Structure and infrared (IR) assignments for the OLED material: N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB)

Halls, Mathew D.; Tripp, Carl P.; Schlegel, H. Bernhard

doi:10.1039/b101619i

Cited by 58 publications

(40 citation statements)

References 25 publications

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“…Since cations are usually accompanied by radical spins, electron spin resonance (ESR) spectroscopy is a useful technique for this purpose. Although the thin films of NPB have been characterized by various methods like photoemission, 5 time-of-flight, 6 infrared, 7 and Raman spectroscopies, 8 the ESR study of NPB film has not been reported to our best knowledge. We have utilized this technique for other organic materials in thin films 9 and in organic devices.…”

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confidence: 99%

Electron Spin Resonance of Thin Films of N,N′-Di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) Doped by Iodine Vapor

et al. 2012

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We have successfully observed the ESR signals of radical cations in the thin films of N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB), typical hole-transport material, for the first time to our best knowledge. In order to characterize the radical cation state, ESR spectra obtained upon chemical doping with iodine are analyzed combined with density functional theory (DFT) calculation. The ESR line width is inversely correlated with doping concentration.Organic light-emitting diodes (OLEDs) have been actively studied because of their excellent characteristics such as high efficiency, low cost productivity, and flexibility.1,2 Aromatic amines are widely used as hole-transport materials in the devices. Among them, N,N¤-di(1-naphthyl)-N,N¤-diphenylbenzidine (NPB) (Scheme 1) is a typical one with high thermal stability.3 Blue-color luminescence was also reported in an OLED containing NPB layer.4 In order to understand the holetransport properties, it is quite important to elucidate the nature of cationic state. Since cations are usually accompanied by radical spins, electron spin resonance (ESR) spectroscopy is a useful technique for this purpose. Although the thin films of NPB have been characterized by various methods like photoemission, 5 time-of-flight, 6 infrared, 7 and Raman spectroscopies, 8 the ESR study of NPB film has not been reported to our best knowledge. We have utilized this technique for other organic materials in thin films 9 and in organic devices. 1012 In the present letter, we carried out chemical doping by exposing the NPB films to iodine vapor and successfully observed ESR signals arising from the induced radical cations. We also performed density functional theory (DFT) calculations to obtain insights to the radical cation state.By thermal vacuum evaporation under the pressure of 4 © 10 ¹4 Pa, NPB was deposited on a quartz substrate cleaned with isopropyl alcohol and acetone. The NPB film is 80nm in thickness and 0.84 cm 2 in area. Subsequently, the film was doped using iodine vapor under vacuum of 1 © 10 ¹1 Pa for 1 h, then it was sealed in an ESR sample tube. The doping concentration was reduced by exhausting the sample tube using a diffusion pump and was controlled by the length of the exhaustion time. ESR measurements for the sample were performed with a JEOL JES-FA200 ESR spectrometer.13 DFT calculations of the B3LYP/6-31G(d) level were carried out for an isolated NPB cation.14 The molecular geometry was fully optimized. The principal values of g tensor were computed using the gauge-independent atomic orbital method. Figure 1 shows observed ESR spectra: (a) spectrum observed right after the iodine doping and (b) that measured after an exhausting time of five hours. As seen in this figure, clear ESR signals were observed in both cases. In Figure 1a, g value, peak-to-peak ESR line width ¦H pp , and spin concentration of the ESR spectrum are 2.0040, 0.676 mT, and 2.65 © 10 20 cm ¹3, respectively. Meanwhile, in Figure 1b, they are 2.0036, 1.06 mT, and 2.12 © 10 19 cm ¹3, respectively. The spi...

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confidence: 99%

Electron Spin Resonance of Thin Films of N,N′-Di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) Doped by Iodine Vapor

et al. 2012

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“…Beyond this, measurements of transition in the middle infrared region [3] on thermal stressed organic layers suggest no change in the organic material in a chemical sense. If we take into consideration that the glass transition temperature of α-NPD (95 °C [4]) is lower than the temperature used for annealing (120 °C), this effect might be caused by a thermal induced molecular diffusion changing the conditions at the interface between the organic and the conducting layers.…”

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confidence: 96%

Characterization and optimization of the deposition process of aluminum top electrodes for organic devices

et al. 2008

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We investigated the effect of thermal stress, caused by the deposition of aluminum on top of an organic light emitting diode (OLED), on the device performance and proved our simulated results by experimental tests. Concerning the temperature of the substrate, we found a much larger influence of thermal radiation, caused by the evaporation source and the environmental setting compared to the kinetic and thermal energy of the deposited material itself. Due to these results, we developed a new system for metal deposition, using the flash-evaporation technique. Using it, we were able to minimize the influence of thermal radiation and geometry on the evaporation. Therefore the substrate heating was reduced by more than 90 % and the photometric efficiencies of test-devices were improved slightly. Additionally the time of deposition and retention was lowered by 90 %, with an increased material yield of more than 55 % at the same time. The resistance of the conducting layer decreases by two orders of magnitude, caused by emerging micro crystals. Surprisingly, the roughness of the surface actually decreased slightly.

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“…The proper interpretation of complex condensed phase spectra is facilitated by computations of IR spectra of isolated ligands. The DFT/B3LYP method used here usually gives overestimated vibrational frequencies [38]. However, despite of its limited quantitative predictive power, some general trends in a series of similar compounds are often studied.…”

Section: Vibrational Spectramentioning

confidence: 99%

A comparative DFT study of substrates and products of industrial enzyme nitrile hydratase

Pepłowski

Kubiak

Zelek

et al. 2007

Int J of Quantum Chemistry

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ABSTRACT:Nitrile hydratase (NHase) is a metalloenzyme used in industrial biotechnology for a large scale production of common chemicals. NHases convert nitriles to the corresponding amides. Although the structures of some forms of NHases containing nonheme low spin Fe(III) or low spin noncorrinoid Co(III) are known, neither a catalytic mechanism nor the reasons of high selectivity towards aromatic ligands are recognized. Optimized geometries, molecular electrostatic potential maps and infrared spectra of commercially important aromatic substrates of the NHase (nicotinonitrile, o-, m-, p-methylbenzonitrile) and the corresponding products (nicotinamide, o-, m-, p-methylbenzamid) were investigated using the density functional theory method with the B3LYP functional and the 6-31G(d,p) basis set. Calculated hypothetical intrinsic reaction paths indicate that benzimidic acids may be involved as intermediates. This study elucidates differences in the electronic properties of substrates and products of NHases, provides an insight into the molecular basis of the catalytic reaction and helps to explain varying enzymatic activities of microbial NHases.

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Structure and infrared (IR) assignments for the OLED material: N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB)

Cited by 58 publications

References 25 publications

Electron Spin Resonance of Thin Films of N,N′-Di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) Doped by Iodine Vapor

Electron Spin Resonance of Thin Films of N,N′-Di(1-naphthyl)-N,N′-diphenylbenzidine (NPB) Doped by Iodine Vapor

Characterization and optimization of the deposition process of aluminum top electrodes for organic devices

A comparative DFT study of substrates and products of industrial enzyme nitrile hydratase

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