Multi-color solid-state emission of β-iminoenolate boron complexes tuned by methoxyl groups: aggregation-induced emission and mechanofluorochromism

A series of new beta-iminoenolate boron complexes modified by methoxyl groups in different positions of the benzene rings were synthesized. They all exhibited AIE behaviours, for example, the emission intensity of B2B in THF/H2O (v/v = 1/19) was ca. 91-fold higher than that in THF. Interestingly, the emitting color for B4B changed from green to yellow during the AIE process on account of the different aggregated states. In addition, it was found that the molecular stacking modes enormously affected the solid emission of beta-iminoenolate boron complexes. B2B emitted blue light due to the weakest pi-pi interaction in the crystal. BB, B3B and B24B emitted blue-green light on account of their similar packing modes in crystals, in which moderate pi-pi interactions were involved. In the case of crystal B4B, pi-aggregates were generated directed by strong pi-pi interactions, leading to yellow emission. On the other hand, B4B exhibited multi-color emission upon stimulation by mechanic forces. When B4B was ground into ground powder 1 in a mortar, pi-aggregates were damaged and the emitting color changed from yellow to green. If a small amount of ground powder 1 was further ground on a parchment, it was turned into ground powder 2 emitting blue light derived from the isolated molecules. The fluorescence of ground powder 1 could recover to yellow upon exposure to DCM. This meant that the emitting color changes between yellow and green were reversible. Similarly, reversible mechanofluorochromic processes between ground powder 1 and 2 were observed. Although the crystal with yellow emission could be changed into ground powder 2, only green emitting ground powder 1 was gained after treating with DCM. This kind of mechanofluorochromic materials with multi-color emission might have potential applications in anti-fake labels and mechanic sensors.