Tuning The Emission Of An ?-cyanostilbene Derivative By Host-guest Interaction

Due to their unique molecular structure,?-cyanostilbene derivatives possess excellent self-assembly properties,desirable optical and electrical properties and sensitivity to external stimuli.Therefore,they are considered as a suitable and versatile option for developing multifunctional materials.The desired optical properties of ?-cyanostilbene derivatives depend to a large extent on the molecular structure and intermolecular interactions.However,the general regulation of the emission position and intensity of ?-cyanostilbene derivatives requires organic synthesis steps,which is too inconvenient to carry out.So,it is essentially important to control the emission properties of a-cyanostilbene derivatives through a simply methodPreviously,our group synthesized an ?-cyanostilbene derivative,denoted by BCPB.As being dissolved in aqueous solution,BCPB emits strong emission of excimers at around 660 nm due to the formation of micelles.Three distinctly different emissions of BCPB are realized through two-step host-guest assembly driven by concentration of cucurbit[7]uril(CB[7]).For convenience of description,we define the molar concentration ratio of CB[7]to BCPB as Rm.When Rm?4,CB[7]reduces the PL intensity of excimer(660 run),and the wavelength of PL gradually changes from the emission of excimer to that of monomer(540 nm);when Rm>4,the emission bathochromic shifts from 540 nm to 588 nm.The shift of fluorescence emission is attributed to the two-step assembly of Rm between BCPB and CB[7].When Rm?4,the first CB[7]mainly binds to the pyridine group at the end of the BCPB alkyl chain,destroying the excimer formed by the BCPB molecules.As a result,the fluorescence emission peak shifted from the fluorescence emission of the monomer to 588 nm.When Rm>4,the second CB[7]began to bind to the diethylamino group(-N(Et)2)at the end of the BCPB conjugated group.which increasing the electron donating ability of the-N(Et)2 group.Therefore,the second CB[7]enhances the push-pull electron effect of the BCPB conjugated structure,resulting in red shift of the fluorescence emission peak from the fluorescence emission of the monomer to 588 nm.The cavity of CB[7]restricts the rotation of the-N(Et)2 single bond in BCPB and the hydrophobic environment provided by its cavity causes fluorescence enhancement.Using the B-H equation,we obtain the binding constants K1 and K2 of the two-step combination as 4.99× 103 and 2.27 × 103 L mol-1,respectively.This result indicates that the ion-dipole interaction between pyridinium and CB[7]is stronger than the hydrophobic interaction between-N(Et)2 and CB[7].The addition of NaCl can restore the fluorescence emission of the complex to the fluorescence emission of the excimer.In general,we need to consider many complex factors to achieve near-infrared fluorescence emission of organic fluorescent molecules.Here,we use CB[8]as the host molecule to control the luminescence of BCPB molecules to the near-infrared region through host-guest recombination under acidic conditions,avoiding tedious synthesis steps.BCPB can not only help CB[8]dissolve in aqueous solution,but also produce strong host-guest interaction with CB[8].We suspect the binding position of BCPB and CB[8]is deduced to be the pyridine position at the end of the long alkyl chain of BCPB molecule.When pH=3,after the protonated BCPB molecules interact with CB[8],the wavelength of emission peak shifts from 645 to 800 nm,which contributes to realize near-infrared fluorescence emission through host-guest interaction.By Job's plot and MALDI-TOF-MS,it was proved that the binding ratio of BCPB to CB[8]was 1:1 at pH=3.Because of long carbon chains of BCPB molecules,we suspect that BCPB molecules may enter the CB[8]cavity by U-folding,which further affects the stacking mode of BCPB molecules,resulting in red shift of flourescence.