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

## TL;DR

The study uses advanced optical spectroscopy to track the full energy relaxation pathway of a light-sensitive iron complex, revealing key excited-state transformations.

## Contribution

This is the first time the complete deactivation trajectory of an iron(II) polypyridyl sensitizer has been traced using optical spectroscopy.

## Key findings

- The photoexcited Fe-amido chromophore evolves from a 1PALCT state through 3PALCT and 3MC to a 5MC state.
- Spin-parity transformations are identified via isosbestic points in the oTA spectral dynamics.
- Coherent oscillations observed are impulsive Raman-induced from the ground state.

## Abstract

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.

Combining high-resolution, wide-band oTA experiments with the unique absorptive properties of diarylamido ligand-metal complexes, we trace the complete deactivation trajectory of an iron(ii) polypyridyl sensitizer using optical spectroscopy.