Phenyl-C61-butyric Acid Methyl Ester - American Chemical Society

Mar 18, 2014 - fullerene derivatives, phenyl-C61-butyric acid methyl ester. (PCBM) is the best studied and also one of the best performing electron ac...
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Thermal [6,6] → [6,6] Isomerization and Decomposition of PCBM (Phenyl‑C61-butyric Acid Methyl Ester) Bryon W. Larson,† James B. Whitaker,† Alexey A. Popov,*,§ Nikos Kopidakis,*,‡ Garry Rumbles,*,†,‡ Olga V. Boltalina,*,† and Steven H. Strauss*,† †

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States § Department of Electrochemistry and Conducting Polymers, Liebniz Institute for Solid State and Materials Research, Dresden D01069, Germany ‡

S Supporting Information *

ABSTRACT: For the first time, the thermal stability limits of one of the most highly cited and well-studied fullerene derivative electron acceptors, phenyl-C61butyric acid methyl ester (PCBM), have been investigated under thermal annealing and vapor deposition conditions. Significant decomposition is observed when PCBM is heated, even briefly, to and beyond its melting temperature in an inert atmosphere, as evidenced and quantified here by proton nuclear magnetic resonance, atmospheric-pressure chemical ionization mass spectrometry, and UV− vis spectroscopy, as well as high-performance liquid chromatography. The major thermally induced decomposition product of PCBM has been isolated, characterized, and identified as a new pentacyclic [6,6]-addition motif isomer of PCBM (iso-PCBM). Cyclic voltammetry studies show no difference in electrochemical properties between PCBM and iso-PCBM, and our quantum chemical calculations predict the new isomer to be ∼43 kJ/mol more thermodynamically stable than PCBM.

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INTRODUCTION High thermal stability is one of the desired properties of fullerenes and fullerenes derivatives that makes them useful in applications in organo-electronic technologies such as organic photovoltaic (OPV) devices, organic light-emitting diodes (OLEDs), and organic field effect transistors (OFETs). Of all fullerene derivatives, phenyl-C61-butyric acid methyl ester (PCBM) is the best studied and also one of the best performing electron acceptor materials in these devices1 and has been widely used in both fundamental and applied organic semiconductor studies.2−7 Knowledge of the thermal stability limits of this technologically important compound is therefore essential; however, no systematic and reliable studies of the thermal behavior of PCBM have been conducted. In fact, even existing data on the effects of the thermal treatment of PCBM are controversial in many cases. On one hand, postfabrication thermal annealing treatments of PCBM-containing OPV devices have been shown to drastically improve device efficiency and are now common practices in polymer solar cell research.8,9 On the other hand, it has been shown that prolonged thermal treatments decrease cell performance10 and that high-temperature annealing leads to decreased charge carrier lifetimes in microcrystalline PCBM.11 At the same time, numerous studies involve films of PCBM for layered organoelectronic applications12−16 or for the study of the solid-state properties of the film, such as solid-state electron affinity (EA) or ionization potential (IP),17,18 prepared by high-vacuum thermal evaporation. A few general remarks that PCBM © 2014 American Chemical Society

thermally decomposes have been made in the literature, although they were not accompanied by compelling relevant supporting data.19,20 Furthermore, the decomposition temperatures that have been mentioned in the literature for PCBM and PCBM-like fullerene derivatives are inconsistent, ranging from 200 to 400 °C.11,21−25 Our interest in the fundamental and applied properties of PCBM and other fullerene electron acceptors,26−28 and the ambiguous, if not contradictory, reports cited above, convinced us that a detailed study of the thermal stability or instability of PCBM was needed. In this paper, the physical, chemical, and electrochemical effects of (i) thermal annealing of PCBM films or powders over a temperature range of 180−380 °C and (ii) vapor deposition of PCBM are described, which are based on high-performance liquid chromatography, cyclic voltammetry, nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis, and mass spectrometry. In addition, the major product of thermal decomposition of PCBM has been isolated, characterized, and identified as a new, more thermodynamically stable isomer of PCBM that forms at high temperatures.

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EXPERIMENTAL SECTION

Commercially available PCBM powder (Nano-C, 99%) and all solvents [Fisher Scientific, high-performance liquid chromatography (HPLC) grade] were used as received. Vacuum sublimation was Received: February 18, 2014 Revised: March 18, 2014 Published: March 18, 2014 2361

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conducted in an Edwards Auto306 vapor deposition chamber by resistively heating the sample in a tungsten boat at a base pressure of 7 × 10−6 Torr and deposition rate of 0.7 Å/s as monitored by an LQC crystal mass balance. HPLC analysis and separation were performed using Shimadzu instrumentation (CBM-20A control module, SPD20A UV detector set to 300 nm, LC-6AD pump, manual injector valve) equipped with a 25 mm (inside diameter) × 250 mm preparative Cosmosil Buckyprep column (Nacalai Tesque, Inc.) with toluene or a toluene/hexane mixture as the eluent at a flow rate 16 mL/min. Negative and positive ion atmospheric-pressure chemical ionization mass spectra were recorded using a Finnigan LCQ-DUO mass spectrometer with an acetonitrile carrier solvent (samples injected in toluene/acetonitrile mixtures) at a flow rate of 0.3 mL/ min. 1H NMR spectra were recorded using a Varian INOVA 400 MHz spectrometer with samples in CDCl3 (tetramethylsilane as the internal standard). Cyclic voltammograms were recorded using an in-house, three-electrode (0.5 mm platinum wire working and auxiliary electrodes and a 0.5 mm silver wire as the quasi-reference electrode), one-compartment electrochemical cell controlled by a PAR 273 potentiostat/galvanostat at room temperature in a dinitrogen atmosphere glovebox. Thermogravimetric analysis (TGA) was accomplished using a TA Instruments 2950 Series TGA device at a heating rate of 3 °C/min in a He atmosphere using a platinum sample boat. Optimization of the molecular structures at the GGA PBE29 level was performed with the Priroda code.30,31 Additional density functional theory (DFT) and MP2 computations were performed using the Orca suite.32

insoluble, but after the residue had been thoroughly washed with toluene, the extract was analyzed. It was found to contain primarily C60 based on its UV−vis spectrum and the presence of a peak with the same HPLC retention time (Figure 1, inset) as a C60 reference under identical analytical conditions. This means that while some C60 was indeed formed as a decomposition product, the majority of the mass of the sample was retained and became insoluble, most likely because of polymerization. To determine the nature and onset temperature threshold of these thermally induced chemical changes to PCBM, a series of experiments involving its thermal treatment were conducted in the temperature range of 180−380 °C. This range was chosen for two reasons. (i) It includes the temperature range over which mass loss was observed in the TGA thermogram, and (ii) it practically covers annealing temperatures commonly applied for studies related to PCBM’s thermal properties, especially phase behavior studies. Each PCBM sample (approximately 1 mg) was heated for 20 min in a TGA instrument at a constant temperature under a N2 flush and then analyzed by HPLC (Figure 2) and 1H NMR spectroscopy (Figure SI-1 of the

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RESULTS AND DISCUSSION Thermal Behavior of PCBM. TGA is a useful technique for determining thermally induced changes in physical and chemical properties of a compound and therefore was used to study the thermal behavior of PCBM. Figure 1 shows the

Figure 2. HPLC chromatograms for a series of PCBM samples that were heated for 20 min at temperatures in the range of 180−340 °C. The eluent was toluene and the flow rate 5 mL/min for each analysis (10 mm × 250 mm Cosmosil Buckyprep column) at 300 nm detection.

Supporting Information). No detectable decomposition of PCBM was found by HPLC or 1H NMR spectroscopy in the samples heated at ≤260 °C. This indicates that the mass loss feature observed in the thermogram (Figure 1) at 240 °C is not due to a chemical change in PCBM, but more likely due to a loss of trace solvent molecules. Interestingly, heating at 300 °C resulted in some decomposition as evidenced by the appearance of new peaks in the HPLC chromatogram and 1 H NMR spectrum (see Figure 2); however, the entire sample dissolved readily in toluene after such a heating treatment (i.e., no insoluble material was formed). When the sample was heated to 340 °C, massive decomposition was observed by HPLC analysis and 1H NMR spectroscopy, and an insoluble char was formed; i.e., the sample did not fully dissolve after the thermal treatment. At 380 °C, the sample was almost entirely insoluble. Heating PCBM for 60 min at 280 °C resulted in the same degree of decomposition as when the sample was heated

Figure 1. TGA of PCBM under a He atmosphere at a heating rate of 3 °C/min. The inset shows the HPLC chromatogram [100% toluene, 16 mL/min, Cosmosil Buckyprep column (25 mm × 250 mm), at 300 nm detection] of the toluene-soluble part of the PCBM sample after the TGA experiment.

TGA thermogram of PCBM heated to 600 °C. A gradual mass loss of ∼0.5% was observed up to 345 °C, including a small feature in the thermogram at 240 °C (discussed below), followed by a mass loss of 9.6% up to 600 °C. This result is consistent with the literature TGA thermograms for PCBM11,21,23,25,33 and PCBM-like21,22 derivatives. The calculated mass change associated with full cleavage of PCBM’s methano adduct to C60 is a loss of 20.9%, which is not consistent with the observed mass loss of 9.6%. After the TGA experiment, the remaining residue was char-like and mostly 2362

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for 20 min at 300 °C (Figure SI-2 of the Supporting Information), indicating decomposition does occur at lower temperatures during longer periods of heating. The rate of degradation appears to increase drastically as temperatures exceed 300 °C. This observation merits caution when PCBM-containing devices or sample preparation techniques require prolonged heating at elevated temperatures (i.e., >280 °C). In an experiment aimed to determine the long-term thermal stability of PCBM at a slightly lower temperature, we heated a PCBM sample in the TGA instrument for 6 h at 220 °C and found negligible decomposition (280 °C thermal isomerization of PCBM to iso-PCBM. If breaking one of the two C(methano)−C(cage) bonds occurs at 180 °C in the [5,6]-isomer and is therefore much faster at ≥280 °C, breaking one of these bonds in PCBM may be reversible and may indicate that the rate-determining step for the isomerization shown in Figure 6 (black and blue C atoms) is abstraction of an ortho H atom by an insipient methinyl radical. This requires that both isomerizations, [5,6]-PCBM → PCBM and PCBM → iso-PCBM, involve a common intermediate. The [5,6]-PCBM → PCBM photochemical isomerization at 25 °C has been reported45 but is not relevant here because all of the thermal treatments of PCBM in this work were performed with rigorous exclusion of light. Sublimation Study of PCBM. Low-pressure vapor deposition is a technique well suited for the preparation of high-quality and impurity-free thin films and has been successfully implemented in the fabrication of OPVs containing C60 and copper(II) phthalocyanine.46 Thermal evaporation has been utilized for the fabrication of organic electronic devices such as OPVs12,13,15,16 and OLEDs,14 as well as for fundamental property measurements such as solid-state EA.17,18 However, the consequences of using this technique for the preparation of thin films of PCBM have not been addressed in the literature. Sublimation of a 45 mg sample of as-received PCBM at ≥340 °C for ∼15 min, in a vapor deposition chamber typical of those used by many workers in the field to prepare OPV active layers, resulted in a toluene-soluble sublimate and a solid residue that appeared to have been molten during the sublimation (see Figure SI-4 of the Supporting Information for photos). The residue consisted of toluene-soluble compounds and insoluble material; the latter was not characterized further. The toluenesoluble sublimate and residue were analyzed (HPLC chromatograms shown in Figure 7, along with that of pristine PCBM), and they contained the same mixture of fullerene species as

Figure 7. HPLC chromatograms of PCBM, its sublimate, and residue (top to bottom, respectively) using a 25 mm × 250 mm Cosmosil Buckyprep column with toluene as the eluent at a flow rate of 16 mL/ min and 300 nm detection. Percentages given correspond to the PCBM peak areas of the integrated chromatogram.

PCBM samples heated to 340 °C in a dinitrogen atmosphere, although in different relative amounts. When the sublimate and residue HPLC traces in Figure 7 are added together, the resulting composite trace is virtually congruent with the 340 °C HPLC trace in Figure 2, as shown in Figure SI-5 of the Supporting Information; the 1H NMR spectra are also in accord (Figure SI-6 of the Supporting Information). Significantly, as far as OPV active layer fabrication is concerned, sublimation of PCBM results in the deposition of a mixture of fullerenes consisting of only ∼41 mol % PCBM (and ∼24 mol % iso-PCBM). Typical vapor deposition chambers, including the one used in this work, do not allow for postdeposition air-free handling of the sample, because these instruments are generally too large to house in an inert-atmosphere glovebox. For this reason, a sample of PCBM was sealed in a glass ampule under vacuum and sublimed at 340 ± 5 °C in a tube furnace by inserting only part of the ∼30 cm long ampule into the furnace. The ampule was subsequently opened in a purified dinitrogen-filled glovebox, and the major components of the sublimate were identical to those contained in the sample prepared as described above (i.e., their 1H NMR and mass spectra were the same). This demonstrates that the principle thermal decomposition products of PCBM are not air sensitive, at least on the time scale of these analyses. To date, this stands as the first experimental evidence characterizing and quantifying the decomposition of PCBM accompanying vapor deposition. These results indicate that while it is possible to vapor-deposit PCBM under these conditions, the sublimate contains at best only 40−50% of PCBM, the rest being its new isomer, and 2365

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reversible first and second reductions, but a less well-defined third quasi-reversible reduction due to peak shouldering, likely due to differing third reduction properties of minor impurities that co-eluted during HPLC separation. Broader peaks and only two defined redox events were found for fraction C, as expected, because this fraction contained a complex mixture of compounds (Figure 7, middle). Even so, there is no observable difference in the E1/20/− potentials of these three fractions (−0.08, −0.08, and −0.07 V vs C600/− for A−C, respectively) compared with that of pristine PCBM (E1/2 = −0.09 V vs C600/−), within the experimental error of ±10 mV.

other decomposition products. Although these results are specific to PCBM, they suggest that high-temperature vapor deposition of similar organic fullerenes, and possibly organic fullerenes in general, may also lead to the deposition of unknown amounts of decomposed derivatives of the parent organic fullerene. In this vein, the results of OPV active-layer experiments that involved the vapor deposition of an organic fullerene, including OPV device figures-of-merit, should be reexamined. The electrochemical properties of the sublimed mixture of PCBM isomers and other decomposition products were of interest because thermal evaporation has been utilized for the fabrication of organic electronic devices such as OPVs as well as for measurements of fundamental physical properties such as solid-state EA.17,18 It is known that the presence of small amounts of impurities that possess different electrochemical properties has drastic and detrimental effects on mobility and recombination rates of free carriers in OPV materials. For example, the efficiency of a PCBM-based OPV device was greatly reduced when the active layer was doped with small amounts of PC84BM, whose E1/2 is anodically shifted 350 mV relative to that of PCBM (PC84BM is 350 mV easier to reduce than PCBM).47,48 On the other hand, why the mixture of isomers comprising indene-C60-bisadducts resulted in some of the best performing photovoltaic devices despite differences in the electronic properties of the acceptor molecules has yet to be explained.27,49 To study the electrochemical properties of the components present in the thermally evaporated films of PCBM, the sublimate was separated by HPLC into three fractions (Figure 7, middle): A (retention time of 5.5−6.0 min), B (retention time of 6.0−6.8 min), and C (the rest of the material eluting at 2.8−5.5 and 6.8−12.0 min). These fractions, along with PCBM and C60 for reference, were studied by cyclic voltammetry. The voltammograms are depicted in Figure 8. Fractions A and B were confirmed as PCBM and iso-PCBM, respectively, by 1H NMR spectroscopy. Fraction A (PCBM) had three sharp quasireversible reductions, while fraction B exhibited sharp quasi-

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SUMMARY AND CONCLUSIONS In this study, the behavior of PCBM during thermal annealing and thermal evaporation was studied, and decomposition products were characterized and quantified. It was found that PCBM decomposes after a brief (20 min) thermal treatment at 300 °C but also decomposes to the same degree after being heated for 1 h at 280 °C. Above 300 °C, decomposition is massive; a 20 min thermal treatment of PCBM at 340 °C results in nearly 80% sample degradation. The main product of thermal decomposition is a new and more thermally stable isomer of PCBM, characterized here for the first time, which contains a five-membered cycloadduct. PCBM can be thermally evaporated, but with significant decomposition, as the sublimate contains only ∼41% PCBM, in addition to the cyclo-pentyl isomer and other decomposition products. Even so, the [6,6]-addition motif is retained in nearly all of the decomposition products, and therefore, the electrochemical properties of the sublimation products remain unchanged compared to those of PCBM, measured here by cyclic voltammetry. Nevertheless, the figures of merit for molecular electronic active layers and devices involving fullerene derivatives are believed to depend critically on intermolecular interactions and the morphologies of, and interactions between, blended and composite phases therein, and there is no reason to believe that these interactions and domains will be similar for PCBM, iso-PCBM, and the other lower-m/z decomposition products. Therefore, the interpretations of the results of materials and/or devices fabricated by (i) vacuum sublimation of PCBM or (ii) heating PCBM at or above 280 °C should be carefully and critically re-examined.

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ASSOCIATED CONTENT

S Supporting Information *

Additional NMR, MS, HPLC, and absorption spectra, as well as photos of molten PCBM char. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-1012468) and the Colorado State University Research Foundation for generous support. G.R., B.W.L., and N.K. acknowledge funding by the Solar Photochemistry Program of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy through Grant DE-AC36-08GO28308 to the National Renew-

Figure 8. Cyclic voltammograms of C60, PCBM, and the sublimate fractions, recorded at a scan rate of 100 mV/s in a 0.1 M solution of tetrabutylammonium tetrafluoroborate in o-dichlorobenzene (ferrocene was used as an internal standard). 2366

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able Energy Laboratory. A.A.P. thanks Deutsche Forschungsgemeinschaft (project PO 1602/1-1) for support.

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