Electrochemical Studies of a Redox Responsive 4 H-Bond Array Capable of Self-Dimerization

Tuesday, 30 May 2017: 16:40
Churchill A2 (Hilton New Orleans Riverside)
G. M. Darzi and D. K. Smith (San Diego State University)
Supramolecular structures can change in response to external signals, such as change in pH, temperature, or voltage of an electrode. This is important for many applications such as self-healing polymers and gels, triggered release of entrapped molecules for drug delivery, and smart material. Ureidopyrimidinone (UPy) array with an alkyl-pyridinium (RP) redox center has been synthesized for this study. This array prefers the tautomer that presents an ADAD H-bond motif in the starting oxidation state. Due to electrostatic repulsions and unfavorable secondary H-bond interactions, this motif would form a dimer with relatively weak H-bonding. Upon 2e- reduction, where 1e- is gained per R-pyridinium redox center, the H-bond strength should increase due to the loss of the repulsive charges, making the nitrogen a stronger hydrogen acceptor. Because the nitrogen is now a better hydrogen acceptor, there could be a possibility of an intermolecular proton transfer. This would encourage the tautomer to have an AADD motif that will make the H-bonding stronger by increasing the favorable secondary H-bond interactions.

The UPy(RP) array is not soluble in dichloromethane using the NBu4PF6 electrolyte, but it is soluble in acetonitrile. The concentration dependent 1H NMR spectra suggest that UPy(RP) is a monomer in acetonitrile. The cyclic voltammetry scans of the UPy array in CH3CN show three reductions. The first occurs at a potential very similar to that seen with simple model compounds. The second is considerably positive of that observed for the model compounds and the third is very close to the second reduction peak of the model compounds. The first reduction is reversible if the scan direction is switched immediately after the peak, but irreversible if the scan direction is switched after the second reduction peak. As the scan rate increases, the second reduction peak disappears. An oxidation peak at far positive potentials starts to appear only after going through the second and third reduction. DFT calculations suggest the preferred tautomer changes upon reduction, and this may explain the unexpected voltammetry observed in acetonitrile where the compound is not dimerized. To improve the solubility of the UPy array in methylene chloride, TFAB was used as a counterion instead of the PF6. The CV’s of UPy/TFAB array in methylene chloride show similar behavior to that in acetonitrile with an additional fourth reduction wave observed at far negative potential. 1H NMR spectrum of UPy/TFAB at room temperature suggests the presence of two tautomeric forms in methylene chloride. However, at low temperatures, only one tautomer is observed in 1H NMR spectrum that’s believed to be the dimer. Further studies to help elucidate what is actually happening in this system will include taking CV’s at low temperatures in CH2Cl2, and conducting UV-Vis spectroelectroanalytical experiments for the system.