The high- and low-spin states of [FeII(bpy)3]2+

The molecule iron(II) tris-bipyridine, in aqueous solution, shows an ultrafast photocycle with a variety of par ticularly interesting features. As in oxygenated myoglobin and hemoglobin, the central Fe2+ ion in [FeII(bpy)3]2+ 6-fold coordinated, providing an approximately octahedral environment. Similar to the crystal field splitting experienced by octahedrally-coordinated metal ions in transition metal oxides (see Chapter V), the bipyridine ligand fields in [FeII(bpy)3]2+ cause a splitting of the five Fe 3d-orbitals into a three-fold t2g and a two-fold eg state (see Fig. II.13). This splitting results in an S=0 low-spin singlet groundstate and an S=2 high-spin quintet excited state. The high-spin state is accompanied by a lengthening Δr(Fe-N) of the Fe-N bonds, since the eg states are of anti-bonding character. Molecular complexes having low- and high-spin states with a small energy difference, and for which transitions between them can be induced by temperature or pressure, are called spin-crossover systems. The state energies and transition rates are sensitive to the nature and the bonding distance of the ligand groups. As discussed in Chapter V, a large ligand-field splitting allows Hund’s rule to be violated, stabilizing a low-spin state. In the biologically important hemoglobin, this ligand sensitivity leads to a cooperative effect in oxygen affinity: In the oxygenated state, the local environment of the iron ion is nearly planar, and a large ligand splitting stabilizes the S=0 low-spin state, reinforcing the oxygen binding. In a reducing environment, the iron-oxygen bond is broken, producing a “domed” geometry around the iron, with a small ligand splitting, for which the high-spin S=2 state is more stable. In this way, the oxygen affinity shows a non-linear on-off dependence on the local oxygen concentration. It has been found spectroscopically that photoexcitation of [FeII(bpy)3]2+, in which an iron 3d-electron is excited into the continuum of metal-to-ligand charge transfer states, leads to a shor t-lived high-spin state, via the photocycle shown in Figure II.14. By probing the photoexcited system with 70 ps synchrotron X-ray pulses, the geometric structure of the high-spin quintet state could be measured [11], revealing an increase in the Fe-N bond distance of 0.2 Å. The relaxation back to the singlet ground-state was measured to be 660 ps, in agreement with laser spectroscopic results [12]. By careful selection of a feature in the XANES region of the energy spectrum sensitive to the Fe-N bond distance, the arrival of the molecule in the high-spin state from the initially excited MLCT states, within 150 fs, was measured for the first time [13] using the femtosecond hard X-ray slicing beamline at the Swiss Light Source [14]. Of par ticular interest are the unusually fast singlet→quintet and quintet→singlet intersystem crossings – much faster than the conventional wisdom shown in Figure II.1 – which avoid the 1T and 3T states. These unexpectedly high conversion rates are indicative of relaxation in the strongly non-Born-Oppenheimer regime.

The laser-sliced synchrotron beamline produces approximately 200 X-ray photons per pulse, at 1% bandwith and a 2 kHz repetition rate, resulting in long measurement times at only a few selected settings of photon energy and pump-probe delay. With of the order of 1011 photons per pulse at 100–400 Hz, the SwissFEL will bring a vast improvement in flux to such experiments.