Metal-mediated diradical reactivity: Activation and kinetics of photochemically labile ligands

The function of metal ions in determining pathways for excited-state deactivation and the fundamental chemical intermediates produced upon photolysis of metal diazo and metalloenediyne compounds has been investigated. First, ligand-to-metal charge transfer (LMCT) photolysis of a [Fe(triazine) ₃] (triazine = 3-hydroxy-1,2,3-benzotriazine-4(3H)-one) complex demonstrates reactivity comparable to that of the n-π* reactivity of the free ligand, indicating that a reactive state of the -N=N- ring system can be accessed through LMCT excitation. Second, the photoreactivity of [Cu(4,5-DAF) ₂]2+ (4,5-DAF = 9-diazo-4,5-diazafluorene) reveals electron-transfer reactivity can be obtained via a ligandcentered n-π* transition, demonstrating that optically dark, low-energy excited states can be populated via unrelated electronic transitions. The electronic and structural aspects of porphyrinic-enediynes show that alkyne substitution at the beta position of the porphyrin macrocycle introduces a delocalization of the porphyrin-based excited states onto the enediyne functionalities. Similarly, the electronic structure of [Ru(bptt)(phen)₂]2+ (bptt = 2,3-bis(phenylethynyl)-1,4,8,9-tetraaza-triphenylene) reveals that the frontier molecular orbitals of the ligand and corresponding ruthenium complex contain significant character on the enediyne portion of the ligand. This leads to a bichromophoric complex with two, nearly degenerate, emissive states in an aqueous environment. Both the porphyrinic-enediynes and ruthenium metalloenediyne demonstrate excited-state deactivation pathways bypassing the reactive enediyne state. Lastly, the non-radiative decay of a V 5+-catechol-enediyne complex with strong near-IR electronic transitions is employed to drive the thermal enediyne reactivity with photons, presenting a novel photo-thermal mechanism for the activation of metalloenediyne constructs.; Structural and mechanistic studies on a series of regulatory proteins probing both the local Cu2+ binding sites and novel flavin-based photocycle are also presented. Characterization of a flavin-based photocycle of the regulatory protein AppA reveals that H-bonding interactions at the flavin are altered upon photoexcitation. This initiates a global conformational change of the protein. Lastly, the metal-binding sites in a series copper-containing regulatory proteins (RegA, SenC, and RegB) are probed. While further studies are required to elucidate the structure of the Cu2+ site in RegA, the spectral properties of RegB and SenC indicate that RegB is likely 6-coordinate with mixed N/O ligation, and the metal-binding site in SenC is 4-coordinate with a CuS₂NO distorted tetrahedral geometry.