The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First‐Row Transition Metal Complexes

Abstract

The expansion of d-orbitals as a result of metal-ligand bond covalence, the so-called nephelauxetic effect, is a well-established concept of coordination chemistry, yet its importance for the design of new photoactive complexes based on first-row transition metals is only beginning to be recognized. Until recently, much focus has been on optimizing the ligand field strength, coordination geometries, and molecular rigidity, but now it becomes evident that the nephelauxetic effect can be a game changer regarding the photophysical properties of 3d metal complexes in solution at room temperature. In Cr III and Mn IV complexes with the d 3 valence electron configuration, the nephelauxetic effect was exploited to shift the well-known ruby-like red luminescence to the near-infrared spectral region. In Fe II and Co III complexes with the low-spin d 6 electron configuration, charge-transfer excited states were stabilized with respect to detrimental metal-centered excited states, to improve their properties and to enhance their application potential. In isoelectronic (3d 6 ) isocyanide complexes of Cr 0 and Mn I , the nephelauxetic effect is likely at play as well, enabling luminescence and other favorable photoreactivity. This minireview illustrates the broad applicability of the nephelauxetic effect in tailoring the photophysical and photochemical properties of new coordination compounds made from abundant first-row transition metals.

Design Strategies for Luminescent Titanocenes: Improving the Photoluminescence and Photostability of Arylethynyltitanocenes

“…Photocatalysis driven by charge-transfer (CT) excited states in transition-metal complexes is a central area of investigation in organic synthesis − and in the conversion of solar energy into electricity − (dye-sensitized solar cellsDSSCs) or fuels. − These catalysts are dominated by rare and expensive second- and third-row transition metals such as Ru and Ir . Thus, there has been significant effort over the past decade to replace these metals with earth-abundant metals. − Significant progress has been made using complexes of Cr, − Mo, , W, , Mn, Fe, − Co, , Ni, Cu, − and Zn. , One feature that complicates the use of many metals with d 1 through d 9 configurations is the presence of low-lying metal-centered (MC) or d–d excited states. , Such states are typically highly distorted and have very short lifetimes. Thus, thermal access of such states provides a rapid, nonradiative decay pathway, rendering the overall excited-state lifetime too short to undergo the type of collisional energy or electron transfer necessary for photocatalysis.…”

Host Dependency of Boundary between Strong and Weak Crystal Field Strength of Cr3+ Luminescence

“…Although first-principles calculations can reproduce the potential energy surfaces of Cr 3+ in the octahedral coordination environment and accordingly simulate the luminescence, such as the multipeaks observed in Cr doped La-gallogermanate compounds, Racah parameters together with the T-S diagram are favorable for fast estimation of Cr 3+ luminescence. Coulomb energies of different spectral terms are expressed by the Racah parameters B and C, which rely on the covalency between Cr 3+ and the p -orbitals on ligands. − The basic crystal-field theory gives a strong dependence of bond length on Dq, , which arises from the admixture of deep s -orbitals of ligands into e g orbitals of Cr 3+ . , …”

Luminescent Cyclometalated Chromium(III) Complexes

“…Replacing two out of the six pyridines in [Cr III L 6 ] n complexes by π-donating carbazolates or isoindolinates in the complexes [Cr(dpc) 2 ] + (dpc – = 3,6-di- tert -butyl-1,8-di(pyridine-2-yl)-carbazolato; λ em = 1067 nm; Φ < 0.00089% at 77 K) and [Cr(bpi) 2 ] + (bpi – = 1,3-bis(2′-pyridylimino)-isoindolinato; λ em = 930 nm; Φ = 0.11% at 298 K) shifts the emission energy above 900 nm (Scheme ). ,− However, the PL quantum yields are rather poor. ,− …”