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Langmuir 1992,8, 1366-1371
Behavior of Pyrene in Air/Water Monolayers of Eicosanoic Acid Erik Wistus, Emad Mukhtar, Mats Almgren,' and Sten-Eric Lindquist Department of Physical Chemistry, University of Uppsala, Box 532, 5'-751 21 Uppsala, Sweden Received October 25, 1991. In Final Form: January 27, 1992
Mixtures of pyrene and eicosanoic acid have been studied by surface pressure-area (r-A) isothermsand steady-state and time-resolved fluorescence spectroscopy. Addition of pyrene in a monolayer at the airwater interface did not affect the area of the film.Steady-state fluorescence spectra showed only monomer emission even at very high pyrene concentrations. Time-resolved fluorescence spectroscopy showed that decays were described by a single exponentialfunction. Addition of iodide ions in the subphase quenched the fluorescence strongly and this quenching was found to be independent of the degree of compression. These observations suggest that the pyrene molecules are located at the water-fatty acid interface.
Introduction In the early 1960s Kuhn and co-workers pioneered the use of Langmuir-Blodgett (LB) films for designing units with photophysical behavior dependent upon a supermolecular organization of functional molecules.1 Numerous spectacular photophysical effects were early demonstrated,2 but it took a long time before a real interest awakened for the opportunities these organized units offered in a variety of possible application^.^ The new enthusiasm then created has now calmed down, quenched by the buckets of problems thrown in by the harsh reality. A more realistic attitude has been reached, and the lack of fundamental knowledge about the LB film, and its precursor, the monolayer at the aidwater interface, is well recogni~ed.~?~ Beside the obvious problem of getting stable films which are only a few molecules thick, one of the worst obstacles in the utilization of LB films has been the tendency of active molecules,even when present at low concentrations, to aggregate into dimers or other inactive oligomers. This is a problem which appears already in the monolayer, as do also the segregation of the components into separate phases when a mixture of different amphiphiles is used. The segregation, which may be more or less clearly seen already from the P A profiles, has been beautifully demonstrated in fluorescence microscopy work.6 The reason for the segregation, and the increased tendency to aggregation as compared to normal solutions, is clearly due to the solidlike character of the monolayers: the long saturated alkyl chains, which normally are employed as hydrophobic tails in the molecules forming the monolayer are not melted, with a mobility as in a liquid, but pack into ordered arrays. The best solvent properties would be expected when chains with some unsaturation are used, and a liquid-expanded type of monolayer is present up to reasonably high film pressures. In any case, a prerequisite for the formation of LB films (1) Zwick, M. M.; Kuhn, H. 2.Naturforsch., A 1962, 17a, 411; Pure Appl. Chem. 1965, 11. (2) Kuhn, H. PhysicalMethodsof Chemistry;Weiasberger, A., Rossiter, B., Eds.; John Wiley and Sons, Inc.: New York, 1972; Vol. 1, part 3b. (3) Barlow, W.A,, Ed. Langmuir-Blodgett Films. Thin Films Science and Technology; Elsevier Scientific Publishing: Amsterdam, 1980. (4) Swalen, J. D.;Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, s.; Israelachvilli, J.; McCarthy, T. J.; Murray, R.; Pease, F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (5)Langmuir-BlodgettFilms. Current status and prospects for further development, Vol. 1; Report of the Langmuir-Blodgett Working Party, Science & Engineering Research Council, U.K., 1984. (6) Losche, M.; MBhwald, H. Rev. Sci. Instrum. 1984,55, 1968.
with desired properties is that the precursor monolayer have a controlled composition and homogeneity. Measurements of Forster energy transfer in the monolayer7t8 can give information on the aggregation into dimers and small clusters, and investigations of fluorescence quenchingg and excimer formationlo*" report on the mobility of probes in the monolayer. These investigations have employed amphiphilic derivatives with long hydrophobic anchors; the pyrene chromophore has often been utilized due to its long lifetime and excimer formation. Pyrene itself has long been used as a probe in membranes, micelles, and other self-assembled structures. In its underivatized form it has a further function as a polarity probe, due to the sensitivity of the intensity distribution in the fluorescence spectrum to the environment, the socalled I11 to I ratio.12J3 Pyrene adsorbed on the surface of hydrophobized silica particles (porous silica particles for chromatography,chemically modified by reaction with n-alkylchlorosilanes with 2, 8, or 18 carbons in the alkyl chain), which were suspended in water or water-methanol mixtures, reported a rather hydrophobic environment, in water corresponding to l - o c t a n ~ l .Furthermore, ~~ it was shown that excimer formation occurred dynamically, with pyrene mobile in a liquid-like environment.16 It appeared worthwhile to try underivatized pyrene as a probe in monolayers at the aidwater interface as well. High concentrations of pyrene could readily be accommodated in fatty acid monolayers but, surprisingly,showed no evidence of excimer formation and indicated clearly a very hydrophilic environment at all surface pressures. An investigation of this peculiar behavior is reported in this contribution. Experimental Section Materials. Eicosanoic(arachidic)acid (henceforthcalled EA), dipalmitoylphosphatidylcholine(DPPC), and dioleylphosphati(7) Urquhart, R. R. S.; Hall, A.; Thistlethwaite, P. J.; Grieser, F. J. Phys. Chem. 1990,94,4173. (8)Urquhart, R. S.; Grieser, F.; Thistlethwaite, R.; Wistus, E.; Almgren, M.; Mukhtar, E. Submitted for publication in J. Phys. Chem. (9) Caruao, F.; Grieser, F.; Murphy, A.; Thistlethwaite, P.; Urquhart. R.; Almgren, M.; Wistus, E. J. Am. Chem. SOC.1991,113,4838. (10) Yliperttula, M.;Lemmetyinen,H. Chem. Phys.Lett. 1988,152,61. (11) Vanderkooi, J. M.; Fischkoff, S.; Andrich, M.; Podo, F.; Owe, S. J. Chem. Phys. 1975,63, 3661. (12) Kalyanasundaram, K.;Thomas, J. K. J. Am. Chem. SOC. 1977,99, 2039. (13) Nakajima, A. Bull. Chem. SOC. Jpn. 1971,44, 3272. (14) Almgren, M.; Medhage, B.; Mukhtar, E. J. Photochem. Photobiol. A: Chem. 1991, 59, 332. (15) Stihlberg, J.; Almgren, M. Anal. Chem. 1985, 57, 817. (16) Stihlberg, J.; Almgren, M.; Alsins, J. Anal. Chem. 1988,60,2487.
0743-7463/92/2408-1366$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8, No. 5, 1992 1367
Pyrene in AirlWater Monolayers dylcholine (DOPC) was purchased from Sigma. Dihexadecyldimethylammonium bromide (DHDAB) was obtained from Sogo Pharmaceutical Co., Ltd. These chemicals were used without further purification. Pyrene, obtained from Aldrich, was recrystallized twice from ethanol. The chemicalswere dissolved separately in chloroform (Merck, Uvasol) to 1mM solutions and these solutions were mixed to the desired molar ratio. The mixed solution was spread onto a pure water surfacewith a Hamilton syringe. The water used was deionized and distilled water purified by a three-step filtering system (MilliQ)from Micropore. The conductivity waa always less than lo-' S m-l. Apparatus. The Langmuir trough used was a conventional poly(tetrafluoroethy1ene)(PTFE) trough (KSV 2200) equipped with a quartz window at the bottom. Surface pressure was monitored with a platinum Wilhelmy plate attached to a microbalance. The output from the balance was used to control the film compressingbarrier, thus making it possible to maintain a constant surface pressure (&lmN/m) over a long time. The isotherms, surface pressure against area ( P A ) , were recorded at compression rates of approximately 2 A*/min and molecule. The data were collected and analyzed on an IBM PC with software from KSV, Helsinki. A PTFE tip, placed above the water surface and barely touching it, made it possible to reproduce the water level in the trough, which was important for the spectroscopic measurement due to geometrical reasons. Fluorescence measurements were made with the following setup: A silica lens was used to focus the excitation light on the water surface. The horizontal excitation beam was reflected through the water surface so that it would pass down through the window in the bottom of the trough and be trapped by a light sink placed under the trough. The incident angle was 50-55O. For the steady-state measurements a fused silica optical fiber bundle was used to collect and transmit the emission to an optical multichannel analyzer, OMA (EE&G model 1460). A UV cutoff filter was attached to the rear end of the fiber to reduce the scattered light from the excitation beam. The fiber was positioned to give a good Raman scattering signal from the pure water surface and at right angle to the surface. A pulsed (20 Hz) nitrogen laser at 337 nm (Laser Science, Inc., WSL-337ND)was used as light source. Typical exposure times were 20-60 8. Fluorescence decays were measured with a time-correlated single photon coupling system. The excitation light (320 nm) was produced by a frequency doubled mode-locked DCM dyelaser (Spectra Physics Model SP 375 and 3449) synchronously pumped by a mode-locked NdYAG laser (Spectra Physics Model SP 3800). The emission from the surface was collected and focused by a fused silica lens on a microchannel plate photomultiplier tube (MCP) from Hamamatau (Model R15640). A narrow band-pass filter at 400 nm and a low fluorescent cut-off filter were used to select the wavelength of observation. The emission photon pulses from the MCP were discriminated in a constant factor discriminator, CFD (Tennelec TC 455), and used as stop or start pulses in the time to amplitude converter, TAC (Tennelec 861A). The output from the TAC was connected via an analog to digital converter (ADC) to an IBM computer (IBM PS/2, Model 302869)which with hard- and softwarefrom Nuclear Data worked as a multichannel analyzer,MCA. The fluorescence decays were transferred to a Digital Equipment VAX station 2000 and fitted to a single or double exponential function by a nonlinear least-squares procedure. In both the steady-state and the dynamic measurements a background signal was recorded on illumination of the pure water surface. The steady-state background consisted of the Raman peak, stray light, and the dark current through the photodiodes in the OMA. The last component was reduced considerably by coolingthe detector part to -20 "C with a water-ethanol mixture. The temperature was then kept stable by a Peltier element in the detector. The time-resolved background showed a fast decay probably arising from scattered excitation light reaching the PTFE which fluoresces strongly. These backgrounds were later subtracted from the recorded spectra and decays. In order to ensure that the recorded backgroundand the fluorescencedecay were exposed
5
1 0 1 5 20 25 30 3 5 40 Mean area per molecule (A2)
Figure 1. F A isotherms for pyrene/EA mixtures on a 0.3 mM CdClzsubphase. The pyrene concentration is, from right to left, 0%, 5 % , 12%, 25%, and 53% per mole. The mean area per molecule is calculated as an average of both pyrene and EA molecules. The inset shows the slope drldA (at ?r = 35 mN/m) as function of pyrene concentration. by the same energy, a photodiode connected to a custom-built voltage/frequency converted monitored the excitation beam and gated the detection system. Fluorescencefrom Langmuir-Blodgett deposita was measured with a Spex Fluorolog 1680 0.22-m double spectrometer. The slides were positioned so that incident angle of the excitation beam was 45O and emission was monitored in the "front face" mode. Due to the low intensity these spectra were recorded with poorer resolution than the ones recorded on the OMA.
Results and Discussion Isotherm Data. When fatty acids are spread and compressed on pure water, they show a liquid-phase region before they reach the close-packed, solid phase. Addition of doubly charged metal ions in the subphase such as Cd2+ or Ca2+gives an extension of the solid region as the metal ion links two fatty acid ions and thus preforms microdomains of solid phase in the uncompressed monolayer. When these domains are compressed to close packing, the solid phase is reached without passage through the liquidcondensed phase. Pure pyrene did not form compressible monolayers on the air-water interface and attempts to compress large amounts of pyrene resulted in visible crystals at the water surface. Mixtures of EA and pyrene gave reasonable stable monolayers. The stability seemed to fall with increasing amounts of pyrene, which was observed as a decrease in T with time. This decrease seemed to be proportional to the pressure, but even at pyrene concentrations of 55 % by mole the stability was sufficient during the times of measurement which however did not exceed 2 h. Rigorous stability studies were not performed. The mixed monolayers showed isotherms similar to pure fatty acid isotherms. Incorporation of pyrene did not affect the area of the compressed film, implying that pyrene was located so that it did not occupy any area. Shifts in area per molecule were all less than experimental error, however increasing amounts of pyrene did tend to decrease the slope (d?rldA) of the solid region of the isotherm. This was observable on a CdC12 subphase where the linear, solid part of the isotherm extends over a large region. The slope dTldA is directly proportional to the surface compressional modulus of the monolayer.16 A low value indicates that the presence of pyrene influences the close packing process of the film. Figure 1 shows isotherms of films containing EA mixed with pyrene at concentrations from 0 to 53% per mole. The area per molecule is calculated as an average over
Wistwr et al.
1368 Langmuir, Vol. 8,No. 5, 1992
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.-VI
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Figure 2. Fluorescence spectra from 2 % pyrene in EA. Emission waa collected at different surface pressures. I = 2,5,10,15,20, 30,35 and 40 mN/m. Only monomer emission was seen. Each spectrumgives a point in fluorescence intensity as shown in Figure 3.
slope, dr/dA, at 35 mN/m as function of pyrene concentration. As incorporation of pyrene in the monolayer could not be detected from isothermdata, the possibility that pyrene was dissolved in the subphase was tested. A mixture of 10% pyrene and 90% EA was left on a water subphase in the trough overnight; a sample was taken with a Voll pipet which was inserted under the film from the backside of the barrier. The sample was analyzed with an absorbance spectrophotometer and a fluorometer and showed no traces of pyrene. As the area of the film is not affected by the presence of pyrene, the location of the pyrene molecules at the interface must be such that they do not take up space or area from the fii-forming matrix. One can roughly divide a Langmuir film into two parts, one hydrophobic part consisting of the aliphatic tails and one polar part consisting of the polar heads at the water-monolayer interface. T w o possible, nonintruding locations for the pyrene molecules in the monolayer can be suggested: the pyrene molecules can be located either on top of the monolayer at the surfactant-air interface or in the polar part at the water-surfadant interface. The fluorescencestudies can discriminate between these possibilities. Steady-StateFluorescence. Fluorescence spectra of Langmuir films containing pyrene in different matrices showed only monomer emission (Figure 2). The same spectrum was obtained at different surface pressures and at different pyrene concentrations, up to over 50% per mole. When the fluorescence from pyrene in an EA matrix on pure water or a 3 mM CdCl2 subphase was measured as function of area per molecule (Figure 3) the intensity increased with decreasing area with a jump in intensity followed by a local minimum or plateau in the phasetransition region. Although the isotherms are different for fatty acids on pure water and the Cd2+ solution, the rise in intensity occurs at the same mean area per fatty acid molecule, independent of pyrene concentration. Other workers7J8using other probes have found a sudden change in the fluorescence intensity as the monolayer reaches the liquid-condensed phase. This behavior has been attributed to a rise in local viscosity around the fluorophore, altering the rate of some radiationless deacti(17) Davies, J. T.; Rideal, E. K. Znterjacial Phenomena; Academic Press: New York, 1961; p 265. (18) Grieeer, F.; Lay, M.; Thietlethwaite, P. J. J. Phys. Chem. 1985, 89,2068.
20
25
30
35
Mean area per fatty acid molecule ( A Z )
40
Figure 3. P A isotherm of 2 5% pyrene/EA on a pure water subphase. Squares with error bars show fluorescence intensity as function of mean molecular area per EA molecule. Dotted line shows theoretical area dependence of the fluorescenceintensity. 60
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*
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*
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n
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Figure 4. Fluorescence intensity as function of surface concentration of pyrene molecules. Fluorescence was measured at I = 1 and 30 mN/m. The pyrene concentration was varied from 5.5% to 53%.
vation process. If this were the mechanism responsible for the intensity variation observed in the present case, then there should also be a concomitant change in the fluorescence lifetime. Such a change was not observed however (see later), and a different mechanism must be involved. This can probably be due to optical effects, for example changes in the turbidity and scattering at the surface during compression or a change in the refractive index at the interface which altere the optical path for the excitation or emission radiation. The fluorescence intensity was measured at a = 1and r = 30 mN/m, in order to avoid the phase-transition region, where the anomalieswere observed. The intensity plotted against surface concentration of pyrene is shown in Figure 4. An initial linear portion was observed u to a pyrene concentration of 0.02-0.03pyrene molecule/& corresponding to an area per pyrene molecule of 35-50A2. That area is approximately the area occupied by a single pyrene molecule lying flat at the interface. This linear part was followed by a saturation region where the fluorescence intensity was constant. This indicates that the surface concentration of pyrene reaches a limit corresponding to close packing of pyrene molecules on the surface. Observations of the ratio between the intensity of the vibronic peaks III/I in the pyrene emission spectrum is often used as a measure of the polarity of the microenvironment around the probe.l2J3 This ratio varies between 0.63 for apolar solvent (H2O) and 1.65for nonpolar solvent (cyclohexane). The III/I ratio observed for pyrene in the monolayer immediately after spreading of the chloroform/ pyrene solution was about 1,decreasing to 0.63 following the evaporation of chloroform. This value was maintained during compression indicating that the pyrene was at all times in a polar environment. In order to investigate if pyrene mixed with surfactants forming a less ordered structure in monolayers would show
Pyrene in AirlWater Monolayers the same fluorescence characteristics, the two phospholipids DOPC and DPPC were employed. These surfactants differ from each other by a double bond at the ninth carbon in the hydrophobic tails. The unsaturated DOPC gives an isotherm without a phase transition at room temperature. Aa the film is in its liquid-expanded state, its hydrophobic part was expected to be a better solvent for pyrene. If pyrene was dissolved in the hydrophobic part, then increasing concentrations of pyrene would give excimer emission. When pyrene was mixed with DOPC, the fluorescence intensity was found to be irregular and the results difficult to reproduce. The anomalies that were present with EA mixtures at the phase transition region could not be observed. The III/I ratio indicated a polar environment. Two observations were made: only monomer emission could be seen, and the variance of intensity fluctuations decreased with increasing compression. This can be understood if pyrene/DOPC mixtures are less homogeneous than pyrene/EA mixtures and microdomains of pyrene move into and out from the observation area. An increasingcompression gives an increasingviscosity of the film and decreases the lateral movements of the domains within the film. Pyrene/DPPC mixtures showed some similarities to the EA matrix: only monomer emission and increasing intensity with increasing surface concentration of pyrene. The deviations in intensity from a pure concentration dependence (Figure 3) were not observed in this case. Again the III/I ratio indicated a polar environment for the probe. DHDAB was tried as a twin-tailed matrix molecule with a positively charged headgroup in order to see if the charge of the matrix and the known interactions between the quaternary ammonium group and pyrene14 would alter the fluorescence behavior. Also in this case only monomer emission was observed, and the intensity increased with compression up to a maximum where it started to diminish. The decrease is probably due to quenching by the Br- ions which are located near the interface. Increasing the surface concentration of the positively charged surfactant by compression increases the local concentration of Br- at the interface, which is sensed by the pyrene molecules. The polarity around the pyrene molecules was about the same as in the other matrices. Time-Resolved Fluorescence. Fluorescence decay curves were recorded at different surface pressures with EA on six different subphases: pure water, 0.03 M CaC12, 0.03 M Cd2, 0.003 M Cd2, 0.06 M KI, and 0.006 M KI. The surface pressures were chosen so that measurements were made as soon as a rise in surface pressure was noticed and also in the liquid and the solid regions of the isotherms. Decay data were fit to a single or double exponential function of the form
I ( t ) = I, exp(-kt) + I, exp(-k’t) (1) On pure water and on the CaClz subphase the decays were found to fit best to the single exponential model, confirming that pyrene was not aggregated in the system. The decay time of 125 ns showed no dependence on surface pressure and the influence of Ca2+ions had a negligible effect (see Figure 6). The measurements on iodide solutions were done to study iodide quenching of the pyrene fluorescence. These measurements showed that pyrene was strongly quenched by iodide ions in the subphase (Figure 5 ) , giving double exponential decays with a fast component, 1-1.5 ns, and a slower component 13-65 ns (see Table I). Again no significant effect was noticed by the presence of Ca2+ions.
Langmuir, Vol. 8, No. 5, 1992 1369
2 1 0
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Figure 5. Fluorescence decays from pyrene/EA mixtures on subphases with varying concentrations of iodide ions. The uppermost decay is without iodide, the one in between is measured with 6 mM iodide ions in the subphase, and the lowest is measured on a subphase with a iodide ion concentration of 60 mM.
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25 30 35 40 Film pressure (mNlm) Figure 6. Fluorescence lifetimes as functionof surface pressure: open circles measuredon KCI;open squares on CaC12;f i e d circles on 60 mM KI; X on 30 mM CaI2; filled diamonds measured on 6 mM KI. Table I. Fluorescence Lifetimes Measured at Different Surface Pressures with Different Subphases [W m M subphase */(mN/m) r/ns 0 60 mM KCl 0.5 126 0 0 0 0
0 0 0 0 0 60 60 60 60 60 60 60 60 6.0 6.0 6.0 6.0
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15 35 0.5 5.0 15 35 0.5 5.0 15 35
121 121 123 121 123 128 128 126 126 13.2 12.9 12.9 12.9 14.2 14.1 14.1 14.0 66.0 65.0 63.0 65.0
The fast component which is due to static quenching by I- close to the pyrene molecules represents approximately 4-5% of the total amplitude. This component is overlapping with the fast background fluorescence and is therefore difficult to analyze with good accuracy. Rate constants for the quenching k, were calculated from the Stern-Volmer equation
Wistus et al.
1370 Langmuir, Vol. 8, No. 5, 1992 70/7 = 1 + kq70C$ (2) where T O is the unquenched lifetime, 7 is the lifetime of the slow decay of the quenched molecules, and CQis the quencher concentration in the bulk subphase. k, values were found to be the same within experimental error: 1.1 and 1.2 X 109 s-l M-1 for the two iodide concentrations, the higher value at the lower concentration. The quenching was also found to be independent of surface pressure or area per molecule, in contrast to what has been found with substituted pyrene probes such as pyrenedocosanoic acid or pyrene-bearing lipids in phospholipids at the nitrogen-water interface by Bohorques and Patter~0n.l~ They reported a rate constant for the quenching of (3-4) X los s-l M-'at the uncompressed monolayer and a quenching decreasing down to 1.5 X lo8 s-l M-'in the compressed monolayer of DOPC which has no solid crystalline state. In DPPC the quenching seemed to vanish as the monolayer was compressed to a close packed crystalline state. The decreased quenching was taken to indicate increasing distance from the iodide ions in the aqueous subphase to the fluorophore in the monolayer as it was compressed. The matrix also forms a barrier which prevents the quenchers from reaching the pyrene fluorophores. The permeability of this barrier decreases as the monolayer is compressed. Pyrene can also be quenched by oxygen from the atmosphere or the aqueous subphase, The lifetime of pyrene in oxygen-saturated aqueous solutions was determined to be 135 ns, close to the value in the monolayer in the absence of quenchers. The constant and rapid quenching of pyrene by iodide from the water solution and by oxygen indicates that the location and orientation of pyrene remain mainly constant during compression of the monolayer and that the access of the quenchers to pyrene is unchanged under compression. If pyrene molecules were located on top of the layer or within the alkyl part, a pressure or packing dependence on the lifetimes should be observed, as the possibility for iodide ions to penetrate the fatty acid matrix would be decreased at a higher degree of packing. That such a dependence was not observed, that no excimers were formed, and that the III/I ratio indicates a polar environment suggest that the pyrene molecules are lying flat at the interface between the polar group of the lipids and the water subphase. The similarity between the lifetimes of pyrene in the monolayer and in aqueous environment further supports this conclusion. It is very remarkable that no excimer formation is observed with any of the matrices. The pyrene molecules must be well stabilized in the positions they occupy to resist excimer formation after excitation. The fact that no partitioning of pyrene into the fluidlike hydrophobic part of the DOPC monolayer is observed also indicates a good stability in the polar sites. Depositions. Depositions of pyrene/EA mixtures were made onto hydrophilic quartz and silicon slides by the conventional Langmuir-Blodgett technique. The depositions on quartz were of poor quality with transfer ratios = 0.6-0.8. It was found that the first layer had a much lower adhesion to the substrate than what pure fatty acids have, resulting in a strong tendency for the first layer to peel off as the substrate was passed through the surface during deposition of the second layer. This was perhaps to be expected from the presence of a layer of pyrene between the support and the fatty acids. Several series of depositions on n-phosphorus single crystalline (100) silicon wafers were made. A desired
(19)Bohorques, M.; Patterson, L. K. Thin Solid Films 1988,159,133.
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Figure 7. Fluorescence intensityfrom deposita on Si as function of numbers of deposited layers. C
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Figure 8. Steady-statefluorescence from LB film consisting of 19 layers of pyrene/EA (a) immediately after deposition; (b) after 48 h.
feature of the silicon slices, apart from a smooth surface suitable for LB depositions, was that they showed no fluorescence interfering with the pyrene fluorescence.The first layer had a better adhesion to the siliconsurface giving better deposits; transfer ratios were around 0.9. The fluorescence intensity from the deposita shows a nonlinear increase with the number of layers deposited (Figure 7). This can be due to a selective deposition of mainly fatty acid which results in an increased local pyrene concentration on the surface around the area from where the deposition occurs. Increasing pyrene concentration will then give an increasing increment in concentration on the deposit. An alternate or cooperative explanation can be that as the number of layers are increased, the density or cross section of defects is increased nonlinearly which may cause scattering of the excitation light and thus increase number of excited pyrene molecules. Furthermore, fluorescence quenching by the semiconducting substrate cannot be excluded. Such quenching would decrease with increasing distance from the substrate. The fluorescence spectrum from deposita did not reflect the observation from the monolayer at air-water interface where only monomer emission was observed. Immediately after deposition the fluorescencespectrum showed a maximum at 430 nm (spectrum a, Figure 8)which has been attributed to a ground-state dimer with one of the rings overlapping.20.21After being stored a t room temperature for 48 h the deposits showed a fluorescence spectrum that was more like the normal distribution (20) Yamazaki, I.; Tamai, N.; Yamazaki, T.J. Phys. Chem. 1987,91, 3572. (21) Suib, S. L.; Kostapapas, A. J. J . Am. Chem. SOC.1984,106,7705. (22) Vithknovsy, A. G.; Sluch, M. I. h o g . Colloid Polym. Sci. 1991,84, 288.
Pyrene in AirlWater Monolayers
Langmuir, Vol. 8, No. 5, 1992 1371
between monomer and excimer as normally seen in pyrene solutions (Figure 8b). A similar spectrum was obtained when pyrene/EA solution was applied thinly on a quartz support and allowed to dry at room temperature. However, a thin layer of pure pyrene crystals shows only excimer emission. The change in spectra from the LB deposits over time can be explained by migration of pyrene in the EA. The depositions on quartz slides behaved somewhat differently. Immediately (15 min) after deposition they showed uniform excimer fluorescence, seemingly evenly distributed over the entire slide, whereas after 24 h, strong emission could be seen from discrete spots indicating that crystallization had occurred. Fluorescence spectra from mixed LB deposita of perylene and a fatty acid have been rep0rted.~3 In this case the spectrum gave evidence for the presence of monomers, dimers, and at low temperature also excimers, in some similarity with our observations for pyrene. No study of perylene at the airlwater interface was reported. The fact that the pyrene layer could not be transfered undisturbed into a LB deposition is not surprising. Whatever the reason for the stability of the arrangement at the monolayer/water interface, it is not due to specific interactions between the polar heads of the amphiphilic molecules and pyrene; the presence of water is also required. The interface between a solid support (which moreover is not molecularly smooth) and the lipid head-
groups presents a quite different environment, as does also the headgrouplair interface on the uplift and the space between the polar groups at two layers; in the latter, dimers and excimers could be easily formed.
(23) Johansson, L.B.-A.; SundstrBm, V.Chem. Phys. Lett. 1990,167, 383.
Registry No. I-,20461-54-5; pyrene,129-00-0; eicosanoic acid, 506-30-9.
Conclusions The intensity ratio of vibronic peaks III/I indicates that pyrene is in contact with water. The constant lifetimes (and constant quenching) when the monolayer is compressed indicate that iodide ions can reach the pyrene molecules at the same rate, regardless of how densely the matrix is packed, and allow us to conclude that the pyrene moleculesremain located at the water-surfactant interface. The somewhat unexpected influence on the isotherms and the absence of excimer formation make it tempting to draw the conclusion that pyrene lies flat on the water surface,and the EA is pushed up onto the pyrene molecules so that close packing of the aliphatic tails of EA is achieved. Investigations by means of time-resolved fluorescence anisotropy mea~urements~~ would give further information about the orientation of the pyrene molecules on the air/ water interface.
Acknowledgment. This work has been supported by The Swedish Natural Science Research Council, The Swedish Board of Technical Development, The Knut and Alice Wallenberg Foundation, and The Goran Gustafsson Foundation.
