Poly(aryleneethynylene) type polymers containing a ferrocene unit in the pi-conjugated main chain. Preparation, optical properties, redox behavior, and Mossbauer spectroscopic analysis

Palladium-catalyzed polycondensation between diiodoferrocenes (1,1'-diiodoferrocene and 1,6-diiodo-1',6'-biferrocene) and diethynyl aromatic compounds HC=C-Ar-C=CH (e.g., 2,5-diethynyl-pyridine and 2,5-diethynyl-3-hexylthiophene) gives poly(aryleneethynylene) (PAE) type polymers containing the ferrocene unit in the pi-conjugated main chain. The prepared polymers include (-Fc-C=C-Ph-C=C-)(n), PAE-Fc-1, (-Fc-C=C-Py-C=C-)(n), PAE-Fc-2, and (-Fc-C=C-hexTh-C=C-)(n) PAE-Fc-3 (Fc = 1,1'-ferrocenylene; Ph = p-phenylene; Py = pyridine-2,5-diyl; hexTh = 3-hexylthiophene-2,5-diyl). H-1-NMR and IR spectra of the polymers are reasonable for their structures. The PAE type polymer containing the pyridine unit (PAE-Fc-2) is soluble in formic acid, and the polymer containing the hexylthiophene unit (PAE-Fc-3) is soluble in common organic solvents such as CHCl3, THF, and benzene. UV-visible spectra of the polymers exhibit a main pi-pi* absorption peak at about 330 nm and a d-d absorption peak at about 450 nm. The cyclic voltammogram of the polymers in a CH3CN/CH2Cl2 solution shows a reversible Fe(II) reversible arrow Fe(III) redox cycle at about 0.25 V vs Ag/Ag+, and the redox peaks are broadened compared with those of low-molecular-weight ferrocenes. Exchange of electrons along the main chain is considered to be the origin of the broadening. The polymers themselves are insulating, however, they are converted into semiconducting materials with conductivity of 10(-7) to 10(-4) S cm(-1) by formation of adducts with iodine. Mossbauer spectra of the polymers reveal oxidation of Fe(II) in the ferrocene unit of PAE-Fc-1 to Fe(III), and the ease of the oxidation reflects the electronic properties of the polymer.