| Abstract
| - Various π-conjugated copolymers constituted of π-excessivethiophene, selenophene, or furan units(Ar) and π-deficient pyridine or quinoxaline (Ar‘) units have beenprepared in high yields by the followingorganometallic polycondensation methods: (i) nX−Ar−Ar‘−X + n Ni(0)Lm →(-Ar−Ar‘)-n(X = halogen, Ni(0)Lm = zerovalent nickel complex), (ii) n X−Ar−X +n Me3Sn−Ar‘−SnMe3 →(-Ar−Ar‘)-n(palladium catalyzed),and (iii) a X−Ar−X + b X−Ar‘−X +(a + b)Ni(0)Lm →(-Ar)x(Ar‘)-y.Powder X-ray diffraction analysis confirmsan alternative structure of a polymer prepared by the method ii.The copolymers have a molecular weight of 5.4 ×103 to 3.3 × 105 and an [η] value of 0.37to 4.4 dL g-1. π−π* absorption bands of thecopolymers generally showred shifts from those of the corresponding homopolymers,(-Ar)-n and(-Ar‘)-n,and the red shifts are accounted for bycharge-transferred CT structures of the copolymers. For example,an alternative copolymer of thiophene and 2,3-diphenylquinoxaline gives rise to an absorption band atλmax = 603 nm, whereas homopolymers of thiopheneand2,3-diphenylquinoxaline exhibit absorption peaks at about 460 and 440nm, respectively. The CT copolymers areelectrochemically active in both oxidation and reduction regions,showing oxidation (or p-doping) peaks in a rangeof 0.39 to 1.32 V vs Ag/Ag+ and reduction (orn-doping) peaks in a range of −1.80 to −2.22 V vsAg/Ag+,respectively. Copolymers of pyridine give unique cyclicvoltammograms exhibiting p-undoping peaks at potentialsmuch different (about 2−3 V lower) from the corresponding p-dopingpotentials, and this large difference betweenp-doping and p-undoping potentials is explained by an EC mechanism.They are converted into semiconductors bychemical and electrochemical oxidation and reduction. Copolymersof thiophene with pyridine and quinoxalineshow the third-order nonlinear optical susceptibilityχ(3) of about 5 × 10-11 esu at thethree-photon resonant wavelength,which is 5−7 times larger than those of the correspondinghomopolymers and related to the CT structure in thecopolymers.
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