AMINOETHYLGLYCINE-DERIVATIZED RUTHENIUM COMPLEXES: MODEL ARCHITECTURES FOR PHOTOCHEMICAL PROCESSES

Open Access
Author:
Myers, Carl Philip
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 17, 2010
Committee Members:
  • Mary Elizabeth Williams, Dissertation Advisor
  • Mary Elizabeth Williams, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Alexander Thomas Radosevich, Committee Member
  • Theresa Stellwag Mayer, Committee Member
Keywords:
  • photoinduced energy transfer
  • photoinduced electron transfer
  • peptide coupling chemistry
  • metal complex coordination chemistry
  • heterometallic
  • self-assembly
  • photocatalysis
Abstract:
Transition metal coordination chemistry and amide peptide coupling chemistry can be exploited to synthesize a wide variety of compounds with ever increasing complexity. Employing these methodologies in the field of artificial photosynthesis could be a tractable solution to the often difficult synthetic challenge of linking donors and acceptors together in a systematic fashion. Successful application of these techniques would provide a wide ranging library of metals and ligands to fine tune compounds by which excited state energy or electrons may be passed from one complex to another, ultimately to do work, e.g. catalyze a reaction. This thesis describes the design and synthesis of peptide sequences capable of binding transition metal ions to self-assemble heterometallic complexes and the resulting photophysical effects of that complexation. Tris(bipyridine)ruthenium(II) complexes were derivatized with aminoethylglycine (aeg) backbones that are arranged in a “hairpin” configuration. Pendant bipyridine (bpy) ligands are complementary to each other and addition of transition metal ions (copper (II), zinc (II), or palladium (II)) form [M(bpy)2]2+ complex(es) through intra-molecular ligand coordination. Peptide coupling chemistry was used to design and synthesize sequences that bind solution metals in a predetermined fashion. To accomplish this, advances in peptide coupling techniques were improved greatly through application of solution phase syntheses that produced gram scale quantities of di-and tri-peptides, a significant advancement over milligram scale yields through solid-phase supported syntheses. Metal binding of Cu2+ ions was measured through fluorescence quenching measurements (steady state and transient) that indicated stoichiometric metal binding to the peptide scaffolds. Through peptide coupling chemistry, we successfully increased the donor-acceptor (Ru-Cu) distance to provide information as to whether energy or electron transfer dominated the emission quenching event. To answer unexpected observations of [Ru(bpy)3]2+ quenching by Zn2+, the effects of metal ion binding on the peptide scaffold and [Ru(bpy)3]2+ were measured through NMR spectroscopy. The Ru-hairpin structure was assigned using two-dimensional NMR techniques and indicated two major conformations at room temperature in dimethylsulfoxide solutions thus complicating the spectra. A model compound was synthesized excluding sources of hindered rotation to paint a clearer picture and the effect of Zn2+ binding was measured through NMR titrations that indicated a significant change in chemical environment of Ru-coordinated bpy ligands. An advantage of [Ru(bpy)3]2+ complexes is bpy ligands maybe be charged with a varying number of reactive acyl chloride groups. Reaction of these with amine-terminated peptides can easily increase the number of aeg strands. To demonstrate this, the number of hairpins was also increased from one to two or three placing [Ru(bpy)3]2+ at the center of a multimetallic complex and imparting metal coordination in three dimensions. These complexes were compared with single hairpin complexes capable of binding two Cu2+ centers to further emphasize peptide dictation on [Ru(bpy)3]2+ photophysical properties. These complexes also served as a starting point to synthesize a tri-heterometallic complex. The peptide scaffold also offers the unique possibility of linking known donor-acceptor pairs to determine if photocatalysis is possible on our peptides. [Ru(Me2 bpy)3]2+ is liked with one, two, and three [(bpy)Pd(CH3)(CH3CN)]+ complexes through the aeg backbone, that when irradiated with light (> 455 nm) are able to photocatalyze the dimerization of á-methyl styrene. These compounds represent the first example of a photocatalytic reaction performed on our scaffolds. Finally, a series of compounds is presented that demonstrate the modularity of these species along with a discussion of transient absorption spectroscopy measured for single hairpins. The complexes presented are designed and synthesized with the specific intension of exploiting the modularity in metal complex coordination chemistry and peptide coupling chemistry to increase spectroscopic complexity, complex rigidity, and metal identity. These are presented with basic characterization and a discussion of their intended purpose.