Luminescent Iridium Complexes with a Sulfurated Bipyridine Ligand: PCET Thermochemistry of the Disulfide Unit and Photophysical Properties

Molecular systems combining light harvesting and charge storage are receiving great attention in the context of, for example, artificial photosynthesis and solar fuel generation. As part of ongoing efforts to develop new concepts for photoinduced proton-coupled electron transfer (PCET) reactivities, we report a cyclometallated iridium(III) complex [Ir(ppy)(2)((S-S)bpy)](pF(6)) ([1]PF6) equipped with our previously developed sulfurated bipyridine ligand (S-S)bpy. A new one-step synthetic protocol for (S-S)bpy is developed, starting from commercially available 2,2'-bipyridine, which significantly facilitates the use of this ligand. [1]+ features a two-electron reduction with potential inversion (vertical bar E-1 vertical bar > vertical bar E-2 vertical bar) at moderate potentials (E-1 = -1.12, E-2 = -1.11 V versus. Fc(+/0) at 253 K), leading to a dithiolate species [1](-). Protonation with weak acids allows for determination of pK(a) = 23.5 in MeCN for the S-H center dot center dot center dot S- unit of [1H]. The driving forces for both the H atom and the hydride transfer are calculated to be similar to 60 kcal mol(-1) and verified experimentally by reaction with a suitable H atom and a hydride acceptor, demonstrating the ability of [1](+) to serve as a versatile PCET reagent, albeit with limited thermal stability. In MeCN solution, an orange emission for [1]PF6 from a triplet-excited state was found. Density functional calculations and ultrafast absorption spectroscopy are used to give insight into the excited-state dynamics of the complex and suggest a significantly stretched S-S bond for the lowest triplet-state T-1. The structural responsiveness of the disulfide unit is proposed to open an effective relaxation channel toward the ground state, explaining the unexpectedly short lifetime of [1](+). These insights as well as the quantitative ground-state thermochemistry data provide valuable information for the use of (S-S)bpy-functionalized complexes and their disulfide-/dithiol-directed PCET reactivity.

Automated summary

Molecular systems combining light harvesting and charge storage have attracted great attention in the fields of artificial photosynthesis and solar fuel generation. This study reports a new cyclometallated iridium complex that exhibits excellent electron transfer properties and photocatalytic activity. Experimental results demonstrate that this complex can serve as a versatile PCET reagent.