Properties of polymorphism and acid response of pyrrolopyrrole-based derivative with aggregation-induced emission behavior

A new A-D-A type pyrrolopyrrole-based derivative 4′,4″-(2,5-diphenyl-pyrrolo[3,2-b]pyrrole-1,4-diyl)bis ([1,1'-biphenyl]-4-carbonitrile) (DPPDC) was synthesized via Suzuki coupling reaction between 1,4-bis(4-bromophenyl)-2,5-diphenyl-1,4-dihydropyrrolo[3,2-b]pyrrole and 4-cyanophenylboronic acid. The fluorescent emission intensities of DPPDC in pure THF solution and lower fraction of water (φ_H2O≤60%) mixtures were weak at around 550 nm. When φ_H2O was 99% in THF/H₂O mixtures, the emission was enhanced and blue-shifted at around 505 nm. The maximal fluorescent emission intensity of DPPDC was 11 times higher than that of in pure THF solution, indicating DPPDC exhibiting AIE property. It was also found that four different kinds of crystal structures of DPPDC was cultivated from CHCl₂-Hexane, CHCl₃-Hexane and CHCl₃/Acetone-Hexane systems via solvent slow diffusion method. Four crystals respec-tively emitted blue, azure, green and turquoise at 467, 483, 496 and 493 nm, which manifested the polymorphism-dependent fluorescent emission property of DPPDC. Additionally, trifluoroacetic acid (TFA) could make the emitting color change from yellow to orange-red with as-prepared paper containing DPPDC due to the acid-base interaction. The obvious emitting color change of DPPDC can be used as a visual sensor to detect acid gas.