Photophysics of Up‐Conversion Nanoparticles: Radical Photopolymerization of Multifunctional Methacrylates Comprising Blue‐ and UV‐Sensitive Photoinitiators

Abstract

UCNPs based on NaYF 4 :Yb 3 + /Tm 3 + @NaYF 4 generate blue (450 nm, 476 nm) and UV light (345 nm, 362 nm) upon exposure with a laser emitting at 980 nm. Time-resolved measurements indicated the same origin of the emission monitored at 362 nm and 450 nm by a rise time of 226 μs while their decay occurs with a similar time frame of 260 μs. The observation can be attributed to an energy transfer where successive absorption of three photons results in population of the 1 I 6 state. Intensity-dependent exploration of the upconverted emission indicated a cubic dependence between the luminescence intensity I f and laser intensity I (I f ∝ I 3 ) monitored at 362 nm and 450 nm. The emission at 345 nm involves successive excitation with four photons. The up-converted light excites radical photoinitiators selected from Ivocerin, thioxanthone and camphorquinone where a coinitiator, sulfonium salt, helped to generate the initiating radicals. These different systems comprising the UCNP and the photoinitiator generate crosslinked materials based on urethane dimethacrylate (M1) as model monomer applying laser exposure at 980 nm. Real-time FTIR monitoring of M1 exposed indicated a different initiation time as a function of the laser intensity applied. Stress relaxation measurements complemented the understanding between material parameters on the one side and loading with UCNPs, light intensity and kinetics on the other side.

Mediated Generation of Conjugate Acid by UV and Blue Sensitizers with Upconversion Nanoparticles at 980 nm

“…Assisted photoreactions via upconversion nanoparticles ( UCNP ) gained increased attention in the last years [1–10] . They convert longwave radiation into shortwave emission, namely NIR light into UV and blue light.…”

“…The solutions obtained appeared in an acceptable optical appearance for the experiments and exhibited overall less activity to generate con‐H + (Supporting Information Figure SI6). The high intensity of the NIR laser was necessary to enhance the amount of emitted UV radiation [7] even though some sensitizers possess a higher reactivity with lower intensity (Supporting Information Figure SI7). These measurements were compared with conventional UV light using an UV‐LED ( λ = 395 nm and an intensity of 230 mW cm −2 ).…”

3D printing of multi-scalable structures via high penetration near-infrared photopolymerization

“…Despite the convenience offered by UV and blue lights, intensity of short wavelength lights is a trade-off for better mechano-properties by uniformed polymerization and health concern of UV exposure. Among the photopolymerization methodologies, near-infrared light (NIR) has a more salient role in rapid deep-curing for its remarkable penetration in various media by employing up-conversion strategies 24 – 27 . Recently, we have applied NIR light in free radical as well as cationic polymerization by utilizing up-conversion nanoparticle (UCNP), and successfully photocured resins for over 10 cm with relatively uniformed conversion 28 – 30 , which is promising to improve the curing manner and valuable to additive manufacturing.…”

NIR‐Sensitized Activated Photoreaction between Cyanines and Oxime Esters: Free‐Radical Photopolymerization

“…Complementary studies about 3D and 4D printing extended the potential of photopolymerization . Furthermore, NIR excitation based on up‐conversion nanoparticles enabled blue and UV light photoinitiators, while excitation at 980 nm resulted in a nonlinear four‐photon absorption process in the generation of UV light . An extensive review discussed the potential of free‐radical and living radical polymerization .…”

“…[23] Furthermore,NIR excitation based on upconversion nanoparticles enabled blue and UV light photoinitiators, [24,25] while excitation at 980 nm resulted in an onlinear four-photon absorption process in the generation of UV light. [26] An extensive review discussed the potential of free-radical and living radical polymerization. [27] This potential is based mainly on UV and visible-light excitation, but NIR excitation did was not addressed.…”