Noncovalent Intermolecular Interaction in Cofacially Stacked 24π Antiaromatic Hexaphyrin Dimer

Abstract

p-p Stackingi so mnipresent not only in nature but in aw ide variety of practical fields appliedt oo ur lives. Becauseo fi ts importance in ap erformance of natural and artificial systems, such as light harvesting system and working layer in device, many researchers have put intensive effort into identifying its underlying nature. However,f or the case of p-p stacked systemsc omposed of antiaromatic units, the understanding of the fundamentalm echanisms is still unclear.H erein, we synthesized an ew type of planar b,b'-phenylene-bridgedh exaphyrin (1.0.1.0.1.0),r eferred as naphthorosarin whichp ossesses the 24p-electronc onjugated pathway.E specially,t he corresponding antiaromaticp orphyrinoid shows the unique property to form dimeric species adopting the face-to-face geometry which is unprecedentedi nc ases of known annulated naphthorosarins. In order to elucidate the intriguing properties derived from the stacked dimer,t he current study focuses on the experimental support to rationalizet he observed p-p interactions betweent he two subunits.

Dithieno[a,e]pentalenes: Highly Antiaromatic Yet Stable π‐Electron Systems without Bulky Substituents

“…This concept has been investigated theoretically over the past several decades, [12] and has recently been corroborated experimentally for the dimers of norcorrole by Shinokubo and co‐workers for the first time [13] . Nevertheless, experimental studies on stacked antiaromatic π‐systems have been limited to several porphyrinoids [14] and benzene‐fused dehydroannulenes [15] . The biggest challenge in this area of chemistry is the inherently high reactivity of antiaromatic π‐systems.…”

Post Synthetic Modification of Planar Antiaromatic Hexaphyrin (1.0.1.0.1.0) by Regio-Selective, Sequential SNAr

“…Such a larger shift is unique in these classes of macrocycles considering the fact that the meso-substituents adopt nearly orthogonal conformation against the rigid macrocyclic core. For instance, peripherally modified rosarrin analogues displayed minimal changes in absorption spectra compared to the parent rosarrin 2 [10,11]. We envisaged that macrocycle core possesses a modified dioxine ring at meso position which alters its dihedral angle and results such a bathochromic shift.…”

“…Furthermore, upon varying the meso-substituents from pentafluorophenyl group to bis(trifluoromethyl)phenyl group displayed strong, intermolecular 'face-to-face', π-π interaction between antiaromatic molecules. Formation of the 'face-to-face' stacked dimer showed temperature dependency with intriguing photophysical properties [11]. However, experimental exploration of the rosarrins often encountered difficulties because varying meso-substituents are synthetically challenging due to low yield and difficult purification process.…”

Three-Dimensional Fully pi-Conjugated Macrocycles: When Truly 3D-Aromatic and when 2D-Aromatic-in-3D?

“…Is the aromaticity in 1 -3 and 1 6+ instead related to the Hückel-aromaticity of two-dimensional polycyclic aromatic hydrocarbons (PAHs)? Compounds 1 -3 and 1 6+ are unusual and intriguing, yet, even though their aromatic character is apparent from both experimental and computational observations, 33,35,39 the cause of this aromaticity has not been analyzed in depth. In particular, there has been no search and investigation of macrocycles that potentially disprove the hypothesis that 1 and its hexacation comply with the 6n + 2 rule for 3D-aromaticity and Hirsch's 2(n + 1) 2 rule for spherical aromaticity.…”

“…One form of aromaticity in three dimensions, which recently has been labelled as 3D-aromaticity, is the through-space (face-to-face) aromaticity found in methano-bridged superphanes involving π-stacked [4n]annulenes and explored experimentally in cyclophanes and hexaphyrin dimers. [31][32][33][34] Another highly interesting and intriguing compound is the π-conjugated cage compound 1 (Figure 1) and its cations up to the hexacation 1 6+ , which were reported by Wu and co-workers and considered to be "3D globally aromatic". 35 This bicyclic macrocycle consists of three equally long -conjugated arms, and in its neutral form it was, according to computations, found to possess one Hückel aromatic cycle with 38π-electrons while the lack of aromaticity in the other two cycles is a result of the C2 symmetric structure and poor -conjugation in the third bridge.…”