Delving into the Variability of Supramolecular Affinity: Self-Ion Pairing as a Central Player in Aqueous Host-Guest Chemistry.

The determination of binding constants is a key matter in evaluating the strength of host-guest interactions. However, the profound impact of self-ion pairing on this parameter is often underrated in aqueous solution, leading in some cases to a misinterpretation of the true potential of supramolecular assemblies. In the present study, we aim to shed further light on this critical factor by exploring the concentration-dependent behavior of a multicharged pillararene in water. Our observations reveal an extraordinary 1-million-fold variability in the affinity of this macrocycle toward a given anion, showcasing the highly dynamic character of electrostatic interactions. We argue that these findings bring to the forefront the inherent determinism that underlies the estimation of affinity constants, a factor profoundly shaped by both the sensitivity of the instrumental technique in use and the intricacies of the experimental design itself. In terms of applications, these results may provide the opportunity to optimize the operational concentrations of multicharged hosts in different scenarios, aiming to achieve their maximum efficiency based on the intended application. Unlocking the potential of this hidden variability may pave the way for the creation of novel molecular materials with advanced functionalities.

Molecular Recognition by Pillar[5]arenes: Evidence for Simultaneous Electrostatic and Hydrophobic Interactions.

The formation of inclusion complexes between alkylsulfonate guests and a cationic pillar[5]arene receptor in water was investigated by NMR and ITC techniques. The results show the formation of host-guest complexes stabilized by electrostatic interactions and hydrophobic effects with binding constants of up to 107 M-1 for the guest with higher hydrophobic character. Structurally, the alkyl chain of the guest is included in the hydrophobic aromatic cavity of the macrocycle while the sulfonate groups are held in the multicationic portal by ionic interactions.

Pillar[5]arene-Based Supramolecular Plasmonic Thin Films for Label-Free, Quantitative and Multiplex SERS Detection.

Novel plasmonic thin films based on electrostatic layer-by-layer (LbL) deposition of citrate-stabilized Au nanoparticles (NPs) and ammonium pillar[5]arene (AP[5]A) have been developed. The supramolecular-induced LbL assembly of the plasmonic nanoparticles yields the formation of controlled hot spots with uniform interparticle distances. At the same time, this strategy allows modulating the density and dimensions of the Au aggregates, and therefore the optical response, on the thin film with the number of AuNP-AP[5]A deposition cycles. Characterization of the AuNP-AP[5]A hybrid platforms as a function of the deposition cycles was performed by means of visible-NIR absorption spectroscopy, and scanning electron and atomic force microscopies, showing larger aggregates with the number of cycles. Additionally, the surface enhanced Raman scattering efficiency of the resulting AuNP-AP[5]A thin films has been investigated for three different laser excitations (633, 785, and 830 nm) and using pyrene as Raman probe. The best performance was shown by the AuNP-AP[5]A film obtained with two deposition cycles ((AuNP-AP[5]A)2) when excited with a 785 laser line. The optical response and SERS efficiency of the thin films were also simulated using the M3 solver and employing computer aided design models built based on SEM images of the different films. The use of host molecules as building blocks to fabricate (AuNP-AP[5]A)2) films has enabled the ultradetection, in liquid and gas phase, of low molecular weight polyaromatic hydrocarbons, PAHs, with no affinity for gold but toward the hydrophobic AP[5]A cavity. Besides, these plasmonic platforms allowed achieving quantitative detection within certain concentration regimes. Finally, the multiplex sensing capabilities of the AuNP-AP[5]A)2 were evaluated for their ability to detect in liquid and gas phase three different PAHs.

Investigation of the binding modes of a positively charged pillar[5]arene: internal and external guest complexation.

The selective binding behavior of a trimethylammonium-derived pillar[5]arene towards different guests in aqueous media and under neutral conditions is reported. Although it is known that this macrocycle has the capability to form complexes with guests, we anticipate that the intrinsic pillar shape of the macrocycle with two positively charged rims should allow a diversity of binding modes. The three guests were selected based on their charge and size. The inclusion binding modes and the affinity of the macrocycle to form host-guest complexes were determined by ITC and NMR techniques. We reveal the ability of a cationic water soluble pillar[5]arene to effectively complex two guest molecules, one in each rim, evidencing the diversity of binding modes. Two different structures for 1 : 1 and three for 1 : 2 complexes are reported showing the pillararene ability for internal/external binding.

Supramolecular phosphate transfer catalysis by pillar[5]arene.

A kinetic study on dinitrophenylphosphate monoester hydrolysis in the presence of a cationic pillararene, P5A, has been carried out. Formation of the supramolecular complex between phosphate ester and P5A has been studied by NMR showing complexation-induced upfield proton shifts indicative of aromatic ring inclusion in the pillararene cavity. Molecular dynamic calculations allow structure characterization for the 1 : 1 and 1 : 2 complexes. As a result of the supramolecular interaction both the acidity of DNPP and its hydrolysis rate constants are increased. Catalysis results from combination of both electrostatic stabilization reducing the negative electron density on the PO3(=) oxygens and monoester dianion destabilization by the steric effects of close NMe3(+) groups hindering the hydrogen-bonding with water and destabilising the monoester dianion.