| Abstract
| - We have prepared and fully characterized by means of multinuclear NMR and UV−vis spectroscopya series of pyridoneimines and pyridonemethides in order to show how it is possible to finely tuneπ-electron structure properties by properly exploiting substituent and solvent effects. Substituentswith different electron-withdrawing capacities were introduced in pyridoneimines 2−4, pyridonemethides 5 and 6, and pyridine sulfonamido derivatives 7−9. The anisochrony of the carbonposition of the azinium ring (geometric isomerism) and the exploitation of previously reported 13Cand 15N shift/π-electron density relationships allowed the investigation of the extent of intramolecular charge transfer from the donor group to the acceptor pyridinium moiety. By combiningdifferent substitutions with the polarity of the surrounding media, we were able to access a wholerange of push−pull electron structures in solution, from fully aromatic-zwitterionic to quinoid-neutral, through many possible intermediate situations along the path. Due to the strict correlationbetween the π-electron structure of push−pull derivatives and many photonic properties suchas nonlinear optical activity, we believe that the achieved results should be valuable for thedevelopment of new efficient, tailor-made, heteroaromatic systems with optimized features asadvanced organic materials.
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