Fluorophore unit controlled photoswitching of hydrazone derivatives: reversible and irreversible off-on/dual-color fluorescence photoswitches

Hydrazone photoswitching functionalities integrated with three different fluorophore units (perylene (PY-PH), tetraphenylethylene (TPE-PH) and triphenylamine (TPA-PH)) were synthesized and demonstrated fluorophore unit dependent fluorescence photoswitching and mechanofluorochromism. TPE-PH and TPA-PH exhibited reversible off-on and dual-state fluorescence photoswitching, respectively, due to conformational changes upon light irradiation. In contrast, PY-PH showed irreversible photoswitching. Further, TPA-PH displayed fluorescence switching between two fluorescence states whereas TPE-PH showed off-on switching upon crushing and heating. PY-PH did not show clear mechanofluorochromism. Thus, the present work explored the structural impact on the fluorescence photoswitching and mechanofluorochromism that might be useful for developing new photofluorochromic/mechanofluorochromic smart materials for multifunctional applications.