The Azine Bridge as a Conjugation Stopper: An NMR Spectroscopic Study of Electron Delocalization in Acetophenone Azines

Dipole parallel-alignment of organic molecular crystals of azines has been achieved with a design that was based on the hypothesis that the azine bridge is a conjugation stopper. This hypothesis has now been tested in detail, and ¹ H and ¹³ C NMR spectroscopic data of symmetric and unsymmetric acetophenone azines are presented in support of this design concept. Previous structural, ab initio, and electrochemical studies have shown that the azine bridge largely inhibits through-conjugation in molecules with the general structure DPhC(Me)=N-N=C(Me)PhA, where D is a donor group and A is an acceptor group. NMR spectroscopy is an excellent tool to probe the degree of conjugation through the azine bridge. The NMR results reported here for nine symmetrical and 18 unsymmetrical azines show in a compelling fashion that the hypothesis holds. Varying the donor group does not change the chemical shifts of the aromatic hydrogen and carbon atoms on the acceptor-substituted phenyl ring. Likewise, varying the acceptor group does not change the chemical shifts of the atoms in the donor-substituted phenyl ring.