Langmuir 1993,9, 302-3008
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Naphthalene Chromophore Tethered in the Constrained Environment of a Self -Assembled Monolayer Klemens Mathauer and Curtis W. Frank' Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received April 13,1993. I n Final Form: July 12,199P [ll-(2-Naphthyl)undecyl]trichlorosilaneand octadecyltrichlorosilanewere coadsorbedfrom hexadecane solution onto quartz. The absorbance ratio of the A-Bb to A-La transitions of naphthalene in the UV spectraof a monolayer comparedto solution spectraindicatean orientationof the naphthalene chromophore with its long axis toward the normal of the substrate surface. Contact angles, transmission FTIR,and fluorescence spectra of adsorbed monolayers with different ratios of the two components show that the distributionof chromophoresis homogeneousover the whole range of composition. The exclusiveappearance of a high-energy naphthalene excimer with increasing surface concentration of the naphthalene-tagged component is observed and explained by the special geometry in the monolayer. A simple two-dimensional lattice model taking energy migration into accounthas been applied to explain the dependence of the ratio of excimer to monomer fluorescence on the surface concentration of naphthalene chromophores.
Introduction The adsorption of alkyltrichlorosilanes is a well-known procedure for the modification of silica and related surfaces.'P2 With the work of Sagiv? who found that longchain n-alkyltrichlorosilanes form highly ordered closepacked monolayers, this form of surface modification became part of a field called self-assembly(SA). The most studied systems in this area are long-chain n-alkylsulfur compounds (alkanethiols,PGdialkyl sulfide^,^,^ and dialkyl disulfidesgJO)which form similar films on gold. A distinct advantage of self-assembly of the n-alkyltrichlorosilanes or n-alkylsulfur compounds over the classical Langmuir-Blodgett technique1' is the stability of self-assembled monolayers due to the possibility of chemical reaction with the surface.12 Thus, a subsequent chemical treatment of the chemisorbed monolayer surface is possible when appropriate functional groups are present at the tail of film-forming molecules. For example, a new hydroxylated surface can be formed from terminal olefin13 or ester g r 0 ~ p s . lWith ~ this approach it is even possible to assemble multilayers by the adsorption process.14*16 Functional or chromophoric groups are necessary in any
* Abstract published in Advance ACS Abstracts, September 1, 1993. (1)Leyden, D. E., Ed. Silylated Surfaces; Gordon & Breach New York, 1980. (2)Grushka, E., Ed. Bonded Stationary Phases in Chromatography; Ann Arbor Science Publication: Ann Arbor, 1974. (3) . Saniv., J. J.Am. Chem. SOC.1980. 102.92. (4)Porter, M.D.;Bright, T. B., Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559. (5)Nuzzo,R.G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. SOC. 1987,109,733. (6)Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.;Nuzzo,R. G. J. Am. Chem. SOC.1989,111,321. (7) Li T, T.-T.; Weaver, M. J. J. Am. Chem. SOC.1984,106,6107. (8)Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D.L.;Porter, M. D. Langmuir 1988,4,365. (9)Tanirmchi, I.: Tozosawa, K.: Yamamchi, H.: Yasukouchi. K. J. Chem. Sot, Chem. Commun. 1982,1032.(10)Nuzzo, R.G.; Fusco, R. A.; Allara, D. L. J.Am. Chem. SOC.1987, 109,2358. (11)Ulman, A. An introduction to ultrathin organic films: from Langmuir Blodgett to self-assembly;Academic Press: San Diego, 1991. (12)Recently it has been shown that in the case of the trichlormilanes there is no direct reaction of octadecyltrichlorosilane with the surface hydroxy groups or the first adsorbed water layer but with subsequent layers of water: Tripp, C. P.; Hair, M. L. Langmuir 1992,8,1120. (13)Netzer, L.;Sagiv, J. J. Am. Chem. SOC.1983, 105, 674. (14)Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989,5,101. (15)Pomerantz, M.;Segmuller, A.; Netzer, L.;Sagiv, J. Thin Solid Films 1985,132,153. I
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conceivableapplication'e of SA films like nonlinear optics, molecular electronics, or sensor devices.ll Moreover, coadsorption of tagged and untagged surfactants is required in order to control the surface concentration of the functional groups. To study the incorporation of chromophoric groups, we have selected the alkyltrichlorosilane system because optically transparent substrates can be used. The first experiments in this field have been performed by S a g i ~however, ;~ he used dyes with head groups which could not irreversibly bind to the surface and were thus squeezed out by the coadsorbant octadecyltrichlorosilane (OTS)after longer adsorption times. Coadsorption of w-substituted alkanethiols on gold surfaces was investigated by Bain et 81.l' With regard to the distribution of chromophores in a monolayer, the self-assembly process has another advantage over the LB technique. Most two-component monolayers phase separate a t the air-water interface to form large domains of each compound, whereas this is not expected17for mixed self-assembledmonolayers prepared by coadsorption. Chen and Frank showed that for the reversibleadsorptionof fatty acids onto aluminum surfaces the contribution of the head group substrate interaction is about equal to the van der Waals interaction of CIS chains.ls Thus, it is reasonable to assume that the influence of the head groups dominates in the case of OTS and van der Waals interactions of the alkyl chains are less important. Low-angle X-ray reflection studies on partial OTS layers show that these layers do not grow in an island fashion,lg and the structure of a complete monolayer exhibits a short-range in-plane liquidlike order.20 Nevertheless, it is obvious that van der Waah interactions between the surfactant molecules become important in the final stage of the adsorption process in order to provide the possibility for a close-packed hydrocarbon structure. We can thus expect that a homogeneoustwo-dimensional distribution of two components can be achieved by (16)(a) Roberta, G. G. In Langmuir-Blodgett F i l m ; Roberta, G. G., Ed.; Plenum Press: New York, 1990; p 317. (b) See ref 11. (17)(a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. SOC. 1989,111,7155.(b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC. 1989.111.7164. ~ . _ . ~-~ , ---,
(18)Chen, S.H.; Frank, C. W. Langmuir 1989,5,978. (19)Wasserman, S.R.;Whitesides, G. M.; Tidswell, I. M.; Ocko,B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. SOC.1989,111,5852. (20) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Kosomky,S. D.; Folkers, J. P.; Whitesides, G. M. J. Chem. Phys. 1991,95,2854.
0743-7463/93/2409-3002$04.00/00 1993 American Chemical Society
Langmuir, Vol. 9, No. 11, 1993 3003
Naphthalene Chromophore in a Self-Assembled Monolayer coadsorption if the following conditions are fulfilled: (1) both componentsmust have the samesurface-bindinghead group and show a similar solubility in the solvent; (2) no specific interaction (like hydrogen bonding) can exist between the molecules of one component; (3)the geometry of both molecules should allow a close packing in the monolayer. From a photophysics point of view it seemed reasonable to use naphthalene as a model chromophore. This chromophore is often used in the study of polymer conformation and miscibility by photophysical methods.21 The existenceof local aggregation or potentially interesting constraints should be detectable by optical absorption and emission spectroscopy. The short-range neighborhood of a naphthalene chromophore in the monolayer can be investigated by the excimer-to-monomer ratio from the fluorescence spectrum. At higher chromophore concentrations energy migration among the naphthalene chromophores is expected to affect the excimer-to-monomer ratio in the fluorescence spectra. With regard to the architecture of self-assembled monolayers of n-alkyl chains,the naphthalene should fit into the monolayer with minor disturbance when attached through its 2-position to an alkyl chain surfactant. In this paper we will describe the synthesis of a 2-naphthyl-tagged n-alkyltrichlorosilane and the formation of mixed films with octadecyltrichlorosilane (OTS) onto quartz. We characterize the films by several independent methods (wettability and FTIR, W, and fluorescence spectroscopies) and derive a model for the architecture of these films from the results.
Experimental Section General Procedures. NMR spectra were recorded on a Varian XL-400 spectrometer in CDCls solvent and referenced to CHCL ('H NMR 7.24 ppm, 13C NMR 77 ppm). o-Undecenyl bromide (Pfaltz&Bauer),2-chromonaphthalene(Aldrich),trichle rosilane (Aldrich),and odadecyltrichlorosilane (Huels)were used as received. Hexadecane (Aldrich) used for the adsorption solutions was passed throughneutral alumina (BrockmanActivity I) prior to use. Ether was distilled from LiAW. Substrates. The quartz slides used as Substrates (Hellma cuvette windows) were first cleaned with 2-propanol and wiped with Kimwipes, rinsed with chloroform, and then treated with a mixture of concentrated H2SO4 and H202 (7/3 v / v )for ~ ~30 min at 90 "C. After cooling to room temperature, the slides were rinsed with deionized water and dried with a stream of nitrogen. Used substrates were cleaned with the same procedure, then treated with a 5% HF solution for 30 s, rinsed with deionized water, and once again cleaned with the HfiOdH202 mixture as described above. Monolayer Preparation. The freshly cleaned quartz slides were immersed in a 2 mM solution of alkyltrichlorosilane in hexadecane in 20-mL scintillation vials that had been cleaned by storing them for a minimum of 24 h in a 20 % KOH solution in 2-propanol and thoroughly rinsed with deionized water. All solutions were prepared and handled in a nitrogen atmosphere (Aldrich atmosbag). After 20 h the substrates were transferred to scintillation vials containing methylene chloride and then removed from the nitrogen atmosphere. In order to remove any solution-deposited material, the substrates were wiped off with 100% cotton tissues that had been dipped in 2-propanol. The substrates were then rinsed with chloroform and ethanol and dried with a stream of nitrogen. Contact Angle. Advancing contact angles were determined by the sessile drop technique with a RambHart Model 100contact (21)Semerak, S. N.;Frank, C. W. Adv. Polym. Sci. 1983,54,31. (22)Pintchovski, F.; Price, J. B.; Tobin, P. J.; Peavey, J.; Kobold, K. J. Electrochem. SOC.1979,26,1428.
angle goniometer at room temperature.= The contact angles were measured by applying a 2-pL drop to the surface, and while retrieving the needle, the volume of the drop was increased by about 50%. The contact angles obtained with this method are typically 2-3O lower than the maximum advancingcontact angles obtained by the captive drop technique.ll FTIR Spectroscopy. FTIR transmission spectra of the monolayers on the quartz slides were recorded on a BIORAD Digilab FTS-GOA single-beam spectrometer equipped with a HeNe laser and a TAB detector. Spectra were recorded at 4-cm-' reaolution,and 1024scans were coadded. A single-beam reference spectrum of a freshly cleaned quartz slide was recorded before each series of measurements (usually five spectra) and used as a background spectrum in the recording of the monolayer spectra UVSpectroscopy. UVspedraon [11-(2-naphthyl)undecyl]trichlorosilane 2-Np-containing monolayers were obtained with a Varian CARY 3 W-vis spectrometer with a clean quartz slide as reference. Fluorescence Spectroscopy. Steady-state fluorescence spectra were recorded on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. Corrected emission spectra were recorded in the front-face mode in air with an excitation wavelength of 285 nm and slits of 4 mm. 11-(2-Naphthyl)undec-l-ene (la) 11-(2-Naphthyl)undec2-ene (lb). A portion of a solution of 12 g of o-bromoundecin 100 mL of dry ether was added to 2 g of 1-ene (51.4 "01) magnesium turnings under nitrogen. After addition of catalytic amounta of iodine,the mixture was slightlyheated without stirring until the Grignard reaction started, and then the rest of the solution was added over a period of 30 min. After stirring for 1 h under reflux, the Grignard solution was decanted off and added all at once to a water-cooled mixture of 10.6 g (51.2 "01) of 2-bromonaphthaleneand 80 mg of Ni(DPPE)2ClP in ether. After stirring for 20 h at room temperature, the mixture was acidified with diluted HCl and the organic phase washed with NazCOs solution and water and fiially dried over NaaSO4. By chromatographyonsilicagel (n-hexanes)a byproduct (RF = 0.8, heptane, probably the undecenyl dimer) can be separated. Unreacted 2-bromonaphthalenewas removed under vacuum (50mTorr, 100 OC oil bath temperature). Yield 7.15 g (50%),1a:lb = (53:47). lH NMR (400 MHz, CDCls): 6 = 7.82 (m, 3H, Np), 7.65 (8, lH, 1-Np), 7.46 (m, W , Np), 7.37 (pd,lH, Np), 5.87 (m, lH, H-2 (la)),5.47 (m, 2H, H-2,3 (lb)), 5.02 (m, 2H, H-2 (la)),2.81 (t, 2H, 11-H), 2.08 (m, 2H, H-3 (la),H-4 (lb)), 1.75 (m, 2H, H-ll), 1.67 (d, 3H, H-1 (lb)), 1.36 (m, CH2). [ll-(2-Naphthyl)unde1cyl]trichlorosilane(2-Np). A 4-g (14.26-"01) sample of the mixture l a / l b was dissolved under nitrogen (Aldrich atmosbag) in 20 mL of trichlorosilane in a pressure tube (Teflon cap) and 14 p L of a 0.0131 M solution of H&Cb in 2-propanol added. The tube was sealed, and the mixture was heated under stirring at 95 'C for 20 h. After cooling to room temperature, the solution was tranferred under nitrogen to a round-bottom flask. Trichlorosilane was removed by distillation. The residue consists mainly of a mixture of ca. 80 mol 7% 2-Np and ca. 20% unreacted lb. 2-Np was isolated by fractionated shorbpath distillation at 50 mTorr (230 OC oil bath temperature). About 2 g of forerun was taken. Yield 2.1 g of 2-Np (35%). 1H NMR (400 MHz, CDCl3): 6 = 7.79 (m, 3H, Np), 7.76 (8, lH, Np), 7.44 (m, 2H), 7.34 (dd, lH, Np), 2.78 (t, 2H, H-ll), 1.71 (m, 2H, H-lo), 1.57 (m, 2H, H-1). 13CN M R (100.6 MHz,CDCL): 6 = 140.40,133.61,131.89,127.70,127.58,127.36, 126.26,125.79,36.1,31.8,31.36,29.52,29.31,29.25,29.08,28.99,
24.28, 22.23.
Results Synthesis. The synthesisof [11-(2-naphthyl)undecylltrichlorosilane (2-Np) was performed by coupling the Grignard reagent of w-bromoundecene with 2-bromonaphthalene followed by a platinum-catalyzed (HzPtCb) hy_________
~~
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(23)Waaserman, S.R.;Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 6,1074. (24)Tamao, K.; Sumitani, K.; Kim, Y.; Zembayashi, M.; Fujioka, A.; Kodama, S.; Nakajiama, I.; Minato, A.; Kumada, M. Bull. Chem. SOC. Jpn. 1976,49,1968.
Mathauer and Frank
3004 Langmuir, Vol. 9, No. 11, 1993
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Mole fraction 2-Np in solution Wavelength [nm] Figure 1. UV absorption spectrum of a coadsorbed monolayer (solid line) of OTS and 2-Np (89.7 mol % 2-Np in solution) compared to a solution spectrum (dotted line) of 2-Np in n-heptane (C = 3.26 X 1V moVL). The monolayer spectrum is obtained with a clean quartz slide as reference.
drosilylation of the terminal double bond with trichlorosilane (HSiC13).25s26Although the platinum catalyst causes an almost 50% migration of the terminal double bond in the 2-pwitionythe hydrosilylation yields only the 1-substituted product. This is in agreementwith previous r e ~ u l t a . ' ~Following *~~ the reaction with 'H NMR shows that the 1-ene compound reacts faster with the trichlorosilane than the a-ene, which itself reacts under isomerization to the 1-trichlorosilyl product. No effort was undertaken to complete the isomerization. The product can be separated from unreacted 11-(2-naphthyl)undec2-ene and other side products by short-path distillation. The purity of the compound was estimated by NMR to be greater than 95% The main impurity is the unreacted 2-ene compound which should not adsorb on quartz. The formation of other trichlorosilane compounds during the hydrosilylation reaction was prevented by careful purification (distillation and chromatography) after the Grignard step. UV Spectroscopy. UV spectra were taken of coadsorbed monolayers over the whole range of composition. Figure 1shows a UV spectrum of two monolayers (one on each side of the quartz slide) adsorbed from a solution containing 89.7 mol % 2-Np compared with the solution spectrum of 2-Np in heptane. The 'La band of the naphthalene around 280 nm shows a red shift of 5 nm in the monolayer spectrum compared to the solution. Mab suki et al. observed a similar red shift for chromophores in Langmuir-Blodgett films.27 The higher dielectric constant of the solid environment in the monolayer compared to the heptane solution can account for this solvatochromic effect. Another striking feature in Figure 1 is the difference in the ratio between the absorbances of the 'Bb band at 224 nm and the 'La band around 280 nm for the solution and film. The transition dipole moment for the 'A-'Bb transition is parallel to the long axis of naphthalene28 whereas the A-lL, transition is parallel to the short axis. For transmitting light the electric field vector lies parallel to the substrate surface, and thus the smaller ratio between both peaks for the monolayer shows that there is a preferential orientation of the long
.
'
(25) Speier, J. L.; Webster, J. A.; Barnes, G. H. J. Am. Chem. SOC. 1967, 79, 974. (26) Ryan, J. W.; Speier, J. L. J. Am. Chem. SOC. 1964,86, 895. (27) Matsuki, K.; Fukutome, H. BUZZ.Chem. SOC.Jpn. 1983,56,1006. (28) (a) Klevens, H. B.; Platt, J. J. Chem. Phys. 1949, 17, 470. (b) Platt, J. J. Chem. Phys. 1949, 17, 484.
Figure 2. Absorbance of the 'L,band of naphthalene at 280 nm and the 'Bb band at 224 nm of coadsorbed monolayers of OTS and 2-Np as a function of the mole fraction of 2-Np in solution.
axis of the chromophores toward the normal of the substrate surface. The peak maxima of both bands in the monolayer spectra are plotted in Figure 2 against the composition in solution. Both band intensities increase with increasing mole fraction of the 2-Np naphthalene compound in solution. The uncertainties for both series are given as a standard deviation derived by measuring 10 spots on the same sample. The reason for the higher uncertainty at 224 nm is probably due to slight thickness changes of the quartz substrate, which has an absorbance of ca. 0.04 at this wavelength. The total absorbancefor the monolayers lies in the range that is expected for two monolayers of naphthalene. The absorbance clearly increases with increasing 2-Np concentration in solution. Given that the adsorption of trialkylsilanes onto hydroxylated surfaces is irre~ersible,2~ this indicates that OTS and 2-Np adsorb with the same rate. Contact Angles. The sessile drop technique was used to obtain quasi-equilibrium static advancing contact angles. Figure 3 shows the contact angle data for a series of coadsorbed monolayers of OTS and 2-Np. The linear relationship for both water and hexadecane contact angles is in agreement with Cassie's law:30
e = x1 cos el + x 2 cos e2
(1) where x1 and xp are the mole fractions of the two components in the monolayer and 61 and 62 are the contact angles of the pure monolayers. This shows that the contribution of each component to the surface energy is additive and no specific interactions between the surfactants are present. Recently, Israelachvili and Gee pointed out that Cassie's law is only valid for a surface made of discrete patches of the two component^.^^ By averaging molecular quantities like polarizability and dipole momenta rather than cohesive energy, they derived a new relation for homogeneous binary systems: COS
(1 + COS el2 = x,(i + COS ell2 + x2(i + COS e2i2 (2) The expected values for cos 8 according to this relationship are shown as a dotted line in Figure 3. The error for contact angle measurementa does not allow a distinction between the two laws for our experiment. A linear dependence of cos 8 on composition was also found for (29) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. J. Am. Chem. SOC. 1992,114,5436.
(30)Cassie, A. B. D. Discuss. Faraday SOC.(London)1948,3,11. (31)Israelchavili, J. N.; Gee, M. L. Langmuir 1989,5, 288.
Naphthalene Chromophore in a Self-Assembled Monolayer
Langmuir, Vol. 9, No. 11, 1993 3005
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Mole fraction 2-Np in solution Figure 3. Advancing contact angles of water and hexadecane on coadsorbed monolayers of 2-Np and OTS. The solid line represents the course for the cosine of the water contact angle which isexpectedby Caasie's law and the dotted line that expected by the expression of Gee and Israelachvili.
coadsorbed f i i s of tetradecyltrichlorosilaneand w-tetradecenyltrichlorosilane on silicon wafers32and bromoand methyl-terminatedn-alkanethiols on gold surfaces.17a Transmission FTIR Spectroscopy. Quantitative FTIR measurements in the CH stretching region on monolayers can be used to obtain information about the surface coverage and the average orientation of the hydrocarbon tail of the amphiphilic molecules as long as well-definedreference standards are available. L a n g m u i ~ Blodgett fiis of n-alkanoic acid cadmium salts show an almost perfect orientation of the hydrocarbon chains perpendicular to the substrate.33 Using these films, Chen previously determined an absorption coefficient for the asymmetric CH2 stretching peak va(CH2) in monolayers that show a tilt angle of ' 0 to be 0.00425 absorbance-A2/ CH2 unit.% In Figure 4 the CH stretching vibration regions in the FTIR spectra of several mixed monolayers of 2-Np and OTS (on quartz) are shown. Going from low to high surface concentrations of 2-Np, the CH3 asymmetric inplane C-H stretching va(CH3) band at 2960 cm-' due to the OTS component disappears, and the antisymmetric CH2 stretching vibration peak va(CH2)decreasesand shifts from 2918 cm-' for a pure OTS monolayer to 2922 cm-l for a pure 2-Np layer. Snyder et al. have shown that the location of the (CH3 peak is indictive of the extent of lateral interaction between polymethylene chain^.^^?^^ This has been used by Porter et al. to determine the state of aggregation in n-alkanethiol monolayers on gold as a function of chain length.37 They found that with chain lengthssmaller than 16 carbons the peak poiition approaches that of an octanethiol liquid (2924 cm-9. The value for a pure 2-Np (32) Silberzan, P.;Lbger, L. D.; D. A w e d , Bernattar, J. J.Langmuir 1991, 7, 1647. (33) (a) Chollet, P. A.; Messier, J. J. Chem. Phys. 1982, 73,235; Thin Solid Films 1983,99, 197. (b) Bonnerot, A.; Chollet, P. A.; Frisby, H.; Hoclet, M. Chem. Phys. 1986,97, 365. (34) Chen,S. H.;Frank,C.W.Langmuir 1989,5,97&ACSSymposium Series 447; Scheming, D. R., Ed.;American Chemical Society: Washington, DC, 1990, p 160. (35) Snyder, R. G.;Straws, H. L.; Elliger, C. A. J.Phys. Chem. 1982, 86, 5145. (36) Snyder, R. G.;Maroncelli, M.; S t r a w , H. L.; Hallmark, V. M. J . Phys. Chem. 1986, 90,5623. (37) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109,3559.
2950
2900
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Wavenumbers [cm-'] Figure 4. CH stretching vibration region in the FTIR spectra of several coadsorbed monolayers of 2-Np and OTS on quartz. The antisymmetric CHZstretchingvibration peak v.(CHz) shifte from 2918 cm-l for a pure OTS monolayer to 2922 cm-1for a pure 2-Np layer. "'""-
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Mole fraction 2-Np Figure 5. Antisymmetric CH2 stretchingvibration peak height as a function of monolayer composition. The dashed line represents expected values for a constant tilt of Oo and 30°, respectively,with minimal areas of 20 Az for OTS and 26 A2 for 2-Np.
monolayer (2922 cm-') agrees with the frequencyexpected for a monolayer consisting of C-11 alkyl chains, and thus, it can be concluded that the naphthalene groups do not disturb the packing tendency of the alkyl chains.% Figure 5 shows the absorbance at the va(CH2) peak maximum as a function of 2-Np mole fraction. A pure 2-Np monolayer shows an absorption of 0.0015 f 0.0002. This value is in agreement with a close-packed structure of 2-Np molecules when one molecule occupies an area of 25 A2 (which is the value found in close-packed planes of naphthalene crystals) and a tilt of the hydrocarbonchains of about 30°. In Figure 5 two dashed lines are shown which represent the calculated absorbance of va(CH2) for the mixed monolayers with constant tilts of Oo and 30°, where alkyl chains show maximal van der Waals interaction^.^^ The calculations are based on a smallest possible area per molecule of 20 A2 for OTS at a tilt angle of Oo and 25 A2 for 2-Np and tilted OTS. It can be seen that a pure OTS monolayer favors a tilt angle of Oo and a pure 2-Np monolayer an angle of 30°. For low and high concentrations the average tilt angle of the mixtures seems to be determined by the major component. (38) Tillman, N.; UL", A.; Schildkraut, J. S.;Penner, T. L. J. Am. Chem. SOC.1988,110,6136. (39) b a n , A.; Eilers, J. E.; Tillman, N. Langmuir 1989,5, 1147.
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Energy [Id cm'] Figure 6. Fluorescence spectra of mixed monolayers of OTS and 2-Np. Correctedemissionspectra were recorded in the frontface mode in air with an excitation wavelength of 286 nm and slits at 4 mm. Depicted is a spectrum for a monolayer with 89.7 mol % 2-Np (solid line) and a deconvoluted spectrum for the excimer (dottedline)usinga fluorescence spectrumof a monolayer with 4.5 mol % 2-Np.
2-Np Surface Coverage Figure 7. Excimer-to-monomer ratio zD/zM in coadsorbed monolayers of 2-Np and 0"s as a function of the molar ratio of 2-Np compared with a fit of the energy migration model. The integrated areas for the monomer and excimer envelope were determined in order to cdcuhted zD/zM.
Fluorescence Spectroscopy. The fluorescence spectra of the monolayer with various amounts of 2-Np show the appearance of an excimer emission at 365 nm with increasing surface concentration of 2-Np, but this is considerably blue shifted compared to the more usually observed sandwich excimer of 2-naphthyl derivatives21 around 400nm. In Figure 6 the corrected emission spectra of a coadsorbed monolayer of 2-Np and OTS (89.7 % 2-Np) is depicted. A deconvolution is shown with the spectrum of a film containing 4.5 mol % 2-Np (which consists primarily of monomer emission) to determine the excimer peak. Figure 7 shows the excimer-to-monomer ratio ID/ IM for the 2-Np/OTS mixed monolayers as a function of the molar fraction of 2-Np. The integrated areas for the monomer and excimer envelope were determined in order to calculate ID/IM.Although the spectra in Figure 6 show a slight shoulder in the region where the normal excimer is expected, most of the observed excimer emission stems from the high-energy excimer. Excitation spectra are found to be independent of the observation wavelength. A dual excimer fluorescence of naphthalene has been observed in the fluorescence spectraof l,3-bis[(4methoxyl-naphthyl)1propane and 1,3-bis[(4hydroxy-1-naphthyl)]propane and has been ascribed to two different excimer geometries.40 The high-energy excimer shows a partly overlapping structure between two aromatic rings and has its emission maximum around 370 nm. The appearance (40)Itagaki, H.; Obukata, N.; Okamato, A.; Horie, K.; Mita, I. Chem. Phys. Lett. 1981, 78, 143.
of the second excimer is caused in these cases by bulky substituents and intramolecular hydrogen-bondformation, respectively. De Schryver et al. showed that both excimer geometries also exist in meso- and rac-1,l'-di-l-naphthyldiethyl ether.41 A partially overlapped pyrene excimer has been observed recently in Langmuir-Blodgett films of pyrene-labeled cellulose octadecanoate and its existence explained by the restricted conformationalchangeof chains in these However, this could only be observed in time-resolved fluorescence spectra. In static fluorescence the usual pyrene excimer dominates the spectrum. Discussion
The linear dependence of the cosine of the water and hexadecane contact angles on the mole fraction of 2-Np and the fact that the water contact angle for a pure 2-Np monolayer surface of 85O f 3' is in close agreement with the contact angle of naphthalene crystals (88°)43suggest that the monolayer surfaces of these coadsorbed films are close packed. A naphthalene crystal" exhibits a monoclinic body-centered lattice (B = 123'1, composed of planes in which the long axes of the molecules are oriented along the c axis. Each molecule has four neighbors in the plane at a distance of 5.1 A. Both 2- and 2,6-substituted naphthalene derivativesretain the same close-packed layer structure of the aromatic rings.46 The only effect of the substituents is to increase the crystal dimensions along the naphthalene long axis. In principle, a 2-Np monolayer can thus obtain a naphthalene packing similar to that observed in these three-dimensional bulk crystale. An example of the packing of hydrocarbon chains and naphthalene can be found in the crystal structure of 2,6di&ylnaphthalene.4 The structure is determined in this case by the close-packed naphthaleneswhich show exactly the same arrangement as in a naphthalene crystal. The bond between the C-2 of naphthalene and the first alkyl C is oriented in crystals with an angle of 27O to the surface normal. Rotation around this bond thus allowstilt angles for the alkyl chain between 8.5' and 65.5' with respect to the naphthalene plane. Because of the symmetryof these layers (glidemirror planes parallel to ac),the hydrocarbon chains have to be in the ac plane of the monoclinic cell and thus cannot adopt a tilt angle of 30' with respect to the plane normal. The alkyl chains show an anomalouslyhigh separation, which is also reflected in the difficulty to crystallize this substance. As shown by Ulman et al.39the best interaction energy for close-packed n-alkyl chains is found for tilt angles around either 0' or 30' with correspondinglattice spacings around 4.3 and 5 A. The latter is on the order of the preferred naphthalene distance. However, no commensurable tilted alkyl sublayer exists for the naphthalene packing. This general problem of commensurability of intraassembly planes has been recently discussed by Ulman."' The present situation is comparable to the bulk behavior of 4,4'-diodyl-l,l'-biphenyl where a liquid crystalline state results because of a similar mismatch. (41) De Schryver, F. C.; Demeyer, K.; van der Auweraer, M.; Quantan, E. Ann. N. Y.Acad. Sci. 1981,366,93. (42) Tsujii,Y.; Itoh, T.;Fukuda, T.;Miyamoto, T.;Ito, S.;Yamamoto, M. Langmuir 1992,8,936. (43) Jones, J. B.; Adameon, A. W. J. Phy8. Chem. 1968, 72,647. (44)Robertson, J. M. Organic Crystals and Molecules; Cornell University Press: Ithaca, NY, 1953. (45)~taigorodskii,A. I. Organic Chemical Cry8tallography; Consultants Bureau: New York, 1961; pp 225-227. (46)Kitaigorodsky, A. Acta Phy8iCOChim. URSS 1946,21, 1047. (47) Ulman, A. Langmuir 1992, 8, 984.
Naphthalene Chromophore in a Self-Assembled Monolayer
hngmuir, Vol. 9, No. 11, 1993 3007
monomer intensity ratio is given by where M is the probability that the absorbed photon ultimately decays from the monomer state and &/QM is the fluorescence quantum yield ratio of excimer and monomer. If energy transfer can be neglected, then every excited monomer which is not part of an excimer-forming site can either lead to monomer fluorescence or decay radiationlessly; thus, where q is the number fraction of chromophoresthat are in an excimer-forming site. To determine q, we apply a simpletwo-dimensionallattice approachwhere the sizeof a lattice site is taken equal to the area per 2-Np molecule; then Figure 8. Possible conformation of a high-energy excimer of naphthalene in a monolayer containing 2-Np. The naphthalene planes are parallel with overlap of one benzene unit at a distance of 3.3 A. The hydrocarbon chains show a normal distance of 4.3
A.
A possible explanation for the exclusive appearance of the partially overlapped excimer in the 2-Np monolayers can be given on the basis of a maximal alkyl chain contact in these monolayers. The smallest possible distance between two naphthalene chromophoresin the monolayer is thus determined by the distance between two parallel alkylchains, and a distanceof 3.3 A to form a fullsandwich excimer cannot be achieved. However,the attachment of the alkyl chain in the 2-position of the naphthalene does allow a partial overlap of two adjacent chromophores as is shown in Figure 8. To generate this model, we fixed the geometry of the naphthalene pair with partial overlap of the naphthalene chromophoresat a distance of 3.3 A and calculated the minimum energy conformation for the alkyl chains with an MM2 force field. The hydrocarbon chains show a normal distance of 4.3 A. Whereasthe fluorescenceof pyrenetaggedmonolayers42 consists predominantlyof the excimer emission at higher chromophore concentrations, this is not observed in the present case. It seems reasonableto assume that at higher surface concentrations of 2-Np adjacent chromophores would adopt the herringbone packing arrangement found in the crystal of naphthalene and its 2-derivativeswhere the short axis of neighboringnaphthalene nuclei make an angle of 55O. Pyrene, however, tends to form groundstate dimers at higher concentrations.48 Excimer fluorescence is a well-established molecular probe for the analysisof the morphology of two-component systems where one component bears an aromatic The convenient experimental quantity is the ratio of excimer-to-monomeremission intensity ZD/IM.The dependence of ZD/ZM on the concentration of naphthalene chromophorescan give informationabout the distribution of the chromophoresin the system. For a phase-separated system a relation with a steep rise at low concentration and a decreasingslopeat higher concentrationis expected. In the followingwe will show that the observed dependence of ID/ZMon the composition of the monolayers can be explained in terms of a homogeneous distribution of naphthalene chromophores with energy migration and trapping of the exciton. For systems in which the triplet state decays quickly and radiationlessly to the ground state, the excimer-to(48)Stevens, B. Spectrochim. Acta 1962,18,439. (49) Frank, C. W.; Gashgari, M.A.; Semerak, S. N.In Photochemical and Photophysical Tools in Polymer Science; Winnik, M. A., Ed.Series C Mathematical Sciences 1986,182,523.
q = N&
where N is the number of neighbors in the lattice (which we assume to be four in the present case), x is the molar fraction of the chromophores,and 5 definesthe probability that two adjacentchromophoresare in an excimer-forming geometry. However, relation3cannot explainthe observed dependence of ID/ZMon the concentration of naphthalene chromophores. Unreasonably high values for either QD/ QMor 5 result, when the relation is fitted to the data points. Gelles and Frank developed a simple lattice model for energy migration and trapping and applied it to concentrated blends of polystyrene with poly(viny1 methyl ether).50 By simply allowing energy migration to nearest neighbors in the lattice, A4 can be expressed in terms of q and an additional parameter, a, the possibility that monomer radiative or nonradiative emission takes place before a step can be made. As shown in ref 50 a-qa
(4) = q + a - qa The probability M that the absorbed photon stays with the monomer until decay is now further reduced due to the possibility that the exciton can hop to an adjacent chromophore. If ke is defined as the rate constant for transfer to one nearest neighbor and kmis the rate constant for monomer emission, then Nxke is the net transfer rate constant and a=
km
k,
+ Nxk,
Combination of eqs 3-5 gives
The mathematical form of eq 6 is able to'reproducethe behavior of ZD/ZM with concentration. The model describes an increased excimer-to-monomer ratio a t higher chromophore concentrations due to increased hopping of the exciton. Thus, the possibilityfor one absorbed photon to decay in an excimer-forming site increases. However, a fit of the experimental data with this relation to obtain quantitative information about the number of excimerforming sites or the hopping distance is not attempted due to the large number of assumptions that have to be made. Nevertheless, it can be shown that reasonable parameters result from a curve that comes closest to the experimental values. The ratio QD/QM is 0.55 in poly(2vinylnaphthalene), where the full sandwich excimer is (60)Gellea, R; Frank,C. W.Macromolecules 1982,14747.
Mathauer and Frank
3008 Langmuir, Vol. 9, No. 11, 1993 formed.51 The high-energy excimer shows a considerably shorter lifetime than the excimer at 400 nm,52and thus, asmaller value for QD/& is expected. The geometry fador [ should be in the same range as observed for excimers in polystyrene ([ = 0.01).60 The ratio k J k m is the most difficult to predict. The dependence of the transfer rate on the relative distances and orientationof the naphthalene groups according to the theory by Flir~te1.5~ is (7) The Fiirster relation predicts for chromophores with a Ro of 11.75 As4 and a distance of 5.1 A (the distance of the nearest neighbor in naphthalene crystals) which are randomly oriented a value for k J k m of 150. However, not only the exact orientation factor K~ (which can have all values from 0 to 4) but also the lifetime of the monomer and therefore k m in the monolayer are unknown. A very likely factor that can reduce the monomer lifetime is oxygen quenchings5 because the spectra were taken in front-face mode in air. The dotted line in Figure 7 was calculated with QD/QM = 0.32,[ = 0.01, and k,/k, = 46. The low transfer rate is reasonable considering the possible _ _ _ ~
(51) Semerak, S. N.; Frank, C. W. Adu. Polym. Sci. 1983,54, 31. (52) Ysmanaka,T.;Takhhi,Y.;Uchida,K. Chem.Phys.Lett. 1990, 17, 405. (53) FBrster, T.H.Discuss.Faraday SOC.1959,27, 7. (54) Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds; Academic Press: New York, 1973.
quenching reactions. The deviation of ID/IM data from the calculated values at high naphthalene concentration is reasonable because, as we pointed out above, at higher concentrationsthe naphthalene has the poseibility to pack in a crystal-like arrangement which should decrease the number of excimer-forming sites.
Conclusion Using wettability and W, FTIR, and fluorescence spectroscopies we have shown that [ll-(2-naphthyl)undecylltrichlorosie forms complete monolayers on quartz with tilted alkyl chains. The chromophore density in a monolayer can be diluted by coadsorption with octadecyltrichlorosilane. In all mixed monolayers the naphthalene chromophores are oriented with their long axis toward the normal of the substrate surface. Fluorescence spectroscopy showed that the naphthalene chromophores are distributed homogeneously at all concentrations. The course of ID/IMwith concentration in the monolayers gives evidence for energy transfer among the chromophores. Acknowledgment. This work has been supported in part by Raychem Corp. K.M.acknowledgesa postdoctoral scholarship by. BASF-AG granted through the German National Scholarship Foundation. (55) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic Press: New York, 1971.
