High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

Abstract

Cyclometalated Ir(III) complexes are often chosen as catalysts for challenging photoredox and triplet−triplet-energy-transfer (TTET) catalyzed reactions, and they are of interest for upconversion into the ultraviolet spectral range. However, the triplet energies of commonly employed Ir(III) photosensitizers are typically limited to values around 2.5−2.75 eV. Here, we report on a new Ir(III) luminophore, with an unusually high triplet energy near 3.0 eV owing to the modification of a previously reported Ir(III) complex with isocyanoborato ligands. Compared to a nonborylated cyanido precursor complex, the introduction of B(C 6 F 5 ) 3 units in the second coordination sphere results in substantially improved photophysical properties, in particular a high luminescence quantum yield (0.87) and a long excited-state lifetime (13.0 μs), in addition to the high triplet energy. These favorable properties (including good long-term photostability) facilitate exceptionally challenging organic triplet photoreactions and (sensitized) triplet−triplet annihilation upconversion to a fluorescent singlet excited state beyond 4 eV, unusually deep in the ultraviolet region. The new Ir(III) complex photocatalyzes a sigmatropic shift and [2 + 2] cycloaddition reactions that are unattainable with common transition metalbased photosensitizers. In the presence of a sacrificial electron donor, it furthermore is applicable to demanding photoreductions, including dehalogenations, detosylations, and the degradation of a lignin model substrate. Our study demonstrates how rational ligand design of transition-metal complexes (including underexplored second coordination sphere effects) can be used to enhance their photophysical properties and thereby broaden their application potential in solar energy conversion and synthetic photochemistry.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

“…Metal-to-ligand charge-transfer (MLCT) excited states play a key role in many coordination complexes and organometallic compounds because they enable a range of different applications in photophysics and photochemistry. Precious and rare elements such as ruthenium(II), 1 − 500 osmium(II), 5 − 9 rhenium(I), 10 − 18 or iridium(III) 15 , 19 − 25 in polypyridine or cyclometalating coordination environments often feature a long-lived MLCT excited state, 26 , 27 whereas among first-row d 6 transition metal elements, this is yet a very rare occurrence. Iron(II) is by far most investigated in this regard, 28 − 44 yet only a handful of iron(II) complexes with MLCT lifetimes in the nanosecond time regime are known.…”

Energy transfer (EnT) photocatalysis enabled by gold-N-heterocyclic carbene (NHC) complexes

“… 26 Nevertheless, the triplet energy level ( E T ) of 64 kcal mol −1 has represented a plateau until very recent advances for iridium systems. 27 In addition to this limitation, the cationic nature of iridium complexes and the corresponding low solubility in many organic solvents often prevents the use of first choice sustainable solvents for catalytic reactions. 28 In the context of solvent compatibility, the exact amount required to enable catalysis is oftentimes overestimated.…”

Boronated Cyanometallates

“…44 Most recently, Wenger et al have boronated heteroleptic Ru(II) and Ir(III) diimine complexes and demonstrated their high activity for energy transfer and photoredox catalysis. 45,46 Studies of cyanometallates have been driven by the promise of applications in magnetic materials involving single-molecule/Prussian-Blue analogues and in electrode materials for solid-state batteries. 18,[47][48][49] Of direct relevance to our research is the performance of reversible cyanometallates as electrolytes in redox flow batteries.…”