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

Aggregation-induced emission luminogens (AlEgens) 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 AlEgens based on 2,5-bis(4-alkoxycarbonylpheny0-1,4-diaryl-1,4-dihydropyrrolo[3,2-b] pyrrole (AAPP). The AlEgens can be facilely synthesized through a single-step reaction under mild conditions with satisfactory yields. Interestingly, AAPP was found to have multiple luminous mechanisms that result in variable fluorescence properties. The propeller-like structure of AAPP enables a restricted intramolecular rotation (RIR) process which significantly enhances its fluorescence in the aggregated state (i.e. AIE fluorescence). In addition, there is a donor-acceptor interaction between the heterocycle center and the alkoxycarbonyl units in AAPP 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, AAPP compounds exhibit multifunctional applications: (1) as a reversible fluorescent thermometer, AAPP exhibited excellent fatigue resistance. There was a good linear relationship between its fluorescence intensity and temperature from 10 degrees C to 60 degrees C. (2) The desethyl AAPP derivative (CAPP) 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.