Reversible 2D Supramolecular Organic Frameworks encompassing Viologen Cation Radicals and CB[8]

Abstract

Reversible 2D supramolecular organic frameworks encompassing branched viologen architectures and cucurbit[8]uril (CB[8]) were constructed and investigated. UV-vis investigation clearly indicates the formation and intermolecular dimerization of monocation radicals and their encapsulation into the hydrophobic CB[8] cavity which is further complemented by EPR (electron paramagnetic resonance) spectroscopy. Particle size measurements by dynamic light scattering method showed particle sizes in the range of several µm indicating larger aggregates. Zeta potential measurements suggested the instability of these particles and their tendency to form aggregates. TEM (transmission electron microscope) analysis further revealed the formation of supramolecular polymer (monocation radical with cucurbit[8]uril) whose diameter were in the range of several µm as indicated by DLS measurements; however the oxidized form, i.e., the viologen dication with cucurbit[8]uril showed dotted spots in the range of sub nanometer level. The internal periodicities of the supramolecular polymers were analyzed by SAXs (small angle X-ray scattering) measurements. Additionally, we have demonstrated that these supramolecular organic frameworks can be depolymerized by oxidation in air and again can be polymerized (intermolecular radical dimerization) by reduction under inert atmosphere demonstrating that these systems will be of broad interest.

Figure 1

Structure of cucurbit[8]uril (CB[8]) host and viologen guest molecules (EV, EDV, ETV & ESV).

Figure 2

Synthesis of star shaped-viologen (ESV): Reaction conditions; a) CH₃CN, 80 °C, 2 d; b) 3 M NH₄PF₆/H₂O.

Figure 3

The absorption spectrum of (a) EV, EDV, ETV & ESV (0.25 mM) and (b) the absorption spectrum in the presence of CB[8], [CB[8]] = 1.0, 2.0, 3.0 and 6.0 equiv. for EV, EDV, ETV & ESV respectively) in sodium phosphate buffer (0.1 M) solution containing sodium dithionite (50 mM), RT.

Figure 4

EPR spectra of EV, EDV, ETV & ESV (0.50 mM) in sodium phosphate buffer (0.1 M) containing sodium dithionite (50 mM) (a) in the absence and (b) in the presence of CB[8], RT. (CB[8] = 1.0, 2.0, 3.0 and 3.0 equiv. for EV, EDV, ETV and ESV respectively).

Figure 5

Cyclic voltammograms of Viologen derivatives (a) EV, (b) EDV, (c) ETV and (d) ESV (1.0 mM) in aqueous KCl (0.1 M) on glassy carbon (0.07 cm²) in the absence (black line) and the presence (red line) of CB[8] (1.0 mM, (1.0 equiv. for all viologens)) at V = 0.1 V/s, RT.

Figure 6

Particle size analysis of EV, EDV, ETV & ESV (0.2 mM), measured in aqueous solution containing excess of sodium dithionite (a) in the absence and (b) in the presence of CB[8] (0.1 mM), RT.

Figure 7

TEM images of ETV (0.1 mM) + CB[8] (3 equiv.) before (a,b) and after reduction (c,d) with sodium dithionite.

Figure 8

Thin layer SAXs profile of EDV, ETV, ESV (c = 0.2 mM) in the presence of CB[8] (0.1 mM) and sodium dithionite.

Figure 9

Scheme depicting the supramolecular self assembly process involved in EDV, ETV and ESV.

Figure 10

UV-vis reversibility study of EDV (0.25 mM) with CB[8] (2.0 equiv.) was monitored in the aqueous solution containing excess of sodium dithionite; (a) after degassing with N₂; inset shows the absorption of dimer, and (b) after O₂ bubbling, RT.