Insight into the Isoelectronic Character of Azomethines and Vinylenes Using Representative Models: A Spectroscopic and Electrochemical Study

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.