Construction And Application Of Thermochromic Fluorescent Materials With Highly Adjustable

Stimuli-responsive fluorescent materials refer to a new type of smart materials whose fluorescent properties will change significantly under external stimuli(e.g.,light,mechanical force,electricity,heat and solvents).Due to their wide potential applications in molecular switching,display,sensing,biological imaging,optical recording,and information protection,it have aroused extensive attention during the past few years.Among them,heat has the advantages of easy availability,low cost,abundant sources,simple control,etc.,especially related to an important physical parameter — temperature.With these characteristics,thermo-responsive fluorescent materials have great application potential in temperature sensing and indication,thermo-responsive functional decorations,ect.,and with the improvement of material performance,they also have application potential in a series of advanced applications,such as thermal-control information display.Therefore,the development and application of thermo-responsive fluorescent materials are very necessary.In the first chapter,we present a comprehensive review of recent progress in the thermo-responsive fluorescent materials,and their types,mechanism and applications are summarized.Due to their late development,there is no relevant system introduction yet.According to different composition characteristics and thermoresponsive modes(or paths),we divide them into intrinsic,binary and ternary thermoresponsive fluorescent materials.From the literature review,existing systems often involve complex molecular synthesis or material preparation processes,and the most critical problem is the poor adjustability(e.g.,fluorescence color,response temperature),resulting in the poor overall material performance.This also makes the application of thermo-responsive fluorescent materials mainly focus on temperature sensing and indication.Therefore,the development,performance optimization and application expansion of highly adjustable thermo-responsive fluorescent materials system are of great significance.In Chapter 2,from the perspective of matrix design,a phospholipid-like phase change material with excellent crystallinity—long alkyl chain saturated fatty acids(FA)is used as the thermo-responsive matrix,and its crystallization/melting upon cooling/heating is used to control the "passive" aggregation/dispersion of the doped fluorophores to construct a highly adjustable thermochromic fluorescent system.Due to the strong "aggregation" effect(i.e.,crystallinity)of the thermo-responsive matrix,the system does not need to be doped with fluorophores with special intermolecular force or structure,so it can be applied to some common fluorophores,such as coumarin,rhodamine 6G and tetraphenylethylene.Taking five coumarin compounds with different substituents as an example,they are respectively doped into dodecanoic acid(DA)to prepare thermochromic fluorescent materials(TFMs),which can produce high-contrast reversible fluorescent color transition before and after heating.Their thermochromic properties have been studied and compared in detail by a variety of characterization methods,including fluorescence spectrum,fluorescence quantum efficiency and fluorescence lifetime.By comparing with other phase change materials with poor crystallinity(such as polyethylene glycol,polycaprolactone and paraffin wax),and using characterization data such as fluorescence spectrum,X-ray diffraction pattern,scanning electron microscope pattern,we proved that the excellent crystallinity of DA is playing a key role in the thermochromic properties.Variable temperature fluorescence spectroscopy and differential scanning calorimetry technology show that the fluorescence transition temperature of the material is consistent with the phase transition process of FA.Therefore,by selecting FA with different alkyl chain lengths from 12 to 22,a series of TFMs with different thermoresponsive temperature can be obtained.Subsequently,various application modes of the TFMs were explored,including thermo-responsive fluorescent paper,writable fluorescent crayons,and thermal printing,and the cycling stability of the material in the application mode was monitored.In addition,the TFM also has the advantages of simple preparation,low cost,good reversibility(> 200 cycles),and high compatibility,which endows it a good practical application prospects in temperature indication and sensing,and anti-counterfeiting and encryption.In the third chapter,based on the high adjustability of the above system,by optimizing its performance in response mode,color scalability and application mode,an excellent integrated TFM with full-color,off-on switching mode,and printability were achieved.After introducing a light-absorbing dye-methylene blue(MB)with special absorption characteristics,the crystallization/melting of FA simultaneously regulate the synergistic changes in the molecular existence state as well as spectrum of MB and related fluorophores,resulting in the switching on-off of F(?)rster resonance energy transfer(FRET)process,thereby achieving the off-on of fluorescence.The involved high-efficiency FRET process was proved in detail by calculated F(?)rster radius and fluorescence lifetime measurements.Based on the principle of complementary primary colors,by adopting the three primary color fluorophores with high fluorescence quantum efficiency in the system,the thermo-responsive full-color emission(including white light emission)is successfully realized.At the same time,our TFMs can be combined with inkjet printing technology in the form of ink,so that the high-resolution thermal-responsive full-color images and text can be easily obtained on a flexible substrate.With merit of it,such TFMs show great potentials in high-end applications,such as thermal control dynamic interior decoration,programmatic temperature-control information display,and advanced information encryption.In Chapter 4,based on the highly adjustable system,another crystalline phase change material-cholesterol pelargonate(CP)is introduced to effectively affect the crystallization rate of FA,thereby realizing the optical memory-forgetting properties of thermal-responsive fluorescence,that is difficult to achieve in the field of stimulusresponsive materials.Through the fluorescence spectrum test,the thermal response fluorescence intensity-time curve of the material presents a characteristic memoryforgetting curve.Through the investigation of different influencing factors of TFMs(such as CP doping concentration,FA chain length,temperature,etc.),and with the help of confocal fluorescence microscopy technology,the interaction process between CP and FA has been preliminarily explored.Finally,based on the ability of long-term memory-forgetting to modulate optical information in space and time dimensions,it demonstrates its application potential in the field of high-end multi-dimensional information security.In summary,from the perspective of matrix design,we have constructed a highly adjustable thermochromic fluorescence system.Based on the system's highly adjustability,further optimize its comprehensive performance,an excellent integrated TFM with full-color,off-on switching mode,and printability were achieved.At the same time,based on the high adjustability of the system,TFM with new optical memory-forgetting properties was developed.Based on the substantial improvement of the comprehensive performance of materials or the development of new optical properties,the application potential of TFMs in the high-end field is expanded.