The Chemical Structure, Dynamics and Mechanisms Program supports the efforts of Professor Thomas J. Meyer of the University of North Carolina at Chapel Hill for the investigation of proton coupled electron transfer (PCET) and concerted electron-proton transfer (EPT) in two areas. The first project focuses on the role of PCET and EPT in electrochemical reactions and electrocatalysis. The group's results document a significant role for PCET and EPT in electrode reactions with potentially important implications for electrocatalysis by solution and surface-bound catalysts. The second research area focuses on excited state, light-driven EPT reactions that produce highly reactive intermediates for carrying out solar fuel and green chemical reactions.
Proton coupled electron transfer reactions, reactions in which both electrons and protons are transferred, are of considerable interest in both chemistry and biology. These reactions are at the core of biological systems and artificial photosynthesis and are utilized in both water oxidation and carbon dioxide reduction. This research examines the mechanistic details of these reactions and may lead to the development of alternative energy systems. Graduate and postdoctoral students participate in the supervision of undergraduate research, in demonstrations and presentations at UNC and at local schools, in a summer programs developed to broaden participation.
for the Public - Dramatic rate enhancements have been observed in the oxidation of the phenols, tyrosine (TyrOH), tyrosine methyl ester (TyrOMe, MeC(O)O-TyrOH), and 4-methylphenol (4-MeArOH), at ITO electrodes modified by addition of the electron transfer relays [MII(bpy)2(4,4′-(HO)2P(O)CH2)2bpy)]2+ (M = Ru, Os) due to introduction of concerted electron-proton transfer (EPT). - Oxidation of the modified amino acid N-acetyl-tryptophan, which is unique among the redox active amino acids owing to its weakly acidic indolic proton with pKa = 16, which undergoes electron transfer oxidation followed by proton transfer and not EPT. - Transient absorption, time-resolved emission, and steady-state emission applied to the investigation of excited state PCET quenching and back electron transfer in the MLCT excited state of [Re(bpy)(CO)3(4,4′-bpy)]PF6 (bpy is 2,2′-bipyridine and 4,4′-bpy is 4,4′-bipyridine) by hydroquinone. - Excited state PCET has been shown to occur in hydrogen-bonded adducts between an organic dye (either para-nitrophenyl-phenol or hydroxyl-coumarin) and an external, nitrogen containing base in solution. Intramolecular charge transfer (ICT) occurs to an internal electron acceptor accompanied by proton transfer to the external base by photoEPT. - Cysteine, with its acidic thiol proton (pKa = 8.2) relative to the –O-H proton of tyrosine (pKa = 10.1) or the weakly acidic N-H proton of tryptophan (pKa ~ 16-17), utilizes EPT pathways in its oxidation chemistry. - In the oxidation of H2Q to Q by transition metal complex oxidants, an EPT pathway appears with AcO- acting as the acceptor base and with added strong acids, in the reduction of Q. - Excitation of phenol-to-MQ+ charge transfer bands in H-bonded association complexes leads to concerted, light driven EPT, ArOH---MQ+ + hν -> ArO.,HMQ.+, as shown by ultrafast transient absorption monitoring. - Following MLCT excitation of the assembly, (bpy)2Ru(Mebpy-tpy)Ru(bpy)(OH2)4+, ultra-slow proton transfer occurs from the (Mebpy-tpy-)RuIII-OH2 excited state with proton loss induced by added bases, acetate and H2PO4−.
