Coumarin-based Oxime-esters As Visible Light Photoinitiators:Molecular Design,Synthesis And Application

Photocuring technology with characteristics of“5E”?efficiency,enhanced quality,energy savings,economics,environmentally friendliness?,has been widely used in various traditional applications including coatings,adhesives and inks,and high-tech applications like 3D printings.In recent years,energy-saving and environment-benign light emitting diode?LED?has gradually replaced traditional UV mercury lamps and has become the most popular irradiation source.However,currently it's still difficult for traditional UV photoinitiators?PIs?to match the LED lamps due to the narrow emitting wavelength range combined with long emtting wavelength?UV-A and visible light?of LED.Therefore,to develop efficient LED PIs via rational molecular design is one of the greatest challenges in photocuring research.This project aims to design and synthesize a series of efficient coumarin oxime-ester PIs by modifying the core structure of the coumarin oxime-ester via adjusting the structures of the terminal groups,changing the substitution positions and increasing the conjugation length.After systematically studying their structure-property relationship,we continued to applied the photoinitiators to the photocuring of thick materials and 3D printings.In the first section,four heterocyclic coumarin-based oxime-ester PIs?2-O,2-S,3-O,3-S?containing coumarins as chromophores,oxime esters as cleavable functionalities and heterocyclic groups as latent initiating moieties were synthesized.The novel heterocyclic oxime ester PIs exhibit broad absorption band ranging from 400 to 480 nm,well matching the emitting spectra of the popularly used 405 and 450 nm LED light sources.Under irradiation,the N-O bond cleaves and the formed heterocyclic free radicals after decarboxylation can efficiently initiate polymerization of acrylates.2-S is the most efficient one and its initiation activity is comparable to benzene radicals.The heterocyclic PIs exhibits good thermal stability?the decomposition temperature is above 160??and excellent photobleaching property in both diluent soluiton and resins.More than 10 mm of curing depth can be obtained when using 2-S as a PI.The heterocyclic PIs shows potential application in dental restorative materials.In the second section,a series of long-conjugated coumarin oxime-ester PIs?OEC4,OEC3-1and OEC3-2?were designed and synthesized by introducing p-dimethylamine phenylacetylene moiety at the 7 position of the coumarin.The impact of the substitution positions?OEC4 and OEC3-1?and the types of terminal moieties?OEC3-1 and OEC3-2?on the photoinitiation efficiency was studied systematically.These PIs exhibit absorption band ranging from 380 to480 nm.The?_max of OEC3-1 with good coplanarity is two times greater than that of OEC4.The photolysis rate of OEC3-2 with terminal vinyl moiety was faster than that of OEC3-1 with terminal benzene moiety.Under irradiation of 450 nm LED light,the PIs can efficiently initiate the photopolymerization of acrylates and OEC3-2 has the best performance.The initiation efficiency of OEC3-2 is higher than that of commercial PI Lucirin TPO.Additionally,the long-conjugated coumarin oxime-ester PIs possess large two-photon cross sections?208-536 GM?and can be applied to two-photon 3D printings to construct delicate 3D structures with submicron size.Concerning the safety concern derived from the migration of the residual PIs,in the third section,a polymerizable oxime ester PI?DCCA?containing terminal carbon-carbon double bond was synthesized and the PI?DCCB?with terminal benzene moiety was also prepared as a reference.Under irradiation of 450 nm LED light,DCCA can efficiently initiate the photopolymerization of acrylates and thiol-ene system.The initiation efficiency of DCCA is higher than that of DCCB and commercial PI Lucirin TPO.DCCA can be applied to DLP 3D printings to construct delicate 3D logo of Jiangnan University.Since the double bond of DCCA are involved in the photopolymerization process and therefore the unreacted PIs would be confined within the polymeric network,the migration rate of the 3D objects using DCCA is much lower than that using DCCB.The polymerizable PIs show potential application in biomedicine and food packaging.