Bridging the Gap between Polyfluorene and Ladder-Poly-p -phenylene:  Synthesis and Characterization of Poly-2,8-indenofluorene

We present here a new derivative of poly-p-phenylene (PPP), the poly-2,8-indenofluorene (8a,b), to bridge the gap between 2,7-poly-9,9‘-dialkylfluorene (PF) (2) and ladder-poly-p-phenylene (LPPP) (4). The monomer 7a,b was synthesized starting from 6,12-dihydroindeno[1,2b]fluorene (5), which was four times alkylated, brominated with copper(II) bromide on an aluminum oxide matrix, and polymerized via a Ni(0)-mediated coupling reaction (M n ∼ (3.3−3.9) × 104 (tuluene, polystyrene standards)). As expected, the wavelength data for absorption and emission are intermediate to those of PF and LPPP and prove the suitability of the polymer to be used as blue-light-emitting material in polymer light-emitting diodes. The polymer 8a,b is stable up to 380 °C and shows thermotropic liquid crystalline behavior at high temperatures (250−300 °C). Oligomers of the 6,6‘,12,12‘-tetraoctylindenofluorene (6a) have been synthesized by Ni(0)-induced coupling of monobromo and dibromo (7a) compounds, to determine the effective conjugation length (n ∞ ,abs = 6−7, n ∞ ,em = 5−6).