Preparation And Characterization Of Novel Photochromic And Mechanochromic Materials Based On Spiropyrans

Spiropyran derivatives have drawn great attention due to their excellent stimuliresponse properties,and many photochromic and mechanochromic materials have been designed based on these compounds.However,with increasing application demand,the single-stimulus-responsive materials are becoming more and more inadequate.For example,advanced anti-counterfeiting encryption requires multi-level encryption and decryption,which also strongly demands the development of compounds with multistimuli responsiveness to prepare multi-level anti-counterfeiting materials.In order to prepare this kind of materials,we modified spiropyrans with large steric hindrance groups to provide sufficient free volume for their ring-opening isomerization,making them more sensitive to external stimuli.In this paper,the effects of rigid benzene rings,flexible long chains and hydrogen bonds on the stimuli-response of spiropyrans were discussed,including the response of novel spiropyran derivatives to ultraviolet light,mechanical force and thermal stimuli.Further,we also demonstrated their application in the field of anti-counterfeiting and encryption.The main research contents are as follows:1.We synthesized a compound P3 by modifying spiropyran(SP-OH)with rigid benzene rings,which shows dual sensitivity of photochromism and mechanochromism.The relatively loose molecular packing of P3 is constructed by weak van der Waals force,which makes it sensitive to external stimuli.Therefore,it shows excellent stimuliresponse performance in both powder and single-crystal states,including extremely fast response speed(discoloration after 10 s ultraviolet irradiation),reversible high-contrast color change and dynamic adjustability.In addition,single crystal structure analysis is helpful to further explore the mechanism of stimuli-response of the compound.The excellent thermal stability of powder and its simple recovery method after grinding(heating at 85 ℃ for 5 min)are also very important for the practical application of P3.Based on the fluorescence resonance energy transfer during P3 discoloration,we presented a time-resolved encryption strategy: modulating the fluorescence emission and color change by simply changing the ultraviolet light stimulation time or grinding time,and constructing a time-resolved multi-dimensional encryption and dynamic anticounterfeiting model.2.We synthesized the compound TDBA-SP by introducing rigid benzene ring and flexible long chains into spiropyran(SP-OH)at the same time,which can respond to both ultraviolet light and mechanical force in solid state.After the introduction of the long chains,the spiropyran derivative was more sensitive to force stimulation,and the powder showed a significant fluorescence change(from light yellow to red)after grinding for 10 min.In addition,TDBA-SP powder has good solubility and a lower melting point(58 ℃),which is convenient for processing.Furthermore,the prepared photochromic film showed good fatigue resistance and can be reused for least 50 times.TDBA-SP has dual stimuliresponsiveness,good fatigue resistance and excellent processing performance,which makes it have great application potential in ultraviolet printing and anti-counterfeiting.3.On the basis of TDBA-SP structure,we synthesized a spiropyran derivative TDBA-SP2 by introducing hydrogen bonds(amide bonds)into spiropyran(SP-COOH).This compound shows good reversible photochromism,mechanochromism and thermochromism and its isomerization was explored by UV-vis absorption spectra,fluorescence emission spectra and infrared spectra.We found that TDBA-SP2 has excellent reversible stimuli-response and fatigue resistance in both solution and powder states,and there is no obvious performance attenuation after five cycles.Therefore,we designed a multi-level information encryption model based on the multi-stimuli response of TDBA-SP2,which is helpful to further guarantee information security.