A systematic series of four novel homo‐ and heteroleptic Cu I photosensitizers based on tetradentate 1,10‐phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9‐position (X=SMe or OMe) were designed. Their solid‐state structures were assessed by X‐ray diffraction. Cyclic voltammetry, UV‐vis absorption, emission and X‐ray absorption spectroscopy were then used to determine their electrochemical, photophysical and structural features in solution. Following, time‐resolved X‐ray absorption spectroscopy in the picosecond time scale, coupled with time‐dependent density functional theory calculations, provided in‐depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu−X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers.

Time‐resolved X‐ray (Tr‐XAS) and optical transient absorption (OTA) spectroscopy on the pico‐microsecond timescale coupled with density functional theory calculations are applied to study the light‐induced spin crossover processes of a Fe‐based macrocyclic complex in solution. Tr‐XAS analysis after light illumination shows the formation of a seven‐coordinated high‐spin quintet metastable state, which relaxes to a six‐coordinated high‐spin configuration before decaying to the ground state. Kinetic analysis of the macrocyclic complex reveals an unprecedented long‐lived decay lifetime of approximately 42.6 μs. Comparative studies with a non‐macrocyclic counterpart illustrate a significantly shortened approximately 568‐fold decay lifetime of about 75 ns, and highlight the importance of the ligand arrangement in stabilizing the reactivity of the excited state. Lastly, OTA analysis shows the seven‐coordinated high‐spin state to be formed within approximately 6.2 ps. These findings provide a complete understanding of the spin crossover reaction and relaxation pathways of the macrocyclic complex, and reveal the importance of a flexible coordination environment for their rational design.