Charge separated, MLCT excited states of ruthenium(II) coordination compounds have received intense interest for application in solar energy conversion and photoredox catalysis. Efforts to use the vastly more earth abundant iron congeners, however, have often suffered due to the rapid deactivation of the initially generated MLCT excited state to an energetically lower ligand field excited state. We performed ultrafast optical spectroscopy measurements on a series of sterically hindered low-spin Fe(II) polypyridyl compounds in an attempt to understand the role of nuclear motion on the ultrafast decay mechanism of this class of compounds. By monitoring the absorbance of the bipyridyl radical anion, we show that the MLCT lifetime of a sterically hindered FeCu<sub>2</sub>-Cage complex is 2.8 ps, a marked increase relative to the sub-200 fs lifetime of the Fe-Cage analogue in which the removal of the Cu(I) ions allows for greater structural flexibility to the chromophore. Furthermore, coherent oscillations assigned to vibrational coherences in excited electronic states are observed for each complex. The stark difference in dephasing times observed between the Fe-Cage and its rigid FeCu<sub>2</sub>-Cage analogue provide important insight into the nature of the nuclear motion that is coupled to the MLCT deactivation.

Steric control of MLCT deactivation in low spin Fe(II) polypyridyls

2019 Spring National Meeting

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