Unraveling the mechanism of NO ligand photoisomerism by time-resolved infrared spectroscopy

► Time-resolved infrared spectroscopy is used to study photo-induced NO linkage isomerisation. ► The vibronic coupling in the excited state allows for the ultrafast NO rotation of 90° within 300 fs. ► The vibrational-rotational relaxation leads then to the metastable side-on NO configuration. ► The latter state has a lifetime of about 300 ns at room temperature.