Synthesis and Photoinduced Processes of Donor-Acceptor Substituted Diarylbutadienes

Several donor-π-acceptor systems based on diarylpolyene scaffold were synthesized and examined for their fluorescence and photochemical properties. The fluorescence investigations of these dienes reveal weak to strong solvatochromism attributed to intramolecular charge transfer (ICT). The systems containing nitro as acceptor, exhibit very weak quantum yields. Changing the position of nitro group from para, meta or to ortho results in further loss in emission. Interestingly, simple para (or meta) nitro diarylbutadienes show inefficient photoisomerization. However, introduction of a methyl group on the double bond in the same system yields regioselective isomer of methyl substituted double bond. Likewise presence of nitro group at ortho position also yields regioselective isomer of the double bond distal to the aromatic nitro group. Such behaviour is explained by (a) steric effect of methyl group that induces non-planarity and (b) the intramolecular C − H … . O hydrogen bonding that inhibits the isomerization of the proximal double bond. Similar photoisomerization observations were seen with a pyridyl group in place of phenyl ring. NMR and single crystal X-ray data substantiates these observations. Incorporation of suitable donating groups on phenyl ring of an arylpolyene scaffold bearing pyridine as an acceptor reveals remarkable fluorescence properties with strong bathochromic emission shifts. Methylation of pyridine results in a positively charged pyridinium ion which enhances the electron withdrawing ability and show strongly red-shifted emission near ~700nm. Such a strong emission, coupled with water-solubility can have useful applications as fluorescence probes for biological media.