Design And Synthesis Of Solid-state Fluorescent Squaraines For The Study Of Micro/Nanocrystal Optical Properties

Squaraine dye molecules are widely used in photovoltaic cells,biomarkers,biosensing,photodynamic therapy,photocatalysis and nonlinear optics.However,almost all common squaraine dye molecules suffer from aggregation-caused quenching(ACQ)phenomenon,and the fluorescence will become weak in the solid state,or even not emit light.Therefore,the development of its application in organic solid-state devices is hindered.In recent years,squaraine dye molecules with solid fluorescent properties have gradually appeared,but the study of their luminescence mechanism has remained at the macroscopic aggregation-induced emission(AIE)phenomenon and the change of the electronic transition state of the molecule.At present,there is a lack of more in-depth research on the luminescence mechanism of solid-state emission squaraine dye molecules to guide the design and synthesis of their molecules.On the basis of the molecular structure of squaraine dye molecule containing aniline structure,the intermolecular interaction is enhanced by expanding the intramolecular conjugation structure and molecular planarization,thereby promoting the formation of excimer complexes after light excitation.In excimer complexes,the generation of a new electron radiation transition pathway realizes the solid-state emission of squaraine dye molecules.This aggregation-enhanced excimer emission(AEEE)mechanism provides an idea for the design of novel solid-state emission squaraine dye molecules.The thesis includes the following parts:1.Designed and synthesized a series of solid-state emission squaraine dye molecules with aniline structure,and studied the influence of different substituent groups on their fluorescence properties.Based on the squaraine dye molecules with aniline structure as the basic skeleton,a series of squaraine dye molecules with solid fluorescent properties were designed and synthesized by introducing strong electron attracting/pushing groups or changing their conjugate structure.They were subsequently subjected to basic optical characterization.It is found that there is almost no fluorescence in the solution state,but there is obvious fluorescence in the solid state,which is an obvious aggregation-induced emission(AIE)effect.This is different from the typical aggregationcaused quenching(ACQ)phenomenon of squaraine dye molecules reported in the past.It was also found that the introduction of strong electron-withdrawing/donating groups could not well control the emission wavelength of solid state squaraine dye molecules,but by restricting intramolecular rotation and expanding the conjugated system,the emission wavelength could be better shifted to the near-infrared area.This work has guiding significance for the design of solid-state emission squaraine dye molecules with aniline structure.2.Through the experiment of femtosecond transient absorption spectroscopy,it is the first time to find the contribution of excimer to solid-state emission in squaraine dye molecules.The dynamic behavior of excited state electrons of Bu Ph A-SQD,DPh A-SQD and CZ-SQD squaraine dye molecules in solution and film state was studied by femtosecond transient absorption spectroscopy.It was found that after the sample was excited,the excited state absorption signal of the new substance was generated in the DPh A-SQD and CZ-SQD films.Considering the experimental results of AIE and ammonia fumigation,the new substance was classified as an excimer.And through the ultrafast detection method of femtosecond transient absorption experiment,the halochromic behaviors of DHHAP solution caused by tautomerism was studied.Different sizes of antimony nanosheets are distinguished by transient absorption signals.3.The application of Bu Ph A-SQD,DPh A-SQD and CZ-SQD micro/nanocrystals as optical waveguide devices and the application of CZ-SQD molecules as ammonia gas detectors are preliminarily studied.Three solid-state light-emitting micro/nanocrystals of squaraine dye molecules were prepared,Bu Ph A-SQD,DPh A-SQD and CZ-SQD,and their morphology was measured by fluorescence microscope,scanning electron microscope(SEM),transmission electron microscope(TEM)and other characterization.The three morphologies are all nanoneedles with a length of several tens of microns,accompanied by obvious optical waveguide effect.Then,by collecting the angle-resolved spectra of the three kinds of micro/nanocrystals,it was found that the CZ-SQD micro/nanocrystals still have a double emission phenomenon,indicating that it has application prospects as controllable optical waveguide,stimulated emission,Bose-Einstein condense device.By fumigating the CZ-SQD solid powder with ammonia gas,the ammonia gas enters the molecules to change the intermolecular interaction and hinder the formation of excimer complexes,resulting in the relative decreasing of fluorescence signal(749 nm)of the excimer and increasing of the single molecule emission signal(587 nm).This experimental result shows that the CZ-SQD molecule is expected to be applied to the detection of ammonia gas.