Nanoscale Order and Structure in Organic Materials

Jun 13, 2011 - on gold is an ideal system to understand thin film processes occurring at the interface .... In particular, the values given in this wo...
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Nanoscale Order and Structure in Organic Materials: Diindenoperylene on Gold as a Model System M. B. Casu* Institute of Physical and Theoretical Chemistry, University of T€ubingen, Auf der Morgenstelle 18, D-72076 T€ubingen ABSTRACT: Device performances in organic electronics are strongly influenced by the structure and morphology of the active layer. We investigated the thin film of diindenoperylene, a promising candidate for electronics, deposited on polycrystalline gold, and Au(100) and Au(111) single crystals using X-ray absorption and photoemission together with atomic force microscopy. The results show the influence of the substrate morphology, in this case via the roughness, on the structure of the obtained films. This affects the intermolecular interactions as evidenced by different details in X-ray absorption comparing the three cases. A further source of influence is the surface lattice geometry of the substrate that affects the shape of the islands. This behavior mimics very closely the behavior of atoms on metal growth, showing a general pattern in matter organization in the nanoscale regime.

’ INTRODUCTION Diindenoperylene (DIP, C32H16, Figure 1) is a perylene-based molecule known since the beginning of the last century when it was synthesized in 1934.1 The interest in this molecule has recently enjoyed a renaissance because of the properties of its thin films that may have applications in organic electronics. DIP single crystals exist under two different phases investigated by M. A. Heinrich et al.:2 A triclinic low temperature Rphase that converts to a monoclinic high-temperature β-phase above 403 K. The β-phase also corresponds to the molecular arrangement in DIP thin films on SiO2.3 DIP thin films have been investigated with a variety of techniques including X-ray diffraction, atomic force microscopy (AFM), transmission electron microscopy (TEM), and ultraviolet photoelectron spectroscopy (UPS)3 7 to explore their electronic, morphology, and structural properties. DIP thin films show high hole mobility, good filmforming properties, and thermal stability,3 12 and DIP is a good candidate as a donor material in organic photovoltaic cells.13 In this paper, we report on an extensive investigation of DIP thin films deposited on polycrystalline gold and gold single crystals with different surface lattice geometries, based on soft X-ray spectroscopies together with AFM. The results reveal that DIP on gold is an ideal system to understand thin film processes occurring at the interface between metals and organic molecules. ’ EXPERIMENTAL SECTION All preparations have been performed in situ, in ultra high vacuum (UHV). The substrates were polycrystalline gold foil and Au(100) and Au(111) single crystals. Gold foils (Goodfellow, purity 99.99%) were cleaned in UHV by means of repeated cycles of Ar-sputtering and checked in situ by using X-ray photoelectron spectroscopy (XPS) and by ex situ AFM. r 2011 American Chemical Society

Clean Au(100) single crystals were prepared by several cycles of Ar+ ion bombardment (800 V), followed by annealing in UHV at approximately 800 K. The surface presents large terraces, and the presence of steps is observable. It has been investigated with low energy electron diffraction (LEED) and dark field low energy electron microscopy (LEEM), which gave a pattern with the expected Au(100) reconstruction.14 Clean carbon-free Au(111) (MaTeck GmbH) single crystal surfaces were prepared by several cycles of Ar+ ion bombardment (600 V), followed by annealing in UHV at approximately 830 K, which gave the well-known herringbone pattern.15 DIP thin films have been deposited by using organic molecular beam deposition (OMBD) (evaporation rate = 0.3 nm/min), keeping the substrates at room temperature. OMBD represents the application to organic small molecules of the process that governs molecular beam epitaxy (MBE) in inorganic materials. MBE has led to the fabrication of a wide range of new devices, and research and technology have substantially gained advantageous properties from the high control of thin film thickness and structure. The same relevance has been achieved by OMBD. The organic material is evaporated by using a Knudsen cell. The molecules are collimated by passing through an orifice and then deposited on a substrate. The deposition rate is monitored by a quartz microbalance. The flux is controlled by the cell temperature and a shutter. All precautions for a clean environment have been taken to ensure the reproducibility of the experiments: the cell has been outgassed before use, and the material is purified by using thermal gradient sublimation. The nominal film thickness was determined by using the quartz microbalance and crosschecked by using the attenuation of the XPS substrate signals (Au 4f). The measurements on DIP on polycrystalline gold have been performed at the Institut f€ur Festk€orperphysik soft X-ray dipole beamline WERA at ANKA (Karlsruhe, Germany). This beamline covers photon energies that range from 100 to 1500 eV, with an energy resolving power Received: May 22, 2011 Published: June 13, 2011 3629

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Figure 1. (a) Thickness-dependent C 1s core level photoemission spectra of DIP deposited on polycrystalline gold are shown. (b) A 5 μm  5 μm AFM image of a 15 nm thick (nominal thickness) film together with the profile analysis of the same sample. The profile was extracted as indicated by the line. (c) C1s NEXAFS spectra obtained from a 3.4 nm (upper panel) and a 12 nm thick film (lower panel). The spectra were taken in grazing incidence for p (black curve) polarization and in normal incidence for s (gray curve) polarization. The geometry of the NEXAFS experiment and the molecular structure are also shown. of E/ΔE up to 10000. The preparation chamber (base pressure 2  10 10 mbar) is equipped with ion sputter guns, separated gas dosing systems, LEED optics, a quartz microbalance, and an evaporation cell for OMBD. The spectroscopy main chamber is equipped with a SCIENTA SES2002 electron energy analyzer for photoemission spectroscopy. Standard NEXAFS measurements have been carried out with energy resolution ∼0.095 eV and P = 0.95 in the partial electron yield (retarding potential, 76 V) mode in grazing incidence (θ2 = 70°) and normal incidence (θ1 = 0°). Standard NEXAFS and XPS measurements on DIP on Au(100) and Au(111) were performed at the beamline UE52-PGM at BESSY (Berlin, Germany). This beamline is characterized by a plane grating monochromator. The photon energy ranges from 100 to 1500 eV, with an energy resolving power of E/ΔE= 10500 at 401 eV (cff = 10 and 20 μm exit slit). The main chamber (base pressure 2  10 10 mbar) is equipped with a standard twin anode X-ray source, a SCIENTA R4000 electron energy analyzer, and a homemade partial electron yield detector. All NEXAFS spectra were normalized by taking the ring current and the clean substrate signal into account, as discussed in ref 16; the photon energy scale has been corrected according to ref 17. Afterward, all spectra were scaled to give an equal edge jump. The quantitative approach to determine the molecular orientation from NEXAFS spectra is based on the fact that the polarization dependence stems from the dipole selection rules.16 The π* or the σ* resonances are largest when the electric field vector of the incident radiation is along the π* or the σ* orbitals, respectively. This not only explains the dichroic NEXAFS behavior but allows one to determine quantitatively the molecular orientation. It can be shown that measuring the NEXAFS intensity for at least two different polarization directions with respect to the surface, it is possible to calculate the angle between

the molecular plane and the surface, as discussed in detail in refs 11, 12, and 18. We performed molecular orientation calculations by using only the intensities of the π* resonances because of the fact that in our spectra, they are very sharp. In particular, the values given in this work are calculated using the π* resonance with the highest intensity. The AFM measurements were performed under ambient conditions in tapping mode with a Nanoscope IIIa (Digital Instruments) scanning probe microscope. No degradation of the samples was observed on the time scale of all presented results.

’ RESULTS In Figure 1, the soft X-ray characterization together with the AFM image of a thick DIP film on polycrystalline gold are shown. The C 1s core level spectra (hν = 320 eV) depending on film thickness of the DIP film are dominated by a strong peak at 284.4 eV for coverage higher than a monolayer (Figure 1a). As discussed in detail in ref 19, because of the molecular symmetry (the free DIP molecule belongs to the D2h point group), the C 1s core level features are expected to be due to at least nine contributions, one for each different nonequivalent carbon site, leading to a broad peak shape. Because of their similar binding energy and the finite resolution of our experiment, their separation and energy determination by curve fitting would be quite speculative. In addition, a shakeup satellite at around 286.3 eV (HOMO LUMO shake up satellite20,21) is also visible. Comparing the signal of the C1s core levels of the multilayer and the monolayer, no change in the shape of the features neither in the details is present, indicating a weak interaction of the molecules with the substrate. However, a 0.4 eV shift toward the 3630

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Figure 2. (a) Thickness-dependent C 1s core level photoemission spectra of DIP deposited on Au(100) are shown. (b) A 5 μm  5 μm AFM image of a 15 nm thick (nominal thickness) DIP film together with the profile analysis of the same sample. The profile was extracted as indicated by the line. (c) C1s NEXAFS spectra obtained from a 3.6 nm DIP. The spectra were taken in grazing incidence for p (black curve) and s (gray curve) polarization. The geometry of the NEXAFS experiment is also shown.

lower binding energy is clearly observable for thinner films. It is due to the substrate-enhanced efficiency of the screening effect of the C 1s core hole in the layer on top of the gold substrate because at the interface of metals/organics, an additional screening of the core hole is present due to the occurrence of an image potential screening.21 It has been shown that this shift is independent from a change in molecular orientation.22 In addition, we have not observed surface core level shifts, that is, differences in the photoemission binding energy of a core level of a surface with respect to a bulk atom molecule.23,24 Thus, we can rule out this effect as contributions to the observed 0.4 eV shift. We also note that when comparing the shift between films deposited on gold and rutile TiO2(110), the shift is 0.2 eV smaller in the latter case,19 evidencing the different screening strength in the two systems, due to the different nature of the substrate (metal or metal oxide). A second effect is also present: the increase of the line width with increasing film thickness, that is, the larger full width at half-maximum (fwhm) of the C 1s peak for the multilayer (fwhm = 1 eV) with respect to the first layers on top of the substrate (fwhm = 0.9 eV). In a previous work, the attenuation of the substrate signal also has been monitored as a function of time under constant evaporation rate, evidencing Stranski Krastanov growth mode, that is, monolayer plus islands.12 The AFM (Figure 1b) image confirms this result; in particular, DIP islands on polycrystalline gold show the average grain size above 300 nm, and the minimum grain size is about 200 nm, while the average film roughness is 25 nm. Figure 1c shows the C 1s highly resolved NEXAFS spectra taken for a 3.4 and 12 nm thick DIP film. The black curve has been recorded in grazing incidence, while the gray one has been recorded in normal incidence (see the geometry of the experiment in inset in Figure 1). Two main groups of resonances dominate the spectra. We can identify the π* region up to about 290 eV and the σ region above 290 eV.12 Figure 2 contains analogous information for DIP thin films deposited on Au(100) single crystals. The C 1s core level spectra

(hν = 330 eV) as a function of film thickness of the DIP film are shown in panel a. They show similar characteristics as in DIP on polycrystalline gold and similar effects due to the screening of the C 1s core hole, and the different fwhm of the C 1s peak can be observed. The AFM (Figure 2b) shows island formation; also, in this case, the growth follows the Stranski Krastanov mode, as proved by LEEM investigations.25 The C 1s highly resolved NEXAFS spectra taken for a 3.6 nm thick DIP film are finally shown in Figure 2c. The features are similar as in DIP on polycrystalline gold, but details that will be discussed in the following section already indicate a possible dissimilarity in the structure of the films deposited on the two geometrically different substrates. Figure 3 is focused on DIP thin films deposited on Au(111) single crystals. While the C 1s core level spectra (hν = 330 eV), and the highly resolved NEXAFS spectra are very similar to the case of DIP films deposited on Au(100) single crystals (Figure 1a,c), the AFM image (panel b) shows striking differences in the island shape when compared to both films deposited on polycrystalline gold and Au(100) . The islands do not show a compact shape, but they have distinctive branches that depart from a central body and run parallel to certain directions. The C 1s core level spectra (Figures 1 3) do not show changes in the peak shape depending on thickness, indicating that in all cases DIP molecules are physisorbed on gold, also in agreement with previous works.7,12,25

’ DISCUSSION A model study of film growth typically involves deposition of a controlled amount of atoms/molecules onto a well-characterized crystalline substrate at a defined set of growth conditions. The precisely defined growth conditions, coverage, and deposition rates in such studies make it possible to decipher the rules governing the evolution of the growth front and to explore ways to tailor film morphology to obtain specific characteristics. 3631

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Figure 3. (a) Thickness-dependent C 1s core level photoemission spectra of DIP deposited on Au(111) are shown. (b) A 5 μm  5 μm AFM image of a 20 nm thick (nominal thickness) DIP film together with the profile analysis of the same sample. The profile was extracted as indicated by the line. (c) C1s NEXAFS spectra obtained from a 3.4 nm DIP. The spectra were taken in grazing incidence for p (black curve) and s (gray curve) polarization. Geometry of the NEXAFS experiment are as in Figure 2.

Classical theories have been developed by J. A. Venables,26 M. Ohring,27 and Z. Zhang and M. G. Lagally28 focusing on works on inorganic materials. H. Brune29 also performed pioneer work in the 1990s on homoepitaxial and heteroepitaxial metal on metal growth with results that are milestones in understanding thin film processes. Organic materials have been the focus of intensive investigations in the recent years. A variety of techniques have been applied to investigate systems of very different nature: Among small molecules, the most investigated ones are certainly acenes, phthalocyanines, and perylene-based molecules.30 40 This is due not only to their importance as a model system but also because they are attractive candidates as an active layer in devices.41 44 By using the results presented in the previous paragraph, it is possible to go a step further in understanding some general aspects of thin film processes in organic small molecules focusing on thin film structure and morphology. The possibility for a molecule to arrange itself in the film is the most striking difference with respect to the growth of atomic films: NEXAFS is the ideal technique to investigate this characteristic.16 The NEXAFS spectra presented in this work show similar features: A dichroic behavior is shown in all three cases. In all three films, the molecules have a very similar average orientation that varies between 44° [DIP deposited on Au(100)] and around 48° (DIP deposited on polycrystalline gold). Previous highly laterally resolved photoelectron emission microscopy (PEEM) investigations have confirmed that this value is not an artifact due to the averaged signal in standard NEXAFS, but the same dichroic behavior is also visible in high resolution.11,45 However, comparing the three NEXAFS spectra, they present several differences when looking at their details. In particular, the most evident difference is the quenching of feature A in the film deposited on polycrystalline gold. This depends neither on the experimental resolution nor on the spectra normalization, but it is due to the different structure found in this film. This can be

easily cross-checked comparing the presented spectra with NEXAFS measurements performed on a thicker DIP film deposited on polycrystalline gold measured and normalized exactly as the thinner film (see Figure 1c). Feature A is clearly visible, indicating a different evolution of the intensity of feature A with thickness in DIP deposited on polycrystalline gold. An important experimental protocol characterizes the results presented here: The preparation conditions are exactly the same in the three cases, as it is well-known that parameters like evaporation rate and substrate temperature influence thin film properties.6,18,30 40,45 47 In addition, the molecules are physisorbed on the three different gold substrates as obtained from the comparison between XPS performed on monolayers and multilayers. These two points allow a direct comparison of morphology and structure of the obtained films. DIP molecules organize in films on Au(100) and Au(111) following the herringbone structure typical of the single crystal with the long lattice vector c of the crystal unit cell, parallel to the substrate plane.25,45 On the contrary, they have been shown to prefer the upright standing orientation on substrates where the molecules are more strongly bound to each other than to the substrate itself, as in case of SiO26 and TiO2.19 Recent Raman investigations coupled with calculations of the deformation of a relaxed excited molecule with density functional theory have shown that the specific structure of DIP on Au(111) favored Raman signals from combinations of breathing modes, a feature that could not be observed in DIP films on SiO2 with its larger and differently oriented crystallites.48 This observation is in agreement with the structure of DIP on Au(111) identified by PEEM/LEEM investigations.45 Under the perspective of reducing the number of parameters that influence growth processes in the present work, there is only a left variable that influences the film structure inducing the differences that are visible in NEXAFS, that is, the different substrate morphology. The used polycrystalline gold is very 3632

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Crystal Growth & Design rough and with no preferential surface lattice geometry.10,49 Illdefined substrates with roughness larger than the molecular size have been shown to cause differences in molecular orientation, favoring the upright standing arrangement of the molecules due to a further weakening of the substrate molecule interaction.49,50 Consequently, the change in intensity of feature A may point to the existence of an intermediate thickness region in the films induced by the roughness of the polycrystalline gold where a different intermolecular interaction (lower degree of order, strong competition between upright/flat lying arrangement) affects the molecular orbitals contributing to feature A.51 This interpretation is further supported by the presence of subtle differences, with respect to the single crystal cases: a shift in the absorption onset and different energy position of the features above 287 eV. However, this experimental observation needs urgent dedicated theoretical modeling, including solid state effects, that is still missing for X-ray absorption spectroscopy, in contrast to band structure calculations and optical spectroscopy where this degree of comparison between experiments and theory already has been achieved.48,52 The change in the fwhm of the C 1s peak seems to be a general pattern in DIP deposited on gold, since it has been observed also in highly lateral resolution comparing nano-XPS measured on islands and monolayers in the same films.45 We can attribute it to a change from flat-lying molecules to a different arrangement, according to NEXAFS results. The second aspect that can be addressed regards the island morphology: There is a remarkable difference in the island shape of the thin film deposited on Au(111) with respect to the remaining two substrates. While on polycrystalline gold and Au(100) single crystals the islands are similar and compact, on Au(111), they are characterized by branches. The former is not the only case where DIP thin films show compact islands (where compact must be intended as antonyms of branched): In all previously published works on DIP, compact islands were reported. Again, the same preparation conditions in the present data give the opportunity to analyze this difference: Keeping the same nature of the substrate, that is, always gold, and the same nature of interactions, that is, physisorption, there must a fundamental parameter that plays the major role. Zhang et al. summarize in ref 28 most of the basic concepts that govern thin film processes at the atomistic scale: There are four classical regimes that describe thin film growth: The hit and stick diffusion limited aggregation (DLA), extended fractal growth regime, island-corner barrier effect, and compact islands. DLA regime fractal growth is a regime in which the landing molecule sticks to an island at the point where it hits the island itself. In DLA, the branches of the fractal islands are expected to be one building unit (molecules/atoms) large. It was previously reported for organic materials, for example, in the case of pentacene deposited on SiO253 and for para-sexiphenyl.54,55 Observing the DIP islands, we do not find a perfect fractal behavior, their branches are formed by more than one molecule, and they show distinctive features, also different from pentacene on SiO2.53 Thus, we can exclude that this regime applies to the present case. In addition, pentacene and pentacene-based molecules show pure fractal growth on a variety of substrates. This is the crucial difference between pentacene and DIP: Pentacene shows fractal growth, and DIP does not; fractal growth in pentacene does not depend on the substrate. It depends on the preference for a kinetic growth along the b-axis of the in-plane pentacene unit cell.56

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As mentioned, there are still two regimes that allow fractal growth: extended fractal growth regime and island-corner barrier effect. The difference between the two cases is that the latter is temperature dependent. Our experiment has been performed at a fixed temperature (300 K). AFM images reveal that the surface islands are characterized by strong roughness, as previously seen with PEEM investigations.45 This characteristic evidences a strong effect of the island-edge barrier. The vertical growth in a film is due to the interlayer transport of molecules, which is controlled by the barrier for crossing steps.26,28 A molecule landing on top of an island may experience a higher potential barrier when hopping to the lower layer because of its coordination reduction; thus, it is more likely that it will remain on top of the island, causing a rough growth front, as observable in DIP islands. This is the so-called Ehrlich Schw€obel barrier, evidenced via the additional energy required for the diffusing adparticles to surmount a downward step.26,28,57 The surface roughness in DIP islands implies that the temperature at which the system is grown is not high enough to have relevant diffusion along the island edges. Analogously, we may expect that molecules have a hindered capacity to cross island-corner barrier. The same considerations regarding temperature hold for the three substrates because DIP thin films have been deposited using the same preparation: Temperature play a minor role in this context. The molecules prefer a higher coordination, giving rise to islands with branches on surfaces with triangular lattice geometry. This means that the growth of branched island in DIP films is related to the triangular symmetry of the substrate surface, as seen in homoepitaxial and heteroepitaxial metal on metal growth.28,29 Still, there is a difference between molecules and atoms: In the case of homoepitaxial and heteroepitaxial metal on metal growth, the fractal growth mode is mainly Volmer Weber; that is, islands directly nucleate on the substrate. In the case of DIP on gold, first there is always a monolayer on top of gold, and then, island nucleation occurs. It is known that the first layer of DIP molecules on top of the Au(111) single crystal does not affect the herringbone reconstruction that is perfectly preserved upon deposition of the first layer.58 We can suppose that from the analogy of XPS and NEXAFS results at the interface with Au(111) and Au(100) evidencing a weak physisorption on both surfaces, also in the latter case, the reconstruction is preserved upon deposition of the first layer. This suggests that geometry and surface energy characteristics of the gold substrates are transmitted through the first layer to the successive layers that respond to the substrate influence similarly as in homoepitaxial and heteroepitaxial metal on metal Volmer Weber growth mode. We also obtained analogous results in real time in situ experiments performed by using PEEM and LEEM, leading to the consequent exclusion of postgrowth effects in the data here presented.25,45 Our results raise several questions: Does this phenomenon have a general relevance; that is, does it occur also for other molecules besides DIP? Which is the role of the degree of interaction with the substrate (chemi- vs physisorption)? Does the stoichiometry make a difference (completely carbon-based molecules vs different atoms)? These questions can be answered looking at the extensive investigations performed on 3,4,9,10-perylene-tetracarboxylicdianhydride (PTCDA) that not only is the most investigated organic molecule being a prototype since its first evidence for organic MBE,59 but it is also a perylene-based molecule like DIP. Recent works performed using scanning tunneling microscopy 3633

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Crystal Growth & Design have been published by Ikonomov et al.60,61 on Ag(100) and by Kilian at al.62 on Ag(111). The comparison is straightforward because the experiments are performed in a very similar way, with the same technique, and under similar preparation conditions with Ag(100) kept at 120 K and Ag(111) at 100 K. An additional interesting aspect is that the noble metal is silver, supporting a larger generality of our discussion. Submonolayers of PTCDA form islands that have a quadratic shape on Ag(100), while submonolayers of PTCDA deposited on Ag(111) are characterized by dendritic islands. In both cases, PTCDA is chemisorbed on silver with a stronger bonding and a smaller vertical adsorption height on Au(100) since the surface is more open. This example shows how our interpretation has a general relevance: The dependence of the island shape on the lattice geometry of the substrate is not affected by the stoichiometry of the particular molecule, neither by the degree of the chemical interaction with the substrate.

’ CONCLUSIONS In this work, we have investigated DIP thin film deposited on polycrystalline gold and Au(100) and Au(111) single crystals by using X-ray absorption and photoemission together with AFM. The results show the strong influence of the substrate morphology, via the roughness, on the structure of the obtained films. This affects the intermolecular interactions as evidenced by different details in NEXAFS. A further source of influence is the lattice geometry of the substrate that affects the shape of the islands. This behavior mimics very closely the behavior of atoms in homoepitaxial and heteroepitaxial metal on metal growth. Our results demonstrate that on one hand the morphology of the substrate induces changes in the film structure, and on the other hand, thin film processes follow a general behavior. Hence, our work is of global relevance, indicating also that intense theoretical modeling is needed regarding intermolecular interaction, solid state effects, and effects induced by the environment on which the film nucleates, including the lattice geometry of the substrate. This has an obvious impact also on thin film-based devices. ’ AUTHOR INFORMATION Corresponding Author

*Tel: +49 7071 29 76252. Fax: +49 7071 29 5490. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the Helmholtz-Zentrum Berlin, Electron storage ring BESSY II, in particular Dr. W. Braun, and the Angstroemquelle Karlsruhe ANKA for providing beamtime; I. Biswas, M. Nagel, B.-E. Schuster, and S.-A. Savu for taking part in the beamtime; P. Hoffmann, P. Nagel, and S. Schuppler for beamline support; Wolfgang Neu and Stephan Pohl for technical support; and Prof. T. Chasse for helpful discussions. Financial support from the Helmholtz-Zentrum Berlin is gratefully acknowledged. ’ REFERENCES (1) von Braun, J.; Manz, G. Ber. d. D. Chem. Gesellschaft 1937, 70, 1603–1610.

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