title: Excited state properties of β-carotene analogs incorporating a lactone ring.
abstract: Carotenoids possessing a carbonyl group along their polyene backbone exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In fact, the ICT characteristics of naturally occurring carbonyl carotenoids play an essential role in the highly efficient energy transfer that proceeds in aquatic photosynthetic antenna systems. In the present study, we synthesized two short-chain polyene carotenoids incorporating a lactone ring, denoted as BL-7 and BL-8, having seven and eight conjugated double bonds (n = 7 and 8), respectively. The excited state properties of these compounds were directly compared to those of their non-carbonyl counterparts to clarify the role of the carbonyl group in the generation of ICT. The energies of the optically allowed S2states for BL-7 and BL-8 were found to be more than 0.3 eV (2400 cm-1) below those of non-carbonyl short β-carotene homologs. Ultrafast spectroscopic data demonstrated various solvent polarity-induced effects, including the appearance of stimulated emission in the near-IR region in the case of BL-7, and significant lifetime shortening of the lowest-lying singlet S1excited states of both BL-7 and BL-8. These results suggest that these compounds exhibit ICT characteristics.
authors.affiliation: Institute of Pulsed Power Science, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555, Japan. kosumi@kumaoto-u.ac.jp.
authors.forename: Daisuke
authors.identifier: http://orcid.org/0000-0001-9339-7913
authors.initials: D
authors.lastname: Kosumi
authors.affiliation: Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Kajikawa
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Sakaguchi
authors.affiliation: Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan and Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
authors.forename: Shigeo
authors.initials: S
authors.lastname: Katsumura
authors.affiliation: Department of Applied Chemistry for Environment, Faculty of Science and Technology, Kwansei Gakuin University, Japan. hideki-hassy@kwansei.ac.jp.
authors.forename: Hideki
authors.initials: H
authors.lastname: Hashimoto