Effect of the acceptor moiety on the electrochemical and electrochromic properties of Donor-Acceptor-Donor polymer films

The aim of this work is to synthesize EC polymers having green or blue colour in the neutral state and transparent oxidized state by using the D-A-D approach. Two different monomers, diphenylthienopyrazine (M1) and diphenylquinoxaline (M2) are synthesized. The monomers are successfully electropolymerized to P1 and P2, respectively, by cyclic voltammetry on ITO substrates. The effect of the acceptor moiety on the electrochemical and electrochromic properties of the polymer films is investigated. Both polymers exhibit the characteristic redox couple due to their oxidation/reduction at 0.95 V/0.62 V for P1 and at 1.05 V/0.35 V for P2; their doping/dedoping reaction is reversible. The two polymers have different optical properties; namely, the phenyl ring in the acceptor unit (P2) caused hypsochromic shift. The acceptor moiety affects mainly the LUMO level; specifically, the LUMO level shifted to less negative values by replacing thiophene with phenyl. The electrochromic properties of the polymer films are strongly affected by the acceptor moiety. Polymer P1 changes its colour from green (neutral state), to grey (intermediate oxidised state), to transparent grey/sky blue (oxidized state) and to lemon green (reduced state). Correspondingly, P2 starts from royal blue (neutral state), going to blue purple, grey-blue and light grey (intermediate oxidised states) and then to green (oxidized state); additionally, it became also oil green and light blue (reduced states). Both polymers are multi-coloured and were good candidates for application in electrochromic devices.