Infinitene: A Helically Twisted Figure-Eight [12]Circulene Topoisomer

Abstract

New forms of molecular nanocarbon particularly looped polyarenes adopting various topologies contribute to the fundamental science and practical applications. Here we report the synthesis of an infinity-shaped polyarene, infinitene (1) (cyclo[c.c.c.c.c.c.e.e.e.e.e.e]dodecakisbenzene), comprising consecutively fused 12-benzene rings forming an enclosed loop with a strain energy of 60.2 kcal•mol −1 . Infinitene (1) represents a topoisomer of still-hypothetical [12]circulene, and its scaffold can be formally visualized as the outcome of the "stitching" of two homochiral [6]helicene subunits by both their ends. The synthetic strategy encompasses transformation of a rationally designed dithiacyclophane to cyclophadiene through the Stevens rearrangement and pyrolysis of the corresponding S,S′-bis(oxide) followed by the photocyclization. The structure of 1 is a unique hybrid of helicene and circulene with a molecular formula of C 48 H 24 , which can be regarded as an isomer for kekulene, [6,6]carbon nanobelt ([6,6]CNB), and [12]cyclacene. Infinitene ( 1) is a bench-stable yellow solid with green fluorescence and soluble to common organic solvents. Its figure-eight molecular structure was unambiguously confirmed by X-ray crystallography. The scaffold of 1 is significantly compressed as manifested by a remarkably shortened distance (3.152−3.192 Å) between the centroids of two π−π stacked central benzene rings and the closest C•••C distance of 2.920 Å. Fundamental photophysical properties of 1 were thoroughly elucidated by UV−vis absorption and fluorescence spectroscopic studies and density functional theory calculations. Its configurational stability enabled separation of the corresponding enantiomers (P,P) and (M,M) by a chiral HPLC. Circular dichroism (CD) and circularly polarized luminescence (CPL) measurements revealed that 1 has moderate | g CD | and |g CPL | values.

Non-intersecting ring currents in [12]infinitene

“…Consequently, drastic changes rise on the shieldings of the nuclei as compared to those of the neutral infinitene (see ESI †). The resulting NMR chemical shifts for the neutral and charged infinitene are in good agreement with the experimental chemical shifts reported by Krzeszewski et al 1 The integration of the current-density on the dication yielded a ring-current strength of À46.5 nA T À1 . When the external magnetic field is applied parallel to y axis, largely the same topology of the current density is obtained for the dication as for the neutral molecule.…”

“…Recently, Krzeszewski et al reported the synthesis of an infinityshaped polyarylene with twelve six-membered rings (6-MRs) belonging to the D 2 point group. 1 The molecular structure can formally be obtained by fusing a [6]helicene with its chiral counterpart at their terminal rings. Consequently, [12]infinitene has two chiral isomers, i.e., (P,P)-infinitene and (M,M)-infinitene (see Fig.…”

“…They also noted that [12]infinitene can be considered as a twisted helical isomer of [12]circulene. 1 Among the family of novel non-planar aromatic systems, [12]infinitene represents an unexplored challenge to understand the three-dimensional electron delocalization. The 6-MRs experience p-p stacking due to the molecular topology, which has been shown to strongly affect the interpretation of the aromaticity because in general p-p stacked rings show strong magnetic shieldings.…”

“…Planar circulenes exhibit concentric counterrotating currents, 12,13 making simple classifications of the aromaticity of infinitene even harder. In the study by Krzeszewski et al, 1 nucleus-independent chemical shift (NICS) calculations 14 on [12]infinitene suggest that the aromatic character varies from one ring to another. However, NICS(0) calculations on nonplanar molecules, especially in helical structures, are not reliable for elucidating the p delocalization, since they only give a very local picture of the magnetic behavior.…”

“…The structural parameters differ by less than 0.02 Å. 1 The Cartesian coordinates of the optimized structures are given in the ESI. † Magnetic properties computed with hybrid functionals have been found to be in close agreement with those calculated at the CCSD(T) level.…”