Hyperbranched blue to red light-emitting polymers with tetraarylsilyl cores: Synthesis, optical and electroluminescence properties, and ab initio modeling studies

A series of hyperbranched alternating copolymers were synthesized by either Suzuki or Grignard coupling polycondensation reactions between two AB(4)-type tetrahedral monomers [tetra(4-bromobiphenyl)silane (1) and tetra(2-bromo-9,9-dihexylfluorenyl)silane (2)] with AB(2)-type dibromo or diboronic acid monomers of 9,9-dihexylfluorene or oligothiophenes. Polymers (3, 5-8) (derived from 1 and 9,9-dihexylfluorene-2,7-diboronic acid, 2,2'-bithiophene-5,5'-diboronic acid, 2,2':5',2-terthiophene-5,5-diboronic acid, 5,5'-dibromo-3,3'-dihexyl-2,2'-bithiophene, and 5,5-dibromo-3,3'-dihexyl-2,2':5',2-terthiophene, respectively) emitted blue to red light in the solid state and their absorption and emission maxima were red shifted with the increase in conjugation length. Polymer 4 (derived from 2 and 9,9-dihexylfluorene-2,7-diboronic acid) emitted blue light highly efficiently in both solution and the condensed state, but its absorption and emission maxima were slightly blue-shifted from those of 3, revealing that no effective electron delocalization occurred among the fluorene units. Polymer 9 (derived from 2 and 5,5'-dibromo-3,3'-dihexyl-2,2'-bithiophene) emitted intense blue light in solution with only the structured peaks from the fluorene units being observed in its PL spectra. The film of 9 emitted green light and emissions from both the fluorene and bithiophene chromophores were observed in the PL spectra with the latter being stronger, due to the intramolecular energy transfer from fluorene units to bithiophene units. The ab initio studies were carried out to elucidate the intrigue optical properties of the present hyperbranched polymers. Double layer devices with configurations ITO/PEDOT/polymer (4 or 7)/LiF/Ca/Ag, which emitted blue and green light, respectively, were fabricated and preliminary data on the device performances are reported. We present two strategies for the design and synthesis of RGB-emitting hyperbarched polymers: (1) using a structurally not crowded core 1 and adjusting the emission by conjugation length; (2) using a structurally crowded core 2 and adjusting the light emission by intramolecular energy transfer.