Control over rotary motion and multicolour switching in 3-hydroxyphthalimide fluorophores: An interplay between AIE and ESIPT

In this paper, we demonstrate the ability of N-substituted 3-hydroxyphthalimides with pyridine and isoquinoline rings to perform multicolour switching by different stimuli. Restriction of the C–N rotary motion of the heterocycles allows control over their photoluminescence performance (PLQY = 54–65%) in water-induced aggregation by an additive interplay of excited-state proton-mediated processes in the aggregates. Restriction of the rotations by the affinity of the heterocyclic nitrogen atoms to Zn2+ and switching to new emission bands (PLQY = 8–29%) in solution and solid state were also achieved. Protonation of the pyridine/isoquinoline rings allows the formation of intramolecular hydrogen bonds with the imide carbonyl groups, resulting in restricted rotations. The protonated forms switch to strong blue emission due to local excitation of the protonated heterocycles, which are unable to undergo proton transfer. Understanding the interplay between both photophysical processes would allow the switching of emissions by multistimuli.