Molecular dynamic staircases: all-carbon axial chiral “Geländer” structures

Abstract

Expanding on previous Geländer models, we designed and characterized two chiral all-carbon Geländer molecules with enhanced thermodynamic stabilities.

An Ortho‐Tetraphenylene‐Based “Geländer” Architecture Consisting Exclusively of 52 sp2‐Hybridized C Atoms

“…Comparable maximal Δɛ values were recorded around 240 nm for both bicyclic structures with 83 L • mol À 1 for 1 and 68 L • mol À 1 for 2, resulting in similar dissymmetry factors (g CD ) of about 1.4 • 10 À 3 at this wavelength (Figure S2 -15). These Δɛ values are substantially higher than the ones measured for previously reported helical chiral Geländer molecules with terphenyl backbones, [28] or naphthol derivatives (10-20 L • mol À 1 ), [52] but considerably smaller than the ones reported for more extended systems, such as a twisted cycloparaphenylene derivative (~200 L • mol À 1 ). [53] While at first glance both bicyclic helical hydrocarbons 1 and 2 display absorptions of the same polarity for the same enantiomers, an inversion of the Cotton bands at 250-290 nm was recorded.…”

“…These pioneering Geländer structures not only interconvert between enantiomers easily, the combination of the molecule's symmetry with the independence of both interlinking junctions also allows their meso forms, having opposed axial chirality along the molecule's axis. We developed the concept of Geländer molecules further by attaching a second phenyl‐based oligomer of different step‐size alongside the para ‐terphenyl axle (Figure 1c) [25–29] . While the improved molecular design avoids meso ‐forms by propagating the axial chirality along the oligomers backbone, it came at the prize of considerably increased synthetic challenges.…”

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

“…[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. [20] More recently, our focus moved to axial chiral systems like bicyclic 'Geländer'-type structures [21] or the macrocyclization of the ligands assembled in a M(II) terpyridine complex resulting in a helical macrocycle with an arrangement resembling a propeller. [22] Cyclo[n]thiophenes are an interesting class of conjugated macrocycles; they are model compounds for polythiophenes, with well-defined self-assembling and electronic features.…”

Bicyclic Phenyl–Ethynyl Architectures: Synthesis of a 1,4‐Bis(phenylbuta‐1,3‐diyn‐1‐yl) Benzene Banister

“…Consequently, isomers displaying faster racemization are labeled as tropos [18, 19] . Another challenge observed in the syntheses/characterizations of these “Geländer” oligomers was the formation of different bicyclic systems with varying ring‐sizes [16, 20] …”

“…[18,19] Another challengeo bserved in the syntheses/characterizations of these "Geländer"o ligomersw as the formation of different bicyclic systems withv arying ring-sizes. [16,20] From am aterials property perspective, "Geländer"s tructures with ac onjugated banister would be particularly interesting as model compounds with electrons delocalizedo nah elical sub-unit. We thus recently reported the synthesis of macrocycle 2 (Scheme 1), the shortestm ember of an ew "Geländer" design with ah elical oligo-para-p henylene-di-ethynylenes (OPDE) banister comprising sp-ands p 2 -hybridized carbon atoms exclusively.…”