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A Description of Vibrational Modes in Hexaphyrins: Understanding the Aromaticity Reversal in the Lowest Triplet State
Author(s) -
Sung Young Mo,
Oh Juwon,
Naoda Koji,
Lee Taegon,
Kim Woojae,
Lim Manho,
Osuka Atsuhiro,
Kim Dongho
Publication year - 2016
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201603631
Subject(s) - aromaticity , excited state , chemistry , triplet state , infrared spectroscopy , spectral line , spectroscopy , infrared , photochemistry , computational chemistry , atomic physics , molecule , physics , organic chemistry , optics , quantum mechanics , astronomy
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.