Water-Soluble Tris(cyclometalated) Iridium(III) Complexes for Aqueous Electron and Energy Transfer Photochemistry

Abstract

Conspectus Cyclometalated iridium­(III) complexes are frequently employed in organic light emitting diodes, and they are popular photocatalysts for solar energy conversion and synthetic organic chemistry. They luminesce from redox-active excited states that can have high triplet energies and long lifetimes, making them well suited for energy transfer and photoredox catalysis. Homoleptic tris­(cyclometalated) iridium­(III) complexes are typically very hydrophobic and do not dissolve well in polar solvents, somewhat limiting their application scope. We developed a family of water-soluble sulfonate-decorated variants with tailored redox potentials and excited-state energies to address several key challenges in aqueous photochemistry. First, we aimed at combining enzyme with photoredox catalysis to synthesize enantioenriched products in a cyclic reaction network. Since the employed biocatalyst operates best in aqueous solution, a water-soluble photocatalyst was needed. A new tris­(cyclometalated) iridium­(III) complex provided enough reducing power for the photochemical reduction of imines to racemic mixtures of amines and furthermore was compatible with monoamine oxidase (MAO-N-9), which deracemized this mixture through a kinetic resolution of the racemic amine via oxidation to the corresponding imine. This process led to the accumulation of the unreactive amine enantiomer over time. In subsequent studies, we discovered that the same iridium­(III) complex photoionizes under intense irradiation to give hydrated electrons as a result of consecutive two-photon excitation. With visible light as energy input, hydrated electrons become available in a catalytic fashion, thereby allowing the comparatively mild reduction of substrates that would typically only be reactive under harsher conditions. Finally, we became interested in photochemical upconversion in aqueous solution, for which it was desirable to obtain water-soluble iridium­(III) compounds with very high triplet excited-state energies. This goal was achieved through improved ligand design and ultimately enabled sensitized triplet–triplet annihilation upconversion unusually far into the ultraviolet spectral range. Studies of photoredox catalysis, energy transfer catalysis, and photochemical upconversion typically rely on the use of organic solvents. Water could potentially be an attractive alternative in many cases, but photocatalyst development lags somewhat behind for aqueous solution compared to organic solvent. The purpose of this Account is to provide an overview of the breadth of new research perspectives that emerged from the development of water-soluble fac-[Ir­(ppy)]3 complexes (ppy = 2-phenylpyridine) with sulfonated ligands. We hope to inspire the use of some of these or related coordination compounds in aqueous photochemistry and to stimulate further conceptual developments at the interfaces of coordination chemistry, photophysics, biocatalysis, and sustainable chemistry.

Cyclometalated Spirobifluorene Imidazolylidene Platinum(II) Complexes with Predominant 3LC Emissive Character and High Photoluminescence Quantum Yields

“…The high triplet energy of both complexes was further confirmed in an LED (370 nm)-driven irradiation reaction using 4a or 4b as the photocatalyst for the cis – trans isomerization of methyl cinnamate (Figure S31 and Table S2), which is known to proceed via TTET. ,, …”

“…The high triplet energy of both complexes was further confirmed in an LED (370 nm)-driven irradiation reaction using 4a or 4b as the photocatalyst for the cis−trans isomerization of methyl cinnamate (Figure S31 and Table S2), which is known to proceed via TTET. 78,81,82 In the photostationary state, the cis−trans ratio is close to 1:1; therefore, the E T of 4a and 4b seems to surpass that of both cinnamate isomers (i.e., amounts to >2.4 eV 77 ), resulting in similar rates for the cis-to-trans and the trans-to-cis conversion via energy transfer. 78,82,83 These initial NMRscale irradiation experiments in DCM gave turnover numbers for both photosensitizers in the order of 40.…”

“…78,81,82 In the photostationary state, the cis−trans ratio is close to 1:1; therefore, the E T of 4a and 4b seems to surpass that of both cinnamate isomers (i.e., amounts to >2.4 eV 77 ), resulting in similar rates for the cis-to-trans and the trans-to-cis conversion via energy transfer. 78,82,83 These initial NMRscale irradiation experiments in DCM gave turnover numbers for both photosensitizers in the order of 40. These results are quite promising considering the much lower photostability of the complexes in this solvent compared to THF (see above).…”

Unravelling the Effect of Water Addition in Consecutive Photocatalysis with Naphthalene Diimide

“…Photostimulated reactions using water as reaction media have also been explored in view of the benefit of using a cheap, environmentally friendly solvent. − An interesting noncatalytic example is a photodecarboxylative acylation of α-oxocarboxylic acids to afford 3-acylquinoxalin-2­(1 H )-ones via radicals using visible light; in this case, the authors propose an oxidation promoted by a direct excitation of an hypervalent I­(III) which could be done without any catalyst . Several photoredox catalysis reactions have been performed in water using water-soluble photocatalysts such as Ir or Ru complexes. , In these situations, the solubilities of the reactive compounds usually change the strategy, and for non-water-soluble substrates the use of micelle systems or another supramolecular arrangement is required. − …”

“…25 Several photoredox catalysis reactions have been performed in water using water-soluble photocatalysts such as Ir or Ru complexes. 27,28 In these situations, the solubilities of the reactive compounds usually change the strategy, and for nonwater-soluble substrates the use of micelle systems or another supramolecular arrangement is required. 29−32 Goez and Neumann claimed that a protic medium could influence photocatalyst and substrates properties like reduction potential (E°), being noteworthy the higher reductant power of some ruthenium polypyridyl complexes in aqueous medium toward pinacol coupling due to their more negative E°.…”