J. Phys. Chem. C 2009, 113, 4227–4228
Reply to “Comment on ‘Compression Induced Chirality in Dense Molecular Films at the Air-Water Interface Probed by Second Harmonic Generation’” G. Martin-Gassin, E. Benichou,* G. Bachelier, I. Russier-Antoine, Ch. Jonin, and P. F. Brevet Laboratoire de Spectrome´trie Ionique et Mole´culaire, UniVersite´ Claude Bernard Lyon 1- CNRS (UMR 5579), Baˆtiment Alfred Kastler, 43 BouleVard du 11 NoVembre 1918, 69622 Villeurbanne cedex, France ReceiVed: December 18, 2008; ReVised Manuscript ReceiVed: January 23, 2009 In our paper published recently in The Journal of Physical Chemistry C,1 we have reported the observation of chirality in a dense molecular film of the achiral stilbazolium dye 4-(4(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) at the air-water interface. This result has been obtained in a Langmuir trough using the surface sensitive technique of second harmonic generation (SHG) with a detailed polarization analysis of the signals. In this study, evidence of chirality was essentially observed in the highly compressed films after severe compression. From the polarization analysis, it appeared that the origin of this chirality at the microscopic level was dominated by the magnetic dipole contributions. At the reading of Wang’s comments, it appears that our point-of-view and some of the ideas raised in our paper have been misunderstood. We will not discuss the problem of data reproducibility and reliability, since these are routinely reproduced in our laboratory, but rather we will focus on the three scientific questions pointed out by these authors: • The choice of solvent and the phase diagram of the film. • The role of the film compression in the appearance of chirality. • The magnetic dipole contribution to the SH intensity. 1. The Choice of Solvent and the Phase Diagram of the Film First of all, Wang and co-workers have pointed out the unusual choice of methanol as the solvent for the DiA solution spread at the air-water interface. If we agree with this statement regarding this unusual choice of solvent, chloroform for instance is the most common solvent in this type of experiments, we firmly believe that this choice led us to the original experimental conditions reported for these films of achiral compounds. It hence also led us to the data reported which appear rather unusual. Another motivation supporting this choice of solvent comes from previous experimental work where we have reported that DiA is soluble in methanol but fully insoluble in water owing to its too long alkyl chains. Hence, we have been able to perform hyper Rayleigh scattering (HRS) experiments of DiA aggregates in bulk methanol-water mixtures.2,3 As a result, methanol was found to be a good solvent choice to initiate aggregation at the air-water interface. There are consequences in using methanol as the solvent for the DiA solution. For example, isotherm plots cannot be directly compared to those obtained in the literature with the more * To whom correspondence should be addressed. E-mail: benichou@ lasim.univ-lyon1.fr. Telephone: +33 472 431 914. Fax: +33 472 445 871.
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conventional solvent chloroform. This is indeed the conclusion of Wang and co-workers. We did not perform a comparison in our paper and therefore did not claim that using methanol or chloroform was without consequences. In particular, the surface pressure versus area per molecule diagram is strongly shifted toward small molecular areas for methanol as compared to chloroform (see Wang and co-workers’ plots). This result underlines that indeed the choice of methanol was adequate to initiate aggregates in the film. With aggregation present, the concept of phase diagram has no longer a defined meaning. This is a thermodynamical concept where a monolayer is assumed. As pointed out by Wang and co-workers, the stability too is not obtained, but this was clear in our data. Furthermore, the DiA aggregates at the air-water interface may well take a tridimensional organization. Hence, it is impossible to ensure that the film does form a monolayer. For this reason, we have never used the term “phase diagram” in our paper, but we agree that the word “monolayer” could be confusing. Here, we only wanted to say that, compared to the wavelength of light, the film thickness remained very small, probably of the order of three molecular lengths. Furthermore, surface pressure plots were only reported as surface pressure versus average molecular density, the only parameter at our disposal, because the area per molecule could not be recalculated since aggregates may be present. Wang and co-workers have commented on several occasions our data. In their first point, they claim that it is possible to observe a transfer of molecules beyond the trough barriers, as seen through the color taken by the water surface in this part of the trough. We have never observed such a colorful water surface beyond the barriers. Wang and co-workers have also pointed out that, when compression is paused to perform SHG polarization measurements, the surface pressure dropped. As they mentioned in their Comment, this drop was observed for both methanol and chloroform cases. The reason for these pressure drops is the film relaxation after the barrier stop. We agree that we could have discussed these pressure drops in greater detail in our paper, although they are not at the center of our attention. All SHG measurements were anyhow recorded after a few minutes of settling time in order to obtain a stable optical signal as shown in Figures 5 and 6 of our paper. This settling time is in agreement with the settling time measured by Wang and co-workers, supporting similarities in the observation and experimental practices. It must be pointed out here that the film relaxation supports the idea that thermodynamical equilibrium is not obtained during the compression of the film. Hence, it is difficult to talk about phase diagrams (see above). In conclusion, the surface pressure measurements performed in our study were not analyzed in terms of phase diagrams due to the non-thermodynamical conditions of the experiment. The molecular aggregation observed in the film, leading to a possible formation of tridimensionnal aggregates, confirmed that methanol was a good choice of solvent for the DiA solution in the present context. We also agree with Wang and co-workers that chloroform is a better choice to obtain strict monolayers and measure phase diagrams. Complementary studies on the thermodynamical properties of a DiA film at the air-water interface spread from a methanol solution, although not the focus of our attention, could be interesting to add to this general problem.
10.1021/jp811191k CCC: $40.75 2009 American Chemical Society Published on Web 02/18/2009
4228 J. Phys. Chem. C, Vol. 113, No. 10, 2009 2. The Role of the Film Compression in the Appearance of Chirality The second point is concerned with the role of compression in the appearance of chirality. The question is actually an excellent point raised by Wang and co-workers and would probably require an in-depth investigation and a full manuscript in itself. The question may be reformulated as whether the chirality is already present even at low compression in the film or if it appears owing to the compression in an initially achiral film. To demonstrate that the chirality was already present at low pressure, Wang and co-workers have digitized our experimental data. With and without this new treatment, it appears at first sight that chirality may be present at low compression. In this case, and that is quite intriguing, it is possible to envisage that initial aggregates in the film are chiral and act as nucleation points for the later stages of compression. We have tried to be as cautious as we could in our conclusions because the fluctuations in the SHG signal were not negligible at these weak average molecular densities. If we are looking carefully our experimental data given in Figure 6-I, SHG intensities should be equal at the fundamental polarization angles of 45° and 225°. Similarly, the SHG intensities at the fundamental polarization angles of 135° and 315° should be equal too. Fluctuations of the signal are of the order of 5%, and therefore, it seemed difficult to conclude on a clear evidence of chirality. For this reason, we have been cautious in our paper and stated that chirality was not present at low molecular densities. To prove further that chirality was present at low-pressure conditions, Wang and co-workers have also performed experiments on a monolayer obtained from the spreading of chloroform DiA solution. We believe that the presence of chirality at low surface pressure and its origin at the molecular level remain an open question and could be the center of future studies. Finally, once it has been assumed that chirality is not present at low average surface densities, it is clear that the strong deformations of the polarization plots at higher densities are induced by the film compression.
Comments 3. The Magnetic Dipole Contribution to the SH Intensity We have shown in our paper that the strong deformation of the polarization plots stems from the magnetic dipole contribution. This statement was fully supported by a discussion on the microscopic models already available in the literature. To refute this approach, Wang and co-workers did similar experiments albeit using chloroform as solvent. They propose also a discussion with other compounds, such as HTC. If their discussion is interesting per se, it is difficult to see the point in the Comment and we look forward to seeing these data published as a regular paper. The interesting point though is that Wang and co-workers briefly mentioned that similar results were obtained in conditions very close to the collapsing density. Since chirality is observed in our case in strongly compressed films, therefore close to the collapsing density, and since it is possible that the DiA aggregates are tridimensional, their results seem to be in close agreement with ours. 4. Conclusions A careful commenting of the questions addressed to us by Wang and co-workers allowed us to clarify a few points discussed in our paper. It seems that Wang and co-workers misunderstood us principally on the experimental conditions (choice of solvent, thermodynamical conditions, tridimensional aggregates, etc.), and we agree that some points needed clarification. The results of Wang and co-workers presented in Figure 4 of their comment are not in contradiction with our results at low pressure shown in Figures 5 and 6-I. The dramatic deformations of the polarization plots at higher pressure appeared for conditions not investigated by Wang and coworkers. References and Notes (1) Martin-Gassin, G.; Benichou, E.; Bachelier, G.; Russier-Antoine, I.; Jonin, C.; Brevet, P. F. J. Phys. Chem. C 2008, 112, 12958. (2) Revillod, G.; Duboisset, J.; Russier-Antoine, I.; Benichou, E.; Bachelier, G.; Jonin, C.; Brevet, P. F. J. Phys. Chem. C 2008, 112, 2716. (3) Revillod, G.; Russier-Antoine, I.; Benichou, E.; Jonin, C.; Brevet, P. F. Nonlinear Opt., Quantum Opt. 2006, 35, 135.
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