Nuclear magnetic resonance spectra of pyrido- and azapyridocyanines

The nuclear magnetic resonace spectra of six pyridocyanines, four azapyridocyanines, one monoquaternized 2,4′-dipyridylamine and two dipyridylamines were measured and fully analyzed. The spectra of the 2,2′-pyrido- and azapyridocyanines support previously published results that these dyes exist in the all-trans form and not in the mono-cis form, as stated by others. Strong evidence was obtained that N,N′-ethylene-2,2′-pyridocyanine is twisted. The spectra of dyes with p-connected pyridine rings indicate that these nuclei may rotate around the bond between the meso group and the p-position. Relative chemical shifts in the spectra of the homologous pyrido- and azapyridocyanines can be explained by electronic effects rather than by magnetic anisotropy of the aza group or changes in ring currents.