Resolving consecutive excited-state evolution in Fe-amido chromophores by wide-band optical transient absorption...

A detailed understanding of excited-state evolution is critical to realizing the full potential of abundant-metal coordination complex photosensitizers. Here, we show how wide-band optical transient absorption spectroscopy (oTA) can delineate the complete energy relaxation pathway of the photoexcited state of Fe(ii) polypyridyl complexes supported by benzannulated diarylamido ligands. By covering a broader spectral region from 370 to 1200 nm, we resolve consecutive evolution of a photoexcited Fe-amido chromophore from an initially generated singlet 'π-antibonding-to-ligand' charge transfer (¹PALCT) excited state to a long-lived metal-centred quintet (⁵MC) via both a ³PALCT and what we assign as a ³MC state. Notably, we identify spin-parity transformations by observing photogeneration of the ¹PALCT followed by its conversion into a ³PALCT state, and the subsequent ³MC-to-⁵MC transformation via observation of an isosbestic point in the oTA spectral dynamics. The state-to-state transformations are accompanied by coherent oscillations which are impulsive Raman-induced, originating in the ground state. Combining high-resolution, wide-band oTA experiments with the unique absorptive properties of diarylamido ligand-metal complexes, we are thus able, for the first time, to trace the complete deactivation trajectory of an iron(ii) polypyridyl sensitizer using optical spectroscopy.