Independent Control of Singlet Oxygen and Radical Generation via Irradiation of a Two-Color Photosensitive Molecule

Abstract

Free-radical polymerizations are used for a wide range of applications but are detrimentally impacted by the presence of oxygen. Zinc phthalocyanines have been previously used as singlet oxygen generators to excite radical-consuming ground-state triplet oxygen into its less reactive singlet state prior to photoexcitation of a photoinitiator. We report for the first time that polymerization can be achieved via irradiation of UV band of phthalocyanine and that photosensitization and photoinitiation can be independently achieved via irradiation of its two distinct absorption bands to reduce oxygen inhibition and initiate polymerization without the need for additional treatment. We propose a mechanism for this unique photoinitiation phenomenon and verify its feasibility via computational and experimental approaches. This new class of dual-photosensitive molecules shows promising utility in applications that are adversely impacted by the presence of oxygen, such as coatings and stereolithography.

Enhancing UV Photopolymerization by a Red‐light Preirradiation: Kinetics and Modeling Strategies for Reduced Oxygen Inhibition

“…A new induction time ( T ID ) under UV light is then calculated for various t ′ (the preirradiation time of red light). Our numerical results of T ID versus t ′ will be compared with the measured data of Childress et al…”

“…The PS and PI have the selection feature given by their distinct difference in absorption bands, without overlap. For example, the reported system of Childress et al is based on a monomer (M) of di(ethylene glycol) ethyl ether acrylate mixed with zinc 2,9,16,23‐tetra‐tert‐butyl‐29H,31H‐phthalocyanine (ZnTTP), having distinct absorption peak at UV 365 nm and red 635 nm, such that it can be independently excited by a UV light and a red light, respectively. We note that Childress et al is our special case that monomer [M] = [M'] and [A] = [B], that is, one PI (ZnTTP) with the absorption in both UV and red.…”

“…In their techniques, the induction time can be partially or completely eliminated (by the red light) to control (tailor) the conversion rate of UV‐light‐initiated photopolymerization. While Childress et al reported the first measured data for the novel strategy for red‐light‐controlled oxygen inhibition, this study will present, for the first time, the detailed kinetics and theoretical modeling. The dual‐wavelength kinetic equations will be derived and numerically solved for various red‐light preirradiation time with specified PS, PI, and oxygen concentrations.…”

“…Recently, Childress et al reported a dual wavelength of red and UV, where red light preirradiates the ethyl ether acrylate mixed by phthalocyanine, followed by a UV excitation of the PI, such that photosensitization and photoinitiation can be independently achieved via irradiation of the two distinct absorption bands to reduce oxygen inhibition and initiate polymerization. In their techniques, the induction time can be partially or completely eliminated (by the red light) to control (tailor) the conversion rate of UV‐light‐initiated photopolymerization.…”

“…The dual‐wavelength kinetic equations will be derived and numerically solved for various red‐light preirradiation time with specified PS, PI, and oxygen concentrations. Analytic formulas for the induction time and UV conversion efficacy will be derived and utilized to analyze the measured data of Childress et al, including the roles of red‐light preirradiation time and UV‐light intensity on the induction time and overall conversion rate. Finally, strategies for controlled initiation–inhibition switch based on two mechanisms, (a) oxygen‐inhibition for improved conversion and (b) radical‐inhibition for spatial confirmation in 3D printing, are presented.…”