Spin-flip luminescence

Abstract

In molecular photochemistry, charge-transfer emission is well understood and widely exploited. In contrast, luminescent metal-centered transitions only came into focus in recent years. This gave rise to strongly phosphorescent CrIII complexes with a d3 electronic configuration featuring luminescent metal-centered excited states which are characterized by the flip of a single spin. These so-called spin-flip emitters possess unique properties and require different design strategies than traditional charge-transfer phosphors. In this review, we give a brief introduction to ligand field theory as a framework to understand this phenomenon and outline prerequisites for efficient spin-flip emission including ligand field strength, symmetry, intersystem crossing and common deactivation pathways using CrIII complexes as instructive examples. The recent progress and associated challenges of tuning the energies of emissive excited states and of emerging applications of the unique photophysical properties of spin-flip emitters are discussed. Finally, we summarize the current state-of-the-art and challenges of spin-flip emitters beyond CrIII with d2, d3, d4 and d8 electronic configuration, where we mainly cover pseudooctahedral molecular complexes of V, Mo, W, Mn, Re and Ni, and highlight possible future research opportunities. Graphical abstract

Supramolecular cis-“Bis(Chelation)” of [M(CN)6]3− (M = CrIII, FeIII, CoIII) by Phloroglucinol (H3PG)

“…The photoluminescence spectra of 1 and the (PPh 4 ) 3 [Cr(CN) 6 ]•2H 2 O reference at 77 K are presented in Figure 8, both revealing three distinguishable bands of the location and maximum lines specified in Table 3. The photoluminescence pattern observed in the range 750-875 nm is assigned to the 2 E g → 4 A 2g phosphorescence characteristic of various inorganic solids and hybrid molecular solids involving [Cr(CN) 6 ] 3− moiety [9,69,75,84,85] and may be interpreted in terms of its vibronic properties. The positions of these bands are usually indicated in respect to the R 1 (0 -0) emission line (here not measured, however, expected to be located at ca.…”

Molecular Rubies in Photoredox Catalysis

“…In contrast, Cr III complexes with tridentate ligands, in particular those inducing a very strong ligand field by large bite angles, are highly photostable, often strongly luminescent with very high excited state lifetimes, and are hence called “molecular rubies” ( Otto, et al, 2015 ; Otto, et al, 2018b ; Jiménez, et al, 2019 ; Treiling, et al, 2019 ; Reichenauer, et al, 2021 ). The long-lived excited states of the molecular rubies are metal-centered spin-flip states (doublet states, 2 E/ 2 T 1 ) in contrast to the typically exploited triplet MLCT excited states of Ru II or Ir III complexes ( Kitzmann, et al, 2022 ). Important prerequisites for the high lifetime and the photostability of molecular rubies are the large ligand field splitting resulting in dissociative metal-centered quartet states ( 4 T 2 ) being shifted to high energy and fast intersystem crossing (ISC) processes that rapidly depopulate the 4 T 2 states to arrive at the long-lived 2 E/ 2 T 1 states with high efficiency and without detrimental back-ISC processes ( Otto, et al, 2018a ; Kitzmann, et al, 2022 ).…”

“…The long-lived excited states of the molecular rubies are metal-centered spin-flip states (doublet states, 2 E/ 2 T 1 ) in contrast to the typically exploited triplet MLCT excited states of Ru II or Ir III complexes ( Kitzmann, et al, 2022 ). Important prerequisites for the high lifetime and the photostability of molecular rubies are the large ligand field splitting resulting in dissociative metal-centered quartet states ( 4 T 2 ) being shifted to high energy and fast intersystem crossing (ISC) processes that rapidly depopulate the 4 T 2 states to arrive at the long-lived 2 E/ 2 T 1 states with high efficiency and without detrimental back-ISC processes ( Otto, et al, 2018a ; Kitzmann, et al, 2022 ). Potential disadvantages of the molecular rubies for photoredox applications are their comparably small extinction coefficients in the visible spectral region due to the Laporte-forbidden nature of the 4 A 2 → 4 T 2 excitation and the comparably low excited state energies (1.6–1.75 eV) ( Otto, et al, 2018b ; Kitzmann, et al, 2022 ).…”

Efficient Triplet‐Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

“…Fundamentally different from long‐lived 3 MLCT (or 3 LMCT) excited states are intraconfigurational MC excited states which are characterized by a spin‐flip within the lower‐energy d orbitals (t 2g orbitals in octahedral symmetry) [13] . Thanks to the spin‐forbidden and often Laporte‐forbidden character of these spin‐flip transitions, the excited state lifetimes can even reach milliseconds [13a, 14] . Certain Cr III complexes are called “molecular rubies” owing to their excited state landscapes resembling that of the oxidic mineral ruby, and they possess extraordinarily long excited state lifetimes and record photoluminescence quantum yields [13a, 14] .…”

“…Energy transfer from the π‐π* states of appended anthracenyl substituents to Cr III leads to quenching of the anthracene fluorescence and sensitized phosphorescence from the metal‐centered doublet state [19] . The reverse process of sensitizing the triplet state of anthracenes or other organic dyes by electronically excited Cr III complexes (doublet‐triplet energy transfer, DTET), relevant for sTTA‐UC, has not been observed so far due to the lack of Cr III complexes with suitably high doublet state energies and long excited state lifetimes [13a, 14a–d, 15] . The recently reported second generation molecular ruby [Cr(bpmp) 2 ] 3+ has a comparably high doublet energy (1.75 eV) and a high excited spin‐flip state lifetime in the low ms range (e.g., 1550 μs in H 2 O/HClO 4 ) along with a comparably low‐energy absorption band at 462 nm (bpmp=2,6‐bis(2‐pyridylmethyl)pyridine) [14e] .…”