“Living” luminogens: light driven ACQ-to-AIE transformation accompanied with solid-state actuation

In situ photo-irradiation of trans-2-(4-fluorostyryl)benzo[d]oxazole (t-FSBO) and trans-2-(2,4-difluorostyryl)benzo[d]oxazole (t-2FSBO) with aggregation-caused quenching (ACQ) characteristics in the solid-state afforded unprecedented aggregation-induced emission (AIE) luminogens through [2+2] cycloaddition. Although the photoreaction products were weakly conjugated, they were emissive. Interestingly, the product cisoid-2,2′-(3,4-bis(2,4-fluorophenyl)cyclobutene-1,2-diyl)bis(benzo[d]oxazole) (c-2FPCBO) could form blue and yellow emissive crystals due to the different packing modes. Crystallographic results and computational calculation suggested that this anomalous phenomenon was attributed to intramolecular and intermolecular through-space conjugation. To the best of our knowledge, this is the first reported in situ ACQ-to-AIE transformation achieved by UV irradiation, deepening the understanding that non-conjugated molecules can be emissive. In addition to this novel and accessible ACQ-to-AIE transformation, the photosalient effect was also observed during the reactions. Notably, a macroscopic actuator was achieved by simply coating t-FSBO on a nitrile butadiene glove, which could bend over 70° towards UV light source. This system thus offers a new class of AIE luminogens with a “living” character.