π-Conjugated Donor−Acceptor Copolymers Constituted of π-Excessive and π-Deficient Arylene Units. Optical and Electrochemical Properties in Relation to CT Structure of the Polymer

Abstract

Various π-conjugated copolymers constituted of π-excessive thiophene, selenophene, or furan units (Ar) and π-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 = zerovalent 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 × 10 3 to 3.3 × 10 5 and an [η] value of 0.37 to 4.4 dL g -1 . π−π* 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 λ 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 χ (3) of about 5 × 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.