Structure and spin of the low- and high-spin states of Fe2+(phen)3 studied by x-ray scattering and emission spectroscopy.

Abstract

The structure and spin of photoexcited Fe²⁺(phen)₃ in water are examined by x-ray scattering and x-ray emission spectroscopy with 100 ps time resolution. Excitation of the low-spin (LS) ground state (GS) to the charge transfer state ¹MLCT^* leads to the formation of a high-spin (HS) state that returns to the GS in 725 ps. Density functional theory (DFT) predicts a Fe-N bond elongation in HS by 0.19 Å in agreement with the scattering data. The angle between the ligands increases by 5.4° in HS, which allows the solvent to get 0.33 Å closer to Fe in spite of the expansion of the molecule. The rise in solvent temperature from the return of photoproducts to the GS is dominated by the formation dynamics of HS, ¹MLCT^* → HS, which is followed by a smaller rise from the HS → GS transition. The latter agrees with the 0.61 eV energy gap E(HS)-E(LS) calculated by DFT. However, the temperature rise from the ¹MLCT → HS transition is greater than expected, by a factor of 2.1, which is explained by the re-excitation of nascent HS^* by the 1.2 ps pump pulse. This hypothesis is supported by optical spectroscopy measurements showing that the 1.2 ps long pump pulse activates the HS^* → ⁵MLCT^* channel, which is followed by the ultrafast return to HS^* via intersystem crossing. Finally, the spins of the photoproducts are monitored by the K_β emission and the spectra confirm that the spins of LS and HS states are 0 and 2, respectively.