Electrochemical Color Change of 1,4‐Diketo‐3,6‐Diphenyl‐Pyrrolo‐[3,4‐c]‐Pyrrole

The title compound (DPP) is a red pigment based on the diketopyrrolopyrrole chromophore. DPP is, however, transparent in solution and appears slightly greenish‐yellow. It has been found that DPP turns red when electrochemically reduced (“electrochromism”). Electrochromic properties of DPP have thus been investigated in the presence of sodium ferrocyanide or ferrocene used as a sub‐redox system. Attention has been paid to the elucidation of the coloration mechanism by means of voltammetry and spectroscopy. The first reduction of DPP proceeds in a one‐electron process and is reversible, whereas the second reduction is irreversible. The electrochromic red color arises as a result of deprotonation at the NH group which follows the cathodic reduction. The mono and di‐deprotonation give absorption maxima at 582 and 620 nm, respectively.