Langmuir 1991, 7, 1719-1726
1719
Fluorescence Probe Studies of Self-Assembled Monolayer Films Shaun H. Chen and Curtis W. Frank' Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received July 6,1990. I n Final Form: February 14,1991 Fluorescence probe techniques were used to study the effects of incor orating "guest molecules" into organized organic self-assembled monolayers formed by spontaneous a sorption from solutions. Selfassembled n-alkanoic acid monolayers containing pyrene fluorescence probes were the model systems studied. The fluorescenceemission intensity ratio between the first and the third highest energy (frequency) emission peaks I l a , has been shown to reflect the polarity of the medium surrounding the pyrene group. For systems in w ich the host acid chains are longer than the probe chains by six or more CH2 chain segments, i.e., when the pyrene groups are completely 'buried" inside the packed hydrocarbon chains, the low I 1 / I svalues obtained indicate a nonpolar environment. When host acids of shorter chain lengths are used, the pyrene groups become less buried in the film, and a more polar environment is indicated by the increased I1/Is values. For films with host acids shorter than (212, however, 1 1 / 1 9 decreases, perhaps indicating some disordering in the monolayers, analogous to similar results for Langmuir-Blodgett films. Fluorescence quenching experiments were performed on the pyrene-doped monolayers with aqueous solutions of nitromethane, CHd02, which quenches the pyrene fluorescencevia a dynamic process. The apparent quenching constant KD showed a similar trend as the I1/Is results. The apparent diffusivity of CHsN02 in the vicinity of the pyrene groups was determined after accounting for the partitioning of CHsNOz between the aqueous and the film phases and the fluorescence lifetime reduction of the Py-Cls and Py-C12 probes anchored on aluminum substrates. As the host acid chain becomes longer than the probe (i.e., the pyrene group is more deeply embedded inside the film),the effectivediffusivityis decreased. For the case in which the pyrene group is totally buried inside the monolayer, the effective diffusivity is decreased by about 50% relative to the fully exposed case.
B
Introduction Organized organic monolayers formed by spontaneous adsorption of amphiphilic molecules from solution, known as self-assembled (SA)films, have received a great deal of research interest recently.l+ On the basis of various characterization techniques, previous workers have shown that in these filmsthe hydrocarbon chains are well-ordered (oriented approximately perpendicular to the substrate) and closely packed (comparable to a solid crystalline state). The SA films have structures closely resembling those of the Langmuir-Blodgett (LB) monolayers, but unlike the latter, which are artificially constructed with external pressure, the former are formed spontaneously. The attainment of an ordered equilibrium structure and the capability of molecular-level structural control in fabricating the SA films provide a good means of 'molecular engineering" materials of desired properties. In addition, SA films form good model systems for electrical,'+ physical,1° chemical,b.J1J2and mechanicaP experiments. For many applications it is necessaryto introduce certain ~
~~
~~
(1) Maoz, R.; Sagiv, J. J. colloid r n t e r f a i Sci. 1984,100,465. Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101,201. Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986,112,457. (2) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1,45,52. (3) Maoz, R.; Sagiv, J. Langmuir 1987,3, 1034. (4) Nuzzo, R. G.; Fueco, F.A.; AUara, D. L. J. Am. Chem. SOC.1987, 109,2368. (5) Troughton, E. B.; Bain, C. D.; Whitasidee, G.M.;Nuzzo, R. G.; A h a , D. L.; Porter, M. D. Langmuir 1988,4,365. Strong, L.; Whitesides, G. M.Langmuir 1988,4,548. (6) Chen, S.H.; Frank, C. W. Langmuir 1989,5,978. (7)Sabatani, E.; Rubinatein, I.; Ma-, R.; Sagiv, J. J. Electroaol. Chem. Interfacial Electrochem. 1987,219, 365. (8)Polymeropoulon, E. E.; Sagiv, J. J. Chem. Phys. 1978,69, 1836. (9) Widrig, C.; Majda, M. Lungmuir 1989,5, 689. (10) Cohen, S. R.; Naaman, R.; Sagiv, J. Phys. Reu. Lett. 1987, 58, 1208, J. Chem. Phy8. 1988,88, 2757. (11) Waeeerman, S.R.; Tao, Y.; Whitaides, G. M.Langmuir 1989,5, 1014. --. -.
(12) Tillman, N.; Ulpan, A.; Elman, J. F. Langmuir 1989,5,1020. (13) DePalma, V.; T h a n , N.Langmuir 1989,5, 868.
useful "active" extrinsic chemical groups into the films. For example, in nonlinear optical applications"-16 some optically active molecules need to be incorporated in the thin films, and in the prospective chemical sensor applic a t i o n ~ ' some ~ * ~ chemically ~ active functional groups need to be incorporated. In some cases the whole film can be constructed of the active molecule, while in others the active compound can be introduced into the films as the 'guest" molecule. When anchored or enclosed in an organized structure, a chemical functional group is expected to be of a different character from that of a group in the bulk. Sagiv and co-workers's studied the reactivity of intrinsic alkene groups within monolayer films with oxidizing agents penetrating into the films. They found that the penetrability to the enclosed groups decreased as the packing density of the hydrocarbon chains of the molecules is increased. Rubinstein et al. also constructed selfassembled monolayers containing electroactive ligands on Au electrodes and showed that these electrodes are ionic size selective,lg based on electrochemical measurements. In the investigation of systems containing small "guest" components, indirect methods such as the spectroscopic techniques are desirable because of their nondestructive nature. UV-vis absorption and fluorescence spectroscopic probe techniques have been used to study the structure ~~
(14) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandrw, E. A,; Garoff, S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F. W.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3, 932. (15) Swalen, J. D. J. Mol. Electron. 1981, 2, 155. (16) Dirk, C. W.; Tweig, R. J.; Wagniere, G.J. Am. Chem. SOC.1986, loR.5887. - - -, - - - .. (17) T a f w , I.; Kurihara, K. Jpn. Kokai Tokkyo Koho JP 62209350, 1987; Chem. Abstr. 1989, 110, 150918j. (18) Ma-, R.; Sagiv, J. Langmuir, 1987,3, 1034, 1045. (19) Rubinstein, I.; Steinberg, 5.;Tor, Y.; Shanzer, A,;Sagiv, J. Nature 1988,332,426.
0743-7463/91/2407-1719$02.50/0 0 1991 American Chemical Society
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aqueous phase of dye-containing self-assembled monolayer films20and to study the chemical reactivity of active amphiphilic compounds forming21*22 or incorporated into23Langmuir * CHSN02 films. The various fluorescent probe methods can provide molecular-levelinformation about local microenvironment (e.g., configuration and molecular association around the probe) or dynamics (e.g., probe-probe interaction). These have also been demonstrated for a variety of other types of systems, e.g., micelles and polymer~,26~~~ and Langmuir-Blodgett fi1ms.B In this work we use fluorescence probe techniques to study the guest-containing organized mixed monolayer film films. We incorporated amphiphilic pyrene-containing fluorescent probes as the guest molecules into selfassembled n-alkanoic acid monolayers. The fluorescence properties of the probes were then examined to yield molecular-level information on the systems. The fine structure of pyrene emission reflects the environment (i.e., the Figure 1. Fluorescence quenching of a mixed monolayer film microscopic film structure) surrounding the probe, and containing anchored pyrene probes by CHsNO2 quencher from the fluorescence quenching experiments manifest the an aqueous phase in contact with the film surface. interaction between the surface-bound pyrene groups with substances in the surroundings. Our major conclusions vital to the subsequent adsorption process. The general cleaning are (a) the fluorescence 11/13 of the pyrene probes procedures used in a previous s t u d 9 were followed. incorporated into SA n-alkanoic acid monolayers shows Adsorbate solutions were prepared by dissolving weighed the probe environment polarity change with expected local portions of the fatty acids in hexadecane (HD) to near-saturation film structural variations and (b) fluorescence quenching concentrations (10-L10-9MI. Py-C12 and Py-Cl6 were first of the surface-bound pyrene probes shows the effect of dissolved in HD/toluene (1:l)by warming to about 50 "C. The mixed solutionswere then prepared by diluting more concentrated film packing on the diffusion of external quencher Py-Cl2 or Py-C16/(HD + toluene) solutions and mixing with a molecules in the vicinity of the probes.
I
Experimental Section Materials. The adsorbates consisted of the homologous series of n-alkanoic acids, abbreviated as Clo through Cm, which were obtained from Sigma Chemical Co. Those samples with nominal purities less than 99% were recrystallized twice from ethanol. The pyrene end-tagged palmitic (hexadecanoic) acid (Py-cl6) and pyrene end-tagged lauric (dodecanoic) acid (Py-Clp) (Molecular Probes, Inc.) were used as received. Pyrenebutyric acid (Py-CJ (Kodak) was recrystallized from ethanol. The various spectroscopic-grade solvents (Sigma or Aldrich), were used as received. Distilled and deionized water was used for substrate cleaning. Aluminum surfaces prepared by evaporating approximately lo00 A of ultrapure aluminum onto Si wafers were used in this study as the substrate. The surface of these substrates actually consists of 20-50 A of amorphous aluminum oxide grown naturally. Straight-chain alkanoic acids are known to easily adsorb in monolayers on this type of substrate2S6forming closely packed SA monolayers of nearly perfectly ordered structure, with the alkyl chains aligned approximately perpendicular to the substrate surface. Sample Preparation. Under normal laboratory conditions the surfaces of the freshly prepared aluminum substrates become contaminated quickly by adsorbing small organic molecules from the air, and precleaning of the solid substrates was found to be ~~~~
(20) Sagiv,J. J . Am. Chem. SOC. 1980,102,92. Isr. J. Chem. 1979,18, 339, 346. (21) Grieser, F.; Drummond, J. J . Phys. Chem. 1988,92,5580. (22) Loechek, R.;MBbius, D. J . Chim. Phys. 1988,86, 1041. (23) Whitten, D.G.; Frederick, R. H.; Quina, F. H.; Sprintachnik, G.; Sprintachnik, H. W. Pure Appl. Chem. 1977,49,379. (24) Jay, J.; Johnston, L. J.;Scaiano, J. C. Chem.Phys. Lett. 1988,148, 517. Somasundaran, P.; Turro, N. J. Macromolecules 1988,21,950. (26) Thulborn, K. R.;Sawyer, W. H. Biochim. Biophys. Acta 1978, 511,125. Blatt, E.;Chatelier, C.; Sawyer, W. H. Photochem. Photobiol. 198439,477. (26) Char, K.;Frank, C. W.; Gast, A. P.; Tang, W. T. Macromolecules 1987,20,1833. Char, K.; Gaet, A. P.; Frank, C. W. Langmuir 1988,4,989. (27) Chander, P.; Somasundaran, P.; Turro, N. J.; Waterman, K. C. Langmuir 1987,3,298. Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner, E.; Zachariaeee, K. A. J. Phys. Chem. 1985,89, 3521. (28! Murakata, T.; Miyashita, T.; Matsuda, M. Langmuir 1986,2,786. Tamlu, N.; Yamazaki, T.; Yamazaki, I. J . Phys. Chem. 1987,91, MI. Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987, 91, 3572.
host fatty acid solution to the desired molar ratios. The final solutions used for adsorption, of total acid concentration O.OOO& 0.005M in solvent mixtures containing about 10% by volume of toluene in HD, all have a small fraction of Py-Cla of Py-Cl?probe (molar fraction = 1-5%). During the initial dissolution it was necessary to warm the liquid contents to about 50 OC. The same procedure was also used to redissolve the crystals occasionally formed during solution storage, especially for the more concentrated (>0.01M)solutions of Cm through Cm. The preparation of the monolayer films was carried out at 22 OC. Pyrene-labeled mixed monolayers were prepared by adsorption from solutions of the desired concentration of a particular host/probe molar ratio. The substrates were immersed in the solutions for a predetermined period of time, removed, and then blown dry of any remaining liquid droplets by nitrogen. All monolayers were prepared under equilibrium adsorption conditions? Fluorescence Measurement a n d Sample Characterization. Fluorescence emission spectra of the pyrene-doped monolayers were obtained with a Spes Fluorolog 212 spectrofluorometer with a 450-W xenon arc lamp. The excitation wavelength was set at 343 nm and the spectra were taken in the front-face mode. Because of the low signal intensity from these monolayer samples, wide slits (2mm) were used. The fluorescencespectroscopy measurements were performed on fluorescence-probe-labeled monolayers prepared on the Al substrates. With fluorescence probes positioned very close to a metal surface, the fluorescenceis strongly attenuated due to nonradiative energy transfer to the metal.2BlmIn our mixed-monolayer systems, the pyrene groups were within 100 A from the Al surfaces, considering the native aluminum oxide layer to be 2050 8, thick and the all-trans C12 and hydrocarbon chains to be about 15 and 20 A in length, respectively. Thus, the quenching effect of the metal surface had to be taken into account. However, fluorescence spectra of reasonable quality can still be obtained. To study the interaction between the surface-bound fluorophore groups and the surroundings, fluorescence quenching experiments were devised. Figure 1 schematically shows the (29) Kuhn, H. Pure Appl. Chem. 1966,11,345. J. Chem. phy8.1970,
53, 101.
(30) Waldeck, D. H.;Aliviaatoe, A. P.; Harris,C. B. Surf. Sci. 1986, 158,103. Chance, R. R.; Prock, A.; Silbey, R. In Advances in Chemical Physics; Rice, S. A,, Prigogine, I., Eds.;Wiley: New York, 1978; Vol. 37, P 1.
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Self-Assembled Monolayer Film fluorescencequenchingprocess. In our system the surface-bound film is immersed in a liquid phase containing a small molecule that is known to quench, i.e., reduce the intensity of, pyrene fluorescence. The quencher molecule will partition into the film phase, diffuse to the vicinity of the fluorophore,and quench the fluorescencethrough a collisionalprocess. Because the fatty acid film will start to dissolve when immersed in a nonpolar liquid that is similar in nature to the fatty acid long chain, we couldnot use such a liquid as the phase for the quencher. In this work we chose nitromethane, CI-bNO2,aa the pyrene fluorescence quencher, and water (in which the long chain acids will not dissolve)as the solvent for the quencher. Mixed SA monolayers containing the pyrene probes were immersed in aqueous CHsNOz solutions of specified concentrations in a fluorescence cell and the fluorescencespectra were taken. In order to optimize the spectrophotometer output for the solid-in-cellgeometry, the optical path for the excitation light was modified to focus on the sample surface. Several experimental methods were used for macroscopic characterization of the monolayer films. A Ram6-Hartcontactangle goniometer was used to measure the contact angles of HD and water on film surfaces. A Gaertner ellipsometer was used to measure the film thickness, using a He-Ne 632.8-nm laser source and fixing the film refractive index nf = 1.50 during the calculations. A Perkin-Elmer 1710FTIR spectrometerwas used to obtain the transmission and attenuated total reflectance infrared (ATR-IR) spectra of the films. These routine measurementswere done in the same manner as in previous Solution fluorescence measurements for Py-C, were also performed on the Spex Fluorolog 212. Solution UV absorption measurements for CHaNOz were performed on a Varian Cary 210 spectrophotometer. Solution Fourier transform infrared (FTIR)absorption spectra for CHaNOz were also taken with the Perkin-Elmer 1710 spectrometer using a short-path KC1 liquid cell. Rssults SA Monolayers Characterization. After the mixedmonolayers were prepared, we used a number of more conventional methods to characterize the pyrene-probecontaining SA monolayers. The results were, however, very similar to those reported earlier2I6for pure n-alkanoic acid SA monolayers. For c18 host monolayers containing Py-Cl6 probes prepared from 1-5% probe solutions, the water and hexadecane static contact angles were 95 f 3’ and 45 f 2O,3l respectively. These values were, within the experimental error, identical with those obtained for equilibrium pure CIS monolayers.2 Apparently, the presence of a small amount of pyrene groups on the CH3 surfaces did not significantly affect the (average) surface property. Ellipsometry measurements of film thickness did not unambiguously show any significant difference between a Py-C,/C,, mixed monolayer and a pure C,, monolayer. For example, a Py-C16/C16 monolayer prepared from 1-5 5% probe solution gave thickness values of 22 f 3 A, as compared to 21 f 2 A for a pure C16 monolayer. No conclusive evidence on film thickness variation can be drawn from such measurements, judging from the experimental error associated with them. Similarly, FTIR measurements for the mixed monolayers failed to yield any peak resulting from the pyrene groups on the surface even at the highest solution probe mole fraction ( 5 % ) , while the CH2 stretch peaks (2850 and 2920 cm-l) for all Py-C,/C,, mixed monolayers were identical with those of the pure Py-C,, acid monolayers, within the experimental error. It thus appears that FTIR is not sensitive enough (31) In this work and ab0 in a previous paper (ref 6), static contact angles were reported. Later comparisons with aduancing contact angle mensurementashowed, for exnmple, a static water contact angle of 96 3O correspondr to an advancing water contact angle of 107 3O. This value in in good ngreement with those previously obtained for similar systems (refs 1-5).
*
to detect the extremely low surface concentration of the pyrene groups, for which the aromatic C-H stretch peaks were also weaker than the alkyl CH2 stretch peaks. All of these characterization results seem to indicate that the quality, or the global structure, of the SA monolayers was not altered by the incorporated pyrene groups, at least on the average. The only tools that could be used to perceive the presence of the pyrene probes were the fluorescence techniques, as will be discussed in the following sections. Fluorescence Probe Polarity Measurements. The vibronic fine structure of the fluorescence emission of pyrene and pyrene derivatives in solutions is known to be an indicator for the solvent polarity around the fluoroph0re.3~ The intensity ratio between the first and third highest energy (frequency) emission peaks, known as the 11/13 ratio, has been shown to correlate well with solvent polarity.32 The 11 peak, which arises from the 0-0 transition from the lowest excited electronic state to the lowest ground state, is a “symmetry-forbidden” transition that can be enhanced by the distortion of the ?r-electron cloud. On the other hand, the 1 3 peak is not forbidden, and thus is relatively solvent-insensitive. Consequently, the ratio 11/13 is a measure of the net band enhancement. The observed 11/13 of unsubstituted pyrene ranges from about 0.5 in nonpolar solvents such as hexane to about 1.8 in highly polar solvents such as methanol. For the l-substituted pyrene groups such as the probes used in this study, substitution on the aromatic group intrinsically distorts the symmetry and thus decreases the probability of 0-0 transitions. For such compounds, the extent of solvent-enhancement of 11/13 is reduced and thus is not as sensitive an index as the unsubstituted pyrene. However, the 11/13 ratio of l-substituted pyrene has still been found to be sensitive to solvent polarity.26 To confirm the applicability of the 11/13 measurement to our systems, we first performed a series of reference measurements of 11/13 for a l-substituted pyrene in solvents covering a wide range of polarity. In order to achieve reasonable solution concentration (lO-s-lO* M), we used a smaller l-substituted pyrene molecule, pyrenebutyric acid (Py-C4), which is similar to the long chain Py-C, probes in structure. The 11/13 measurement was then applied to the self-assembled films containing pyrene end-tagged acids and used to obtain film structural information. For the solution 11/13 measurements, the solvents included hexane, diethyl ether, toluene, ethanol, chloroform, methanol, methylene chloride, ethylene glycol, acetone, and water. Figure 2 shows two typical fluorescence emission spectra of Py-Cd in methanol and in heptane. It is apparent that the 11/13 intensity ratios for the two are indeed different. Because of the complicated fine structure, curve-fitting of these peaks is quite difficult. Here we calculated11/13 simply by taking the ratio between the first and the third maxima of the spectra, at approximately 376 and 384 nm, respectively. In nonpolar hydrocarbons such as heptane and toluene 11/13 is at a low of about 2.3, while in highly polar solvents it is above 3.2. The enhancement due to polarity is indeed reduced in comparison to the 11/13range of 0.5-1.8 for unsubstituted pyrene. A plot of 11/13 vs e, the solvent dielectric constant, is shown in Figure 3a. 11/13increased monotonically with increasing e, but most of the variation only occurred within the range o f t = 0-10. As shown in (32) Nakajima, A. Bull. Chem. SOC.Jpn. 1971, 44, 3272. Kalyanasundnrnm, K.; Thomas, J. K. J . Am. Chem. SOC.1977,99,2039. Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982,96,17. Glushko, V.; Thaler, M. S. R.; Knrp, C.D.Arch. Biochem. Biophys. 1981,210,33.
1722 Langmuir, Vol. 7, No. 8,1991
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-,
3
e
(CH,),-COOH
I 440
I
360
400
380
420
1
A
4
1
360
380
Figure 4. Typical fluorescence emission spectra of Py&/Czx mixed monolayers on Al substrate: (a) Py-C16/C14; (b) Py-C113/ (2%. 2.8
I
i
2.7 2.6
l
t
r
I
I
00’
20
80
60
40
100
2.2
1 1
10
15
20
I
Host Chain Length
E , Dielectric Constant
b
440
Wavelength (nm)
Wavelength (nm)
Figure 2. Typical fluorescence emhion spectra of Py-Cd in a polar (methanol) and a nonpolar (heptane) solvent.
420
400
Figure 5. ZI 19 of Py-Cl&,
and Py:Cl6/Czz films vs length (carbon num er) of host molecule cham.
d
41
I
I
0
0.1
0.2
0.3
I 0.4
Af Figure 3. Correlation of Py-Cd Zl/Za with (a) dielectricconstant and (b) Lippert function: Af = [(e - 1 ) / ( 2 t + 111 - [(n2- 1)/(2n2 + l ) ] .For comparison,the data of Kalyanasundaramand Thomas (ref 32) for pyrene are also shown in part b. Figure 3b, the measured 11/13 values of Py-C4 were found to correlate well linearly with the Lippert function, A f , which is also known as the “orientation polarizability”
n2-1 Af =---c - 1 2~ + 1 2n2 1 The Lippert function is normally related to spectral shifts of emission peaks of polar-substituted aromatic fluorophores that result from solvent relaxation of excited states. For pyrene and derivatives, there are no significant spectral shifts due to solvent relaxation. However, the solvent polarity apparently has an effect similar to solvent relaxation. Polar solvents, as discussed previously, can stabilize the nonsymmetric excited state of pyrene, thus enhancing the probability of the 0-0 transition and thereby increasing the 11/13ratio. The linear correlation between I1/&and Af indicates that the microscopic solvent and chromophoreorientation processes are closely related. For comparison, the pyrene 11/13 data previously obtained by
+
Nakajima and Kalyanasundaram and Thomas32are also plotted in Figure 3b. As can be observed, the correlation of pyrene 11/13 and Af and that of pyrenebutyric acid and Af are quite similars3except one is vertically shifted from the other. This demonstrates the usefulness of 11/18 of both the unsubstituted and substituted pyrene for studying local environment polarity or ordering around the probe. Typical fluorescence emission spectra of Py-C16/C14 and Py-C16/C22 mixed monolayers prepared from 5% probe solutions are shown in Figure 4. The shapes of these spectra are similar to the usual pyrene emission, with a number of peaks between 370 and 430 nm. For the pyrene fluorescence emission, a peak at about 470 nm would indicate the formation of an excimer between excited and unexcited chromophores, which would complicate the subsequent analysis. Excimer emission was absent in all the samples prepared in this work, indicating that the Py-Cls or PpC12 molecules did not aggregate within the adsorbed monolayers. The 11/13 values of a series of Py-C12/C,, and py-clS/ C,, mixed monolayers are plotted against the host fatty acid chain length x x in Figure 5. The mixed monolayers of the longest host chains for Py-C12/CX,and Py-C16/Czt gave the lowest 11/13 values, which (-2.32) are similar to the solution values obtained with Py-Cr in nonpolar solvents. For shorter host fatty acid chains, the11/13 value increased with decreasing host chain length, until it (33) In Nakajima‘s original work, pyrene ZI ISWBB correlated with the parameter = [(e - 1)/(2c + 1)1- (1/2)I(nh- 1)/(2n* + 111, which very similar to the Lippert function. The fit was good except for beneene and dioxane, which are known to have an anomalouseffect on electronic spectra In thin work we elected to w e a eeriea of “regular”solventa to demonstrate the existence of Zl/Za polarity correlation. We plea chow to use the Lippert function BB the parameter becauae of ita phynical significance. ~,ieshown,theI~/I~4correlationfor uneubetitutedpyrene data is also reciaonably good.
Langmuir, Vol. 7, No. 8, 1991 1723
Self-Assembled Monolayer F i l m reached a maximum (- 2.75)at host chain length of about 14. However, for host acids shorter than C12, Illla again dropped to lower values, but here the data points scatter significantly. For Py-C12/c,, systems, the 11/13 values follow a similar trend, but changes were only observable for Py-Clz/C1, and Py'C12/c16, and the maximum value was smaller than in the Py-C16/Crz systems. Fluorescence Quenching Measurements. In a homogeneous system, the most frequently observed type of fluorescencequenching is the dynamic quenchingprocess, in which a quencher molecule goes through collision with the excited chromophore and causes nonradiative energy lossthat reducea the fluorescence On the other hand, the static quenching process involves the formation of a nonfluorescent complex between the quencher and the fluorophore. Dynamic quenching has been observed for the CHsNOz/pyrene system in solutions.% For a dynamic quenching reaction, the Stern-Volmer equation describes the fluorescence intensity variation
5 .o 4.0
3.0
Io I
2.0 1.o
0.0
Figure 6. Stern-Voher plots of P ~ - C ~ ~ / C fluorescence, ,Z quenched by aqueous phase CHsN02.
t 2) Zo/I = 1 + KD [QI where 10 and 1 are the unquenched and quenched fluorescence emission intensity,KD is the dynamic quenching constant, and [&] is the quencher concentration. The bimolecular quenching constant k,, a molecular parameter, is related to KD by (3)
where TO is the unquenched fluorescence lifetime. In a nonhomogeneous system, a couple of complications arise for the quenching dynamics. First, for a system in which the fluorophore resides within a region of constrained geometry, the anisotropic diffusion will lead to complicatednonexponential fluorescence decay.% Second, for a two-phase system in which the fluorophore may reside in one phase while the quencher may be present in both phases, the entry and exit of the quencher molecules will also cause the fluorescence decay to deviate from simple exponential behavior.3' Determination of these complicated dynamics,however, would require fluorescencedecay measurements. For our monolayer/CH3NO~/watersystems, these types of dynamic behavior are probably present. However, because the weak fluorescence signal from the samples were overwhelmed by large scattered intensity from the aluminum substrates, we were unable to measure the fluorescence decay with a flashlamp fluorescence lifetime instrument. Withonly the stationary fluorescenceemission measurement available, we were thus in effect taking the exponential approximation of the fluorescence decays. Although this simplistic treatment may lead to deviations in the absolute values of the rate constants obtained, we believe that it will be useful in examining the relatiue changes semiquantitatively. CHfl02 solution concentrationsof up to 1 M were found to produce significant quenching effects. For different Py-c12/Czzand Py-cl6/czz systems, different extents of quenching were observed. However, for all systems the Stern-Volmer dynamic quenching relationship was followed. As shown in Figure 6,for the Py-C16/czx systems withxx = 14-22,straight lines were obtained in the SternVolmer plots. The apparent dynamic quenching constant (34)Lakowicz,J.R.Principles of Fluorescence Spectroscopy;Plenum PM: New York, 1983. (36)Arora, K.5.;Turro, N.J. J . Polym. Sci., Polym. Chem. 1987,25, 269.
(36)Infelta, P.P.;Gratzel, M.; Thomas, J. K.J. Phys. Chem. 1974,78, 190. Almgren, M.; Alsm, J.; Mukhtar, E.;van Stan, J. J . Phys. Chem. 1988,92,4479. (37)Thomas, J. K.Acc. Chem. Res. 1977,10,133.
1.0 ! 5
10
15
20
25
30
Host chain length Figure 7. Dynamic quenching constant KD based on aqueous phase quencher concentration v8 host chain length for Py-C12/ c,, and Py-cv/c,,systems. KD (based on the aqueous phase CHsNOz concentration), or the slope of the lines, decreased monotonically with increasing host molecule chain length x x . For the PyC12/Cxxsystems, similar plots were obtained. A plot of KD vs the chain length for both of Py-C12/Czz and Pyc16/cz, systems with nx = 10-30 is shown in Figure 7.For the two probes, the data points follow curves of similar shapes but of different minimum and maximum values.
Discussion Localization of Pyrene Probes Followed by 11/13 Measurements. The trend of 11/13 variation in Figure 5, along with the correlation of 11/13 with polarity shown in Figure 3,suggests a continual change in the local microenvironmentalsurrounding the pyrene group with the length of the host molecule forming the monolayers. In Figure 8 schematic representations of Py-C16/czz monolayers are depicted, assuming that the orientation of the hydrocarbon chain in each is perfect (which is generally a good assumption1*2*6). The length of c16 is about 22 A, and the length increases by about 1.22 A with each additional carbon in the main chain. The pyrene group has a dimension of about 4-6 A, depending on its orientation. For systems in which the host chains are equal in length or shorter than the probe chain, e.g., the PyCl6/C14 mixed monolayer, the pyrene groups of the probe molecules could stick out of the film surface or at least be located right at the surface. On the other hand, for systems in which the host chains are much longer than the probe chain, the pyrene groups of Py-c16 would be completely enclosed inside the packed aliphatic tails of the host acids. Figure 6 shows a minimum Zl/Z3 value, about 2.3, for both Py-Cl2- and Py-Cle-containing systems in which the
1724 Langmuir, Vol. 7, No. 8, 1991
Chen and Frank interaction among the alkyl chains is present. It has been shown in Langmuir-Blodgett= or self-aseembled2monolayer studies that for the alkanoic acid systems,a minimum hydrocarbon chain length of about 12 carbons is required to form well-ordered films. Allara and Nuzzo2 have suggested the possibility of random, disordered multilayer formation of C12 and shorter acids on Al based on ellipsometry measurements. The sudden change of for host chains having less than 12 carbons agrees with this proposal. In addition, we note that the reproducibility of the Ill&data for the short host fatty acid films was not as good as for the long host acid films, indicating that the mechanism of film formation for Cl2 and shorter acids is probably quite complicated and difficult to control. Calculationof Effective QuencherDiffusivity. The fluorescence quenching data can be used to obtain information on the quencher mobility in the neighborhood of the fluorophore. The bimolecular quenching constant k, is related to the diffusion coefficient by the Smoluchowski-Einstein equation
Figure& Schematic structuresof ideal Py-Cie/C,,mixed monolayer films: (a) PY-CM/CU; (b) Py-Cie/Cie; (c) PY-CdCis;(d)
PY-Cv/Cao;(9) Py-Cie/Cm
host molecule has six or more carbons longer than the probe. This value corresponds to a state similar to the nonpolar hydrocarbon case (e.g., heptane), as shown in Figure 3. This would correspond to the fully enclosed pyrene groups localized in a hydrocarbon packing in Figure 8e. As the length of the host molecule is decreased, the 11/13 value increases, reaching a maximum for systems with host chains of 14 carbons. This trend may be correlated with the change in the location of the pyrene groups relative to the film surface such as suggested by the varying film structures shown in Figure 8. When the host molecules become shorter, they can no longer totally enclose the pyrene groups, and the probe environment is no longer the nonpolar situation. As the probe becomes more and more exposed to the exterior of the hydrocarbon films, the apparent polarity of its environment increases. As the host chain length is further decreased from 14 or 12 or less carbons, however, the Z1/Z3 value decreases from its high value at C14 to a lower value. There are two possible reasons for this: (1) If the portion of Py-ClS outside of the film is long enough, the unsupported segments of the probe chain may bend and cause the pyrene end to “lie downmon the surface and be covered by other chain segments;hence, they may become confined in a somewhat less polar environment. (2) When the host fatty acid is very short, the adsorption of the molecules may be incomplete or the resultant films may not be well-organized structures because not enough cooperative van der Waals
where N A is Avogadro’s number, DQ is the effective diffusion coefficient, R is the sum of the collisional radii of the quencher and the fluorophore, and y is a proportionality constant. To perform such further analysis, two complicating factors have to be considered. The first is the partitioning of the CH3N02 quencher between the aqueous solution and the hydrocarbon film phases. The situation is completely analogous to the partitioning of a solute between two dissimilar solvating phases. We measured this partition constant with long chain alkanes, including a liquid (hexadecane) and a solid (octadecane), in equilibrium with aqueous CHsNO2 solutions using FTIR and UV absorption measurements. A numerical value for the partition coefficient,defined as the CHaNO2 concentration ratio between the hydrocarbon phase and the aqueous phase, was obtained Second, the effect of the aluminum substrate on the fluorescence lifetime has to be considered. As shown schematically in the insert in Figure 9, when a fluorophore is very near a reflecting/absorbing metal surface, the partially reflected wave can cause interference with the original wave, and the nonradiative energy transfer to the metal can reduce the fluorescencelifetime significantly. Figure 9 shows the results of a calculation using the approximate theory of Kuhn.29 The two curves are for the fluorophoresin parallel and perpendicular orientation (with respect to the substrate surface), respectively. At a certain distance the interference effect starts causing a sinusoidal fluctuation in the lifetime, while at very short distances the nonradiative energy transfer dominates. For our systems the distance of separation between the fluorophore and the surface is less than 100 A, and the nonradiative energy transfer should be important. According to a later exact theoretical calculation by Chance, Prock, and Silbey,mKuhn’s approximation underestimates T for the parallel case. The important issue to be noted here, however, is that within the range of these distances the lifetimewill decrease by a factor of about 0.9 when d is decreased by about 5 A (from Py-Cl6 to Py-ClZ, four carbon chain segments). (38)Gaines, G. L. Insoluble Monolayers at Liquid-Cos Interfaces; Interscience: New York, 1966.
Self-Assembled Monolayer Films
Langmuir, Vol. 7, No. 8, 1991 1725
f 4
9.0 1
k2.8
8.0-
2.7
zx 7.0 -
2.6
0
h
0 v) Q)
2.5
6.0 0
0
v
d5.0 -
0 0
.\9L*
--
12.4 12.3
0
-8
0.5
-4 0 4 8 12 (Host chain length - Guest chain length)
Figure 10. Apparentdiffusivityof CHsNOzin Py-C,/C, mixed monolayers and 11/13 of the monolayers plotted against hostguest chain length difference.
For comparison, the 11/13 data for host molecules longer than C12, previously shown in Figure 5, were also shifted 0 2 4 6 8 and shown in Figure 10. The apparent microenvironment 2x11 ,d inhomogeneity variation as observed with the pyrene II/ I 3 measurements is shown closely followed with another h independent fluorescence quenching measurement of a Figure 9. Relativefluorescencelifetime vs distanceof separation transport parameter. Although based on only a semiquanbetween the fluorophore and a metal surface, calculated by Kutitative analysis and not to be used in an absolute manner, hn's approximationtheory. The insert showsthe resulting values for the Py-C1e/C,, and Py-Cle/CXxmonolayers on Al. the relative D,variation definitively reflects the existence of a nonhomogeneous region formed in the vicinity of Since there are yet many other uncertainties including pyrene groups incorporated in the ordered SA monolaythe exact fluorophore orientation and position and their ers. variation from one system to another, we only with to For host chains shorter than C12, the 11/13 values do not provide a reasonable semiquantitative analysis of our fit on the master curve, but fall significantly below and results based on a first-order calculation using Kuhn's have not been included in Figure 10. This kind of anomaly simple theory. To accommodate the unknown orientation in 11/13, which is probably due to the structure disordering effect, an average was taken between the parallel and the discussed previously, was not observed in the fluorescence perpendicular cases, after multiplying by an arbitrarily quenching results. It seems that for the systems of very chosen "correction factor" of 2 for the parallel orientation short host molecules, although the film structures are no case to compensate for the underestimation by Kuhn's longer well-organized, the probes are still fully exposed to approximation. Using estimated numerical values of 50 the external phase since the randomized films do not form and 55 A for the distances of separation39d for Py-C12/Cxx an effective "barrier" to quencher diffusion. / and Py-C16/CXx,respectively, we obtained ~o(Py-C12/C~~) It should, however, be noted that the above treatment 7O(Py'c16/cxx) 0.91. is not the only possible interpretation of the fluorescence The quenching constant data can then be used to quenching results. For example, the increase in KD with calculate the effective quencher diffusivity according to decreasing host chain length may be thought of as due to the proportionality given in eq 4. To do this, we compared "pores" formed in the preturbed structure, allowing the our data to literature values35of k, = 3.04 X lo9 (L/mol) quencher to access the fluorophore. This relationship sand D, = 1.4X loa cm2/sfor CHsNOs/pyrene in aqueous between KDand the "accessibility" of the enclosed pyrene systems. For our systems, we neglected the mobility of groups is probably useful for considerations in some the anchored pyrene groups because the rate of carbonapplications such as for immobilized chemical sensors. carbon bonds rotation required for this motion should be small, compared to the freely diffusing quencher. The Summary relative CH3N02 diffusivity in the vicinity of the pyrene In summary, the fluorescence probe method was used groups thus obtained is shown in Figure 10, in which D,, to study the incorporation of a foreign chemical group normalized to the value of Py-C16/cxx with x x > 20, is into the organized hydrocarbon chain packing region in plotted against the host-probe chain length difference. the self-assembled monolayers. Although other macroFor the two probes a "universal curve" was found, scopiccharacterization methods (FTIR, contact angle gosuggesting the unique effect of probe location relative to niometry, and ellipsometry) showed that the global the film surfaces, as was also seen from the 11/13 result. monolayer structures were not significantly changed upon For the maximally exposed pyrene case, the calculated D, incorporating foreign groups, fluorescencemeasurements is at a maximum. As the host chain length is increased, showed that the local structure may be altered. The pyrene D, decreases, indicating that the hydrocarbon chain probes used in this work were especially useful in that the packing somehow hinders the diffusive motion of CH311/13 of the fluorescenceemission was shown to follow the N01. For the totally enclosed pyrene case D, reaches its microscopic variation in the environment polarity with minimum and can be thought of as the diffusivity within probe location. The anticipated monolayer structures for the hydrocarbon packing. On the other hand, the value systems with host molecules of 14 or more carbons were forthe exposed pyrene case is probably related to a "surface supported by these data. Structural disordering for diffusivity". systems with host chains shorter than 12 carbons was also indicated. The fluorescencequenching experiments show (39) An estimated thicknees of native A1103 layer of 30 A was included in d. the effects of monolayer structure on quencher diffusion. 0
1726 Langmuir, Vol. 7, No. 8, 1991 The effective diffusivity of CHaNOz in the vicinity of the pyrene group was decreased by half when the pyrene group switches from a totally exposed case to a fully enclosed case. These results not only show the variation in the localized probe environments with monolayer structure but also demonstrate the effect of the perturbation of the localized probes on the accessibility and reactivity of the surface-anchored chemical groups. This work also dem-
Chen and Frank onstrates the versatility of the fluorescence techniques and also shows the molecular-level structure design and control capability of the self-assembly method.
Acknowledgment. This work was supported by the Office of Naval Research under Contract N00014-87-0426. Helpful discussions with Professor R. Fabian W.Pease are also gratefully acknowledged.
