Ultrafast charge-transfer dynamics in a visible-light-excited iridium(III) terpyridine 2-phenylpyridine complex studied by femtosecond X-ray absorption spectroscopy

• Published in: Journal of photochemistry and photobiology. A, Chemistry. Chemistry., 2023-02, Vol.435, p.114267, Article 114267
• Main Authors: Yamanaka, Ken-ichi, Sato, Kosuke, Sato, Shunsuke, Nozawa, Shunsuke, Lee, Sunghee, Fukaya, Ryo, Fukuzawa, Hironobu, You, Daehyun, Saito, Shu, Takanashi, Tsukasa, Katayama, Tetsuo, Togashi, Tadashi, Nonaka, Takamasa, Dohmae, Kazuhiko, Adachi, Shin-ichi, Ueda, Kiyoshi, Yabashi, Makina, Morikawa, Takeshi, Asahi, Ryoji
• Format: Article
• Language: English
• Subjects: CO2-reduction photocatalyst; Femtosecond X-ray absorption spectroscopy; Intersystem crossing; Iridium complex; Metal-to-ligand-charge-transfer state
• ISSN: 1010-6030; 1873-2666
• DOI: 10.1016/j.jphotochem.2022.114267

Femtosecond X-ray absorption study of [Ir(tpy)(ppy)Cl]+ in acetonitrile under metal-to-ligand-charge-transfer (MLCT) excitation revealed that the structural change around Ir atom occurs along with intersystem crossing from 1MLCT to hot 3MLCT (114 fs), and subsequent internal conversion to relaxed 3MLCT mainly occurs not around the Ir atom but at the ligand such as the tpy. [Display omitted] •Photocatalyst for CO2 reduction: Iridium (III) terpyridine (tpy) 2-phenylpridine (ppy) ([Ir(tpy)(ppy)Cl]+).•Femtosecond time-resolved X-ray absorption near-edge structure spectroscopy (TR-XANES) of [Ir(tpy)(ppy)Cl]+.•The TR-XANES directly detected a fast structural change around the Ir atom associated with the ISC to the hot triplet state. Femtosecond time-resolved X-ray absorption near-edge structure (TR-XANES) spectroscopy was used to elucidate the visible-light excitation process in mononuclear iridium (III) terpyridine (tpy) 2-phenylpridine (ppy) ([Ir(tpy)(ppy)Cl]+), which is a known CO2-reduction photocatalyst. Ultrafast intersystem crossing (ISC) was detected by TR-XANES spectroscopy with a time resolution of 90 fs, and the lifetime of the excited singlet state of the metal-to-ligand-charge-transfer state (1MLCT) was found to be 114 fs. Theoretical calculations reproduced the changes in the XANES spectra corresponding to those in the molecular structure. The TR-XANES directly detected a fast structural change around the Ir atom associated with the ISC to the hot triplet state. By contrast, the subsequent relaxation process to the lowest triplet state by internal conversion was not detected, suggesting that the internal conversion mainly occurs not around the Ir atom but at the ligands such as the tpy.