pi-conjugated donor-acceptor copolymers constituted of pi-excessive and pi-deficient arylene units. Optical and electrochemical properties in relation to CT structure of the polymer

Various pi-conjugated copolymers constituted of pi-excessive thiophene, selenophene, or furan units (Ar) and pi-deficient pyridine or quinoxaline (Ar') units have been prepared in high yields by the following organometallic polycondensation methods: (i) n X-Ar-Ar'-X + n Ni(0)Lm --> (Ar-Ar')(n) (X = halogen, Ni(0)-Lm = zerovlent nickel complex), (ii) n X-Ar-X + n Me(3)Sn-Ar'-SnMe(3) --> (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 confirms an alternative structure of a polymer prepared by the method ii. The copolymers have a molecular weight of 5.4 x 10(3) to 3.3 x 10(5) and an [eta] value of 0.37 to 4.4 dL g(-1). pi-pi* absorption bands of the copolymers generally show red shifts from those of the corresponding homopolymers, (Ar)(n), and (Ar')(n), and the red shifts are accounted for by charge-transferred CT structures of the copolymers. For example, an alternative copolymer of thiophene and 2,3-diphenylquinoxaline gives rise to an absorption band at lambda(max) = 603 nm, whereas homopolymers of thiophene and 2,3-diphenylquinoxaline exhibit absorption peaks at about 460 and 440 nm, respectively. The CT copolymers are electrochemically active in both oxidation and reduction regions, showing oxidation (or p-doping) peaks in a range of 0.39 to 1.32 V vs Ag/Ag+ and reduction (or n-doping) peaks in a range of -1.80 to -2.22 V vs Ag/Ag+, respectively. Copolymers of pyridine give unique cyclic voltammograms exhibiting p-undoping peaks at potentials much different (about 2-3 V lower) from the corresponding p-doping potentials, and this large difference between p-doping and p-undoping potentials is explained by an EC mechanism. They are converted into semiconductors by chemical and electrochemical oxidation and reduction. Copolymers of thiophene with pyridine and quinoxaline show the third-order nonlinear optical susceptibility chi((3)) of about 5 x 10(-11) esu at the three-photon resonant wavelength, which is 5-7 times larger than those of the corresponding homopolymers and related to the CT structure in the copolymers.