Photo-Induced Spin-State Conversion in Solvated Transition Metal Complexes Probed via Time-Resolved Soft X-ray Spectroscopy

Abstract

Solution-phase photo-induced low-spin to high-spin conversion in the Fe II polypyridyl complex [Fe(tren(py) 3 )] 2+ (where tren(py) 3 is tris(2-pyridylmethylimino-ethyl)amine) has been studied via picosecond soft x-ray spectroscopy. Following 1 A 1 → 1 MLCT (metal-to-ligand charge transfer) excitation at 560 nm, changes in the iron L 2 and L 3 edges were observed concomitant with formation of the transient high-spin 5 T 2 state. Charge-transfer multiplet calculations coupled with data acquired on low-spin and high-spin model complexes revealed a reduction in ligand field splitting of ~1 eV in the high-spin state relative to the singlet ground state. A significant reduction in orbital overlap between the central Fe-3d and the ligand N-2p orbitals was directly observed, consistent with the expected ca. 0.2 Å increase in Fe-N bond length upon formation of the high-spin state. The overall occupancy of the Fe 3d-orbitals remains constant upon spin-crossover, suggesting that the reduction in σ-donation is compensated by significant attenuation of π-back-bonding in the metal-ligand interactions. These results demonstrate the feasibility and unique potential of time-resolved soft x-ray absorption spectroscopy to study ultrafast reactions in the liquid phase by directly probing the valence orbitals of first-row metals as well as lighter elements during the course of photochemical transformations.

Tracking reaction dynamics in solution by pump–probe X-ray absorption spectroscopy and X-ray liquidography (solution scattering)

“…The resemblance of the two high-spin spectra indicates that π-backbonding in the low-spin state of [Fe(tren(py) 3 )] 2+ is strongly inhibited due to orbital re-hybridization in the highspin state. This interpretation is also supported by the CTM calculation, 169 which supports the dilation of ligand cage as shown by the Fe K-edge measurement. 154 We note that the net change of the integrated absorption for all Fe-2p transitions, which corresponds to the integrated area under the TRXAS spectrum in Fig.…”

“…169 In the Fe L 2,3 -edges TRXAS spectrum measured at 90 ps shown in Fig. 3(A), (positive) transient absorption and (negative) ground state bleaching (i.e.…”

“…However, due to large absorption cross section of solvents in the soft X-ray regime, TRXAS in the transmission mode requires the sample in a liquid sheet of sub-micron thickness that remains stable in vacuum environment under typical photoexcitation conditions. 163,[169][170] For this purpose, based on a previously reported design, 188 we have developed liquid cells using silicon nitride membranes (100 nm thick) that can generate liquid sheets of less than 200 nm to 10 µm thickness. Ultimately, it is desirable to develop nanofluidic cells for flowing samples, and several promising approaches have been proposed with recent technological advances.…”

“…For comparison with the low-spin and high-spin spectra of [Fe(tren(py) 3 169 The high-spin spectrum of [Fe(tren(py) 3 )] 2+ reconstructed from the TRXAS spectrum is similar to that of the high-spin compound, [FeCl 6 ] 4-. In [FeCl 6 ] 4-, π-symmetric Cl-3p orbitals are fully occupied and therefore Cl ligands cannot serve as a π-acceptor.…”

Electronic structure of Fe- vs. Ru-based dye molecules

“…4 and 7-11. For three-dimensional complexes, the electronic structure [12][13][14][15][16][17] and charge transfer [18][19][20][21][22][23][24] have been investigated using many spectroscopic [25][26][27][28][29][30][31] and theoretical [32][33][34][35][36][37][38][39][40] methods.…”