Photo-induced electron transfer in cu(i) bis-phenanthroline based assemblies. Part I: Chromophore-acceptor diads. Part II: Donor-chromophore-acceptor triads
Date
2013
Authors
Lazorski, Megan, author
Elliott, C. Michael, advisor
Shores, Matthew P., committee member
Chen, Eugene, committee member
Bailey, Travis S., committee member
Sites, James R., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The photophysical behavior of [Cu(I)P2] (P=2,9-disubstituted-1,10-phenanthroline ligands) in donor-chromophore-acceptor (D-C-A) triads and chromophore-acceptor (C-A) diads is a complex and fascinating area of under developed, yet fundamental, electron transfer chemistry. In metal polypyridyl D-C-A and C-A triads/diads, metal polypyridyl chromophores (C) in which the polypyridyl ligands are covalently linked to acceptor (A) and/or donor (D) moieties, photo-excitation of the chromophore initiates a series of electron transfer events that result in the formation of a charge separated (CS)/charge transfer (CT) state, respectively. The majority of high-performing metal polypyridyl D-C-A/C-A complexes, on which [Cu(I)P2] D-C-A/C-A research is based, incorporate ruthenium (as [Ru(II)L3] where L=polypyridyl ligand) or other rare, expensive, and sometimes toxic metals such as osmium, rhenium and platinum. Although [Ru(II)L3] D-C-A/C-A's have historically set the benchmark for metal polypyridyl D-C-A/C-A performance, it is clear that these complexes are not a practical choice if D-C-A's or C-A's were incorporated into a device for large scale production. However, bisphenanthroline complexes of copper, a much more earth abundant, cheaper and less toxic metal, exhibit very similar photophysical properties to [Ru(II)L3] and have thus gained recognition as promising new materials for D-C-A/C-A triad/diad construction. In order to understand the electron transfer (ET) events occurring in [Cu(I)P2] D-C-A/C-A triads/diads, a complex must be synthesized that is capable of forming a CS with high quantum efficiency (Φcs/ct) and a long CS/CT lifetime (τcs/ct). Therefore, the intent of the research reported herein is to synthesize novel, yet functional heteroleptic [Cu(I)P2] D-C-A/C-A triads/diads and study their fundamental, photo-initiated electron transfer chemistry, specifically the formation of a CS/CT state. Many challenges, which are not present for [Ru(II)L3], make the design and synthesis of [Cu(I)P2] D-C-A/C-A assemblies an art in itself. Therefore, a significant amount of effort was spent on fabricating ligand architectures that (1) are appended with acceptor and/or donor moieties capable of being reduced/oxidized resulting in the formation of a CS/CT, (2) are able to be easily modified so the amount of energy stored in the CS/CT can be tuned, (3) favor the self-assembly of [Cu(I)P2] complexes, (4) are able to facilitate processes that maximize the Φcs/ct. Once the ligands were obtained, the complexation equilibria behavior of these [Cu(I)P2] triads and diads were studied. Despite efforts to design ligand architectures that favor heteroleptic formation, the thermodynamic driving force for heteroleptic D-C-A triad formation is less favor-able than expected. Thus, mixing stoichiometric quantities of D, C and A results in a statistical mixture of C-A, C-D and D-C-A products. Furthermore, since the ligands are labile and will re-arrange to the most thermodynamically stable configuration of products when these complexes are dissolved, isolation of the D-C-A product is impossible. However, recent advances in ligand design have shown promise for resolving this on-going issue. Despite having a mixture of products with the D-C-A, the electron transfer processes of the [Cu(I)P2] D-C-A triads and C-A diads were investigated. Using Transient Absorption (TA) laser spectroscopy, the CT state in the constructed C-A diads and the CS state in the D-C-A triads were detected and the lifetimes were determined. However, it was found that those lifetimes could be modulated to a small degree by solvent in the C-A diads (c.a. 6x longer in polar solvents), and drastically via the application of a magnetic field in D-C-A triads (c.a. 60x longer). The ability to modulate the lifetimes enabled the deconvolution of the effects due to the C-A diad vs D-C-A triad in the statistical product mixtures. Although the response in a magnetic field was a somewhat expected result, as similar effects occur in the [Ru(II)L3 D-C-A/C-A's, the magnitude of change in the lifetime and the quantum efficiency offers new insight into the electron transfer events that occur in the CS/CT formation process for [Cu(I)P2] D-C-A/C-A complexes.
Description
Rights Access
Subject
copper bis-phenanthroline
charge separated state
solar fuels
D-C-A
donor-chromophore-acceptor
chromophore-acceptor diads