Impact of the fac/mer Isomerism on the Excited-State Dynamics of Pyridyl-carbene Fe(II) Complexes

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.

Hot Branching Dynamics in a Light‐Harvesting Iron Carbene Complex Revealed by Ultrafast X‐ray Emission Spectroscopy

“…As already mentioned above, the two ultrafast processes 1 MLCT to 3 MLCT and partial decay to the 3 MC state cannot clearly be distinguished as two separate processes because they both are similar or faster than the measurement response function. In principle, the ultrafast formation of part of the 3 MC population could occur from the 1 MLCT state, but we choose the relaxation scheme summarized in the conclusions, based on results from optical TA on similar molecules . A comprehensive discussion of the other, less successful, tested models as well as the used rate equations is given in the Supporting Information.…”

“…This is consistent with the generally fast cooling of charge transfer states.Optical TA measurements on similar FeNHC complexes suggesting ! 100 fs 1 MLCT-3 MLCT ISC [3,32,39,43,45,46] would support this scenario.T ime constants and associated amplitudes of the processes are summarized in the conclusions.…”

“…[37,38] Then ew results obtained here in particular show that the conventional picture of ac onsecutive or parallel progression through an umber of excited states has to be revised to abranched decay involving hot (vibrationally unrelaxed) and cold (vibrationally relaxed) de-excitation pathways through CT and MC states as has also been recognized by other groups. [39] This significantly enriches our understanding of the factors limiting the photochemical efficiencyo ft his type of complexes.…”

“…In principle,t he ultrafast formation of part of the 3 MC population could occur from the 1 MLCT state,b ut we choose the relaxation scheme summarized in the conclusions,b ased on results from optical TA on similar molecules. [3,32,39,43,45,46] Acomprehensive discussion of the other, less successful, tested models as well as the used rate equations is given in the Supporting Information.…”

“…Ther eference spectra now show that the decrease/increase in the signal at 7057.6/7060.5 eV is aresult of increasing spin density on the iron atom with ar ise time of about 150 fs,i ndicating the presence of an ultrafast intersystem crossing, in agreement with many previous studies on iron complexes. [3,32,39,43,45,46] At later times,w eo bserve ar ecovery of the excitation-induced spin momentum, but it does not fully recover to the groundstate level within the 1nse xperimental window because of the long-lived oxidized/ionized state formed through excitedstate absorption.…”