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 w ith highly electron-withdrawing pyridol[3,4-b]pyrazine derivatives have bee n prepared by using the Stille reaction and electrochemical oxidative polym erization 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 copoly mers are observed at a longer wavelength by about 30 nm than those of simil ar CT-type copolymers of thiophene with pyridine and quinoxaline. These W-v is data are considered to reflect a stronger CT interaction between thiophe ne and pyrido[3,4-b]pyrazine, which has a higher electron-withdrawing abili ty than pyridine and quinoxaline. The copolymers are electrochemically acti ve in both oxidation and reduction regions. In the reduction (n-doping) reg ion, the copolymers show normal three couples of n-doping and n-undoping be tween -1.55 and -2.25 V vs Ag/Ag+. On the other hand, they receive oxidatio n (p-doping) at E-pa of 0.9 V vs Ag/Ag+. The electrochemical p- and n-dopin g of the film of the copolymers is accompanied by changes in its UV-vis spe ctra (electrochromism), and new absorption bands emerge in the range 900-15 00 nm by the p- and n-doping. The X-ray diffraction pattern of the copolyme r having a long alkyl side chain suggests self-assembling of the polymer as sisted by side chain crystallization.