  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.
