Pseudo‐Octahedral Iron(II) Complexes with Near‐Degenerate Charge Transfer and Ligand Field States at the Franck‐Condon Geometry

Abstract

Increasing the metal‐to‐ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo‐ and heteroleptic complexes [Fe(cpmp)2]2+ (12+) and [Fe(cpmp)(ddpd)]2+ (22+) with the tridentate ligands 6,2’’‐carboxypyridyl‐2,2’‐methylamine‐pyridyl‐pyridine (cpmp) and N,N’‐dimethyl‐N,N’‐di‐pyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal‐ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six‐membered chelate rings. The push‐pull ligand cpmp provides intra‐ligand and ligand‐to‐ligand charge transfer (ILCT, LL'CT) excited states in addition to MLCT excited states. Ground and excited state properties of 12+ and 22+ were accessed by X‐ray diffraction analyses, resonance Raman spectroscopy, (spectro)electrochemistry, EPR spectroscopy, X‐ray emission spectroscopy, static and time‐resolved IR and UV/Vis/NIR absorption spectroscopy as well as quantum chemical calculations.

Can range-separated functionals be optimally tuned to predict spectra and excited state dynamics in photoactive iron complexes?

“…Using the set of optimal parameters from the DSCF method (set A) for dynamics simulations results in the population of 3 MLCT states that are stable within our 2 ps simulation time. This result is in contrast to TAS experiments, 34 which show the depopulation of MLCT states within 200 fs. Using the set which is in agreement with multireference CASPT2 calculations (set B) leads to relaxation processes within both the singlet and triplet manifolds.…”

“…Such states are the lowest-lying excited states, as shown in ref. 34, and they have been assigned to the experimentally observed longlived signal.…”

“…34. Those experiments 34 report two additional time constants for the excited-state decay, i.e., s 1 < 200 fs and s 2 = 30 − 40 ps. The ultrafast s 1 could, in principle, also contribute to the TA signal in our experiments, which have a time resolution of ca.…”

“…In ref. 34, the s 1 component is associated with small spectral changes and ascribed to the lifetime of the 3 MLCT state due to the decay into MC states. Note that it was not possible to spectroscopically differentiate between triplet and quintet MC states.…”

“…1), where cpmp = 6,2 ′′ -carboxypyridyl-2,2 ′ -methylamine-pyridyl-pyridine is a push-pull ligand used previously in an analogous ruthenium(II) compound. 33 This complex, recently synthesized and experimentally characterized, 34 follows two strategies for achieving long-sought long-lived MLCT states: 35 forming a close to octahedral coordination of the central iron that maximizes the ligand-eld splitting and destabilizing the MC states by electron-rich ligands while simultaneously stabilizing the MLCT states by p-acceptor groups, such as in other iron(II) push-pull complexes. [36][37][38][39][40] We rst investigate different routes to tune the rangeseparation parameters of the LC-BLYP functional 18 (Section 2).…”