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

Abstract

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]PF 6 ) 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 (|E 1 | > | E 2 |) 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•••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]PF 6 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.

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer

“…17 The disulfide-centered excited state is generally lower in energy compared to the parent complexes. However, we noticed that in the ground state, the disulfide unit does not interact strongly with the complex framework, 16,18,33 and thus substitution with the disulfide does not significantly influence the overall electronics of the system, in contrast to functional groups with charges, inductive effects, or extended aromatic systems. 34 Vlcěk and co-workers for example showed that a positively charged methylviologen ligand partially directs charge flow toward it.…”

“…Top: Energy diagrams of Kasha (left) and anti-Kasha behavior (right) due to slow internal conversion (IC) between higher- and lower-energy states (adapted from ref ). Bottom: Structures of complexes with sulfurated ligand S–S bpy 2,2 A , , B , and complex [1] PF 6 investigated herein.…”

“…Recently, we investigated the excited-state dynamics of disulfide-functionalized complexes [Ru­(bpy) 2 ( S–S bpy 2,2 )] 2+ ( A ; bpy = 2,2′-bipyridine) , and [Ir­(ppy) 2 ( S–S bpy 2,2 )] + ( B ; Hppy = 2-phenylpyridine) containing the sulfurated 2,2′-bipyridine derivative S–S bpy 2,2 (Figure , bottom left) . The electron-accepting disulfide moiety was able to fully direct charge flow in the excited state toward the S–S bpy 2,2 ligand, where a direct response via S–S bond elongation was revealed by computational simulations, indicating that the excited electron populates the antibonding σ* orbital of the S–S bond .…”

Iron(III) Carbene Complexes with Tunable Excited State Energies for Photoredox and Upconversion

“…Coordination complexes and organometallic compounds based on precious metals such as ruthenium, iridium, platinum, or gold remain among the most widely used compounds for applications in lighting, − sensing, , photocatalysis, − solar energy conversion, − dye-sensitized solar cells, artificial photosynthesis, − and phototherapy. − Further exploration of precious-metal-based compounds remains crucially important, due to their stability and sometimes remarkable performance, − but aspects such as greater abundance and cost-effectiveness drive a substantial portion of current research in the direction of first-row transition metals. This new focus also represents an opportunity for fundamentally new discoveries, as the photophysics and photochemistry of first-row transition metal complexes can be very different from those of the more widely investigated precious metals named above. , …”

Hydrogen sharing between two nitroxyl radicals in the gas phase and other microsolvation effects on the infrared spectrum of a bulky hydroxylamine

“…9 A natural choice for X is the persistent, sterically hindered TEMPO radical 10 (T) or some of its derivatives, 11 which have shown to be relevant in many fields of molecular science such as controlled radical polymerisation, 12 spin labeling, 13 and redox reactions. 14 T can accept an OH• • • O hydrogen bond from its associated hydroxylamine TEMPO-H (TH). This complex TH-T (Fig.…”