Fe^II Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-to-Ligand Charge-Transfer Excited-State Lifetime

Abstract

The iron carbene complex [Fe(btz)](PF) (where btz = 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-to-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a MLCT state with a 528 ps excited-state lifetime in CHCN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported Fe complex. The low potential of the [Fe(btz)]/[Fe(btz)] redox couple makes the MLCT state of [Fe(btz)] a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [Fe(btz)] form of this complex, these results show that the Fe and Fe oxidation states of the same Fe(btz) complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.

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.…”

Is Iron the New Ruthenium?

“…Key strategies and concepts for obtaining Fe II complexes with long-lived MLCT states, along with exemplary molecular structures: a) [Fe(dcpp) 2 ] 2 + (dcpp = 2,6-bis(2-carboxypyridyl)pyridine; [11] [14] R = tBu, [15] and R = iPr; [16] e) bis(terdentate) complexes with cyclometalating 2,2':6',2''-terpyridine(tpy)-derivedligands; [17] f) [Fe(btz) 3 ] 2 + (btz = 4,4'-bis(1,2,3-triazol-5-ylidene);Ar= p-C 6 H 4 -Me). [18] Figure 3. Ligand field effects depending on the type of bonding interactions between metal do rbitalsand ligand groupo rbitals(LGOs) or atomic orbitals (AOs).…”

Blue‐Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons

“…Based on the O 2 -sensitive nature of the emission, its origin in 1 is attributed to atriplet 3 LMCT state.The bi-exponential decay profile in 2 suggests that the emission of 2 arises from adifferent excited state than that of 1.I nc omparison to an isoelectronic Fe II -NHC (NHC = N-heterocyclic carbene) complex (E T1 % 1240 nm, t PL = 528 ps) the t PL values of 1 and 2 are found to be about 950 times higher, owing presumably in part to the very low energy of the triplet state in the iron complex (Supporting Information, Table S11). [20] In contrast, 3 exhibits no emission. Thus,t he combination of 6-membered chelates and electron-rich ligands is key to turn on the emission in Co III complexes by separating the 3 LMCT and/or 3 LC states from the non-radiative 3 MC state.…”