Effect of Connecting Units on Aggregation-Induced Emission and Mechanofluorochromic Properties of Isoquinoline Derivatives with Malononitrile as the Terminal Group

Two isoquinoline derivatives IQ-BIM and IQ-BM with a terminal malononitrile unit, in which the double bond and single bond are used as the connecting units, respectively, are obtained simultaneously by the reaction of an isoquinoline derivative with malononitrile in the presence of a catalytic amount of triethylamine. The different connecting units show an important effect on their photophysical properties. IQ-BIM with an acceptor−π–acceptor structure exhibits an obvious aggregation-induced emission (AIE) phenomenon in the tetrahydrofuran–water mixed solvent owing to the restriction of intramolecular rotation but no obvious solvatochromic property, whereas IQ-BM with a donor−π–acceptor structure shows obvious solvatochromic and dual-state emission properties, and its emissions in the mixtures are mainly dominated by the intramolecular charge transfer effect. This result indicates that the existence of double bonds is beneficial to the occurrence of the AIE phenomenon. Although both these compounds exhibit outstanding mechanofluorochromic (MFC) activities through the crystalline-to-amorphous transition, the red shift of the fluorescence spectrum of IQ-BIM is attributed to the increased molecular conjugation, whereas that of IQ-BM is due to the generation of the excimers induced by the π–π interactions. Moreover, the former displays a higher contrast MFC phenomenon than the latter due to the looser stacking arrangement. This work gives important reference values for understanding the influence of the connecting units on the photophysical properties of fluorescent materials.