title: A Description of Vibrational Modes in Hexaphyrins: Understanding the Aromaticity Reversal in the Lowest Triplet State.
abstract: Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of [26]- and [28]hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of [26]bis(rhodium) hexaphyrin (R26H), those of [28]bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.
authors: Spectroscopy Laboratory for Functional π-electronic systems and Department of Chemistry, Yonsei University, Seoul, 120-749, Korea.
authors: Young Mo
authors: YM
authors: Sung
authors: Spectroscopy Laboratory for Functional π-electronic systems and Department of Chemistry, Yonsei University, Seoul, 120-749, Korea.
authors: Juwon
authors: J
authors: Oh
authors: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan.
authors: Koji
authors: K
authors: Naoda
authors: Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 609-735, Korea.
authors: Taegon
authors: T
authors: Lee
authors: Spectroscopy Laboratory for Functional π-electronic systems and Department of Chemistry, Yonsei University, Seoul, 120-749, Korea.
authors: Woojae
authors: W
authors: Kim
authors: Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan, 609-735, Korea. mhlim@pusan.ac.kr.
authors: Manho
authors: M
authors: Lim
authors: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto, 606-8502, Japan. osuka@kuchem.kyoto-u.ac.jp.
authors: Atsuhiro
authors: A
authors: Osuka
authors: Spectroscopy Laboratory for Functional π-electronic systems and Department of Chemistry, Yonsei University, Seoul, 120-749, Korea. dongho@yonsei.ac.kr.
authors: Dongho
authors: D
authors: Kim
keywords: Baird's rule; aromaticity reversal; hexaphyrins; time-resolved IR spectroscopy