Photophysical Studies of Ultrathin Films: Characterization of PS and

Ultrathin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) coated ... M. Angeles Izquierdo , João C. Lima , Santiago V. Luis , A. Jorge...
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Photophysical Studies of Ultrathin Films: Characterization of PS and PMMA on Fused Quartz by Fluorescence Spectroscopy of Pyrene and (4-(1-Pyrenyl)butyl)trimethylammonium Bromide E. H. Ellison and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received July 31, 1995. In Final Form: December 28, 1995X Ultrathin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) coated on fused quartz plates (or slides) by dip coating are examined by using fluorescence spectroscopy. Cations of the pyrene derivative (4-(1-pyrenyl)butyl)trimethylammonium bromide (PBN) are adsorbed onto the quartz slides from aqueous solutions. The rate of dynamic quenching and prompt quenching by O2 of the PBN fluorescence on the slide in the presence of PMMA and PS films at varying degrees of coverage indicates that, at low coverage, PS films are patchy while PMMA films are continuous. These results are in agreement with published data from AFM studies of similar systems. The fluorescence of pyrene dissolved in ultrathin films on quartz and on hydrophobic (surface derivatized) quartz slides is also examined. As revealed by the trend in pyrene III/I and average fluorescence decay time with coverage, a fraction of the pyrene molecules in PS films diffuse to the polar quartz surface, where they become adsorbed. PMMA, on the other hand, blocks access of pyrene to adsorption sites on the quartz surface presumably by an interaction between carbonyl groups and quartz surface hydroxyls. This fluorescence technique thus provides additional information to AFM in the penetration of reactive species, such as O2, to the silica-polymer interface and in the polymer-mediated interaction of molecular dopants, such as pyrene, with the silica surface.

Introduction Thin polymer films are frequently prepared by evaporation of solvent from polymer solutions coated on solid substrates. For example, spin coating involves the removal of excess polymer solution on a horizontallyinclined planar substrate by rapid spinning followed by evaporation of the solvent from a liquid film adhered to the substrate. Dip coating involves the deposition of a liquid film onto a vertically-inclined planar substrate by controlled withdrawal of the substrate from a polymer solution followed by evaporation of the solvent. Both methods can be used to prepare ultrathin films or films less than 1000 Å thick. In recent years, there has been a surge of interest in the physical properties of ultrathin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) spin-coated onto silicon wafers (see references below). The intent of such studies is to characterize the interfacial polymer domains and also to gain knowledge of the unusual physical properties of ultrathin films relative to thicker films. Studies of ultrathin films have used ellipsometry1 and neutron2 or X-ray3 reflectivity to determine the thickness dependence of the thermal expansion and glass transition temperature, Tg, of films. Keddie et al.,1 from measurements of ellipsometric angles with temperature, observed an increase in the glass expansivity (or thermal expansion) below Tg and a decrease in Tg with decreasing film thickness of PS spin-coated onto hydrogen-passivated silicon wafers. This was explained by a difference in the mobility of polymer molecules at or near the air-polymer interface relative to that of bulk molecules. Molecules at the air-polymer interface are less constrained, and as the film thickness decreases the proportion of molecules X

Abstract published in Advance ACS Abstracts, March 1, 1996.

(1) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (2) Wu, W.; van Zanten, J. H.; Orts, W. J. Macromolecules 1995, 28, 771. (3) Orts, W. J.; van Zanten, J. H.; Wu, W.; Satija, S. K. Phys. Rev. Lett. 1993, 7, 867.

with enhanced mobility increases and thus Tg decreases. On the other hand, Orts et al.,3 from measurements under high vacuum of X-ray reflectivity profiles with temperature, observed a decrease with decreasing film thickness in the thermal expansion of PS below Tg. The contrasting trends in the thermal expansion of PS in refs 1 and 3 may arise from a difference in substrate surface character. In ref 3, measurements were made on acid-cleaned silicon wafers, which may be more polar and thus more adsorbing than the less polar H-passivated wafers used in ref 1. However, the difference could also be due to the gas pressure during measurements. The experiments in ref 1 were performed at atmospheric pressure while those in ref 3 were performed under high vacuum. It may be that dissolved gases plasticize the films. In studies of ultrathin PMMA films, neutron reflectivity measurements of deuterated PMMA by Wu et al.2 revealed an increase in the thermal expansion below the bulk Tg with decreasing film thickness. Although Tg values were not measured due to instrumental constraints, it was inferred from a similar study4 that the Tg of PMMA films increases with decreasing thickness on acid-cleaned silicon. In a somewhat unrelated study of silica-filled polymers,5 two glass transitions were observed from DMTA measurements: one corresponding to the usual bulk species and another 50-100 °C higher than the bulk Tg depending on the polymer. The higher temperature transition was assigned to the adsorbed polymer. Both PMMA and PS displayed a higher-than-bulk Tg. Other studies have used X-ray reflectivity to examine PS films on float glass slides. As revealed by Reiter,6 dewetting of continuous ultrathin films was observed below Tg. The flow of polymer below the bulk Tg was taken as evidence for a reduction in Tg with decreasing film thickness. PS films at low coverage were also examined (4) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. J. Chem. Soc., Faraday Discuss., in press. (5) Tsagaropoulos, G.; Eisenberg, A. Macromolecules 1995, 28, 396. (6) Reiter, G. Macromolecules 1994, 27, 3046.

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by Stange et al.,7 who used STM and AFM to describe the coverage of PS spin coated from dilute solutions onto surface-oxidized silicon wafers. The results indicate that pseudocontinuous films result from the use of 0.05 and 0.075 wt % PS solutions in toluene and that continuous films result at and above 0.1 wt %. Lower concentrations than 0.05 wt % result in a patchy morphology with regions of exposed SiO2 between the PS. In this report we develop an alternative approach that utilizes fluorescence spectroscopy to characterize ultrathin films of PMMA and PS on fused quartz plates (or slides). This is the first photophysical study of ultrathin polymer films that we are aware of. Two fluorescent probe species are employed: pyrene and the pyrene derivative (4-(1pyrenyl)butyl)trimethylammonium bromide (PBN). Pyrene is analyzed as a dopant in the film by dissolving small amounts of pyrene in the polymer solutions from which the films are prepared. PBN is adsorbed as the cation onto the quartz surface prior to coating the slide with polymer. Because PBN strongly interacts with the quartz surface by an ionic interaction, it does not diffuse into the bulk film. The photophysics of PBN thus report on the solid-polymer interface. Measurements of the pyrene fluorescence ratio, III/I, are used to assess the microenvironment polarity encountered by pyrene in ultrathin films. The pyrene III/I (or three-to-one ratio, which is the intensity ratio of the third and first vibronic bands in the fluorescence spectrum) is a useful gauge of the microenvironment polarity, since “forbidden” vibronic transitions, such as the 0-0 transition or the “one” band, are known to exhibit intensity enhancements at constant pyrene concentration with increasing solvent or microenvironment polarity.8 Other more allowed transitions such as the “three” band exhibit no intensity enhancements with increasing polarity. Examples of III/I values given in ref 8 are 1.65, 1.02, 0.90, 0.75, 0.68, and 0.63 in hexane, diethyl ether, toluene, methanol, acetone, and water, respectively. The intensity of forbidden transitions in pyrene is also strongly enhanced by covalently-attached alkyl groups at the 1-position. Such groups reduce the electronic symmetry of the aromatic ring by electron withdrawal. Therefore, pyrene derivatives such as PBN and 1-methylpyrene exhibit a weak dependence of the forbidden transitions on the microenvironment polarity and thus are not very useful as polarity probes.9 The pyrene fluorescence decay profile is also examined in ultrathin films. There are numerous factors that can influence the fluorescence decay of pyrene, and these are discussed where appropriate. Prior to the discussion of the pyrene photophysics in ultrathin films, we first assess the coverage and morphology of ultrathin films on the quartz slide. This is achieved from time-resolved measurements of the rate of quenching by molecular oxygen (or the rate of O2 quenching) of the PBN fluorescence in the presence and absence of films. In the absence of a film, O2 quenches with high efficiency the fluorescence of PBN adsorbed on the quartz surface. In the presence of a film, the quenching efficiency is reduced, relative to the open surface, to an extent that depends on the coverage and morphology of the film. In this study the ultrathin films are prepared by dip coating. The deposition of organic coatings onto planar substrates by dip coating occurs during withdrawal of the (7) Stange, T. G.; Mathew, R.; Evans, D. F.; Hendrickson, W. A. Langmuir 1992, 8, 920. (8) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (9) Zachariasse, K. A.; Uaz, W.; Sotomayor, C.; Ku¨hnle, W. Biochim. Biophys. Acta 1982, 688, 323.

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substrate from a polymer solution in which it was immersed. Prior to withdrawal, polymer adsorption is necessary to reduce the surface tension of the polymer solvent at the liquid-solid interface so that a film of polymer solution will adhere to the substrate as it is withdrawn. Provided that polymer adsorption has occurred, then the polymer coverage increases with increasing withdrawal speed and polymer concentration.10 An important aspect of this study was the characterization of the PBN adsorption and photophysics on the quartz slide in the absence of a film. Following a discussion of this topic, the photophysics of PBN and pyrene in the presence of ultrathin films are described. The results using PBN as a fluorescent probe indicate that, at low coverages of PMMA, the film morphology of PMMA is clearly different from that of PS. As indicated by quenching of the PBN fluorescence by O2, there are exposed regions on the PS surface, or regions where the O2 concentration is much higher relative to that of covered regions. On the other hand, PMMA films are more continuous, even at low coverage. The results using pyrene as a probe indicate that an interaction between pyrene and the quartz surface is inhibited by PMMA but not by PS, evidently because of the stronger interaction between PMMA and the polar quartz surface relative to PS. Materials and Methods Materials Used. PBN was obtained from Molecular Probes, Inc. (Eugene, OR) and used as received. Pyrene (Kodak) was purified by liquid chromatography using silica gel and cyclohexane. PS standards (MW ) 900 000, Mw/Mn ) 1.10; MW ) 35 000, Mw/Mn ) 1.06) were obtained from Supelco, Inc. and used as received. PMMA standards (MW ) 838 000, Mw/Mn ) 1.04; MW ) 26 900, Mw/Mn ) 1.11) were obtained from Pressure Chemical and used as received. The polymer solvent used was LC-grade methylene chloride (CH2Cl2) containing small amounts of cyclohexene as a stabilizer. H2O2 (30% unstabilized), HF (49%), cupric chloride, and chlorodimethylphenylsilane were obtained from Aldrich. Equipment Used. Emission spectra were obtained using an SLM Instruments SPF-500C spectrofluorimeter, and absorption spectra were obtained using a Varian Cary-3 UV-visible spectrophotometer. Emission decays were collected by exciting at 337.1 nm using a PRA LN-100 N2 laser (fwhm ) 150 ps). The sample emission was focused onto a Bausch and Lomb grating monochromator (1200 grooves/mm) using a series of lenses and detected using a Hammamatsu R1664U microchannel plate PMT. Amplification and digitization of the signal were achieved using a Tektronix 7A29 amplifier and 7912AD digitizer, respectively. Molecular oxygen was delivered in controlled amounts to the evacuated samples from an O2 reservoir equipped with a regulator. Oxygen pressures were measured using a Hastings model 760 vacuum gauge and a Hastings DV-760 vacuum gauge tube. Preparation of Quartz Surfaces. Chemisorbed surface contaminants were removed from the quartz surface by a 30-s etch in a hot 1% HF solution followed by rinsing in distilled and deionized H2O. Slides were then treated according to a procedure for chemically removing surface contaminants from silicon wafers and quartz tubes.11 This treatment involved the submersion of slides in a boiling mixture of concentrated NH4OH, 30% unstabilized (electronic grade) H2O2, and H2O (1:1:3 by volume) followed by rinsing in H2O and submersion in a boiling mixture of concentrated HCl, 30% H2O2, and H2O (1:1:3 by volume). Slides were then exposed to concentrated H2SO4 for 1 h and rinsed with H2O immediately prior to use. This treatment resulted in the most repeatable PBN photophysics and level of PBN adsorption on the quartz surface. (10) Yang, C.; Josefowicz, J. Y.; Alexandru, L. Thin Solid Films 1980, 74, 117. (11) Kern, W.; Puotinen, D. A. RCA Rev. 1970, 31, 187.

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When necessary the quartz surface hydroxyls were reacted with chlorodimethylphenylsilane (CDPS). Quartz slides were refluxed overnight in toluene and excess CDPS using pyridine as a catalyst, and the derivatized slides were subsequently washed with acetone, methanol, and water. Deposition of Polymer Films on Quartz Slides. Polymer films were deposited on the quartz slides by using a modification of a method used to prepare plastic replicas for use in electron microscopy.12 This method involves the drainage of a polymer solution in a glass funnel (3 cm diameter, 10 cm length) through a capillary tube (0.20 cm bore, 12 cm length). Slides were submerged in the solution and fixed in position via a clamp attached to a two-holed rubber stopper acting as a lid. After 2 min of exposure, the stopcock was opened and the solution was withdrawn by gravity at a rate of 0.2 in/s. A low pressure and nonagitating stream of dry N2 was used to purge the funnel and to expedite solvent evaporation from the film after withdrawal. During withdrawal of the polymer solution, certain combinations of polymer and substrate resulted in “streaming”. Streaming is described here as the flow of the polymer solution in small streams on the vertically-oriented slide that results from low levels of polymer adsorption. The result is a nonuniform, opaque film. In contrast, when polymer adsorption is such that the solvent is stabilized on the surface and a uniform liquid coating is observed, a clear film results. We therefore chose as a criterion for the analysis of films that the films be clear or that streaming was not observed. Films prepared from PMMA or PS solutions are heretofore referred to as x-PMy or x-PSy films, where x corresponds to the polymer concentration (in mmoles of monomer per liter of solution) during withdrawal and y refers to the number-average molecular weight of the polymer in thousands. For example 10-PM838 refers to films prepared from 10 mM solutions of PMMA monomer and a polymer molecular weight of 838 000. Sample Measurement. In order to measure the fluorescence of PBN on the quartz slide, it was necessary to reduce the concentration of O2, which quenches the fluorescence, by housing the PBN-derivatized slides in an evacuable cuvette during measurement. During the collection of emission decays, slides were positioned tightly along the diagonal of the cuvette and the laser pulse positioned at 45° to the surface normal. In order to reduce the amount of scattered excitation light on the PMT, an excitation cutoff filter was placed at the monochromator entrance and the cuvette positioned so that specular reflection was away from the PMT. A 1 mm slit was placed 15 mm from the sample at the excitation port of the cuvette holder in order to limit excitation to the middle portion of the slide and to avoid excitation of and/or scatter from the cut edges of the slide. Annihilating pulses were eliminated by placing glass slides between the sample and laser, which reduces the pulse energy via reflective losses. Acquisition of steady-state fluorescence emission spectra utilizing a 150 W Xe arc lamp was more troublesome due to much higher levels of scattered excitation light. A similar sample orientation to that for the time-resolved analysis was ineffective at reducing scattered light intensities. It was necessary to obtain the undistorted spectrum, especially when analyzing values of the pyrene III/I. This meant that an excitation filter could not be used, since the filter overlaps the fluorescence spectrum. In order to obtain the fluorescence spectrum of PBN on the quartz slide, a special dimension and orientation of the slide was necessary. For the time-resolved analysis, a slide dimension of 0.45 × 1.5 in2 was used; such slides fit tightly along the diagonal of the cuvette. For the steady-state analysis, a dimension of 0.39 × 1.5 in2 was used. Such slides could be rotated and positioned inside the cuvette so that scattered excitation was reduced to small levels. However, the use of such slides involved the excitation of the corners and cut edges of the slide. From a time-resolved analysis of O2 quenching of the PBN fluorescence we were able to show that the coverage of polymer films on the roughened, cut edges was less than that on the smooth planar surface. Therefore, under evacuated and controlled oxygen conditions, the steady-state fluorescence spectral analysis was limited to the analysis of PBN adsorbed on quartz slides in the absence of films. However, when air-born O2 did not significantly (12) Bradley, D. E. In Techniques for Electron Microscopy; Desmond, K., Ed.; Blackwell Publishing: Oxford, 1965; p 58.

Ellison and Thomas quench the fluorescence, as revealed by a time-resolved analysis, then the fluorescence spectrum could be collected in air outside the cuvette. The collection of the spectrum outside the cuvette eliminated the scattered light problem while still allowing excitation of the middle portion of the slide.

Results and Discussion Adsorption of PBN onto the Quartz Slide Surface. The number of PBN molecules adsorbed on the quartz slide surface was estimated from the absorbance of a 5.0 mL aqueous PBN solution before and after equilibration with slides of dimension 0.45 × 1.5 in2. The level of PBN uptake was 2.5 and 0.5 nmol/slide when the PBN concentration was 9.3 × 10-6 M and 9.3 × 10-7 M, respectively. The slide with 2.5 nmol adsorbed PBN (or the 2.5-PBN slide) was removed clean, or it appeared to be dry after removing it from the PBN solution. The slide with 0.5 nmol adsorbed PBN (or the 0.5-PBN slide) was not clean after removing it from solution; droplets of H2O were adhered to its surface. A uniform film of H2O was adhered to surfaces free of PBN. These results indicate that, as expected, the degree of surface hydrophobicity increases with increasing PBN concentration. The effect of pH on the adsorption of PBN was also evaluated. The pH should influence the degree of ionization of the surface SiOH group, which may influence the adsorption of PBN cations. We have found that in the pH range between 3.2 and 9.0, using either HCl or NH4OH to adjust the pH, the level of PBN adsorption from a 9.3 × 10-6 M solution is constant at 2.5 ( 0.1 nmol/slide. Since the pKa of silica is near 6.0,13 then ionization of the silica is apparently not necessary to adsorb PBN cations. This supports an ion exchange mechanism. However, early IR studies by Blackman and Harrop14 of the adsorption of quaternary ammonium surfactants on fumed silica indicated that the OH band of the SiOH group was weakened upon interaction with quaternary ammonium ions but not enough to support an ion exchange mechanism. Such a mechanism could be operative here. A calculation of the surface area of the slide, including the cut edges, is 9.5 cm2. The true surface area is probably higher due to surface irregularities, but assuming that it is not greater than twice the calculated value, each PBN molecule is estimated to occupy 65-130 Å2 on the 2.5PBN slide. In units of the coverage, this corresponds to 1.3-2.6 µmol/m2. These values are in fair agreement with the value of 1.8 µmol/m2 determined by Bijsterbosch15 for the maximum packing density of a monolayer of the quaternary ammonium surfactant dodecyltrimethylammonium bromide adsorbed on fumed silica. Fluorescence Spectrum and Decay Profiles of PBN Adsorbed onto Quartz Slides. Figure 1A illustrates the dependence of the PBN fluorescence spectrum on the concentration of PBN on the quartz surface. Prior to measurement, each slide was removed from the solution and rinsed thoroughly with H2O to remove any adhered PBN solution and/or traces of bromide ion, which may quench the fluorescence. Adsorbed H2O was removed by heating under vacuum (at less than 1 mbar pressures) for 10 min at 120 °C in an evacuable quartz cuvette. In Figure 1A, the region between 370 and 425 nm indicates the PBN monomer spectrum; the region between 425 and 600 nm indicates the PBN excimer (or excited dimer) spectrum. PBN excimers form on the surface because the butyl group of PBN allows the motion necessary for closely-spaced pyrenyl groups to form the required sand(13) Hansen, S. H.; Helboe, P.; Thomsen, M. J. Chromatogr. 1986, 368, 39. (14) Blackmon, L. C. F.; Harrop, R. Nature 1965, 208, 777. (15) Bijsterbosch, B. H. J. Colloid Interface Sci. 1974, 47, 186.

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Figure 2. Decay profiles of the PBN monomer and excimer fluorescence on quartz slides: A, 2.5-PBN monomer; B, 0.5PBN monomer; C, 2.5-PBN excimer. λex ) 337.1 nm (N2 laser). Observation λ of the monomer and the excimer was 400 and 500 nm, respectively.

Figure 1. Fluorescence spectrum of PBN adsorbed on the quartz slide (A) and silica gel (B) surfaces. λex ) 337 nm (150 W Xe arc lamp), emission and excitation slit width ) 1 and 10 nm, respectively, T ) room temperature (i.e., 296 K).

wich complex.16 As the concentration of PBN on the surface increases, the average distance between PBN molecules decreases and a greater proportion of molecules participate in excimer-forming reactions. The result is that the monomer to excimer ratio (M/E), or the ratio of the fluorescence intensity at 385 nm to the maximum intensity of the excimer fluorescence, is lower at higher PBN concentrations. Figure 1B illustrates the PBN fluorescence spectrum on a silica gel, which represents the pure monomeric spectrum. The PBN coverage on the silica gel is four orders of magnitude smaller than the PBN coverage on the 2.5PBN slide. Unlike the high surface area silica gel, the high coverage on the slide surface is necessary to observe the fluorescence of PBN. The PBN fluorescence decay profile is also dependent on the concentration of PBN on the quartz surface. The monomer decay rate increases with increasing PBN coverage, as illustrated by the decay profiles A and B in Figure 2. Excitation scans of the monomer and the excimer fluorescence on the 2.5-PBN surface indicate that the excimer originates according to the same absorption profile as that of the monomer (data not shown). This was also observed on the 0.5-PBN surface. These results imply that the excimer is formed by a dynamic mechanism. Growing-in of the excimer fluorescence was observed from the time-resolved analysis, which also supports a dynamic mechanism. The rate of the growing-in was higher on the 0.5-PBN surface relative to the 2.5-PBN surface. The more rapid excimer formation at low coverage probably originates from a heterogeneous (or clustered) arrangement of quartz surface hydroxyls that results in clustered regions of PBN. Another observation was that the rate of growingin on the 2.5-PBN surface was visibly much higher than (16) Viaene, K.; Schoonheydt, R. A.; Crutzen, M.; Kunyima, B.; De Schryver, F. C. Langmuir 1988, 4, 749.

Figure 3. Dependence of the PBN monomer fluorescence decay profile on O2 pressure at room temperature on the 0.5-PBN surface at 25 ns, from top to bottom: 0, 0.83, 2.9, and 6.9 Torr of O2. Gaussian fits (see text) to the decay profiles are given.

the rate of monomer decay. Since, according to the excitation scans, the excimer is not formed from ground state dimer absorptions, the mismatch between the rate of growing-in and the monomer decay also originates from surface heterogeneity. Previous studies of PBN adsorbed on clay surfaces indicated that the PBN fluorescence is quenched on the surface by interactions with the quaternary ammonium group. Such quenching reactions may be intra- or intermolecular. It is possible that, at the higher PBN concentration, crowding among ligands enhances the quenching reaction by decreasing the size of the cage in which PBN interacts with the ammonium ion. However, given the results above, it is more likely that quenching of the PBN monomer fluorescence at monolayer coverage is dominated by the dynamic excimer formation. A comparison of the excimer decay profile (collected at an emission wavelength of 500 nm) with that of the monomer on the 2.5-PBN surface (compare decay profiles A and C in Figure 2) reveals a much higher monomer decay rate. The excimer decay rate on the 0.5-PBN surface (data not shown) is very similar to that on the 2.5-PBN surface. O2 Quenching of the PBN Fluorescence. Figure 3 illustrates the dependence on O2 pressure of the PBN monomer fluorescence decay profile on the 0.5-PBN surface. Molecular oxygen quenches the fluorescence of the pyrenyl group by the formation of a contact complex

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Figure 4. Stern-Volmer analysis of the monomer fluorescence quenching by O2 on the 0.5-PBN (open triangles) and the CTABPBN (closed triangles) surfaces.

with charge-transfer character.17 This can occur by collisional encounters between adsorbed pyrenyl groups and gaseous O2, as in the Eley-Rideal or ER mechanism, or by encounters between adsorbed O2 and pyrenyl groups, as in the Langmuir-Hinschelwood or LH mechanism. The LH quenching mechanism dominates at temperatures where O2 adsorption is significant. This was indicated in recent studies of O2 quenching of the fluorescence of pyrene adsorbed on silica gel18 and alumina19 and of the fluorescence of Ru(bppy)32+ adsorbed on silica gel and controlled pore glasses.20 According to ref 20b, the ER mechanism is a significant factor in O2 quenching at room temperature because the level of O2 adsorption is small. The dependence on O2 pressure of the average decay rate of the PBN monomer fluorescence on the 0.5-PBN surface is given in Figure 4 (open triangles). Average decay rates are given because the decay profiles in Figures 2 and 3 and all other decay profiles encountered in this study were heterogeneous. When there is no physical reason to assume a multiexponential distribution, such as a biexponential one, then it is customary in this laboratory when dealing with heterogeneous decays to determine the 1/e decay time or the average decay rate, kavg. Values of kavg can be conveniently determined by modeling the decay rate distribution as a Gaussian. The details of this approach are described elsewhere.21 The Gaussian approach has been utilized successfully in our laboratory to describe the decay kinetics in numerous microheterogenous systems including but not limited to zeolites, porous silica gel, alumina, and polymer films.22 The previous studies of pyrene and Ru(bppy)32+ fluorescence on silica gel and alumina utilized the Gaussian model and the Stern-Volmer relation to describe the rate of O2 quenching. The Stern-Volmer relation for quenching of heterogenous decays is given by kavg ) k0,avg + kq,avg[Q], where k0,avg is the average decay rate in the absence of a quencher and kavg is the average decay rate at some concentration [Q] of quencher species. The slope of each plot in Figure 4 yields the average quenching rate constant, kq,avg (s-1 Torr-1). In SI units, kq,avg at 295 K on (17) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; p 500. (18) Krasnansky, R.; Koike, K.; Thomas, J. K. J. Chem. Phys. 1990, 94, 4521. (19) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991, 95, 7385. (20) (a) Samuel, J.; Ottolenghi, M.; Avnir, D. J. Phys. Chem. 1992, 96, 6398. (b) Samuel, J.; Ottolenghi, M.; Avnir, D. Physica A 1992, 191, 153. (21) Albery, J. W.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J. Am. Chem. Soc. 1985, 107, 1854. (22) Thomas, J. K. Chem. Rev. 1993, 93, 301.

Ellison and Thomas

the 0.5-PBN surface is 2.4 × 1011 s-1 M-1 compared to 1.9 × 1011 s-1 M-1 on the 2.5-PBN surface (data not shown). Quenching of the excimer fluorescence was also dynamic (kq,avg ) 1.4 × 1011 s-1 M-1), indicating an interaction between O2 and the excited sandwich complex. At this stage it is not necessary to interpret the observed differences in kq,avg between the monomer and the excimer. The PBN fluorescence was also examined in the presence of the cationic surfactant CTAB. Adsorption of CTAB was achieved by first adsorbing PBN at the 0.5 nmol level followed by immersion of the slide in a 6.5 × 10-5 M CTAB solution for 30 s. The PBN derivatized slide was removed clean from the CTAB solution. After exposure to the CTAB solution, the slide was rinsed thoroughly with H2O to remove any traces of adhered CTAB and bromide ions and then dried by heating under vacuum for 10 min at 120 °C. A significant drop in the concentration of PBN was observed from the difference in I0 of the decay profile before and after exposure to CTAB. Evidently, the CTAB cations remove some of the PBN cations by an exchange mechanism. Longer exposure times completely remove the PBN from the surface. The PBN fluorescence spectrum in the presence of CTAB is very similar to the spectrum on silica gel as in Figure 1b. This indicates that excimer formation is inhibited. There was also a significant drop in k0,avg. Apparently the CTAB molecules inhibit a quenching reaction between PBN and the quartz surface or other PBN molecules. A plot of kavg versus O2 pressure for O2 quenching of the monomer on the CTAB-PBN surface is also given in Figure 4 (closed triangles). The value of kq,avg is 1.3 × 1011 s-1 M-1, which is on the same order as kq,avg in the absence of CTAB. It was speculated that CTAB may shield the PBN molecules from collisions with O2. Evidently, any shielding that does occur is not very extensive. O2 Quenching of the PBN Fluorescence in the Presence of PMMA Films. The coverage and morphology of ultrathin polymer films deposited onto PBNderivatized quartz slides were evaluated from the analysis of the PBN monomer fluorescence decay profile and its dependence on added O2. Two features of the decay profiles were examined: the decay rate and the extent of prompt quenching. Prompt quenching can be described as dynamic quenching inside the time scale of the laser pulse that results in a reduced I0 value in the presence of a quencher relative to the I0 value in the absence of a quencher. The effect of prompt quenching on the decay analysis of heterogeneous decay profiles is to emphasize the longer decaying components and underestimate the true average decay rate. Figure 5A illustrates the O2 pressure dependence of kavg when PMMA films are coated on the 2.5-PBN slide. None of the films corresponding to the data in Figure 5 were annealed, and the polymer solvent was removed under vacuum at room temperature. Referring first to the higher molecular weight species (open symbols in Figure 5A), values of kavg in the presence of the 10- and 2.5-PM838 films (or on the 10- and 2.5-PM838 surfaces) are relatively insensitive to changes in O2 pressure in the region 0-1 atm (760 Torr). The low sensitivity implies that O2 quenching is controlled by the mobility and/or solubility of O2 in the polymer film or that the high O2 concentration in the vicinity of PBN molecules in the absence of films (or on the open surface) is no longer present. However, on the 1.0-PM838 surface, kavg is more sensitive to O2 pressure, which indicates exposed regions on the surface and/or regions of lower coverage where the O2 concentration is higher relative to higher PMMA coverages. On the open surface the rise in kavg is most

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Langmuir, Vol. 12, No. 7, 1996 1875 Table 1. Dependence of kq,avg (in the Region 0-10 Torr of O2) and I0,p/I0 (at 760 Torr of O2) on the PBN Coverage in the Presence of the 1.0-PM838 Film

Figure 5. Dependence of kavg (A) and I0,p/I0 (B) of the PBN monomer fluorescence on O2 pressure in the presence of PMMA films coated on the 2.5-PBN surface. Open symbols: 1.0-PM838 (circles), 2.5-PM838 (triangles), and 10-PM838 (squares). Filled symbols: 1.0-PM27 (circles), 2.5-PM27 (triangles), and 10-PM27 (squares). Filled inverse triangles: 2.5-PBN surface in the absence of a film.

abrupt, and above 50 Torr it levels off around the decay rate of the laser pulse, which is about 1 × 109 s-1. Another useful indicator of the coverage is the extent of prompt quenching of the fluorescence by O2. The extent of prompt quenching is expressed as the ratio of the initial intensity of the decay profile at a given O2 pressure, I0,p, to the initial intensity under evacuated conditions, I0. Figure 5B illustrates that values of I0,p/I0 on 10- and 2.5-PM838 surfaces are relatively insensitive to O2 pressure in the region 0-800 Torr of O2. This is in accord with the decay rate analysis. However, the drop in I0,p/I0 is much more significant on the 1.0-PM838 surface, which is also in accord with the decay rate analysis. On the open surface, prompt quenching at 1 atm of O2 was always lower than in the presence of polymer films and varied between 9% and 13% regardless of the PBN concentration or whether or not CTAB molecules were coadsorbed. The rate of O2 quenching of the PBN monomer fluorescence on the 1.0-PM838 surface can be determined in the region 0-10 Torr of O2, where prompt quenching does not interfere with the decay analysis. A Stern-Volmer analysis yields a kq,avg value of 3.5 × 1010 s-1 M-1, which is roughly 25% of the value on the open surface in the presence of CTAB. The quenching rate on the 10-PM838 surface, estimated from a Stern-Volmer analysis in the region 0-800 Torr, is 3.0 × 108 s-1 M-1. It should be noted that comparisons of kq,avg in polymer systems where prompt quenching is not significant in the region 0-800 Torr of O2 are not made here because the values of kavg in vacuum and at 1 atm of O2 differ by only 10-15%. Thus, any comparisons would be prone to error. One factor that could influence the O2 quenching is the extent of the PBN surface coverage. Since the number of

PBN coverage (nmol/slide)

10-10kq,avg (s-1 M-1)

100I0,p/I0

0.1 0.5 2.5 CTAB-PBN

4.8 4.2 3.5 2.8

70 59 66 70

adsorbed PBN molecules affects the surface character, the extent of polymer adsorption could be influenced as well. The dependence of O2 quenching on the PBN coverage was evaluated on the 1.0-PM838 surface, which is prepared from the most dilute PMMA solution studied (i.e., 1.0 mM). Values of kq,avg, determined in the region 0-10 Torr of O2, and values of I0,p/I0 at 1 atm of O2 are given in Table 1. The data indicate no large increases or decreases in kq,avg and I0,p/I0 with the PBN coverage. Furthermore, when the 1.0-PM838 film is coated on the CTAB-PBN surface, O2 quenching is similar to when the film is coated on the PBN surface. At lower PBN coverages, the PMMA-surface interaction should be stronger due to the interaction of the carbonyl group in PMMA with the polar quartz surface hydroxyls relative to pyrenyl groups. On the CTAB surface, the polymersurface interaction should be weaker. Evidently, these changes in the surface character do not significantly influence the adsorption of PM838. Although the PBN coverage and the presence of CTAB did not play a large role in the extent of O2 quenching, the molecular weight of the polymer did. Figure 4 reveals that quenching is more extensive on the 1.0- and 2.5-PM27 surfaces relative to the same PM838 surfaces, as indicated by the greater sensitivity of kavg and I0,p/I0 to O2 pressure. A Stern-Volmer analysis in the region 0-10 Torr on the 1.0- and 2.5-PM27 surfaces indicates kq,avg values of 1.1 × 1011 and 3.4 × 1010 s-1 M-1, respectively. The quenching rate on the 2.5-PM27 surface is very similar to that on the 1.0-PM838 surface and so are the values of kavg and I0,p/I0 in the region 0-800 Torr of O2, as indicated in Figure 4. The difference in quenching due to molecular weight most likely arises from a difference in coverage due to polymer adsorption, which scales with molecular weight. Another result worth noting was the adsorption of PM27 on the CTAB surface. Wtihdrawal of a CTAB-PBN slide from the 2.5-PM27 solution resulted in streaming (see Materials and Methods section) or an opaque, nonuniform film. Apparently, the adsorption of PM27 was not extensive enough to stabilize the polymer solvent on the surface. Since a uniform film was observed on the PBN surface, a stronger adsorption interaction occurs on the PBN surface relative to the CTAB surface. The higher molecular weight species (i.e, PM838), which is significantly adsorbed on the CTAB surface, overcomes the weak interaction by additive effects. O2 Quenching of the PBN Fluorescence in the Presence of PS Films. Polystyrene films were analyzed using the same approach as for PMMA films. Trends in kavg and I0,p/I0 on PS900 surfaces at varying levels of coverage are illustrated in Figure 6. Values of kq,avg determined in the region 0-10 Torr of O2 are given in Table 2 for those surfaces where prompt quenching in the region 0-800 Torr of O2 was significant. On the PS surfaces, kavg rises to a maximum at less than 100 Torr of O2. This was also observed at lower PBN coverages (data not shown). Evidently there are regions of the PS films where access by O2 to PBN is similar to that on the open surface and covered regions where quenching is much less efficient. Quenching of the PBN

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Figure 7. PBN fluorescence spectrum in the presence of 10PS900 (A and B) and 10-PM838 (C and D) films. A and C, on the 2.5-PBN surface; B and D, on the 0.5-PBN surface. λex ) 337 nm (150 W Xe arc lamp), emission and excitation slit width ) 1 and 7.5 nm, respectively.

Figure 6. Dependence of kavg (A) and I0,p/I0 (B) of the PBN monomer fluorescence on O2 pressure in the presence of PS films coated on the 2.5-PBN surface. Open symbols: 2.5-PS900 (triangles), 5.0-PS900 (diamonds), and 10-PS900 (squares). Filled symbols: 5.0-PS35 (diamonds), 10-PS35 (squares). Filled inverse triangles: 2.5-PBN surface in the absence of a film. Table 2. Dependence of kq,avg (in the Region 0-10 Torr of O2) on the Coverage of PS900 and PS35 on the 2.5-PBN Surface PS surface

kq,avg (S-1 M-1)

2.5-PS900 5.0-PS900 5.0-PS-35 10-PS35

1.6 × 1011 5.6 × 1010 1.4 × 1011 2.1 × 1010

fluorescence in the “exposed” regions initially increases kavg until prompt quenching begins to eliminate the exposed component from the decay analysis. The drop in kavg above 100 Torr of O2 results from a decrease in the percent contribution to the decay profile from the exposed component and an increase in the less quenched components. At higher coverage, kavg rises to a smaller maximum and values of kq,avg are lower, as indicated in Table 2. At 760 Torr of O2, values of kavg in Figure 6A, where prompt quenching was significant, are approaching values on the 10-PS900 surface, where prompt quenching was not significant. This indicates that quenching in the covered regions is similar to quenching on the 10-PS900 surface, where the overall coverage is much higher. According to these results, the morphology of PMMA at low coverage is clearly different from that of PS. It is apparent that the PMMA films are more continuous at low coverage than the corresponding PS films because there is a more evenly distributed range of quenching rates on the PMMA surface. On the other hand, the PS films are patchy or aggregated, which results in a bimodal distribution of quenching rates. When the solvent is removed by evaporation, PMMA films retain the continuous morphology of the thin liquid film adsorbed on the slide surface prior to solvent evaporation. PS films possess less affinity for the surface than for other PS molecules.

Although no thickness or density measurements were made here, we expect that PMMA films are thinner and less dense than the aggregates of PS. The morphological features of PS films at low coverage have been observed in other studies. For example, dewetted PS films have been characterized by optical phase interference microscopy23 and X-ray reflectivity.6 One particular study used AFM to gather images of PS films at low coverage on silicon substrates.7 In that study, patchy PS films were observed when using 0.004 M (or lower) solutions, respectively. The results here indicate that PS900 films prepared from 0.010 M solutions are continuous and that films prepared from 0.005 M (or lower) solutions are patchy. These similarities are surprising considering the different procedures used to prepare the coatings (i.e., spin coating versus dip coating) but may indicate that the coverage and morphology are not highly dependent on substrate surface character (i.e., PBNderivative versus native SiO2 surface). This would be true provided that polymer adsorption was extensive enough to prepare a stable liquid coating. Of further interest is the influence of annealing the ultrathin films above the bulk Tg. Any frozen structures in the film that may result from rapid solvent evaporation should be relaxed by the annealing procedure. However, the results of experiments where either patchy or continuous films were annealed by heating in vacuum at 120 °C for 30 min were not significantly different from the results prior to heating. For example, annealing the 10PS900 film did not produce exposed regions of the surface. Longer annealing times may produce different results, but this was not tested here. PBN Fluorescence Spectrum in the Presence of PMMA and PS Films. Figure 7 illustrates the PBN fluorescence spectrum in the presence of air following deposition of 10-PM838 and 10-PS900 films onto the 2.5PBN and 0.5-PBN surfaces. As indicated above, exposed regions on the quartz surface are eliminated by such films. Heating the samples under vacuum above the glass transition temperature at 120 °C for 0.5 h did not significantly alter the spectrum. When comparing the spectra in Figures 7 and 1A, it is apparent that M/E values are significantly higher in the (23) Reiter, G. Phys. Rev. Lett. 1992, 68, 75.

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Figure 8. Fluorescence spectrum of pyrene in 10 µm PM838 and PS900 films.

presence of the polymer film. The time-resolved analysis indicated a significant drop (by a factor of 2-3) in the monomer decay rates following exposure to the films. The decrease in decay rate and increase in M/E is thus due to an increase in the rigidity of the PBN microenvironment that inhibits the monomer quenching reaction and converts a portion of the excimer-forming species to monomer. This is possible only by intimate contact between PBN and the polymer. The lowered M/E value is not due to the desorption of PBN into the polymer solvent. Exposure of the slides, which correspond to the spectra in Figure 1A, to methylene chloride followed by exposure to vacuum conditions for a few minutes does not significantly alter the spectra in Figure 1A. This was expected, since the organic solvent cannot accommodate the PBN cation. Photophysics of Pyrene Doped in Ultrathin Films. Pyrene was dissolved in 100-PM838 and 100-PS900 solutions at a level of 50 mM (moles of pyrene per liter of polymer), and a few drops of the resulting solutions were cast onto horizontally-inclined quartz slides to form a uniform liquid coating. After solvent removal by slow evaporation and evacuation at room temperature for 4 h, the resulting films were clear and approximately 10 µm thick. The fluorescence spectra of pyrene in the two films are given in Figure 8. There was no evidence of pyrene excimer in either film. Each spectrum was collected in air and normalized about the three position. There is a small amount of fluorescence quenching by O2 in the air, as revealed by a small increase in the fluorescence decay rate in air relative to vacuum conditions, but this does not interfere with the spectrum analysis. An analysis of the pyrene III/I indicates values of 0.88 and 0.67 for PS and PMMA, respectively. Identical III/I values were determined in films doped with 5 mM pyrene. The difference in polarity is also reflected in the relative positions of the emission band; the PMMA spectrum is blue shifted relative to that of PS. The lower III/I or higher polarity in PMMA is due to the carbonyl group, which is more polar than the benzyl group in polystyrene. The pyrene III/I was also evaluated in ultrathin films on quartz and derivatized quartz slides. Polymer films were coated on slides that had been dried in vacuum at 150 °C and transferred hot to the withdrawal funnel, where they were cooled to room temperature under a nitrogen purge. Solvent was removed from the films immediately after their preparation by evacuation at room temperature for 1 h. Prior to analysis, samples were equilibrated overnight in a sealed, evacuated chamber containing activated molecular sieves. Fluorescence decay times were collected under vacuum unless otherwise stated. The highest polymer concentration used to prepare ultrathin films was 100 mM. The thickness of the 100-

Figure 9. Dependence of the pyrene III/I (A) and average fluorescence decay time (B) in the presence of air on the logarithm of the PS900 concentration used to prepare ultrathin films on quartz (filled circles) and phenyl-quartz (open circles) surfaces.

PS900 film, which was estimated from the optical density of pyrene in the film, is 500 Å. Approximately the same thickness was observed for a 100-PM838 film. In the 500 Å films, values of the pyrene III/I and average decay rate of the excited singlet were not significantly different from the values in the 10 µm films. Figure 9A illustrates a plot of the III/I value in PS900 films exposed to air versus the logarithm of the polymer concentration used to prepare the film. The lowest polymer concentration used was 5.0 mM because lower concentrations produced fluorescence intensities too weak for a reliable III/I analysis. On the quartz surface, the III/I in PS films (Figure 9A) decreases with decreasing coverage but remains approximately constant on a hydrophobic quartz surface prepared by the reaction of quartz slides with chlorodimethylphenylsilane (heretofore referred to as the phenyl-quartz surface). The trend in the average decay time (τavg ) 1/kavg) of the pyrene fluorescence with decreasing coverage, as illustrated in Figure 9B, follows the trend in the pyrene III/I. These results are discussed below. With respect to PM838, neither the III/I nor τavg varied significantly with either the coverage or the quartz surface hydrophobicity. A III/I value of 0.66 ( 0.01 was determined in the 5.0-PM838 film, which is not significantly different from that for the 100-PM838 (500 Å thick) film. Even though the III/I is lower in PMMA than in PS and is thus a less sensitive indicator of a decrease in the polarity, plots of τavg in PMMA films on the quartz and the phenyl-quartz surfaces versus the logarithm of the polymer concentration (see Figure 10) bear no resemblance to the result for PS. The difference between PMMA and PS with respect to the trends in III/I and τavg with coverage can be explained by the difference in affinity of the two polymers for the quartz surface. Pyrene can diffuse to the quartz surface, where more localized interactions will lower its free energy. A lowering of τavg could result from quenching interactions

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Figure 10. Dependence of the pyrene average fluorescence decay time in the presence of air on the logarithm of the PM838 concentration used to prepare ultrathin films on quartz (filled squares), phenyl-quartz (open squares), and Cu-quartz (open triangles) surfaces.

with the quartz surface. A lowering of the III/I could result from the increased polarity of the quartz surface relative to the polymer.24 Depending on the strength of the interaction between the polymer and the quartz surface, the adsorption sites on the quartz surface may or may not be accessible to pyrene. PMMA apparently blocks access to the quartz surface more effectively than PS. This is probably due to an interaction between the carbonyl groups in PMMA and the quartz surface hydroxyls. On the phenyl-quartz surface, the polar sites are eliminated. On the quartz surface, as the coverage increases, so does the proportion of pyrene molecules in the bulk. This reduces the influence of the quartz surface on the values of τavg and III/I. If the film thickness of PS on the quartz surface was much less than that on the phenyl-quartz surface, then this could be used to explain the different values of τavg and III/I on the two surfaces. However, we expect PS adsorption on the quartz surface to be more extensive relative to that on the phenyl surface, due to more localized interactions with silanol groups relative to phenyl groups. Since the film thickness scales with adsorption, a greater film thickness on the phenyl-quartz surface relative to the quartz surface is therefore not expected. Of concern to us was whether a certain proportion of pyrene molecules in ultrathin films were quenched by O2 in the air. This could be the case when analyzing the 5.0-PS900 film, where the patchy morphology was observed on the PBN surface. A certain proportion of pyrene molecules may diffuse to open regions on the surface, where they are promptly quenched by the high concentration of O2. However, this was not observed. Decay rates of the pyrene fluorescence in the 5.0-PS900 and 5.0PM838 films on the quartz and phenyl-quartz surfaces were only slightly greater in air than in vacuum, and there was no evidence of prompt quenching. This indicates that pyrene prefers the covered regions of the quartz surface. This seems reasonable because on the open quartz surface there is no solubilization of pyrene from air or vacuum. Figure 10 also illustrates the affect of adsorbed Cu(II) cations on the lifetime of pyrene in PMMA films. Cu(II) was adsorbed on quartz surfaces by exposing four clean slides simultaneously to an aqueous solution containing (24) An independent determination of the pyrene III/I on dry silica gel is 0.55.

Ellison and Thomas

0.01 M CuCl2. After exposure of the slides two times to 15 mL aliquots of H2O, the slides were dried in vacuum for 1 h at 150 °C. It is assumed that the Cu(II) concentration is approximately the same on all of the slides. The reduction in pyrene fluorescence decay time in ultrathin PMMA films on the Cu-quartz surface is presumably due to electron transfer via a tunneling mechanism. Previous studies of electron transfer from pyrene to Cu(II) cations in rigid cellophane indicated that electron tunneling was responsible for the formation of pyrene cation upon singlet excitation.25 Since electron tunneling occurs over short distances (10-15 Å), an analysis of the decay rate can be used to infer donoracceptor spacings. In our case, decreasing film thickness results in a greater proportion of pyrene molecules undergoing faster electron transfer rates, which decreases the average decay time. Conclusions We have indicated that fluorescence spectroscopy provides information about ultrathin films that is in agreement with previous studies. For example, a patchy morphology of PS was observed at low coverage from the analysis of O2 quenching of the PBN fluorescence on the quartz slide, and these results were complementary to the AFM studies of similar systems. Therefore, this fluorescence technique not only provides information complementary to AFM but also provides additional information about the approach of small reactive molecules, such as O2, to the surface which AFM can in no way provide. We have also shown that the photophysics of pyrene in ultrathin films is sensitive to adsorbed quenchers and the polymer-surface interaction. This indicates that the photophysical response from bulk polymer-probe interactions can be reduced to levels which allow for the study of interfacial phenomena. It seems likely that other fluorescent species that report directly on the state of the polymer, such as the “molecular rotor” probes or probes for polymer free volume,26 may also yield useful information about ultrathin films. Future developments of the photophysical approach should also involve spectroscopy of excited triplet species. Because the excited triplet decays on a much longer time scale (microseconds as opposed to nanoseconds for the excited singlet), it is a more sensitive indicator of the oxygen concentration. Observation of the triplet species would allow the measurement of O2 quenching rates of triplet dopants as well as the glass transition temperature.27 This may provide direct and unambiguous information about the microstructure of ultrathin films. Of course such developments will be difficult to achieve because the observation of the triplet species, either by transient absorption spectroscopy or by luminescence spectroscopy, is hampered by the low number of probe species in the ultrathin films. We are currently searching for ways to overcome this problem. Acknowledgment. The authors wish to thank The National Science Foundation for support of this work. LA9506436 (25) Milosavljevic, B. H.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 2513. (26) Loutfy, R. O. Pure Appl. Chem. 1986, 58, 1239. (27) Chu, D. Y.; Thomas, J. K.; Kuczynski, J. Macromolecules 1988, 21, 2094.