Toward charge separation through conformational control in copper coordination complexes

Griffin, Paul J

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Description

Title

Toward charge separation through conformational control in copper coordination complexes

Author(s)

Griffin, Paul J

Issue Date

2023-07-13

Director of Research (if dissertation) or Advisor (if thesis)

Olshansky, Lisa

Doctoral Committee Chair(s)

Olshansky, Lisa

Committee Member(s)

• Mirica, Liviu
• Vura-Weis, Josh
• Rodríguez-López, Joaquín

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

copper, photochemistry, electron transfer, coordination chemistry

Abstract

The continued development of solar energy as a renewable resource requires the design of cheap, earth-abundant photosensitizers to remain sustainable. State-of-the-art technologies employ second and third row transition metals such as Ru and Ir in dye-sensitized solar cells, as photoelectrode dopants, and as photoredox catalysts, but these elements are both rare and expensive. The development of first-row transition metal photosensitizers to address these needs has garnered significant attention in recent years. While most photosensitizers of first-row transition metals suffer from drawbacks associated with the primogenic effect – excited state deactivation through metal-centered states – copper provides a promising alternative, if it can be tamed. The geometry of copper complexes is intimately related to their oxidation states. CuI is a d10 metal ion with no ligand field stabilization and consequently adopts tetrahedral, trigonal planar, or linear coordination environments. The CuI ion commonly binds “soft” ligands in Pearson’s “hard and soft acid and base” heuristic. CuII, on the other hand, is a d9 metal ion which undergoes Jahn-Teller distortion to adopt tetragonal geometries in molecular systems and binds “hard” ligands. The large structural reorganization associated with the CuII/I redox couple – from tetrahedral to tetragonal, for instance – results in slow and irreversible electron transfer for many copper systems. Paradoxically, nature leverages CuII/I redox chemistry to promote rapid electron transfer. These blue copper proteins hold the copper ion in a constrained geometry intermediate between those common to CuI and CuII – this structural rigidity of the protein has been called the “entatic state” by Vallee, Williams, and Malmström. The entatic state lowers the reorganization energy of the CuII/I redox event and consequently facilitates rapid electron transfer. This thesis begins with the synthesis and characterization of dpaR (dpa = dipicolylaniline, R = H, OMe, SMe), a series of ligands capable of stabilizing both CuI and CuII. Solution-phase measurements suggest that CuCl(dpaOMe) and CuCl(dpaH) are conformationally dynamic in their univalent states between trigonal planar and a 4-coordinate species, most likely tetrahedral, as assessed by variable-temperature nuclear magnetic resonance (NMR) spectroscopy. The corresponding CuII complexes are rigorously square pyramidal. Conversely, CuCl(dpaSMe) forms rigid CuI complexes in the solid state and solution while having solution-phase dynamics in the CuII state, as assessed by single crystal X-ray diffraction, powder and variable temperature electron paramagnetic resonance (EPR) spectroscopies supported by density functional theory (DFT). The dynamic behavior of these motifs appear to be mediated by a weak Cu–aniline bond in conjunction with the bonding interactions of the -OMe and -SMe. Cyclic voltammetry of these complexes revealed reversibility in the CuII/I redox couple, suggesting that the reorganization energy associated with electron transfer might be low. The reorganization energies of the CuII/I redox couple for [CuCl(dpaSMe)]+/0 and [CuCl(dpaOMe)]+/0 were assessed by NMR linewidth broadening experiments in dichloromethane. The electron transfer self-exchange rate constants of k11 = 2.21(9) x 106 and 2.48(6) x 105 M-1 s-1 for [CuCl(dpaSMe)]+/0 and [CuCl(dpaOMe)]+/0, respectively, are among the fastest recorded for molecular CuII/I and compare favorably with natural systems (k11 = 105 – 106 M-1 s-1). [CuCl(dpaSMe)]+/0 is the fastest reported for a CuII/I redox couple to date. Marcus analysis supported by DFT calculation revealed low inner sphere reorganization energies in both complexes (inner = 0.62 and 0.71 eV for SMe and OMe, respectively) as a consequence of the facile solution-phase dynamics they engage in. Comparisons to rigid CuII/I complexes and implications for the rigidity-based definition of the biological entatic state, in the formulation of Vallee and Williams, are discussed. Finally, ligands incorporating twisted intramolecular charge transfer (TICT) chromophores into the dpaR framework, that exhibit their own conformational dynamics upon illumination, were designed and synthesized. The CuCl(dpaaR) (dpaa = dipicolylaminoacetophenone, R = H, OMe) complexes were structurally characterized by conventional means and their steady-state photochemistry assessed by UV-Vis absorption/emission spectroscopies. Dynamic characterization using time-resolved photoluminescence (TR-PL) and transient absorption (TA) spectroscopies was undertaken to disentangle the kinetics involved in excited state procession. A reduction in F, the fluorescence quantum yield, was observed in CuCl(dpaaOMe) with respect to its ligand – a result not observed in the control complexes – by ~17x. The compound exhibits excellent photostability with less than 2% decay after 9 x 106 laser shots at 400 W. However, TR-PL revealed that the emission quenching was a direct consequence of changes to the radiative rate constant (kr) rather than to accelerated charge separation in the excited state manifold. Ideas to improve these dynamic copper photosensitizers are discussed.

Graduation Semester

2023-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/121364

Copyright and License Information

Copyright 2023 Paul Griffin

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