Unravelling the fine structure of stacked bipyridine diamine-derived C₃-discotics as determined by X-ray diffraction, quantum-chemical calculations, Fast-MAS NMR and CD spectroscopy

Abstract

An in depth investigation of the fine structure adopted by the helical stacks of C 3 -discotics 1 incorporating three 3,3 0 -diamino-2,2 0 -bipyridine units is described. In the bulk the molecules display liquid crystalline behaviour in a temperature window of >300 K and an ordered rectangular columnar mesophase (Col ro ) with an inter-disc distance of 3.4A is assigned. X-Ray diffraction on aligned samples has also revealed a helical superstructure in the liquid crystalline state, and a rotation angle of 13-16 between consecutive discs. The proposed superstructure in the bulk phase has been further substantiated by a combination of quantum-chemical calculations and solid-state NMR spectroscopy. Dilute solution NMR spectroscopy and elaborate CD spectroscopy on aggregated samples have revealed an isodesmic growth pattern of the C 3 -discotics. From the combined results it has become evident that the fine tuning interaction responsible for the highly ordered helical architectures is not weak intermolecular hydrogen bonding, but rather rigidification, due to propeller formation after preorganisation by p-p interactions. Although all the techniques used underpin the structural features proposed, none of them individually is able to point to a unique structure. However, together the techniques give very strong evidence for a confined ship-screw arrangement in which all amidic carbonyl oxygens point in one direction.

What Molecular Features Govern the Mechanism of Supramolecular Polymerization?

“…[50] On the other hand, if a molecular stack lacks this kind of long-range interaction, then the stabilization gained by the stack is purely local, arising from interaction with the nearest neighbours, and this contribution remains more or less constant throughout the course of polymerization. [27] Thus, about 19-20 molecules constitute a helical pitch and the observed helical twist is not complete for a tetramer, as can be seen in Figure 7. [7] To test the validity of the above hypothesis, we chose three different systems (one cooperative and two isodesmic) to perform computational studies.…”

“…[23] In a BTA stack, the amide groups of the molecules are rotated out of the benzene plane by 458 to form triple helical hydrogen bonds, although the isolated molecule in the gas phase is planar. In MCH, 2 aggregates through an isodesmic pathway, as shown in Figure 2 B. Quantum chemical calculations [27] show no evidence of intermolecular hydrogen bonding. Dispersion, calculated through the empirical correction to DFT, [23b] is found to contribute about 30 % to the stabilization of the stack.…”

“…The dominant contributor to the stabilization of the stacks stems from these hydrogen bonds. X-ray diffraction studies [27] and quantum chemical calculations suggest a rotation angle of 10-408 between neighbouring molecules in a stack, and this small angle does not allow for intermolecular hydrogen bonding. As the hydrogen bonds are all directed along the long axis of the stack, the latter acquires a macrodipole.…”

“…[25] Recent computational work has elucidated cooperativity in this class. [27] A detailed study of the superstructure of 2 has been carried out by using combined experiments and theory. The dominant contributor to the stabilization of the stacks stems from these hydrogen bonds.…”

C3‐Symmetrical π‐Scaffolds: Useful Building Blocks to Construct Helical Supramolecular Polymers

“…This generates a more planar, discotic geometry that favors self‐assembly via an isodesmic mechanism. In BTAs 1 a–c , the helical arrangement of the stacks arises from the dipolar helical alignment of the amides stemming from the C 3 symmetry of these units …”