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–Sbpy)](PF6) ([1]PF6) equipped with our previously developed
sulfurated bipyridine ligand S–Sbpy. A new one-step
synthetic protocol for S–Sbpy 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 (|E1| > |E2|) at moderate
potentials
(E1 = −1.12, E2 = −1.11 V versus. Fc+/0 at 253 K), leading to a dithiolate species [1]–. Protonation with weak acids allows for determination
of pKa = 23.5 in MeCN for the S–H···S– unit of [1H]. The driving forces for both the H atom
and the hydride transfer are calculated to be ∼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 T1. 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–Sbpy-functionalized complexes and their
disulfide-/dithiol-directed PCET reactivity.