An NMR study on the conformation of naphthalene-viologen linked compounds:effect of flexible spacer length

The conformational equilibria of naphthalene-viologen compounds linked with a flexible polymethylene bridge [-(CH2)(n)-; n = 3, 5, 7, 9, 12] were exam ined by NMR spectroscopy and molecular modeling. The chemical shift of the donor-acceptor dyad molecules depends on the linkage length and concentrati on. The equilibrium constants for-the intramolecular and the intermolecular complexation were obtained from the chemical shift variation with concentr ation at high salt concentration. The complexation-induced chemical shifts (CISs) of the intra- and intermolecular complexes are obtained for molecule s of n = 3 and 5. The molecules with longer linkages (n = 9 and 12) show ag gregation behavior and the critical coagulation concentrations were obtaine d as 1.7 and 10 mM, respectively. Comparing the NMR results with molecular modeling calculations indicates that the naphthalene moiety can take variou s positions and orientations with respect to the bipyridinium moieties depe nding on the length of the linkage.