Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar‐Energy Conversion: Current Status and Open Questions

Abstract

One major challenge of future sustainable photochemistry is to replace precious and rare transition metals in applications such as energy conversion or electroluminescence by earth‐abundant, cheap, and recyclable materials. This involves using coordination complexes of first row transition metals such as Cu, Cr, or Mn. In the case of iron, which is attractive due to its natural abundance, fundamental limitations imposed by the small ligand field splitting energy have recently been overcome. In this review article, we briefly summarize the present knowledge and understanding of the structure‐property relationships of Fe(II) and Fe(III) complexes with excited state lifetimes in the nanosecond range. However, our main focus is to examine to which extent the ultrafast spectroscopy methods used so far provided insight into the excited state structure and the photo‐induced dynamics of these complexes. Driven by the main question of how to spectroscopically, i. e. in energy and concentration, differentiate the population of ligand‐ vs. metal‐centered states, the hitherto less exploited ultrafast vibrational spectroscopy is suggested to provide valuable complementary insights.

Towards panchromatic Fe(ii) NHC sensitizersviaHOMO inversion

“…The physical picture of the photodynamics of these molecules upon MLCT optical excitation that emerges from these results resembles previous results from us 19,20,[41][42][43] and others 18,44 on similar tridentate complexes with 4 NHC ligands. Namely, the initially created 1 MLCT state is converted through an ultrafast (<100 fs) spin-crossover to a hot non-emissive (no noticeable stimulated emission observed) 3 MLCT state which then undergoes a sub-ps vibrational relaxation.…”

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes

“…However, for the 3d 6 and 3d 8 valence electron configurations, the presence of low-lying distorted MC states represents a major challenge . In second- and third-row transition metal complexes with the d 6 and d 8 configurations, the respective MC states are often at higher energies than emissive MLCT or other charge-transfer (CT) states, , making nonradiative relaxation less prevalent and leading to favorable photophysical and photochemical behavior of complexes made from Ru II , Ir III , − Pt II , − or Au III . , Among 3d metals, Fe II has received the most attention concerning the relaxation of MLCT into MC states, − complemented recently by studies of isoelectronic Co III , Mn I , and Cr 0 compounds. − Thus, there is now a substantial body of literature on the photophysics of 3d 6 compounds, whereas 3d 8 complexes have remained underexplored in comparison …”

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…Tetrahedral copper­(I) complexes with luminescent and redox-active metal-to-ligand charge-transfer (MLCT) excited states have long been known , and nowadays represent a well-developed compound class with applications in light-emitting devices, − dye-sensitized solar cells, and photocatalysis. − In the 3d 10 valence electron configuration of Cu I , there are no metal-centered (MC) excited states that can depopulate the photoactive 3 MLCT states, , unlike for analogous 3d 6 (Co III , Fe II , Mn I , Cr 0 ) − or 3d 8 (Ni II ) compounds, , in which nonradiative MLCT deactivation by MC states can be undesirably fast . Consequently, complexes with the 3d 10 configuration are predisposed for obtaining long-lived and strongly emissive excited states, as illustrated for example by recently reported linear two-coordinate Cu I compounds with exceptionally strongly luminescent ligand-to-ligand charge-transfer (LLCT) excited states. − …”