Direct Tyrosine Oxidation Using the MLCT Excited States of Rhenium Polypyridyl Complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration
of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional
ReI(bpy)(CO)3CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved
emission quenching and transient absorption spectra of Re(bpy-Y)(CO)3CN and Re(bpy-F)(CO)3CN show
that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it
is more compatible with intramolecular Y oxidation, polypyridyl ReI complexes have been prepared with
the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)2](PF6) (PP
= bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized.
Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state
potential for tyrosine oxidation, similar to that for the (bpy)ReI(CO)3CN framework. The excited-state oxidation
potential can be increased by introducing a monodentate phosphine to the ReI(NN)(CO)3+ framework (NN
= polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of
[Re(phen)(P-Y)(CO3)](PF6). Analysis of the pH dependence of the rate constant for tyrosyl radical generation
is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.