Conversion of light energy to chemical or electrical energy requires control of excited-state reactivity such as photoinduced electron transfer. Transient absorption spectroscopy is a time-resolved technique that allows one to measure the excited state dynamics of molecules. In this technique, the sample is excited with a “pump” laser pulse, and the changes in absorption due to excitation are monitored with a “probe” pulse as a function of pump-probe time delay. In this manner, one can measure the lifetimes and spectral signatures of excited states.<br/>Here, I present transient absorption measurements of a bimetallic Fe(II)Co(III) complex with a metal-to-metal charge transfer (MMCT) absorption band in the visible region. By pumping this band with a green laser, we populate the Fe(III)Co(II) MMCT state, which then relaxes by undergoing ultrafast back electron transfer. However, this complex uniquely forms long-lived excited states following the back electron transfer. In order to assign the excited states, we have performed transient absorption spectroscopy with probe pulses in the visible, extreme ultraviolet (XUV), soft X-ray, and hard X-ray regions. XUV and X-ray spectroscopies are element-specific, allowing us to determine the oxidation and spin state at each metal independently. These transient absorption measurements indicate that the complex undergoes a sub-picosecond spin flip and back electron transfer to a local, Fe(II)-centered triplet state. Then, on a 30 ps timescale, this triplet state undergoes an additional intersystem crossing to a local, Fe(II)-centered quintet state with a 500 ps lifetime. Despite the complicated electronic structure of this bimetallic complex, the combination of multiple spectroscopic probes has allowed us to elucidate the flow of charge and spin in this molecule, which is of paramount importance to the development of advanced photocatalysic and photovoltaic technologies.

Monitoring electron transfer in a bimetallic molecule with ultrafast light pulses across the electromagnetic spectrum

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