Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a Cr^III Polypyridine Complex and Their Use in Photoredox Catalysis

Abstract

The photoredox activity of well-known Ru II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr III complex is very similar to that for a Ru II polypyridine, and thereby validate the concept of estimating the redox potential of d 3 spin-flip excited states in analogous manner as for the MLCT states of d 6 compounds. Building on this insight, we use our Cr III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N -phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…The ILCT states of our Zn II complexes complement the different types of photoactive excited states reported recently for first-row and other Earth-abundant transition-metal complexes, 28 which includes the classical MLCT states for d 6 complexes (Cr 0 , Mn I , Fe II , Co III ), 19 , 24 , 25 , 115 − 117 square-planar d 8 compounds (Ni II ) 26 , 118 and four-coordinate d 10 complexes (Cu I ), 3 , 78 ligand-to-metal charge-transfer (LMCT) states for Ti IV , Zr IV , 119 , 120 Mn IV , Fe III and Co III , 121 − 126 121 − 126 metal-centered (MC) states for V III , Cr III and Co III , 127 − 132 as well as ligand-to-ligand charge-transfer (LLCT) excited states for two-coordinate Cu I complexes. 29 − 33 Given these findings, it seems reasonable to conclude that Zn II complexes with charge-transfer and triplet excited states would perhaps deserve greater attention in future studies aiming to discover new photophysics and photochemistry in first-row transition-metal complexes.…”

Photosensitized activation of diazonium derivatives for C–B bond formation

“…This plateau was reached at a driving force of around -DG ET = $1.2 eV, which is in line with or slightly larger than the $1 eV commonly observed in the literature for other systems. [41][42][43][44][45] Interestingly, the quenching rate constants also trended with Hammett parameters (Figure 3C). In all cases, a similar slope afforded reasonable fits of the data.…”

“…To date, there is no clear theory that allows prediction of the efficiency with which these oxidized and reduced species separate, yet it is of paramount importance for further improvement of light-driven applications. 19,41,[55][56][57][58] Cage-escape yields The literature of cage-escape yields (4 CE ) for inorganic PS is mostly centered on [Ru(bpy) 3 ] 2+ , which has often been considered as the prototypical inorganic PS. In a seminal paper, Olmsted and Meyer sought to understand the factors influencing 4 CE .…”

EuIII functionalized silica nanoparticles encapsulating chiral CrIII complexes with simultaneous unpolarized red and polarized NIR-I luminescence

“…39 Therefore, combining large g lum and high quantum yields on Cr(III) complexes is achievable and allow high CPL brightness (B CPL = e l  f l  g lum /2) which is key for the different applications employing chiral photoactive materials, where having good responses under ambient conditions is also advisable. 31 Based on the inertness, 40 inexpensive character of chromium, 41,42 and the interesting photophysical properties of Cr(III) complexes, 39,43,44 these systems have been recently used for molecular upconversion, [45][46][47][48][49] molecular thermometry, 50 pressure sensors, 51 photocalysis, [52][53][54][55][56] NIR-II luminescence, 57,58 and, remarkably, as CPL emitters. 48,59,60 Aiming at transferring the latter application to the nanoscale, herein we present amorphous silica nanoparticles which encapsulate photoactive chiral Cr(III) chromophores.…”