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

• Published in: Chemistry : a European journal 2006-07, Vol.12 (22), p.5840-5848
• Main Authors: Ko, Chi-Chiu, Kwok, Wai-Ming, Yam, Vivian Wing-Wah, Phillips, David Lee
• Format: Article
• Language: English
• Subjects: luminescence; N ligands; photochemistry; photochromism; rhenium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200501325

Synthesis of the diarylethene‐containing ligand L1 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≤340 nm in the free ligand to metal‐to‐ligand charge‐transfer (MLCT) excitation at λ≤480 nm in the complex. The photochromic reactions were studied by 1H NMR, 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 the 3MLCT to the 3IL(L1) state that initiated the ring‐closure reaction. The photophysical and electrochemical properties of the complex were also investigated. Photochromic reactions of diarylethene‐containing ligand L1 and its ReCl(CO)3 complex (see picture) were studied inter alia by ultrafast transient absorption and time‐resolved emission spectroscopy. The photochromism of L1 could be photosensitized and extended from intraligand (IL) excitation at λ≤340 nm in the free ligand to metal‐to‐ligand charge‐transfer (MLCT) excitation at λ≤480 nm in the complex. The results support the depicted mechanism, in which intramolecular energy‐transfer (ET) from the 3MLCT to the 3IL(L1) state initiates ring closure.