Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes

Kristina Martinez; Sydney M Koehne; Kaitlyn Benson; Jared J Paul; Russell H Schmehl

doi:10.1021/jacs.2c10165

Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes

J Am Chem Soc. 2023 Mar 1;145(8):4462-4472. doi: 10.1021/jacs.2c10165. Epub 2023 Feb 20.

Authors

Kristina Martinez¹, Sydney M Koehne², Kaitlyn Benson³, Jared J Paul⁴, Russell H Schmehl¹

Affiliations

¹ Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States.
² Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States.
³ Division of Chemistry and Chemical Engineering, Caltech, Pasadena, California 91125, United States.
⁴ Department of Chemistry, Villanova University, Philadelphia, Pennsylvania 19085, United States.

Abstract

Bimolecular excited-state proton-coupled electron transfer (PCET*) was observed for reaction of the triplet MLCT state of [(dpab)₂Ru(4,4'-dhbpy)]²⁺ (dpab = 4,4'-di(n-propyl)amido-2,2'-bipyridine, 4,4'-dhbpy = 4,4'-dihydroxy-2,2'-bipyridine) with N-methyl-4,4'-bipyridinium (MQ⁺) and N-benzyl-4,4'-bipyridinium (BMQ⁺) in dry acetonitrile solutions. The PCET* reaction products, the oxidized and deprotonated Ru complex, and the reduced protonated MQ⁺ can be distinguished from the excited state electron transfer (ET*) and the excited state proton transfer (PT*) products by the difference in the visible absorption spectrum of the species emerging from the encounter complex. The observed behavior differs from that of reaction of the MLCT state of [(bpy)₂Ru(4,4'-dhbpy)]²⁺ (bpy = 2,2'-bipyridine) with MQ⁺, where initial ET* is followed by diffusion-limited proton transfer from the coordinated 4,4'-dhbpy to MQ⁰. The difference in observed behavior can be rationalized based on changes in the free energies of ET* and PT*. Substitution of bpy with dpab results in the ET* process becoming significantly more endergonic and the PT* reaction becoming somewhat less endergonic.