Multi-stimuli-responsive behaviours of fluorenone-based donor–acceptor–donor triads in solution and supramolecular gel states

Bilaterally asymmetric and symmetrical fluorenone-based donor–acceptor–donor (D–A–D) triads bearing hydrogen-bonded urethane units were synthesised in this study. Both the fluorenone derivatives exhibited supramolecular gelation in organic solvents. While the bilaterally asymmetrical fluorenone-based mono-urethane formed supramolecular gel in medium-chain linear alkanes and n -dodecyl benzene, the bilaterally symmetrical bis-urethane gelated to form n -dodecyl benzene and some mixed solvents. In the absorption spectra of these two compounds, two absorption peaks attributable to π–π* and intramolecular charge transfer (ICT) transitions were found. These D–A–D triads show emission from a twisted intramolecular charge transfer (TICT) state, together with photoluminescence from locally excited states. Solvatofluorochromic and acidifluorochromic behaviours were confirmed in dilute solutions for both compounds. While the fluorenone-based mono-urethane organogel displayed an acid-responsive colour change, the aromatic bis-urethane organogel changed its aggregation behaviour in response to trifluoroacetic acid. This study revealed that these D–A–D triads have the potential to be used as chemoresponsive materials for the multi-mode detection of target species.