Design, Synthesis and Characterization of new Supramolecular Architectures

Supramolecular chemistry is a highly interdisciplinary field that has developed astonishingly rapidly in the last four decades. In the late 1980s, following the award of the 1987 Nobel Prize to Pedersen, Cram, and Lehn, there was a sudden increase of interest in supramolecular chemistry, a highly interdisciplinary field based on concepts such as molecular recognition, preorganization, and self-assembling.

Dendrimers are globular size, monodisperse macromolecules in which all bonds emerge radially from a central focal point or core with a regular branching pattern and with repeating units constituting the branching points. The term dendrimer refers to its characteristic tree-like structure and it derives from the Greek word dendron (tree) and meros (part). From a topological viewpoint, dendrimers contain three different regions: core, branches and surface. Each repetition synthetic cycle leads to the addition of one more layer of monomers in the branches, called generation. Therefore, the generation number of the dendrimer is equal to the number of repetition cycles performed and to the number of branching points present from the core towards the periphery.

All chemicals were purchased from Aldrich and were used without further purification. Solvents were purchased from Aldrich and purified according to literature procedures. Thin-layer chromatography (TLC) was performed on aluminum sheets coated with silica gel 60F (Merck 5554). The plates were inspected by UV light. Column chromatography was performed on silica gel 60 (Merck 40–60 nm, 230–400 mesh). High-performance liquid chromatography (HPLC) grade MeCN was purchased from Aldrich and degassed by bubbling He. Known amounts of the compounds were dissolved in MeCN (HPLC grade) to afford stock solution of known concentration (0.001 M). The resulting solutions (injected volume 10 μL) were analyzed at ambient temperature by HPLC (flow rate 1.0 mL min⁻¹; mobile phase, pump A 0.1% CF₃CO₂H in H₂O, pump B MeCN/0.1% CF₃CO₂H in H₂O/95:5; time (min)/pump A (%) = 0/100, 8/100, 28/0, 42/0, 45/100) by employing a Hypersil BDS C18 column (length 25 cm, inside diameter 5.6 cm) operated by HP 1090 (Hewlett–Packard) connected to thermo surveyor UV–Vis diode array detector. Melting points were determined on an Electrothermal 9200 apparatus and reported uncorrected. ¹H and ¹³C spectra were recorded on Varian Mercury 400 (400 and 100 MHz, respectively), using residual solvents as the internal standard. Samples were prepared using deuterated solvents purchased from Cambridge Isotope Laboratories. The chemical shift values were expressed as δ values, and the couplings constants (J) are in Hertz. The following abbreviations were used for signal multiplicities: s, singlet; d doublet; t triplet; m multiple; br broad. Electron impact mass spectra (EIMS) were obtained from VG Prospec mass spectrometer.

Dendrimers are repetitively branched, yet constitutionally well-defined macromolecules, exhibiting high monodispersity. These nanoscopic compounds can contain selected chemical units in predetermined sites of their structure (core, branches, periphery) whose mutual interactions can lead to unusual, sometimes unpredictable, chemico-physical properties resulting in a wide range of potential applications. Dendrimers containing photoactive and/or electroactive moieties are currently attracting much attention since they can be designed to perform useful functions such as light-harvesting.

Molecular recognition is defined by the information and the energy involved in selection and binding of substrate(s) by a given receptor molecule. Molecular recognition phenomena imply the formation of supramolecular species characterized by peculiar structural, thermodynamic, and kinetic features determined by molecular information stored in receptor and substrate(s) that lead to selective binding. Stability of the supramolecular host–guest complex substantially depends on the medium and results from a subtle balance between solvation (of both receptor and substrate) and complexation (i.e., “solvation” of the substrate by the receptor).

Pseudorotaxanes are supramolecular systems minimally composed of an axle-like molecule surrounded by a macrocyclic component. These peculiar host–guest systems represent convenient precursors for the synthesis of more complex interlocked structures, such as rotaxanes and catenanes, which can operate as simple molecular machines. Essentially, the functioning mode of a pseudorotaxane consists of an external stimulus-promoted reversible threading–dethreading process. The structural information stored in both axle and wheel should match (be complementary) from a dimensional point of view as well as for the inter-component interactions responsible for the stability of the complex. A common feature of these systems is that the macrocycle resides in a precise position (station) along the axle, which is also the structural unit that responds to the external stimuli governing the dethreading–threading process. Typically this unit is positioned in a more or less central portion of the axial component. In a rotaxane, the two stoppers present at the termini of the axial component, by acting as a structural tie, prevent dethreading.

Molecular machines—(supra)molecular systems in which large-amplitude motions of some components can be controlled by appropriate stimuli can be operated by means of chemical, electrochemical, or photochemical processes. However, the use of light stimulation has several advantages. For example, photons can be used to supply energy to the system and to gather information about its state without physically touching it. Light excitation can be carried out by a variety of sources (including the sun), with the possibility of a fine resolution in space and time.

Pseudorotaxanes complexes can be rendered kinetically inert, i.e. transformed into rotaxanes, by attaching bulky groups at the extremities of the axle to prevent dethreading. Therefore, the pseudorotaxane or rotaxane behavior of a given axle-macrocycle pair depends on the threading–dethreading rate constants, which in turn are determined by the intrinsic kinetic parameters of such processes—particularly, the energy barriers—and the experimental conditions.

Here we show that the reversible acid/base switching of the absorption and photoluminescence properties of a fluorophore as simple as 8-methoxyquinoline in solution can form the basis for molecular 2:1 multiplexing and 1:2 demultiplexing with a clear-cut digital response.