Synthesis, crystal structures and solid-state acidochromism of multiaryl-substituted pyridine derivatives with aggregation-induced emission property

Abstract

Pyridine derivatives have been widely reported as fluorescent sensors in solution phase, but they are rarely used in solid state due to aggregation-caused quenching effect. Herein, two new multiaryl-substituted pyridine derivatives with benzofuran (ABFP) and benzothiophene (ABTP) at 6-position were synthesized to study solid-state emissions and fluorescence stimulus-responsive properties. These two compounds have highly twisted molecular conformations, as revealed by the single-crystal structure analyses. They emit slight fluorescence in pure organic solutions due to the free rotations of peripheral aromatic rings, whereas show strong blue/cyan emissions in aggregated state because of the restriction of intramolecular rotation, revealing typical aggregation-induced emission (AIE) characteristic. These compounds adopt loose stacking arrangements and have weak intermolecular interactions, which are advantageous to strong solid-state emissions. Upon the stimulation of trifluoroacetic acid-ethylenediamine, the ABFP and ABTP solid-state samples exhibit a transformation in fluorescence colour between blue and green/yellow-green, revealing reversible acidochromic activities. Furthermore, the protonation of the nitrogen atom on the pyridine ring is unambiguously confirmed to be responsible for the acidochromic activities in solid state by the single-crystal structure analyses. The work provides important information for the design of the fluorescent molecules with AIE and solid-state acidochromism based on the traditional pyridine skeleton.