It has been shown that Ru(II) diimine complexes having hydroxylated diimine ligands (i.e. 4,4'-dihydroxy-2,2'-bipyridine, dHbpy) are capable of transferring protons (PT*) and/or electrons (ET*) upon excitation. Transient absorption spectroscopy in the visible can be used to distinguish the products that emerge from the encounter complex formed preceeding reaction. For appropriate Ru(II) complex chromophore / acceptor systems, unambiguous determination of the reaction in the encounter complex as PT*, ET* or proton coupled electron transfer PCET* can be made. Through synthetic variation of the chromophores and acceptors, the free energies of each of the three reaction types can be systematically varied, yielding ultimately a map of the type of excited state photoreaction as a function of free energy for each of the three processes. This allows identification of the energetics necessary to observe true excited state proton coupled electron transfer in reaction cages.<br/>This presentation will report results for excited state reactions of chromophores of the type [(dHbpy)Ru(NN1)(NN2)]<sup>2+</sup> ( NN1 and NN2 = diimine, diphosphine, diamine ligands) with N-alkyl-4,4'-bipyridinium ions (alkyl = methyl, benzyl, cyanomethyl) in anhydrous acetonitrile. The map of encounter complex escape products vs. the free energies will provide a basis for determining what sort of chromophores might be employed for PCET* interaction with catalysts for substrate reduction/protonation.

Distinguishing excited state electron transfer, proton transfer and proton coupled electron transfer reactions of Ru(II) hydroxy diimine complexes

ACS Spring 2022

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