light. After accepting an electron it gets reduced and form radical cation EV$^{\newmoon +  }$. In the reduced state it shows absorption peak at 396 and 606nm which indicate blue color of the device\[22\]. The advantage with viologen is that it shows reversible switching between the two oxidation states mentioned above. In other words, EV$^{\newmoon +  }$ can go back to EV$^{2+}$ by oxidation, which shows no absorption in the visible region which is transparent state of the device. Details of redox reactions taking place during oxidation and reduction of EV have been provided in SI separately. A solid state electrochromic device can be made using viologens by appropriately controlling the redox process using an electric bias.

![Figure 1: (a) Schematic representation of cross-bar geometry device connecting through the external biasing arrangement (b) photographs of device without and with applied bias, (c) Schematic model of change in oxidation state of viologen inside the device as a result of applied bias.](figure1){width="10cm"}

Many of the viologen applications are largely based on its ability to change in the electron absorption spectrum by reversible oxidation–reduction reaction. This feature of viologen can be tuned by altering the substituents on the nitrogen and used it for 