Electroluminescent diodes from a single component emitting layer of dendritic macromolecules

Abstract

Communications separating the PPV-rich parts and the pyridine-rich segments. This will be explained in more detail elsewhere.The significant red emission at high voltages is probably associated with the higher efficiency for light emission in pyridine-rich segments. As demonstrated by OnodaI7] the EL efficiency for light emission from PPyV is rather high (7 N 0.5 YO ) and it is further enhanced due to the presence of the junctionsr7 in PPV/PPyV. At the present time, we cannot exclude alternative explanations associated with the formation of exciton or impurity bands in the x-x* forbidden gap of PPV upon substitution of the pyridine moieties in the copolymer. ExperimentalThe 2,6-pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) monomer was prepared by adding tetrahydrothiophene (THT) (15 ml) to 2,6bis(chloromethy1)-pyridine (5 g) in methanol (150 ml). The reaction mixture was kept at 50°C for 4 days and was then concentrated to gelation. The hissulfonium salt was precipitated by addition of ice cold acetone (250 ml) and was isolated. The 1,4-phenylenedimethylene-bis-(tetramethylene sulfonium chloride) was prepared according to the literature procedure. The copolymerization was typically carried out as follows. A mixture of 20 YO (molar) of 2,6pyridyl-dimethylene-bis-(tetramethylene sulfonium chloride) (1.49 g) and 1,4phenylene dimethylene-bis-(tetramethylene sulfonium chloride) (5.96 g) in water (200 ml) was prepared. Polymerization was initiated under nitrogen at 0-2°C by addition of a stoichiometric quantity of 1 N NaOH in water (21.2 ml) under constant vigorous stirring for 1 h. The mixture was neutralized with 1 N HCl(9.4 ml) corresponding to 54.2 % conversion of the monomers to the copolyelectrolyte. The copolyelectrolyte solution was homogenized and dialyzed (Mw cut-off 12000) at 4°C for ten days.Films for the various measurements were prepared by casting the dialyzed copolyelectrolyte on relevant substrates by spin coating. The polyelectrolyte was then converted to the final copolymer, co(2,6PyV-PV) by heating in vacuum, 10-6torr, at 280°C for 12 h. Film thicknesses were determined by X-ray reflectivity at grazing angle using a 12 kW Rigaku rotating anode and found to be approximately 1000 A. Absorption spectra were measured on a HP 8452 diode array spectrometer. PL spectra were recorded on Perkin Elmer Luminescence Spectrometer LS 50. EL spectra were recorded using an Oriel monochromator and the light intensity was quantified with a photo-multiplier (products from Research Inc. model no. R955).

Blue Light Emitting Polyphenylene Dendrimers with Bipolar Charge Transport Moieties

“…However, blue OLEDs still suffer from much shorter lifetimes than green and red ones [4,5], even though pure blue light emitters have demonstrated over 20% EQE for phosphorescent materials [6,7] and nearly 20% for thermally activated delayed fluorescence (TADF) molecules [8]. Dendrimer-based OLEDs have been investigated in the last two decades, due to their numerous advantages over small molecules and polymers, i.e., cost-effective solution processability, high performance reproducibility thanks to their well-defined structures, in contrast to polymers, and precise functionalization of dendrimers at multiple positions [9][10][11][12][13]. Dendrimers for OLEDs generally include two types: one designed for better charge transport (conjugated scaffold) [14][15][16][17][18][19] and one for surface-to-core energy transfers (non-conjugated scaffold) [20][21][22][23][24].…”

Charge transport in highly efficient iridium cored electrophosphorescent dendrimers

“…Since early reports of organic electroluminescence ͑EL͒ in small molecules, 1 polymers, 2 and conjugated dendrimers, 3,4 dramatic improvements in the efficiency and stability of these materials have been achieved. Now, external quantum efficiencies ͑EQEs͒ of organic light-emitting diodes ͑OLEDs͒ are of the order of 10%-20% 5 and lifetimes in excess of 10 000 h ͑Ref.…”

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

“…Organic dendrimers 1 have shown great promise for energy funneling processes as well as several other possible applications including light emitting diodes, [2][3][4][5] solid-state organic laser devices, 6 new nanocomposite materials 7 and artificial light-harvesting systems. 8 These applications, and the fundamental physics and chemistry behind novel dendritic macromolecular architectures, have motivated the investigation of the mechanisms involved in the energy transfer processes in novel organic dendrimers.…”