Syntheses of new alternating CT-Type copolymers of thiophene and pyrido[3,4-b]pyrazine units:: Their optical and electrochemical properties in comparison with similar CT copolymers of thiophene with pyridine and quinoxaline

Four kinds of new pi-conjugated copolymers of electron-donating thiophene with highly electron-withdrawing pyridol[3,4-b]pyrazine derivatives have been prepared by using the Stille reaction and electrochemical oxidative polymerization in high yields, and their optical and electrochemical properties have been compared with those of previously reported CT-type pi-conjugated polymers. Chemically prepared polymers show an [eta] value of about 0.3 dL g(-1). The pi-pi* absorption bands (lambda(max) = ca. 633 nm) of the copolymers are observed at a longer wavelength by about 30 nm than those of similar CT-type copolymers of thiophene with pyridine and quinoxaline. These W-vis data are considered to reflect a stronger CT interaction between thiophene and pyrido[3,4-b]pyrazine, which has a higher electron-withdrawing ability than pyridine and quinoxaline. The copolymers are electrochemically active in both oxidation and reduction regions. In the reduction (n-doping) region, the copolymers show normal three couples of n-doping and n-undoping between -1.55 and -2.25 V vs Ag/Ag+. On the other hand, they receive oxidation (p-doping) at E-pa of 0.9 V vs Ag/Ag+. The electrochemical p- and n-doping of the film of the copolymers is accompanied by changes in its UV-vis spectra (electrochromism), and new absorption bands emerge in the range 900-1500 nm by the p- and n-doping. The X-ray diffraction pattern of the copolymer having a long alkyl side chain suggests self-assembling of the polymer assisted by side chain crystallization.