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COMMENTS Comment on “Compression Induced Chirality in Dense Molecular Films at the Air-Water Interface Probed by Second Harmonic Generation” Yan-yan Xu,† Feng Wei,‡ and Hong-fei Wang* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, the Chinese Academy of Sciences, Beijing, China 100190 ReceiVed: NoVember 30, 2008; ReVised Manuscript ReceiVed: January 13, 2009 Recently, Benichou and co-workers1 reported the measurement of the chirality of the Langmuir monolayer formed with the achiral stilbazolium dye molecule 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) probed with the in situ surface second harmonic generation linear dichroism (SHGLD) technique. The main conclusions in this work are the following: (1) the chirality is compression induced and exists in the dense DiA Langmuir monolayer, and (2) there is significant magnetic dipole contribution to both the achiral and chiral second harmonic susceptibility terms in the dense DiA Langmuir monolayer. The formation mechanism and the origin of the chirality in the Langmuir monolayer are important issues because of the general interest in the monolayer chirality in the Langmuir monolayer and Langmuir-Blodgett films, and also because of the theoretical importance of the problems related to the spontaneous chiral symmetry breaking and film formation. In our studies with the Langmuir monolayer of several achiral molecules, we found that the chirality in the monolayer was not likely compression induced, and we did not find evidence for significant magnetic dipole contributions. We also found that the chirality in the Langmuir monolayers was inhomogeneous and changed its chiral signs under compression. These results are to be reported elsewhere.2 Therefore, based on our studies on similar issues and our experiences in the same field, we would like to raise the following concerns on the Benichou paper regarding (1) potential flaws in the phase diagram and SHG data of the DiA monolayer, (2) the validity of the claims for the compression-induced chirality, and (3) the validity of the claim for the significant magnetic dipole contributions. Overall, we believe that these issues are worthy of additional clarification and examination. In this Comment, first we shall examine the phase diagram, that is, the Langmuir isotherm, and SHG responses from the DiA Langmuir monolayer. We shall show that these data in the Benichou paper were potentially flawed. Their Langmuir isotherm of the DiA monolayer was significantly different from * Author to whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 86-10-62555347. Fax: 86-10-62563167. † Also a graduate student of the Graduate School of the Chinese Academy of Sciences. ‡ Also a graduate student at the Hefei National Laboratory for Physical Sciences at Microscale, and Department of Chemical Physics, University of Science and Technology of China, Hefei, China 230026.
the results in the literature, and the reported Langmuir isotherm as well as the SHG data cannot be reproduced under controlled conditions in our laboratory. (The DiA sample was purchased from Molecular Probes (D3883) and was used without further purification. It is from the same source as the DiA sample in the Benichou paper.) We then shall show that with the data in the Benichou paper the conclusion that the chirality in the DiA Langmuir monolayer was compression induced can also be questioned. In the end, we shall discuss the issue on the significant magnetic dipole contribution to the chiral SHG signal from the DiA Langmuir monolayer. We have not found experimental evidence to support the claim for significant magnetic dipole contribution. 1. The Phase Diagram and SHG Response of the DiA Langmuir Monolayer Before going on to discuss the mechanism and origin of the chirality in the DiA Langmuir monolayer, we would like to discuss the issues on the phase diagram as well as the surface states of the DiA Langmuir monolayer, because it is essential to obtain SHG data for Langmuir monolayers with well characterized phase diagrams. Surface pressure isotherms of DiA Langmuir monolayers under different conditions were reported and documented before.3,4 We found that the phase diagram presented in the Benichou paper was significantly different from that in the literature. Because the DiA chromophore head group is charged and with a large size, a liquid expanded type of isotherm is expected. This is indeed the case in the literature.3,4 The surface pressure of the DiA Langmuir monolayer slowly rises around the surface density of 100 Å2 per molecule, which is a relatively low surface density in comparison to those of other neutral chromophore head groups with a similar molecular size.3,4 The surface pressure isotherm in the Benichou paper is shown in the upper panel of Figure 1. We digitized the phase diagram in the Benichou paper and converted the surface density from the not commonly used unit of nmol/cm2 into the commonly used unit of Å2 per molecule. The conversion relationship is that x nmol/cm2 ) 100/6.02x Å2 per molecule. Therefore, 0.2 nmol/ cm2 corresponds to 83.0 Å2 per molecule, and 0.5 nmol/cm2 is 33.2 Å2 per molecule, and so on. On the isotherm in the Benichou paper, the surface pressure rose around 30 Å2, which was significantly shifted from the value of ∼100 Å2 as in the literature.3,4 It is surprising that such a significant difference was not discussed in the Benichou paper. In the lower panel of Figure 1, we present the phase diagram measured from direct spreading without the moving barrier on the Langmuir trough. When the barrier was present, the DiA was only spread on one side of the barrier where the surface pressure was measured. Because the DiA molecule is ionic and may have slight solubility in water, we suspected that a fair fraction of the DiA molecules might be transferred to the other side of the barrier during the compression process. This might have caused the significant shift of the phase diagram as presented in the Benichou paper. This assumption was confirmed from the data presented in Figure 1. It is clear that, in the static measurement, the phase diagrams were significantly expanded
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Figure 2. Relaxation behavior of the SHG signal from the DiA Langmuir monolayer at a surface density of 27.1 Å2 per molecule. Every four peaks represents a complete 360° polarization measurement. The total time for the measurement is about 650 s. One can see that, for the first few cycles, the SHG signal is more noisy and the baseline is significantly above the noise level, while for the relaxed monolayer the SHG signal is less noisy and the baseline is reduced to the noise level.
Figure 1. Surface pressure isotherm of DiA Langmuir monolayer with different solvent by a different process. Upper panel: Data for the compression process. The thin solid curve is the isotherm digitized from the data in the Benichou paper (0.1 mM DiA methanol solution with compression speed of 5 cm2/min at temperature of 15 °C). The other two solid lines in the upper panel are isotherms measured at 20 °C by spreading the 0.1 mM DiA solutions using both methanol and chloroform as solvents. The measurement was performed with the KSV 1100 film balance (Finland). The isotherms were measured using compression speeds in the range 3-18 cm2/min. The isotherm with the chloroform solution agreed well with the reported results in the literature.3,4 Lower panel: Data for the static surface pressure measurement on the directly spread monolayer when the Langmuir trough was without the moving barrier.
to the lower surface density than those in the compression measurement. In addition, in the compression experiment, when methanol was used as the spreading solvent, the surface on the other side of the barrier on the Langmuir trough usually became colorful after compression, confirming that the DiA molecule was transferred from the monolayer side. Even though this visual phenomenon was less apparent for the case using chloroform as solvent, comparison of the phase diagrams obtained with and without the moving barrier also indicated that there was some depletion of the DiA molecules for the chloroform case under compression. Therefore, in general, the DiA phase diagram measured using the Lamgnuir trough compression or using methanol as solvent can be different from batch to batch. On the other hand, as shown in the lower panel of Figure 1, the phase diagram under the static measurement using chloroform as solvent can be well reproducible. In fact, the three batches of data using chloroform as solvent, which nevertheless overlap indistinguishably from each other, were not taken on the same day. Therefore, reproducible SHG measurements are better performed under the spreading condition with chloroform as solvent. Using methanol as solvent was truly an unusual choice, since the most commonly used solvent for making solutions for Langmuir
phase diagram measurements is either chloroform for the molecules insoluble in cyclohexane, or cyclohexane.5 In the literature, sometimes hexane or benzene was also used. As shown in the data in Figure 1, the surface pressure of the monolayer formed with the DiA chloroform solution started rising around the density of 100 Å2 per molecule and the monolayer already broke up around the density of 42 Å2 per molecule, while in the monolayer formed with the DiA methanol solution the surface pressure started rising only at the density of 50 Å2 per molecule. This indicated that almost half of the DiA molecules might have been missing from the air/ water interface for the methanol case. In the static surface pressure measurement as in Figure 1, a significant DiA deficit was also observed for the methanol case than for the chloroform case. This indicated that using methanol as the spreading solvent was not a good choice for the measurement of the DiA phase diagram. More problems could have been introduced in the SHG measurement during the monolayer compression. In the SHG measurement experiment, the compression of the Langmuir monolayer had to be stopped at different surface densities to make the polarization dependent SHG measurement. Therefore, the compression was not continuous, and during the SHG measurement process the surface density and surface pressure gradually decreased. We found that this was true for both the methanol and chloroform cases. As discussed above, the drop of the surface pressure during this time period was due to the slight solubility of the DiA molecule in the water phase.6,7 Such a surface pressure relaxation phenomenon in the Langmuir monolayer was also observed for the charged long chain phospholipids such as DPPC and DPPA.7 However, there was no such surface pressure relaxation when we did the SHG-LD measurement with the neutral PARC18 and TARC18 molecules.2 Consequently, the SHG-LD polarization curve for the DiA Langmuir monolayer was not stable and changed constantly with the measurement time in the compression measurement. This is typified in Figure 2. When the DiA monolayer was compressed to the surface density of 27.1 Å2 per molecule, the initial high surface pressure dropped to about 7 mN/cm in about 10 min during the continuous SHG measurement for 10 complete cycles of 360°. According to the phase diagram in Figure 1, we can see that the DiA monolayer from the chloroform solution was already broken at 27.1 Å2 per molecule. While as the surface
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pressure relaxed back to 7 mN/cm, the nonperiodical SHG polarization curves relaxed back to the periodical ones with a clear chiral signature. The DiA monolayer made with the methanol solution also exhibited similar behavior. Furthermore, as we examine the SHG curves for the different surface densities in the Benichou paper (Figure 6), we observed unbalanced maxima in almost all of the SHG curves. These curves should have been periodical if no time dependent behavior was present. To our surprise, this issue was not examined in the Benichou paper. In conclusion, the phase diagram using methanol as spreading solvent in the Benichou paper might have brought problems to the reproducibility in different batches of the SHG experiment. The SHG data presented in the Benichou paper might be arguable, but the SHG data measured on such DiA Langmuir monolayers cannot be reproduced under controlled conditions in our laboratory. 2. Is the Chirality of the DiA Langmuir Monolayer Compression Induced? To reach the conclusion that the chirality of the DiA Langmuir monolayer is compression induced, one has to show that before the monolayer surface pressure started rising, there was no detectable chirality. In the Benichou paper, it was concluded that, at the surface density of 0.2 nmol/cm2 (83.0 Å2 per molecule), no chirality was observed. From our analysis, this conclusion was not supported with the SHG data as presented in the Benichou paper. In Figure 3, we digitized the SHG data for the 0.2 nmol/cm2 (83.0 Å2 per molecule) density from the Benichou paper. We noticed that, in the two sets of s-polarization detection curves at the density of 83.0 Å2 per molecule, the SHG intensities at the 45° and 225° incident polarization angles were apparently different from those at 135° and 315°. This suggests the existence of the surface chirality, and the data can be fitted satisfactorily with both the dipolar achiral and chiral terms.2,8 By fitting the two sets of SHG-LD data, the degree of chirality excess (DCE) values were obtained as -9.4 ( 4.0% and -11.7 ( 4.0%. Here, the DCE of SHG-LD is defined in eq 1.9 We recently demonstrated that, with the s-polarization detection curve in SHG-LD, the DCE value can usually have an accuracy better than 2%.2 Even though the error bar from the SHG data in the Benichou paper is larger than that in our experiment, the -9.4 ( 4.0% and -11.7 ( 4.0% values obtained from their data were beyond the noise level, and the chirality of the monolayer at the low density of 0.2 nmol/cm2 (83.0 Å2 per molecule) of the DiA Langmuir monolayer cannot be ignored. From the above discussion, the actual surface density of the DiA Langmuir monolayer in this set of data might have been much lower than 83.0 Å2 per molecule. Therefore, even the data in the Benichou paper indicated that the surface chirality actually existed at very low surface pressure. Thus, the argument for the compression induced chirality could not have been reached.
∆I ⁄ I )
2(I-45° - I+45°) (I-45° + I+45°)
(1)
To further illustrate this point, we performed SHG measurements on the directly spread DiA Langmuir monolayer with chloroform as the spreading solvent. Figure 4 presents the SHG data from the DiA Langmuir monolayer at the surface density of 119.6 Å2 per molecule (surface pressure around 0.4 mN/m). The calculated DCE value is -11.5 ( 0.6%. Therefore, the DiA monolayer at such low surface density was already chiral. Because, in the direct spreading experiment, there was no
Figure 3. SHG intensity as a function of the input polarization angle for the DiA monolayer at the surface density of 0.2 nmol/cm2 (83.0 Å2 per molecule). The data were digitized from the Benichou paper. The solid lines are the fitting results with the chiral terms. The fitting results with the chiral term are obviously better than the fitting results without the chiral term as in the original Benichou paper.
compression process involved, the chirality in the DiA Langmuir monolayer cannot be compression induced. This conclusion is also consistent with our experiments with other Langmuir monolayers.2 In conclusion, even with the data in the Benichou paper, the chirality of the DiA Langmuir monolayer at low density was present. Therefore, the chirality could not have been compression induced. Our data on the directly spread DiA Langmuir monolayer also suggested that the conclusion for the compression induced monolayer chirality was not likely. Furthermore, in the SHG-LD measurement analysis on Langmuir films formed with different achial molecules, chiral signals commonly existed at the low surface density before the surface pressure rose.2 Therefore, so far there has been no direct evidence to support the conclusion for the compression induced chirality whatsoever. All the data we have known so far in the Langmuir monolayer measurement actually supported the mechanism for spontaneous symmetry breaking due to molecular self-assembly or aggregation.10
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Figure 5. Molecular structure of DiA and HTC.
Figure 4. SHG intensity as a function of the input polarization angle for the DiA monolayer at the surface density of 119.6 Å2 per molecule. DCE ) -11.5 ( 0.6%, and the chirality was obvious. Filled circles, s-polarization detection; open circles, p-polarization detection; solid line, fitting curves.
3. Is There Significant Magnetic Dipole Contribution to the Chirality of the DiA Langmuir Monolayer? Whether there is a significant magnetic dipole contribution to the SHG of the thin films has been a controversial issue for more than a decade.11,13-15 While in principle it is well-known that the magnetic dipole (and also electric quadrupole) contribution to the χ(2) is often 2-3 orders of magnitude smaller than the electric dipole contribution,11,12 there have been a few experimental studies by Persoons and co-workers, in addition to the Benichou paper under discussion here, which suggested that without the significant magnetic dipole term some of their SHG-CD or SHG-LD data from the molecular thin films could not be quantitatively understood.14-16 The evidence in the Benichou paper to support the significant magnetic dipole contribution in the dense monolayer was that there was significant SHG-LD intensity in the 90°-s and 270°-s polarization combinations in the s-detection curves from the dense DiA Langmuir monolayers. Contrary to the case of the DiA Langmuir monolayer, we did not find apparent SHG intensity in the 90°-s and 270°-s polarization combinations in the s-detection curves of other Langmuir monolayers of achial molecules which did exhibit clear chirality.2 The experimental setup for SHG-LD measurement in our laboratory is almost identical to that in the Benichou paper.2,17 We performed the SHG-LD experiment on the DiA Langmuir monolayer, and we found that there was no SHG intensity in the 90°-s and 270°-s polarization combinations as reported in the Benichou paper, unless the monolayer was compressed to its collapsing density, as shown in Figure 2. When the monolayer collapsed into a macroscopically nonuniform state, the criteria proposed by Benichou and co-workers on the magnetic dipole contribution to the surface chirality was no longer sufficient. In addition to the DiA Langmuir monolayer, we also performed SHG-LD measurement of two ionic stilbazolium dye molecules 4-(4-(dimethylamino)styryl)-N-octadecylpyridinium iodide (HHC) and 4-(4-(N-methyl-N-octadecyl-amino)styryl)N-methylpyridinium iodide (HTC). Both molecules have the same stilbazolium chromophore as in the DiA molecule. The only difference between the DiA and the HTC molecules is that the DiA has two hexadecyl straight carbon chains attached to the nitrogen atom at the end of the stilbazolium chromophore, while the HTC has one octadecyl straight carbon chain and one
Figure 6. Results of the polarization SHG measurements at 400 nm of the HTC Langmuir monolayer at the surface density of 42.0 Å2 per molecule at the air/water interface. The monolayer is clearly chiral. However, the SHG intensity is nearly at the noise level in the 90°-s and 270°-s polarization combinations, indicating no apparent magnetic dipole contribution in the chiral HTC Langmuir monolayer. Filled circles, s-polarization detection; open circles, p-polarization detection; solid line, fitting curves.
methyl group attached to the same nitrogen atom (as shown in Figure 5). More importantly, the HTC molecule not only has a similar UV-vis spectrum as that of the DiA molecule, but its Langmuir monolayer also has a similar surface phase diagram as that of the DiA monolayer.3,4 Our SHG-LD measurement of the HTC Langmuir monolayer did show apparent chirality in the s-detection curves. As shown in Figure 6, the HTC monolayer SHG-LD measurement also showed nearly noise level SHG intensity in the 90°-s and 270°-s polarization combinations in the s-detection curves even at the surface pressure above 25 mN/m. This behavior is the same as that of the PARC18 monolayer and DiA monolayer in our laboratory and different from that of the dense DiA monolayer in the Benichou paper.2 In conclusion, the SHG data to support the significant magnetic dipole contribution to the surface chiral SHG signal presented in the Benichou paper could not be reproduced in our laboratory under controlled conditions. Other measurements we have known of did not show anything similar to those data. Therefore, there has been no concrete evidence to support the magnetic dipole contribution to the Langmuir monolayer chirality. 4. Concluding Remark In summary, we have found evidence in contradiction to the main conclusions in the Benichou paper. This evidence includes the problems to reproduce their data under controlled conditions and also the problems in their data analysis.
4226 J. Phys. Chem. C, Vol. 113, No. 10, 2009 According to examination of the phase diagrams of the DiA monolayer under different conditions, the DiA monolayer itself was not at a stable state under the compression condition using methanol as solvent. The SHG data in the Benichou paper on the DiA Langmuir monolayer might be arguable, but they cannot be reproduced in our laboratory under controlled conditions. Examination on the SHG data of the DiA monolayer at low surface density in the Benichou paper also indicated monolayer chirality. Therefore, the conclusion for compression induced chirality could not be reached. SHG measurement in our laboratory on the DiA Langmuir monolayer at low surface density from direct spreading also confirmed the existence of chirality. This evidence actually supports the mechanism of spontaneous symmetry breaking due to molecular self-assembly or aggregation. The SHG data in the Benichou paper to conclude for the significant magnetic dipole contribution was most likely a measurement of the collapsed monolayer, which was not uniform, and the criteria for such contribution cannot be applied. Moreover, these data could not be reproduced under well controlled conditions in our laboratory. In the measurements on various Langmuir monolayers, including DiA, we could not find evidence to support the assumption for significant magnetic dipole contribution to the chirality in the Langmuir monolayer. We believe that studies on well characterized molecular systems are essential to help us understand the mechanism and origin of the chiral phenomena in the Langmuir monolayer. We are particularly concerned that the main conclusions in the Benichou paper were not supported by the studies in our laboratory. Acknowledgment. We thank Professor Fu-you Li for providing the HTC molecules synthesized and purified in his labora-
Comments tory. We also thank Professor Ming-hua Liu for allowing us use of the KSV 1100 film balance in his laboratory. H.-f.W. is thankful for the support by the National Natural Science Foundation of China (NSFC, Nos. 20425309, 20533070, 20773143) and the Ministry of Science and Technology of China (MOST No. 2007CB815205). References and Notes (1) Martin-Gassin, G.; Benichou, E.; Bachelier, G.; Russier-Antoine, I.; Jonin, Ch.; Brevet, P. F J. Phys. Chem. C 2008, 112, 12958. (2) Xu, Y. Y.; Rao, Y.; Zheng, D. S.; Guo, Y.; Liu, M. H.; Wang, H. F. J. Phys. Chem. C, in press, http://dx.doi.org/10.1021/jp810509u. (3) Lupo, D.; Prass, W.; Scheunemann, U.; Laschewsky, A.; Ringsdorf, H.; Ledoux, I. J. Opt. Soc. Am. B 1988, 5, 300. (4) Lusk, A. L.; Bohn, P. W. J. Phys. Chem. B 2001, 105, 462. (5) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (6) Vollhardt, D. AdV. Colloid Interface Sci. 2006, 123-126, 173. (7) Nin˜o, M. R. R.; Lucero, A.; Patino, J. M. R. Colloids Surf., A 2008, 320, 260. (8) Zheng, D. S.; Wang, Y.; Liu, A. A.; Wang, H. F. Int. ReV. Phys. Chem. 2008, 27, 629. (9) Verbiest, T.; Kauranen, M.; Maki, J. J.; Teerenstra, M. N.; Schouten, A. J.; Nolte, R. J. M.; Persoons, A. J. Chem. Phys. 1995, 103, 8296. (10) Pawlik, A.; Kirstein, S.; De Rossi, U.; Daehne, S. J. Phys. Chem. B 1997, 101, 5646. (11) Belkin, M. A.; Shen, Y. R. Int. ReV. Phys. Chem. 2005, 24, 257. (12) Wampler, R. D.; Zhou, M. K.; Thompson, D. H.; Simpson, G. J. J. Am. Chem. Soc. 2006, 128, 10994. (13) Fischer, P.; Hache, F. Chirality 2005, 17, 421. (14) Kauranen, M.; Verbiest, T.; Maki, J. J.; Persoons, A. J. Chem. Phys. 1994, 101, 8193. (15) Sioncke, S.; Verbiest, T.; Persoons, A. Mater. Sci. Eng., R 2003, 42, 115. (16) Elshocht, S. V.; Verbiest, T.; Kauranen, M.; Persoons, A. J. Chem. Phys. 1997, 107, 8201. (17) Rao, Y.; Tao, Y. S.; Wang, H. F. J. Chem. Phys. 2003, 119, 5226.
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