Synthesis and manifold but controllable emission switching of stilbene-appended polyaromatic terpyridine derivatives via aggregation and trans-cis isomerization

We designed in this work a new series of stilbene-appended terpyridine systems covalently coupled with anthracene, naphthalene and pyrene moieties which can exhibit multiple yet controllable photo-switching behaviors under divergent conditions. Upon aggregation, these emissive compounds display either remarkable blue-shifted emission enhancement or emission quenching with red-shift, depending upon the nature of solvent. Both free-as well as aggregated forms of the compounds undergo trans-* cis isomerization upon treating with visible light, accompanied with significant alteration in their spectral behaviors. The reverse cis-* trans process is achieved on keeping the photolyzed solutions in dark. Hence, 'on-off' and 'off-on' emission switching is feasible upon successive exposure to light and keeping in dark. A gradual decrease in rate constant and quantum yield of photoisomerisation was observed with increase in extent of aggregation. Additionally, DFT and TD-DFT calculations are also carried out to interpret experimental spectral bands of the compounds.