2,7-bis(diarylamino)-9,9-dimethylfluorenes as hole-transport materials for organic light-emitting diodes

2,7-Bis(p-methoxyphenyl-m'-tolylamino)-9,9-dimethylfluorene (1'), 2,7-bis(phenyl-m'-tolylamino)-9,9-dimethylfluorene (2) and 2,7-bis(p-fluorophenyl-m'-tolylamino)-9,9-dimethylfluorene (3') have been synthesized using the palladium-catalyzed reaction of the appropriate diarylamines with 2,7-dibromo-9,9-dimethylfluorene. These molecules have glass-transition temperatures 15-20degreesC higher than those for their biphenyl-bridged analogues, and are 0.11-0.14 V more readily oxidized. Fluorescence spectra and fluorescence quantum yields for dimethylfluorene-bridged and biphenyl-bridged species are similar, but the peaks of the absorption spectra of 1'-3' are considerably red-shifted relative to those of their biphenyl-bridged analogues. Time-of-flight hole mobilities of 1'-3'/polystyrene blends are in a similar range to those of the biphenyl-bridged analogues. Analysis according to the disorder formalism yields parameters rather similar to those for the biphenyl species, but with somewhat lower zero-field mobility values. Density functional theory (DFT) calculations suggest that the enforced planarization of the fluorene bridge leads to a slightly larger reorganization energy for the neutral/cation electron-exchange reaction than in the biphenyl-bridged system. Organic light-emitting diodes have been fabricated using 1'-3'/polystyrene blends as the hole-transport layer and tris(8-hydroxy quinoline)aluminium as the electron-transport layer and lumophore. Device performance shows a correlation with the ionization potential of the amine materials paralleling that seen in biphenyl-based systems, and fluorene species show similar performance to biphenyl species with comparable ionization potential.