Synthesis And Properties Of Novel Photoinitiators Based On Thioxanthones

As a 5E technology?efficient,enabling,economical,energy-saving,and environmental-friendly?,photopolymerization has been widely used in various industrial applications including coatings,paints,inks,adhesives,optoelectronics,stereolithography,and nanotechnology.Traditional photopolymerization commonly employs medium or high pressure mercury lamps as irradiation sources with short UV light?<280 nm?.Excitation by short UV light suffers from some issues including production of ozone,radiation safety and limited curing depth.The inefficient mercury lamps are being substituted by some visible irradiation sources like the light emitting diode?LED?featuring irradiation safety and efficient energy utilization.In order to introduce LED light into photopolymerization applications,optimization of photoinitiating systems is needed so that the absorption of the photoinitiator/photosensitizer matches the emission of visible LED irradiation to ensure efficient photon absorption.Thioxanthones?TX?are quite attractive due to the long wavelength absorption between360–420 nm and low fluorescence quantum yields.Such famous type II photoinitiators are generally applied by combining with amines as efficient co-initiators.However,amines are apt to induce yellowing and toxicity.Moreover,the intrinsic drawback of bimolecular system that the back electron transfer occurs between the excited thioxanthones and the amines,would decrease the initiation efficiency,especially in formulations of high viscosity.Therefore,it is highly desirable to develop one-component visibile thioxanthone-based photoinitiators without the above mentioned disadvantages.In this paper,a series of novel unimolecular photoinitiators were synthesized,such as containing thioxanthones as chromophores and benzodioxoles as coinitiators,introducing amines into thioxanthone backbone in order to reduce the migration and increase photoefficiency.The study on initiating mechanism were confirmed by electron spin resonance?ESR?,photolysis,laser flash photolysis?LFP?measurements.The major work includes the following parts:?1?Efficient benzodioxole-based unimolecular photoinitiators:from synthesis to photopolymerizationThis part described a series of novel unimolecular type II photoinitiators,containing thioxanthones as chromophores and benzodioxoles as coinitiators.The structures of these unimolecular photoinitiators were characterized by ¹H NMR,¹³C-NMR and high-resolution mass spectrometer?HR-MS?.Study on the photophysical properties of the photoinitiators indicated that electron donors/acceptors as spacers between thioxanthone and benzodioxole affected both the UV-Vis absorption and the fluorescence emission.The long wavelength absorptions from 385 nm to 402 nm as well as low fluorescence quantum yields make the investigated benzodioxole derivatives quite attractive as efficient photoinitiators under UV-A and visible LED light irradiation.With a proper molecular design,the unimolecular photoinitiator exhibited higher initiation efficiency than the thioxanthone derivatives from the literature.Possible initiation mechanism was also proposed based on the photolysis study.?2?Quaternized thioxanthone as one-component visible photoinitiators based on thiol–ene chemistryNovel photoinitiators used for negative photoresist based on thiol–ene chemistry were prepared,and characterized through UV-vis,fluorescence,and NMR measurements.Electron spin resonance measurements were also conducted to confirm the generated radical.Real-time FTIR tests illustrated that the photoinitiator exhibited efficient activity in the polymerization of thiol–ene monomers.The photolithographic measurement of the photoinitiator demonstrated large potential for application in photoresists.?3?Highly efficient photobase generators based on thioxanthone and superbases:from synthesis to photopolymerizationA series of photobase generators?PBG?,containing thioxanthone as a chromophore and different quaternary ammonium salts as latent active species,were facilely synthesized in two steps,and characterized by NMR and HR-MS.The study on the photophysical properties of the thioxanthones indicated that these compounds were photosensitive to long wavelength because of the characteristic absorption of the chromophore at 380 nm.Investigation on photolysis confirmed the photodecarboxylation reaction mechanism,through which the thioxanthones efficiently released active basic species.All the novel compounds showed large activity in catalysing photopolymerization.The PBG exhibited faster polymerization rate and higher epoxy conversion than the previously described highly active PBG system.The initiation mechanism of the PBG was proposed according to the photolysis study.?4?Carbanion as superbase based on thioxanthone for catalysing thiol–epoxy photopolymerisation:from synthesis to mechanismIn this part,a series of novel PBG contained thioxanthone as a chromophore,which were able to generate carbanion via decarboxylation under LED light irradiation,were designed and straightforwardly prepared.The formed carbanion can be used as a superbase to catalyze thiol-epoxy polymerisation effciently.Investigation on ¹H NMR and FTIR confirmed the generation of carbanion intermediates.The counteranion significantly affected the photodecarboxylation efficiency.The study of photopolymerisation tests,based on real-time FTIR and dielectric analysis measurements,indicated that the generated carbanion exhibited faster polymerisation rate and higher epoxy conversion than traditional superbase.The results of photopolymerisation tests indicated that the generated carbanion exhibited a high polymerisation rate and high epoxy conversion compared with traditional superbase.In this parper,a straightforward synthesis of novel photoinitiators containing thioxanthone were presented,and all the novel photoinitiators showed high activity and conversion in polymerization.The high initiation ability combined with straightforward synthesis makes these PBG promising candidates for commercialization.