Abstract
The formation of a halogen bonded self-assembled co-crystal physisorbed monolayer containing N···Br interactions is reported for the first time. The co-crystal monolayer is identified experimentally by synchrotron X-ray diffraction and the structure determined. Density functional theory (DFT) calculations are also employed to assess the magnitudes of the different interactions in the layer. Significantly, compared to other halogen bonds in physisorbed monolayers we have reported recently, the N···Br bond here is found to be non-linear. It is proposed that the increasing importance of the lateral hydrogen bond interactions, relative to the halogen bond strength, leads to the bending of the halogen bonds.
Highlights
The study of supramolecular self-assembled networks held together by non-covalent interactions is currently of great interest
In this work we report the phase behaviour of the mixed overlayer of BPY–DBTFB to test the prediction from the Density functional theory (DFT) calculations
The DFT results are in reasonable agreement (4.5% error) with this conclusion, indicating a bent bond angle of 163.41 after geometry optimisation. This combination of species is found to form non-linear halogen bonds in the bulk, the halogen bond in the monolayer is somewhat less linear (À8%) than in the bulk structure (176.401/177.711), very similar to some of the halogen bond angles observed for related perfluoroaryl bromide species (B1631).[35]
Summary
The study of supramolecular self-assembled networks held together by non-covalent interactions is currently of great interest. These overlayers are not covalently bound to the substrate (like thiols on gold) but are only weakly physisorbed. There are a number of non-covalent interactions that might be used to control the self-assembly process In particular those with a strength and directionality which can be used to control materials properties such as corrosion resistance, wettability[1,2,3] and the formation of templated monolayer structures for recognition by molecular engineering. A variety of other non-covalent interactions have been observed in physisorbed layers, including halogen–halogen interactions.[21,22,23,24] An extension of this, the halogen bond – the electrostatic interaction between a halogen atom and a Lewis base – represents an important alternative and complement to the hydrogen bond.[25,26,27] The halogen bond is increasingly recognised as an important non-covalent interaction in 3D crystal engineering due to a strength, directionality and robustness comparable to the hydrogen bond.[28,29] the halogen bond provides a parallel set of non-covalent interactions to the hydrogen bond, and has even been demonstrated to be stronger than hydrogen bonding in some self-assembly processes.[30]
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