Dehalogenation polycondensation of corresponding dihalo compounds with a zerovalent nickel complex gives pi-conjugated polymers constituted of pyridine units and 2,2'-bipyridine units in high yields. Poly(pyridine-2,5-diyl) (PPy), poly(2,2'-bipyridine-5,5'-diyl) (PBpy), 3-, 4-, and 6-methylated poly(pyridine-2,5-diyl)s (PMePy's), poly(6-hexylpyridine-2,5-diyl) (P6HexPy), poly(3,3'-dimethyl-2,2'-bipyridine-5,5'-diyl) (P3MeBpy), and poly(6,6'-dihexyl-2,2'-bipyridine-5,5'-diyl) (P6HexBpy) are constituted of 42-300 pi-conjugated pyridine rings as measured by light-scattering methods. PPy and PBpy have a rigidly linear rodlike structure as revealed by their showing a theoretically limiting rho(Y) (degree of depolarization) value of 0.33, and they exhibit a large refractive index increment (Delta n/Delta c = 0.59 cm(3) g(-1)) and a large refractive index of n(D) = 2.2. Stretching of poly(vinyl alcohol) film containing the PPy or PBpy molecules in its surface region affords a polarizer which shows a dichroic ratio of 45. The PBpy molecules stand upright on a carbon substrate in a PBpy film vacuum deposited on the carbon substrate as revealed by electron diffractometry. On the other hand, PBpy molecules in a film vacuum deposited on a glass substrate are oriented in parallel with the surface of the glass substrate as revealed by analysis of optical second-harmonic generation from the PBpy film, which shows alignment of all the PBpy molecules attached to the glass substrate (1 x 1 cm) in a same direction: coordination of a PBpy molecule to the Si-O-H group is proposed to explain such orientation of the PBpy molecules. PPy exhibits fluorescence with a peak at 440 nm in a dilute solution (2 x 10(-6) M monomer unit), whereas PPy shows an additional excimer-like emission at 550 nm in a saturated solution (0.5 M monomer unit) and PPy and PBpy films emit only the excimer-like emission at 550 nm. Picosecond time-resolved fluorescence also supports the excimer-like fluorescence. PPy, PBpy, and their alkyl derivatives are electrochemically reduced or n-doped more easily than poly(p-phenylene) and poly(thiophene-2,5-diyl), reflecting pi-electron-deficient nature of the pyridine ring, and E(0) values of -2.2 through -2.5 v vs ag/ag(+) are observed for the polymers; the n-doping and its reverse reaction (n-undoping) are accompanied by a color change (e.g., yellow in the n-undoped state and blue in the n-doped state for PPy and PBpy). Chemically n-doped PPy and PBpy with sodium naphthalide have electrical conductivities of 1.1 X 10(-1) and 1.6 X 10(-1) S cm(-1), respectively, as measured with compressed powder. On the contrary, PPy, PBpy, and their alkyl derivatives undergo p-doping neither electrochemically nor chemically (e.g., by treatment with AsF5), reflecting the pi-deficient nature of the pyridine ring. PBpy and P6HexBpy form complexes with Ru(II), Ni(II), Ni(0), and Fe(III) species, and cyclic voltammetry of the PBpy-Ru complex reveals electron exchange between the coordinated Ru species, which is considered to occur through the pi-conjugation system of the conjugated polymer ligand. The complexes are active for photoevolution of H-2 from aqueous media as well as for reduction of CO2.
