D-A-D type cyclohexyl and cycloheptyl-modified benzo[d]imidazole derivatives with different aggregation-induced emission enhancement (AIEE) and mechanofluorochromic properties

Abstract

Four tetraphenylethylene or triphenylamine-functionalized donor-acceptor-donor type benzo[d]imidazole derivatives 1–4 with different cycloalkyl substituents were successfully developed and characterized. All of these fluorogenic compounds showed remarkable aggregation-induced emission enhancement characteristics, and these luminogens emitted high brightness orange or green fluorescence in the aggregation state. Furthermore, these cyclohexyl or cycloheptyl-modified benzo[d]imidazole molecules exhibited tunable solid-state fluorescence effect, and their solids demonstrated contrasting mechanically triggered fluorescence responses. More specifically, luminogens 1–3 showed no observable fluorescence changes under mechanical stimulation, while tetraphenylethylene-based cycloheptyl-modified benzo[d]imidazole luminogen 4 displayed reversible mechanochromic fluorescence conversion between green and yellowish-green emission colors. The mechanofluorochromic mechanism of solid 4 was revealed by the powder X-ray diffraction measurements, and the experimental results proved that the phase interconversion between crystalline and amorphous states was responsible for the noticed switchable mechanoresponsive fluorescence. In addition, the mechanical force-sensitive emission of fluorogenic molecule 4 afforded its promising application in ink-free writing.