Interplay between Solution Processing and Electronic Structure in

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Interplay between Solution Processing and Electronic Structure in Metal-Free Organic Magnets Based on a TEMPO Pentacene Derivative C. Arantes,†,‡,∥ E. T. Chernick,§ M. Gruber,§ M. L. M. Rocco,‡ T. Chassé,∥ R. R. Tykwinski,§ and M. B. Casu*,∥ †

Materials Metrology Division, Inmetro, National Institute of Metrology, Quality and Technology, Duque de Caxias, Brazil Institute of Chemistry, Federal University of Rio de Janeiro, Rio de Janeiro 21941-909, Brazil § Department of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), Friedrich-Alexander-University Erlangen-Nürnberg (FAU), 91054 Erlangen, Germany ∥ Institute of Physical and Theoretical Chemistry, Eberhard Karls Universität Tübingen, Tübingen, Germany ‡

S Supporting Information *

ABSTRACT: We have used X-ray photoelectron spectroscopy (XPS) to describe the electronic structure of two newly synthesized derivatives of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) carrying the 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) radical as a substituent. XPS results underline the existing interplay between electronic structure and preparation methods. Our observations suggest a strong influence of the film processing on the shakeup satellite intensities of the N 1s core level spectra. Preparation methods, by tuning the different degree of intermolecular screening, may be use as a tool to influence the properties and the behavior of these materials as thin films, including their local magnetic-exchange coupling.

1. INTRODUCTION Organic-based magnets and single molecular magnets are rapidly evolving toward being a class of materials with strong potential for applications.1 In particular, metal-free organic radicals have attracted considerable attention in the last years due to the flexibility in processing and tuning the physical and chemical properties and their potential magnetic properties associated with their unpaired electrons.2−5 (2,2,6,6-Tetramethylpiperidin-1-yl)oxidanyl (TEMPO) is one of the first synthesized stable (i.e., persistent) nitroxide radicals, and it has been used as a building block of more complex heterocyclic nitroxides.6,7 Open-shell systems have magnetic properties that make them interesting, for example, in the field of spintronics and sensors.3,8,9 In this respect, we have recently focused our activities toward the understanding of thin-film processes (in the nanoscale regime) of open-shell systems by investigating two pyrene derivatives of the nitronyl nitroxide radical, which has been deposited on a number of technologically relevant surfaces, such as titanium dioxide, silicon dioxide, and sapphire.10−15 Our efforts are motivated by the immense technological potential of such materials, as they bring together magnetism and the flexibility of chemical synthesis.16−19 The choice of the substituent attached to the radical plays an important role, not only from a chemical stability point of view but also for the processes of growth and for potential © 2016 American Chemical Society

applications. The vapor pressure of the molecules impacts their film stability: molecules with higher vapor pressure at room temperature (RT) are volatile,20−22 making it difficult to reach the necessary thermodynamic equilibrium for a stable film at device working temperatures.20−22 We have clearly demonstrated this effect in thin films of open-shell systems by comparing the thin-film stability of the mentioned pyrene derivatives characterized by different vapor pressures.23,24 In addition, it is well-known that a free radical coupled to a fluorophore suppresses the normal fluorescence emission process.25,26 In such a molecule, the radicals, e.g., nitroxides, are efficient quenchers of the fluorescence, due to the relaxation of the fluorophore excited states caused by electron-exchange interactions with the radical.26−28 Pentacene and its derivatives are very promising and widely investigated for application in organic thin-film-based devices.29−35 Especially, solution-processed pentacene derivatives emerge as an alternative for devices in order to lower the manufacturing costs for large-scale thin-film area and to ease the deposition onto flexible substrates. Among them, 6,13bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) is a wellReceived: October 13, 2015 Revised: January 6, 2016 Published: January 21, 2016 3289

DOI: 10.1021/acs.jpcc.5b10028 J. Phys. Chem. C 2016, 120, 3289−3294

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The Journal of Physical Chemistry C

taking into account the Wagner sensitivity factors.40 A linear background subtraction was considered for all spectra. Corelevel spectra were fitted using a least-squares fitting procedure as discussed in ref 41. Special care was taken during the experiments to avoid radiation damage because the films are radiation sensitive on a relatively short time scale (several tens of minutes). Atomic force microscopy (AFM) measurements (Digital Instruments Nanoscope III Multimode) were carried out under ambient conditions in tapping mode.

known high-performance, air-stable, solution-processable organic semiconductor.36,37 The covalent linkage of two units such as a TEMPO derivative and TIPS-PEN may pave the way to use this class of molecules as candidates for new applications in electronics. Here, we focus on the thin-film characterization of two pentacene-based nitroxide radicals under different preparation conditions, namely, Ra (C49H55N2OSi) and Rb (C51H55N2OSi) (Figure 1).

3. RESULTS AND DISCUSSION The N 1s core level spectra are very sensitive to perturbations of the nitroxide radicals upon either damage23 or chemical interactions,15 thus allowing a detailed investigation of the electronic structure of the systems containing such substituents. Figures 2a−d show the N 1s core-level spectra of drop-casted

Figure 1. Molecular structures of (a) Ra and (b) Rb.

We investigate solution-processed films deposited on SiO2/ Si(111) surfaces by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Our work shows a clear interplay between the deposition method and the electronic structure in thin films of these materials. A specific characteristic of Ra and Rb is that the TEMPO radical does not quench the pentacene photoexcited singlet state but the photoexcited triplet excited state.38 The quenching depends on the TEMPOto-pentacene distance.38 Time-resolved electron paramagnetic resonance experiments showed that the triplet quenching is accompanied by electron spin polarization transfer from the pentacene triplet state to the TEMPO doublet state in the weak coupling regime.38 This implies that adopting the appropriate chromophore and radical coupling the decay channel of the excited state can be tuned in view of applications in electronics, thus exploring Ra and Rb thin-film properties is also of technological interest.

Figure 2. N 1s core level spectra: (a) Ra and (b) Rb drop-casted films and (c) Ra and (d) Rb spin-coated films, with their respective fitting analyses. See Supporting Information for fit details.

and spin-coated films of the two pentacene-based nitroxide radicals, together with their fitting curves (see Supporting Information for the experimental binding energies, Tables S1− S3). From a quantification point of view, XPS can provide information on the stoichiometry of the investigated films: the area of the main lines together with their relative satellites for a given element is proportional to the percentage of the same element in the chemical formula of the investigated system. In highly resolved XPS spectra, the rich fine structure allows fitting the lines including contributions for different atomic sites of the same element that show differences in their binding energies due to variations in chemical environment.23,41 The lifetime of the core-hole, and consequently the intrinsic peak width, is determined basically by the Heisenberg uncertainty principle. The quantitative analyses of the XPS signals (Table S4 in Supporting Information) agree with the stoichiometry of the molecule, indicating that both pentacene-based nitroxide radicals are intact in the freshly solution-processed films. We apply constraints based on stoichiometry, electronegativity, and bond strength, as extensively discussed in ref 23 for organic radicals and more generally in ref 41 for organic molecules. In principle, each chemically inequivalent atom site should yield a different contribution to the spectrum. Thus, for both molecules Ra and Rb, two main photoemission peaks with the intensity ratio of 1:1 are expected, which can be attributed

2. EXPERIMENTAL METHODS The synthesis of Ra and Rb has been achieved as described in ref 38. Drop-casted and spin-coated films of Ra and Rb from 40 μL of dichloromethane (DCM) solutions with concentration of 8 mg/mL were deposited on native SiO2/Si(111) substrates with area of about 1 cm2 under ambient conditions. The thickness of the films was estimated as ∼400/500 nm for dropcasted and ∼40/50 nm for spin-coated films by using AFM. Powder samples were obtained embedding the powder in an indium foil. All substrates were cleaned with methanol, acetone, and isopropanol under ultrasonic bath (10 min) before use. XPS was carried out at a base pressure of about 9.0 × 10−10 mbar, using a hemispherical electron analyzer (Specs Phoibos 150) and a monochromatized or an unmonochromatized Al Kα radiation (1486.6 eV) as the excitation source. The films were measured with the monochromatized source and the powders with the unmonochromatized source in order to overcome charging effects. Survey XPS were acquired with electron pass energy of 50 and 20 eV for the detailed C 1s, N 1s, O 1s, and Si 2p core-level spectra. The binding energy calibration was carried out using the In 3d and Si 2p signals of the substrates as a reference.39 XPS quantitative analyses were carried out by 3290

DOI: 10.1021/acs.jpcc.5b10028 J. Phys. Chem. C 2016, 120, 3289−3294

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The Journal of Physical Chemistry C to the nitrogen atoms with the two different chemical environments, N and N−O (see Figure 2). In the N 1s spectrum of the Ra drop-casted film (Figure 2a), the lower binding energy peak (N) at 398.06 eV is assigned to contributions due to photoelectrons emitted from the imine group (CN−C), and the higher binding energy peak (N−O) at 400.68 eV corresponds to contributions from the nitroxide radical (N−O). The shift in energy position between these two photoemission peaks is due to the electron-withdrawing capability of the oxygen atoms that reduces the charge density on the nitrogen atoms and consequently increases the photoelectron binding energy. Both photoemission peaks shift to lower binding energy in the spectrum of Rb, and they appear at 397.81 eV (N) and 400.47 eV (N−O) (Figure 2b). The only difference between these two molecules is the ethynyl spacer of Rb linking the pentacene substituent and the phenyl group attached to the nitroxide-based radical of the molecule Rb (Figure 1). Rigid spacers, such as ethynyl groups, are commonly used to hold substituent groups away from the aromatic core. This substitution eases the torsional/steric strain upon bond rotation of the phenyl group with respect to the pentacene plane,42 and it promotes a better interaction between the aromatic rings,37 favoring the intra- and the intermolecular coupling, improving the electron delocalization in the molecule. Consequently, the 0.2 eV energy shift observed for the N 1s spectroscopic lines with respect to the Ra case may be explained with the increase of the intramolecular core-hole screening due to the enhanced coplanarity between the pentacene substituent and the nitroxide-based substituent (see also Figure 1 that evidences the planar/coplanar arrangement of the phenyl/ethynyl phenyl spacer and ref 38 for further details). The spectra of the drop-casted films of Ra and Rb (Figures 2a and 2b) present a small broadening of N 1s peaks and negligible energy shift (∼0.1 eV) with respect to the spin-coated films (Figures 2c and 2d), which can either be related to the increase of film thickness or to inhomogeneous broadening because of local morphological and structural differences.13,43 For the sake of simplicity in the case of dropcasted films, we first performed the best fit procedure including the shakeup satellite intensities in the main line intensities. However, the best fit procedure for the spin-coated films necessarily had to take into account the presence of a shakeup satellite (contribution S1 in Figures 2c and 2d) to achieve the best agreement with the experimental curves. This indicates that in the spectra of the spin-coated films the shakeup satellites are more intense than in the spectra of the drop-casted films. Shakeup satellites are characteristic in photoemission core-level spectra of aromatic hydrocarbon molecules.44 They are due to electronic relaxation effects: different contributions to the satellite spectrum are given by the large number of nonequivalent carbon sites originating from the symmetry reduction due to the core-hole formation. Beyond these phenomena, what is most relevant is that the spin-coated film spectra of both molecules present an additional peak at higher binding energy (405.21 eV), while the N 1s spectra of the drop-casted films show negligible/absent intensity in the same energy range. To shed light on the nature of this spectral feature and on the different spectroscopic behavior of drop-casted and spin-coated films, we performed XPS investigations on the powders of the two molecules (Figure 3). For both powder samples, the two N 1s photoemission peaks present small broadening and energy

Figure 3. N 1s core level spectra of (a) Ra and (b) Rb measured as powders. The arrows indicate the shakeup satellite at higher binding energies.

shifts, due to typical charging effects occurring in organic crystals. Also the spectra obtained for the two powders reveal a feature at higher binding energy, similarly to what we observe for the spin-coated films. The presence of this peak in the powder spectra and its simultaneous absence exclusively in the spectra of the drop-casted films allow us to rule out degradation and oxidation effects as a reason for this feature. If the feature were due to oxidized/degraded powder before preparation, we would obviously expect its presence in all spectra, irrespective of preparation method. Note that the same solution was used for spin-coating and drop-casting the films, to process the two different preparations in a parallel way, easing their comparison. Thus, we can also exclude that differences in the solutions affect our results. In addition, the film deposition was carefully performed to avoid photodegradation due to UV-light exposure.45−47 Consequently, we assign the peak at around 405.2 eV to a shakeup feature, in analogy to the nitrosobenzene electronic structure.48,49 Note that this assignment is fully supported by stoichiometry arguments. The spin-coated films (see Figures 2c and 2d) show lower intensity of the nitroxide main peaks and a N:N−O ratio between imine and nitroxide peaks far from the expected molecular stoichiometry of 1:1. This is because the shakeup satellites contribute to the total intensity of the photoemission peak12,13 (see Discussion and Tables S4 and S5 in the Supporting Information). Therefore, the shakeup satellite at lower binding energy, S1, is associated with the imine photoemission peak, while S2 is related to the nitroxide main line. Its higher intensity in the spectrum of Rb (Figure 2d) is in agreement with the expected molecular stoichiometry for this molecule. Unusually intense shakeup satellites at higher binding energies have been previously observed in several small organic molecules containing nitrogen groups with strong electronacceptor character, such as −NO and −NO2.50,51 Basically, when the core-hole is located in the acceptor part of the molecule, i.e., the nitroxide radical, the electron excitation process (shakeup transitions simultaneous with the photoemission) takes place with higher probability because the electron flow during the charge transfer is favored. On the other hand, when the core-hole is created in the donor part (TIPS3291

DOI: 10.1021/acs.jpcc.5b10028 J. Phys. Chem. C 2016, 120, 3289−3294

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Figure 4. AFM images of Ra thin films. A 10 μm × 10 μm of (a) drop-casted film and (b) spin-coated film. We used the same solution for both preparations.

From the morphological point of view, the drop-casted films have lower uniformity and higher roughness than the spincoated films (the root-mean-square (RMS) roughness being 54 and 2 nm, respectively, calculated on 10 μm × 10 μm AFM images, see Figure 4 as well as Figures S3 and S4 in Supporting Information). Due to the drying effects caused by the solvent evaporation, their AFM images are characterized by distributed circular valleys, as typically observed in films obtained by using this processing method (see Figure 4 and Figure S3 in Supporting Information). The spin-coated films (Figure 4 and Figure S4 in Supporting Information) reveal smooth featureless domains with additional sparse crystallites separated by 2−4 μm, resembling very closely the morphology observed in TIPSPEN films.56,63 The intense shakeup structure observed for the spin-coated films may be related to the presence of these crystalline regions, similarly to the influence of the analogous TIPS-PEN crystallites on the film optical-absorption spectra that are responsible for an additional signal in the range between 680 and 750 nm.56

pentacene) of the molecule, it hinders the charge transfer, and then the concomitant shakeup excitation is quenched.52 Charge transfer shakeup excitations may be especially pronounced in the N 1s core-level spectra of electron−acceptor groups when the core-hole is located in this part of the molecule, as observed in this work and for other nitrogen-based molecules.48,50,53 The different behavior of the feature at around 405 eV when comparing powder/spin-coated films versus drop-casted films is, however, puzzling. We observe a clear dependence of this feature on the preparation conditions. To elucidate the reason for this behavior, we can compare our experiments with the extensive published work that demonstrates that different preparation methods lead to different structural thin-film properties and that polymorphism is widely present in organic solids, including thin films.22,54−56 In TIPS-PEN, the effects of the evaporation behavior in solution-processed films play a paramount role on the film morphologies and molecular structural organization.57−60 Indeed, TIPS-PEN films show a variety of different molecular packing induced by the fast/slow evaporation during preparation. These differences also substantially impact the device performance.61,62 This indicates that another important aspect may influence the shakeup intensity, namely, the film preparation, because spin-coating and drop-casting methods have different evaporation rates leading to different film properties.57−59,62 Thus, we assign the feature at around 405 eV to a shakeup contribution whose intensity is preparation-dependent. Our interpretation agrees very well with previous works: Freund et al.49,52 pointed out that shakeup transitions in gas and solid phases, in particular in the case of nitrogen- and oxygen-containing polar molecules, are very dissimilar because of the major role played by the intermolecular interaction in the solid state. Shakeup satellites are due to relaxation effects of the system that reacts upon the photoemission event in order to screen the core-hole. Differences in the molecular distance/ arrangement may lead to a more/less efficient screening of the core-hole via different channels, enhancing specific satellite intensities. If we compare the results that we have obtained from different sample preparations, we can infer that the spincoated films behave similarly to the crystalline samples because they present a similar shakeup satellite structure. We also observe that the decrease of the coplanarity in Ra should reduce the intramolecular screening in this molecule with respect to the Rb, lowering the shakeup satellite intensity in the spectra of the Ra. This effect is actually observed for the powder samples when comparing the spectra of Ra and Rb (Figure 3).

4. CONCLUSIONS In conclusion, we have used XPS to describe the electronic structure of two newly synthesized derivatives of the TIPS-PEN carrying a radical as a substituent. Our results clearly underline the existing correlation between electronic structure and preparation methods. We explain our results in terms of a different degree of intermolecular coupling. Our observations suggest a strong influence of the preparation methods and the consequent intermolecular screening on the shakeup satellite intensities of the N 1s core-level spectra. Preparation methods strongly influence the electronic structure in organic thin film: in particular they have relevant consequences on the photoemission event. A quantitative description of the phenomena reported here undoubtedly needs supporting simulations. Our result may stimulate the necessary theoretical modeling and calculations. Last but not least, intermolecular interactions influence the magnetic and optical properties of radical-based molecules;19,64,65 therefore, different preparation methods impacting the film properties, that is, causing a different degree of intermolecular interaction, may be used in the future as a tool to tune the magneticexchange coupling and to control the electronic and the magnetic properties of these materials. 3292

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10028. Stoichiometry of the Ra and Rb, drop-casted, and spincoated films. C 1s core level spectra of drop-casted films, spin-coated films, and powders. AFM Images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49 7071 29 76252. Fax: +49 7071 29 5490 (M.B.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank W. Neu, E. Nadler, H. Adler, and R. Kakavandi for technical support. Financial support from DFG under the contract CA852/5-1 and CA852/5-2, DAAD (PPP Brasilien 2j, Project ID: 56266057) under the funding of the German Federal Ministry of Education and Research (BMBF) and the CAPES/DAAD/PROBRAL (390/13) is gratefully acknowledged. Funding at FAU is gratefully acknowledged from the “Solar Technologies go Hybrid” − an initiative of the Bavarian State Ministry for Science, Research, and Art, the “Excellence Initiative” supporting the Cluster of Excellence “Engineering of Advanced Materials”, and the FAU Emerging Fields initiative “Singlet Fission”.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b10028 J. Phys. Chem. C 2016, 120, 3289−3294

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DOI: 10.1021/acs.jpcc.5b10028 J. Phys. Chem. C 2016, 120, 3289−3294