Hyperbranched and thermally cross-linkable oligomer from a new 2,5,7-tri-functional fluorene monomer

New hyperbranched (HOFV) and linear oligomers (LOFV) were prepared from 2-bromo-5,7-divinyl-9,9-dihexylfluorene (AB₂) and 2-bromo-7-vinyl-9,9-dihexylfluorene, respectively, by the Heck reaction to study the effect of hyperbranched structure. The oligomers were readily soluble in common organic solvents. The weight-average molecular weights (M_w) of HOFV and LOFV, determined by gel permeation chromatography using polystyrene as standard, were 2350 and 3950, respectively. Optical properties, both in solution and film state, were investigated using absorption and photoluminescence (PL) spectra. In film state, the absorption and PL spectra peaked at 416-425 nm and 473-503 nm, respectively. The HOFV showed energy funnel effect and enhanced fluorescence efficiency owing to the hyperbranched structure. The HOMO and LUMO levels of HOFV (LOFV), estimated from their cyclic voltammograms, were -5.25 (-5.34) eV and -2.66 (-2.75) eV, respectively. Thermal curing of HOFV to form cross-linked HPFV (hyperbranched poly(fluorenevinylene)) was studied by IR, DSC, UV-visible spectra, NMR, AFM and SEM. The terminal vinyl groups in HOFV film almost disappeared to provide smooth, homogeneous and solvent-resistant films of HPFV. Two-layer PLED devices (ITO/PEDOT/HPFV/Ca/Al) exhibited maximal luminance and luminous efficiency of 1480 cd/m² and 0.18 cd/A, respectively, which were superior to its linear counterpart LPFV (352 cd/m², 0.06 cd/A).