Photo-induced primary processes of <i>trans</i>-[Co(acac)₂(N₃)(py)] in liquid solution studied by femtosecond vibrational and electronic spectroscopies

The elementary light-triggered dynamics of the transition metal complex, trans-[CoIII(acac)2(N3)(py)] (trans-[1]), was investigated using femtosecond mid-infrared (mIR) and UV-Vis spectroscopies following ultrafast acetylacetonate-to-cobalt charge-transfer excitation at 266 nm. The spectro-temporal response is indicative of an ultrafast return to the electronic ground state via a sequential intramolecular electron transfer. The locally excited ligand-to-metal charge transfer state, 1LMCTacac, decays within 200 fs via azide-to-acac electron transfer to another charge-transfer state, 1LMCTazide, that features an azidyl radical ligand attached to a cobalt(II) center. A subsequent cobalt-to azidyl electron transfer occurring within 1.8 ps generates an excited metal centered state, 1MC, thereby re-establishing the fully closed-shell character of the anionic azido and acac ligands. The fully relaxed electronic ground state of trans-[1] is then recovered via internal conversion and vibrational energy relaxation on a 10 ps-time scale.