Molecular Rubies in Photoredox Catalysis

Abstract

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.

Charge‐Transfer und Spin‐Flip‐Zustände als sich ergänzende Gegensätze

“…Auswahl von Anwendungen für Komplexe mit angeregten CT-und SF-Zuständen (siehe Haupttext für Details). [69,164,[235][236][237][238][239] Deaktivierung begrenzen, wie für [Ru(bpy) 2 (py) 2 ] 2 + mit flexiblen Pyridinliganden (py) gezeigt wurde. [186,187] Hydrostatischer Druck kann ebenfalls die Energie der SF-Emission beeinflussen.…”

“…Cr III -Komplexe können diesen Nachteil teilweise durch ihre sehr langen Lebensdauern der angeregten Zustände von μs bis ms ausgleichen. [44,69,223] Durch geschicktes Ligandendesign wurden CT-und SF-Emitter erfolgreich für eine Vielzahl von Sensoranwendungen eingesetzt (Abbildung 8): Die Einführung von sauren bzw. basischen Gruppen an den Liganden ermöglichte die Messung des pH-Werts, [63,67,[247][248][249][250] Quantifizierung von Sauerstoff wurde durch Ausnutzung der Löschung der Phosphoreszenz durch 3 O 2 unter Bildung von 1 O 2 erreicht, [179,251] und eine Vielzahl von Mechanismen wie TADF, thermische Äquilibrierung energetisch ähnlicher SF-Zustände oder thermisch aktivierter strahlungsloser Zerfall kann Temperaturmessung ermöglichen (siehe Abschnitte 3.2 und 4.1).…”

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.…”

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

“…1,11 With respect to photooxidizing properties, many strongly photooxidizing complexes operate as inner-sphere oxidants in hydrogen atom transfer and M-X bond homolysis reactions, such as high valent oxido and halido complexes of manganese(IV), tungsten(VI), cerium(IV), uranium(VI) and copper(II), [12][13][14][15][16][17][18][19] while strong genuine single-electron outer-sphere photooxidants, which fully retain their coordination sphere, are rare. Recent advances in the field of earth-abundant photocatalysts operating as strong single-electron oxidants include zirconium(IV), 20,21 cobalt(III), 22 iron(III) [23][24][25][26][27] and chromium(III) complexes [28][29][30][31] in their respective excited states, which are of ligand-to-metal charge transfer character (LMCT) 32 for the former and of spin-flip character for chromium(III) complexes. 10,33 Photosensitizers possessing excited states with potentials more positive than +0.80 V vs. SCE (+0.42 V vs. ferrocene) 34 are considered as photosensitizers with extreme redox potentials.…”

“…35,36 The hexacarbene iron(III) complex [Fe(phtmeimb)2] + and various polypyridine chromium(III) complexes realize potentials of Ered* = 1.25 -1.84 V vs. SCE (0.87-1.46 V vs. ferrocene) (phtmeimb = [phenyl(tris(3-methylimidazolin-2ylidene))borate] − ) with blue-green light excitation. [23][24][25][26][27][28][29][30][31] The formally strongest reported photooxidant based on a first row transition metal is [Co(dgpz)2] 3+ with Ered* = 2.75 V vs. SCE (2.37 V vs. ferrocene), yet its strong oxidizing power towards challenging substrates has not yet been exploited (dgpz = 2,6diguanidylpyrazine). 22 Strong organic photooxidants encompass substituted acridinium salts, 37 2,3dichloro-5,6-dicyano-1,4-benzoquinone 38,39 and the electrochemically generated trisaminocyclopropenium radical dication with Ered* = 2.06-3.33 V vs. SCE (1.68-2.95 V vs. ferrocene).…”