Impact of the fac/mer isomerism on the excited states dynamics of pyridyl-carbene Fe(II) complexes

TL;DR

This study investigates how fac/mer isomerism influences the excited state dynamics of pyridyl-carbene Fe(II) complexes, revealing isomer-dependent lifetimes and proposing a mechanism involving spin-crossover regions, advancing the design of photoactive iron complexes.

Contribution

It provides new insights into the role of fac/mer isomerism in excited state decay of iron complexes with pyridyl-NHC ligands, combining experimental and computational approaches.

Findings

Distinct excited state populations with different lifetimes depending on isomer

Longer excited state lifetime than similar complexes with four bonds

Proposed mechanism involving differential trapping in spin-crossover regions

Abstract

The control of photophysical properties of iron complexes and especially of their excited states decay is a great challenge in the search for sustainable alternatives to noble metals in photochemical applications. Herein we report the synthesis and investigations of the photophysics of mer and fac iron complexes bearing bidentate pyridyl-NHC ligands, coordinating the Fe with three ligand-field enhancing carbene bonds. Ultrafast transient absorption spectroscopy reveals two distinct excited state populations for both mer and fac forms, ascribed to the populations of the T1 and the T2 states, respectively, which decay to the ground state via parallel pathways. We find 3-4 ps and 15-20 ps excited state lifetimes, with respective amplitudes depending on the isomer. The longer lifetime exceeds the one reported for iron complexes with tridentate ligands analogues involving four iron-carbene bonds. By combining experimental and computational results, a mechanism based on the differential trapping of the triplet states in spin-crossover regions is proposed for the first time to explain the impact of the fac/mer isomerism on the overall excited-state lifetimes. Our results clearly highlight the impact of bidentate Pyridyl-NHC ligands on the photophysics of iron complexes, especially the paramount role of fac/mer isomerism in modulating the overall decay process, which can be potentially exploited in the design of new Fe(II)-based photoactive compound.