Langmuir 1999, 15, 1215-1222
1215
Fabrication of Extended Conjugation Length Polymers within Diacetylene Monolayers on Au Surfaces: Influence of UV Exposure Time Mei Cai, Mark D. Mowery, Henning Menzel,† and Christine E. Evans* Department of Chemistry, University of Michigan, 930 N. University Ave., Ann Arbor, Michigan 48109-1055 Received September 9, 1998. In Final Form: November 16, 1998 The effective conjugation length of the delocalized polymer backbone is one of the key factors in designing monolayer polymers for sensor or nonlinear optical applications. In this manuscript, the photopolymerization behavior of self-assembled diacetylene-containing disulfide monolayers is assessed on gold surfaces. Formation of the long conjugation length, so-called blue form, of the polydiacetylene backbone structure is exclusively monitored using resonance Raman spectroscopy as a function of UV exposure time. In these studies, initial formation of the blue polymer form is followed by an irreversible loss with prolonged exposure. This behavior mirrors the chromatic phase transition to shorter conjugation lengths exhibited for multilayer Langmuir-Blodgett films upon extended UV exposure. Although the exact nature of this phase transition remains elusive, most theories focus on factors affecting the alignment of the polymer backbone and influencing the effective conjugation length. Three such factors are examined here: the Au-S bond with the surface, the crystallinity of the alkyl side chains, and the strain induced by hybridization changes. When the reductive desorption technique is used, the Au-S bond is shown to not be correlated with the polymerization process. In addition, no change in chain crystallinity is observed upon polymerization, but the twist of the methylene chain exhibits significant changes with prolonged UV exposure. This result is consistent with the hybridization-induced strain being translated into the polymer backbone as well as the methylene chains, resulting in a decrease in the effective conjugation length.
Introduction Spontaneously organized assemblies on gold substrates fabricated from thiol or disulfide compounds containing internal functionalities are important for applications ranging from sensor design to photolithography. The incorporation of these internal functionalities creates molecular scaffolding within the monolayer structure through interaction mechanisms including π-stacking,1,2 hydrogen bonding,3,4 dipole coupling,5-7 and covalent attachment.8-15 This approach can be used to design * To whom correspondence should be addressed. Fax: (734) 6474050; e-mail:
[email protected]. † Permanent address: Institute for Macromolecular Chemistry, University of Hannover, Am Kleinen Felde 30, 30167 Hannover, Germany. (1) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Small, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. J. Am. Chem. Soc. 1997, 119, 10563. (2) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (3) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124. (4) Clegg, R. S.; Hutchinson, J. E. Langmuir 1996, 12, 5239. (5) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (6) Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1990, 6, 1512. (7) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700. (8) Peanasky, J. S.; McCarley, R. L. Langmuir 1998, 14, 13. (9) Mowery, M. D.; Evans, C. E. J. Phys. Chem. B 1997, 101, 8513. (10) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (11) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189. (12) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (13) Kim, T.; Ye, Q.; Sun, L.; Chan, K. C.; Crooks, R. M. Langmuir 1996, 12, 6065. (14) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (15) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. J. Phys. Chem. B 1998, 102, 9550.
interfacial structures with novel optical, electronic, and structural properties. Recently, the incorporation of conjugated diacetylene groups within thiol or disulfide compounds has permitted the fabrication of robust monolayer polymers9-15 with structural control in three dimensions.14,15 Similar to diacetylene bulk crystalline solids and LangmuirBlodgett (LB) films,16-31 formation of the conjugated polydiacetylene backbone structure can be initiated by photoirradiation or thermal treatment. As illustrated in Figure 1, the spatial constraints for polymerization are considerable, with tolerances of approximately 0.5 Å for the spatial alignment of diacetylene units on neighboring molecules. However, no significant expansion or contraction of the crystal structure is anticipated upon crosslinking.26,27,29 In contrast with LB films, self-assembled monolayers are surface attached, limiting the degrees of (16) Wegner, G. Z. Naturforsch. 1969, 24b, 824. (17) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1976, 15, 764. (18) Tieke, B.; Wegner, G. In Topics in Surface Chemistry; Kay, E., Bagus, P. S., Eds.; Plenum Press: New York, 1978; p 121. (19) Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (20) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478. (21) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1483. (22) Tieke, B.; Enkelmann, V.; Kapp, H.; Lieser, G.; Wegner, G. J. Macromol. Sci. Chem. 1981, A15, 1045. (23) Tieke, B.; Lieser, G. J. Coll. Interface Sci. 1982, 88, 471. (24) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. (25) Enkelman, V. In Polydiacetylenes; Cantow, H.-J., Ed.; SpringerVerlag: New York, 1984. (26) Lando, J. B. In Polydiacetylenes; Bloor, D., Chance, R. R., Eds; Nijhoff: Dordrecht, The Netherlands, 1985; p 363. (27) Schott, M.; Wegner, G. In Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, 1987, Chapter 3. (28) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1991, 7, 2336. (29) Cao, G.; Mallouk, T. E. J. Solid State Chem. 1991, 94, 59. (30) Kuriyama, K.; Kikuchi, H.; Kajiyama, T. Langmuir 1996, 12, 2283. (31) Tieke, B.; Bloor, D. Makromol. Chem. 1979, 180, 2275.
10.1021/la981219i CCC: $18.00 © 1999 American Chemical Society Published on Web 01/27/1999
1216 Langmuir, Vol. 15, No. 4, 1999
Cai et al.
Furthermore, both reversible and irreversible phase transitions, particularly between the blue and red phases, have been observed extensively in polydiacetylene films ranging from amorphous thin films to bilayer assemblies. Indeed, this blue-to-red transition is the foundation of several recent sensor designs.34-37 In these designs, the conjugation length of the polymer backbone is conjectured to be diminished upon interaction of the target molecule with a polydiacetylene bilayer, leading to a color change response. Extension of these sensor designs to the more robust, spontaneously assembled monolayer requires a more thorough understanding of the formation of the bluephase polymer within a monolayer assembly. Recent studies have extended these polymerized assemblies to the surface-attached monolayer regime.9-15 In fabricating these films, a broad range of conditions were utilized in the polymerization process including the type and power of UV lamp, distance from the sample, and exposure time.10-14 Batchelder et al.10 reported that polymerization was complete after 30 s and a phase transition from blue to red did not occur within 50 s of UV irradiation. Crooks et al. monitored the visible absorption of blue-phase polymer with increasing UV exposure.13 Although an increase in absorbance with UV exposure was observed, absorption spectroscopy of polydiacetylene monolayers on gold suffers from low detectability and interference from the gold surface plasmon absorption, making detailed studies of polymerization kinetics and phase behavior difficult.38 In this paper, resonance Raman spectroscopy is used to monitor the formation of the important blue-phase polymer within these monolayer assemblies as a function of UV irradiation time. The following conjugated diacetylene disulfide is used to fabricate spontaneously organized assemblies on gold substrates: Figure 1. Schematic diagram of spatial constraints for polymerization.
[CH3(CH2)15 CtCsCtC(CH2)9Ss]2
freedom of the diacetylene-containing molecules. Nonetheless, recent studies have demonstrated the feasibility of fabricating these polydiacetylene monolayers under a range of substrate conditions.32,33 In addition, the position of the polymer backbone can be varied vertically by adjusting the alkyl chain spacer length,15 and lateral control is afforded through phototemplating.14 This combination of factors leads to significant control over the molecular architecture of these monolayer polymers at the nano-to-micrometer level. Studies of diacetylene crystal solids and LB films indicate an intricate dependence between the conjugated backbone electronic structure and the extent of UV exposure. Characterized extensively as crystals and LB films,16-31 polydiacetylenes are known to exist in several phases which appear blue, purple, or red in color. Although the exact nature of this chromism remains elusive, it is generally attributed to variations in the effective conjugation length of the polydiacetylene backbone. Because the excitonic absorption of polydiacetylenes is directly related to this effective conjugation length, the blue phase is characterized by the longest conjugation length, with the purple and the red forms indicative of successively diminished conjugation length.18-23,28,30 The maximum optical absorption characteristic for the blue, purple, and red forms appear at 640, 600, and 540 nm, respectively.
where the methylene units in the “tail” region is 15 and the “spacer” region from the surface to the polymer backbone is 9. The monomeric layer is designated 15,9DA, and the polymerized layer is designated as 15,9-PDA. The gold substrate is formed using high-temperature, ultrahigh vacuum (UHV) vapor deposition onto mica in order to create atomically flat gold domains that are expected to facilitate longer polymer chains.32 As a result, any change in the long conjugation length polymer is expected to arise solely from UV irradiation.
(32) Mowery, M. D.; Menzel, H.; Cai, M.; Evans, C. E. Langmuir 1998, 14, 5594. (33) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. Adv. Mater. 1998, in press.
Experimental Methods Chemicals. Dinonacosa-10,12-diyn-disulfide (15,9-DA) was synthesized using a procedure described previously in this laboratory.39 Chloroform was purchased from Aldrich Chemical Co. Water from a Milli-Q UV Plus ultrapure water system (Millipore, >18 MΩ) was used in this study. Mica (ASTM V-2), obtained from Asheville-Schoonmaker Mica Co., and gold (99.99%), purchased from Kurt J. Lesker Company, were used to form gold films on mica. Monolayer Fabrication. Evaporated gold films were prepared using a custom-built UHV thin film deposition system. Both sides of a mica substrate were cleaved immediately before insertion into the chamber. These mica substrates were sus(34) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993, 261, 585. (35) Reichert, A.; Nagy, J. O.; Spevak, W.; Charych, D. J. Am. Chem. Soc. 1995, 117, 829. (36) Pan, J.; Charych, D. Langmuir 1997, 13, 1365. (37) Cheng, Q.; Stevens, R. C. Adv. Mater. 1997, 9, 481. (38) Barker, A. S. Phys. Rev. B 1973, 8, 5418. (39) Mowery, M. D.; Evans, C. E. Tetrahed. Lett. 1997, 38, 11.
Fabrication of Extended Conjugation Length Polymers pended 21 cm above the source using a tantalum mask and degassed at 380 °C for 12-24 h using two 500 W halogen lamps positioned approximately 8 cm below the substrate. Raman spectroscopic experiments indicated that the extended degassing at high temperature is necessary to remove contaminants. Gold was vapor deposited from a K-Cell (Oxford Instruments) onto the heated substrate (250 °C) at a rate of 0.03 Å/sec to a final thickness of ∼2000 Å (Leybold Inficon Inc.). During the deposition process, the pressure was maintained at less than 1 × 10-7 Torr. Substrates were subsequently annealed for 3 h at the deposition temperature and then allowed to cool to room temperature and a base pressure of 2 × 10-10 Torr. When this technique is used, the large atomically flat domains (>100 nm) on the resultant gold substrate are not expected to limit the formation of bluephase polymer.32 After gold deposition, the chamber was backfilled with dry nitrogen and the substrates were removed. The resulting gold films were immediately immersed into a 1 mM chloroform solution of the 15,9-DA and allowed to equilibrate at room temperature for 40-48 h. Strict light control was maintained during the preparation and storage of the diacetylene solutions and monolayer films. The substrates were subsequently removed and rinsed thoroughly with chloroform and water, and dried with a stream of nitrogen. Polymerization of the diacetylene monolayers was performed under nitrogen with UV irradiation from a low-intensity UV lamp (model UVG-11; Ultra-Violet Products Inc.; 4 W, 250-260 nm) at a distance of 2 cm above the sample surfaces. The intensity of the UV lamp at 2 cm away is estimated by the manufacturer to be 4 mW/cm2. Raman Spectroscopy. Resonance Raman spectra were obtained using an imaging system consisting of a microscope objective (10×, 0.25 NA), a spectrograph (Holospec f/1.8i VRT; Kaiser Optical System), and a charge-coupled device (CCD) detector (TK1024AB; Photometrics). The 632.8 nm line from a He-Ne laser (model 05-LHP-991, Melles Griot) was utilized for excitation at an incident laser power of 4-5 mW. The CCD detector was cooled with liquid nitrogen to -110 °C. Spectra were calibrated using emission lines of known wavelength from a neon lamp. FTIR Spectroscopy. FTIR external reflection spectroscopy (FTIR-ERS) experiments were accomplished using a nitrogenpurged Nicolet 550 Magna IR spectrometer with a liquid nitrogencooled MCT detector. Spectra were obtained with a SpectraTech Inc. specular reflectance accessory using p-polarized light incident on the samples at 85° with respect to normal. All spectra were taken as the average of 1024 scans and referenced against a freshly prepared, unmodified gold film at a resolution of 2 cm-1. Electrochemistry. The reductive desorption experiments were performed using a potentiostat (CV-27, Bioanalytical Systems Inc.) with a computer data acquisition system. All experiments were performed in a standard three-electrode cell configuration which included a Ag/AgCl reference electrode (saturated KCl internal solution) and a platinum wire counter electrode. The area of the working electrode was defined by an elastomer O-ring as 0.95 cm2. All measurements were performed at 20 ( 1 °C in 0.5 M KOH which was degassed with nitrogen for 1 h prior to scanning from -0.5 to -1.35 V at 100 mV/sec. All solutions were prepared immediately before use with ultrahigh purity water.
Results and Discussion The unique optical and electronic properties of polydiacetylene monolayers have important potential applications in the areas of sensor design and photolithography. In many sensor applications and nonlinear optical applications, advances center on the formation of a highly delocalized polymer backbone. In the solid state or as LB films, the polymerization behavior of diacetylene compounds is commonly evaluated using UV-vis absorption spectroscopy.17-28 Although this method is clearly successful for solid-state or multilayer samples, it is not practical for monolayer assemblies on gold because of limited detectability and possible convolution with visible absorption from the gold surface plasmon absorption. Alternatively, the visible absorption of the polymerized monolayers renders these assemblies highly amenable to
Langmuir, Vol. 15, No. 4, 1999 1217
Figure 2. Resonance Raman spectra of 15,9-DA on Au surfaces upon UV exposure under nitrogen for (a) 2 min, (b) 4 min, (c) 7 min, (d) 12 min, (e) 33 min, (f) 53 min, and (g) 93 min. Experimental conditions as noted in the text.
sensitive resonance Raman spectroscopic investigations when the laser excitation wavelength is tuned to the excitonic absorption of the polymer backbone.40 Utilizing this approach, formation of the longest conjugation length polymer backbone can be monitored directly. Indeed, alkylbased monolayers and nonpolymerized diacetylenes exhibit no signal under the measurement conditions utilized in this study. As a result, the significant resonance enhancement is utilized to monitor polymer formation within these single molecular layers. On the basis of the excitonic behavior of similar LB films, excitation at 633 nm is chosen to monitor the formation of the so-called blue polymer phase. At this frequency, the intensity of the resonance Raman signal is expected to be exclusively proportional to the number density of the longer conjugation length, blue-phase polymer. When this approach is used, resonance Raman spectroscopy is a sensitive measure of the relative polymerization efficiency as well as the phase of the polymer. Polymerization Behavior. A representative series of resonance Raman spectra of 15,9-PDA monolayers acquired with 633 nm excitation are shown in Figure 2 as a function of UV irradiation time. The peaks in each spectrum are attributed exclusively to the polymer backbone within the monolayer assembly, as no peaks are observed from unpolymerized or n-alkyl assemblies. Bands assigned to the unsaturated double and triple bond stretching vibrations in the polymerized assemblies are located at 1459 and 2082 cm-1, respectively.40,41 The significant diminution in frequency of these bands relative to isolated double and triple bonds (1620 and 2260 cm-1, (40) Batchelder, D. N.; Bloor, D. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley Heyden: London, 1984; p 133. (41) Bloor, D.; Preston, F. H.; Ando, D. J.; Batchelder, D. N. In Structural Studies of Macromolecules by Spectroscopic Methods; Iven, K. J., Ed.; John Wiley & Sons: London, 1976, Chapter 8.
1218 Langmuir, Vol. 15, No. 4, 1999
Figure 3. Resonance Raman intensity of the v(CdC) vibration as a function of UV irradiation time under nitrogen for 15,9-DA on Au surfaces.
respectively)42,43 is consistent with the expected delocalization in the polymerized structure.43 In addition, the frequencies of these bands are in excellent agreement with those of multilayer blue-phase, polydiacetylene LB films,23,31 indicating selective monitoring of the highly conjugated polymer form in these monolayers. The peaks occurring at 705 cm-1 and between 1100 and 1400 cm-1 have not been assigned conclusively in the literature, but are conjectured to arise from the in-plane bending vibrations of the polydiacetylene backbone and the vibrations from the methylene units in close proximity to the polymer backbone.10,42,44,45 Overall, the resonance Raman spectra of the 15,9-PDA are in good agreement with similar measurements of blue-phase polydiacetylene in multilayer LB films. Not only are the vibrational transitions in the assemblies on gold consistent with multilayer studies, the polymerization kinetics are also in good qualitative agreement. As illustrated in Figure 2, the resonance Raman intensities of these polymer vibrations increase to a maximum after 7 min of UV exposure. This is a direct result of a systematic conversion of monomer adsorbates into a highly conjugated polydiacetylene scaffolding within the monolayer. Upon additional UV exposure, the Raman intensities of these bands decrease, owing to a proportional alteration in structure away from the blue-phase polymer. Furthermore, the lack of an increase or decrease in Raman intensities after several days indicates the irreversible nature of these structural alterations in the monolayer polymer. In Figure 3, a quantitative representation of the polymerization kinetics of these monolayers on gold is illustrated. Here, the Raman intensity of the alkene vibration as a function of UV exposure shows a rapid increase in highly conjugated polymer followed by a steady decline of the blue-phase form within the monolayer. Absorption spectroscopy of multilayer polydiacetylene LB films exhibits a similar rapid increase in the blue-phase polymer, indicated by an absorbance at 640 nm.23 This increase in absorption in multilayer systems was followed (42) Angkaew, S.; Wang, H.-Y.; Lando, J. B. Chem. Mater. 1994, 6, 1444. (43) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: San Diego, 1991. (44) Bower, D. I.; Maddams, W. F. The Vibrational Spectroscopy of Polymers; Cambridge University Press: Cambridge, 1989. (45) Tieke, B.; Bloor, D.; Young, R. J. J. Mater. Sci. 1982, 17, 1156.
Cai et al.
by a gradual decrease in the intensity of the peak associated with the blue-phase polymer and accompanied by a systematic increase in the lower conjugation length red-phase polymer at 540 nm. Unfortunately, quantitative comparison of the polymerization kinetics for diacetylene monolayers on gold and multilayer LB films is not possible because of differing polymerization conditions. However, the relatively rapid rise in the formation of blue-phase polymer followed by a gradual decline exhibits similar rates between surface-attached monolayers on gold and multilayer LB films.23 This decrease in the presence of the highly conjugated polymer form has not been previously reported for polydiacetylene monolayers on gold. It is known from multilayer studies and monolayer studies at the air-water interface that the decrease in blue-phase polymer is accompanied by an increase in the shorter conjugation length red form, but the mechanism underlying this change remains elusive. Two primary approaches have been highlighted in the literature. In the first, the initial blue phase is a solid solution of polymer in a monomer matrix, whereas the red phase is the polymer and arises upon further irradiation.20,21 In the second approach, the blue phase is the more stable polymer form which is converted to the shorter conjugation length red phase from strain-inducing forces.46 At present, these two models are not easily distinguishable. However, the degrees of freedom within the surface-attached monolayers of interest here are considerably more limited than their Langmuir monolayer and LB multilayer counterparts. Indeed, the alkyl side chains, thought to be important in determining the conjugation length, are now constrained to a narrow distribution of geometries. As a result of this constrained mobility, the second model appears to be favored here. In this case, a strain-induced transition to a lower conjugation length polymer upon extended UV exposure could be initiated by a change in the steric environment caused by photooxidation of the sulfur headgroup, a change in the crystallinity of the alkyl side chains, or strain induced from the hybridization change. In addition, more destructive mechanisms such as direct desorption of the monolayer or photoinduced cleavage of the polymer backbone must be considered. Phase-Transition Mechanism. Consistent with previous multilayer and solid-state studies, the decrease in the presence of the highly conjugated blue-phase polydiacetylenes on gold surfaces upon prolonged UV irradiation can be attributed to a phase transition from blue to red. Although resonance Raman measurements do not permit the determination of an isobestic point indicating interconversion, the presence of the blue-to-red transition is reinforced by recent dual-frequency measurements of red phase within the monolayer.47 The exact mechanism of this shift in chromism is not well understood, however, several hypotheses can be systematically investigated. An initial possibility is that the decrease in Raman intensities of the polymer backbone vibrations is due simply to the cleavage of the Au-S bond through photooxidation of the sulfur headgroup. A cleavage of the Au-S bond could alter the strain on the polymer backbone by increasing the degrees of freedom of the monolayer. Ultimately, this mechanism could also lead to the deg(46) (a) Eckhardt, H.; Boudreaux, D. S.; Chance, R. R. J. Chem. Phys. 1986, 85, 4116. (b) Campbell, A. J.; Davies, C. K. L. Polymer 1995, 36, 675. (47) (a) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. Macromolecules submitted for publication. (b) Cai, M.; Mowery, M. D.; Pemberton, J. E.; Evans, C. E. Appl. Spectrosc., to be submitted for publication.
Fabrication of Extended Conjugation Length Polymers
radation of the monolayer through material loss. Recent reports have implicated the oxidation of the sulfur headgroup by ozone as a primary mechanism of selfassembled monolayer (SAM) degradation.48-51 Specifically, the photooxidation of alkanethiol SAMs on gold upon UV exposure at wavelengths below 200 nm has been observed.49 Second, as has been proposed for multilayer and solid-state polydiacetylenes, the decrease in blue-phase polymer may arise from a decrease in the alkyl chain crystallinity upon UV exposure. Any decrease in crystalline order within the alkyl chains may induce strain on the polymer backbone and decrease the effective conjugation length, leading to the predominance of red-phase polymer.17-19,22-24,28,30 Third, although no significant expansion or contraction of the monolayer is expected upon polymerization, a hybridization shift from sp to sp2 at the carbon positions above and below the polymer backbone must be accommodated upon polymerization. The strain induced by such a hybridization change would likely be translated into the alkyl side chain portion of the monolayer as a change in the alkyl chain orientation. Further exposure to UV irradiation may produce increased strain which cannot be translated away from the polymer backbone into the alkyl chains because the sulfur headgroup is covalently bound, limiting the degrees of freedom to accommodate polymerization-induced strain. In this case, the strain may be transferred to the polymer backbone, altering the effective conjugation length. Finally, it is also possible that phase transition is caused by direct polymer cleavage from extended UV exposure, resulting in a decreased conjugation length. To better understand the polymerization behavior observed for 15,9DA on gold surfaces, the hypotheses proposed above are systematically investigated in this study. Reductive Desorption. The decrease in resonance Raman intensity of the blue-phase monolayer polymer upon prolonged UV exposure may be related to changes in the structure of the gold-sulfur interface. Photooxidation of the sulfur headgroup would alter the steric environments of the conjugated backbone by breaking the Au-S covalent bond. In addition, cleavage of the Au-S bond, if accompanied by removal of the adsorbates, could also lead to destruction of the monolayer from simple material loss. The Au-S bond integrity is evaluated as a function of UV exposure using reductive desorption experiments. The breaking of the gold-sulfur bond during these measurements occurs via the following one-electron reduction to a thiolate species:52
AusSsR + 1e- f Au(0) + -SsR The potential at which this reduction occurs is dependent on a number of parameters including the monolayer thickness, the domain structure of the assembly, and factors which influence the gold-sulfur bond strength such as substrate crystallinity and the sulfur binding site (step or terrace).52-55 Although reduction of the gold-sulfur bond (48) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305. (49) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654. (50) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656. (51) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (52) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (53) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (54) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (55) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2787.
Langmuir, Vol. 15, No. 4, 1999 1219
Figure 4. Reductive desorption of diacetylene monolayers after various irradiation times. The solid line is the first scan and the dashed line is the second scan. Experimental conditions as noted in the text.
does result in desorption from the surface, factors such as solution agitation and the solubility of the individual adsorbates determine if the monolayer is actually physically removed. Indeed, reductive desorption studies by Crooks et al. indicate that the monomeric species can be physically removed from the surface more easily than the polymer can be removed.11 However, with no physical agitation, the limited solubility of both monomer and polymer are expected to keep them near the surface. Thus, in contrast with previous studies, the reductive desorption method is utilized here to assess the nature of the Au-S bond to determine if photooxidation occurs under these UV exposure conditions. Monolayers on nominally atomically flat gold (111) surfaces are characterized by a single reduction peak on the negative scan direction. The charge associated with this reduction is related to the monolayer surface coverage through the following relation:
Γ ) Q/nFA where Γ is the surface coverage in moles/cm2, Q is the charge obtained by integrating the reductive desorption peak, n is the number of electrons, F is Faraday’s constant, and A is the area of the electrode. The reverse scan is characterized by a small peak resulting from readsorption of a portion of the monolayer. The second scan exhibits a reductive desorption peak smaller and more positive than the original because of a partially readsorbed monolayer. This shift is likely caused by the presence of pinholes in the monolayer created upon incomplete readsorption. Indeed, these profiles are nearly identical to those observed for analogous monolayers formed from alkanethiols onto gold.55 UV-induced cleavage of the goldsulfur bond by a photooxidation mechanism is expected to yield a decreased intensity and increased broadening of the reduction peak with irradiation time. Figure 4 shows representative scans of reductive desorption experiments performed in triplicate for 15,9DA monolayers after various UV exposure times. For an
1220 Langmuir, Vol. 15, No. 4, 1999
unpolymerized monolayer with no UV exposure, a single peak located at -1.20 V ( 15 mV is evident on the first scan which is attributed to the one-electron reduction of the Au-S bond. The surface coverage, after accounting for the capacitive charging of the gold surface,55,56 is 8.8 ( 1 × 10-10 mol/cm2. This result, after minimal surface roughness is considered, is consistent with the expected coverage of 7.6 × 10-10 mol/cm2 coverage for a x3 × x3 R30° adlayer on gold (111).55 The second scan indicates a broader peak shifted substantially to more positive potentials, consistent with observations for n-alkyl thiols. Upon UV irradiation for 7 min to the maximum resonance Raman intensity, the same reductive desorption behavior is observed with a peak potential located at -1.18 V ( 12 mV and a surface coverage of 9.3 ( 2 × 10-10 mol/cm2. Finally, after more than 90 min of UV exposure, when the resonance Raman signal is largely depleted, the structure of the gold/sulfur interface remains unaltered as measured electrochemically. In this case, the reductive desorption peak is located at -1.20 V ( 14 mV with a surface coverage of 8.5 ( 2 × 10-10 mol/cm2. Small changes in the desorption peak may be attributed to structural alterations in the monolayer upon UV exposure such as changes in tilt, twist, or polymerization efficiency which could slightly alter the thickness or dielectric constant of the monolayer. However, the consistency in peak potential and surface coverage indicates that the decrease in conjugation length shown by resonance Raman spectroscopy is, in fact, not related to changes in strain from cleavage of the gold-sulfur bond. Furthermore, the reductive desorption results demonstrate no alterations in the gold/sulfur interface through a photooxidation mechanism. The lack of photooxidation is further confirmed by XPS measurements which show no sulfonate species on the gold surface. Alkyl Chain Crystallinity. The structure and conformational order within the alkyl chain regions have been considered to significantly influence the effective conjugation length of the polymer backbone in diacetylene LB films.23,28,30 In one mechanism, the phase transition from blue to red is initiated by a decrease in alkyl chain crystallinity, which induces strain to the polymer backbone, thus decreasing the effective conjugation length.30 In this study, the structure and conformational order of the alkyl side chains in the DA and PDA monolayers are assessed as a function of UV exposure using FTIR-ERS. Surface selection rules indicate that only transition moments with components perpendicular to the surface will have a significant intensity in external reflection. The transition moment directions for the CH stretching transitions are illustrated in Figure 5, together with a schematic of the alkyl chain surface orientation. An alltrans alkyl chain can be described on the basis of the tilt relative to the surface normal (R) and the twist of the CCC plane relative to the tilt direction (β). As illustrated in Figure 1, DA and PDA monolayers have two distinct alkyl regions, above and below the sites of unsaturation (tail and spacer regions, respectively). It is important to note that both regions are probed in the external reflection measurement, leading to a spectrum representing a convolution of the tail and spacer regions. Figure 6 shows the FTIR-ERS spectra in the C-H stretching region of 15,9-DA monolayers on gold surfaces at increasing UV exposure times of (a) 0 min, (b) 7 min, (c) 23 min, and (d) 93 min. Vibrational transitions at 2919 and 2852 cm-1 are assigned to the νa(CH2) and νs(CH2) stretching modes, respectively. Additional peaks at 2878 and 2963 cm-1 arise from the Fermi resonance methyl (56) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1997, 13, 243.
Cai et al.
Figure 5. Schematic diagram of alkyl chain orientation in the laboratory frame and CH stretching transition moments in the molecular frame.
Figure 6. FTIR-ERS spectra of 15,9-DA on Au surfaces upon UV exposure under nitrogen for (a) 0 min, (b) 7 min, (c) 23 min, and (d) 93 min. The arrows indicate the direction of the peak intensities for increasing UV exposure. Experimental conditions as noted in the text.
symmetric stretch and the methyl asymmetric stretch, respectively.57-60 The peak positions of the νa(CH2) and νs(CH2) modes and their band shapes are sensitive indicators of the alkyl chain crystallinity. The peak frequency of the νa(CH2) and νs(CH2) modes occur at ca. (57) Snyder, R. G.; Schachtsneider, J. H. Spectrochim. Acta 1963, 19, 85. (58) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (59) Synder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
Fabrication of Extended Conjugation Length Polymers
2919 and 2850 cm-1, respectively, for all-trans crystalline alkyl chains.58,60 These transitions shift to higher frequencies and broaden for liquidlike disordered chains.60 Therefore, the peak positions for the νa(CH2) and νs(CH2) vibrations observed for 15,9-DA monolayers (Figure 6a) indicate that the alkyl side chains are predominantly in a highly crystalline environment. However, the highfrequency shoulder on the νa(CH2) is indicative of a disordered component in the methylene chains, conjectured to arise from the spacer region.15 Significantly, the peak positions for the νa(CH2) and νs(CH2) stretches are negligibly shifted within the experimental resolution of (1 cm-1 upon polymerization and prolonged UV irradiation (Figure 6b-d), suggesting negligible perturbation of the alkyl side chain crystallinity. The lack of alteration of the alkyl chain crystallinity demonstrates that crystallinity changes with UV exposure are not responsible for the decrease in conjugation length of the polydiacetylene backbone. In addition, the νs(CH3) and νa(CH3) modes at 2878 and 2963 cm-1, respectively, do not change in either intensity or peak position, suggesting a negligible effect of polymerization on the orientation of the outermost methyl groups. Finally, constant absorbances for the methyl transitions indicate that no material was lost because of prolonged UV irradiation, consistent with observations in the reductive desorption experiments. Alkyl Chain Structure. Although the peak positions of the methylene stretches indicate that alkyl chain crystallinity is not perturbed upon polymerization and prolonged UV exposure, the band intensities change dramatically (Figure 6). The methylene asymmetric stretching transition exhibits a systematic decrease in intensity with a concomitant increase in the methylene symmetric transition. In external reflection infrared spectroscopy, only those vibrations with transition dipole moments oriented perpendicular to the metal substrate are observed. As a result, peak intensities are directly related to the component of each transition moment that is perpendicular to the metal surface. For the methylene asymmetric and symmetric stretching transitions, the transition moments are perpendicular to the chain axis (that is, in the HCH plane) with the νs(CH2) moment bisecting the HCH bond angle and the νa(CH2) moment oriented perpendicular to the νs(CH2) moment (Figure 5). Therefore, the chain axis of an all-trans polymethylene chain must have a significant tilt angle relative to the surface normal for the symmetric band to have any intensity. Moreover, the CCC plane of the methylene chains must be rotated or twisted with respect to the plane formed by the chain axis and the surface normal for the asymmetric transition to be observed. Upon UV exposure, the band intensities shift in opposite directions. If only the tilt angle is decreased, a unidirectional decrease in band intensities would be expected. Thus, concomitant with a decrease in tilt angle, the twist angle is also changing upon UV exposure. The simultaneous increase in the intensity of νs(CH2) and decrease in the intensity of νa(CH2) arises solely from a decrease in the twist angle with UV exposure. Indeed, this concomitant decrease in the alkyl chain tilt and twist angles upon polymerization was observed previously for similar surface-attached diacetylene monolayers.13 Several methods have been proposed to calculate the ensemble average tilt and twist angles.61-64 Although the spectral simulation approach appears to yield the most (60) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1982, 88, 334. (61) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
Langmuir, Vol. 15, No. 4, 1999 1221 Table 1. Calculated Twist Angle as a Function of Exposure Time exposure time (min)
(I(νaCH2)/I(νs(CH2))
twist angle (β)
0 7 23 93
11.6 ( 2.1 6.3 ( 0.9 3.7 ( 0.4 2.2 ( 0.3
72° 66° 60° 53°
information,61-63 the presence of a disordered alkyl chain component within the DA and PDA monolayers leads to significant difficulties in fitting. However, changes in the alkyl chain twist (β) with UV exposure may be estimated using an approach recently reported in the literature.64 Using the projection of transition moments onto the surface normal, the twist angle may be calculated presuming all-trans methylene chains within the monolayer. Although a disordered alkyl chain component is present in the DA and PDA monolayers, this simple model is useful in estimating the change in twist angle with UV exposure for illustrative purposes. The twist angle may be approximated from the intensity ratio of the methylene asymmetric and symmetric stretches in conjunction with the intensity ratio from a reference SAM (denoted with subscript 0) of known twist (β0).64
[(
tanβ ) tanβ0
)(
I(νaCH2) I0(νsCH2) I(νsCH2) I0(νaCH2)
)]
1/2
Using analogous alkanethiol monolayers as a reference, a twist of 52° measured in previous studies62,63,65 is utilized along with an intensity ratio for the reference of 0.5. The estimated twist angles with UV exposure shown in Table 1 indicate the clear decrease in β with exposure time. For the unpolymerized monolayer, a relatively high twist angle is estimated with the angle decreasing to near the value measured for simple alkanethiol monolayers upon prolonged UV exposure. This decrease in β may arise from a combination of decreased average twist angle and increased distribution of angles within the monolayer assembly. In fact, the intensity ratio has been determined to be near 0.5 (β ) 52°) for an isotropic distribution of twist angles about the chain axis.66 These apparent changes in the methylene chain twist may be attributed to structural changes upon polymerization. Because of ensemble averaging, however, it is not possible to localize the twist angle within the monolayer assembly. However, the primary structural change expected upon polymerization is in the diacetylene region, where adjacent molecules in the assembly are covalently linked to form the delocalized polymer backbone (Figure 1). During this process, the acetylenic carbons at the top and bottom of the diacetylene monomers undergo a change in hybridization from sp to sp2. Because the polymer backbone is parallel to the substrate, this hybridization shift results in a decrease in the tilt angle of the bonds above and below the backbone. As a result, it is perhaps not surprising that this hybridization change could induce a torque force on the methylene chain region near the polymer backbone. This structural change could result both in a decrease in the average twist angle in polym(62) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (63) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 2, 558. (64) Hou, Z.; Abbott, N. L.; Stroeve, P. Langmuir 1998, 14, 3287. (65) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (66) Snyder, R. G. J. Chem. Phys. 1965, 42, 1744.
1222 Langmuir, Vol. 15, No. 4, 1999
erized regions and an increased distribution in the twist angle for the overall monolayer. Interestingly, this significant change in the twist angle does not appear to be translated into the outermost regions of the alkyl chains, as no change in the νs(CH3) and νa(CH3) modes is observed with exposure time (Figure 6). In addition, although a decrease in the localized tilt angle results from this hybridization shift, the lack of change in the methyl transitions indicates that the tilt angle is also not translated to the outermost regions of the monolayer. On the basis of these results, the change in hybridization upon polymerization appears to be the most likely origin of the decrease in extended conjugation length polymer with exposure time. Upon initial exposure, it appears that the chain regions near the polymer backbone can accommodate the changing twist with polymerization leading to an increase in the formation of the blue polymer form. Upon further exposure, the chains can no longer accommodate the torque and the strain is transferred to the polymer backbone, resulting in a decreased polymer conjugation length as indicated by the decrease in resonance Raman intensity for the blue form. Using this model, the temporal behavior of the CH stretching transitions need not mirror the resonance Raman measurements. Indeed, it is more likely that some threshold will be reached and then the strain will be transferred into the polymer backbone. The impact of this hybridization effect may be exacerbated in these SAMs because of the constrained methylene spacer chain. This hypothesis suggests an increased formation of the so-called red phase, resulting from a decrease in the effective conjugation length of the polymer backbone. Recent studies indicate the presence of the red-phase polymer within the monolayer, lending further support for this chromatic phase transition model.47
Cai et al.
Conclusions Photoinduced polymerization of the diacetylene-containing monolayer assembly is shown to yield the highly conjugated blue-phase polymer. Monitored selectively using resonance Raman spectroscopy, formation of the highly conjugated polymer reaches a maximum at 7 min UV exposure before decreasing. This turnover is consistent with similar observations for LB multilayer assemblies of polydiacetylene, and is likely caused by a decrease in the polymer conjugation length. Reductive desorption experiments show no correlation between cleavage of the Au-S bond and the chromatic transition. Moreover, both desorption and FTIR studies indicate that no material loss is observed upon prolonged UV exposure. In addition, increased strain in the polymer backbone induced by alterations in the alkyl chain crystallinity is not indicated by infrared spectroscopy. However, significant changes in the methylene chain twist are observed with increasing UV exposure, likely a result of the hybridization change from sp to sp2 above and below the backbone during polymerization. Upon increased exposure, it is conjectured that the strain induced by the change in hybridization can no longer be accommodated by the alkyl chains, and is transferred to the polymer backbone, leading to a decreased conjugation length. Studies are presently underway to monitor multiple polymer phases within the single molecular layer and evaluate the feasibility of direct polymer cleavage during prolonged UV irradiation. Acknowledgment. The authors acknowledge financial support by the National Institute of General Medicine Sciences, National Institute of Health (#GM52555-01 A1). H.M. thanks the Fulbright Commission for a travel grant. LA981219I
