Design,Synthesis,Photophysical/Photochemical Properties And Application Of Conjugated Enone Compounds

Light-responsive organic small molecules can undergo chemical or physical changes under illumination,and they have been applied in different fields based on different light-responsive properties.The ability of a molecule to absorb light is the basic condition for its response to light.In the field of photopolymerization,photoinitiators can generate free radicals in response to light to initiate polymerization.The LED light source has the advantages of environmental protection,high efficiency,and energy-saving,and will gradually replace the mercury lamp as the mainstream irradiation light source in the field of photopolymerization.Compared with the broad emission of mercury lamps,LED light sources have narrow emission bands and their wavelengths are generally greater than 365 nm.However,most of the current commercial photoinitiators have very weak light absorption above 365 nm(ε<1000 M^-1 cm^-1),which cannot be well matched to LED light sources.The key to solving this problem is to develop photoinitiators with strong light absorption capacity and high initiation efficiency under the luminous wavelength band of LED light sources.Conjugated enones can be prepared by the Claisen-Schmidt condensation reaction between aldehydes and ketones.The cheap and wide range of raw materials endow them with the convenience of synthesis and certain structural design.Due to the presence of carbonyl groups,these compounds can undergo photochemical hydrogen abstraction to generate free radicals.Through rational structural design,the absorption spectrum of these compounds can be extended to visible light region,which can meet the needs of the light absorption capacity of the LED photoinitiator.However,due to the lack of in-depth understanding of the relationship between the photochemical/photophysical properties and building units of conjugated enones,there is a certain blindness in the structural design of such hydrogen abstraction photoinitiators/photosensitizers.Thus,its performance cannot be well played.Moreover,as a class of easily synthesized compounds,the properties of conjugated enones should be more explored to enhance their added value in practical applications.Given the demand for high-efficiency LED photoinitiators in the field of photopolymerization and the lack of deep understanding,wide application,and poor performance of conjugated enone compounds,a variety of conjugated enone compounds were designed and synthesized in this thesis.The structures were characterized and verified,and the structure-activity relationship between the structural units in conjugated enones and photochemical/photophysical properties was systematically explored by combining experimental methods and theoretical calculations.Based on the summarized structure-activity relationship,a LED photoinitiator with low migration and excellent initiation performance was further designed and synthesized,and a class of tropinone-based conjugated enone derivatives with high photothermal conversion ability was designed and synthesized and applied as light-switched adhesives.Moreover,the acid-induced color change of a pyrrole-based conjugated enone was discovered and the acidochromism mechanism and application were explored.Specifically,in response to the problem that the photophysical/photochemical properties of conjugated enones are poorly understood,the following work has been done in this thesis:（1）Four pyrrole-based conjugated enone compounds（PYOs）with different ketone units were designed and synthesized.The maximum absorption wavelength of PYOs is around 410 nm(ε_max=～40000 M^-1 cm-¹),which is significantly improved compared with traditional photoinitiators(eg:ITX,ε₄₀₅=～500 M^-1 cm^-1).PYOs,PYOs/TEOA,and PYOs/ONI systems were able to initiate radical photopolymerization under LED light sources at 405 nm and 460nm.The existence of the six-membered ring not only makes the energy of the excited state of the molecule dissipated mainly through nonradiative transitions but also reduces the driving force for the electron transfer reaction of the molecule,which is not conducive to the photo-initiated polymerization reaction.C3PY with acetone as a building block performed the best in each system,but its photoinitiation efficiency was still poor.（2）Three conjugated enone compounds（C3s）with peripheral groups of N-methyl pyrrole,N-methyl indole,and N-ethyl carbazole were designed and synthesized using acetone as the building block.The maximum absorption wavelength of C3s is around 400 nm(ε_max=～40000 M^-1 cm^-1).C3CZ can undergo efficient photochemical hydrogen abstraction,while the photochemical reactions between C3PY/C3ID and TEOA are not obvious.It is speculated that after the cis-trans isomerization of C3PY and C3ID,an intramolecular hydrogen transfer reaction occurs.This intramolecular reaction reduces the photostability of the molecule,which is not conducive to its use as a photoinitiator.Carbazole as a building block is favorable for the photo-initiated process,but its large conjugated area leads to a decrease in solubility.（3）Because of the solubility problem caused by the large conjugated peripheral groups,four conjugated enone compounds（BKs）containing different electron donating groups were designed and synthesized using acetone as the building unit.With the enhancement of the electron-donating ability of the group,the absorption wavelength of BKs is red-shifted,and the maximum absorption wavelengths of C3PH,C3PO,C3PS,and C3PN are 323 nm,356 nm,372 nm,and 435 nm,respectively.However,at the same time,the driving force for the reaction of BKs with TEOA is reduced.The photoinitiation efficiency of BKs under a specific wavelength LED light source depends on the balance between its photochemical hydrogen abstraction reactivity and light absorption capacity.Among BKs,C3PO with methoxy group as electron-donating group exhibits excellent photoinitiation activity under 365 nm,405 nm,and 460 nm LED light sources.Based on the structure-activity relationship obtained in the above three parts,aiming at the problems of poor performance and narrow application of conjugated enone compounds,the following works were carried out in this thesis:（4）C3POAC was designed and synthesized by introducing two acrylate groups into C3PO,to obtain a conjugated enone photoinitiator with excellent photoinitiation performance and migration stability.The molar extinction coefficients of C3POAC at 365 nm,405 nm,and 460 nm are 40100 M^-1 cm^-1,4400 M^-1 cm^-1,and～2 M^-1 cm^-1,respectively.The initiation activity of C3POAC was comparable to that of the commercial initiator ITX under 365 nm and 405nm LED light sources and was significantly better than that of the commercial photoinitiator ITX under the 460 nm LED light source.The bulk polymerization of butyl acrylate proves that C3POAC can participate in the polymerized network,so it will not migrate out of the obtained polymer samples.（5）The first part of the work demonstrated that cyclic ketones facilitate the conversion of absorbed light energy into thermal energy by molecules.Further,in this part,four kinds of tropinone-based conjugated enones（TPs）with different lengths of alkyl chains were designed and synthesized using nitrogen-containing heterobicyclic tropinone and 3,4-dialkoxy benzaldehyde as building blocks.TPs were applied to the adhesive field.TP6（75°C）and TP8（83°C）have relatively low melting points and can undergo solid-liquid transition under irradiation with a 405 nm LED light source（light intensity of 800 m W/cm²）.The adhesion strength of TPs to glass is about 0.5 MPa.Among them,TP8 has suitable crystallization rate and bonding strength,and can maintain the bonding force of 0.25 MPa after five photothermal debonding-bonding cycles,which can be used as reusable,light-controlled switched small molecular adhesives.（6）In the first part of the work,it was found that C3PY has an acidochromism phenomenon.Furthermore,this paper took TFA as an example to study the acidochromism of C3PY in different organic solvents and solid state,and summarized its discoloration mechanism.It is proposed that there were hydrogen bonding and protonation between C3PY and TFA.In high dielectric constant solvents,TFA interacted with C3PY by protonation.In medium and low dielectric constant solvents,hydrogen bonding and protonation coexisted,and the specific ratio of the two depended on the concentration of TFA in the solution.In proton acceptor solvents,acidochromism was not obvious.The application of C3PY as an acid sensor in organic solvents was explored and its detection limit for TFA in some solvents was calculated.