Stimuli-responsive aggregation-induced fluorescence in a series of biphenyl-based Knoevenagel products: effects of substituent active methylene groups on π-π interactions.

A series of three biphenyl-based Knoevenagel products (denoted 1a, 1b, 1c) with active methylene groups has been synthesized. Compounds 1a and 1b show strong solid-state fluorescence, whereas 1c displays low emission. Effects of substituent groups in condensed phase packing of the molecules have been investigated and correlated with their photophysical properties. Interestingly, compound 1a exhibits mechanofluorochromism with emission color changes from yellow to green (wavelength shift of 40 nm) after mechanical grinding. Furthermore, fluorescence of 1a and 1b is turned off under alkaline conditions, making them potential candidates for aggregation-enhanced emission-based pH sensors.