The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This communication presents the design, synthesis, and device stability data for novel bis-5'-alkylthiophen-2'yl-2,6-anthracene organic semiconductors. When incorporated into thin-film field-effect transistors, mobilities as high as 0.5 cm2/Vs and on/off current ratios greater than 107 are observed. We have investigated device stability in terms of both shelf life and operating lifetime. Devices incorporating the reported semiconductors display an average field-effect mobility of 0.4 cm2/Vs for DHTAnt and an on/off current ratio of 106 even after 15 months of storage. Furthermore, there is no decrease in performance during continuous operation of the devices over several thousand cycles.
The development of new organic semiconductors with improved electrical performance and enhanced environmental stability is the focus of considerable research activity. This paper presents the design, synthesis, optical and electrochemical characterization, crystal packing, modeling and thin film morphology, and organic thin film field effect transistor (OTFT) device data analysis for a novel 2,6-bis[2-(4-pentylphenyl)vinyl]anthracene (DPPVAnt) organic semiconductor. We observed a hole mobility of up to 1.28 cm2/V.s and on/off current ratios greater than 107 for OTFTs fabricated using DPPVAnt as an active semiconductor layer. The mobility value is comparable to that of the current best p-type semiconductor pentacene-based device performance. In addition, we found a very interesting relationship between the charge mobility and molecule crystal packing in addition to the thin film orientation and morphology of the semiconductor as determined from single-crystal molecule packing study, thin film X-ray diffraction, and AFM measurements. The high performance of the semiconductor ranks among the best performing p-type organic semiconductors reported so far and will be a very good candidate for applications in organic electronic devices.
The electrical performance of organic thin-film transistors (TFTs) often degrades when the devices are exposed to air. This is generally ascribed to the generation of trap states, [1] possibly as a result of the oxidation of the organic semiconductor.[2] One strategy to improve the stability of p-channel organic TFTs is the synthesis of conjugated semiconductors with a relatively large ionization potential. [3][4][5][6][7][8] However, most of the TFTs based on organic semiconductors with large ionization potentials reported up till now have shown carrier mobilities that are smaller than that of pentacene. Here, we report on a new organic semiconductor, di(phenylvinyl)anthracene (DPVAnt), [9] that combines large carrier mobility (similar to that of pentacene) with increased ionization potential and improved stability as compared to pentacene. DPVAnt has been synthesized by a Suzuki coupling reaction between 2,6-dibromoanthracene and 4,4,5,5-tetramethyl-2-[2-phenylvinyl]-[1,3,2]dioxaborolane [9] with a yield of 85%.Pentacene has been purchased from Fluka. Both semiconductors have been purified by temperature gradient sublimation in a stream of inert gas. Cyclic voltammetry indicates a highest occupied molecular orbital (HOMO) energy of -5.4 eV for DPVAnt, as compared to -5.0 eV for pentacene. From UV-vis absorption spectroscopy we have determined an optical bandgap of 2.6 eV for DPVAnt and 1.8 eV for pentacene. These results are consistent with the general observation that molecules characterized by a smaller conjugated p-system have more negative HOMO energies and larger bandgaps.Simple TFT test structures have been prepared on heavily doped silicon substrates (serving as the gate electrode) with a thermally grown SiO 2 gate dielectric. The dielectric surface has been treated with octadecyltrichlorosilane (OTS), [10] and the organic semiconductor has been vacuum deposited onto the substrate. Gold source/drain contacts have been thermally evaporated through a shadow mask (Fig. 1a). During the deposition of the semiconductor, the substrates are held at a temperature of 60°C for pentacene and 80°C for DPVAnt. The carrier mobilities extracted from the transfer characteristics measured in air are 1 cm 2 V -1 s -1 for pentacene and 1.3 cm 2 V -1 s -1 for DPVAnt (Fig. 1b). Both TFTs have an on/off current ratio of 10 7 and a subthreshold swing of 500 mV decade -1. Perhaps the most striking differences between the two devices are the much more negative turn-on and threshold voltages of the DPVAnt transistor (V turn-on = -14 V, V th = -16 V) as compared to the pentacene TFT (V turn-on = -2 V, V th = -5 V). The exact reason for this difference is not known, but it may be related to the more negative HOMO energy of DPVAnt as compared to pentacene. As shown by the atomic force microscopy (AFM) images in Figure 1c and d, both semiconductors form well-ordered polycrystalline films, which is a prerequisite for obtaining large carrier mobilities.For practical applications, a transistor structure with patterned gate electrodes ...
The rearrangement of 2-bromomethyl-2-methylmonothiomalonates to succinyl derivatives was found to take place in quantitative yields in the presence of one molar equivalent of Co(I) generated by the reduction of heptamethyl Co(II)yrinate perchlorate with NaBH4 or electrochemically. The chiral thiomalonate gave racemic succinate.
The vitamin-B 12 derivative 11, incorporating a peripheral N 4 -acetylcytosine moiety, was alkylated under reductive conditions with 2-(iodomethyl)-2-methylmonothiomalonate 8 bearing the complementary guanine moiety. The reaction yielded a mixture of vitamin-B 12 -derived complexes with variations in the cytosine moiety: products 16 ± 18 with a cytosine, a N 4 -acetylated cytosine, and a N 4 -acetylated reduced cytosine moiety were formed (see Scheme 5). The complexes were photolyzed in CHCl 3 /MeCN to yield the dimethylmalonate derivative 22 (Scheme 6) but not the rearranged succinate, in contrast to the results obtained earlier with complexes incorporating the A ¥ T base pair (see Scheme 1).
Pentacene is among the most popular organic semiconductors for organic thin-film transistors (TFTs), due to its relatively large carrier mobility [1,2]. However, the stability of pentacene TFTs under continuous dynamic operation may be insufficient for future applications. Here we compare the static and dynamic performance and the operational stability of low-voltage organic TFTs based on pentacene and a recently synthesized organic semiconductor, di(phenylvinyl)anthracene (DPVAnt). DPVAnt was synthesized by a Suzuki coupling reaction [3], and pentacene was purchased from commercial sources. Both materials were purified by temperature-gradient sublimation. TFTs and circuits were fabricated on glass substrates using an inverted staggered TFT structure with evaporated aluminum gates, a thin gate dielectric based on a solution-processed self-assembled monolayer (0.7 iF/cm2), an evaporated organic semiconductor layer, and evaporated gold top contacts [4]. All layers were patterned using manually aligned shadow masks. The device structure and the chemical structures of the semiconductors are shown in Figures 1 and 2. All electrical measurements were carried out in ambient air. Figures 3 and 4 show the electrical characteristics of a pentacene TFT and a DPVAnt TFT before, during, and after an operational stability test. During the test a continuous square-wave signal with a period of 10 sec is applied to the gate electrode, and the drain current is monitored using a Semiconductor Parameter Analyzer. As can be seen, the performance of both TFTs changes during the test. In particular, carrier mobility and on-current of the pentacene TFT are reduced by more than an order of magnitude (mobility from 0.4 cm2/Vs to 0.02 cm2/Vs, on-current from 2 pA to 80 nA), while the threshold voltage appears to be unchanged. On the other hand, the mobility of the DPVAnt TFT is stable at 0.3 cm2/Vs. The threshold voltage of the DPVAnt TFT has shifted by about -0.5 V during the test, reducing the on-current by about 50% (from 1.3 pA to 700 nA), and the subthreshold behavior of the DPVAnt TFT is somewhat degraded. Overall, the DPVAnt TFT shows better operational stability compared with the pentacene TFT. In order to obtain TFTs with a high cut-off frequency, it is necessary to reduce the parasitic capacitances and increase the transconductance. The latter can be accomplished by reducing the channel length, but it also requires a small contact resistance. Figure 5 shows a shadow-mask-patterned pentacene TFT with a channel length of 10 ptm, a channel width of 100 ptm, a total gate capacitance of about 15 pF, and a transconductance of 4 iS (40 pS/mm) at low voltage (VDS =-1.5 V, VGS -2.0 .. -2.5 V). For DPVAnt TFTs we have obtained a transconductance of 30 pS/mm. Analyzing pentacene and DPVAnt TFTs with channel length ranging from 50 ptm to 10 ptm we have extracted the gate-bias-dependent contact resistance (sum of drain and source resistance [5]) in the linear regime and found a value of 850 Q.cm for pentacene and 1.3 kQ.cm for DPVAnt (see Fig...
Rearrangements O 0140The Co(I) Induced Methylmalonyl-Succinyl Rearrangement in a Model for the Coenzyme B 12 Dependent Methylmalonyl-CoA Mutase. -Brominated and iodinated (not shown) thioesters undergo methylmalonyl rearrangement in the presence of equimolar amounts of Cob(I)ester, generated in situ by NaBH4 or electrochemical reduction of Cob(II)ester perchlorate (COB). Alkylation of the complex is not observed and the reaction is specific for thioesters. With enantiopure thioesters racemized rearrangement products are obtained. -(SUN, F.; DARBRE*, T.; Org. Biomol. Chem. 1 (2003) 18, 3154-3159; Inst. Chem. Biochem., Univ. Bern, CH-3012 Bern, Switz.; Eng.) -Nuesgen 02-040
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