Molecular Structure and Spectroscopic Properties of Kekulene

Abstract

Kekulene (1), the synthesis of which was recently reported!2 1 , is the first example of a new class of aromatic compounds in which the annellation of six-membered rings leads to a cyclic system enclosing a cavity lined with hydrogen atoms. Compound (1), for which 200 Kekule structures with different arrangements of double and single bonds can be formulated[3J, was of interest with regard to 'IT-electron delocalization and the related problem of the diatropicity in the macrocyclic system for which, as long ago as 1951, different theoretical approaches were shown to lead to contradictory predictions in the specific case of (1]141. Experimentally, the 'H-NMR absorption of the internal hydrogens did not show any evidence for a diatropicity in the macrocyclic system!21, in qualitative agreement with MO calculations of chemical shifts!4.51. In this context, and in connection with some spectroscopic properties of (1), the determination of the molecu-... In the crystal lattice of (1) the molecules are stacked along the b axis with the stacking axis forming an angle of 42.9° with the molecular planes. Neighboring molecules in such a stack have an interplanar distance of 336 pm and are parallel-shifted by 312 pm. The molecular planes of adjacent stacks are inclined by 86° to each other, resulting in the "herringbone pattern" shown in Figure 1 as a side view along the a axis.lar structure of kekulene by X-ray analysis was of special interest. (1) crystallized from pyrene (purified by zone-melting) in a tube sealed under high-vacuum on slow cooling from 450With a mean deviation of carbon atoms from the mean plane through the 48 carbon atoms of only 3 pm and a maximum deviation of 7 pm, (1) has an almost perfectly planar structure. This includes, with deviations of 5 and 10 pm, respectively, even the six internal hydrogens, although the non-bonding distances of 196(2) pm between adjacent hydrogen atoms are unusually short.

Electronic properties of kekulene

“…a Figure 7. Labelling of the chemical bonds in kekulene used in table 3. order to reproduce the standard resonance integral [13], we obtain bond lengths (self-consistent within 0·01 A) which agree quite well with the experimental ones [2], cf. table 3.…”

“…This assumption is in any case justified for those sites in which only one of the triplet sublevels radiates [10]. Accordingly, using perturbation theory we obtain, following Lemaistre and Zewail [17], for the uncorrelated case for the ODMR transition frequency between the spin orbitally active magnetic sublevel (ax) and the inactive sublevel aj (j = y, z) (2) with w x / being the ODMR transition of the specially selected site (0) with energies Eo (T) and b o the SOC mixing coefficient for this site.…”

“…INTRODUCTION Kekulene [1,2] is the first example of a new class of aromatic compounds in which the annelation of six-membered rings leads to a cyclic system enclosing a cavity lined with hydrogen atoms (figure 1). On the basis of the X-ray structure analysis [2] the sextet notation for kekulene shown in figure 1 is undoubtedly the best representation of the actual bonding situation out of the 200 possible structures with different arrangements of double and single bonds which can be formulated [3]. The X-ray structure analysis has further shown that kekulene has an almost perfect planar structure in the ground state (the maximum deviation of carbon atoms from the mean plane through the 48 carbon atoms is only 7 pm).…”

“…2. EXPERIMENTAL Purified kekulene [1,2] was dissolved in zone-refined liquid 1,2,4,5-tetrachlorobenzene or 1,2,4,5-tetrabromobenzene. Polycrystalline samples were prepared by rapidly cooling the liquid samples and by tempering them at room temperature for several weeks.…”

A Chiral Macrocyclic Oligothiophene with Broken Conjugation – Rapid Racemization through Internal Rotation

“…[4][5][6][7] A synthetic milestone in the field of conjugated aromatic compounds was the synthesis and investigation of kekulene by the young François Diederich in the labs of Heinz Staab. [8] [9] Various molecular motifs have been reported as subunits of conjugated macrocyclic compounds, like e. g. pyridines, benzenes, acetylenes, as well as fivemembered aromatic heterocycles, like furans and thiophenes. [10] [11] We reported the assembly and investigation of a variety of macrocycles consisting of aromatic subunits in the past, among others structures comprising functional subunits like redox chromophores [12] [13] or optically addressable azo-benzenes, [14] macrocycles designed as single molecule switches [15 -17] or with pronounced π-stacking features, [18] [19] and giant macrocycles as model compounds for persistent ring currents.…”