π-Conjugated and Light Emitting Poly(4,4‘-dialkyl-2,2‘-bithiazole-5,5‘-diyl)s and Their Analogues Comprised of Electron-Accepting Five-Membered Rings. Preparation, Regioregular Structure, Face-to-Face Stacking, and Electrochemical and Optical Properties

Poly(4,4‘-dialkyl-2,2‘-bithiazole-5,5‘-diyl)s (PRBTz, alkyl = methyl (PMeBTz), butyl (PBuBTz), and heptyl (PHepBTz)) and their analogues comprised of 33−150 thiazole rings have been prepared by organometallic polycondensation, and their chemical and physical properties are compared with those of π-conjugated poly(thiophene-2,5-diyl) (PTh) and poly(pyridine-2,5-diyl) (PPy). Electrochemical n-doping of PRBTz takes place at E ° = −1.77 to −2.30 V vs Ag/Ag + and is accompanied by the appearance of a new absorption band in the near-infrared. PMeBTz assumes a relatively stiff structure in solution and shows a large refractive index increment of 0.55 cm 3 g - 1; powder X-ray diffraction analysis of PMeBTz supports a face-to-face type stacking of the polymer chains in the solid state. All of the polymers show photoluminescence in solutions and in the solid, and an electroluminescence device using PMeBTz as the emitting layer gives emission of light at λ max = 680 nm with 100 cd m - 2 at 8 V. A thin film of PMeBTz gives an optical third-order nonlinear susceptibility χ (3) of 2.5 × 10 - 11 esu, which is larger than observed with PTh and PPy films, and comparison of the χ (3) value with that (0.3 × 10 - 11 esu) of nonregioregular poly(4-methylthiazole-2,5-diyl) (PMeTz) reveals the importance of the regioregular structure of PMeBTz to give the larger χ (3) value.