Solvent- and Temperature-Dependent Conformational Changes between Huckel Antiaromatic and Mobius Aromatic Species in meso-Trifluoromethyl Substituted [28]Hexaphyrins

We investigated the photophysical properties of figure-eight-like meso-hexakis(trifluoromethyl) [26]- and [28]hexaphyrins-(1.1.1.1.1.1) 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 Huckel's [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S-1-state lifetime of similar to 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 Racket antiaromatic and Mobius 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 H-1 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.