Stimuli-responsive fluorescence switching of cyanostilbene derivatives: ultrasensitive water, acidochromism and mechanochromism.

A novel donor-π-acceptor structure stimuli-responsive fluorescent material of ()-2-(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl)-3-(pyridin-2-yl)acrylonitrile (N-TPA) was designed and synthesized, with the cyano-group and pyridine as the acceptors (A) and triphenylamine as the donor (D). N-TPA exhibits an obvious solvatochromic effect and the excited state is confirmed to be a hybridized local and charge-transfer (HLCT) state that simultaneously possesses the locally-excited (LE) state and charge transfer (CT) state characters. The LE state ensures relatively high fluorescence efficiency while the CT state provides multi-stimuli responsive fluorescence behaviors because it is easily tuned by the surrounding environment. Firstly, N-TPA exhibits "on-off-on" fluorescence properties in the mixture of water/tetrahydrofuran (THF) with the increasing water content. For the "on-off" part, a good linear relationship between fluorescence intensity and water fraction is achieved, which is ascribed to the synergistic effect of protons in water and intramolecular charge-transfer (ICT) effect depending on solvent polarity. The "off-on" part demonstrates the aggregation-induced enhanced emission (AIEE) character of N-TPA. Secondly, N-TPA can be used as a protonic acid sensor to detect trifluoroacetic acid (TFA) in solvent and HCl vapour in the solid state due to the binding of the proton to the pyridine group. Finally, N-TPA presents remarkable and reversible mechanochromic fluorescence switching between 552 nm and 642 nm (90 nm red-shift) during the pressurizing-depressurizing process. This work not only comprehensively demonstrates the stimuli-responsive fluorescence behaviors of N-TPA, but also provides a D-π-A structure fluorescent material possessing potential applications in detection and sensing with remarkable fluorescence changes.