New Cleavable Photoinitiator Architecture with Huge Molar Extinction Coefficients for Polymerization in the 340–450 nm Range.

Abstract

A new Type I photoinitiator Tr_DMPA is described. It consists in three 2,2′-dimethoxy 2-phenyl acetophenone (DMPA) units grafted onto a truxene (Tr) scaffold. Compared to DMPA itself, the lowest electronic transition exhibits a ππ* character and the corresponding molar extinction coefficients ε are increased from about 400 M–1 cm–1 (at about 332 nm for DMPA) to 63 000 M–1 cm–1 (at 338 nm for Tr_DMPA); such huge values are exceptional in Type I photoinitiators at this wavelength. Tr_DMPA undergoes a fast cleavage and efficiently initiate an acrylate polymerization upon a Xe–Hg lamp, a halogen lamp or a laser diode exposure at 405 nm (upon very low light intensities: 2–12 mW/cm2) in the 300–450 nm range. The polymerization of epoxides or divinylethers is also feasible in the presence of an iodonium salt at 405 nm where reference photoinitiators cannot efficiently operate. The chemical mechanisms analyzed by ESR, fluorescence, steady state photolysis, and laser flash photolysis experiments are discussed.

2,2,2-trifluoroacetophenone-based D-π-A type photoinitiators for radical and cationic photopolymerizations under near-UV and visible LEDs

“…[8][9][10][11][12][13] However, most reported and commercial PIs exhibit less-than-perfect performance upon LED irradiation because they have a maximum absorbance below 365 nm where the most common and cost-effective UV/LED lamps irradiate. 14 In the reported PIs, substituted ketones (e.g., the well-known 2,2-dimethoxy-2-phenylacetophenone (DMPA), 15,16 benzophenone (BP), [17][18][19][20][21] or thioxanthone (TX) [22][23][24][25][26] ) can work as cleavable (Type I) or uncleavable (Type II) PIs, and their photogenerated radicals is used for universal application in free-radical polymerization (FRP), free-radical-promoted cationic photopolymerization, or in cationic polymerization (CP) (as photosensitizers). The excellent properties of ketone derivatives provide them with interesting potential applications for polymer synthesis.…”

Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry

“…The current efforts result in an amazing series of proposals of new PISs . For example, we have recently introduced PISs ( [99][100][101][102][103][104][105][106][107]) exhibiting really novel absorption properties (red-shift absorptions, multicolor absorptions, enhanced molar extinction coefficients, ε): e.g., colored substituted or functionalized ketones [60], modified organometallic derivatives [70][71][72][108][109][110][111][112][113][114][115][116][117] (ruthenium-, iridium-, platinum-, zirconium-and zinc-based complexes, titanocene derivatives…), various series of dye-based skeletons [61][62][63][65][66][67][68][74][75][76] (e.g., phenylenediamine, polystilbene, polyazine, violanthrone, acridinedione, 2,7-di-tert-butyldimethyldihydropyrene, bodipy, boranyl, thiophene, perylene bis-dicarboximide, hydrocarbons, pyrromethene, pyridinium salt…), di-and tri-functional architectures of photo initiators [64], light harvesting compounds [57,58] (where a strong molecular orbital coupling occurs, leading to ε huge values) and push-pull and multicolor photoinitiators (novel chromophores; donor--acceptor arrangements; unusual broad absorptions from the blue to the red wavelengths…).…”

“…Therefore, visible photons can be successfully used and Hg lamps avoided. The recent development of di-and tri-functional architectures of PIs, light harvesting PIs and push-pull and multicolor PIs opens a route towards highly absorbing PIs in the 400-800 nm range [57,58,64,73].…”

“…A smaller number of systems can operate at 395 nm. Recently developed PISs operating in the near-UV/visible range (e.g., [57,58]) noticeably extend the scope of the existing structures and should be efficient upon a 395 nm LED irradiation. Laser diodes also lead to efficient FRP, CP and FRPCP .…”

“…As the polymerization efficiency in the presence of EPOX, LDO or ESO in the same experimental conditions are relatively close, it is obvious that renewable monomers can be successfully used in photocurable formulations operating in a large range of excitation wavelengths delivered by polychromatic (Xe lamps, household lamps) and (quasi) monochromatic (LED and laser diodes) light sources and sun. Some high performance PISs developed in the last year (see, e.g., the 2012 and 2013 references in [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76]) should ensure a faster formation of tack-free coatings upon sunlight exposure under air: work is under progress. …”