Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a long-lived quintet state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)3 2+ and conclusively identify a 3T intermediate that forms in 170 fs and decays to a vibrationally hot 5T2g state in 39 fs. A coherent vibrational wavepacket with a period of 249 fs and damping time of 0.63 ps is observed on the 5T2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe–N symmetric stretch. The results show that the shape of the M2,3-edge X-ray absorption near-edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high-spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.
Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(CN), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe–N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid–base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.
Cobalt complexes that undergo charge-transfer induced spin-transitions or valence tautomerism from low spin CoIII to high spin (HS) CoII are potential candidates for magneto-optical switches. We use M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy with 40 fs time resolution to measure the excited-state dynamics of CoIII(Cat-N-SQ)(Cat-N-BQ), where Cat-N-BQ and Cat-N-SQ are the singly and doubly reduced forms of the 2-(2-hydroxy-3,5-di-tert-butylphenyl-imino)-4,6-di-tert-butylcyclohexa-3,5-dienone ligand. The extreme ultraviolet probe pulses, produced using a tabletop high-harmonic generation light source, measure 3p → 3d transitions and are sensitive to the spin and oxidation state of the Co center. Photoexcitation at 525 nm produces a low-spin CoII ligand-to-metal charge transfer state which undergoes intersystem crossing to high-spin CoII in 67 fs. Vibrational cooling from this hot HS CoII state competes on the hundreds-of-fs time scale with back-intersystem crossing to the ground state, with 60% of the population trapped in a cold HS CoII state for 24 ps. Ligand field multiplet simulations accurately reproduce the ground-state spectra and support the excited-state assignments. This work demonstrates the ability of M2,3-edge XANES to measure ultrafast photophysics of molecular Co complexes.
In this work, we perform femtosecond M 2,3 -edge XANES on low-spin iron complexes using a high harmonic generation (HHG) light source. Low-spin iron complexes hold promise as affordable alternatives to ruthenium-based photosensitizers. Conventional iron complexes with polypyridyl ligands are unsuitable because the excitation energy is dissipated in 200 fs by ultrafast population of the low-lying quintet metastable state. This formally ∆S=2 process is conjectured to proceed through a metal-centered triplet intermediate state, but spectroscopic evidence of such an intermediate has remained elusive. Here we use femtosecond M 2,3 -edge XANES to elucidate the role of metal-centered intermediate states. The transient response of Fe(phen) 3 (SCN) 2 following excitation in the MLCT band clearly shows the presence of a metal-centered 3 T 1 state. The transient response also shows oscillations consistent with the evolution of a vibrational wave-packet on the quintet surface. We also recorded the M 2,3 -edge transient response of Fe[(CF 3 ) 2 bpca] 2 , a complex with a weaker ligand field. A comparison of the photophysical behaviors of Fe(phen) 3 (SCN) 2 and Fe[(CF 3 ) 2 bpca] 2 sheds light on the influence of crystal field strength on the relaxation mechanism of the MLCT state. Our findings add a hitherto missing piece to the existing picture of photophysics of iron complexes.
<p>Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a <sup>5</sup>T<sub>2g</sub> state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)<sub>3</sub><sup>2+</sup> and conclusively identify a <sup>3</sup>T intermediate that forms in 170 fs and decays to a vibrationally hot <sup>5</sup>T<sub>2g</sub> state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the <sup>5</sup>T<sub>2g</sub> surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M<sub>2,3</sub>-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.<br></p>
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