11974
J . Phys. Chem. 1993,97, 11974-1 1978
Characterization of Hemicyanine Aggregates in Air-Water Monolayers Robert A. Hall, Peter J. Thistlethwaite,’ and Franz Grieser School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
Nobuo Kimizuka and Toyoki Kunitake Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 81 2, Japan Received: April 27, 1993; In Final Form: August 6, 1993’
Absorption spectra of a docosyl derivative of the hemicyanine chromophore were measured a t the air-water interface as a function of surface concentration. At low compression, a broad band is observed corresponding to the monomer species. With increasing surface pressure two new bands grow in concurrently a t the expense of the monomer band. Principal factor analysis of the spectra indicates that the two aggregate bands are produced by a single aggregate species. In terms of exciton theory, this corresponds to the aggregate geometry having two molecules per unit cell. The sensitivity of the aggregate structure to the local environment is shown by the shift in position of the absorption bands for a mixed hemicyanine-cationic diluent monolayer. The polyion complexed hemicyanine monolayer exists only as the monomer a t all surface concentrations, in contrast to the monomer-aggregate coexistence observed on a pure water subphase.
Introduction There is considerable interest in the fabrication of nonlinear optical devices from Langmuir-Blodgett (LB) films.lS2 The ordering provided by the layer-by-layer deposition and the wide range of compounds which can be incorporated into a film make the LB method attractive. Hemicyanine compounds are highly polarizable, a characteristic which makes them, at first glance, suitablecomponentsin the production of non-linear optical devices. Consequently, much of the early workon hemicyanines has focused on the production of a large second harmonic signal and the orientation of the chromophore.3-5 Recently more attention has been paid to the structure of the monolayers as this is a significant factor in determining the signal strength. The orientation of the alkyl chains has been studied via infrared transmission and reflectance spe~troscopy6-~ and the formation of aggregates which produce a blue-shifted absorption due to exciton interaction has also been investigated.*-’ The poor stability and durability of many LB films has led many workers to seek to improve these properties. Stabilization of a charged monolayer can be achieved by the formation of an electrostatic complex between the charged amphiphiles and a polymeric counterion dissolved in the subphase.12J3 In the course of stabilizing the monolayer, the monolayer structure is modified and such changes have been observed by spectroscopic and surface pressure t e c h n i q u e ~ . ~Several ~ J ~ studies of the effects of complexation have been carried out on monolayers that contain chromophores whose spectral response is sensitive to aggregation. The disruption of J aggregates,*6the formation of aggregates in condensed “islands”l7 and no observable spectral change’s are some consequences of complexation of a polymer to a charged monolayer. In this paper the absorption spectra of hemicyanine air-water monolayers are interpreted using curve resolution methods and exciton theory. This reveals a complex aggregate structure, involving two molecules in the unit cell. The effect of polyion complexation on aggregate formation is also studied. Experimental Method The chemical structure of the hemicyanine dye, 4-[4-(dimethy1amino)styryll-1-docosylpyridinium bromide is shown here:
* To whom correspondence should be addressed. *Abstract published in Aduunce ACS Abstracts. October 1, 1993. 0022-365419312097-11974$04.00/0
The dye was purchased from Aldrich Chemical Co. Dimethyldioctadecylammonium bromide (DDOAB), sodium dextran sulfate (MW approximately 8000) and potassium polystyrene sulfonate (degree of polymerization 10 500) were obtained from Sago Pharmaceutical Co., Sigma Chemical Co., and Tokuyama Soda Co., respectively. All nonaqueous solvents were of spectroscopic grade. Water used.to prepare subphases was purified by a Milli-Q system. Theconcentrationof polymer in thesubphase was 10-4 M per repeat unit. Experiments were conducted on a Teflon Langmuir trough housed in a cabinet which was maintained at a temperature of 20 OC. Monolayers were formed by spreading a chloroform solution of the dye or a 2:1 molar mixture of the dye and DDOAB on to the appropriate subphase. The surface pressure was measured by the Wilhelmy plate technique, employing a 1.0cm-wide piece of filter paper suspended from a pressure transducer. Ten minutes was allowed for evaporation of the solvent prior to film compression. Absorption spectra at the air-water interface were recorded on an Otsuka ElectronicsMCPD-100 diode array ~pectrometer.1~ A bifurcated optical fiber was positioned 2-3 mm above the water surface and a mirror placed directly underneath the fiber at the bottom of the trough. Spectra were recorded during compression of the monolayer, the time to measure each spectrum being approximately 2 s.
Results and Discussion Surface Pressure Measurements. Surface pressurearea isotherms of monolayers of the hemicyanine dye on different subphases are shown in Figure 1. The isotherm of the dye on pure water shows considerable surface pressure at large areas indicating a significant electrostatic contribution to the pressure. On dissolved polymer subphases the monolayers are condensed, with a reduction in the electrostatic contribution to the pressure. The collapse pressures of the dye on the polymer subphases are lower than that of the dye on pure water. This is in contrast to previous reports which have indicated an improvement in stability 0 1993 American Chemical Society
Hemicyanine Aggregates in Air-Water Monolayers
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11975
0.0125 0.01
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Figure 1. Surface pressurearea isotherms of the hemicyanine dye (a) on pure water, (b) on sodium dextran sulfate solution (10-4M repeat unit), and (c) on potassium polystyrene sulfonatesolution (10-4M repeat
Figure 3. Absorption spectra of hemicyanine dye/DDOAB mixed monolayer (molar ratio 2:l) on pure water at pressures of 4, 16,25, 35, and 49 mN m-l. The arrow indicatesthe directionof increasingpressure.
unit). 0.035 0.03
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Figure 2. Absorption spectra of hemicyanine dye on pure water at pressures of 6, 10, 12, 14, and 35 mN m-l. The arrow indicates the direction of increasing pressure.
by polyioncomplexing.12J7~l* Also the limiting area per molecule of the film on the polystyrene sulfonate subphase (0.55 nmz molecule-l) is larger than that on pure water or dextran sulfate (0.36and 0.38 nm2 molecule-1, respectively). This suggests the polymer backbone of polystyrene sulfonate hinders the monolayer from achieving a close packed arrangement. Absorption Spectra. Absorption spectra of the dye on a pure water subphase a t various pressures are shown in Figure 2. At low pressure a single absorption maximum at 465 nm is seen, which has been observed previously and assigned to themonomer.19 As the surface pressure increases two new bands (maxima a t approximately 425 and 600 nm) grow in. These spectral changes are reversible as expansion of the monolayer results in an almost instantaneous reversion to the original spectrum. Moreover if the spreading and compression are repeated the two bands can be reproduced with the same intensity ratio. This and the observation that the two bands increase concurrently suggests they result from a single species. The shorter wavelength band has been observed previously in cast films at around 405 nm9J0.aandattributed to an H aggregate, while the 600-nm band has also been reported previously but no assignment was made.10 The discrepancy with regard to the position of the short-wavelength band between the present observations and earlier reports is not surprising as both the number and position of the aggregate bands of transferred films has been shown to vary markedly with preparation conditions.20.21 The present observations contrast with a recent study by Evans and Bohnll of a zwitterionic hemicyanine in which only a single hypsochromic band was seen. This might suggest that the present observation of two aggregate bands requires the postulate of two
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Figure 4. Absorption spectra of hemicyanine dye on sodium dextran sulfate solution at pressures of 2, 5, 13, and 40 mN m-l. The arrow indicates the direction of increasing pressure.
separate aggregates. Leaving aside the question of whether the same aggregate structure would be expected for different hemicyanines, other evidence to be presented later argues against this idea. Moreover as will be discussed later we postulate a different aggregate structure to that proposed by Evans and Bohn and one that is compatible with two bands. The absorption spectra of a mixed monolayer of dye and DDOAB at various surface pressures are shown in Figure 3. Similar behavior is observed to the pure dye monolayer; with monomeric absorption a t low pressure and two new bands a t higher pressure. The long-wavelength absorption peak of the aggregate is shifted relative to that of the pure dye monolayer, suggesting that the aggregate is sensitive to changes in the monolayer environment. Figure 4 displays absorption spectra of the pure dye monolayer complexed to dextran sulfate. At low pressure the spectrum is similar to that on pure water, and a broadening of the band to longer wavelengths is observed as the pressure increases. It is clear that no aggregate is formed, even a t high surface pressure, in contrast to themonomer-aggregate coexistenceon a pure water subphase. The absorption spectra of the monolayer complexed to polystyrene sulfonate (not shown) are very similar to those for the dextran sulfate complexed monolayer. These results support the proposition that highly ordered aggregates, Le., those which have a strong exciton interaction, are disrupted by complexation to a polymer. Aggregates with less order are not as susceptible to disruption, and complexation may even enhance chromophore interaction by condensing the monolayer. Curve Resolution. The concurrent growth of the two aggregate bands in the absorption spectra in Figures 2 and 3 suggest that
11976 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 TABLE I: Results from Separate Principal Factor Analysis of the Dve and Mixed Dve/DM)AB Svstems no. of components %variance IF eigenvalue ~~
1 2 3
4 5
94.262 99.834 99.968 99.998 100.000
Dye
2.6234E-5 9.1625E-6 1.1 159E-4 1.1510E-5
2.1303E-3 1.2591E-4 3.0205E-6 6.1683E-7 5.6416E-8
100
1
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Hall et al.
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I
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99 .
93.121 99.905 99.912 99.996
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2.699lE-5 6.5146E-6 9.8254E-6 2.0585E-5
1.8606E-3 1.3543E-4 1.3291E-6 4.9045E-7 1.1969E-8
these two bands are due to a single aggregate species. Principal factor analysis22 is a matrix manipulation method which can be used to estimate the number of absorbing species in a set of “combination” spectra. In focusing on the differences between the “combination” spectra, the analysis provides a number of eigenvalues and eigenvectors which each account for a certain amount of the variance in the data. Of these eigenvectors or factors, a few (the principal factors) contain all the spectral information while the rest contribute to the noise in the data. The number of principal factors corresponds to the number of components which contribute to the combination spectra. The nature of the pure components is contained in the principal eigenvectors, but further analysis is required to extract the pure component spectra. There are several indicators used to find the most probable number of principal factors. Firstly, the empirical indicator function (IF)22 is at a minimum when the correct number of factors is employed. Another commonly used criterion is the number of factors which produce greater than 99% of the variance.23~2~ A plot of the logarithm of the eigenvalueversus the number of factors can also give some indication of the number of significant factors. To determine the number of species present in each of the dye and mixed dye/DDOAB systems a principal factor analysis (PFA) of each set of absorption spectra was undertaken. In both cases, the quality of the eigen analysis was good as indicated by the performance index values (0.0720 and 0.0007 for the dye and dye/DDOAB systems, respectively) being considerablyless than one.25 Results of the analyses are presented in Table I. For both systems the number of species as determined by the minimum in the indicator function is two. The first two factors account for 99.8 and 99.9% of the variance for the dye and mixed systems respectively, and there is little variation in the logarithm of the eigenvalues for greater than two components, as illustrated in Figures 5 and 6. Thus the three absorption maxima observed in each system can be accounted for by two species, an aggregate band with two maxima and a monomer band. The principal factor analysis results cannot exclude the possibility of the coexistence of the monomer and two different single absorption band aggregates. However as was noted earlier theintensityratioof the two bandsobserved is constant in repeated experiments. Thus in the present case the postulate of two aggregates would require that the conversion of the monomer of each of the two aggregates as the monolayer is compressed be the same from experiment to experiment. This seems most unlikely. From the principal factor analysis the spectra can be well explained on the basis of two species-monomer and a single aggregate, without the need to introduce the notion of a second aggregate. Several methods are available for the generation of the purecomponent spectra from the principal eigenvectors. The presence of an aggregate spectrum with two maxima requires a curve analysis method which makes no assumptions about the spectral shape, so the self modelling curve analysis of Lawton and
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Fgue 5. Plot of percent variance and logarithm of the eigenvalue versus the number of components for the pure dye system. 100
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Figure 6. Plot of percent varianceand logarithmof the eigenvalue versus the number of components for the mixed dye/DDOAB system. aggregate j--monomer
c
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Figure 7. Area-normalized pure-component curves of the monomer and aggregate for the dye monolayer absorption generated by the self-modeliig analysis.
Sylvestre26 was used. For the pure dye system the two generated pure-component curves are shown in Figure 7. It is clear from the figure that the monomer component curve is almost the same as the absorption spectrum at low surface pressure, having a maximum of 463 nm compared with the measured maximum of 465 nm. The aggregate curve has two peaks, 423 and 601 nm, and can be explained by the exciton model of Hochstrasser and Kasha.27 In their application of excition theory to monomolecular lamellar systems the unperturbed excited state of the isolated chromophore is, in the lamellar aggregate, broadened into a band of levels. This excited-state resonance interaction between chromophores in the aggregate requires a considerable degree of both orientational and translational order in the monolayer,
The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11977
Hemicyanine Aggregates in Air-Water Monolayers nnidt "."' ' I
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Figure 8. Area-normalized pure-component curves of the aggregate for the dye and dye/DDOAB systems generated by the self-modelinganalysis.
implying a well-defined lattice structure. In the case of an aggregate based on a single molecule per unit cell this exciton band is symmetrically spread around the unperturbed excited state. Transition is permitted only to the upper or lower limit of the exciton band (depending on the orientation of the transition dipoles in the aggregate) leading to a single blue- or red-shifted absorption. For an aggregate containing two molecules per unit cell the exciton band of the aggregate may be asymmetrically positioned with respect to the original excited state and transition to two levels of the band is now allowed, leading to an asymmetric splitting of the spectrum into two peaks. The two absorption bands observed for the aggregate in the present work are thus indicative of an ordered lattice containing two translationally nonequivalent molecules per unit cell. The hemicyanine aggregate exhibits an asymmetric splitting, with the narrow blue band and broad red band being shifted from the monomer by 2040 and 4960 cm-I, respectively. The calculated exciton splitting of 7000 cm-1 is large compared to a cyanine dye-monolayer complex which produced a splitting of 25OO cm-1.28 However, a recent study of transferred hemicyanine monolayers2I found a monomeric and two aggregate absorptions at 18 800, 21 400, and 30 100 cm-l, respectively, indicating a very strong interaction between the two oscillating dipoles. This suggests that the high splitting energy observed is not unreasonable. The curve resolution for the dye/DDOAB system produced similar results, with the monomer component curve very similar to the absorption spectra at low compression and an aggregate band with two maxima. For this system the aggregate band maxima occurred a t 422 and 591 nm which gives a splitting of 6800 cm-1. The aggregate component curves from both systems are shown in Figure 8 for comparison. The significant difference in the band shape and position of the maxima between the pure dye and mixed monolayers indicates the sensitivity of the aggregate structure to the local environment. Closer inspection of the long-wavelength band in the aggregate component curves reveals considerable asymmetry. This may be due to a limitation in the self-modeling analysis which can distort the generated curve. An assumption applied to the self-modeling analysis is that each of the components must have an intensity of zero at a wavelength where the other component has finite intensity. The minimum in intensity at approximately 485 nm is expected considering the positions and shapes of the two bands; however, the intensity value of zero a t this point may result from the above requirement. That is, the basic shape of the generated curve is reasonable, but the depth of the minimum may be an artifact of the curve analysis procedure. To verify the generated curve obtained from the self-modeling analysis a curve analysis method which does not use this assumption, entropy minimization29830 was employed. For both the pure dye and the dye/ DDOAB systems, the entropy minimization analysis produced
aggregate curves that were almost identical to those obtained from the self-modeling analysis. Confirmation of the two molecule per unit cell aggregate by other methods is not easily accessible but some comparisons with other data are possible. An electron diffraction study of hemicyanine monolayers transferred to alumina and silanized silica substrates revealed both crystalline and disordered regions.31 The analysis produced a calculated area per unit cell of 0.249 nm2, indicating that the ordered portions of the film can be described in terms of a lattice with one molecule per unit cell. Other workers have reported the dissociation of the H aggregate during the transfer process.1° Thus the aggregate structure observed a t the air-water interface is unlikely to be preserved in multilayer films. Recent work by Peterson and c o - w o r k e r ~has ~ ~demonstrated ,~~ a complex liquid-crystal-like polymorphism in air-water monolayers of classical fatty acids. Earlier conclusions based on surface pressure area isotherms were later confirmed by grazing incidence X-ray diffraction studies. These studies confirmed the existence of surface structures with substantial orientational and translational ordering existing over the order of 50 lattice spacings. The X-ray data were explicable in terms of a single molecule per unit cell, although the authors noted that the expected weak reflections indicative of structures containing two molecules per unit cell would be difficult to detect. The exciton model invoked to explain our own observations is based on the notion of the existence of medium-range orientational and translational order in the aggregate. The extent of the ordered aggregate to produce such an exciton interaction has been estimated to be in the range 50-1000 m01ecules~~J~ The present results which are compatible with an ordered structure of two molecules per unit cell thus appear to provide some support for the X-ray data.
Conclusion In this paper the structure of the air-water hemicyanine monolayer was investigated by analysis of the absorption spectra. The usefulness of the principal factor analysis and associated curve resolution techniques has been demonstrated. The analysis reveals that the two aggregate bands are due to a single aggregate species. This exciton splitting corresponds to a monolayer lattice structure with two molecules per unit cell. This structure is considerably more complex than observed previously for the hemicyanine monolayer. When complexed with a polyion the film shows no aggregate absorption, indicating that the complexation of a charged monolayer with a polyion has a significant effect on the structure of the monolayer. Acknowledgment. The authors thank Dr. Trevor Smith for helpful discussions and the use of the curve resolution programs. R.A.H. acknowledges the receipt of an Australian Postgraduate Research Award. Financial assistance from the Australian Research Council and the Advanced Mineral Products Research Centre (University of Melbourne) is gratefully acknowledged.
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(8) Schildkraut, J. S.;Penner, T. L.; Willand, C. S.; Ulman, A. Opr. Lerr. 1988,13, 134. (9) Marowsky, G.; Steinhoff, R. Opr. Lett. 1988,13, 707.
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