Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

Abstract

The possibility of achieving luminescent and photophysically active metal-organic compounds relies on the stabilization of charge transfer states and kinetically and thermodynamically blocking non-radiative dissipative channels. In this contribution we explore the behavior of bidentate iron complexes bearing N-heterocyclic carbene ligands with extended conjugation systems by a multidisciplinary approach combining chemical synthesis, ultrafast time-resolved spectroscopy, and molecular modeling. Lifetimes of the metal-to-ligand charge transfer and metal-centered states reaching up to ≈20 picoseconds are evidenced, while complex decay mechanisms are pointed out, together with a possible influence of the facial and meridional isomerism. The structural degrees of freedom driving the non-radiative processes are highlighted and their rigidification is suggested as an effective way to further increase the lifetimes.

Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties

“…The positive DADS of τ 1 in the 380–450 nm range can be related to a reshaping of ESA, i.e., vibrational relaxation of 3 MLCT. On the other hand, the DADS of τ 2 and τ 3 are very similar to the situation we recently reported for complexes with bidentate ligands [ 14 ], where two excited states seem to co-exist, and is rather at odds with the single excited state scenario we reported for tridentate Fe–NHC complexes [ 12 ]. Indeed, for the latter τ 3 was dominant in intensity and τ 2 with minor amplitude rather related to structural relaxation in the single excited state.…”

“…The most relevant electronic states involved in the photoresponses of C1 and C2 are displayed in Figure 8 . We have adopted the computational methodology used in our previous works on Fe–NHC complexes with bidentate [ 11 , 13 , 14 ] and tridentate ligands [ 12 ] in order to obtain fully comparable results.…”

“…On longer time scales, the triplet state must evolve to reach non-adiabatic crossing points with the ground state that mediate non-radiative decay channels. These mechanisms are intrinsically different between Fe–NHC complexes with bidentate [ 11 , 13 , 14 ] and tridentate [ 12 ] ligands and have different implications for the longer ESA components τ 2 and τ 3 . In the former type of complexes, relaxation of the lowest-lying triplet state T 1 leads to a 3 MC region through a barrierless path encompassing an extended area of spin-crossover character, i.e., an area where S 0 lies above T 1 , in which the system can be trapped for several picoseconds [ 13 ].…”

“…N -heterocyclic carbenes (NHC) ligands, ideally combining a strong σ-donating and a weak-to-moderate π-accepting character, have been reported to destabilize the MC states over the MLCT manifold, thus impressively slowing down the deactivation pathways. In this context, tridentate pyridyldicarbene ligands with appropriate modifications of both the NHC side and the central azine have been reported by us and other groups as leading to remarkable 3 MLCT lifetimes in the 9–32 ps range for the corresponding iron (II) complexes [ 4 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Other ligands giving rise to more rigid complexes [ 15 ] or different coordination architectures [ 16 ] have achieved impressive MLCT lifetimes beyond hundreds of picoseconds.…”

“…However, in contrast to tridentate ligands, the dissymmetrical bidentate ligands generally form mixtures of fac and mer isomers, which can be problematic when a vectorial electron transfer is required [ 26 ]. In addition, it has also been shown that the fac and mer isomerism may lead to unexpected photophysical consequences [ 11 , 13 , 14 ]. Thus, the possibility of achieving tridentate iron complexes with reduced angular strain still appears extremely attractive for the design of photoelectrochemical active agents.…”