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

## TL;DR

Scientists created gels that change color when exposed to certain ions, which could be useful for colorimetric sensing materials.

## Contribution

The study introduces a new class of supramolecular gels that exhibit fluorescence color changes in response to ionic species.

## Key findings

- The length of the OEG linker affects the self-assembly and solubility of the fluorenone derivatives.
- The gels show significant fluorescence color changes when exposed to NaCl and alkali metal hydroxides.
- The color change is due to altered balance between locally excited and twisted intramolecular charge transfer states.

## Abstract

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.

Fluorenone‐based donor–acceptor–donor‐type supramolecular gelators with urethane groups separated by oligo(ethylene glycol) linkers were developed. The addition of alkali metal cations induces a significant fluorescence color change by altering the balance between the emission from the locally excited and the twisted intramolecular charge transfer states. These supramolecular gels respond to chemical stimuli and thus have strong potential as colorimetric materials.