Structure-Function Relationship And Fluorescence Sensing Of Naphthalimide Containing Conjugated Polyelectrolytes

Conjugated polyelectrolytes(CPEs)play an important role in the fields of biosensing,bioimaging,and drug delivery.The introduction of electron donor-receptor(D-A)units on the conjugated polymer backbones typically results in lower band gaps and larger Stokes shifts due to the nature of intramolecular charge transfer(ICT).The appearance of such polymers is of great significance for improving the photophysical properties of common CPEs,thereby enhancing their competitiveness in the field of fluorescence sensing and fluorescence imaging.However,due to the symmetrical forbidden nature of the twisted intramolecular charge transfer(TICT)state and the nonradiative relaxation of TICT excited state by intramolecular rotations,many D-A type CPEs with simple side chains usually showed quite low fluorescence quantum yields(QYs)in high polar solvents such as water and methanol,which greatly limits their application.In addition,common methods for increasing the fluorescence quantum yields of D-A type CPEs in highly polar solvents,such as encapsulating polymer with large-sized hydrophilic shell,constructing rigid backbone structure,and building aggregation induced emission(AIE)system,are still many shortcomings.Therefore,it is of great significance to develop new methods for improving the fluorescence quantum yields of D-A type CPEs in water.In this paper,a series of new D-A type CPEs were designed and synthesized.Their structure-function relationship and application in fluorescence sensing were studied indepth.The details are as follows:(1)Several D-A type CPEs with different linkage,p PNPEs and m PNPEs,were synthesized using 1,8-naphthalimide monomer and phenylene acetylene monomers.Compared with typical PPEs,the CPEs have many excellent properties,including redshifted fluorescence emission and large Stokes shift,which have potential in fluorescence sensing and bioimaging.(2)The photophysical properties of p PNPEs and m PNPEs were characterized and their structure-function relationships were studied.The results showed that,the backbone of p PNPEs remained stretched in aqueous solution similar to the previously reported DA type CPEs with simple side chains,resulting in extremely weak fluorescence emission due to the intramolecular charge transfer(TICT)feature.While the meta-substituted m PNPEs can form order helical structures due to hydrophobic interaction in water,and inhibited the TICT effect to achieve an increase in fluorescence quantum yields.This method of controlling the conformation of the polymer by changing the linkage of monomrs provided a simple and effective strategy for improving the fluorescence quantum yields of D-A type CPEs in water.(3)The helical m PNPE-2 with relatively high fluorescence quantum yield,good solubility and large Stokes shift in water was applied to fluorescence sensing.A visual,fast and sensitive method for detection trace methylene blue in water was developed in this artical.