Insight into the isoelectronic character of azomethines and vinylenes using representative models: a spectroscopic...

A series of azomethine and vinylene dyad and triad analogues were prepared. Their absorbance, fluorescence, and redox properties were examined experimentally and theoretically using density functional theory (DFT) calculations. These measurements were done to determine the effect of the heteroatom of the azomethine relative to its all-carbon counterpart and to assess the isoelectronic character of the two bonds. The orientation of the azomethine was found to have little effect on the absorbance, fluorescence, and electrochemical properties. In contrast, the spectral and electrochemical properties were highly contingent on the electronic groups and degree of conjugation. The spectral properties could be tuned 200 nm across the visible region. More importantly, the heteroatom in the conjugated bond was found to give rise to only a 20 nm bathochromic shift in the absorbance and fluorescence spectra. The fluorescence quantum yield (ΦFl) of the vinylene derivatives varied between 5% and 20% with fluorescence quenching occurring by photoisomerization from the E to Z isomers. In contrast, the fluorescence of the analogous azomethine derivatives was completely quenched. The collective spectroscopic and electrochemical ab initio DFT data additionally confirmed that the azomethine and its analogous vinylene are isoelectronic. It was also found that a conjugated thiophene vinylene dyad with primary amines in the α,α'-positions could be prepared and isolated. The compound was stable under aerobic conditions providing electron withdrawing (either ester or nitrile) groups were located in the adjacent positions.