Ultrafast competition between CO release and spin crossover upon photoexcitation of a cyclopropenone-bipyridyl Fe^II complex

The photo-triggered release of carbon monoxide (CO) from a cyclopropenone-bipyridyl ligand and its iron(II) complex was investigated using a combination of stationary and ultrafast time-resolved spectroscopies. Upon continuous irradiation, the complex undergoes CO release and forms the corresponding acetylene photoproduct, albeit at <i>a</i> ∼ 3 times lower rate than the free cyclopropenone ligand, indicating a drastic reduction in quantum yield. We attribute this to the ultrafast dynamics revealed by femtosecond time-resolved electronic absorption spectroscopy, where photoexcitation of ligand-centered states of the complex is predominantely followed by energy-transfer, leading to spin-crossover of the Fe^II ion rather than CO release. This ultrafast response stands out as highly remarkable as it virtually outcompetes the photo-decarbonylation, which in the free ligand occurs in only ∼130 fs.