Definitive examples of excited state proton coupled electron transfer are still relatively rare. In part, the discovery of such systems is complicated by the large number of pathways for excited state decay possible coupled with changes in the free energy of proton transfer and electron transfer between the ground and excited states. Very recently we have begun expanding upon observations made by Paul and Meyer (Coordination Chemistry Reviews 254, no. 21–22 (2010): 2459–2471. doi:10.1016/j.ccr.2010.03.001) several years ago on proton coupled electron transfer in systems involving a Ru(II) hydroxyphenanthroline complex chromophore and the N-methyl-4,4'-bipyridine cation (figure). Excitation of the chromophore to form the 3MLCT state results in a significant increase in the acidity of the hydroxyl substituent. In the presence of a variety of pyridine derivatives, none of which react with the ground state of the complex, excited state proton transfer occurs with rate constants that increase with increasing pKa of the pyridinium ion up to the diffusion limit. Excited state electron transfer to the N-methylpyridinium ions is too slow to be measured in all the systems examined. However, when N-methyl-4,4'-bipyridine or related derivatives is used as acceptor, transient absorption spectroscopy reveals featuers of the reduced, protonated 4,4'-bipyridinium derivative; this will only occur if proton transfer and electron transfer occur within the encounter complex formed between the 3MLCT state of the Ru(II) complex chromophore and the N-methyl-4,4'-bipyridine. Details of the analysis will be presented along with consideration of yields for formation of proton transfer products and PCET products.

Excited State PT, ET and PCET in Hydroxybipyridine Ru(II) Complexes in Nonaqueous solution

255th National Meeting (2018)

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