Spectroscopy and Photochemistry of Pyrazolyl-Bridged Binuclear Iridium(I) Complexes

Citation

Marshall, Janet Layne (1987) Spectroscopy and Photochemistry of Pyrazolyl-Bridged Binuclear Iridium(I) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/63cx-a405. https://resolver.caltech.edu/CaltechTHESIS:02192019-111735898

Abstract

Spectroscopic studies of a series of pyrazolyl-bridged and substituted-pyrazolyl-bridged binuclear iridium(I) complexes indicate that the description of the metal-metal interactions in previously studied D_4h d⁸-d⁸ rhodium(I), iridium(I), and platinum(II) species may be extended to these lower symmetry (C_2v) d⁸-d⁸ molecules. Specifically, the ¹A₁(dσ)² (dσ^*)² ground state exhibits weak metal-metal bonding, and the lowest excited states are a singlet (¹B₂) and triplet (³B₂) derived from the (dσ)²(dσ^*)¹(pσ)¹ electronic configuration. The ¹B₂ and ³B₂ excited states, which are expected to feature strong metal-meta1 bonding, are 1uminescent at ambient temperature in fluid solution.

Electronic absorption and emission spectroscopic studies and photophysical investigations of the emissive singlet and triplet excited states of bis(1,5-cyclooctadiene)bis(µ-pyrazolyl)diiridium(I), [Ir(µ-pz)(COD)]₂, and analogous substituted-pyrazolyl complexes are presented in Chapter 2. The absorption spectrum of [Ir(µ-pz)(COD)]₂ exhibits an intense band attributable to ¹A₁ → ¹B₂ at 498 nm (ε = 8100 M^-1cm^-1). Both fluorescence (λ_max = 558 nm, Φ_em = 0.0001, τ < 20 ps) and phosphorescence (λ_max = 684 nm, Φ_em = 0.0078, τ = 250 ns) from the ¹B₂ and ³B₂ excited states, respectively, are observed at ambient temperature for this complex. The absorption and emission spectra of the substituted-pyrazolyl complexes show similar features. In addition, ground-state resonance Raman studies of these complexes suggest the presence of a reasonable metal-metal bonding interaction in the formally nonbonded binuclear center; excitation into the bands corresponding to the metal-metal localized ¹A₁ → ¹B₂ transition results in resonance-enhancement of vibrations at frequencies of of 58 cm^-1 to 80 cm^-1 that are assigned to ν(Ir-Ir ).

The long lifetime of the ³B₂(dσ^*pσ) excited state of [Ir(µ-pz)-(COD)]₂ implies that it should be able to participate in bimolecular reactions. The results presented in Chapter 3 show that this strongly reducing excited state undergoes photoinduced electron transfer with a variety of substrates including reversible electron transfer to one-electron acceptors such as methyl viologen and pyridinium monocations. For pyridinium acceptors with reduction potentials ranging from -0.67 V to -1.85 V vs. SSCE, the rates of electron-transfer quenching range from a diffusion-limited rate of 2.0 x 10¹⁰ M^-1s^-1 to 1.1 x 10⁶ M^-1s^-1 and obey Marcus-theory predictions for outer-sphere electron transfer in the "normal free-energy region." However, the rates do not decrease as predicted for the "inverted free-energy region." With acceptors such as halocarbons, the unproductive back-electron-transfer reaction can be circumvented, and net two-electron, photoinduced electron transfer yields iridium(II)-iridium(II) oxidative addition products.

Chapter 4 focuses on spectroscopic and photophysical investigations of pyrazolyl-bridged binuclear iridium(I) complexes containing carbon monoxide ligands. Spectroscopic studies of tetracarbonylbis(µ-pyrazolyl)diiridium(I), tetracarbonylbis(µ-3-methylpyrazolyl)diiridium(I), and tetracarbonylbis(µ-3,5-dimethylpyrazolyl)diiridium(I) reveal interesting features in the electronic absorption spectra at ambient temperature and 77 K that may be assigned to dπ(xz,yz) → [σ(p_z), π^*(CO)] transitions and predominantly metal-metal localized σ^*(d_z2) → [σ(p_z),π^* (CO)] transitions that reflect the degree of metal-meta1 interaction. Photophysical studies of the emissive ^1,3B₂(dσ^* pσ) excited states suggest that a higher-energy d-d excited state may provide a pathway for thermal deactivation of these states.

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