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

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.