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Photochemical Stability of Pentacene and a Substituted Pentacene in Solution and in Thin Films Ashok Maliakal,*,† Krishnan Raghavachari,‡ Howard Katz,† Edwin Chandross,† and Theo Siegrist† Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, New Jersey 07974 and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Received June 11, 2004. Revised Manuscript Received August 19, 2004
The organic semiconductor pentacene (1) has shown the highest field effect mobilities in thin films of any organic semiconductor, yet suffers from instability toward oxidation. 6,13-Bis(triisopropylsilylethynyl)pentacene (2) has been reported as an interesting functionalized pentacene which is soluble in common organic solvents and exhibits high carrier mobility (>0.1 cm2/Vs) in thin film transistor devices. In our investigations of 2, we were surprised by its remarkable stability in solution. Using UV-vis spectroscopy we observe that under ambient light conditions, 2 is approximately 50× more stable toward degradation in air-saturated tetrahydrofuran solution as compared to unsubstituted pentacene. Previous investigators have implicated oxygen in the mechanism of photodegradation of pentacene. In this study, quantum chemical calculations have been performed which demonstrate that alkynyl functionalization at the 6 and 13 positions reduces the rate of photooxidation in two ways. First, alkynyl substitution reduces the triplet energy of 2 considerably, thereby preventing singlet oxygen sensitization. Second, alkynyl substitution lowers the LUMO energy for 2 as compared to that of pentacene. We propose that the lower LUMO energy hinders photooxidation by reducing the rate of electron transfer from photoexcited 2 to oxygen. In thin films, pentacene is more stable to photooxidation than 2 when exposed to UV irradiation. The stabilization of pentacene in the solid state is discussed in the context of solid-state interactions.
Introduction Organic transistors offer a potential entry route into new, inexpensive organic electronics for a variety of applications including large-scale flexible displays, active packaging, and large-area sensor arrays.1,2 Much attention has been devoted to pentacene, as it is the best organic semiconductor, showing the highest mobilities and on/off ratios in thin film transistors for p-type semiconductors.3 Despite its promise, pentacene does present limitations in terms of processability owing to its extremely poor solubility, and its instability in solution.4,5 Furthermore very few examples of substituted pentacenes exist in the literature owing to the lack of air stability of these compounds in solution.6,7 One notable example of a substituted pentacene which is stable in air-saturated solutions is 6,13-bis(triisopro* To whom correspondence should be addressed. E-mail: maliakal@ lucent.com. † Bell Laboratories. ‡ Indiana University. (1) Gamota, D., Brazis, P., Kalyanasundaram, K., Zhang, J., Eds. Printed Organic and Molecular Electronics; Kluwer Academic Publishers: Boston, MA, 2004. (2) Dimitrakopoulos, C.; Malenfant, P. Adv. Mater. 2002, 14, 99. (3) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron Device Lett. 1997, 18, 87. (4) Laquindanum, J.; Katz, H.; Lovinger, A. J. Am. Chem. Soc. 1998, 120, 664. (5) Herwig, P. T.; Mullen, K. Adv. Mater. 1999, 11, 480. (6) Takahashi, T.; Kitamura, M.; Shen, B. J.; Nakajima, K. J. Am. Chem. Soc. 2000, 122, 12876. (7) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao, Z.; Siegrist, T.; Kloc, C.; Chen, C. H. Adv. Mater. 2003, 15, 1090.
pylsilylethynyl)pentacene (2), which can be chromatographed and stored in solution for relatively long periods of time.8,9 Reported mobilities for 2 are high, with the best value reported to be 0.4 cm2/Vs.10 In our investigations of 2, we were surprised by its anomalous photostability. This report seeks to identify the mechanism of photostabilization. Early investigators have noted that pentacene degrades in the presence of light and air.11-14 The byproducts of photooxidation of pentacene were identified to be the transannular endoperoxide and also a dimeric peroxide.14 Early investigators into the mechanism of photooxidation proposed a singlet oxygen sensitization mechanism, which has been observed for other polycyclic aromatic hydrocarbons (PAHs).15 Typically, singlet oxygen sensitization requires a sensitizer with a triplet energy that is larger than the singlet-triplet energy gap in singlet oxygen (i.e., greater than 22.5 kcals/mol).16,17 Pentacene has a triplet energy of 17.9 kcals/mol which (8) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15. (9) Anthony, J.; Brooks, J.; Eaton, D.; Parkin, S. J. Am. Chem. Soc. 2001, 123, 9482. (10) Sheraw, C.; Jackson, T.; Eaton, D.; Anthony, J. Adv. Mater. 2003, 15, 2009. (11) Lewis, I.; Singer, L. J. Phys. Chem. 1981, 85, 354. (12) Birks, J.; Appleyard, J.; Pope, R. Photochem. Photobiol. 1963, 2, 493. (13) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Chem. Soc. Jpn. 1988, 61, 1057. (14) Clar, E. Polycyclic Aromatic Hydrocarbons; Academic Press: London, 1964; Vol. 1. (15) Stevens, B.; Perez, S.; Ors, J. J. Am. Chem. Soc. 1974, 96, 6846.
10.1021/cm049060k CCC: $27.50 © 2004 American Chemical Society Published on Web 11/06/2004
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Scheme 1. Reactions of PAHs with O2
is not large enough to sensitize singlet oxygen.18 It is plausible that pentacene triplets can be thermally activated to sensitize singlet oxygen during their excitedstate lifetime (110 µs).18 However, in the case of 2, whose triplet energy is calculated to be 5.5 kcals/mol (vide infra), singlet oxygen sensitization is not expected to be thermodynamically feasible. An alternative mechanism for photooxidation of pentacene and 2 is electron transfer and superoxide formation (see Scheme 1).16,17 This pathway, which has been observed in related PAHs,19 involves the transfer of an electron from the first excited singlet state of pentacene to ground-state oxygen resulting in the pentacene radical cation and the superoxide anion. Subsequent reactions of the pentacene radical cation with superoxide would explain the formation of the endoperoxide and dimeric peroxide photoproducts which have been observed previously. In the current study, photodegradation of pentacene and 2 were investigated in tetrahydrofuran solutions and in films. Quantum chemical calculations were performed to determine HOMO-LUMO and excitedstate energies and orbital profiles. Our results demonstrate that in tetrahydrofuran solution, 2 is roughly 50× more stable than pentacene. Quantum chemical calculations on 1, 2, and 6,13-bis(trimethylsilylethynyl)pentacene (3) (a computationally simpler analogue of 2) demonstrate that alkynyl substitution lowers both the triplet energy and the LUMO orbital energy for 2 as compared with 1. The low triplet energy of 2 prevents photodegradation through singlet oxygen sensitization. The lowering of the LUMO in 2 is proposed as the source of photostabilization by reducing the rate of electron transfer to oxygen. In the case of thin films, pentacene shows greater photostability than 2. This result suggests that solid state interactions also have an important effect on photostability, possibly by modifying excited-state lifetimes and/or oxygen permeability. These results suggest that in designing new p-type organic semiconductors, care must be placed not only on HOMO orbital energies to ensure activity,4 but also on triplet and LUMO energies as well as excited-state photophysics to ensure photostability in both thin film form and during solution processing. Experimental Section Materials. Pentacene was purchased from Aldrich and used after it was washed with dichloromethane. Tetrahydrofuran was HPLC grade. Compound 2 was synthesized as described (16) Turro, N. Modern Molecular Photochemistry; University Science Books: Sausalito, CA, 1991. (17) Kavarnos, G. Fundamentals of Photoinduced Electron Transfer; VCH: New York, 1993. (18) Murov, S.; Carmichael, I.; Hug, G. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993. (19) Mazur, M.; Blanchard, G. J. Phys. Chem. B. 2004, 108, 1038.
Figure 1. UV-Vis spectra of 1 in THF (dashed line), and in thin film (solid line). previously.9 Quartz slides were purchased from SPI supplies (West Chester, PA). Films of Pentacene and 2 were evaporated onto quartz slides using an Edwards 306 auto evaporator. Photodegradation studies were carried out in quartz cuvettes in a Rayonet photoreactor with Rayonet RPR-1849Å/2537Å UV lamps (South New England Ultraviolet Co., Middeltown, CT). UV-Vis Spectroscopy. UV-Vis spectra were obtained using a HP 8453 spectrophotometer. Air-free samples were prepared in a glovebox and sealed with rubber septa prior to measurement. Calculations. We have performed quantum chemical calculations using the Gaussian 03 program20 on the ground state (S0) as well as the lowest excited singlet and triplet states (S1 and T1) of 1 and 2. Initially, the ground-state structures of 1 and 2 were determined by complete geometry optimization using the B3-LYP hydrid density functional theory (Becke’s three-parameter exchange functional21 and Lee, Yang, and Parr’s correlation functional22) and the polarized 6-31G* basis set. In addition, a slightly simplified version of 2 where the isopropyl groups were replaced by methyl groups [6,13-bis(trimethylsilylethynyl)pentacene, 3] was also optimized.
Results UV-Vis Absorption Spectra. Figure 1 shows the UV-vis absorption spectra for pentacene in both THF solution and in a 500 Å film on quartz. In the case of pentacene, there is a large red-shifted absorption band with a peak at roughly 670 nm for the thin film sample which is not present in the solution sample.23 In Figure 2 , the UV-vis absorption spectra for 2 are presented for THF solution and for a 500 Å film on quartz. In comparing these spectra with those of pentacene, although a slight red shift of the long wave(20) Gaussian 03, Revision B.5. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian, Inc.: Pittsburgh PA, 2003. (21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (22) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (23) Lim, S.; Bjorklund, T.; Spano, F.; Bardeen, C. Phys. Rev. Lett. 2004, 92, 107402.
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Figure 2. UV-Vis spectra of 2 in THF (dashed line), and in thin film (solid line).
Figure 4. Stability of oxygen-free (1500
oxygen-free, dark 250+/-90 >1500
saturated and ambient light (fluorescent light) conditions. The half-life of 1 increases by over an order of magnitude when the air-saturated solutions are stored in the dark between measurements. In the case when oxygen is excluded (140 11+/-1
>140 64+/-13
a function of UV irradiation time in a Rayonet photoreactor (illumination λmax ) 254 nm). Data are presented in Figure 6 for samples in air and for samples monitored under airfree conditions. In this case pentacene in the thin film is shown to be surprisingly stable to UV-irradiation even in air despite its instability in solution.13 Minimal degradation is observed over the 140-min time span of irradiation. Contrary to the behavior of pentacene, 2 is sensitive to UV-irradiation as a thin film, with degradation observed in both air and air-free cases. The half-lives for these samples are presented in Table 2. Calculations. The excitation energies of 1-3 were determined by two different procedures. In the simplest method, the B3-LYP/6-31+G* level of theory was used to get the HOMO and LUMO levels (b2g and b3u, respectively, for pentacene in D2h symmetry) to derive the associated band gap. The calculated band gaps for 1 and 3 are 2.21 and 1.90 eV, showing a notable difference. For comparison, the gap calculated for 2 (1.89 eV) is virtually identical to that for 3. Thus, the difference in the electronic structures between 1 and 2 is clearly due to conjugation with the silylethynyl group and can be described accurately by using 3 instead of 2. A careful analysis reveals that the difference in the gaps between 1 and 3 (or 2) is entirely due to the lowering of the LUMO level in the latter, with the HOMO levels staying virtually identical. The HOMO and LUMO orbital plots for both 1 and 3 are shown in Figure 7 and illustrate the above points. The calculated gaps are in excellent agreement with the corresponding experimental values. The electronic excited states of 1 and 3 were also derived by time-dependent density functional theory (TDDFT)25 at the B3-LYP/6-31+G* level. The calculated singlet excitation energies are 1.92 and 1.67 eV, respectively. These are slightly lower than the corresponding experimental values by 0.2-0.3 eV. The difference in (25) Stratmann, R. E.; G. E. S.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218.
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excitation energies (0.25 eV) is very similar to that seen in the HOMO-LUMO gap (0.31 eV), displaying the same trend. The lowering of the excitation energy from 1 to 2 seen experimentally, 0.21 eV, is in good agreement. To get relaxed excitation energies, the geometries of the excited singlet states were optimized using the configuration interaction method with single excitations at the CIS/6-31+G* level of theory.26 Single point calculations with TDDFT (B3-LYP/6-31+G*) at these geometries yield lower excitation energies for 1 and 3 as expected (1.71 and 1.53 eV, respectively). Thus, our best estimate for the difference in the relaxed singlet excitation energies is 0.18 eV. For comparison, we also carried out calculations on the corresponding triplet excited states. The triplet excitation energies computed by TDDFT at the ground state geometries are 0.60 and 0.37 eV. The geometries were then completely optimized at the B3-LYP/6-31G* level. Using these relaxed geometries, we obtained the final estimates for relaxed triplet excitation energies of 0.77 and 0.24 eV for 1 and 3, respectively (B3-LYP/ 6-31+G*). The computed value for pentacene is in excellent agreement with the corresponding experimental value (0.78 eV). As noted earlier, the triplet excitation energy for 1 is lower than the singlet-triplet energy difference for O2 (0.98 eV) while that of 3 is substantially lower. Finally, we have performed B3-LYP/6-31G* optimizations on pentacene positive ion (2B2g). Since solvation effects are important for such ions, we have also carried out B3-LYP/6-31+G* calculations including the solvent reaction field with the SCI-PCM model (self-consistent isodensity method using the polarizable continuum model) using water as the solvent.27 Similar calculations have also been carried out for O2-. Solvation favors electron transfer since the individual ions are stabilized substantially relative to the uncharged reactants. Starting from the S1 excited state of pentacene, the electrontransfer reaction from 1 to O2 is exothermic by ∼0.8 eV after including such solvent stabilization. Discussion Photooxidation of polycyclic aromatic hydrocarbons (PAHs) by oxygen is known to occur via two major processes (Scheme 1). In the first, S1 of the PAH transfers an electron to oxygen, generating the PAH radical cation and superoxide radical anion. The radical cation can react further with the superoxide or other nucleophiles.16,19 Alternatively, triplet states of PAHs can sensitize the formation of singlet oxygen. Singlet oxygen can then react with ground-state PAH to generate endo-peroxides and other oxidation products.15 In pentacene, the triplet energy (17.9 kcals/mol)18 is less than the 22.5 kcals/mol triplet singlet energy gap for oxygen. Compound 3 is calculated to have a significantly lower triplet energy (0.24 eV, 5.5 kcals/mol). Compound 2 is expected to have a triplet energy similar to that of the computational model 3. In the case of pentacene, although singlet oxygen sensitization would (26) Foresman, J. B.; M. H.-G.; Pople, J. A.; Frisch, M. J. J. Phys. Chem. 1992, 96, 135. (27) Foresman, J. B.; T. A. K.; Wiberg, K. B.; Snoonian, J.; Frisch, M. J. J. Phys. Chem. 1996, 100, 16098.
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Figure 7. Quantum chemical calculations for HOMO and LUMO orbitals and energies for 1 and 3 (a computationally simpler model for 2).
In the case where kO2 is small, then
Scheme 2. Photophysics for Pentacene or 2
ΦO2 ≈ kO2τs
be endothermic by 4.6 kcals/mol, the long triplet lifetime of pentacene (110 µs)18 makes thermally activated singlet oxygen sensitization plausible. In the case of 2, the extremely low triplet energy suggests that the singlet oxygen sensitization mechanism for photodegradation can be excluded, endowing 2 with enhanced photostability. A second source of photostabilization is through the lower LUMO energy of 2 as compared to 1. We will subsequently analyze the relative rates of photooxidation for 1 and 2 via electron-transfer based on the energies of these systems. The excited singlet state of pentacene or 2 is subject to a set of processes illustrated in Scheme 1. Pentacene can undergo fluorescence, internal conversion, or intersystem crossing at rates kF, kIC, kISC, respectively (see Scheme 2). In addition to these processes, pentacene can react with oxygen at a rate kO2. The quantum yield of reaction with oxygen can be written as
ΦO2 ) kO2/(kO2 + kF + kIC + kISC)
(1)
(2)
where τs is the singlet excited-state lifetime for pentacene. The lifetime τS can be estimated using values for the fluorescence and intersystem crossing quantum yields for pentacene as well as the internal conversion rate kIC.16,28 These values allow an estimate of ∼3 ns for τs. If we assume that internal conversion is the dominant photophysical process for 2, we can estimate the τs for 2 to be ∼1 ns. The value for kO2 can be expressed by the Arrhenius equation as
kO2 ) A exp(-Ea/RT)
(3)
where Ea is the activation energy for the process. To calculate the relative yields of photoreaction we can set up the ratio of yields as follows:
ΦO2(1)/ΦO2(2) ) kO2(1)τs(1)/kO2(2)τs(2)
(4)
∼exp((Ea(2) - Ea (1))/RT)*τs(1)/τs(2)
(5)
∼exp(∆Ea/RT)*τs(1)/τs(2)
(6)
In a first-order approximation, the excited singlet state of pentacene is formed by promotion of an electron from the HOMO orbital to the LUMO orbital. This highenergy electron is transferred to oxygen in the photooxidation reaction. Ιf the difference in activation energies (∆Ea) is considered to be due to the energy of (28) Birks, J. Photophysics of Aromatic Molecules; Spottiswoode, Ballantyne & Co. Ltd.: London, 1970.
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Scheme 3. Electron Transfer Photooxidation of 1 and 2*
*Circles represent electrons. Absorption of light promotes electron from LUMO to HOMO. The energy of the LUMO orbital determines the activation energy for electron transfer between 1 or 2 and oxygen.
ionization of 1 or 2, then this amount would correspond to the difference in LUMO energies which is calculated to be 0.31 eV (∼7 kcals/mol) (see Figure 7 and Scheme 3). An improved calculation is arrived at using the relaxed singlet energies for 1 and 2. In this case ∆Ea is calculated to be 0.18 eV (∼4 kcals/mol). Using the value 0.18 eV, the estimated lifetimes for 1 and 2, and eqs 4-6 we calculate the stability enhancement ΦO2(1)/ΦO2(2) to be ∼3 × 103. This estimated value is based on equivalent rates of photon absorption. The value of ∼50 measured in this study is based on integrated photon absorption under ambient light conditions. More importantly, the difference in singlet energies used to determine ∆Ea was calculated in the gas phase which may overestimate this activation energy difference. What this calculation demonstrates is that in 2, the LUMO orbital drops compared to that of 1, which imparts significant photostabilization to 2 in solution. Furthermore very small differences in orbital energies translate into large changes in solution photostability. In the case of 1 and 2, this difference corresponds to the difference between a compound which can be handled and processed in solution under atmospheric conditions and a compound which must be handled under oxygen-free conditions in solution. In the case of films, we might expect a parallel result of photostabilization of 2 with respect to 1. This was not observed. Instead (see Figure 6), pentacene proved to be more stable toward photooxidation than 2 as a polycrystalline thin film. One possible explanation of this photostability could be that the crystal structure of pentacene does not allow for oxygen diffusion,13 whereas the crystal structure of 2 does allow for oxygen permeability. Data on the crystal structures for 1 and 2 do not support this hypothesis. In the case of 1, which packs in a herringbone arrangement, the spacing between aromatic planes is found to be ∼4 Å29 which is larger than the spacing for 2 (∼3.5 Å).9 However, the possibility of oxygen diffusion through the side chains in 2 cannot be ruled out. An alternative hypothesis is that there is a difference in the photophysics of pentacene in the solid state as compared to that in solution which shortens its excited(29) Haddon, R. C.; Chi, X.; Itkis, M. E.; Anthony, J. E.; Eaton, D. L.; Siegrist, T.; Mattheus, C. C.; Palstra, T. T. M. J. Phys. Chem. B 2002, 106, 8288.
state lifetime, and hence reduces its photooxidation yield. The UV-Vis spectrum of pentacene thin films does exhibit a long wavelength band which is not present in films of 2 (see Figure 1). The long wavelength band for pentacene films has been observed previously.30-33A similar long wavelength band is observed in the UV-Vis absorption spectra for tetracene films (which have a crystal structure similar to that of pentacene), and this band is accompanied by a dramatic reduction in the excited-state lifetime.23 The dramatic change in the absorption spectrum of thin film pentacene indicates similar strong interactions between pentacene molecules in the ground state. It is interesting to note that in single crystals of 2, a long wavelength (∼800 nm) absorption edge has been reported.24 In polycrystalline thin films of compound 2, only a slight red shift is observed in the absorption spectra (see Figure 2). However, the long wavelength absorption is observed in a 0.5-mm-thick crystal, which is 10,000 times thicker than the films used in this study. This result suggests that the interactions responsible for this long wavelength absorption are much weaker for 2 than for 1. Films of 2 may have longer excited-state lifetimes than 1, resulting in greater photooxidation yields. Although this hypothesis is consistent with our data, further experimental work is required to exclude other possibilities. Conclusion Pentacene has so far shown the best thin film mobility of any organic p-type semiconductor. Yet improvements through synthetic modification are hindered by the intrinsic instability of the core toward photooxidation during solution processing. Solutions of 2 shows exceptional photostability in air under ambient light conditions, and the source of this photostabiliziation has been demonstrated to be the result of (1) a low energy triplet in 2 which prevents singlet oxygen sensitization, and (2) stabilization of the LUMO orbital as a result of alkynyl substitution. This hypothesis is consistent with (30) Schlosser, D.; Philpott, M. Chem. Phys. 1980, 49, 181. (31) Hesse, R.; Hofberger, W.; Bassler, H. Chem. Phys. 1980, 49, 201. (32) Peter, G.; Bassler, H. Chem. Phys 1980, 49, 9. (33) Lee, J.; Kim, S.; Kim, K.; Kim, J.; Im, S. Appl. Phys. Lett. 2004, 84, 1701.
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an electron transfer/superoxide mechanism of photooxidation for 2. Quantum chemical calculations of excited-state energies may serve to predict other substituted pentacene structures that would be expected to also exhibit enhanced photostabilization. Efforts are underway to explore the generality of these photostabilization mechanisms in other substituted pentacenes. Acknowledgment. We thank Andrew Lovinger and Brian Northrup for experimental assistance, and Nicho-
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las Turro for helpful discussions. We are grateful to the reviewers for their insightful suggestions. This work is supported in part by a NIST ATP grant (Cooperative Agreement 70NANB2H3032). Supporting Information Available: X-ray diffraction data for 2 as a thin film on quartz. This material is available free of charge via the Internet at http://pubs.acs.org. CM049060K
