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

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.