Color Changes in Gels Composed of Hydrogen‐Bonding Donor–Acceptor–Donor‐Type Fluorenone Derivatives and Short‐Chain Polyethylene Glycol in Response to Ionic Species

A series of donor-acceptor-donor (D-A-D)-type fluorenone derivatives with urethane units and oligo(ethylene glycol) (OEG) linkers of different lengths (F-EG_n-BU, n = 2-4) are synthesized to investigate the effect of the length of the OEG linker on the self-assembling behavior and photophysical properties of the fluorenone derivatives. In alcoholic media such as ethanol and 2-propanol, the fluorenone-based D-A-D triad with triethylene glycol linkers (F-EG₃-BU) specifically form supramolecular gels via hydrogen bonds between the urethane units. The extension of the OEG linkers (F-EG₄-BU) reduced the strength of molecular aggregates, resulting in higher solubility in various organic solvents. Each compound formed intramolecular charge transfer (ICT) states, as evidenced by the ICT band in the UV-vis absorption spectra and the twisted intramolecular charge transfer emission bands in the photoluminescence spectra. Changes in the absorption and emission spectra of short-chain poly(ethylene glycol)s (SC-PEGs) gelated with F-EG₂-BU depend on the coexisting ionic species. In this study, we describe the unique chemical stimuli-response behaviors of fluorenone-based SC-PEG gels that cause significant changes in the spectroscopic properties upon ion doping with NaCl and alkali metal hydroxides.