Photodegradation of Azobenzene-Based Self-assembled

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Photodegradation of Azobenzene-Based Self-assembled Monolayers Characterized by In-Plane Birefringence Guanjiu Fang,† Yue Shi,† Joseph E. Maclennan,† David M. Walba,‡ and Noel A. Clark*,† Departments of †Physics and ‡Chemistry and Biochemistry, and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: Azobenzene-based self-assembled monolayers (azo-SAMs) are photoactive and become orientationally ordered when illuminated with linearly polarized light (LPL), making them attractive as dynamic alignment layers in liquid crystal cells. Azo-SAMs, however, are chemically unstable when exposed to both air and light. We have characterized the photodegradation of a methyl redbased SAM by measuring with a high-sensitivity polarimeter the optical anisotropy induced by illumination with linearly polarized actinic light after the sample is irradiated with circularly polarized light (CPL) in air. The number of unbleached, photoactive molecules in the SAM decays exponentially with CPL exposure time, lowering the reorientation rate during photowriting with LPL. Azo-SAMs in an argon atmosphere, in contrast, are chemically stable and remain photoactive even after exposure to CPL.

I. INTRODUCTION In the presence of light, azobenzene-based molecules undergo reversible photoisomerization between their trans and cis forms.1,2 When orientationally ordered, such dye molecules show anisotropic optical properties because of their anisotropic molecular polarizability. Birefringence can be induced in an initially isotropic sample via photoisomerization, by illuminating with linearly polarized light (LPL), and erased again using circularly polarized light (CPL).1,2 Films incorporating azobenzene-based dyes have wide application in optical devices for information storage2,3 and waveguiding,4,5 and in liquid crystal (LC) alignment.6
11 Particularly exciting are the very high sensitivities for LC photoalignment achievable with azobenzene-based selfassembled monolayers (azo-SAMs).9
11 However, a concern in such SAM applications is the gradual but substantial loss of LC alignment photosensitivity when the azobenzene dyes are exposed to light in the presence of gaseous oxygen.8 Such photodegradation requiring both light and oxygen has been characterized previously in polymer films by measuring the decrease of the optical absorption of dispersed dyes with the film exposed to air.12
14 Here, we present the first detailed study of such photodegradation effects in SAMs and discuss likely candidates for the photodegradation process. Using dMR, the methyl red derivative shown in Figure 1, as the precursor, we have prepared covalently bonded selfassembled monolayers (SAMs) on glass microscope slides.10,11 The SAMs are first azimuthally randomized by exposure to circularly (or randomly) polarized green (514 nm) light in air and then irradiated with LPL. We probe the photoinduced molecular alignment by measuring the in-plane birefringence induced in the SAM.11,15 Since the dMR monolayers do not absorb at long visible wavelengths whether they are oxidized or not, 632.8 nm light can safely be used to probe in-plane birefringence as a measure of the number of active dye molecules. r 2011 American Chemical Society

Figure 1. Chemical structure of dMR, the azobenzene precursor used to prepare self-assembled monolayers. dMR is 2-[(1E)2-[4-(dimethylamino)phenyl]diazenyl]-N-[3-(triethoxysilyl)propyl]benzamide.

Because the in-plane retardance of even a fully active SAM is very small, we employ a high-contrast polarimeter15 to measure the in-plane birefringence.

II. EXPERIMENTAL SECTION A. Sample Preparation. dMR, the derivative of methyl red used to prepare the self-assembled monolayers,10,11 is shown in Figure 1. Monolayers were created by dipping glass microscope slides into a solution of dMR in toluene (272 mg of dMR in 70 mL of toluene, with 0.25 mL of n-butylamine as a catalyst). The triethoxysilyl group of the dMR is partially hydrolyzed by traces of water in the dipping solution, forming R
Si
OH units. These then react with Si
OH groups on the glass surface by dehydration, forming Si
O
Si linkages of the dMR molecules on the surface. The role of the n-butylamine is not completely Received: April 10, 2011 Revised: July 2, 2011 Published: August 03, 2011 10407

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Figure 2. Sealed azo-SAM cell. The SAMs on the outer glass surfaces have been removed, while the remaining photoactive surfaces (shown in red) are bathed in an inert argon atmosphere.

clear but it seems likely that the butylamine assists in catalyzing monolayer formation. The condensation reaction occurs if the silane is hydrolyzed or the glass surface has hydroxyl groups. A mechanism involving monolayer formation mediated by the formation and selective deposition of a monohydroxydiethoxysilane intermediate, followed by amine-catalyzed covalent attachment to the surface, has been proposed by Walba et al.16 The basic properties of such formed dMR-SAMs on glass have been characterized by Yi et al.10 dMR-SAMs were prepared for photodegradation study either exposed to air or, for comparison, sealed as the inner surfaces in a cell filled with argon gas (Figure 2). The periphery of such cells was sealed with epoxy, and all SAM material was removed from the outer glass surfaces using piranha solution. Planar-aligned LC cells, observed in a polarized light microscope, were also used to assess the azimuthal alignment of LCs on dMR-SAMs deliberately exposed to light before the cells were filled. These hybrid cells were made by sandwiching the nematic LC Merck E31 (nematic-to-isotropic transition temperature TNI = 61.5
C) between glass plates coated respectively with rubbed nylon and with the azo-SAM, both of which align LCs parallel to the substrate. The nylon surfaces were prepared by spin-coating a nylon solution (0.5 wt % Aldrich Nylon 6 [
NH(CH2)5CO
]n in methanol) for 30 s at a speed of 3000 rpm. The LC cells were filled through capillary action in the isotropic phase. B. Probing the Number of Photoactive Molecules. A high extinction polarimeter15 was used to measure the optical anisotropy induced in azo-SAMs by exposure to LPL as a function of cumulative prior exposure to intense CPL, irradiation that erases any in-plane optical anisotropy and partially oxidizes the azoSAM molecules. The exposure to LPL in each such experiment was small, less than 0.5% of the fluence of the CPL. The polarimeter, which has an extinction ratio of 2.4  10
10, uses a 0.7 mW probe beam from a helium
neon laser and includes two 514 nm pump beams from an argon ion laser, one of which is LPL and the other CPL. For an anisotropic organic monolayer between air and a substrate of index ng, illuminated with visible light, the retardance is very small and the transmission T is well approximated by15 ( " # )2 πΔnd 4nm 2 ð1Þ T = sin 2θ λ ðng þ 1Þ2 where a principal optic axis is along the mean in-plane molecular orientation of the monolayer OA, θ is the angle between OA and the polarization of the probe light, d is the film thickness, and nm = (n|| + n^)/2 is the mean refractive index of the sample in the film plane. For a monolayer with anisotropy typical of a liquid crystal (nm = 1.6, ng = 1.5, and Δn = 0.13) we have 4nm/(ng + 1)2 ∼ 1. With θ = 45
and assuming small Δnd, we can solve eq 1 for the retardance, obtaining Δnd ≈ λ(T)1/2/π, proportional to the number of active molecules.

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Figure 3. Alignment textures of nematic liquid crystal cells with the LC (Merck E31) between a rubbed nylon surface (rubbed along R) and a dMR-SAM surface, imaged by polarized transmitted light microscopy. (a) Twist domains separated by a reverse twist wall obtained by exposing a fully photoactive azo-SAM to linearly polarized actinic light (UV radiation centered at 365 nm) with P oriented parallel to the rubbing direction R of the nylon surface. The LC director field could be made uniform in this cell by rotating the actinic polarization direction by 90
. (b) LC alignment on a photo-oxidized azo-SAM. The white threads in the texture are not photoresponsive. Both cells are 5 μm thick and are observed between parallel polarizer and analyzer.

III. RESULTS AND DISCUSSION A. Liquid Crystal Photoaligment. Azo-SAMs are chemically stable and remain photoactive for a long time when stored in air in the dark, with azo-SAMs exposed to air in a dark box for several years showing the same dynamic response to linearly polarized light as fresh samples. Azo-SAMs are also stable when kept under argon even in the presence of light. However, they lose their ability to photoalign if exposed to both light and air concurrently for an extended period, implying that azo-SAMs degrade through photo-oxidation of the excited azobenzenes. Farrow17characterized the degradation by monitoring the ability of azo-SAMs to align nematic liquid crystals after being allowed to photooxidize, reporting that the photoactivity decreased significantly after 2 h exposure to ambient light and air and ceased altogether after 20 h. Our experiments confirm these observations and show that LCs align even if there is only partial ordering of the SAM surface. We prepared two hybrid LC cells comprising rubbed nylon and azo-SAM substrates. The azo-SAM surface of a fresh cell was exposed to 5 J/cm2 of LPL at 365 nm polarized parallel to the nylon rubbing direction R to obtain the twist alignment shown in Figure 3a. A second cell was filled after first exposing the azo-SAM to 40 J/cm2 of LPL polarized perpendicular to R, and then to 5 J/cm2 of LPL polarized parallel to R, yielding the inhomogeneous twist alignment texture shown in Figure 3b. Although this state is still fairly dark when observed between parallel polarizers, the white threads in the texture apparent in the photomicrograph are regions of the azo-SAM that are not photoactive, confirming that after extended exposure to light some dye molecules completely lose their ability to photoisomerize. Liquid crystal alignment is an indirect, macroscopic indication of the degree to which the azo-SAM molecules are damaged. There are damaged azo-SAMs molecules in both the white and gray areas of Figure 3b, but with a higher concentration of damaged molecules in the white areas. The angle ψ between the substrate normal and the optic axis (the long axis of azobenzene) varies depending on the local surface roughness and the surface density of the SAM molecules. This variation leads to spatial variations in the number of absorbed photons when the sample is exposed to CPL, giving inhomogeneous degradation. The white threadlike regions presumably correspond to molecules with bigger ψ, that is, the molecular axis tilted further from the glass normal. 10408

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increasingly longer as the oxidation exposure time increases. There is no such shift when the azo-SAM is sealed in argon (Figure 4b), although in this case too the saturated birefringence is somewhat reduced as the CPL irradiation time increases. To describe the photodegradation of the monolayers, we employ a first-order kinetic model5,13,14,18 for the active dMR, with areal density N1(tCPL), to an inactive state, of density N2(tCPL), where the overall density N0 = N1(tCPL) + N2(tCPL), and tCPL is the total exposure time of the CPL photodegradation beam. Once exposed to the much weaker LPL writing beam, starting at t = 0, the active molecules are assumed to become ordered with an in-plane orientational order parameter, S(t) = Æcos2 θ
sin2 θæ, where θ is the azimuthal angle, so that the induced in-plane SAM birefringence is given by ΔnðtÞ ¼ ΔRSðtÞN1 ðtCPL Þ

ð2Þ

where ΔR is a constant associated with the anisotropy of the molecular polarizability of the dye molecule. We assume that Ssat(tCPL), the saturation value of S(t) achieved at long times, will be independent of the number of the active molecules after exposure to CPL for time tCPL, making nsat(tCPL), the saturation value of n(t) at large t, also proportional to N1(tCPL) and thus a function of N1(tCPL): Δnsat(tCPL)  N1(tCPL). For first-order kinetics and CPL illumination, the rate at which dye molecules are bleached is proportional to the density of active molecules N1(tCPL) and the incident intensity ICPL (mW/cm2)

Figure 4. Development of optical retardance of azo-SAMs first randomized for different times (shown in the plot legends) using 514 nm CPL (a) of 2000 mW/cm2 incident on a SAM in air and (b) of 1000 mW/cm2 incident on a SAM in argon, with both samples then written with LPL. In all experiments, the 514 nm LPL writing beam, polarized at 45
from that of the probe beam (632.8 nm), was turned on at time t = 0 and was of intensity 50 mW/cm2. The retardance was calculated using eq 1 with transmission probed by the polarimeter with the monolayer placed between crossed polarizer and analyzer. The solid yellow lines in Figure 4a are fits to the first (0 s) and last (8000 s) plots in Figure 4a using eq 5. The steady reduction in saturated birefringence with increasing exposure to air and CPL illumination is evident.

B. Photoinduced SAM Birefringence. We performed photodegradation experiments on azo-SAMs in air by exposing them to CPL (λ = 514 nm) at an intensity ICPL = 2000 mW/cm2, and on an azo-SAM cell filled with argon exposed to CPL at ICPL = 1000 mW/cm2. Both bleached molecules and randomly oriented active molecules show zero transmission under crossed polarizers, but the unbleached randomly oriented molecules can be aligned along a preferred azimuth by exposure to LPL, giving an optical transmission related to how many molecules are still active. After the azo-SAM was exposed to the CPL beam for a time tCPL, this beam was blocked for 5 min to allow any cis dye molecules to relax back to trans, after which the sample was exposed to the LPL beam polarized at 45
to the crossed polarizer/analyzer of the polarimeter to induce orientational anisotropy while monitoring the writing dynamics. The results are plotted in Figure 4. In the sample exposed to air, the azimuthal anisotropy induced by writing with LPL decreases significantly as tCPL increases, showing that the number of unbleached dye molecules in the SAM is being reduced (Figure 4a). The delay time to the appearance of induced birefringence becomes

∂N1 ðtCPL Þ ¼
ϕσICPL N1 ðtCPL Þ=hν ∂tCPL

ð3Þ

where ϕ is the quantum efficiency for photobleaching, σ is the absorption cross section (cm2), and hν is the photon energy. The number of unbleached molecules and thus the saturation birefringence decreases exponentially with CPL exposure time as Δnsat ðtCPL Þ  N1 ðtCPL Þ ¼ N0 expð
tCPL =τCPL Þ

ð4Þ

where τCPL = (hv)/(ϕσICPL). As discussed above, the time for birefringence to appear becomes longer as the CPL exposure time tCPL increases as shown in Figure 4a, leading to the saturation of the birefringence at different long times. To extract the saturated birefringence for different tCPL, the following function is selected to fit the writing dynamics ( ) 1 ð5Þ ΔnðtÞ ¼ Δnsat ðtCPL Þ 1
½1 þ t=τwr ðtCPL ÞR where τwr(tCPL) is the characteristic response time to LPL after exposure to CPL for time tCPL. The exponential function Δn(t)  1
exp(
t/τ), with a single characteristic time τ, does not fit the experimental data. Equation 5, however, fits very well as shown in Figure 4a, indicating that the azo-SAMs have a distribution of characteristic times H(τ) corresponding to different molecular environments. TheR term in parentheses in eq 5 is given by 1
1/(1 + t/τwr)R = H(τ)[1
exp(
t/τ)] dτ, where H(τ) = (1)/(twr)(exp(
twr/τ))/(Γ(R)(τ/twr)R+1), Γ is the Gamma function, and the integration is over all of the environments. With the fitting parameters nsat(tCPL) and τwr(tCPL), we obtained the active molecule ratio fpa = nsat(tCPL)/ nsat(0), and the characteristic time ratio fτ = τwr(tCPL)/τwr(0) for oxidation time t = tCPL as shown in Figure 5a. The number of 10409

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The relative delay for birefringence to appear is characterized by the characteristic time ratio fτ shown in Figure 5a. The characteristic time τwr increases superlinearly with photobleached molecule fraction fpb = 1
fpa as shown in Figure 5b, indicating that the viscosity η(tCPL) is significantly changed as fpb increases. This is in contrast to azo-SAMs in which the characteristic time becomes smaller as the concentration of photoactive molecules decreases with air surrounding them.19 We have fit the characteristic time ratio dependence using a function20 fτ = exp [2.5  fpb/(1
β  fpb)] with the self-crowding factor β = 0.8 (Figure 5b), giving the viscosity ratio before and after exposure to CPL as fη = η(tCPL)/η(0) = fτ, which is ! 2:5fpb fη ¼ exp ð7Þ 1
0:8fpb

Figure 5. (a) Photodegradation of dMR-SAM as a function of the duration of exposure to CPL at 514 nm. The active molecule ratio (fpa) is given by the ratio of the saturated birefringence induced by writing with LPL after and before exposure to CPL illumination, measured after the same time interval using a polarimeter with a 632.8 nm probe beam. fτ is the ratio of the characteristic times for writing with LPL measured after and before exposure to CPL. The saturated birefringence/characteristic time was obtained by fitting the writing dynamics shown in Figure 4a with eq 5. The solid line in Figure 5a is an exponential fit. (b) The characteristic time ratio depends on the proportion of photobleached molecules. The azo-SAM molecules reorient more slowly when exposed to LPL as the number of photobleached molecules increases, with the effective viscosity of the active molecules increasing superlinearly. The solid line in Figure 5b is a fit to the data using eq 7.

active SAM molecules decays exponentially, with τCPL = 1584 s for the SAMs in air exposed to 2000 mW/cm2 CPL radiation. By knowing the SAM absorbance A and the surface coverage C (the areal density), the absorbed photon number for a dMR-SAM molecule to be completely photobleached is Nphoton ¼

ICPL τCPL ð1
10
A Þ Chν

ð6Þ

where hν is the photon energy of 514 nm light. With A = 0.0015 and C
1 = 0.55 nm2 per molecule,10 the photon number for one molecule becoming nonactive is about 154,536, that is, ϕ
1 = 154,536. Galvan-Gonzalez et al.12 studied the photodegradation of disperse red one and some of its derivatives in polymer systems, finding a quantum efficiency of the photodegradation in the range 10
7 to 10
9, possibly depending on the operating wavelength, local atmosphere, and chromophore structure. It seems that the monolayers are more susceptible to photodegradation than the relatively protected dyes in the polymer system since the SAMs are completely exposed to air.

The viscosity change caused by photoisomerization observed in a solution of polyamides composed of azobenzene and phenylenediamide residues has been called photoviscosity.21 The viscosity here is a measure of the resistance to molecular reorientation when the azo-SAM is exposed to LPL. Since the dMR molecules are chemically bonded to the glass slide, the conformational changes between trans and cis driven by photoisomerization occur within a limited area around each molecule so this viscosity ratio fη mainly reflects the rotational viscosity, that is, the resistance to active molecules reorienting in 2D. The viscosity dependence on photobleached molecule fraction shows that the inactive SAM molecules tend to slow down the reorientation of the active molecules. Similar behavior has been observed in a monodisperse system of spherical particles in liquid, with the spherical particles increasing the system viscosity as predicted by eq 7.20 In mixtures of glycerol and water, components which differ significantly in viscosity, an empirical function fη(x) = 1 + ax exp(bxc) is widely used to fit the viscosity as a function of the glycerol fraction x,22,23 where a, b, and c are fitting parameters. The viscosity dependence in our system can also be fit very well using this function. The interaction between the photobleached and active SAM molecules thus appears to be analogous to those in suspensions of particles or in glycerol/ water mixtures. The azo-SAMs inside a sealed argon cell were also exposed to 514 nm CPL illumination at only 1000 mW/cm2, with birefringence then induced by LPL as shown in Figure 4b. The saturated birefringence shows some initial reduction with accumulated CPL exposure that might be caused by trace amounts of oxygen in the cell but then remains unchanged after a certain exposure. In total, we have performed more than 400 h of experiments using this sealed cell over the past 7 years and the results of optical writing experiments are still consistent and reproducible. This experiment confirms that photo-oxidation is responsible for the significant degradation of methyl red-based SAMs in air.

IV. CONCLUSIONS In summary, when self-assembled monolayers containing azobenzenes are simultaneously exposed to light and air, some of the molecules photooxidize and lose their ability to respond to subsequent doses of polarized actinic light. Nematic liquid crystals show only partial alignment on such damaged substrates, with regions of the monolayer that are not photoactive appearing as bright threads in hybrid twist cells. The photodegradation process has been precisely characterized by measuring the optically 10410

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Langmuir induced monolayer birefringence using a high-contrast polarimeter. The relative number of photoactive molecules remaining after photodegradation decays exponentially with cumulative exposure time, with a photobleaching quantum efficiency of ϕ
1 = 154,536. The reorientation rate becomes slower as the photobleached molecule fraction increases, caused by the increased viscosity of the active molecules. Monolayers maintained in an inert argon atmosphere and exposed to similar doses of incident circularly or randomly polarized light show only a minimal reduction in subsequent photoactivity, confirming that the azoSAMs are damaged only when irradiated in the presence of oxygen.

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(19) Fang, G. J.; Koral, N.; Zhu, C.; Yi, Y.; Glaser, M. A.; Maclennan, J. E.; Clark, N. A.; Korblova, E. D.; Walba, D. M. Langmuir 2011, 27, 3336. (20) Mooney, M. J. Colloid Sci. 1951, 6, 162. (21) Irie, M.; Hirano, Y.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (22) Chenlo, F.; Moreira, R.; Pereira, G.; Bello, B. Eur. Food Res. Technol. 2004, 219, 403. (23) Cheng, N.-S. Ind. Eng. Chem. Res. 2008, 47, 3285.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors thank Youngwoo Yi and Tom Furtak of the Colorado School of Mines for providing some of the SAMs used in this study. This work was supported by NSF MRSEC Grant No. DMR-0820579 and NSF Grant No. CHE-0079122. ’ REFERENCES (1) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403. (2) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309. (3) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268. (4) Wang, Y.; Klittnick, A.; Clark, N. A.; Keller, P. Appl. Phys. Lett. 2008, 93, 143506. (5) Ma, J.; Lin, S.; Feng, W.; Feuerstein, R. J.; Hooker, B.; Mickelson, A. R. Appl. Opt. 1995, 34, 5352. (6) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991, 351, 49. (7) Palffy-Muhoray, P.; Kosa, T.; Weinan, E. Mol. Cryst. Liq. Cryst. 2002, 375, 577. (8) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601. (9) Yi, Y. W.; Furtak, T. E. Appl. Phys. Lett. 2004, 85, 4287. (10) Yi, Y. W.; Farrow, M. J.; Korblova, E.; Walba, D. M.; Furtak, T. E. Langmuir 2009, 25, 997. (11) Fang, G. J.; Shi, Y.; Maclennan, J. E.; Clark, N. A.; Farrow, M. J.; Walba, D. M. Langmuir 2010, 26, 17482. (12) (a) Galvan-Gonzalez, A.; Belfield, K. D.; Stegeman, G. I.; Canva, M.; Chan, K.-P.; Park, K.; Sukhomlinova, L.; Twieg, R. J. Appl. Phys. Lett. 2000, 77, 2083. (b) Galvan-Gonzalez, A.; Canva, M.; Stegeman, G. I.; Sukhomlinova, L.; Twieg, R. J.; Chan, K. P.; Kowalczyk, T. C.; Lackritz, H. S. J. Opt. Soc. Am. B 2000, 17, 1992. (13) Moshrefzadeh, R. S.; Misemer, D. K.; Radcliffe, M. D.; Francis, C. V.; Mohapatra, S. K. Appl. Phys. Lett. 1993, 62, 16. (14) Zhang, Q.; Canva, M.; Stegeman, G. Appl. Phys. Lett. 1998, 73, 912. (15) Fang, G. J.; Maclennan, J. E.; Clark, N. A. Langmuir 2010, 26, 11686. (16) Walba, D. M.; Liberko, C. A.; Korblova, E.; Farrow, M.; Furtak, T. E.; Chow, B. C.; Schwartz, D. K.; Freeman, A. S.; Douglas, K.; Williams, S. D.; Klittnick, A. F.; Clark, N. A. Liq. Cryst. 2004, 31, 481. (17) Farrow, M. J. Design and Synthesis of High Susceptibility Photoactive Self-Assembled Monolayers for the Photoalignment of Liquid Crystals. Ph.D. Thesis, University of Colorado at Boulder, 2004. (18) Simmons, E. L. J. Phys. Chem. 1971, 75, 588. 10411

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