Formation of Heterodimeric Resorcin[4]arene Capsules on Surfaces: An X-Ray Photoelectron Spectroscopy Investigation

Abstract

The hydrogen bond formation of heterodimeric molecular capsules assembled from complementary functionalized resorcinarene cavitands is of considerable scientific interest from the viewpoint of multicomponent molecular assemblies.[1, 2] Different macromolecular and H-bond oligomeric systems have recently been discussed with a view to providing host assemblies encapsulating molecular guests. [3][4][5] On account of the hydrogen-bond character of these capsules, such assemblies provide templates of flexible heterocomposed functionalities. Potentially encapsulated compounds can for example hence be specifically released once the intermolecular hydrogen bond interaction responsible for the formation of these assemblies is modified or disrupted. [5,6] Herein the formation of hydrogenbonded heterodimeric capsules AB (see Figure 1) based on a monolayer of tetrapyridyl resorcinarenes B is verified by means of X-ray photoelectron spectroscopy (XPS). Therefore the N 1s core-level signatures of benzoic-acid-labelled and of AB-capsulated tetrapyridyl resorcinarene monolayers were analyzed with the aid of model assumptions achieved by fitting the signatures of labelled model amine systems, polyethyleneimine triethoxysilane (PEIs) and branched polyethyleneimine (PEIb). Labelling them with carboxyl provided differently weighted Hbond-and protonating interaction characters, depending on the particular contribution of primary, secondary and tertiary amines. Referring to these model systems, the interaction responsible for the achieved on-surface capsulation was proven to be of hydrogen-bond character. Unlike a monolayer with compact dense pyridyl functionalities, this interaction character turned out to depend on the density provided on the surface. The efficiency of the capsulation on the other hand was found to be affected by the individual availability of the labelling carboxyl functionalities.X-ray photoelectron spectroscopy (XPS) investigations of the mentioned model PEI structures labelled with benzoic acid were first carried out to gain a deeper understanding as to the effect of H-bond-or protonation-dominated N···HO(O=C) interactions on the nitrogen N 1s photoelectron spectra.Whilst the PEIs provide secondary and tertiary amines, effectively weighted by (0:67:33), the branched polyethyleneimine (PEIb) consists of primary, secondary and tertiary amines weighted by (25:50:25). The labellings of these PEI structures were performed in solution to gain a first impression of the interaction specifically affecting the spectra. Thus the obtained spectroscopic modifications due to the N···HO(C=O) interaction of the different amine nitrogens could be investigated without any dependence of labelling affinities on surface or immobilizing aspects having to be considered in this model case. The N 1s spectra of both labelled structures are shown in Figure 2. On fitting these spectra, two shifted N 1s species associated with hydrogen bond and protonating interaction were observed, showing different particular amounts in each case. [7,8] T...