Abstract

In this paper we describe the use of combinatorial vapor deposition techniques for the optimization of blue organic light emitting diodes (OLEDs). In these devices the star shaped molecule 1 with a triphenylamine core and three fluorene side groups serves as hole transport and emitting layer. Compound 2 with a much lower lying HOMO and a larger bandgap is used as hole blocking layer. Using combinatorial vapor deposition 42 OLEDs with thickness gradients of both the hole transport and the hole blocking layer have been simultaneously prepared on one substrate. The physical characterization of the devices clearly shows that a hole blocking layer of the star shaped molecule 2 is necessary in order to obtain pure blue emission with CIE coordinates of x = 0.15 and y = 0.15. A thickness of only 5 nm of the blocking layer is sufficient, and with increasing layer thickness the brightness of the blue devices drops. The blue devices exhibit a brightness of 400 cd m^–2 and a luminous efficiency of 2 cd A^–1. The thickness variations of both the hole transport and the hole blocking layer have been made in one combinatorial evaporation experiment on a single substrate using a set of movable masks. This demonstrates how efficient combinatorial methods can be used for the development of OLEDs.

Summary: In this paper we report the synthesis, characterisation and photopatterning of polydisperse fluorene oligomers. The oligomers were synthesised using the Yamamoto coupling. The molecular weights of the oligomers and as a consequence the nematic-isotropic transition temperatures were tailored by an endcapping reaction. With the endcapping species we introduced reactive acrylate functionalities. Subsequent photocrosslinking through a test mask led to the formation of fluorescent microstructures with a resolution of 1 µm.

Summary: The synthesis of new fluorene containing photocrosslinkable reactive mesogens is described. Both monodisperse trimers or pentamers and oligomeric mixtures containing two photocrosslinkable acrylate end groups were obtained by Suzuki-cross-coupling reactions. The pentamer 12a shows an ideal phase behavior for orientation experiments with a broad nematic phase between the glass transition at −10 °C and 123 °C. In the oligomeric mixtures 14a–g the transition temperature from the nematic to the isotropic phase can be tailored from 100 to 310 °C by adjusting the molecular weight of the oligomers by end-capping. This process can be easily characterized by MALDI-TOF spectroscopy. The pentamer 12a and the oligomeric mixture 14c were oriented on rubbed polyimide layers and orientation ratios of 15:1 in photoluminescence were obtained. Experiments with different film thicknesses show that the orientation is not homogeneous throughout the film but decreases with increasing distance from the orientation layer.

Summary: The photopolymerization of two reactive mesogens with photopolymerizable acrylate endgroups, the methyl substituted 1,4-phenylene-bis{4-[6-(acryloyloxy)hexyloxy]benzoate} 1 and acrylic acid 6-{4-[6-(6-acryloyloxyhexyloxy)naphthalen-2-yl]-phenoxy}hexyl ester 2 has been investigated using Photo-DSC measurements. Photocrosslinking of 1 in the nematic phase at 100 °C leads to a final conversion of 87% of the acrylate groups. It is possible to perform photopolymerization with very small amounts of photoinitiator. Even with 0.001% (10 ppm) of photoinitiator, 47% of the acrylate groups polymerize within 15 min. The polymerization of the reactive mesogen 2 proceeds faster in the smectic A phase at 100 °C compared to the isotropic phase at 120 °C and leads to a higher conversion of 75%. This can be explained by an increased local concentration of the acrylate groups between the layers of the smectic cores.

A series of polymerizations of 3,6-dibromo-9-(2-ethylhexyl)carbazole was carried out in different monomer concentrations using standard Yamamoto reaction conditions. It was found that the molecular weight of the resulting poly(N-(2-ethylhexyl)carbazol-3,6-diyl) strongly depends on the monomer concentration in the reaction mixture. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) measurements confirmed the formation of low-molar-mass cyclic oligomers of the 3,6-disubstituted carbazole. In this paper we describe, for the first time, the formation of large amounts of a cyclic tetramer and of higher macrocycles in the synthesis of poly(N-alkyl-3,6-carbazoles) by the Yamamoto method. This seems to be a limiting factor in the synthesis of high molecular weight poly(N-alkyl-3,6-carbazole)s. The optical, thermal, and electrochemical properties of poly(N-(2-ethylhexyl)carbazol-3,6-diyl) have been investigated. Poly(N-(2-ethylhexyl)carbazol-3,6-diyl) is thermally stable, with 5% weight loss at 460 °C in nitrogen. The polymer exhibits a weak blue fluorescence with a maximum at 425 nm. Poly(N-(2-ethylhexyl)carbazol-3,6-diyl) is electrochemically stable, its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are −5.0 and −1.6 eV, respectively.

A series of N-alkylcarbazol-3,6-diyl trimers and pentamers has been synthesized by stepwise Suzuki coupling. The trimers with isopropyl and longer alkyl substituents at the nitrogen atom and all the pentamers form stable glasses with glass transition temperatures varying from 30 to 148 °C for the trimers and 65 to 104 °C for the pentamers. The ionization potential of the N-alkylcarbazol-3,6-diyl trimers is 5.25 eV and 5.20 eV for the pentamers. N-alkylcarbazol-3,6-diyl compounds typically exhibit hole-drift mobilities in the range of 10⁻⁴–10⁻⁶ cm² · V⁻¹ · s⁻¹. The highest hole-drift mobility observed in an amorphous film of N-octylcarbazol-3,6-diyl pentamer reached 10⁻³ cm² · V⁻¹ · s⁻¹ at an applied electric field of 8.1 × 10⁵ V · cm⁻¹.

Among organic materials vitrification for many years was regarded mainly as a privilege of polymers. However, recently a lot of attention is paid to organic low molar mass compounds that readily form glasses above room temperature. Such compounds are called molecular glasses or amorphous molecular materials. Among these materials the most widely studied are charge-transporting molecular glasses used in copiers and laser printers, organic light-emitting diodes, photovoltaic devices, and as photorefractive materials. Two types of molecular glasses, i.e., p-type (hole-transporting), and n-type (electron-transporting) are discussed. Work of the laboratories of the authors is emphasized. In addition, an overview of current and potential applications for these materials is presented.

Suzuki coupling modified by adding Ag₂O instead of the Na₂CO₃ used in the original reaction protocol, was used as a synthetic tool for the synthesis of a copolymer in which 9,9-dialkylfluorene moieties were coupled to unsubstituted poly(p-phenylenevinylene) (PPV) units. The possibility of performing the polymerisations at moderate temperatures appears to be an advantage related to the use of Ag₂O. The copolymer shows a M_w of 21 000 and M_w/M_n = 2.1 and is suitable for the construction of electroluminescent devices. Monolayer organic light-emitting diodes (OLEDs) have been built which emit greenish light.