Can mono- or di-butyltin chlorides produce tributyltin chloride at elevated temperatures? Implications for applications in chemical vapour deposition

The thermolysis of the butyltin chlorides at 200-300 °C in the liquid phase has been investigated by ¹H, ¹³C, and ¹¹⁹Sn NMR spectroscopy. The stabilities follow the order: Bu₂SnCl₂ > Bu₃SnCl > BuSnCl₃. Only tributyltin chloride showed any evidence of redistribution, giving dibutyltin dichloride, together with metallic tin, butane, and but-1-ene, which would be formed by decomposition of tetrabutyltin. Dibutyltin dichloride decomposed to give mainly butane with no other apparent liquid organotin compound. Butyltin trichloride gave butane, some butene, and metallic tin, and showed no evidence of forming tributyltin chloride by the redistribution reaction, which would have environmental implications for its use in the CVD coating of glass.     

Generalized parallel-server fork-join queues with dynamic task scheduling

This paper introduces a generalization of the classical parallel-server fork-join queueing system in which arriving customers fork into multiple tasks, every task is uniquely assigned to one of the set of single-server queues, and each task consists of multiple iterations of different stages of execution, including task vacations and communication among sibling tasks. Several classes of dynamic polices are considered for scheduling multiple tasks at each of the single-server queues to maintain effective server utilization. The paper presents an exact matrix-analytic analysis of generalized parallel-server fork-join queueing systems, for small instances of the stochastic model, and presents an approximate matrix-analytic analysis and fixed-point solution, for larger instances of the model.     

DISTRIBUTED COGNITION,COMPUTERS AND THE INTERPRETATION OF GRAPHS

This paper reoprts on one aspect of recent research on students' understanding of graphs in computer and in pencil and paper media. Over 200 Year 10 [age 14–15] students in two comprehensive schools in Leeds participated in a learning experiment, 100+ with and 100+ without the use of computers. Pre, post and delayed post-tests and interviews provide data for a comparison of learning with the two media.     

Degradation of HTL layers during device operation in PhOLEDs

Different analytical tools and methodologies are currently employed to determine degradation products of organic blue light emitting devices in order to identify the failure mechanisms which determine the lifetime of these devices. This article provides a deeper understanding of degradation mechanisms of organic light emitting diodes (OLEDs) during device operation. Degradation products of blue emitting devices containing 8% of the phosphorescent emitter iridium(III)bis(4,6-difluorophenyl)-pyridinato-N,C^2′ picolinate (FIrpic) in a matrix containing bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminium (BAlq) as electron transport layer (ETL), 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (α-NPD) were investigated using laser desorption ionization (LDI) coupled with a time of flight mass spectrometry (TOF/MS). Especially chemical degradation pathways of the hole transport materials TCTA and α-NPD were investigated. The comparison of experimental data of unstressed and stressed device revealed that new reaction products are formed during the device operation. The linkage of TCTA fragments to the α-NPD core in an interfacial reaction as well as a dimerization of TCTA itself was observed. Ten new reaction products could be characterized via LDI-TOF-MS. Some of these compounds might possess a negative influence on the drop of efficiency and lifetime of blue light emitting devices based on FIrpic.     

Fluorine cleavage of the light blue heteroleptic triplet emitter FIrpic

The lifetime stability of devices containing FIrpic as emitter has been a major concern for organic blue light emitting devices (OLEDs). To gain a deeper knowledge about the purity of FIrpic (bis[2-(4,6-difluorophenyl)pyridyl-N,C2′]iridium (III)) emitters and how the purity is influenced by sublimation steps, non-sublimated and sublimated FIrpic material was analyzed via liquid chromatography coupled with electron spray ionization mass spectrometry (LC/ESI/MS). Cleavage of an electron-withdrawing group from one of the ligands of the heteroleptic phosphorescent emitter could be identified in sublimated FIrpic material via LC/ESI/MS. A detailed chemical analysis using LC/ESI/MS was carried out for complete blue emitting devices of the following structure: indium-tin-oxide (ITO)/50 nm (α-4,4′-bis[(1-naphthyl)phenylamino]-1,1′-biphenyl) (α-NPD)/10 nm 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA)/100 nm TCTA:8% FIrpic/50 nm 1,1′-biphenyl-4′-oxy)-bis(8-hydroxy-2-methylquinolinato)-aluminum (BAlq)/1 nm LiF/100 nm Al. Two isomers of (FIrpic-1F) could be detected in an aged OLED. Changes in the ligand systems of FIrpic, especially the loss of fluorine during the deposition process can alter the emissive properties of the blue phosphorescent emitter. Beside isomer formation and chemical degradation of FIrpic, substantial degradation was observed for the hole transport material α-NPD in driven OLEDs.     

Investigation of FIrpic in PhOLEDs via LC/MS technique

The commercial breakthrough of phosphorescent organic white light sources is presently hampered due to the unavailability of a stable blue phosphorescent emitter material. Moreover, only few analytical investigations have been made regarding the chemical degradation of the phosphorescent emitter materials during the processing or the operation of the devices. Organic light emitting devices (OLEDs) containing phosphorescent metal complexes with iridium as central ion were investigated. Special attention was paid to the chemical degradation of the material. The devices were analyzed by means of high performance liquid chromatography coupled with mass spectrometry (HPLC/MS). Electron spray ionization (ESI) was employed as ionization source. Isomerization phenomena of the blue-green emitting heteroleptic iridium complex FIrpic could be observed after the device manufacture and after operation. These findings could give hints on the mechanisms that influence the lifetime of PhOLEDs based on FIrpic or similar blue emitters.