Triplet MLCT photosensitization of the ring-closing reaction of diarylethenes by design and synthesis of a photochromic rhenium(I) complex of a diarylethene-containing 1,10-phenanthroline ligand

Synthesis of the diaryle-thene-containing ligand LI based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at λ≤340nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation atλ≤480nm in the complex. The photo-chromic reactions were studied by1HNMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the 3MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the3MLCT to the3IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated. KGaA.