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Keywords:

  • Organic Light-Emitting Diodes;
  • Dyes;
  • Doping;
  • Photoluminescence;
  • Thin Films

Abstract

The present work describes the synthesis of difluoro-boradiazaindacenes (Bodipy) functionalized at the central 8-position by phenylamino moieties easily transformable into phenyl amide scaffoldings. Molecules carrying three linear or branched chains were prepared and characterized. An X-ray crystal structure for the pivotal trimethoxyphenyl-Bodipy derivative was determined, and the packing is discussed in terms of molecular interactions; a key feature for the formation of thin films. All of the dyes are thermally stable up to 170 °C but no liquid-crystalline phases are observed. Reversible reduction and oxidation processes occur around +0.97 and −1.34 V, respectively, versus saturated calomel electrode in solution and the electroactivity and photoluminescence are maintained in thin films produced by vacuum evaporation. Interestingly, two distinct emissions are observed at 550 and 635 nm by electroluminescence of the trimethoxyphenyl-Bodipy derivative, corresponding to the luminescence of isolated molecules and dimers, respectively. Doping Alq3 films with this Bodipy molecule by vacuum evaporation produces organic light-emitting diodes (OLEDs) in which very efficient energy transfer from the Alq3 matrix to the Bodipy occurs by a resonance mechanism involving the first Bodipy excited state. Yellow light (550 nm, 344 cd m−2 at 15 V) is emitted at low doping concentration (7 mol %), whereas red light (635 nm, 125 cd m−2 at 15 V) is emitted at higher concentration (19 mol %). Dispersion of the Bodipy into a fluorescent poly(N-vinylcarbazole) polymer (PVK) (≈3 mol % per repeating unit of PVK) by solution processing exclusively produces yellow emission owing to the isolated Bodipyfluorophore (550 nm, 213 cd m−2 at 15 V). The second excited state of the Bodipy dye is likely involved during energy transfer from the PVK matrix.