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.

Transition metal complexes capable of releasing small molecules such as carbon monoxide and nitric oxide upon photoactivation are versatile tools in various fields of chemistry and biology. In this work,...

Transition metal complexes capable of releasing small-molecule messengers such as carbon monoxide and nitric oxide upon photoactivation are versatile tools in various fields of chemistry and biology. In this work, we report on the ultrafast photochemistry of [Mo(CO)2(NO)(iPr3tacn)]PF6 (iPr3tacn = 1,4,7-triisopropyl-1,4,7-triazacyclononane), which was synthesized and subsequently characterized in continuous illumination studies and with femtosecond UV-pump/UV-probe and UV-pump/MIR-probe spectroscopy, as well as with stationary calculations. The experimental and theoretical results demonstrate that while the photodissociation of one of the two CO ligands upon UV excitation can be inferred both on an ultrafast timescale as well as under exposure times of several minutes, no evidence of NO release is observed under the same conditions. The binding mode of the diatomic ligands is impacted by the electronic excitation, and excited molecules are observed on a timescale of tens of picoseconds before CO is released from the coordination sphere. Furthermore, based on calculated potential energy scans, we suggest that photolysis of NO could be possible after a subsequent excitation of an electronically excited state with a second laser pulse, or by accessing low-lying excited states that otherwise cannot be directly excited by light.

Transition metal complexes capable of releasing small-molecule messengers such as carbon monoxide and nitric oxide upon photoactivation are versatile tools in various fields of chemistry and biology. In this work, we report on the ultrafast photochemistry of [Mo(CO)2(NO)(iPr3tacn)]PF6 (iPr3tacn = 1,4,7-triisopropyl-1,4,7-triazacyclononane), which was synthesized and subsequently characterized in continuous illumination studies and with femtosecond UV-pump/UV-probe and UV-pump/MIR-probe spectroscopy, as well as with stationary calculations. The experimental and theoretical results demonstrate that while the photodissociation of one of the two CO ligands upon UV excitation can be inferred both on an ultrafast timescale as well as under exposure times of several minutes, no evidence of NO release is observed under the same conditions. The binding mode of the diatomic ligands is impacted by the electronic excitation, and excited molecules are observed on a timescale of tens of picoseconds before CO is released from the coordination sphere. Furthermore, based on calculated potential energy scans, we suggest that photolysis of NO could be possible after a subsequent excitation of an electronically excited state with a second laser pulse, or by accessing low-lying excited states that otherwise cannot be directly excited by light.