Photoredox catalysis using infrared light via triplet fusion upconversion

Abstract

Recent advances in photoredox catalysis have made it possible to achieve various challenging synthetic transformations, polymerizations and surface modifications^1-3. All of these reactions require ultraviolet- or visible-light stimuli; however, the use of visible-light irradiation has intrinsic challenges. For example, the penetration of visible light through most reaction media is very low, leading to problems in large-scale reactions. Moreover, reactants can compete with photocatalysts for the absorption of incident light, limiting the scope of the reactions. These problems can be overcome by the use of near-infrared light, which has a much higher penetration depth through various media, notably biological tissue⁴. Here we demonstrate various photoredox transformations under infrared radiation by utilizing the photophysical process of triplet fusion upconversion, a mechanism by which two low-energy photons are converted into a higher-energy photon. We show that this is a general strategy applicable to a wide range of photoredox reactions. We tune the upconversion components to adjust the output light, accessing both orange light and blue light from low-energy infrared light, by pairwise manipulation of the sensitizer and annihilator. We further demonstrate that the annihilator itself can be used as a photocatalyst, thus simplifying the reaction. This approach enables catalysis of high-energy transformations through several opaque barriers using low-energy infrared light.

Figure 1.. Upconversion design for NIR to orange and NIR to blue.

(A) A schematic of infrared photoredox catalysis via triplet fusion upconversion. SET, single electron transfer. (B) A Jablonski description of triplet fusion upconversion and its adaptation to photoredox catalysis. Sen, sensitizer; An, Annihilator; PC, Photocatalyst (energy transfer from ¹[An]* to PC may be by resonance transfer or photon emission/absorbance). (C) NIR-to-orange upconversion photoluminescence using FDPP and PdPc, FDPP, furanyldiketopyrrolopyrrole; PdPc, Palladium (II) octabutoxyphthalocyanine. (D) NIR-to-blue upconversion photoluminescence using TTBP and PtTPTNP. PtTPTNP, Platinum (II) tetranaphthoporphyrin; TTBP, tetratertbutylperylene.

Figure 2.. Select examples of reactions driven by NIR light.

(A) Hydrodehalogenation reaction catalyzed by Eosin Y. (B) Rose Bengal catalyzed amine oxidation. (C) Reductive radical cyclization yielding phenanthridine product. (D) Intramolecular [2+2] cyclization. (E) Pyrrole formation via vinyl azide reduction. (F) Polymerization of methyl methacrylate.

Figure 3.. Material penetration experiments.

(A) Crosslinked PMMA gel reaction used to test the penetration of NIR (730 nm) vs blue (450 nm) light through a variety of media. Reactions which bypass the barrier generate a gel. (B) Table shows results of different materials as light barriers. (C) Experimental set up using laser diode, with water as barrier. (D) 10g scale PMMA gel reaction performed with this lamp. (E) Silicone mold used, together with the PMMA shapes that were synthesized through a 7 mm white silicone pad. 1. Halted after 15 minutes due to fire hazard with the bacon burning upon irradiation

Figure 4.. Application of the Beer-Lambert law to blue and NIR light.

A comparison of extinction coefficients and concentrations of Ru(bpy)₃(PF₆)₂ and TTBP, to those of PtTPTNP reveals 304-fold and 293-fold increase in reaction penetration, according to the Beer-Lambert equation (A = εcL), with IR light compared to blue light, respectively.