An Improved Electrochromic Display

During the past decade there has been a concen-trated effort (1-7) to develop a passive electrochromic display (ECD) based upon the reversible reduction and oxidation of viologens. The basic structure of these 4,4'-dipyridinium salts is shown below where X- is typical ly bromide or dihydrogen phos-phate. While there has been an investigation of a var i-ety of these compounds with symmetrical alkyl end groups, the major i ty of the development work has been with salts of 1,1'-diheptyI-4,4'-dipyridinium, that is, where R1---- R2-- (n-- C7H15). The abi l i ty to deposit a uniform purple film onto a transparent electrode (SnO~) by the one electron reduction of this dipositive cation, has been well established (6). However, it has been demonstrated (3, 7) that while this crystal l in