Langmuir 2006, 22, 1129-1134
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UV Polymerization of Self-Assembled Monolayers of a Novel Diacetylene on Silver: A Spectroscopic Analysis by Surface Plasmon Resonance and Surface Enhanced Raman Scattering Emilia Giorgetti,† Maurizio Muniz-Miranda,‡ Giancarlo Margheri,| Anna Giusti,§ Stefano Sottini,*,† Marina Alloisio,§ Carla Cuniberti,§ and Giovanna Dellepiane§ INSTM and Istituto dei Sistemi Complessi - CNR, Via Panciatichi 64, 50127 Firenze, Italy, Dipartimento di Chimica, UniVersita` di Firenze, Via della Lastruccia 3, 50019, Sesto Fiorentino, Italy, INSTM and Dipartimento di Chimica e Chimica Industriale, UniVersita` di GenoVa, Via Dodecaneso 31, I-16146 GenoVa, Italy, and Istituto dei Sistemi Complessi - CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), Italy ReceiVed May 30, 2005. In Final Form: NoVember 18, 2005 UV polymerization of self-assembled monolayers of a novel carbazolyl-diacetylene (CDS9) chemisorbed on silver films was demonstrated by surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) experiments. SPR tests performed during UV exposure permitted one to observe the growth of the absorption coefficient, associated with the formation of the polymeric backbone. The Raman spectra of polymerized monolayers exhibited the bands associated with the CdC stretching modes of the conjugated backbone, typical of the blue and red polymeric phases usually present in polydiacetylenes, with a clear predominance of the red form. Moreover, the strong surface enhancement of the Raman band corresponding to the aromatic CdC stretching modes suggested that carbazolyl groups arrange nearly perpendicularly to the metal surface. In contrast, the absence of a SERS signal in the region of conjugated CtC bond stretchings confirmed a polymerization scheme with conjugated triple bonds nearly parallel to the plane of the metal.
Introduction UV polymerization of diacetylene monolayers chemisorbed on gold-coated substrates has been reported in the literature mainly by two research groups.1,2 The interest of the subject stems from the possibility of growing and patterning robust single molecule polymeric layers exhibiting the outstanding properties of polydiacetylenes (PDAs), in particular as far as applications to nonlinear optics and sensing devices are concerned.3 Moreover, the possibility of in situ polymerization with UV light, by using, for instance, scanning near field optical microscopy (SNOM), opens the way to nanolithography and nanophotonic. We investigated the in situ UV polymerization of diacetylene monolayers, by using a novel diacetylene containing carbazolyl groups, the bis(14-(9H-9-carbazolyl)tetradeca-10,12-diyne-1-yl disulfide) (CDS9). Due to the electron-donating character of the carbazole ring, polycarbazolyl-diacetylenes have significantly contributed to the development of materials with implemented nonlinear optical properties.4,5 Moreover, the hole-transporting properties of the carbazolyl moieties, which can be further enhanced by proper functionalization, can induce photoconduction6 or electroluminescence.7 Furthermore, although changes in * Corresponding author. Phone: 0039-055-5226660.
[email protected]. † INSTM and Istituto dei Sistemi Complessi - CNR. ‡ Universita ` di Firenze. § Universita ` di Genova. | Istituto dei Sistemi Complessi - CNR.
E-mail:
(1) Mowery, M. D.; Cook Smith, A.; Evans, C. E. Langmuir 2000, 16, 5998 and references therein. (2) Taisun, K.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 198 and references therein. (3) (a) Batchelder, N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (b) Kim, T.; Crooks, R. M. Tetrahedron Lett. 1994, 35, 9501. (4) Margheri, G.; Giorgetti, E.; Sottini, S.; Toci, G. J. Opt. Soc. Am. B 2003, 20, 751. (5) Alloisio, M.; Sottini, S.; Riello, P.; Giorgetti, E.; Margheri, G.; Cuniberti, C.; Dellepiane, G. Surf. Sci. 2004, 68, 554.
the orientation of the aromatic groups could introduce some disorder and favor the formation of the red form of the polymer, the interactions among their π-electrons in the monoassembled structure are expected to stabilize the ordered arrangement required for the polymerization. Self-assembled monolayers (SAMs) of CDS9 were grown on silver films, and their degree of polymerization when exposed to UV light was monitored by surface plasmon resonance (SPR) and by surface enhanced Raman scattering (SERS). Silver was preferred to the more frequently used gold, because its surface morphology can be more easily tuned by evaporation procedures, either to permit an efficient coupling of surface plasmons at the metal/organic interface, or to match the requirements of surface enhancement of the Raman scattering.8 Moreover, the ionic diameter of the two metals in bulk is practically the same (2.89 Å for silver and 2.99 Å for gold), thus meaning that, at least in principle, the distances among adsorbing sites required for optimum polymerization can be satisfied with both metals. However, the chemisorption of organosulfur compounds on Ag or Au films is a complex mechanism, involving the formation of S-Me bonds of a sufficient number of vicinal molecules. For example, the different mechanism of chemisorption on roughened Ag and Au was evidenced by SERS investigation on alkyl mercaptans, showing that the adsorption occurs on the three-fold sites in the case of Ag and on the on-top or bridge sites in the case of Au.9 Spectroscopic studies performed on molecular ordering of organosulfur compounds on atomically flat metal surfaces, such as Au (111), Au (100),10 or Ag(111),11 showed (6) Diduch, K.; Wu¨bbenhorst, M.; Kucharski, S. Synth. Met. 2003, 139, 515. (7) Giorgetti, E.; Sottini, S.; DelRosso, T.; Margheri, G.; Alloisio, M.; Dellepiane, G. Synth. Met. 2004, 147, 271. (8) Weimer, W. A.; Dyer, M. J. Appl. Phys. Lett. 2001, 79, 3164. (9) Seung, I. C.; Eun, S. P.; Kwan, K.; Myung, S. K. J. Mol. Struct. 1999, 479, 83. (10) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (11) Fenter, P.; Eisenberger, P.; Jum, L.; Camillone, N., III; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 2013.
10.1021/la0514157 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/04/2006
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Figure 1. (a) Chemical structure of bis(14-(9H-9-carbazolyl)tetradeca-10,12-diyne-1-yl disulfide) (CDS9). The C-S-S bond angle is ∼104°. (b) Chemisorption of CDS9 on silver and subsequent polymerization under UV exposure.
that on Ag(111) the ordering of the organic molecules is different from the expected (nx3Xx3)R30° structure observed on Au(111) surfaces and is characterized by a lattice constant that is incommensurate with the substrate. Moreover, the alkyl chains are less tilted than on Au(111). As a consequence, the in-plane ordering of adsorbed molecules and the robustness of the organic layer grown on Au or Ag surfaces cannot be easily estimated and compared a priori. In the first section of this paper, we describe the preparation of the samples and present their surface morphology, as observed by atomic force microscopy (AFM). In the second section, we give evidence of polymerization of the monomolecular layer, as observed by SPR. The last section is devoted to the Raman experiments, which confirmed the formation of polymer monolayers on silver.
Sample Preparation We grew monolayers of CDS9 by self-assembly (SA) on silvercoated soda-lime glass microscope slides. CDS9 was synthesized by oxidative coupling between the acetylenic moiety carrying the carbazole group and the acetylenic moiety carrying the nonylthiol substituent, through a reactions sequence described elsewhere.12 As formed, the thiol derivative spontaneously oxidizes, giving the disulfide of Figure 1a. Diacetylene-containing disulfides are generally considered more stable than their thiol counterparts and can be stored over extended periods without spurious chemical reactions.1 The SA process of these molecules onto silver surfaces is expected to take place through S-S bond breakdown and chemisorption via sulfur atoms.13,14 Figure 1b shows this adsorption scheme and the subsequent polymerization of the diacetylene units, as proposed for analogous compounds on gold in refs 1 and 2. Silver coatings of controlled roughness were obtained by two different procedures: one by electron-gun assisted deposition at a rate of 14.5 Å/s to obtain a smooth silver/air interface, and the (12) Dell’Erba, C.; Cuniberti, C.; Dellepiane, G.; Alloisio, M.; Petrillo, G. Synth. Met. 2005, submitted. (13) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (14) Venkataramanan, M.; Skanth, G.; Bandyopadhyay, K.; Vijayamohanan, K.; Predeep, T. J. Colloid Interface Sci. 1999, 212, 553.
Figure 2. Typical AFM images showing the topography and the roughness profile along the line indicated by the black arrow of a glass substrate (a), a smooth silver coating (b), and a rough silver coating (c). The images were acquired in dynamic mode at a scanning rate of 0.5 s/line (a and c) and of 0.6 s/line (b).
other by thermal deposition at a rate of 0.25 Å/s to obtain a rough, nanostructured silver/air interface. Figure 2 shows the typical surface morphology of a glass substrate (a), a smooth 50 nm-thick Ag layer (b), and a rough 50 nm-thick Ag layer (c). The images, shown in the figure after plane subtraction, were obtained with a Nanosurf EasyScanDFM in air and in dynamic mode. The cantilever, with an integrated silicon tip (spring constant 13-14 N/m), was vibrating slightly below its free resonance frequency (134-136 kHz) and moved over the surface in a raster scan. The surface RMS roughness and the peak-tovalley value calculated over images of 1 µm × 1 µm were, respectively, 1.4 and 11 nm, 1.8 and 16 nm, and 4.2 and 30 nm for the three samples of Figure 2. A granular structure is visible for both samples b and c. In the first case, the size distribution function of the grain diameter peaked at 12 nm, while in the second case a bimodal distribution occurred, with peaks at 160 and 238 nm. The SA was performed by dipping the silver-coated substrates in a 1 mM solution of CDS9 in spectra-grade chloroform. After 48 h of immersion, the samples were extracted and washed with pure chloroform, to dissolve the extra-monomer deposited on the silver surface and leave only the chemisorbed layer. During the whole processing and storage, CDS9 powder, solutions, and films were kept in the dark. The polymerization of the monomer SAMs was performed by exposure to a low intensity UV lamp with peak emission at 254 nm (model UVG-11, Ultraviolet Products Inc.). The lamp was kept at a distance of 1-2 cm from the films, and the estimated intensity on the samples was of the order of 10 mW/cm2.
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Figure 3. Kretschmann’s configuration.
SPR Characterization In principle, the degree of polymerization reached by the monolayer could be monitored by measuring its absorption coefficient. In fact, it is well known that, while diacetylene monomers are transparent in the visible region, the polymers are strongly colored and often exhibit a blue and a red form, the first being characterized by a more extended effective conjugation length.15 In the case of CDS9, this behavior is confirmed by UV-vis spectra of powder samples in KBr pellet. These spectra show that UV irradiation generates a wide absorption band in the visible region peaked at 515 nm with a shoulder extending up to 650 nm. Analogously, the polydiacetylene SAMs are expected to absorb green and red light proportionally to the degree of polymerization. In detail, green light is resonant with the red phase and partially absorbed also by the blue phase, while red light is resonant with the blue phase only. However, due to the small thickness that may range from 2 to 3 nm,16 conventional electronic absorption spectra of CDS9 monolayers cannot be measured directly. In principle, such spectra could instead be obtained by SPR, provided that a finely tunable laser source is available. Although, in principle, SPR in Kretschmann’s configuration allows one to obtain an accurate measure of the complex refractive index, and hence of the absorption coefficient, of single molecule layers deposited on silver,17 its use to recover complete electronic spectra requires a perfect knowledge of the dispersion of the metal layer and is affected by large experimental errors. For these reasons, we adopted SPR only to measure the thickness of CDS9 SAMs and to monitor the changes, at a given wavelength, of their absorption coefficient with UV exposure and relied on Raman spectroscopy to interpret these data and identify the nature of the process induced in the SAMs by UV illumination. To obtain an efficient coupling of surface plasmons, we performed SPR experiments only on samples having a smooth silver/CDS9 interface. Indeed, a moderate interface roughness limits the resolution of the technique, while an interface roughness larger than 5 nm RMS completely inhibits the coupling of the surface plasmons.17 A sketch of the experimental setup is reported in Figure 3. To exclude a polymerization of the diacetylene layer induced by laser light during the measurements, we excited the surface plasmon with very low power beams (of the order of 1 µW on a spot diameter of ∼2 mm), either from an Ar-Kr (514.5 nm) or a He-Ne (633 nm) laser, and kept the monomeric samples under laser irradiation for more than 1 h, monitoring, time by time, the complex index of refraction. We did not detect any significant change with both wavelengths. Subsequently, we exposed the films to UV light and measured the complex index of refraction after different exposure time intervals with both (15) Chance, R. R.; Patel, G. N.; Witt, J. D. J. Chem. Phys. 1979, 71, 2096. (16) Cavalleri, O.; Prato, M.; Chincarini, A.; Rolandi, R.; Canepa, M.; Ghiozzi, A.; Alloisio, M.; Lavagnino, L.; Cuniberti, C.; Dell’Erba, C.; Dellepiane, G. Appl. Surf. Sci. 2005, 246, 403. (17) Raether, H. Surface plasmons on smooth and rough surfaces and on gratings; Springer-Verlag: Berlin, 1988.
Figure 4. Variation of the real (a) and imaginary (b) part of the refractive index of a CDS9 monolayer on Ag versus UV exposure, measured at 633 nm.
514.5 and 633 nm wavelengths. Figure 4 reports the experimental results on the variation of the real (a) and imaginary (b) part of the index of refraction (∆nr and ∆ni, respectively) versus UV exposure time, obtained at 633 nm. Figure 4 demonstrates the formation of the polymer: indeed, both the absorption coefficient and the refractive index grew with UV exposure and saturated after 30 min exposure. The SPR technique was also adopted to measure the thickness of the SAMs. For this purpose, a SAM was deposited on a wellcharacterized silver film, by partial immersion in the CDS9 solution. In this way, we obtained a step between bare and CDS9coated silver. Afterward, the thickness of the SAM could be evaluated by recording the SPR response across the step. We obtained a thickness of 2.4 nm with a 20% error. This value is in agreement with the expected formation of a single molecular layer.16
Raman Experiments We performed Raman studies, to gather information about the arrangement of the polymer monolayer on the metal substrate and on its degree of conjugation.18 Raman spectra were recorded by using both the 514.5 and 488 nm lines of an Ar+ laser and the 647.1 nm line of a Kr+ laser. Samples were irradiated with 40 ÷ 100 mW laser power. Power measurements were performed with a power meter (model 362; Scientech, Boulder, CO) with an accuracy of ∼5% in the 300-1000 nm spectral range. The laser beam was incident at about 30° with respect to the sample surface and was defocused to impair thermal effects, laser-induced polymerization, or quenching of the Raman signal. The scattered light was collected at 90° with respect to the plane of the samples and detected by a Jobin-Yvon HG2S monochromator equipped with a cooled RCA-C31034A photomultiplier (-28 °C) and a data acquisition facility. (18) Mei Cai; Mowery, M. D.; Menzel, H.; Evans, C. E. Langmuir 1999, 15, 1215.
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Figure 5. Raman spectra of CDS9 powder under irradiation with 514.5 nm at 50 mW (A) and at 400 mW (B).
At first, we recorded Raman spectra of CDS9 monomer powder and chloroform solutions. Figure 5 reports the spectra of CDS9 powder recorded at 514.5 nm and at different power levels of the exciting beam. Curve A, which was obtained with 50 mW power, shows the nonconjugated CtC stretching of CDS9 (2260 cm-1),19 but, even if very weak and not resolved, also the CtC (∼2100 cm-1) and CdC (∼1500 cm-1) stretching associated with the polymer. The same spectrum recorded with 400 mW laser power (curve B of Figure 5) showed a considerable growth of the signal belonging to the polymer. In particular, the band associated with conjugated CtC stretching permits observation of both blue (2080 cm-1) and red (2100 cm-1) forms, indicating that a certain degree of polymerization was produced by the 514.5 nm laser beam. This effect, present at a lower extent also under red-light irradiation, was not observed in Raman spectra of CDS9 solutions in chloroform, where polymer bands are absent, thus excluding the possibility that a significant amount of the polymer, already present in the solution, could be deposited and chemisorbed during the SA process. The spectrum of CDS9 powder obtained after 400 mW irradiation at 514.5 nm, but recorded with 647.1 nm exciting light, is shown in Figure 6 as B′ and also there compared with spectrum B of the previous figure. With this exciting wavelength, the absence of fluorescence and the Raman resonance with the blue form permitted one to enhance the vibrational bands associated with this phase of the polymer. Indeed, the sharp peak at 2082 cm-1 and the very intense and sharp band at 1450 cm-1 are assigned to the triple and the double CC stretching vibrations of the blue phase, respectively. This result demonstrates that the 647.1 nm laser line is in full resonance with the blue form of CDS9 powders and that, in absence of other mechanisms of enhancement, the red form, although present (curve B of Figures 5 and 6), is not visible with this exciting wavelength. The shoulder at 1420 cm-1 is probably associated with the H-C-H bending of aliphatic CH2 groups present in the chain substituents. Between 900 and 1400 cm-1, there is a variety of bands originated by alkyl chain deformation modes, C-C stretching and C-C-H bending. The weak band at 980 cm-1 can be reasonably assigned (19) Zheng, L. X.; Hess, B. C.; Benner, R. E.; Vardeny, Z. V. Phys. ReV. B 1993, 47, 3070.
Figure 6. Raman spectra of CDS9 powder, after irradiation with 514.5 nm. Exciting wavelengths: 647.1 nm (B′) and 514.5 nm (B).
to a collective bending mode of the chain,20 while the intense one at 692 cm-1 can be assigned to the C-S stretching vibration. The C-H stretching modes of the aliphatic groups are visible in the high-frequency region, between 2800 and 3000 cm-1. Afterward, we performed Raman spectra on CDS9 monolayers deposited on silver substrates. Due to the very small thickness of the molecular layers, the resonance effects were not sufficient to obtain intense and well-resolved Raman spectra. Thus, we were forced to use either Kretschmann’s configuration or surface enhancement of the Raman signal to perform the spectroscopic investigation. For this purpose, we prepared silver-coated substrates having adequate roughness. Figure 7 shows the reflectivity of two typical silver surfaces, P and R, used in our experiments. Substrate P corresponds to a smooth 50-nm-thick Ag film optimized for the coupling of surface plasmons and identical to that used for SPR measurements (Figures 2b and 4); substrate R corresponds to a rough 50-nm-thick Ag film (Figure 2c), not suitable for the coupling of surface plasmons. The inset of Figure 7 shows the differential reflectivity between samples P and R and evidences the characteristic reflectivity dip around (20) Comoretto, D.; Ottonelli, M.; Musso, G. F.; Dellepiane, G.; Soci, S.; Marabelli, F. Phys. ReV. B 2004, 69, 115215.
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Figure 7. Reflectivity of samples P and R before growth of the CDS9 SAM. The inset shows the differential reflectivity between the two metal surfaces.
350 nm exhibited by silver layers having moderate roughness.17 The spectral and topographical characteristics of sample R, although not optimized for SERS, are expected to provide a considerable surface enhancement of the Raman signal with 488 and 514 nm exciting wavelengths and a negligible enhancement with 647.1 nm exciting wavelength.21 In practice, we tuned the surface roughness of sample R so that it was sufficiently large to provide the SERS effect, but also sufficiently small to reduce its influence on the CDS9 polymerization process. The Raman spectra of CDS9 SAMs deposited on substrates P and R were recorded before UV irradiation and after irradiating the films for different time intervals, up to 120 min. Three exciting wavelengths were used to evidence either the Raman resonance of different polymer phases, surface enhancement, or both mechanisms. With both samples P and R, it was impossible to obtain Raman spectra of the monolayers before UV exposure, because the Raman response of the monomer was too weak with all exciting wavelengths. In the case of sample P, even after UV exposure we could not observe the Raman spectra by using the conventional configuration at 30° incidence adopted in all other experiments. To enhance the Raman response of sample P, we coupled the exciting beam to the surface plasmon (Figure 3). With this procedure, we obtained a considerable intensification of the exciting radiation, which was confined across the thickness of the CDS9 monolayer, with the electric field mainly perpendicular to the metal/dielectric interface, and the magnetic field rigorously parallel to it. The associated spectra are reported in Figure 8a. This sample exhibited only a resonant contribution to the Raman signal, because the SERS efficiency of smooth silver films is very low. As a consequence, the most intense spectra (curves A, B, and C in Figure 8a) were detected with the 514.5 nm exciting wavelength. For comparison, in Figure 8a is also reported the much weaker spectrum recorded at 488 nm (D). Spectra A, B, and C were obtained after different exposure times to UV light. In particular, spectrum A, recorded after 15 min of UV irradiation, confirms the formation of a conjugated backbone in resonance with the exciting wavelength. After 33 min, the spectrum appears more intense and better defined (B), while no relevant changes were detected after 50 min (C). The weak bands at 1512 and 1534 cm-1, usually observed in the CdC stretching mode region of polydiacetylenes, give evidence of the polymerization process. However, the most intense bands present in these spectra between 990 and 1110 cm-1 are not clearly assigned and could be reasonably related to deformation modes of a disordered chain. (21) (a) Saito, Y.; Wang, J. J.; Batchelder, D. N.; Smith, D. A. Langmuir 2003, 19, 6857. (b) Felidj, N.; Aubard, J.; Levi, G. Trends Phys. Chem. 1999, 7, 103.
Figure 8. (a) Raman spectra of sample P, registered in Kretschmann’s configuration for different UV exposure times. Curves A (15 min), B (33 min), and C (50 min) were obtained with 514.5 nm exciting wavelength, while curve D (50 min) corresponds to the same exposure time as curve C, but was obtained with 488 nm exciting wavelength and, for more clarity, was multiplied by a factor of 10. (b) AFM image of the CDS9 film in the area of Kretschmann’s coupling after Raman experiments, showing the topography and the roughness profile along the line indicated by the black arrow. The AFM image was acquired in dynamic mode at a scanning rate of 0.5 s/line.
Indeed, an optical microscope inspection of the sample in the region of Kretschmann’s coupling showed evidence of damage, further confirmed by AFM analysis (Figure 8b). The RMS surface roughness and the peak to valley value of the organic layer, which were 1.8 and 16 nm before plasmon coupling, increased to 8.7 and 75 nm after Raman tests. In the case of sample R, due to the roughness of the interface, we could obtain Raman spectra of the polymerized monolayers with the conventional configuration at 30° incidence and with a defocused beam. The Raman signal of the CDS9, completely absent before UV exposure, was already well visible after 15 min of UV irradiation, confirming that polymerization of the diacetylene layer took place on the same time-scale observed with SPR tests. Figure 9 reports the spectra of the layer recorded after 71 min of UV irradiation and with different exciting wavelengths. For more clarity, Figure 9 is limited to the frequency interval between 800 and 1700 cm-1 because, in all cases, we did not detect any signal in the region around 2100 cm-1 corresponding to the CtC stretching modes. The best spectrum in Figure 9 is that obtained with the off-resonant 488 nm exciting light (C), suggesting that in these experiments the SERS effect is predominant. The most significant features of spectrum C are the bands at 1514 cm-1 (CdC stretching of red polymer phase) and 1464 cm-1 (CdC stretching of blue polymer phase). The surface enhancement also permitted one to observe a well-resolved band at 1608 cm-1 that was completely absent in the spectra of powders (Figures 5 and 6) and corresponds to the aromatic CdC
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Figure 9. Raman spectra of sample R, registered after 71 min exposure to UV light. Curves A, B, and C were obtained with 647.1, 514.5, and 488 nm exciting wavelengths, respectively. For more clarity, curve A was multiplied by a factor of 10.
stretching of the carbazolyl groups.22 The spectrum recorded with the 514.5 nm exciting wavelength (B) was less intense, but did not exhibit relevant differences. Curve B confirms the expected coexistence of a small SERS effect (on the basis of Figure 7) and a resonant behavior of both polymer forms, as was already observed (Figure 5) in the spectrum of powders with the same exciting wavelength. With the 647.1 nm exciting wavelength (curve A), the spectrum was very weak and noisy, but nevertheless it showed the bands associated with the double bonds of the conjugated red phase (1530 cm-1) and of the aromatic substituents (1620 cm-1), suggesting the existence of a certain amount of surface enhancement of the Raman signal also with this wavelength. In contrast, despite the resonance, the complete absence of the band around 1460 cm-1 ascribed to the blue polymer phase demonstrates a strong predominance of the red phase on the blue one. At first sight, a possible explanation for the poor conversion of CDS9 into its blue form could be attributed to the length of the nonyl chain. However, according to the results presented in ref 23, the conversion efficiency of alkyl-substituted diacetylene disulfides into the blue phase is not significantly influenced by the number of methylene units in the spacer, as long as odd. Differences exist only in the kinetic of the polymerization process, which is slower for shorter spacers. For this reason, we attribute the shorter conjugation length of CDS9 polymeric films to some chain distortions induced by the presence of carbazolyl-tail groups. In summary, the Raman results of Figure 9 demonstrate the following: (i) UV light induces polymerization of the CDS9 monolayer chemisorbed on silver, and the time-scale is the same as that observed by SPR experiments. (ii) The polymerization process can take place also on rough silver/air interfaces as was previously observed on rough gold substrates.24,25 (iii) SERS is the dominant mechanism at the basis of the observed Raman spectra at all of the used excitation wavelengths. (22) Lao, W.; Xu, C.; Ji, S.; You, J.; Ou, Q. Spectrochim. Acta, Part A 2000, 56, 2049. (23) Menzel, H.; Horstmann, S.; Mowery, M. D.; Mei Cai; Evans, C. E. Polymer 2000, 41, 8113. (24) Mowery, M. D.; Menzel, H.; Mei Cai; Evans, C. E. Langmuir 1998, 14, 5594. (25) Menzel, H.; Mowery, M. D.; Mei Cai; Evans, C. E. AdV. Mater. 1999, 11, 131. (26) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824.
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(iv) Red and blue polymer phases coexist. (v) The red phase is predominant. Indeed, despite the resonance, no signal associated with the blue phase was detected by exciting at 647.1 nm. (vi) The bands related to the CtC stretching modes were completely absent in the SERS spectra. This indicates that conjugated CtC bonds are nearly parallel to the metal surface, confirming the polymerization scheme of Figure 1b. Indeed, the electromagnetic effect of SERS determines a strong magnification only of the vibrational modes occurring normally to the metal surface. (vii) The intense SERS bands corresponding to the aromatic CdC stretching modes suggest that, in the polymer monolayer, carbazolyl groups are nearly perpendicular to the metal/polymer interface. (viii) The predominance of the red phase over the blue phase of the polymer could be related to some chain distortion induced by a slightly uneven arrangement of the carbazolyl substituents.
Conclusions We have studied the UV polymerization of SAMs of a novel diacetylene containing carbazolyl groups, the CDS9. The SAMs were obtained on silver-coated glass slides. Because of the electronic properties of the carbazolyl moieties in terms of induced photoconduction, electroluminescence, or implemented nonlinearity, the possibility of growing single molecule polymeric layers of CDS9 on silver is very attractive for linear and nonlinear applications based on the propagation of surface plasmons.4,26 The chemisorption of polymer monolayers via sulfur atoms on silver or gold films is a complex mechanism, where the characteristics of the metal/organic interface can play a key role in the structure and ordering of the organic molecules and consequently determine the possibility of application to nanopatterning or nanophotonics. Because of the presence of the metal layer, a comprehensive knowledge of such structures can be obtained by the combined use of SPR and SERS techniques. In particular, we prepared metal coatings with different topographical characteristics, to match the requirements either of SPR (smooth metal/dielectric interfaces) or of SERS (rough metal/dielectric interfaces). The use of SPR spectroscopy allowed us to measure both the thickness of the single-molecule layer and the variation of its complex index of refraction with exposure to low intensity UV light, giving a first evidence of the polymerization process. The SERS investigation of the organic layers after different exposure times to UV light confirmed the UV polymerization of the monolayers and permitted one to observe the coexistence of a blue and a red polymer phase, with a clear predominance of the red form. This result could be related to the presence of the bulky carbazolyl groups, which could favor some chain distortions. The absence of the bands ascribed to CtC conjugated bond stretchings in the SERS spectra of SAMs, in contrast with the presence of those of the corresponding CdC stretchings, confirmed that the polymerization occurs according to the scheme reported in Figure 1b, where the triple and the double bonds of the polymeric backbone are respectively nearly parallel and nearly perpendicular to the plane of the metal. Furthermore, the observation of the intense band associated with the CdC stretching modes of the aromatic rings suggested that the orientation of the carbazolyl groups is nearly perpendicular to the metal/polymer interface, in agreement with the proposed polymerization scheme. Acknowledgment. Funding from the Italian FIRB 20012003 “Molecules and organic/inorganic hybrid structures for photonics” (contract no. RBNE01P4JF) is acknowledged. LA0514157
