Luminescent alternating boron quinolate–fluorenecopolymers exhibiting high electron mobility

Abstract

π-Conjugated copolymers composed of 9,9-didodecylfluorene and boron quinolate were synthesized by an efficient chelating reaction of quinolinol-based polyfluorene derivative, as a polymeric ligand, with arylborane compounds such as triphenylborane (BPh₃) or fluorobis(pentafluorophenyl)borane diethyl etherate ((C₆F₅)BF·OEt₂). The polymeric ligand was prepared from the deprotection reaction of boron tribromide and benzyloxyquinoline-based polyfluorene, which was produced via the efficient Suzuki cross-coupling reaction of 5,7-dibromo-8-benzylquinoline and 9,9-didodecylfluorene-2,7-diboronic acid in the presence of 2-(2′,6′-dimethoxybiphenyl)dicyclohexylphosphine (S-Phos) and palladium(II) acetate. Their optical properties were studied by UV-vis absorption and photoluminescence spectroscopies. The absorption and PL spectra of the obtained polymers were red-shifted in comparison with those of the polymeric ligand due to the decrease of the LUMO level by the formation of boron complexes, explained by theoretical calculations of the model compounds using the density-functional theory (DFT) method. The molar extinction coefficients and emission maxima of boron-chelating polymers were analogous values to those of (κ²-(N,O)-5,7-bis(9,9-dihexylfluoren-2-yl)-8-quinolinolate)diphenylborane (BFluorene₂q). Furthermore, the electron mobilities of the polymers were determined from the space-charge limited current (SCLC) with electron-only device structure of ITO/Ca/polymer/LiF/Al. As a result, the mobilities for the polymers (3.9 or 2.0 × 10⁻⁵ cm² V⁻¹ s⁻¹) were reasonably close to the value (7.9 × 10⁻⁵ cm² V⁻¹ s⁻¹) calculated from the Alq₃-fabricated device.