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 diarylethene-containing ligand L1 based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromo-phenanthroline ligand is reported. On coordination to the rhenium(i) tricarbonyl complex system, the photo-chromism of L1 could be photosensitized and consequently extended from intraligand excitation at lambda <= 340 nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation at lambda <= 480 nm in the complex. The photochromic reactions were studied by H-1 NMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results pro-vided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the (MLCT)-M-3 excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the (MLCT)-M-3 to the (IL)-I-3(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated.