Mechanofluorochromic Properties Of D-A Typed Phenothiazine Derivatives

Mechanofluorochromic(MFC)materials are a kind of materials which are affected by mechanical force,namely their fluorescence emission spectrum will be changed significantly under the stimulation of the external force.Their fluorescence can be restored to its original state by annealing or fuming.It is widely used in various fields such as fluorescent switch,memory chip,fluorescence sensing,data storage and safe ink.At present,it was found that special structure D-π-A molecule generally showed good MFC behavior.The materials include tetraphenylethene,9,10-divinylanthracene and oligo(p-phenylene vinylene),as well as β-diketone boron complexes and phenothiazine derivatives,and so on.As is well known it is difficult to design a molecule which shows mechanofluorochromic properties,it is a challenging task to design a simple,novel MFC dye.Designing of novel structure simple force fluorescence dye is a challenging subject.The non-planar butterfly structure,strong giving-electrons ability and the various easy modifying substitution of phenothiazine make its derivatives as fluorescence materials.In this article,we have designed new phenothiazine derivatives as a starting point,examining the effecting of the receptor and the length of alkyl chain on the performance of MFC,obtained the following innovative results:(1)A dibenzoyl phenothiazine derivative containing two benzoyl groups has been synthesized.The new phenothiazine gave strong emission in solution and in solid state,and the solid fluorescence quantum yield was 0.63.Notably,the solid-state luminescent properties of EPBPA not only depended on the molecular structure,but also on the molecular packing in the solid.In crystal,we found EPBPA had three hydrogen bonds,which were C-H? C(2.864 ?),C-H? C(2.850 ?),C-H? O(2.652 ?).The twisted conformations of the molecule lead to fixe molecular conformation.EPBPA showed reversible mechanofluorochromic properties.The XRD and DSC results show that the mechanofluorochromic nature is generated through crystalline amorphous phase transformation under external pressure.(2)A series of D-A typed phenothiazine derivatives functionalized by formyl group PCAn(n=1,2,4,6)with different lengths of N-alkyl chains have been designed andsynthesized.The results showed that these compounds emitted strong fluorescence in solutions and solid states,the yields of their solid-state were 52%,42%,49% and 45%.We found that PCA1,PCA2 and PCA4 all showed the mechanofluorochromic properties.Interestingly,the color change of PCAn solid can be observed by naked eyes.It was observed that the pressed PCAn samples emit yellowish-green to orange colors from PCA1 to PCA4 under UV illumination of a 365 nm lamp.However,when the pressed samples were annealed before the isotropic melt transition or exposed to solvent vapor(fuming above dichloromethane for 60s),green or yellowish-green fluorescence colors appeared again.But this phenomenon did not happen for PCA6.It was quite clear that,the PCAn with longer alkyl chains(n = 2,4)exhibited more remarkable fluorescence color changes between pressed and annealed(or fumed)states(from orange to yellowish-green)than that with shorter alkyl chain(n = 1,from yellowish-green to green).Furthermore,when the fumed or annealed samples were repressed,the fluorescence colors were again changed as the first pressing.In addition,the process could be repeated many times,which suggested it is a reversible MFC behavior.XRD and DSC reveal that the transformation between crystalline and amorphous states upon various external stimuli is responsible for the MFC behavior.The results also showed that the introduction of different alkyl chains had a certain effect on the property of the mechanofluorochromic materials.