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

Abstract

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 Hückel's [4n + 2] aromaticity as observed in typical aromatic porphyrinoids. The S(1)-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 (1)H 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.

An electronic aromaticity index for large rings

“…The peripheral ring of naphthalene shows similar AV1245 value to the annulene [10] structure, whereas neither the peripheral ring of anthracene or phenanthrene are as aromatic as annulene [14]. Conversely, hexaethynyl-carbo-benzene also exhibits similar aromaticity to annulene [18], which confirms the prominent aromatic character of this molecule. 46 …”

“…15 Despite these benefits, the multicenter indices suffer from a series of problems that prevents its application in large rings as those showing in new interesting problems such as the aromaticity of (expanded 16 ) porphyrins 17 and its relationship with UV absorption spectra or Hückel-to-Möbius topological switches. 18,19 The goal of this paper is introducing a new electronic aromaticity index that can be applied to rings of arbitrary size. The work is organized as follows: first, we will review the expressions for multicenter indices and reveal their strenghts and weaknesses; second, we will describe an algorithm that permits to calculate MCI on larger rings but it is still limited to rings of medium size; then we will provide the expression of a new multicenter index that will be finally analyzed in a series of compounds to assess its performance.…”

meso‐Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology

“…In the twisted Mçbius topology of 7 with moderate aromatic character,t he pyrrole rings connected to the ethylene bridge (N1 and N4) deviate strongly from the mean plane with tilt angles of 79.48(2)8 8 [N1] and 41.18(2) 8 8 [N4] to release the steric strain of the macrocycle,a nd the torsion angle at C20-C21-C1-C2, found to be 67.328 8,i sl ess than in 5,t hus maintaining the moderate aromatic character of the [20]p system (see Table S1 and Figure S27). [11][12][13][14][15][16][17][18] Furthermore,t he upfield shift of the NMR signal for one of the inner NH atomswas confirmed by the close proximity of the hydrogen atom to the metal center, with aN 2-H2···Rh bond distance and angle of 2.848(1) and 128.89(4)8 8 (see Figure S22). Overall, the variabletemperature NMR studies and singlecrystal analysis proves that both the free bases and the Rh complexes exhibit moderate 20p Mçbius aromatic character rather than 20p Hückel antiaromatic character.…”

Aromaticity Reversal in the Lowest Excited Triplet State of Archetypical Möbius Heteroannulenic Systems

“…[19] In the respect that Pd28H is ac ongener of bis(rhodium) complexes of [26]/[28]hexaphyrins (the same molecular framework except for the central metal), its S 0 and T 1 absorption spectra thus resembled those of the bis(rhodium) complexes of aromatic [26]hexaphyrins and antiaromatic [28]hexaphyrins,r espectively. [19] In the respect that Pd28H is ac ongener of bis(rhodium) complexes of [26]/[28]hexaphyrins (the same molecular framework except for the central metal), its S 0 and T 1 absorption spectra thus resembled those of the bis(rhodium) complexes of aromatic [26]hexaphyrins and antiaromatic [28]hexaphyrins,r espectively.…”