Ligand-controlled alteration of nuclear trajectories during photoinduced intersystem crossing in bis-meridional Fe(II) complexes

The intersystem crossing dynamics of two bis-meridional iron(II) complexes are studied by femtosecond transient M-edge X-ray near edge absorption spectroscopy (XANES) with a tabletop high-harmonic extreme ultraviolet (XUV) spectrometer. Visible-light photoexcitation of Fe(tpy)2(BF4)2 (where tpy = terpyridine) creates a metal-to-ligand charge transfer (MCLT) state that decays in 170 fs to a metal-centered triplet state (3MC), followed by 38 fs decay by intersystem crossing to a metal-centered quintet (5MC). Coherent oscillations on the 5MC surface are observed as a modulation in the XANES spectrum with a frequency of 103 cm-1 and a spectral shape that is characteristic of the symmetric Fe-N stretch. These dynamics and spectra are similar to those previously observed for Fe(phen)32+ (phen = phenanthroline). In contrast, transient spectroscopy of Fe[(4-CF3)2bpca]2 (bpca= bis(2-pyridylcarbonyl)amide) reveals a lower-frequency 42 cm-1 coherent oscillation. Ligand field multiplet calculations combined with ab initio ligand field theory identify this oscillation as a ligand bending mode, highlighting the ligand field sensitivity of M-edge XANES. The activation of different vibrational modes in Fe(tpy)2(BF4)2 and Fe[(4-CF3)2bpca]2 is explained by mapping their excited-state potential energy surfaces using density functional theory. In the latter complex, the nuclear trajectory follows initial expansion along the Fe-Naxial coordinate until reaching the 3MC/5MC seam. After intersystem crossing, the quintet state is significantly displaced along the coordinate corresponding to the ligand rocking mode, which therefore dominates the subsequent trajectory.