Drastic Modulation of Stimuli-Responsive Fluorescence by a Subtle Structural Change of Organic Fluorophore and Polymorphism Controlled Mechanofluorochromism

Stimuli-responsive fluorescence modulation of organic fluorophores is closely related to their structural organization, noncovalent interactions, ability to adopt different conformation, and phase change in the solid state. Herein, we have synthesized aggregation enhanced emissive fluorophores, (5-(4-(diphenylamino)­benzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (1), 5-(4-(diphenylamino)-2-methoxybenzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (2), and 5-(4-(diphenylamino)-4-methoxybenzylidene)-2,2-dimethyl-1,3-dioxane-4,6-dione (3)) and demonstrated molecular structure controlled tunable fluorescence (552 to 616 nm, Φ_f = 14.6–41.8%) and stimuli responses. One showed thermofluorochromism between 586 and 558 nm at room and liquid N₂. 2 showed tunable fluorescence via polymorphism (2a (550 nm) and 2b (610 nm)). Interestingly, hard crushed −2b polymorph showed two different mechanofluorochromism (MFC) when heated at 80 and 180 °C as well as topochemical conversion from 2b to 2a. In contrast, 2a and 3 displayed usual MFC. Crystal structure, powder X-ray diffraction, and differential scanning calorimetric studies indicated conformational, structural, and phase changes with different stimuli which are responsible for fluorescence switching/tuning. Computational studies revealed that optical band gap modulation depend on the molecular conformation and support the fluorescence modulation.