Viability of Möbius Topologies in [26]‐ and [28]Hexaphyrins

Abstract

Recently, hexaphyrins have emerged as a promising class of π-conjugated molecules that display a range of interesting electronic, optical, and conformational properties, including the formation of stable Möbius aromatic systems. Besides the Möbius topology, hexaphyrins can adopt a variety of conformations with Hückel and twisted Hückel topologies, which can be interconverted under certain conditions. To determine the optimum conditions for viable Möbius topologies, the conformational preferences of [26]- and [28]hexaphyrins and the dynamic interconversion between the Möbius and Hückel topologies were investigated by density functional calculations. In the absence of meso substituents, [26]hexaphyrin prefers a planar dumbbell conformation, strongly aromatic and relatively strain free. The Möbius topology is highly improbable: the most stable tautomer is 33 kcal mol(-1) higher in energy than the global minimum. On the other hand, the Möbius conformer of [28]hexaphyrin is only 6.5 kcal mol(-1) higher in energy than the most stable dumbbell conformation. This marked difference is due to aromatic stabilization in the Möbius 4n electron macrocycle as opposed to antiaromatic destabilization in the 4n+2 electron system, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity. For [28]hexaphyrins, the computed activation barrier for interconversion between the Möbius aromatic and Hückel antiaromatic conformers ranges from 7.2 to 10.2 kcal mol(-1), in very good agreement with the available experimental data. The conformation of the hexaphyrin macrocycle is strongly dependent on oxidation state and solvent, and this feature creates a promising platform for the development of molecular switches.

Metalated Hexaphyrins: From Understanding to Rational Design

“…We have previously demonstrated that computational chemistry is ap owerful tool in aiding the design of viable Mçbius expanded porphyrins with high aromatic character. [38][39][40][41] Our initial research focusing on metal-free expanded porphyrins varying in ring sizes and oxidation states revealed thatt he conformation of the hexaphyrin macrocycle is clearly dependento n the oxidation state. [38] [26]hexaphyrin 1 strongly prefers aH ückel conformation, planar and highly aromatic, whereas antiaromaticH ückel and aromatic Mçbius conformers coexist in dynamic equilibrium for [28]hexaphyrin 2.H erein, the metalation effect of d 8 metalso nt he molecular topology of 1 and 2 is scrutinized in detail.…”

Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

“…As additional geometrical descriptors, we computed the average dihedral angle between neighboring pyrrole rings (Φ p ) as a measure of the torsional ring strain [ 37 ] and the π-conjugation index ( Π ) to quantify the extent of the effective overlap of neighboring p orbitals [ 1 ]. Π equals 1 for a completely planar system, it is positive for any Hückel (double-sided) conformation and negative for any Möbius (single-sided) surface.…”

“…In our recent computational research on expanded porphyrins, we have proposed a set of descriptors for measuring the global aromaticity of porphyrinoids based on energetic, structural, magnetic and reactivity criteria [ 37 , 38 , 39 ]. Although some discrepancies exist between the different indices, overall the energetic, reactivity, structural, magnetic and electronic indices nicely agree on the general (anti)aromatic features of these compounds in agreement with the annulene model [ 40 , 41 ].…”

New electron delocalization tools to describe the aromaticity in porphyrinoids

“…[45][46][47][48][49][50][51][52][53][54] In this sense, it is not unexpected that expanded porphyrins are among the most challenging test beds for aromaticity descriptors. It should be pointed out that several works 20,[55][56][57][58][59][60][61][62] have recently analyzed the aromaticity of expanded porphyrins with different topologies and number of π electrons using various aromaticity indices based on the energetic, magnetic, structural, and reactivity criteria. The last problem to describe the aromaticity of porphyrinoids is the critical role of the selected density functional approximations (DFA) to evaluate geometric, energetic and magnetic properties.…”