Langmuir 1990, 6, 211-285
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Influence of Deposition Circumstances on the Spectroscopic Properties of Mixed Monolayers of Dioctadecyloxacarbocyanine and Arachidic Acid G. Biesmans, M. Van der Auweraer,* and F. C. De Schryver Chemistry Department, KULeuuen, Celestijnenlaan 200F, 3030 Leuven, Belgium Received September 9, 1988. In Final Form: August 2, 1989 Although examination of surface pressure-area isotherms for the pure and mixed monolayers of dioctadecyloxacarbocyanine and arachidic acid indicates an ideal mixing behavior, ample evidence exists for dye aggregation in these mixed mono- and multilayers. The spectroscopic properties such as absorption, emission, and excitation spectra and the relative fluorescence quantum yields depend strongly on the subphase pH and on the substrata on which the monolayers are deposited. A model, based on the interaction of the dissociated carboxylic acid groups, is proposed to explain these results.
Introduction During t h e last years, t h e interest in LangmuirBlodgett monolayers has increased c o n t i n u ~ u s l y . ~ - ~ Besides the possible technological applications as reviewed by R o b e r t ~ several ,~ groups have concentrated on the thermodynamic^,^,^ and other fundamental properties of these systems.'-' T h e experiments of Kuhn" e t al. revealed the large number of photophysical processes that could be studied by using Langmuir-Blodgett films in which octadecyl-substituted cyanine dyes and other chromophores were incorporated. The deposition of LangmuirBlodgett films on semiconductors proved to influence the (photo)electrochemical properties of the semiconductor solution interface." Deposition of suitable multilayer structures allowed one to obtain a n energy-harvesting system." Deposition of Langmuir-Blodgett films also influences charge generation and separation at the surface of a n insulating organic c r y ~ t a 1 . l ~ Comparison of the spectral properties obtained by different groups for monolayers of apparently identical c o m p ~ s i t i o n l indicates ~-~~ appreciable differences. This could possibly be due to different conditions for the deposition of the monolayer. The aim of this contribution is to examine the influence of the deposition conditions of mixed monolayers of dioctadecyloxacarbocyanine and arachidic acid on the spectroscopic properties. Dioctadecyloxacarbocyanine has been selected because of the (1) Thin Solid Films 1980,May 1,68. (2) Thin Solid Films 1983,Jan 14,99. (3) Thin Solid Films 1987,Sep 14,152. (4) Roberts, G.; Vincett, P.; Barlow, W. Phys. Technol. 1981,12,69. (5) Gaines, J., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966. (6) (a) Laxhuber, L.; Mohwald, H. Langmuir 1987,3,837.(b) Jones, J.; Webby, G. J . Phys. D 1987,20,226. (7) Mohwald, H.Springer. Proc. Phys. 1986,3,166. (8) Peterson, I.; Girling, I. Sci. Prog. (Oxford)1985,69,533. (9) Swalen, J. Thin Solid Films 1987,152, 151. (10) Kuhn, H.;Mobius, D.; Biicher, H. In Techniques of Chemistry; Weisberger, A., Rossiter, B., Eds.; Wiley: New York. 1972: Vol. 1. Part IIIB, p 571. (11) Roberts, G. Sensors Actuators 1983,4,131. (12) Arden, W.; Fromherz, P. J. Electrochem. SOC.1980,127, 370. (13) Van der Auweraer, M.; Willig, F. Isr. J . Chem. 1985,25,274. (14) Fujihara, M.; Nishiyama, K.; Yamada, H. Thin Solid Films 1986,132,77. (15) Bucher, H.;Drexhage, K.; Flech, M.; Kuhn, H.; Mobius, D.; Schafer, F.; Sondermann, J.; Sperling, W.; Tillmann, P.; Wiegand, J. Mol. Cryst. Liq. Cryst. 1967,2,199. (16) O'Brien, D.;Kelly, T.; Costa, L. Photogr. Sci. Eng. 1974,18, 76. (17) Wiegand, J., Thesis, Marburg, 1966.
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extensive and sometimes contradictory spectroscopic information13-17 existing on this dye and because it is often used as a sensitizer for charge-transfer processes in monolayer assemblies.
Experimental Section Dioctadecyloxacarbocyanine perchlorate (TBO) was prepared according to Sondermann.ls It is purified by column chromatography on silica with a mixture of chloroform and ethyl acetate as eluent. Afterwards the dye was recrystallized from acetic acid. Thin-layer chromatography did not indicate the M were prepared presence of impurities. Solutions of 5 X in chloroform(Janssen spectroscopicgrade). Arachidicacid (Janssen p.a.) was recrystallized twice from ethanol. The monolayers were prepared on a circular trough which allowed thermostatization of the subphase at 295 K (Mayer Feintechnik). The subphase was prepared from Milli-Q purified water to which 5 x M cadmium chloride (Aldrich gold label) was added. The pH was adjusted by using HC1 and NaOH p.a. The spread monolayers were allowed to equilibrate for several minutes before deposition on the substrata. As substrata, glass slides and glass slides covered on one side by SnO, were used. The glass slides (10 x 50 mm) were cleaned by heating for 2 h at 383 K in sulfochromic acid, followed by repeated rinsing in Milli-Q purified water and an overnight stay in a M solution of NaOH in Milli-Qpurified water. Before use, the glass slides were rinsed in Milli-Q purified water. The Sn0,-coated glass slides were purchased from Glaverbel, and their resistance was less than 14 Q/square. These slides were cleaned by 30 min of sonication in chloroform p.a. at 323 K followed by 30 min of sonication in 2-propanol at 323 K. Afterwards, the slides were rinsed in Milli-Q purified water and stored overnight at 373 K at a pressure of 1Torr. The monolayers were compressed at a speed of 6.4 cm2 s-'. All the monolayers were deposited at a surface pressure of 30 mN m-l and a dipping speed of 0.34 mm s-l. The monolayers were deposited on both sides of the glass slides. Three different monolayer assemblies were studied. In the first one, the mixed monolayer of TBO and arachidic acid or the pure TBO monolayer was sandwiched between two layers of arachidic acid (system I). The second one consisted of a dyecontaining layer deposited directly on the glass surface. On top of this layer, two more arachidic acid monolayers were deposited (system 11). In the third type, a TBO monolayer or a mixed monolayer of TBO and arachidic acid was deposited on top of two arachidic acid layers (system 111). The Sn0,-coated glass slides were only covered with a multilayer on the Sn0,-coated side. The dye-containing layer was in direct contact with the semiconductor layer and covered with two more layers of pure arachidic acid (system IV). Absorption spectra were recorded on a Perkin Elmer 550 S spectrophotometer. As reference, a glass slide coated on both (18) Sondermann, J. Liebigs Ann. Chem. 1971,749,183.
0 1990 American Chemical Society
278 Langmuir, Vol. 6, No. I , 1990
Biesmans et al. Table I. Area per Dye Molecule as a Function of Molar Fraction of Dye in the Mixed Monolayer X" A,, nmz u,* nm2 0.65 0.48 0.46 0.49 0.47 0.49
1 0.2 0.1 0.05 0.02 0.001
0.03 0.015 0.01 0.025 0.022 0.03
X = molar fraction of the dye in the monolayer. Standard deviation. arra/molttuie (nm'~
Figure 1. Surface pressure-area isotherms for different mixing ratios of TBO and arachidic acid: (-) X = 0.1; (.-) X = 0.05; (- - -) X = 0.02; (- -) X = 0.01. Inset 1: (--) X = 0.2, calculated curve; (- - -) X = 0.2, experimental curve. Inset 2: pure dioctadecyloxacarbocyanine.
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sides with three arachidic acid layers was used. Fluorescence emission and excitation spectra were recorded on a Spex Fluorolog. All emission spectra were corrected for background by subtracting the intensity measured from a glass slide coated with three layers of pure arachidic acid. The samples were excited at 455 nm (aggregate or dimer region) or at 485 nm (monomer region). The spectra were recorded at an angle of 90°, and the slide was positioned to make an angle of 45' with the incoming light to minimize reflection. In order to prevent photooxidation, a sample compartment was built in which the spectra could be obtained under reduced pressure (1 Torr). Knowing the integrated emission intensity and the absorption at the excitation wavelength, it was possible for all samples to determine the ratio of the fluorescence quantum yield versus that of the sample with the lowest TBO/arachidic acid mixing ratio. To determine absolute values for the fluorescence quantum yield of the monolayer, the following procedure was used. The fluorescence quantum yield of a IO-' and a IO-* M solution of TBO in chloroform (0.199) was determined by using rhodamine B in ethanol as a reference (quantum yield of 0.69)." The absorbance of the solution was measured in a 1cm path length cell. The solution was then transferred into a 1-mm path length cell, and the intensity of the emission was determined by using the same setup used for the monolayers. The quantum yields determined in this way were reproducible within 5% for systems I and 11. The quantum yields determined for system I11 were only reproducible within 30%.
Results Surface Pressure-Area Isotherms. The pressurearea isotherms obtained for mixed monolayers of TBO and arachidic acid depend upon the mixing ratio (Figure 1). The first inset compares, for a mixing ratio of TBO and arachidic acid equal to 1/4, the experimental curve and the curve calculated under the assumption that both components behave independently. The large difference between these two curves indicates an "ideal" mixing behavior of the monolayer-forming components." The second inset is the isotherm of the pure TBO layer. Table I gives the area per dye molecule, ADYE,at a surface pressure of 30 mN m-l. This area is considerably larger in the pure TBO monolayer than in the mixed monolayers. Absorption Spectra of TBO Monolayers. The absorption spectra of mixed dye layers sandwiched between two arachidic acid layers (system I) or directly deposited on glass (system II) are given in Figure 2 and Figure 3, respectively. For the same mixing ratio, the dimer absorption is considerably smaller in system 11. In Table 11, the ratios of the absorbance a t 465 nm to the absorbance (19) Parker, C.; Fleming, G.Chem. Phys Lett 1978,57, 526.
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Figure 2. Absorption spectra, for different mixing ratios, of X= dye layer between arachidic acid layers: (-) X = 1; 0.2; (- - -) X = 0.1; (--) X = 0.05. (-e)
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a t 495 nm are given for the three different systems and different mixing ratios in the monolayers. Emission and Excitation Spectra of TBO in Ethanol at 77 K. Figure 4 shows the excitation spectrum of a solution of TBO in ethanol a t 77 K for an emission wavelength of 520 nm a t three different concentrations of TBO. At M, a second maximum at 468 nm can be observed (Figure 4) in addition to a maximum at 490 nm, corresponding to the 0-0 transition of the monomer. The emission spectra (excitation a t 485 nm) recorded at 77 K depend upon the concentration of the dye (lo4, M). The lo4 M solution shows an intense and maximum at 498 nm and a less intense one a t 528 nm. Increasing the dye concentration by 1 order of magnitude leads to an increase of the maximum a t 528 nm although the maximum a t 498 nm remains the most intense. The maximum at 498 nm is also shifted slightly to longer wavelengths due to overlap of the emission spectrum of both emitting species (cf. infra), changes in the refractive index of the matrix, and reabsorption effects.
Langmuir, Vol. 6, No. I, 1990 279
Influence of Deposition on Mixed Monolayers Table 11. Ratio of Absorbance at 465 nm to Absorbance at 495 nm for Different Monolayer Assemblies X" Ib 11' IIId 1.27 2.11 1.83
1 0.2 0.1 0.05 0.02 0.002
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Furthermore, a t lo-' M a third maximum is observed a t 545 nm. Finally, the fluorescence spectrum of the M solution consists of a broad band positioned around 600 nm and a narrower and less intense band a t 498 nm together with an even less intense maximum a t 538 nm. This broad band has been assigned by Cooper to some kind of "excimer" emission.20 As discussed later in this contribution, one should be very careful when using the term "excimer" for this species. Fluorescence Spectra and Quantum Yields of TBO in Different Monolayer Systems Deposited at pH 5.5. The emission spectra of monolayers with different dye concentrations deposited a t pH 5.5 directly on glass and covered with two more layers of arachidic acid are given in Figure 5 (Figure 5a, excitaton a t 455 nm; Figure 5b, excitation at 485 nm). Already for the lowest dye content (X = 0.002) the emission maximum (510 nm) is shifted to longer wavelengths compared t o ethanol a t 77 K. This shift is due partly to the larger electronic polarizability of the monolayer and partly to the smaller rigidity a t room temperature allowing a more important geometric relaxation. An increase of the mixing ratio to X = 0.02 leads to a small shift of the emission maximum to longer wavelengths and to an increase of the fwhm from 1780 cm-' a t X = 0.002 to 1940 cm-'. This can be due to a further increase of the refractive index and to the overlap between the emission of the monomer and aggregates. A further increase of the mixing ratio to X = 0.1 shifts the maximum to 516 nm; furthermore, a second maximum and a broad band are observed a t respectively 550 and 595 nm. The emission spectrum of the (20) Cooper, W.;Liebert, N. Photogr. Sci. Eng. 1972,16, 125.
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Figure 5. (a, Top) Emission spectra for system I1 deposited at pH 5.5 with an excitation wavelength of 455 nm: (-) X = 0.02; (-) X = 0.002; (- - -1 X = 0.1; (--) X = 1. (b, Bottom) Emission spectra for system I1 deposited at pH 5.5 with an
excitation wavelength of 485 nm: (-1 X = 0.002; (--) X = 0.02; (- - -) x = 0.1; (- -) x = 1. -0.h
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Figure 6. Relative quantum yields versus dye content for systems I, 11, and I11 with the excitation wavelength set at 485 nm: (0) total yield of system I; ( 0 ) total yield of system 11; ( a ) total yield of system III; ( 0 )monomer yield of system I; (H) monomer yield of system 11. pure dye layer shows a broad band around 630 nm with a fwhm of 3970 cm-l, together with a small shoulder a t 550 nm. In Figure 6, the logarithm of the relative fluorescence quantum yields is plotted versus the logarithm of X,the molar fraction of the dye in the monolayer for the three different multilayer systems. The excitation occurred at 485 nm, where the monomer of the dye is excited preferentially. Increasing the amount of dye in the mixed monolayer, the ratio of the
280 Langmuir, Vol. 6, No. I , 1990
Biesmans et al.
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total fluorescence quantum yield to that of a diluted sample ( X = 0.002) decreases much faster for system I than for system 11. When it is assumed that all emission between 495 and 525 nm is due to the monomer of the dye it is also possible to determine the ratio of the fluorescence quantum yield of the monomer to that of a diluted sample. Upon increasing the amount of dye in the mixed monolayer, this latter ratio decreases to considerably lower values than that obtained for the total emission spectrum. Upon excitation at 455 nm, where the light is preferentially absorbed by the aggregates, analogous results are obtained, although for system I1 no further decrease of the total fluorescence quantum yield is observed when X becomes larger than 0.1. For system 111, the decrease of the fluorescence quantum yield is more important than for system I and system 11. This difference is considerably larger than the experimental errors on the ratios of the relative quantum yields, which can amount to 30% for system 111. Influence of pH on System 11 Layers. The emission spectra (excitation at 455 nm), of system I1 deposited a t pH 4, are given in Figure 7 for different dilutions. Compared to Figure 5, the same maxima are observed, but the broad band around 600 nm is already observed a t X = 0.02. When X is increased to 0.1, an "excimern band2' a t 630 nm with a weak shoulder at 510 nm is observed. For the pure dye layer at pH 4, this band becomes very broad (fwhm 4210 cm-l) and the maximum shifts to 650 nm. The excitation spectra obtained for different emission wavelengths a t a mixing ratio X = 0.1 for system 11, deposited at pH 4, are shown in Fig-
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x = 0.002; (.-) x = 0.02; (- - -) x = 0.1; (- -) x = 1.
ure 8. The excitation spectrum of the fluorescence a t wavelengths larger than 550 nm shows an intense maximum at 460 nm and a second maximum at 495 nm. In the excitation spectrum of the emission a t 525 nm, an increased contribution of the band at 495 nm is found. Figure 9 shows the emission spectra of system 11, deposited a t a pH of 7.52, for different dilutions (excitation a t 455 nm). When Figure 9 is compared to Figures 5 and 7, the broad emission due to dye aggregates is only found for more concentrated monolayers. The fwhm for X = 0.002 is only 1540 cm-'. Figure 10 shows the emission spectra of system I1 deposited a t pH 7.5 (Figure loa) and a t pH 4 (Figure lob),
Langmuir, Vol. 6, No. 1, 1990 281
Influence of Deposition on Mixed Monolayers
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at 455 nm. and IV. Figure 13 indicates that the drop of the fluorescence intensity is larger for system IV compared to system 11; this decrease is even more pronounced for excitation a t 455 nm.
Discussion Packing of TBO in the Monolayer. In the mixed monolayers, the area per dye molecule, calculated by using eq 1, is larger than the value that would be expected if both components could be compressed independently. In this case, the pressure-area isotherm should be given by 0.20L
eq
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= AAVGJXDYE - A A R A ~ A R A J ~ D Y E (1) where XDyE is the mole fraction of the dye in the mpnolayer, ADYE the area occupied by a dye molecule, AAVG the experimentally determined average area per molecule, A A R A the area occupied by an arachidic acid molecule, and nARA/nDyE the ratio of the number of arachidic acid molecules over the number of dye molecules in the monolayer ADYE
Figure 12. Quantum yield for the monolayer assembly of system I1 as a function of pH and dilution for exitation at 485 nm: (0) = 0.002; (0) x = 1.
x
when the excitation occurs a t 485 nm. In analogy to the spectra obtained by excitation a t 455 nm, the bathochromic emission becomes more important a t lower pH when samples with the same dye content are compared. In Figure 11, a plot of the logarithm of the ratios of the fluorescence quantum yield (excitation at 485 nm) to that of a diluted sample is plotted versus the pH of the subphase for different values of the mixing ratio (system 11). An increase of the pH leads to a decrease of the ratio. Excitation at 455 nm leads to less important decrease of this ratio upon increasing the pH of the subphase. Considering the experimental errors in the determination of these ratios, they are in this case within the experimental error almost independent on the pH but strongly dependent on the dye concentration. Figure 12 shows a plot of the absolute quantum yields as a function of pH for a diluted monolayer excited at 485 nm and a pure TBO monolayer excited at 485 nm (system 11). While the fluorescence quantum yield of the pure TBO monolayer (X = 1) is low and independent upon the pH, an increase of the pH leads to an increase of the fluorescence quantum yield of the dilute layer. TBO on SnO,. Figure 13 gives the logarithm of the ratio of the total fluorescence quantum yield to that of a dilute sample (X = 0.002) versus the logarithm of the mixing ratio of the dye and arachidic acid for systems I1
where ADYE' and AmA0 are the area occupied by respectively a dye molecule and an arachidic acid molecule in a pure monolayer. Table I indicates that the area occupied by a dye molecule does not depend upon the mixing ratio. Furthermore, ADYE is considerably smaller than ADYEO. This would indicate that mixing on the molecular level occurs ("ideal" mixing behavior). This is also indicated by the difference between the experimentally observed pressurearea curve and that calculated by using eq 2. The observed area per molecule of about 0.45 nm2 corresponds to that observed by Kuhn." This area is considerably smaller than any cross section of the dye molecule parallel to the long axis. This means that in the mixed monolayer the arachidic acid molecules intercalate between the octadecyl chains of the dye molecule. For the pure dye monolayer, the area per molecule amounts to 0.65 nm2. This area corresponds to a cross section parallel to the long axis and perpendicular to the short axis of the chro(21) Bucher, H.; v. Elmer, 0.;Mobius, D.; Tillmann, P.; Wiegand, J. Z . Phys. Chem. N.F. 1969,65, 152.
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Table 111. Influence of Molar Fraction of Dye in Mixed Monolayer, X,on Absorption Cross Section at the Maximum of the Monomers, QM, and the Aggregates, QA, Surface Density, and Concentration as a Function of Mixing Ratio in System 11.
the ratio of the absorbance a t 465 nm to that a t 495 nm has a maximum at X = 0.2; for this mixing ratio, the extent of the aggregation is larger than the highest extent of the aggregation observed for system 11. To rationalize those results, one must be aware of the fact that while in system I1 the dye chromophore is in direct contact 1 2.4 3.0 1.54 X 1014 1.28 with the hydrophilic glass surface, the two other systems 3.3 3.8 8.00 X IOl3 0.66 0.2 both bring the dye chromophore in contact with the acid 3.0 0.1 3.1 4.44 x 1013 0.37 (or salt) groups of the neighboring arachidic acid mono4.8 0.05 3.7 2.35 x 1013 0.20 layer. 2.7 9.76 X 1OI2 0.08 3.8 0.02 4.94 X 10l2 0.04 7.0 0.01 Comparing the emission spectra of the monolayers (Fig9.98 X 10" 0.008 5.3 0.002 ure 5b) with those obtained in ethanol glass reveals a 6.1 Etohd 3.6 close resemblance in aggregation behavior. One must, however, take into account t h a t in the monolayers a f l M , maximum of monomers; flA, maximum of aggregates. 'Number of dye molecules per unit area in the monolayer in described here the dye concentration is much higher than molecules cm-2. Three-dimensional concentration of the dye in the ethanol glass. For the pure dye monolayer, the molecules in the monolayer. Absorption cross section of TBO surface concentration, u, amounts to 1.54 X 1014 molein ethanol. cules cm-'; assuming a molecular height of 2 nm, this would correspond to a concentration of 1.3 M! The emismophore. This could be an indication for an edge-on orision maximum at 498 nm with a fwhm of 690 cm-* can entation of the dye molecule in the pure monolayer. As be assigned to emission of the monomers and comethis area is considerably larger than 2 times the cross sponds to the maximum a t 510 nm (fwhm 1780 cm-') section of the alkyl chains, the alkyl chains should be observed for the emission of monolayers deposited at pH tilted in the pure dye monolayer. The different chro5.5. The broadening of the band is mainly due to the mophore packing in the pure dye layer could certainly higher temperature for the monolayer systems. Howinfluence the spectroscopic properties of the monolayer ever, the emission spectrum of system I1 deposited a t and lead to a difference between the spectroscopic proppH 7.52 reveals a fwhm of 1540 cm-'. Systematic expererties of a pure dye layer and that of a mixed monolayer. iments point to a gradual decrease of the fwhm when Probably this tilted orientation of the alkyl chains will the pH of the subphase for monolayer deposition increases. prevent a brick wall packing leading to J - a g g r e g a t e ~ . ' ~ * ~ ~ -This ~ ~ indicates a decreasing aggregation of the dye molSpectroscopic Properties of the TBO Systems. ecules when the pH increases. The emission maxima a t Starting from the absorption spectra of the monolayers, 550 and 600 nm, which are obtained when the dye conone can calculate the absorption cross section of the dye tent is increased, can be ascribed to dimers and higher molecules. The values of this cross section a t the maxiaggregates. The species emitting a t 600 nm is probably mum of the monomer and of the aggregates are given in a very large H-aggregate rather than an "excimer". In Table I11 for different dilutions assuming a random orithis excimer," only two dye molecules would be responentation of the transition dipoles (Table 111). If there sible for this emission, and therefore it is difficult to underare no interactions between the chromophores or between stand why the emission of the dimer would change to chromophores and substrata, these cross sections should excimer emission if the dye content increases. Furthercorrespond to that of TBO in ethanol. Table I11 indimore, it is incorrect to call the species emitting a t 600 or cates that this is only the case for the highest dilutions 650 nm an excimer since the excitation spectrum differs ( X < 0.01). This is an indication for the formation of as well in the monolayer as in ethanol a t 77 K from the dye aggregates. Also, the ratio of the absorbance a t 495 absorption spectrum of the monomer. These changes of and 465 nm (Table 11)indicates that aggregates are formed the emission spectra parallel the increase of the absorwhen the molar fraction of the dye in the monolayer is bance a t 465 nm and the decrease of the absorbance a t increased. From the absorption spectra, it can be con495 nm when the concentration of the dye in the monocluded that the monomer and the dimer have an absorplayer is increased. The large shift of the emission (emistion maximum a t respectively 495 and around 465 nm. sion maximum a t 650 nm) of a M solution a t 77 K According to K a ~ h a ?the ~ absorption maximum of the or of the concentrated monolayers suggests the formainfinite H-aggregate would be expected around 438 nm tion of aggregates larger than the dimers. At 77 K, the if the intermolecular interaction between two neighborfwhm of the emission a t 650 nm amounts to 2680 cm-', ing chromophores would have the same strength as in a which is considerably smaller than the fwhm (4210 cm-') dimer. Although the absorption maximum of the dimer of the emission of the pure monolayer system a t 298 K. and the shape of the absorption band change as the conThis could indicate that in the pure monolayer several centration is changed, even a t the highest concentraemitting species are present. tions only a shoulder a t 430 nm is observed, indicating Using attenuated total reflection, Memming= obtained that only a fraction of the dye molecules is present in an for mixing ratios below 1/16 only an absorption maxi"infinite" aggregate. Even if larger aggregates than dimer mum a t 495 nm which he considered to be due to the or trimers would be formed, an important fraction of the monomer of the dye. Only for larger mixing ratios, the dye molecules is only involved in dimers or trimers. presence of aggregates was observed in the absorption A difference in aggregation behavior is observed for spectrum. the different systems. While system I1 shows a gradual Killesreiter observed that dimer formation, which was decrease of the dimerlmonomer absorbance ratio, this is indicated by an absorption maximum a t 455 nm, occurred not the case for systems I and 111. In the latter systems, to the largest extent a t a mixing ratio of 1/5.27 It is, however, difficult to compare the results obtained by dif(22) Dietz, F. Tetrahedron 1972,28, 1403. (23) Czikkely, V.; Forsterling, H.; Kuhn, H. Chem. Phys. Lett.
1970,6, 11. (24) Steiger, R.; Kitzing, R.; Junod, P. J, Photogr. Sei. 1973,2, 107. ( 2 5 ) McRae, E.; Kasha, M. J. Am. Chem. SOC.1958, 78, 721.
(26) Memming, R. Faraday Discuss. Chem. SOC. 1974,85, 261. (27) Killesreiter, H. Faraday Discuss. Chem. SOC.1974, 85, 271.
Influence of Deposition on Mixed Monolayers ferent authors when the subphase pH and deposition conditions are not mentioned. Costa et a1.16 assigned the emission maximum a t 510 nm of mixed monolayers of dioctadecyloxacarbocyanine and arachidic acid deposited on top of arachidic acid layers (mixing ratio 1/100) to the monomer of the dye. Increasing the mixing ratio to 1/ 10 led to a maximum a t 535 and at 610 nm; both maxima were attributed to the emission of aggregates. This increase of the dye content in the layer also decreased the relative quantum yield of the pure dye layer to 9% of that of a sample with a mixing ratio 1/100. Memming observed for the emission spectra of the diluted monolayer (mixing ratio 1/16) deposited on glass a maximum a t 520 nm and a shoulder around 555 nm. For a mixing ratio of 1/1,an intense bathochromic emission centered a t 615 nm was observed. This emission could not be due to phosphorescence, as the very weak phosphorescence had a maximum around 700 nm. Memming called the species emitting at 615 nm an "excimer" because the observed shift was much larger than 2 times the shift of the dimer emission; the emission at 615 nm corresponds to a shift of 3500 cm-' versus the emission maximum of the monomer while the shift of the dimer emission amounts to only 1250 cm-l. Kasha" calculated that the shift of the absorption maximum of the infinite aggregate must be 2 times the shift observed for the dimer absorption. Depending on the orientation of the transition dipoles, this shift can either be hypsochromic or bathochromic compared to the monomer absorption maximum. If the interactions between the chromophores are limited by exciton interaction and if interand intramolecular relaxation occurs to the same extent in both species, the shift of the emission maximum of the infinite aggregate should be twice the shift observed for the dimer emission. The anomalous bathochromic shift observed for the emission spectrum of the concentrated monolayers could then indicate that in the large H-aggregates, formed in those systems, the interchromophore interaction is not limited to exciton coupling. One could propose as an alternative explanation that in the very concentrated monolayers a different packing of the dye molecules allows for a different and stronger exciton interaction. If this interchromophore interaction is strong enough to modify the potential hypersurfaces along some intermolecular coordinate appreciably, one can expect that this interaction will modify those hypersurfaces differently for the ground state and the singlet excited states with an in-phase and an out-ofphase combination of the transition dipoles. Such strong interactions could lead to a bathochromic shift of the emission spectrum that is much larger than the hypsochromic shift of the absorption spectra and to a very broad emission band. Although this argumentation is plausible for the Langmuir-Blodgett films, it is difficult to imagine why this kind of aggregate, with a different packing, is formed in a M ethanol solution, where the average distance between the chromophores amounts to 158
A.
Comparison of the Different Monolayer Systems. In rationalizing the results obtained for systems I-IV, one must be aware of the presence of at least three different emitting species, such as the monomer at 510 nm, the dimer at 545 nm, and a larger aggregate at 585 nm. Under the broad band centered a t 585 nm, the emission of other species emitting at long wavelengths can be hidden. All these different species may have different flu-
Langmuir, Vol. 6, No. I , 1990 283 orescence quantum yields and are able to quench the monomer emission with different efficiency. Figure 6 indicates that the total fluorescence quantum yield decreases when the surface density of dye molecules increases. This quenching process can be due to the fact that inefficient fluorescing aggregates compete with monomers for the absorbed photons and to very efficient energy transfer toward these traps via a hopping mechanism.28 The small quantum efficiency observed for the fluorescence of the dimers and aggregates is partly due to the small transition dipole moment from the lowest excited state to the ground state proposed for a sandwich dimer.25.29 As the excited monomers are able to transfer their excitation energy toward the dimers and eventually to the higher aggregates, while the dimers on the contrary can only lose their energy to the higher aggregates, the emission of the monomers will be quenched to a larger extent than that of the dimers when the amount of dye in the monolayer is increased. Also, the small oscillator strength and fluorescence quantum yield of the longwavelength transitions of the dimer and the aggregates will make Forster transfer between different aggregates less efficient. When only the monomer and the dimer contribute to the emission, the total fluorescence quantum yield would be given by eq 3
0 = (1- 4ET)4M + C Y ~ +D (1- C Y ~ E T ~ D(3) where 9 is the total quantum yield of emission, 4Mthe quantum efficiency of monomer emission, 4D the quantum efficiency of dimer emission, the quantum efficiency of energy transfer from monomer to dimer, and CY the fraction of light absorbed by the dimers. When CY is small (mainly monomers present), predominantly monomer emission will be observed. Van der Auweraer and Willig13 observed for monolayers deposited at pH 4.7 negligible energy transfer to dimers as long as X was smaller than 0.02. Their results are in agreement with those of Wiegand." However, Drexhage,15 who used a subphase pH of 5.3, observed dimer formation only a t a higher mixing ratio ( X > 0.01). As soon as a becomes more important, dimer emission will become predominant due to direct excitation of the dimers and to excitation energy transfer from monomers to dimers. However when the quantum efficiency of the dimer emission is smaller than the quantum efficiency of the monomer emission,25 increasing the amount of dye in the monolayer will lead to a drop of the total quantum yield. When all excitation energy flows into the excited-state aggregates and if the fluorescence quantum yield does not differ strongly for the different aggregates, the total fluorescence quantum yield will no longer decrease upon a further increase of the amount of dye in the monolayer. Such a behavior was observed for system I1 for X > 0.05 upon excitation a t 455 nm or for X > 0.2 upon excitation at 485 nm. A parallel behavior was observed for the sensitized hole injection into anthracene single c r y ~ t a l s 'for ~ X > 0.1 upon excitation at 500 nm. When the sample is excited at 485 nm (mainly monomers are excited), the monomer emission is quenched more strongly in system I than in system I1 (Figure 6). The fwhm values are 2230 and 1820 cm-I for system I and system 11, respectively ( X = 0.002). The smaller fwhm (1540 cm-l), observed for the emission spectrum of a monolayer ( X = 0.002, system 11) deposited a t pH 7.52, indi@
J
~
~
(28) Willig, F.; Blumen, A.; Zumofen, G. Chem. Phys. Lett. 1984, 108, 222. (29) Davydov, A. Theory and Molecular Excitons; McGraw-Hill: New York, 1962.
284 Langmuir, Vol. 6, No. 1, 1990
Biesmans et al.
cates that in monolayers deposited at pH 5.5 of system I and system I1 aggregation occurs even in very diluted monolayers (X= 0.002). Due to the more extensive aggregation in system I, more light will be absorbed directly by the aggregates, and energy transfer to the aggregates will be more efficient. Both factors will lead to a smaller value for the total fluorescence quantum yield. Considering system 111, where the chromophore is also in contact with the polar head groups of arachidic acid, the decrease of the total fluorescence quantum yield is comparable with that in system I. The smaller reproducibility observed for system I11 does not allow a detailed discussion of the different behavior that is apparently observed for excitation at 455 and 485 nm. This smaller reproducibility is probably due to the fact that in system I11 the dye is incorporated in the top layer. It has been observed that the uppermost layer3' of a monolayer assembly contains a larger amount of defects and that these defects disappear if another layer is deposited. In system 111,the leveling of the fluorescence quantum yield occurs a t a much higher surface density of the dye than in system 11, and it is accompanied by a decrease of the ratio of the absorbances at 465 and at 485 nm for the pure dye layer (cf. Table 11). This spectral change indicates that the packing of the dye chromophores in the pure layer differs from that in the mixed layers. The only emission observed in the pure dye monolayer for excitation at 455 or 485 nm is due to the aggregates. When comparing the influence of the surface density of the dye molecules upon the fluorescence quantum yield of the different monolayer assemblies investigated, one should take into account that in system I1 direct contact between the glass surface and the dye chromophore occurs. This could lead to fluorescence quenching31 of the dye and therefore to a less efficient energy transfer and a slower decrease of the relative quantum yield when the mole fraction of the dye is increased. The difference in the behavior of system I or I11 and system I1 can also be due to a different packing of the first deposited The amount of cadmium ions transferred with the first deposited layer is very small compared to the amount transferred with the consecutive layers.6e The condensing effect of cadmium on an arachidate monolayer has already been recognized by several group^.^ The cadmium ions will lead in systems I and I11 to a tighter packing of the arachidate molecules and will decrease the miscibility between the dye molecules and the arachidate molecules. When systems I1 and IV are compared, a faster and more extensive drop of the fluorescence intensity is observed for system IV (Figure 13) when for both systems the slide with X = 0.002 was used as a reference. For X = 0.002, the observed fluorescence intensity is larger for system IV than for system 11. These phenomena can be understood considering the difference in surface structure for the two systems. As shown by Armstrong et al.,33the surface of SnO, is strongly pH dependent; it is characterized by an equilibrium between dissociated and nondissociated hydroxyl groups at the surface. Furthermore, during the deposition process a hydratation layer
will be formed on the surface of the s e m i c o n d ~ c t o rThe .~~ more extensive decrease of the fluorescence quantum yield observed for system IV can be due to efficient quenching of the dimer emission by the s e m i c o n d ~ c t o r . ~ This ~,~~ also explains why for system IV the fluorescence quantum yield does not level for X > 1. Influence of Subphase pH. The band broadening of the emission of monolayers (system 11) observed upon lowering the pH of the subphase was already mentioned in the previous section. This is a first indication for the aggregation induced by a decrease of the subphase pH. At pH 5.5 or lower, the fwhm indicates that aggregation already occurs a t low surface densities of the dye (X= 0.002). Figure 10 shows the spectroscopic changes occurring in monolayers of the pure dye as a function of pH. Even for X = 1, changing the subphase pH induces a change in the extent of the aggregation of the pure dye layer. Whereas the species emitting at 600 nm is formed at all values of the pH; higher values of the subphase pH lead to a larger amount of smaller aggregates and eventually dimers coexisting with these large H-aggregates. Figure 11 indicates that the energy transfer to the aggregates occurs over the whole pH range studied. The strongest effect of dye content on the emission of the monolayer can be seen at the highest pH. At the highest subphase pH, aggregaton occurs to the smallest extent. This will lead to a less important energy transfer to the dimers in a highly diluted sample deposited a t a high pH than in a highly diluted sample deposited at a lower pH. This explains why the fluorescence quantum yield of diluted samples (X= 0.002) increases when the subphase pH is increased (Figure 12). On the other hand, as the fluorescence quantum yield of the aggregates in the concentrated samples does not depend upon the subphase pH (Figure 12), increasing the amount of dye in the monolayer (from X = 0.002 to X = 1) will lead to a more extensive decrease of the relative quantum yield. As the pK, of the long-chain fatty acid molecules amounts to 5.5 in a Langmuir-Blodgett changing the subphase pH from 4 to 7.5 will change the ionization degree of the arachidic acid groups to a large extent. As a consequence, more negative charges of the dissociated acid groups are incorporated in the layers deposited at pH 7.5 compared to those deposited a t pH 4. As the tendency to incorporate Cd2+ ions in the first layer is rather these negative charges will be neutralized by monovalent cations or cationic chromophores. At high pH, the dye cations will mainly be neutralized by arachidate anions. One can except that due to Coulomb attraction the interaction between arachidate anions and dye cations will be more exothermic than that between arachidic acid molecules and dye molecules. Therefore, one can expect the arachidate ions to mix to a larger extent with the dye cations than the neutral arachidic acid molecules. This will counteract the aggregation of the dye cations. On the other hand, at low pH the dye cations will be neutralized mainly by small inorganic anions that do not prevent the aggregation. This rationalization of pH effect implies that in the first monolayer only a small number of cadmium cations is incorporated. If the cad-
(30) (a) Skita, V.; Richardson, W.; Filipkowski, M.; Garito, A.; Blasie, J. J. Phys. (Les Ulis, Fr.) 1986, 47, 1849. (b) Fuchs, H.; Schrepp, W.; Rohrer, H. Surf. Sci. 1987, 181, 391. (31) Kemnitz, K.; Murao, T.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. Chem. Phys. Lett. 1983, 101, 337. (32) Dierker, S.; Murray, C.; Legrange, S.; Schlatter, N. Chem. Phys. Lett. 1987, 137, 453. (33) Armstrong, N.; Lin, A,; Fujihara, M.; Kuwana, T . Anal. Chem. 1976, 48, 741.
(34) Oelkrug, D.; Fleming, W.; Fulleman, R.; Gunter, R.; Honnen, W.; Krabiekler, G.; Schiifer, M.; Uhl, S.Pure Appl. Chem. 1986, 58, 1207. (35) (a) Vogel, V.; Mobius, D. Thin Solid Films 1985, 132, 205. (b) Vogel, V.; Mobius, D. Third International Conference on LangmuirBlodgett Films; 1987, Gottingen, F.R.G. (36) (a) Tomoaia-Cotisel, M.; Zsakd, J.; Mocanu, A.; Lupea, M.; Chifu, E. J. Colloid. Interface Sci. 1987,117,464. (b) Kobayashi, K.; Takaoka, K.; Ochiai, S. Third International Conference on Langmuir-Blodgett Films; 1987, Gottingen, F.R.G.
Langmuir 1990,6, 285-288
mium ions would be transferred with the first layer, an increase of pH would lead to an increased formation of cadmium arachidate rather than to a neutralization of the dye cations by the arachidate anions. If this would happen, the increase of the pH would not lead to a less extensive aggregation of the dye when the subphase pH is increased'.
Conclusions The deposition conditions influence the spectroscopic properties of mixed monolayers of dioctadecyloxacarbocyanine and arachidic acid. The fluorescence is quenched by energy transfer to dimers and higher aggregates formed upon increasing the dye content in the monolayer. In a multilayer where the chromophores are in contact with the cadmium arachidate or arachidic acid groups of another monolayer, an increase of the dye aggregation is observed. This aggregation occurs on a molecular scale as it is not revealed in the surface pressure-area isotherms. At least two different types of aggregates can be distinguished and can be compared to the ones formed in an ethanol matrix at 77 K. The spectroscopic properties of the dye
285
layer are strongly influenced by the deposition pH.37 An increase of the pH shifts the equilibrium between arachidic acid and arachidate and induces a more efficient mixing between the dye and the matrix (arachidic acid or arachidate). This reduces in turn the extent of the aggregation of the dye molecules. As mixed monolayers are used in microelectronic devices, it will also be important to choose experimental conditions at which the monolayer is deposited very carefully. Upon investigation of such systems, one should always try to determine the organization at the molecular level. If any photochemical or photophysical properties of such systems are investigated, one should always consider eventual intermolecular interactions (aggregation) of the chromophores involved.
Acknowledgment. M.V.d.A. is a research associate of the FKFO. We thank the Belgian Ministry of Scientific Programmation, the University Research Fund, and the FKFO for financial support to the laboratory. (37) Lehmann, U. Third International Conference on LangmuirBlodgett Films; 1987,Gottingen, F.R.G.
Notes Cis-Trans Photoisomerization of a Surfactant 0-Protonated Stilbazolium Betaine in Micellar Systems Calum J. Drummond,*>+ J. Justin Gooding,' D. Neil Furlong,' and Franz Grieserf CSIRO, Division o f Chemicals and Polymers, Private Bag 10, Clayton, Victoria, 3168, Australia, and Department of Physical Chemistry, T h e University of Melbourne, Parkville, Victoria, 3052, Australia Received J u l y IO, 1989
Introduction Recently, much attention has been focused on the cistrans photoisomerization of surfactant derivatives of stilbene and azobenzene when located in assembled surfactant phases, particularly micelles,'V2 vesicles (liposomes/ bilayers),'-4 monolayers spread at the air/water and Langmuir-Blodgett films deposited on optically transparent substrate^.^-^ A number of the azobenzene studies were instigated because it is presently thought that some surfactants which can undergo CSIRO. T h e University of Melbourne. (1) Russell, J. C.;Costa, S. B.; Seiders, R. P.; Whitten, D. G. J. Am. Chem. SOC.1980,102,5679. (2) Suddaby, B. R.; Brown, P. E.; Russell, J. C.; Whitten, D. G. J. Am. Chem. SOC.1985,107,5609. (3) Kano, K.; Tanaka, Y.; Ogawa, T.; Shimomura, M.; Okahata, Y.; Kunitake, T. Chem. Lett. 1980,421. (4) Shimomura, M.; Kunitake, T. J. Am. Chem. SOC.1987,109,5175. (5) Fukuda, K.; Nakahara, H. J. Colloid Interface Sci. 1984,98, 555. ( 6 ) Mooney, W. F.; Brown, P. E.; Russell, J. C.; Costa, S. B.; Pedersen, L. G.; Whitten, D. G. J. Am. Chem. SOC.1984,106,5659. (7) Morgan, C. G.; Yianni, Y. P.; Sandhu, S. S. In Conducting Polymers: Special Applications; Alcacer, L., Ed., D. Reidel: New York, 1987;p 179.
0143-1463f 9012406-0285$02.50/0
reversible cis-trans photoisomerization may be able to serve as components in Lan muir-Blodgett thin-film molecular electronic devices? There have been relatively few studies of other types of molecular cis-trans photoisomers that possess long hydrocarbon chains. A diverse range of long-chain homologues and analogues of the 0-protonated stilbazolium betaine shown in Scheme I can be easily s y n t h e s i ~ e d , ~ and - l ~ the stilbazolium betaine chromophore can be covalently attached to polymer^.'^,'^ Long-chain homologues of this 0-protonated stilbazolium betaine can be spread to form insoluble monolayers a t the air/water interface,16-ls and the preparation of Langmuir-Blodgett thin films based on surfactant derivatives of this 0-protonated stilbazolium betaine is reasonably straightfor~ard.'~-~~ Notably, Langmuir-Blodgett films of the C,, merocyanine (i.e., the C,, deprotonated) form have been found to exhibit very high second-order nonlinear optical susceptibilities, x2.19920In (8) Minch, M. J.; Shah, S. S. J. Chem. Educ. 1977,54,709. (9) Minch, M. J.; Shah, S. S. J. Org. Chem. 1979,44,3252. (10) Donchi, K. F.;Robert, G. P.; Ternai, B.; Derrick, P. J. Aust. J. Chem. 1980,33,2199. (11) Gruda, I.; Bolduc, F. J. Org. Chem. 1984,49,3300. (12) Dion, F.;Bolduc, F.; Gruda, I. J. Colloid Interface Sci. 1985, 106. 465. (13) Drummond, C. J.; Grieser, F.; Healy, T. W. J . Phys. Chem. 1988,92,2604. (14) Kobayashi, T.;Morishima, Y.; Nozakura, %-I. J. Polym. Sci. 1987,25A, 2839. (15) Morishima, Y.; Kobayashi, T.; Nozakura, S.-I. Macromolecules 1988,21,101. (16) Gaines, G.L.Anal. Chem. 1976,48,451. (17) Godfrev, J. S. Doctoral Dissertation, The University of Melbourne, 1987. (18) Daniel. M. F.: Smith. G. W. Mol. Crvst. Lio. . Crvst. - 1984. IO2(Lett.), 193. (19) Girling, I. R.;Cade, N. A.; Kolinsky, P. V.; Montgomery, C. M. Electron. Lett. 1985,21, 169. (20) Girling, I. R.; Kolinsky, P. V.; Cade, N. A.; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985,55,289.
0 1990 American Chemical Society
