Synthesis and optoelectronic properties of air-stable π-conjugated polymers containing both thiophene-2,5-diyl and fused titanacycle units

Abstract

π-Conjugated poly(arylene ethynylene)s containing both thiophene-2,5-diyl and fused metallacycles such as titanafluorene-diyl and dithienotitanacyclopentadiene-diyl units in their alternating sequence were synthesized and their optoelectronic features were studied by the UV-vis spectra and theoretical calculations. The polymerization of 2,5-diethynyl-3-dodecylthiophene and arylene dihalides possessing fused organometallic units such as a 2,7-dibromo-9-titanafluorene and a 5,5′-diiodo-dithienotitanacyclopentadiene was carried out at 70 °C for 48 h in tetrahydrofuran (THF) in the presence of palladium dichloride/4,5-bis(diphenylphosphino)-9,9-dimethylxanthene as a catalyst and diisopropylamine as a base to produce black purple-colored polymers in 51% and 49% yields, respectively. The polymers are soluble in organic solvents such as THF, chloroform, and dichloromethane and they are stable under air at ambient temperature. In the UV-vis absorption spectra of the polymers thus obtained, the absorption maxima (λ_max) were observed at 395 nm and 486 nm, respectively, which are bathochromically shifted compared to those of model compounds, 2,7-bis(2-thienylethynyl)titanafluorene and 5,5′-bis(2-thienylethynyl)dithienotitanacyclopentadiene derivatives (λ_max = 372 and 418 nm, respectively). The optical band gap (E^opt_g) of the polymer containing the titanafluorene unit is estimated to be 2.28 eV on the basis of the absorption onset, which is narrower than that of the model compound, the bis(thienylethynyl)titanafluorene derivative (3.06 eV). In the case of the polymer containing the dithienotitanacyclopentadiene unit, E^opt_g was estimated to be 2.12 eV, which is also narrower than that of the model compound, the 5,5′-bis(2-thienylethynyl)dithienotitanacyclopentadiene derivative (2.51 eV).