A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence

Abstract

Transition-metal complexes are used as photosensitizers, in light-emitting diodes, for biosensing and in photocatalysis. A key feature in these applications is excitation from the ground state to a charge-transfer state; the long charge-transfer-state lifetimes typical for complexes of ruthenium and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron and copper being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs, it remains a formidable scientific challenge to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers. Here we present the iron complex [Fe(btz)] (where btz is 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (LMCT) state that is rarely seen for transition-metal complexes. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

“…Similar ultrafast transitions into 3 MC states have been observed and described for ruthenium sensitizer complexes . By destabilizing the MC scavenger states we, and other groups, have recently developed iron complexes with significantly slower deactivation of the MLCT states …”

“…[11][12][13] By destabilizing the MC scavenger states we,a nd other groups,h ave recently developed iron complexes with significantly slower deactivation of the MLCT states. [14][15][16][17][18][19][20][21] This makes the iron carbene complexes interesting as ap romising new class of photosensitizers, [22] with recently demonstrated capability to inject electrons efficiently into aT iO 2 substrate,a nd carry out bimolecular oxidation and reduction processes. [15,17] Ultrafast optical measurements in combination with quantum-chemical calculations have provided significant insight into the excited state structure underlying this remarkable improvement of photochemical properties.…”

Photoredox Catalysis with Metal Complexes Made from Earth‐Abundant Elements

“…In the course of studies of Cu I amido complexes, [48] the Peters group found that complexes such as [Cu(carbazolide)(PPh 3 ) 2 ]a re photoluminescent, and this sparked interesti n exploitingt he favorable excited-state properties of this class of complexes for synthetic applications.I nacombined effort between the groups of Peters and Fu, ac onceptually novel,s yntheticallyh ighly useful and broadly applicable approacht o photoredox catalysis emerged. In the initial landmarks tudy, [47] it was demonstrated that the [Cu(carbazolide)(PR 3 ) 2 ]c omplex (R = meta-tolyl) (84)( Scheme 21) reacts with iodobenzene and bromobenzene (85)u nder UV photoirradiation at À40 8Ct oa n Ullmann coupling product (86). Experiments with radical probess ignaled the intermediacy of photogenerated radicals, and the reactivity patterns observed with various halogenated arenes were compatible with CÀXc leavage through an SET process.…”

Photoluminescence of Seven‐Coordinate Zirconium and Hafnium Complexes with 2,2′‐Pyridylpyrrolide Ligands

“…[17] Much attention in this area has been devoted to 1str ow late transition metals [18][19][20][21][22][23][24][25][26][27] and low-valent groups 6a nd 7m etals [28][29][30][31][32][33] that retain the fundamental design principle of photoinduced charge separation via MLCT. [34] In pronouncedc ontrast, luminescent metalc omplexes relying on alternative mechanisms such as ligand-tometal charget ransfer (LMCT), [35][36][37][38][39][40][41][42][43] ligand-to-ligand charge transfer (LLCT), [44,45] or metal-centered excitations( i.e. d-d, f-f, or d-f) [46][47][48][49][50][51][52] are not as well-studied, but offer the potentialt o expand the palette of metals suitable for photosensitizer development.…”