2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-b]pyrrole (AAPP) AIEgens: tunable RIR and TICT characteristics and their multifunctional applications

Aggregation-induced emission luminogens (AIEgens) have attracted extensive interest for their outstanding luminescence properties in the aggregated state and even in the solid state. In this work, we developed a series of novel AIEgens based on 2,5-bis(4-alkoxycarbonylphenyl)-1,4-diaryl-1,4-dihydropyrrolo[3,2-<i>b</i>]pyrrole (<b>AAPP</b>). The AIEgens can be facilely synthesized through a single-step reaction under mild conditions with satisfactory yields. Interestingly, <b>AAPP</b> was found to have multiple luminous mechanisms that result in variable fluorescence properties. The propeller-like structure of <b>AAPP</b> enables a restricted intramolecular rotation (RIR) process which significantly enhances its fluorescence in the aggregated state (<i>i.e.</i> AIE fluorescence). In addition, there is a donor-acceptor interaction between the heterocycle center and the alkoxycarbonyl units in <b>AAPP</b> which allows a typical twisted intramolecular charge transfer (TICT) process in the dispersed state, resulting in strong fluorescence emissions in non-polar or low-polarity solvents but fluorescence quenching in high-polarity solvents. Due to the tunable RIR and TICT processes and the multiple fluorescence, <b>AAPP</b> compounds exhibit multifunctional applications: (1) as a reversible fluorescent thermometer, <b>AAPP</b> exhibited excellent fatigue resistance. There was a good linear relationship between its fluorescence intensity and temperature from 10 °C to 60 °C. (2) The desethyl <b>AAPP</b> derivative (<b>CAPP</b>) was successfully applied in the detection of Cd(ii) in aqueous solution at neutral pH, and showed a 500-fold fluorescence "turn-on" response to Cd(ii) with good selectivity.