state is paramount to the goal of engineering prolonged diffusion rates and increased delocalization in energy capture and transfer materials. To this end, I have synthesized, characterized and analyzed the excited state properties of perfluoroalkyl zinc porphyrin monomers as well as donor-acceptor dimers towards the goal of engineering extended oligomeric arrays featuring delocalized and directional triplet state excitons for solar energy conversion materials. These systems have been analyzed by steady state electronic absorption and emission spectroscopies, nsTA spectroscopy, transient continuous wave EPR and electron nuclear double resonance spectroscopy.

Exploring the spatial distribution of triplet states in perfluoroalkyl zinc porphyrins for solar energy conversion

2019 Fall National Meeting

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