Solution electrochemistry of indolenium squarylium cyanine dyes

The redox potential of a class of donor¯acceptor¯donor (D¯A¯D) molecules, bis(5substituent-2,3,3,-trimethylindolenium-2-ylidene)squaraines (Sq 1¯8), were determined. All the dyes exhibit two reversible oxidation potentials in methanol. C-5 substituents at the indolenium moieties exert a small effect on the first oxidation potential, and the substituent effect is attributed to the inductive parameters. The frontier orbital energy levels were calculated by the AM1¯SCF method. A good linear relationship between the highest occupied molecular orbital energy level and the first oxidation potential was found.