Solvent- and temperature-dependent conformational changes between hückel antiaromatic and möbius aromatic species in meso-trifluoromethyl substituted [28]hexaphyrins

Min Chul Yoon, Pyosang Kim, Hyejin Yoo, Soji Shimizu, Taro Koide, Sumito Tokuji, Shohei Saito, Atsuhiro Osuka*, Dongho Kim

*Corresponding author for this work

Research output: Contribution to journalArticle

37 Scopus citations


We investigated the photophysical properties of figureeight- like meso-hexakis(trifluoromethyl) [26]- and [28]hexaphyrins- ( denoted as TFM26H and TFM28H, respectively, using steady-state and time-resolved spectroscopy along with theoretical calculations to explore their electronic and magnetic natures depending on their molecular aromaticity. TFM26H exhibited a well-resolved absorption feature and intense fluorescence, both of which were neither solvent- nor temperature-dependent. These optical properties were in agreement with its Hückel's [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S1-state lifetime of ̃50 ps for TFM26H in solution was shorter than those in planar aromatic hexaphyrins (>100 ps) presumably due to nonplanar figure-eight geometry of TFM26H. However, TFM28H exhibited remarkable changes in solvent- and temperature-dependent absorption spectra as well as excited-state lifetimes indicating that a dynamic equilibrium occurs between the two conformational species. With the help of quantum mechanical geometry optimization and vertical excitation energy calculations, we found that the figure-eight double-sided conformer observed in the solid-state and single-sided distorted one could be the best candidates for the two conformers, which should be Hückel antiaromatic and Möbius aromatic species, respectively, based on their optical characteristics, molecular orbital structures, and excited-state lifetimes. Conformational dynamics between these two conformers of TFM28H was scrutinized in detail by temperature-dependent 1H NMR spectra in various solvents, which showed that the conformational equilibrium was quite sensitive to solvents and that a conformational change faster than the NMR time-scale occurs even at 173 K.

Original languageEnglish (US)
Pages (from-to)14928-14937
Number of pages10
JournalJournal of Physical Chemistry B
Issue number50
StatePublished - Dec 22 2011


ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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