Nitrile-based carotenoid complexes of Ru (II), Re (I) and Pt (II) metals: Coordination synthesis, electrochemical and photophysical characterization
Nitrile-based carotenoid complexes of Ru(II), Re(I) and Pt(II) metal ions have been synthesized. The photolability of two bis-nitrilo ruthenium(II) complexes formulated as [Ru(bpy)₂(L)₂](PF₆)₂, where bpy is 2,2’-bipyridine and L = acetonitrile and succinonitrile have been investigated. The UV/visible absorption spectrum of the sn derivative is dominated by an intense (εmax~58700M⁻¹ cm⁻¹) absorption band at 287 nm assigned as a LC (∏ → ∏*) transition. The peak observed at 418 nm (ε ~ 10400 M⁻¹ cm⁻¹) is an MLCT band while the one at 244 nm (ε ~ 23600 M⁻¹ cm⁻¹) is of LMLCT character. The AN derivative behaves similarly. These Ru(II) complexes undergo photosubstitution of solvent with quantum efficiencies near one. Calculated and experimental results support a stepwise replacement of the nitrile ligands. Based on DFT calculations, the electron density of the HOMO lies on the metal center, the bipyridine ligands and the nitrile ligands and electron density of the LUMO resides primarily on the bipyridine ligands. Ligand pyridyl-cyano and dicyano derivatives of all-trans-retinal and β-apo-8’-carotenal have been synthesized via Knoevenagel Condensation. Density Functional Theory and Time Dependent Density Functional Theory calculations in all these ligand compounds reveal that the HOMO and LUMO are located primarily on the polyene chain and the energy gap between them is consistent with the observed optical spectrum. Spectroscopic treatments establish that their light-harvesting properties are similar to those observed pigment-protein complexes in plants and purple bacteria These observed properties are due to the highly allowed population of the second-lying ¹Bu excited state. These compounds contain an electron pair donor and can be coordinated to rhenium (I) and platinum (II) metal ion centers. These complexes are photosensitizer dyes in dye-sensitized solar cells (DSSC’s) and also serve as photocatalysts and metallointercalators in DNA.
Thesis (Ph.D.)--Wichita State University, College of Liberal Arts and Sciences, Dept. of Chemistry