Intramolecular Electron Transfer in an Iridium d⁸-d⁸ Donor-Acceptor System

Citation

Fox, Lucius Seiberling (1989) Intramolecular Electron Transfer in an Iridium d⁸-d⁸ Donor-Acceptor System. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hapm-b986. https://resolver.caltech.edu/CaltechETD:etd-06072007-081509

Abstract

A series of donor-acceptor complexes, [Ir₂(µ-Pz*)₂(CO)₂(Ph₂POCH₂CH₂-Py⁺-R)₂](Ph₄B)₂ (Py⁺-R = 2,4,6-trimethylpyridinium, 4-methylpyridinium, pyridinium, and 4-phenylpyridinium; Pz* = 3,5-dimethylpyrazole), have been synthesized for studying the rate of photoinduced electron transfer from the metal localized (dσ*pσ) excited states of d⁸-d⁸ chromophores. The pyridinium electron acceptors are covalently attached to the iridium metal centers (donor) via a three atom hydrocarbon linker bound to the terminal phosphine ligands. The x-ray crystallographic structure of [Ir₂(µ-Pz*)₂(CO)₂(Ph₂POCH₂CH₂-Py⁺)₂](Ph₄B)₂ reveals a metal-metal distance of 3.219(1) Å and a solid state donor-acceptor separation of 5.34(1) Å. Additional donor-acceptor separations and orientations are available to the compounds in fluid solution through rotations about the Ir-P and linker group C-O, C- N, and C-C bonds. Steady state emission spectra show that the fluoresence and phosphoresence quantum yields in these compounds are substantially reduced relative to a appropriate model complex. To date the excited state reactivity of d⁸-d⁸ metal dimers has been exclusively attributed to their ³(dσ*pσ) states. These findings represent the first evidence for reactivity from a shorter lived ¹(dσ*pσ) state.

Picosecond and nanosecond laser flash-photolysis techniques were employed to measure the rates of photoinduced electron transfer and charge recombination in these systems. Values of k_ET, obtained from these studies, vary between 10⁶ sec⁻¹ and 10¹² sec⁻¹ as a function of reaction exoergicity (ΔGᵒ' = -0.8 eV to 1.92 eV). Clear evidence for the inverted behavior predicted by classical and semiclassical electron transfer models is seen at high driving force. Rate constants for reactions involving the ¹(dσ*pσ) and ³(dσ*pσ) excited states, as well as the rates of charge recombination are characterized by very similar values of λ and H_ab. Our data are adequately modeled by the classical theory of electron transfer proposed by Marcus (λ = 1.0 eV, H_ab = 35 cm⁻¹) or by a semiclassical model for k_ET where nuclear tunneling involves a low frequency metal-ligand mode (λ = 1.0 eV, H_ab = 35 cm⁻¹, ω = 100 cm⁻¹). These findings are explored with regard to utilizing the donor-acceptor complexes as molecular photochemical energy storage systems.

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