Abstract: Owing to their sensitive chromic behaviour under external stimulation, such as light, heat, and electricity, viologen-based materials have shown considerable potential in optical switching and molecular sensor recognition applications. 4-Fluoroacetophenone disubstituted viologen (VioF₂·2Cl) was synthesised via a one-step reaction. The introduction of fluoroacetophenone substituents expands the conjugated system of the molecule which stabilises the radical structure and facilitates retention of the colour change for a longer period of time. The synthesized VioF2·2Cl exhibited sensitive photochromic properties, showing a rapid colour change from white to light yellow and then to yellow-green under ultraviolet (UV) light irradiation. Analysis of the crystal structure confirmed that the VioF2+ 2 molecules possess a shorter π · · · π∗ interaction distance after discoloration, which induces photoinduced electron transfer (PET) and thus stabilizes the viologen radical. VioF2·2Cl crystals can maintain their coloured radical state in an oxygen environment for three days without significant fading. This novel viologen derivative exhibits better sen- sitive photochromic properties and free radical colour stability essential for its application in the field of photochromic materials.

Key words: viologen derivative, photochromism, viologen radical