Conformational, binding, and electrochemical studies of redox-active calixarenes

The conformational interconversions of a series of quinone substituted calix(4) arenes are studied by {dollar}sp1{dollar}H-NMR, 1D-NOE difference and NOESY/EXSY spectroscopy. A novel finding resulting from this work is that diethylethercalix(4) arenediquinone can undergo a slow conformational change, at temperatures as low as 5{dollar}spcirc{dollar}C, in the presence of Na{dollar}sp+{dollar}. In the absence of Na{dollar}sp+{dollar} this calixquinone has been reported to have fixed anti-aryl conformation at room temperature. The driving force for this process is the stability of the syn-aryl-Na{dollar}sp+{dollar} complex. The rate of interconversion is the same in the presence or absence of Na{dollar}sp+{dollar}. The slow timescale of interconversion may be an important consideration in the design of calixarene sensors. A related calix(4) arenemonoquinone also undergoes a conformational change under similar conditions, however the details of this process are not well understood. Crystal structures of both the mono- and diquinone calix(4) arenes are reported. The presence of Na{dollar}sp+{dollar} exerts a template effect on several other calix(4) arenediquinones, yielding cone conformers. {dollar}sp{lcub}23{rcub}{dollar}Na-NMR was employed in the determination of the Na{dollar}sp+{dollar} binding constants of six calixquinones. A correlation between the Na{dollar}sp+{dollar} binding constant and conformation, as determined by {dollar}sp1{dollar}H-NMR, was established.; The electrochemistry of eight calixquinones and four nitroaromatic calix(4) arenes is discussed in light of the extent of electronic communication between neighboring redox active units. Important factors correlated to the extent of electronic interaction were the distance between the redox active moieties and rotational freedom available in the calixarene framework. Redox-switchable binding of Na{dollar}sp+{dollar} and Ag{dollar}sp+{dollar} was assessed for four calixquinones. Due to their tendency to adsorb on electrode surfaces, systems with more than one electroactive unit appear to be poor candidates for electrochemical switching. A comparison of the cyclic voltammetric response of two different conformers of the diethylethercalix(4) arenediquinone in the presence and absence of Na{dollar}sp+{dollar} reveals that the two conformations have differing stabilities in their reduced states. The syn-aryl conformer and its Na{dollar}sp+{dollar}-complex were found to be more readily reduced than the corresponding anti conformer and its complex.; The ESR spectra of several calix(4) arenes were also examined. The spectra are complicated by exchange processes, including linewidth alternation effects. Conformational exchange may contribute to the spectral complexity.