Side-group switching between metal-to-ligand charge-transfer and metal-centered excited state properties in iron(II) N-heterocyclic carbene complexes

Fe(II) N-heterocyclic carbene (NHC) complexes have emerged over the last decade as a promising class of light-harvesting complexes for a variety of photochemical applications relying on the presence of high-energy excited states of mainly charge-transfer character with excited state lifetimes of tens of picoseconds or longer. Recent spectroscopic investigations have significantly refined the understanding of some of the key prototype complexes of this kind and highlighted the subtle balance between population of triplet metal-to-ligand charge-transfer ((MLCT)-M-3) and triplet metal-centered ((MC)-M-3) states as a key issue to better understand and ultimately control the excited state dynamics in these complexes. To present a broader perspective on this issue, we here re-examine and discuss the excited state properties of a series of complexes with different side-groups on a common Fe NHC scaffold. Both the steady-state absorption spectrum and excited state dynamics are influenced by the side-group substitution, and the changes are rationalized based on shifting of the lowest metal-to-ligand charge-transfer (MLCT) state in energy based on the electron-withdrawing or electron-donating properties of the side-groups. Only electron-withdrawing substituents such as carboxylic acid groups ensured that the majority excited population stays in the (MLCT)-M-3 state for similar to 20 ps rather than rapidly converting into metal-centered (MC) states. In other complexes, the (MLCT)-M-3 state survived <300 fs after which the (MC)-M-3 state was populated for similar to 10 ps. The transient absorption results also show that the dynamics can be switched in a simple manner by deprotonating the carboxylic acid group, which renders some of the complexes pH-sensitive. For the here discussed complexes, the results from transient absorption measurements indicate that the (MLCT)-M-3 and (MC)-M-3 states were close enough in energy to enable the side-group to determine the photophysics. The emerging understanding of the (MLCT)-M-3-(MC)-M-3 balance, as well as the nature and properties of the (MC)-M-3 state in these complexes with intermediate ligand field strength is used to provide a broader fundamental perspective required to improve the ligand-design of Fe carbene complexes for issues such as to ensure a long-lived (MLCT)-M-3 state.