Iron para-trimethylanaliniumphenyl-porphyrin is a water-soluble porphyrin complex capable of reducing CO2 both electro- and photo-chemically. Despite this well-known ability, the mechanism and associated intermediates that describe this conversion process are unknown. In our initial steps to elucidate this mechanism, we characterize the ground-state and excited-state dynamics of [Fe-p-TMA]4+. Ultrafast transient absorption of [Fe-p-TMA]4+ reveals a relaxation that is complete within hundreds of picoseconds. Global fitting of the data to a three-component sequential model is consistent with the mechanism previously reported for FeTPP, whereby relaxation to the ground state proceeds via an initial internal conversion (k1), charge transfer (k2), and eventual retrodonation (k3). However, while the final state, a ligand field excited state, lives on the order of ~20 ps in FeTPP in organic solvents, we find the lifetime of this state to be extended by greater than an order of magnitude in [Fe-p-TMA]4+ in aqueous conditions. Further, as metal bicarbonate species, MHCO3 (M = Na, K, Cs), are common electrolyte additives in these electro- and photo-chemical reactions, we examine the impact of these species on the spectroscopic behavior of this complex. We find that both the ground and excited states of [Fe-p-TMA]4+ are highly sensitive to the presence of MHCO3, implicating bicarbonate as playing a potentially crucial role in predisposing this [Fe-p-TMA]4+ toward reactivity. Examination of MHCO3 concentration dependence on excited state lifetimes supports the identification of the charge-transfer nature of the second state.

Elucidating chemical microenvironment effects on the ground and excited state properties of water-soluble Iron-porphyrin photocatalysts

ACS Spring 2022

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