Changes in the Electronic Transitions of Polyethylene Glycol upon the Formation of a Coordinate Bond with Li⁺, Studied by ATR Far-Ultraviolet Spectroscopy

Abstract

This study investigates the electronic transitions of complexes of lithium with polyethylene glycol (PEG) by the absorption bands of solvent molecules via attenuated total reflectance spectroscopy in the far-UV region (ATR−FUV). Alkali-metal complexes are interesting materials because of their functional characteristics such as good ionic conductivity. These complexes are used as polymer electrolytes for Li batteries and as one of the new types of room-temperature ionic liquids, termed solvation ionic liquids. Considering these applications, alkali-metal complexes have been studied mainly for their electrochemical characteristics; there is no fundamental study providing a clear understanding of electronic states in terms of electronic structures for the ground and excitation states near the highest occupied molecular orbital− lowest occupied molecular orbital transitions. This study explores the electronic transitions of ligand molecules in alkali-metal complexes. In the ATR−FUV spectra of the Li−PEG complex, a decrease in intensity and a large blue shift (over 4 nm) were observed to result from an increase in the concentration of Li salts. This observation suggests the formation of a complex, with coordinate bonding between Li + and the O atoms in PEG. Comparison of the experimental spectrum with a simulated spectrum of the Li−PEG complex calculated by time-dependent density functional theory indicated that changes in the intensities and peak positions of bands at approximately 155 and 177 nm (pure PEG shows bands at 155, 163, and 177 nm) are due to the formation of coordinate bonding between Li + and the O atoms in the ether molecule. The intensity of the 177 nm band depends on the number of residual free O atoms in the ether, and the peak wavelength at approximately 177 nm changes with the expansion of the electron clouds of PEG. We assign a band in the 145−155 nm region to the alkali-metal complex because we observed a new band at approximately 150 nm in the ATR−FUV spectra of very highly concentrated binary mixtures.

Attenuated Total Reflection–Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations of the Electronic Structure of the Top Surface and Bulk of Polyethylenes with Different Crystallinities

“…In another study, alkali metal complexes composed of a Li salt and PEG were investigated, and it was found that the absorption bands of PEG 17 at 155 and 177 nm shifted higher in energy because of the formation of a complex between Li þ ions and PEG. 19 Further, the intensity of the band at 177 nm decreased with increasing Li salt concentration. Thus, these results can be used to understand the ionic environment around PEG.…”

“…Morisawa et al focused on the electronic states of polymers such as nylon, 16 polyethylene glycol (PEG), 17 polyhydroxy butylate (PHB), nanocomposites of PHB and graphene, 18 and complexes of PEG and lithium ions. 19,20 In these studies, the electronic transitions of polymers were investigated by comparing the ATR-FUV spectra and simulated spectra obtained using quantum chemical calculations, thus revealing the potential usefulness of ATR-FUV for polymer analysis. For example, the FUV spectra of a series of nylon polymers have been reported previously, 16 and the major absorption bands at around 150 and 200 nm were assigned to the s-3p y Rydberg and p-p* transitions, respectively.…”

Determining the Coordination Number of Li+ and Glyme or Poly(ethylene glycol) in Solution Using Attenuated Total Reflectance-Far Ultraviolet Spectroscopy

“…Nami UENO,* Tomonari WAKABAYASHI,** and Yusuke MORISAWA** † suggesting that the absorbance of the 177 nm band reflects the concentrations of free COC in the solutions. 12 Therefore, the ATR-FUV spectra of the binary solutions display an LiTFSI concentration dependency, and the coordination number can be determined using the reaction model described below.…”

“…We observed a reflection absorbance at approximately 170 nm due to an n-Rydberg (Ryd) 3s transition, which was correlated to a concentration of oxygen in ether not involved in a coordination bond. 12 As ATR-FUV provided us the opportunity to observe the formation of coordination bonds in PEG ether groups, an analysis of the reflection absorbance of solutions of Li + by ATR-FUV should allow for the determination of the coordination number of Li + .…”

“…ATR-FUV spectra were obtained for the binary mixtures of LiTFSI and polyether for wavelengths between 145 and 300 nm 12 (Fig. 2, see Sect.…”