Liquid PEG Polymers Containing Antioxidants: A Versatile Platform for Studying Oxygen-Sensitive Photochemical Processes

Abstract

This article proposes the exploitation of widely available, inexpensive, innocuous "green" liquid polyethylene glycol (PEG) polymers containing the oxygen scavenger oleic acid (OA) as promising media for studying oxygen-sensitive photochemical processes. Here we report the successful application of this media to detailed investigations of triplet-sensitized photochemical upconversion, previously established as being readily poisoned by dissolved oxygen. Three different PEG materials were investigated with increasing molecular weight from 200 to 600 g/mol, coded as PEG-200, PEG-400, and PEG-600. These fluidic polymers facilitate an oxygen-depleted environment in comparison to commonly employed organic solvents while providing high solubility and diffusion for the dissolved chromophores. Moreover, the low oxygen permeation afforded by these PEG solvents allows them to remain deoxygenated in open containers under ambient conditions for extended time periods. OA, 9,10-dimethylanthracene (DMA), and 2,5-dimethylfuran (DMF) are shown to efficiently and quantitatively consume dissolved oxygen in the PEG environment in the presence of the photoactivated triplet sensitizer platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP). Oxygen consumption was directly correlated with systematically increasing sensitizer excited-state lifetimes that eventually reach the same plateau as achieved through extensive N2 sparging. Diffusion-controlled bimolecular triplet-triplet energy transfer quenching between PtTPBP and the acceptor/annihilator 9,10-bisphenylethynylanthracene (BPEA) was observed in all three PEG formulations investigated. Subsequent triplet-triplet annihilation, between triplet excited BPEA acceptors, achieves bright and stable upconverted singlet fluorescence from BPEA with no decrease in intensity over 20 h under ambient conditions. In the champion composition (PEG 200), the upconversion quantum efficiency reached 31% under conditions where triplet-triplet annihilation was maximized. This is in stark contrast for the same upconverting pair measured in toluene under ambient conditions, which rapidly decomposes upon exposure to visible light. To illustrate that these PEG compositions could be translated into a suitable solid-state format, these viscous solutions were embedded in a transparent polyurethane polymer shell yielding a flexible and long-term stable upconverting cell that can be manipulated for possible real-world applications. Although the current investigation focused on photochemical upconversion, the oxygen-depleted environments developed here can be utilized to study a plethora of oxygen-intolerant photochemical reactions.

Red Light Activation of Ru(II) Polypyridyl Prodrugs via Triplet-Triplet Annihilation Upconversion: Feasibility in Air and through Meat

“…Oxygen was consumed only very slowly in the dark, but was fully depleted after 5 min of red light irradiation. These results confirm that the antioxidant scavenges the singlet-oxygen that is generated by the photosensitizer upon irradiation [25,34]. When the experiment was repeated in presence of 0.25-50 mM L-Asc or 1-20 mM GSH, i.e., close to the biological concentrations of these two species [41], the "lag time" before upconversion progressively decreased with increasing antioxidant concentration to as low as 0.5 min lag time at 50 mM of L-Asc ( Figure S2).…”

“…To solve this issue, we and others recently showed that antioxidants such as L-histidine, ascorbic acid, or glutathione, can protect TTA-UC against oxygen: upon irradiation of TTA-UC vesicles in air, the photosensitizer initially generates singlet oxygen, which is scavenged by the antioxidant until all oxygen in the irradiated volume has been consumed ( Figure 1b) [25,[34][35][36][37]. Once the O 2 concentration reaches a certain threshold, TTA-UC switches on and remains stable for minutes to even hours in spite of the air atmosphere.…”

Nanodroplet‐Containing Polymers for Efficient Low‐Power Light Upconversion

“…DPA/PdOEP: 18% in toluene, [30] DPA/PtOEP: >23% in ethanol or tetrahydrofuran, [3,28,31] perylene/PdTPBP: 38% in tetrahydrofuran), [32] whereas microemulsions (e.g. DNAMe/PdMeTPP: 16%, oil-in-water) [16,33] or more sophisticated mixtures [13,34] have been proposed for efficient upconversion under ambient conditions. Suspensions of nanoparticulate [35][36][37][38] or encapsulated systems [39][40][41][42] and some organic solutions [8,16] display good but usually lower TTA-UC efficiencies when operated under ambient conditions.…”

“…Taken together, these results highlight that the glassy polymer matrix protects the liquid nanophase surprisingly well from atmospheric oxygen and that the use of a volatile liquid component is no fundamental obstacle for longevity, although the stability needs to be clearly improved before technological exploitation in devices is feasible. Strategies to achieve this include the use of dyes with better (photo)stability (DPA is well-known undergo (photo)oxidation to an endoperoxide and dimerize, respectively, [2,49,50] ), the addition of stabilizers, [34,51,52] a less volatile solvent, [53,54] and perhaps additional barrier layers. [7] The fact that the upconversion observed in the dye doped nanodroplet-containing polymer is the result of TTA-UC was confirmed by time-resolved PL measurements (inset of Figure 3d).…”

Water-Dispersible Silica-Coated Upconverting Liposomes: Can a Thin Silica Layer Protect TTA-UC against Oxygen Quenching?

“…In other words, the upconverting drug carrier did not function in oxygen-rich conditions, and the use of such systems in vivo would lead to unreliable performance, because oxygen concentrations vary drastically in biological tissues. 36−38 The oxygen sensitivity of TTA-UC can be reduced by developing upconversion systems that either (i) feature very fast TTA-UC so that upconversion takes place faster than physical quenching by molecular oxygen, 7,17 (ii) have built-in chemical agents that consume molecular oxygen, 8,39−41 or (iii) are protected by a physical barrier that cannot be crossed by molecular oxygen. Most noteworthy examples of the latter strategy include upconverting oil-core nanocapsules embedded in an oxygen protective cellulose material or poly(vinyl alcohol) nanofibers, 19,42 and upconversion in hyperbrached unsaturated polyphosphoesters.…”