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Langmuir 1998, 14, 490-496
Langmuir-Blodgett Films of Retinal Derivatives C. P. de Melo* and M. I. Mosquera-Sa´nchez Departamento de Fı´sica, Universidade Federal de Pernambuco, 50.670-901 Recife PE, Brazil Received January 29, 1997. In Final Form: November 4, 1997 We report characterization studies of Langmuir films and Langmuir-Blodgett multilayers of 13-cisretinoic acid, 13-cis-retinal aldehyde, all-trans-retinoic acid, and all-trans-retinol palmitate. We have examined the stability of the floating monolayers prepared on an air-water interface, and the values estimated for the mean molecular areas before collapse are consistent with previous results for other retinal derivatives. After successful transference of the monolayers to form Langmuir-Blodgett films on different solid substrates, we have applied standard infrared spectroscopic techniques to investigate the structural organization of the deposited multilayers of the latter three compounds. By combining the information gathered from the isotherm compression curves of the Langmuir films to the analysis of the relative intensities of characteristic retinal peaks on the grazing angle and attenuated total reflectance spectra of the solid films, we have qualitatively estimated the orientation of the deposited molecules relative to the substrate. We comment on the implication of these results for the use of LangmuirBlodgett films of retinal derivatives in molecular electronic devices.
1. Introduction Since the identification that the cis to trans photoisomerization of retinal molecules is the primary event of the vision process in animals,1 great interest has been dedicated to the study of the interaction of light with retinal chromophores.2 Retinal-based proteins (such as bacteriorhodopsin and rhodopsin) seem to be prevalent in a large variety of photobiological processes which range from light harvesting in photosensitive bacteria to color vision in human beings.3 On the other hand, it is now well-established that the nonlinear polarizabilities of short-chain oligomers of conjugated molecules are dramatically affected4-6 by conformational changes. We can then expect that a significant change in the nonlinear optical properties of the retinal molecules should follow the characteristic cis to trans isomerization of these systems. In fact, calculations have indicated that the polarizability response of retinal derivatives is extremely dependent on the conformation adopted by the molecules,4,5 a theoretical prediction recently verified experimentally.6 For an efficient incorporation of these molecules into the design of sensors7 and electro-optical devices8,9 which exploit their large nonlinear optical coefficients, it is important to maximize the structural order of the samples used. The reduction of orientational randomness in the spatial distribution of the transferred molecules in a Langmuir-Blodgett (LB) film has made this preparation technique a method of choice for obtaining * Author to whom correspondence should be addressed: E-mail:
[email protected]. (1) Hubbard, R.; Kropf, A. Proc. Natl. Acad. Sci. 1957, 44, 130. also, see: Dowling, J. E. The Retina: An Approachable Part of the Brain, The Belknap Press of Harvard University: Cambridge, U.K., 1987; Chapter 1. (2) Wald, G. Science 1968, 162, 230. (3) Nathans, J.; Piantanida, T. P.; Eddy, R. L.; Shows, T. B.; Hogness, D. S. Science 1986, 232, 203. (4) Sales, T. R.; de Melo, C. P. Chem. Phys. Lett. 1991, 180, 105. (5) Toto, J. L.; Toto, T. T.; de Melo, C. P.; Robins, K. J. Chem. Phys. 1994, 101, 3945. (6) Bezerra, A. B., Jr.; Gomes, A. S. L.; de Melo, C. P.; de Arau´jo, C. B. Chem. Phys. Lett., in press. (7) Maccioni, E.; Radicchi, G.; Erokhin, V.; Paddeu, S.; Facci P.; Nicolini, C. Thin Solid Films 1996, 284-285, 898. (8) Molecular and Biomolecular Electronics; Birge, R. R., Ed.; American Chemical Society: Washington, DC, 1994. (9) Koyama, K.; Yamaguchi, N.; Myasaka, T. Science 1994, 265, 762.
well-organized multilayers that exhibit highly anisotropic nonlinear optical properties.10 However, in spite of the large amount of work done on the preparation of LB films of the rhodopsin pigments,11 to our knowledge not too many previous reports exist12-14 on the properties of Langmuir films of retinal molecules. In this paper we present a systematic study of the conditions of preparation and characterization of LB films of four retinal derivatives and report initial results of the use of infrared spectroscopic techniques to determine the relative orientation of the deposited molecules. 2. Materials and Experimental Setup The compounds investigated in this work (see Figure 1), 13cis-retinal (cRet; molecular weight (mol wt) 284.4), 13-cis-retinoic acid (cRAc; mol wt 300.4), all-trans-retinoic acid (tRAc; mol wt 300.4), and all-trans-retinol palmitate (tRPal; mol wt 524.9), were used as received from the manufacturer (Sigma-Aldrich, Milwaukee, WI), and all subsequent experimental work was performed under dim red light illumination. Ultrapure housedistilled H2O with a final resistivity of 16.5 MΩ‚cm, obtained after passage through a NANOpure Series 550 equipment (Barnstead, Boston, MA), was used as a subphase for the Langmuir films. The spreading solutions were prepared with chloroform p.A. 99.9% grade (Merck, St. Louis, MO), and the experiments at the air-water interface were carried out in a LB-5000 alternate trough (KSV Industries, Finland) with automatic control of the dynamic conditions of the monolayers and of the transfer process. According to the signal-to-noise ratio required, for the spectroscopic investigation we have used either a deuterated triglycine sulfate (DTGS) detector (with a bandwidth between 180 and 7800 cm-1 and operated at room temperature) or a mercury-cadmium-telluride (MCT) narrow band (between 450 and 5000 cm-1) solid-state detector, operated at liquidnitrogen temperature, in a MB-100 Fourier transform infrared (FTIR) spectrophotometer (Bomem, Vanier, Canada) equipped with attenuated total reflectance (ATR) and grazing incidence reflection (GIR) accessories (Graseby Specac Ltd., Kent, U.K.). (10) Williams, D. J. Angew. Chem. 1984, 23, 690. (11) Palings, I.; Pardon, J. A. Biochemistry 1987, 26, 2544. (12) Huang, J. R.; Lewis, A.; Rasing, Th. J. Phys. Chem. 1988, 92, 1756. (13) Li, J.-R.; Li, X.-C.; Ding, J.-Y.; Wang, J.-P.; Jiang, L. Chin. Sci. Bull. 1993, 38, 1265. Li, J.-R.; Wang, J.-P.; Jiang, L. Biosens. Bioelectron. 1994, 9, 147. (14) Sereno, L.; Silber, J. J.; Otero, L.; Bohorquez, M. V.; Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. 1996, 100, 814.
S0743-7463(97)00092-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998
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Figure 2. Room temperature compression isotherms for the retinal derivatives examined. Table 1. Parameters Used for Dispersion of the Retinal Solutions and Principal Characteristics of the Corresponding Isotherms
Figure 1. Molecules investigated in this work: (a) 13-cisretinal (cRet), (b) 13-cis-retinoic acid (cRAc), (c) all-trans-retinoic acid (tRAc), and (d) all-trans-retinol palmitate (tRPal). The suggested orientation of these compounds relative to the airwater interface is consistent with the information gathered from the isotherm experiments (see section 3.1).
3. Experimental Procedures 3.1. Monolayer Characterization. The floating monolayer of each retinal derivative was formed by spreading a small volume of the corresponding chloroform solution on the surface of the ultrapurified water subphase and, after waiting for the evaporation of the solvent, compressing the resulting film by the moving barrier of the KSV instrument. We have tested several experimental conditions until, when using forward and backward barrier speeds of 10 mm/min (50 mm/min for tRAc), reproducible isotherms were obtained (see Figure 2). In Table 1 we present the values of the concentration and of the volume of solution dispersed in each case, as well as the observed collapse pressure πc and the corresponding mean molecular area. (In all cases, the collapse pressure πc was taken to correspond to the point of inflection of the corresponding isotherm.) From the results shown in Table 1, one can see that the mean molecular area before collapse of these molecules was in the range 21-40 Å2. These values are consistent with previous results obtained for retinal molecules:12,14 Even though theoretical calculations estimate in 40 Å2 the area of the β-ionone ring,12,14 it is expected that both the dynamics of the floating molecules on the water interface12 and the close packing of the film right before collapse should lead to a reduction of the mean molecular area. The fact that, in spite of the larger molecular weight of tRPal, it was the cRet molecule the retinal derivative presenting the largest molecular area (∼40 Å2) at the collapse pressure can be rationalized by assuming that, as indicated in Figure 1, on the average the cRet molecules
concentration (mg/mL) volume dispersed (µL) collapse pressure πc (mN/m) mean molecular area (Å2)
cRet
cRAc
tRAc
tRPal
0.14 200 15.2 39.6
0.96 50 14.0 20.9
1.16 100 17.1 30.5
1.34 50 29.0 31.0
anchor at a more tilted angle relative to the normal of the interface. In fact, in their second-harmonic study of the orientation of retinal and retinal Schiff bases, Huang et al.11 have determined that the molecular direction would vary from 70° relative to the surface normal at low surface pressure to values close to 50° near collapse. A simple test was performed to confirm that the inflection in the pressure-area isotherms did, in fact, correspond to a collapse of the film and not to a simple structural phase transition of the condensed film. For all molecules considered we ran a series of experiments in which a larger amount (typically on the order of 4 times the usual volume used in the normal compression) of material was dispersed on the interface, while the change on the surface pressure was monitored: in each case the pressure has increased to approximately the same value we have deemed as the onset of the corresponding collapse and, then, rapidly decreased, as the excess material either dissolved into the subphase monolayers or forced the floating monolayer to fold itself into different multilayer regions. In some of these experiments (especially in the case of the tRAc monolayers) there was clear visual indication of domain formation. To investigate the possibility of solubilization of tRAc molecules into the subphase, we repeated the isotherm experiment for this compound after waiting increasing amounts of time before compressing the floating monolayer. Whenever the normal amount of material was used (and therefore the formation of domains did not occur), the pressure vs mean molecular area profile remained essentially the same (see Figure 2), even when 15 h had elapsed since the original
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Figure 4. Surface relaxation of the floating monolayers as measured by isobaric creep experiments: for a fixed lateral pressure, as the barrier moves the total surface area of the Langmuir films changes as a function of time. Table 2. Best Set of Parameters for Transfer of the Floating Monolayers to Solid Substrates cRet
Figure 3. First four steps of a hysteresis experiment for a floating monolayer of tRPal. These curves are typical of the behavior of the other retinal derivatives when submitted to a similar experiment.
spreading. We have therefore concluded that no significant solubilization of tRAc has occurred in the original isotherm experiments. As a further check of the reproducibility of the results, using the two separate troughs of the LB-5000 instrument, simultaneous isotherm experiments were performed for each one of the compounds investigated: in all cases, each pair of curves corresponded to essentially identical isotherms. The short-term stability of these Langmuir films was confirmed through a series of hysteresis experiments, in which the floating monolayer is subject to successive cycles of compression to a fixed limiting pressure and subsequent expansion. For all of the retinal derivatives examined, the isotherm was seen to retain its original profile as long as the limiting pressure chosen does not exceed the corresponding value of πc. As an example, in Figure 3 we present the first four steps of a hysteresis experiment for tRPal, in which the upper limiting pressure was set to 25 mN/m. The long-term stability of the films, on the other hand, is better investigated through an isobaric creep experiment, in which a fixed pressure is maintained on the floating monolayer by a free-moving barrier. For the different Langmuir films, after a fixed pressure equal to πc was set in the instrument the initial change in the position of the barrier as a function of time was as shown in Figure 4: as can be noted, while the tRPal monolayer experiences a slight contraction on the order of 3%, all others floating films undergo a less than 10% expansion. We can conclude then that, given enough time, in all cases in which no extra pressure is applied the ordering of the retinal molecules in the free-standing film would undergo changes to a more stable configuration. 3.2. Preparation of the Langmuir-Blodgett Films. For each one of the compounds investigated, a long series of experiments was conducted to determine the best parameters for the transfer of the floating monolayer to a given substrate (usually a glass slide). Following a trial
cRAc
tRAc
tRPal
target pressure (mN/m) 13 13 13 25 forward/backward 0.5/0.4 0.5/0.4 0.5/0.4 0.5/0.4 barrier speed (mm/min) dipping speed (mm/min) 1.1 1.1 1.1 1.6
and error procedure, we have found that to get a transfer ratio (defined as the relation between the trough area swept by the barrier to the covered area of the substrate) as close to 1.0 as possible, the displacement of the barrier during the compression of the film should proceed in a continuous and linear manner and that, upon emergence from the subphase, the substrate should be as free as possible from water droplets. Once found for a given substrate (see Table 2), the best set of transfer parameters was robust in the sense that it could be adopted to transfer the molecule considered to any other solid substrate with equal success. As previously observed for other retinal molecules,13,14 all LB films were obtained in the Z-type configuration; i.e., good transfer ratios were only possible for a transference of the monolayer occurring on the upward stroke of the substrate. Please note that, for a successful transfer of the Langmuir film of tRPal, the chosen target pressure had to be almost twice the value used in the other cases, a fact that we attribute to the larger collapse pressure (πc ∼29 mN/m) observed for this molecule. Due to the variety of techniques to be later used in the FTIR characterization, we had to work with a choice of different substrates: silicon, platinum and conducting glass (i.e., glass covered by a thin film of indium and tin oxide, ITO) slides, and crystals of germanium and ZnSe. Before use, all substrates were carefully cleaned with chloroform and sonicated for 15 min and then submitted to a hydrophilic treatment15 by successive immersions in acetone and methanol. (Between consecutive immersions the substrates were thoroughly washed in deionized water.) Finally, before use the substrates were dried in an oven at 100 °C. The thickness of the different LB films prepared varied from 1 to 10 monolayers, and in all cases the transfer ratios were on the order of 1.00 ( 0.01. 3.3. FTIR Characterization. GIR and ATR infrared spectroscopies are especially suited for the analysis of thin organic films.16,17 While in ATR experiments both per(15) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 99, 62.
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Figure 5. Infrared spectra of the four retinal derivatives investigated in this work: while the tRPal spectrum was obtained through a GIR experiment for deposited drops, all other spectra correspond to transmission experiments performed on KBr pellets.
pendicular and parallel components of the electric field vector of the incoming radiation couple to the transition moment dipoles of the film, only the perpendicular component interacts with the sample in a GIR configuration.18 As a consequence, different selection rules are obeyed in each of these experiments, and by comparison the corresponding spectra, it is frequently possible to identify the vibration modes of the deposited molecules which are oriented along the direction perpendicular to the surface: this provides a very reliable technique to obtain information about the relative orientation of molecular groups of interest.19 For cRet, cRAc, and tRAc, a preliminary transmission spectrum of KBr pellets (using 50 scans, with a resolution of 2 cm-1) was obtained for identification of the characteristic absorption peaks. Since the oily nature of tRPal has prevented us from preparing KBr pellets of this compound, the corresponding disordered spectrum has been taken through a GIR experiment after dripping some drops of the chloroform solution of tRPal on silicon slides and Ge crystals. To reduce the time required for the acquisition of the GIR spectra (and therefore to minimize the background changes in the intensities of water absorption), in these experiments 50 scans were used to collect the data and a resolution of 16 cm-1 was adopted. In Figure 5 we present the corresponding infrared spectra of the four retinal derivatives examined in this work. The GIR and ATR spectra of the retinal LB films were then compared in search of information about the relative molecular orientation of the transferred molecules. Unfortunately, we have not been able to obtain good quality (16) Francis, S. A.; Ellison, A. H. J. Opt. Soc. Am. 1959, 49, 130. Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (17) Harrick, N. J. J. Chem. Phys. Solids 1959, 8, 106. Fahrenfort, J. Spectrochim. Acta 1961, 17, 689. Fahrenfort, J.; Visser, W. M. Spectrochim. Acta 1962, 18, 1103. (18) Banga, R.; Yarwood, K. Langmuir 1995, 11, 618. (19) Takenaka, T.; Nogami, K.; Gotoh, H.; Gotoh, R. J. Colloid Interface Sci. 1971, 35, 395.
Figure 6. Comparison of the infrared spectra of cRet molecules obtained in different experiments: (a) transmission (for a KBr pellet), (b) ATR (for six monolayers on Ge), and (c) GIR (for two monolayers deposited in Pt). All spectra were obtained using a DTGS detector. (d) Schematic representation of the average anchorage angle of the cRet molecules, as suggested from the analysis of the infrared spectra and isotherm experiments.
ATR and GIR spectra for the cRAc films. For the remaining three compounds, however, the analysis of the infrared spectra has allowed us to gain some insight into the structural order of the corresponding LangmuirBlodgett films, as we discuss now. 3.3.1. 13-cis-Retinal. In Figure 6a-c we present the assignments for the observed absorption peaks in a transmission spectrum of a KBr pellet of cRet and compare them to the GIR (for two monolayers deposited in Pt) and ATR (for six monolayers on Ge) spectra obtained using a DTGS detector. Due to the lower intensity of the absorption peaks in the reflectance experiments, the scales
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Figure 7. Comparison of the infrared spectra of tRPal molecules obtained in different experiments: (a) GIR spectra for ordered (13 monolayers deposited on Pt) and disordered (drops on Pt), (b) ATR spectra for ordered (13 monolayers deposited on Pt) and disordered (drops on a Ge). All spectra were obtained using a DTGS detector. (c) Schematic representation of the average anchorage angle of the tRPal molecules, as suggested from the analysis of the infrared spectra and of the isotherm experiments.
of the corresponding spectra have been adjusted. Hence, this analysis should be restrained to an internal comparison of the relative intensities in a given spectrum, and no quantitative prediction of the degree of orientation of the molecules can be advanced. Even so, the identification of the CdO stretching (at 1652 cm-1) and of the ionilidine CdC stretching (at 1570 cm-1) in both ATR and GIR spectra can be taken as an indication that the corresponding molecular vibration occurs mostly in the direction perpendicular to the substrate. As a consequence, we suggest that the 13-cis-retinal molecule is deposited with the conjugated backbone tilted at a relatively large angle relative to the normal of the substrate, as schematically represented in Figure 6d. As noted before, the large mean molecular area at the collapse of cRet (∼40 Å2; see Figure 2) is already suggestive of a large angle of anchorage of this molecule relative to the subphase. Hence, the present FTIR result can be taken
as an indication that the same relative order is preserved on the transferred monolayers. 3.3.2. all-trans-Palmitate Retinol. For tRPal, a comparison of infrared spectra of disordered samples and LB films was made using the GIR (drops vs 13 monolayers deposited on Pt) and the ATR (drops vs 10 monolayers on Ge) techniques with a DTGS detector, as seen in Figure 7a,b. The most striking feature of the GIR spectra of the LB film is the fact that while the C-O stretching (seen at 1172 cm-1 in the spectrum for the drops) is present, there are no observable peaks corresponding to either the symmetric or the asymmetric CH2 modes and to the CdO stretching vibration. (Note that in the spectrum for the drops these latter three modes are seen respectively at 2929, 2859, and 1720 cm-1.) The fact that these peaks, which are present in the ATR spectrum of the LB film, are not seen in the grazing angle incidence experiment can be rationalized if we admit that the tRPal molecules on
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Figure 8. tRAc infrared spectroscopic investigation: comparison of the transmission (for a KBr pellet) spectra to GIR (for 7 monolayers deposited on an ITO-covered glass) spectra (a) and ATR (for 10 monolayers on Ge) of Langmuir-Blodgett (b). While the transmission spectra were obtained using a DTGS detector (please note the difference in scales), a MCT detector was used for the GIR and ATR experiments. (c) Schematic representation of the average anchorage angle of the tRAc molecules, as suggested from the analysis of the infrared spectra and isotherm experiments.
the average are deposited on the solid film as indicated in Figure 7c, with the CdO and CH2 groups oriented along planes essentially parallel to the substrate. This would imply a smaller molecular area than if the chains were oriented otherwise. Again, the information about the relative orientation of the transferred molecules inferred from the infrared spectra is consistent with the analysis of the results of the isotherm experiment. There we have found first that in spite of their larger molecular weight the tRPal molecules occupy a relatively low mean molecular area at collapse. Also, of all retinal derivatives examined, the tRPal molecules presented the largest lateral pressure at a given mean molecular area, as one could expect from the higher repulsion between almost parallel alkylic chains belonging to different neighboring molecules. 3.3.3. all-trans-Retinoic Acid. In Figure 8a,b we present the transmission (taken on KBr pellets), GIR (for
7 monolayers deposited on an ITO-covered glass slide), and ATR (10 monolayers on Ge) spectra of tRAc. As can be seen, both the CdO and the ionilidine CdC stretching absorption peaks can be identified in all three spectra. Once again this can be taken as an indication that the corresponding vibrations occur mostly along planes perpendicular to the substrate, as indicated in Figure 8c. 4. Summary and Conclusion Although the biological importance of retinal derivatives has been recognized for a long time1, only recently has the potential application of these compounds in molecular electronic devices begun to receive the due attention.7-8 A possible way to accomplish the macroscopic control of the molecular response of a given material is to obtain a very fine tuning of the organizational structure of the corresponding samples so as to maximize the collective response arising from the probing of the individual
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molecules. The Langmuir-Blodgett technique has appeared in the last decade as a competitive preparation method of obtaining very organized molecular structures.20 In this paper we have described a systematic study of the stability of Langmuir films of four retinal derivatives prepared at an air-water interface. The values determined for the mean molecular areas of these compounds right before collapse of the floating monolayers were consistent with previously known estimates for retinal derivatives, but we have taken the fact that the cRet molecules presented the largest mean molecular area (40 Å2) as an indication that on the average these molecules anchor at a larger angle relative to the normal of the substrate, a conclusion which was later corroborated by the FTIR analysis of the spectra of the deposited LB films. Next, we have examined the conditions for the successful transfer of the floating monolayers to different solid substrates. In all cases, very good transfer ratios (on the order of 1.00 ( 0.01) could be obtained for Z-type deposition of LB multilayered structures. One should consider the possibility that, as observed by Sereno et al. for LB films of carotenoid pigments,14 a subsequent rearrangement of the molecules would convert the solid film to a Y-type (or bilayer) stacked structure. Note, however, that these latter results were obtained for depositions on a water subphase containing cadmium nitrate and acetic acid. Due to charge screening effects, a salt concentration in the subphase would contribute to a stabilization of the floating monolayer and, eventually, to a different geometrical relaxation of the Langmuir film.21 (An extension of the present work to investigate these solvent effects is planned for the near future.) Finally, a FTIR spectroscopic study of the solid films using grazing angle incidence and attenuated total reflection experiments was performed to analyze the relative degree of ordering of the deposited molecules in three of these films. The combination of the results from (20) Petty, M. C. Langmuir-Blodgett Films: an Introduction; University Press: Cambridge, U.K., 1996. (21) See, for instance: Schwartz, C. K.; Viswanathan, R.; Garnaes, J.; Zasadzinski, J. A. J. Am. Chem. Soc. 1993, 115, 7374.
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the infrared analysis to the data collected from the isotherm experiments has made it possible for us to advance some hypothesis about the anchorage geometry of the cis-retinal, the all-trans-retinoic acid, and the alltrans-palmitate retinol molecules. The suggested geometry is consistent with the idea that the structural molecular ordering present in the floating monolayer is preserved in the transferred films. (A similar analysis could not be made for the cRAc films due to the fact that we could not obtain good quality ATR and GIR spectra of the solid films.) A consistent degree of ordering in the deposited films is a very important step in the development of molecular electronic devices based on retinal molecules. For instance, it is well-established from theoretical calculations4,5 for the isolated molecules that the nonlinear polarizabilities of retinal derivatives change by orders of magnitude upon cis to trans isomerization, a fact that seems to be corroborated by experimental evidence.6 While nonlinear optical techniques have been previously used to establish that floating monolayers of retinal and retinal Schiff bases exhibit a certain degree of anisotropy,10 our present results confirm the idea that even after transfer the retinal derivatives examined retain a certain degree of orientation in the solid films. This fact opens up the possibility of devising molecular optoelectronic devices based on the photoisomerization of Langmuir-Blodgett films of retinal derivatives in which, for instance, an external light source could be used to control the anisotropic nonlinear optical response of the sample. Acknowledgment. We acknowledge the financial support of the Brazilian Agencies CNPq and FINEP. We are very grateful for the technical assistance and extremely helpful observations of Mr. C. G. dos Santos and Mr. A. C. Teno´rio at different stages of this work. Finally, we thank the anonymous reviewer, whose pertinent questions have helped us to make more clear some of our arguments. LA970092E
