Development Of Novel Aggregation-induced Emission System Based On Nitration

Recently,organic fluorescent materials with aggregation-induced emission(AIE)characteristics have drawn increasing attention in the application fields of organic light-emitting diodes,organic lasers,chemical sensors,biological probes.In contrast,organic fluorescent molecules with nitro group substitution have rarely been reported.That is reasonable,since nitro group is generally considered to be fluorescence quenching.It will lead to intramolecular charge transfer and intersystem crossing,which are both detrimental to fluorescence.In this paper,we are dedicated to the exploration of nitro-group-based AIE system,investigation on the substitution effect of nitro group on chromophore and development of new AIE materials with excellent fluorescence properties and nitro group substitution.Taking advantage of the Suzuki coupling reaction and aromatic nucleophilic substitution reaction,we synthesized eight organic molecules with nitro group substitution.The eight organic molecules can be classified into two categories:the C-C system,which was synthesized by Suzuki coupling reaction,and the C-N system,which was synthesized by aromatic nucleophilic substitution reaction.All the resulting molecules were characterized with ~1H NMR and high-resolution mass spectroscopy(HRMS),from which satisfactory results corresponding to their molecular structures were obtained.The thermal stability and phase transition temperature of the molecules were determined by TGA and DSC,respectively.Generally,nitro group leads to fluorescence quenching through two pathways,thus we utilized two systems to investigate the quenching mechanism,the C-N system and C-C system.In C-N system,nitrobenzene is directly bridged to the electron-rich N atom,while in the C-C system,nitrobenzene is bonded to the C atom of the chromophore.By use of the UV-Vis spectrometry and fluorescence spectrometry,we measured the UV absorption and fluorescence spectra of the fluorescent materials.The intramolecular charge transfer ability was determined through calculating the red-shift extent of the maximum absorption peak and fluorescence emission peak in solvents with different polarity.The results show that the C-N system has a strong charge transfer effect in the molecules,while in the C-C system the intramolecular charge transfer ability is weaker.By comparison of their luminescence behavior in different forms and in solvents with different polarity,together with the molecular packing in the crystal,we will gain detailed information on the role of AIE chromophore and nitro group on their fluorescence properties.Through the exploration of C-C system and C-N system,the role of nitro group in different systems was uncovered.For the C-N system,the molecules have a large torsion angle and red shift wavelength,so the nitro group affects the fluorescence properties through intramolecular charge transfer.Under the control of ICT and AIE,molecules exhibit different luminescence behaviors in different morphologies.For the C-C system,the molecules have small torsion angle and red shift wavelength,which can be ascribed to the nitro-group-facilitated intersystem crossing.For the dibenzofuran system,when the torsion angle between the benzene ring and the chromophore is 39.72°and 34.02°,the nitro group will promote the excitons in the singlet excited state to the triplet state through intersystem crossing,which will facilely dissipated through the non-radiative channel,resulting in fluorescence quenching.