Design,synthesis And Performance Of Multi-response Fluorescent Molecular Switch Based On Aie

Aggregation-induced emission(AIE)refers to a phenomenon in which a class of molecules that do not fluoresce or weakly fluoresce in a dilute solution has a significantly enhanced luminescence ability in the state of aggregation.The AIE phenomenon has a unique advantage.It fundamentally overcomes the problem of fluorescence quenching caused by aggregation and has been extensively and deeply studied.Stimulus-responsive color-changing materials are a kind of intelligent materials.Under the stimulation of external environment(such as light,heat,electricity,pH,force(pressure),etc.),the emission wavelength/intensity of fluorescence or phosphorescence can correspondingly change.Molecular switches,a class of molecules with at least two completely different stable states,can also switch between different stable states under the external stimulus.The combination of stimulus-responsive color-changing materials and molecular switches enables the designed fluorescent molecules to respond to a variety of stimuli and realize the switching of luminous color and intensity,which can further broaden the practical value of fluorescent molecules.Taking this as a starting point,this thesis designed and synthesized some fluorescent molecular switches with multi-stimulus responsiveness based on the unique nature of AIE,which can achieve multi-color changes in response to multiple stimuli and achieve high fluorescence switch contrast and high sensitivity in the aggregate state.For the synthesized fluorescent molecules,we mainly focus on the force-induced discoloration behavior in the aggregation state upon many stimulus.The main content of this thesis is divided into the following three parts:(1)A new small molecule fluorescent switch TPE=C4 with a twisted structure of D-A(Donor-Acceptor)was designed and synthesized.Its fluorescence emission showed a clear color change from blue to green when it was stimulated by force in the solid state,and after hydrostatic pressure is applied,the powder sample and the single crystal sample will show different fluorescence response behaviors.As the hydrostatic pressure increases,the powder sample shows a phenomenon of turning from the initial blue to grass green to fluorescent color black.The color change mechanism is due to the force-induced separation of HLCT and the quenching of the ICT state caused by high pressure;the powder emits light after pressure release.After the pressure is released,the powder luminescence cannot be completely restored to the initial state,but the high-pressure induced TPE=C4 crystal discoloration is completely reversible.In addition,when acid was used to fumigate the original TPE=C4 powder,fluorescence discoloration from blue to yellow could also be observed.(2)Designed and synthesized a dichotomous RHB=TPE,which has different color-changing response to anisotropic grinding and isotropic hydrostatic pressure stimulation.After being grinded,the emission color of the fluorescent molecules will change from blue to green and then to red,the mechanism of color change is the polarization of TPE and the ring opening of rhodamine lactam;when subjected to hydrostatic pressure,the color of the single crystal will change from blue to green and then darken.In addition,after acid fumigation of RHB=TPE molecules,high-contrast sequential color switching of molecular fluorescence from blue to yellow to red can be achieved.The color change mechanism is due to the protonation of the imine group.(3)A molecule with Excited-State Intramolecular Proton Transfer(ESIPT)luminescence properties was designed and synthesized.This molecule skillfully combined the spiro ring structure with ESIPT.It has the characteristics of mechanochromism in the solid state,significant piezochromic in the crystal,photochromism,unique acidichromism and solvatochromism.When grinding in the solid state or applying hydrostatic pressure,the force-induces the ring-open reaction of the molecule,which causes the ESIPT to switch from "ON" to "OFF",leading to the emission color change from yellow-green to red.In the polymer matrix,the molecules exhibit reversible photochromic behavior.For solvatochromism,the molecule emits yellow-green luminescence(ketone luminescence)in a non-polar solvent,and blue-green luminescence(enol luminescence)in a polar solvent.When acid or base is added to the dilute solution of the molecule,the blue shift of the emission peak and the increase in the intensity of the emitted light are completely different from those of traditional rhodamine and its derivatives.