Strong bis-cyclometalated iridium photoreductants, in combination with a single sacrificial reductant, enable visible-light-promoted reductive activation of a variety of challenging substrates under simple and general reaction conditions.

A series of potent bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ancillary ligands is described. Structure−property analysis reveals that substituent modification of the NacNac ligands has a large effect on the ground-state Ir IV /Ir III potential, which shifts cathodically as the NacNac is made more electron-rich. In addition, the excited-state Ir IV /*Ir III potentials are ca. 300−500 mV more negative than that of fac-Ir(ppy) 3 (ppy = 2phenylpyridine), indicating that these compounds have much more reducing excited states. Rate constants for excited-state electron transfer between these photosensitizers and benzophenone are ∼2−3 times faster than fac-Ir(ppy) 3 , demonstrating that these complexes are both kinetically and thermodynamically more potent for excited-state electron transfer. We use these photosensitizers to optimize a simple reaction procedure for photocatalytic debromination of aryl bromide substrates, which requires only the photosensitizer, blue light, and an amine base, without silanes or other additives that are used in previously reported methods.

Bis-cyclometalated iridium complexes with electron-rich ancillary ligands exhibit high phosphorescence quantum yields in the deep red region.

The molecular ruby [Cr(tpe)2]3+ and the tris(bipyridine) chromium(III) complex [Cr(dmcbpy)3]3+ as well as the tris(bipyrazine)ruthenium(II) complex [Ru(bpz)3]2+ were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4′-dimethoxycarbonyl-2,2′-bipyridine, bpz = 2,2′-bipyrazine), while [Cr(ddpd)2]3+ serves as a control system (ddpd = N,N′-dimethyl-N,N′-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the CrIII driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst [Cr(tpe)2]3+ outcompetes [Cr(dmcbpy)3]3+ and even [Ru(bpz)3]2+ in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of [Cr(tpe)2]3+ in this photoredox catalytic system.

Incorporation of sulfur dioxide into organic compounds is achieved by a photocatalytic approach using sensitizers made from earth-abundant chromium(III) ions and visible light leading to sulfones and sulfonamides. We employed three different chromium(III) sensitizers [Cr(ddpd) 2 ] 3 + , [Cr(bpmp) 2 ] 3 + and [Cr-(tpe) 2 ] 3 + with long excited state lifetimes and different ground and excited state redox potentials as well as varying stability under the reaction conditions (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-yl-pyridine-2,6-diamine; bpmp = 2,6-bis(2-pyridylmethyl)pyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). Key reaction steps of the catalytic cycles are identified by electrochemical, luminescence quenching, photolysis, laser flash photolysis and catalytic experiments delivering a detailed picture of the challenges in these transformations. The reactivity of the reduced chromium complex was identified as a key property to explain the reaction outcomes. Initial cage escape yield determinations with [Cr(tpe) 2 ] 3 + revealed that desired photoreactions occur with unusually high quantum efficiencies, whereas side reactions are almost unproductive.

In this work, we describe bis-cyclometalated iridium complexes with efficient deep-red luminescence. Two different cyclometalating (C^N) ligands‒1-phenylisoquinoline (piq) and 2-(2- pyridyl)benzothiophene (btp)‒are used with six strong π-donating ancillary ligands (L^X) to furnish a suite of 10 new complexes with the general formula Ir(C^N)2(L^X). Improvements in deep-red photoluminescence quantum yields were accomplished by the incorporation of sterically encumbering substituents onto the ancillary ligand, which can enhance the radiative rate constant (kr) and/or reduce the non-radiative rate constant (knr). Five of the complexes were characterized by X-ray crystallography, and all of them were investigated by in-depth spectroscopic and electrochemical measurements.<br>

In this work, we describe bis-cyclometalated iridium complexes with efficient deep-red luminescence. Two different cyclometalating (C^N) ligands‒1-phenylisoquinoline (piq) and 2-(2- pyridyl)benzothiophene (btp)‒are used with six strong π-donating ancillary ligands (L^X) to furnish a suite of 10 new complexes with the general formula Ir(C^N)2(L^X). Improvements in deep-red photoluminescence quantum yields were accomplished by the incorporation of sterically encumbering substituents onto the ancillary ligand, which can enhance the radiative rate constant (kr) and/or reduce the non-radiative rate constant (knr). Five of the complexes were characterized by X-ray crystallography, and all of them were investigated by in-depth spectroscopic and electrochemical measurements.<br>