6868
Langmuir 1999, 15, 6868-6874
Adsorption of 1,4-Phenylene Diisocyanide on Silver Investigated by Infrared and Raman Spectroscopy Hyouk Soo Han, Sang Woo Han, Sang Woo Joo, and Kwan Kim* Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, Korea Received April 6, 1999. In Final Form: June 2, 1999 The adsorption characteristics of 1,4-phenylene diisocyanide (1,4-PDI) on silver have been investigated by means of infrared and Raman spectroscopy. Specifically, in the infrared spectroscopic study, we have employed three different sampling methods, i.e., reflection-absorption infrared (RAIR), surface-enhanced infrared (SEIR), and diffuse reflectance Fourier transform (DRIFT) spectroscopy. The SEIR and DRIFT spectral features of 1,4-PDI assembled, respectively, on vacuum-evaporated thin Ag film and 2-µm-size Ag powders were both consonant with the RAIR spectral features observed on vacuum-evaporated thick Ag film, implying that the usual infrared surface selection rule was applicable even to SEIR and DRIFT spectroscopy. All three kinds of infrared spectral data clearly dictated that 1,4-PDI should be adsorbed on silver via the carbon lone-pair electrons of one isocyanide group assuming a vertical orientation with respect to the silver substrate. We could also obtain the surface-enhanced Raman (SER) spectrum of 1,4-PDI adsorbed on powdered silver that was used in the DRIFT spectroscopic study. Although an unequivocal SER selection rule has not yet been established, the same conclusion as that from the infrared data could be derived from the peak shift, band broadening, as well as from the presence of the ring C-H stretching band in the SER spectrum.
1. Introduction In the past decade, adsorption of molecular monolayers on metal surfaces has attracted tremendous research interest.1,2 In addition to the fundamental interest in such metal adsorbate systems, practical considerations such as the modification of metal surfaces and the preparation of organic thin films has increased research activity in this area. The most widely studied and well-characterized systems include alkanethiols on gold, silver, and copper, dialkyl sulfides and dialkyl disulfides on gold and silver, and carboxylic acids on aluminum oxide and silver.1-7 Aliphatic as well as aromatic dithiols are known to adsorb on gold as monothiolates by forming one single Au-S covalent bond.8,9 The other thiol group is pendent with respect to the gold surface. In contrast, dithiol molecules are usually adsorbed on silver as dithiolates by forming two Ag-S bonds.9 The detailed origin of the different adsorption characteristics of dithiol molecules on gold and silver has not been fully clarified, however. From an application aspect, elucidation of the adsorption behavior of thiols and sulfides on gold and silver surfaces has been one of the most rewarding subjects in surface/ interfacial chemistry.10 It would also be worthwhile to investigate the adsorption of other organic molecules on * To whom all correspondence should be addressed. E-Mail:
[email protected]. (1) Ulman, A. In An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) 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. (5) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (6) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (7) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032. (8) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (9) Murty, K. V. G. K.; Venkataramanan, M.; Pradeep, T. Langmuir 1998, 14, 5446. (10) Ulman, A., Ed. Thin Films; Academic Press: San Diego, CA, 1995; Vol. 20.
metal surfaces. In fact, an increased understanding of the interaction of molecules with metal surfaces has been accompanied by a growing awareness that many surfaceadsorbate interactions have analogues in the field of organometallic and coordination chemistry.11 The chemistry of metal-nitrile complexes has attracted much attention in view of the fact that the -CtN: group is isoelectronic with molecular nitrogen and that organonitrile complexes can serve as convenient precursors for a wide variety of coordination complexes.12 In organometallic chemistry, nitriles are generally known to coordinate to metal atoms via the nitrogen lone-pair electrons.13,14 In our previous surface-enhanced Raman scattering (SERS) studies of various nitriles in silver sols, a majority of aromatic nitriles were found, however, to adsorb via the CtN π system rather than the nitrogen lone-pair electrons.15-21 Thereby, for 1,4-dicyanobenzene (1,4-DCB), both of the nitrile groups seemed to interact effectively with the silver surface via their π systems, assuming a flat molecular orientation with respect to the surface. Using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Robertson and Angelici22 concluded (11) Albert, M. R.; Yates, Jr., J. T. The Surface Scientist’s Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987. (12) Endres, H. In Comprehensive Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Vol. 2. (13) Balahura, R. J.; Purcell, W. L. Inorg. Chem. 1979, 18, 937. (14) Richardson, D. E.; Taube, H. J. Am. Chem. Soc. 1983, 105, 40. (15) Chun, H. A.; Kim, M. S.; Kim, K. J. Mol. Struct. 1990, 221, 127. (16) Boo, D. W.; Kim, K.; Kim, M. S. Bull. Korean Chem. Soc. 1987, 8, 251. (17) Boo, D. W.; Kim, K.; Kim, M. S. Bull. Korean Chem. Soc. 1988, 9, 311. (18) Chun, H. A.; Kim, M. S.; Kim, K. J. Mol. Struct. 1989, 213, 63. (19) Chun, H. A.; Boo, D. W.; Kim, K.; Kim, M. S. J. Mol. Struct. 1990, 216, 41. (20) Chun, H. A.; Yi, S. S.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1990, 21, 7439. (21) Lee, E.; Yi, S. S.; Kim, M. S.; Kim, K. J. Mol. Struct. 1993, 298, 47. (22) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488.
10.1021/la990396w CCC: $15.00 © 1999 American Chemical Society Published on Web 07/29/1999
Adsorption of 1,4-PDI on Silver
that 1,4-phenylene diisocyanide (1,4-PDI, NC-C6H4-NC), a geometric isomer of 1,4-DCB, should adsorb on the gold surface via only one NC group, with the other NC group pendent with respect to the gold surface. This implies that the adsorption characteristics of 1,4-PDI on gold are different from those of 1,4-DCB on silver. A question naturally then arises whether 1,4-PDI would also adsorb on silver via only one NC group or via two NC groups as seems to occur for 1,4-DCB. In connection with the adsorption characteristics of dithiols on gold and silver, we have thus investigated the adsorption behavior of 1,4PDI on silver by infrared and Raman spectroscopy, bearing in mind that self-assembled monolayers (SAMs) with suitably tailored functionality can be used as a platform to build up fruitful multilayers. 2. Experimental Section and Calculations 1,4-Phenylene diisocyanide (1,4-PDI, 99% purity) and silver powder (99.9+% purity) with a nominal particle size of 2-3.5 µm were purchased from Aldrich. 1,4-PDI was used as received. Silver powders were washed consecutively with ethanol (Hayman, >99.9%), acetone (Merck, >99.5%), n-hexane (Junsei, extra pure), and tetrachloromethane (J. T. Baker, HPLC grade); this was performed only to clean the silver powders and to remove any carbon impurities thereon. A stock solution of 1 mM 1,4-PDI in methanol (Carlo Erba, >99.9%) was bubbled with nitrogen before use. For the self-assembly of 1,4-PDI on silver, 0.050 g of silver powder was placed in a cleaned small vial into which 1.5 mL of methanol and 0.5 mL of the stock solution of 1,4-PDI were subsequently added. After 30 s, the solution phase was decanted. The remaining solid particles were left to dry in ambient condition for 2 h. A portion of the powdered sample was transferred to a 4-mm-diameter cup (Harrick microsampling cup) without compression. After leveling by tapping the cup gently, the sample was subjected to diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic analysis. Separately, a portion of the powdered sample transferred to a glass capillary was subjected to surface-enhanced Raman scattering (SERS) analysis. For a comparative study, 1,4-PDI monolayer self-assembled on a thick, flat silver substrate was also prepared. The silver substrate was prepared by the resistive evaporation of titanium (Aldrich, >99.99%) and silver (Aldrich, >99.99%) at 1 × 10-6 Torr on batches of glass slides, cleaned previously by sequentially sonicating in isopropyl alcohol, hot 1:3 H2O2(30%)/H2SO4, and triply distilled H2O. Deposition of titanium prior to that of silver was performed to enhance adhesion to the substrate. After a deposition of approximately 200 nm of silver, the evaporator was back-filled with nitrogen. The silver substrate was subsequently immersed into a 0.4 mM solution of 1,4-PDI in methanol for 30 s. After the substrate was taken out, it was subjected to a strong nitrogen gas jet to blow off any remaining liquid droplets on the surface or the edges of the substrate. Thereafter, the substrate was subjected to reflection-absorption infrared (RAIR) spectroscopic analysis. We also prepared a cast film of 1,4-PDI on a thin silver substrate. The silver substrate with ca. 10 nm thickness was prepared by the thermal heating of silver at 2 × 10-6 Torr on a CaF2 disk (25 mm diameter and 4 mm thickness) that had been cleaned previously with ethanol in an ultrasonic cleaner. Forty microliters of 1 mM methanol solution of 1,4-PDI was spread on the silver film by using a micropipet, and the solvent was allowed to evaporate in ambient conditions. The substrate was then subjected to a surface-enhanced infrared absorption (SEIRA) spectroscopic measurement. Infrared spectra were measured using a Bruker IFS 113v Fourier transform IR spectrometer equipped with a globar light source and a liquid N2-cooled wide-band mercury cadmium telluride detector. To record the DRIFT spectra, a Harrick Model DRA 3CI diffuse reflectance accessory was employed. A total of 256 scans were measured in the range 4000-600 cm-1 at a resolution of 2 cm-1 using the previously scanned pure Ag powder as the background. The method for obtaining the RAIR spectrum
Langmuir, Vol. 15, No. 20, 1999 6869 has been reported previously.23,24 Each RAIR spectrum was obtained by averaging 1024 interferograms at 2 cm-1 resolution, with p-polarized light incident on the silver substrate at 80°. To reduce the effect of water vapor rotational lines, the sample and reference interferograms were recorded alternately after every 32 scans. The SEIRA spectrum was obtained with a transmission method in the range 4000-1000 cm-1 by averaging 256 interferograms at 2 cm-1 resolution; the transmission spectrum of a pure Ag island film on CaF2 was taken as the reference. The Happ-Genzel apodization function was used in Fourier transforming all the interferograms. The RAIR spectrum is reported as -log(R/Ro), where R and Ro are the reflectivities of the sample and the bare clean metal substrates, respectively. Considering that the infrared light will not penetrate into the metal particles, and thus the usual Kubelka-Munk equation25 cannot be applicable to the present system, the DRIFT spectrum is also reported in -log(R/Ro). Raman spectra were obtained using a Renishaw Raman System 2000 spectrometer equipped with a holographic notch filter and an integral microscope. The 514.5 nm radiation from a 20 mW air-cooled argon ion laser (Spectra Physics Model 163C4210) was used as an excitation source. Raman scattering was detected with 180° geometry using a thermoelectric-cooled CCD detector. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer, and the accuracy of spectral measurement was estimated to be around 1 cm-1. For the vibrational assignment of 1,4-PDI, we performed an ab initio calculation at the RHF/6-31G level with the Gaussian 94 program26 running on an IBM-SP2. No symmetry constraint was imposed on the geometry optimization routine. However, since the optimized geometry turned out to possess a D2h symmetry, the same symmetry restriction was imposed on the process of the vibrational frequency calculation.
3. Results and Discussion To deduce the adsorption characteristics of molecules on metal surfaces, a correct vibrational assignment is of essential importance since one has to analyze minute spectral changes occurring in the surface adsorption. One may expect that the vibrational modes of 1,4-phenylene diisocyanide (1,4-PDI) are comparable to those of 1,4dicyanobenzene (1,4-DCB). In our previous SERS study of 1,4-DCB in aqueous silver sol,21 we assigned the Ramanactive modes of 1,4-DCB by referring to the usual vibrational assignments of benzene derivatives with “dilight” substituents as summarized by Varsanyi.27 However, for a more reliable analysis of the infrared spectrum of 1,4-PDI, we have performed an ab initio quantum mechanical calculation on 1,4-PDI in a free state and, on the basis of the calculated vibrational frequencies, the observed frequencies of 1,4-PDI in various states could be properly assigned (see Table 1). (In fact, for a proper assignment, we consulted the infrared absorption as well as the Raman scattering intensities of the calculated and observed spectra of 1,4-PDI. Being planar with D2h symmetry, the 36 fundamentals of 1,4-PDI could be resolved into the following symmetry species: Γ ) 7Ag + 1B1g + 4B2g + 6B3g + 2Au + 6B1u + 6B2u + 4B3u. Among them, only the B1u, B2u, and B3u vibrations are infrared(23) Son, D. H.; Ahn, S. J.; Lee, Y. J.; Kim, K. J. Phys. Chem. 1994, 98, 8488. (24) Kim, S. H.; Ahn, S. J.; Kim, K. J. Phys. Chem. 1996, 100, 7174. (25) Kubelka, P. J. Opt. Soc. Am. 1948, 38, 448. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision D.4; Gaussian, Inc.: Pittsburgh, PA, 1995. (27) Varsanyi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Academia Kiado: Budapest, 1974.
6870
Langmuir, Vol. 15, No. 20, 1999
Han et al.
Table 1. Vibrational Assignment of 1,4-Phenylene Diisocyanide (1,4-PDI) 1,4-PDI RHF/6-31Ga 867 898 1014 1023 1025 1100 1169 1185 1186 1191 1319 1413 1506 1574 1621
TRIRb
1,4-PDI/Ag Raman
RAIRd
SEIRAe
DRIFTf
SERSf
850 (s) 1028 (w) 1104 (vw) 1172 (m)
1165
1195 (w)
1204
1184 (m) 1286 (m) 1508
1510
1510
1326 1392 1510
1604 (m)
1598
1599
1599
1599
2127 (vs)
2118/2180
2121/2176
2119/2180
2144/2178
1409 (w) 1504 (s) 1680 (w) 1780 (w) 1926 (w)
2114 2118 3022 3023 3036 3039 a
2132 (vs)c 3045 (m)
3064 (m) 3066 (vw) 3072 (s) b
c
3072
assignmentg 10a (B1g) 17b (B3u) 18a (B1u) 5 (B2g) 17a (Au) 18b (B2u) 9a (Ag) 13 (B1u) 7a (Ag) 14 (B2u) 3 (B3g) 19b (B2u) 19a (B1u) 8b (B3g) 8a (Ag) combination combination combination νs(NtC) νas(NtC) 20a (B1u) 7b (B3g) 20b (B2u) 2 (Ag)
2125 cm-1 in methanolic solution. d
Scale factor is 0.89. Diluted with KBr. On vacuum-evaporated thick Ag film. e On Ag island film prepared on CaF2 substrate. f On 2-3.5-µm-size Ag powders. g Symmetries based on D2h point group. See text.
active in the free molecule while the Ag, B1g, B2g, and B3g vibrations are Raman-active. Besides, the earlier limited assignment of 1,4-DCB21,28,29 seemed to be consonant with the present assignment of 1,4-PDI.) Prior to presenting the infrared and Raman spectra of 1,4-PDI on silver, it may be worthwhile to recall the recent work of Lin and McCarley30 on the adsorption of 1,6diisocyanohexane (1,6-DCH) on gold and platinum surfaces. Noticing that the infrared spectral feature of the ν(NC) region was dependent on the concentration of 1,6DCH in the solution phase, they claimed the possibility of the formation of oligomerized or polymerized 1,6-DCH on gold and platinum: namely, at low 1,6-DCH concentration as well as with a short exposure in a concentrated solution, i.e., 0.1 M 1,6-DCH, the majority of the adsorbates formed on metal surfaces were claimed to consist of 1,6DCH molecules that are bound to metal through two isocyanide moieties. However, when the metal substrates were in contact with a concentrated solution for a prolonged duration, they insisted on the occurrence of oligomerization or polymerization. The formation of polymeric layers of 1,6-DCH on gold and platinum was conjectured to occur via two paths. One path was that at high concentration soluble oligomers or polymers of 1,6DCH could be formed in the solution phase and, once formed, were rapidly adsorbed on metal through multiple free isocyanide moieties. The other path was that at high concentrations 1,6-DCH molecules were to adsorb on metal through only one isocyanide group and the pendent isocyanide functionalities would participate in oligomerization or polymerization with nearby 1,6-DCH molecules at the monolayer film/dosing solution interface. On the other hand, it would also be worthwhile to recall the recent DRIFT study of Ontko and Angelici31 that diisocyanides [CtN-(CH2)x-NtC, where x ) 2, 4, 6, 8, and 12; m- and (28) Castro-Pedrozo, Ma. C.; King, G. W. J. Mol. Spectrosc. 1978, 73, 386. (29) Arenas, J. F.; Marcos, J. I.; Ramirez, F. J. Spectrochim. Acta, Part A 1988, 44, 1045. (30) Lin, S.; McCarley, R. L. Langmuir 1999, 15, 151. (31) Ontko, A. C.; Angelici, R. J. Langmuir 1998, 14, 3081.
p-xylyl(NC)2, xylyl ) -CH2-C6H4-CH2-], and triisocyanides [1,1,1-tris(isocyanomethyl)ethane (Tripod(NC)3) and tris[2-isocyanoethyl]amine (Tren(NC)3)] were adsorbed on gold powder via all of their -NC groups. Furthermore, they claimed that the relative binding affinities of the isocyanides increased as the number of -NC groups in the ligand increased, i.e., RNC < R(NC)2 < R(NC)3. RAIR Spectrum of 1,4-PDI on Vacuum-Evaporated Thick Ag Film. Figure 1a shows the transmission infrared spectrum of 1,4-PDI dispersed in a KBr matrix. The peaks appearing in Figure 1a are collectively listed in Table 1 along with the appropriate assignment. The most distinct peaks are located at 850, 1504, and 2132 cm-1 which are assignable, respectively, to the benzene ring 17b, 19a, and the antisymmetric NC stretching vibrations; the ring 17b mode is a kind of an out-of-plane mode and the ring 19a mode is one of the CC stretching modes along the two NC groups. Figure 1b shows the RAIR spectrum of 1,4-PDI that has been self-assembled on a vacuum-evaporated silver film in a 0.4 mM 1,4-PDI solution in methanol for 30 s. Such a short self-assembly in a dilute solution was purposely employed in this work to reduce any possibility of oligomerization or polymerization of 1,4-PDI on silver (vide supra).30 It has to be mentioned that insofar as the concentration of 1,4-PDI in methanol was below 1.0 mM, the RAIR spectral pattern (number of peaks, their peak positions, and relative peak intensities) for the adsorbed 1,4-PDI on silver was barely affected by the bulk concentration or the duration of selfassembly. This may suggest that, on the one hand, any oligomerized 1,4-PDI was not formed on silver and, on the other hand, the orientation of the adsorbed species was not noticeably dependent on the coverage of 1,4-PDI on silver, albeit it was uncertain whether a full monolayer was formed on silver at least in 1.0 mM solution. In fact, when 1,4-PDI was self-assembled on silver in a highly concentrated solution, i.e., 1.0 M, the corresponding RAIR spectrum exhibited a complex feature that could be attributed to the formation of multilayers or to the occurrence of certain oligomerization or polymerization
Adsorption of 1,4-PDI on Silver
Figure 1. (a) Transmission infrared spectrum of 1,4-phenylene diisocyanide (1,4-PDI) dispersed in a KBr matrix. (b) Reflectionabsorption infrared spectrum of 1,4-PDI self-assembled on a vacuum-evaporated thick silver film. (c) Surface-enhanced infrared spectrum, taken with a transmission mode, for 1,4PDI deposited on a vacuum-evaporated thin silver film on an IR-transparent CaF2 crystal. (d) Diffuse reflectance infrared Fourier transform spectrum of 1,4-PDI self-assembled on 2-3.5µm-size silver powders. See text.
reactions at the silver/solution interface. At the present stage, we will focus on only the case of self-assembly in a dilute solution; the characteristics of the self-assembly of 1,4-PDI on silver in a concentrated solution are under investigation and will be reported in the near future. In the RAIR spectrum in Figure 1b, four peaks are clearly identified, namely, at 1508, 1598, 2118, and 2180 cm-1. The former two peaks can be assigned, respectively, to the benzene ring 19a and 8a modes while the latter two peaks have to be attributed to the NC stretching vibrations. This spectral feature seems to contain several noteworthy implications. First of all, the appearance of the benzene ring 8a mode would be intriguing since the mode should be infrared-inactive in a free molecular state; as to be discussed later, the mode is actually Raman-active in a free state. This, however, can be understood by invoking the fact that the symmetry of 1,4-PDI has to be reduced from a D2h to either a C2h or C2v or even to C1 species upon adsorption on metal surfaces. On the other hand, the appearance of two NC stretching bands implies that the two isocyanide groups of 1,4-PDI are no longer equivalent in the adsorbed state. That the one ν(NC) peak is strongly blue-shifted by 48 cm-1 from the position in a free state while the other peak is red-shifted by 14 cm-1 from that in a free state suggests that 1,4-PDI is adsorbed on silver mainly through one of the two isocyanide groups (con-
Langmuir, Vol. 15, No. 20, 1999 6871
sidering that the ν(NC) band of 1,4-PDI is observed at 2125 cm-1 in a methanol solution, the amount of blueshift is as large as 55 cm-1 whereas the amount of redshift is only 7 cm-1). The blue-shift of the ν(NC) mode can be understood by invoking the fact that the carbon lonepair electrons have antibonding character;32 the donation of these electrons to silver should increase the strength of the NC bond. Considering that 1,4-PDI is a conjugated molecule, it is not unreasonable that a small amount of red-shift in the one ν(NC) mode should occur. It is also very informative that the benzene ring 17b mode is hardly detectable in the RAIR spectrum although the corresponding band is one of the most intense bands in the transmission spectrum of 1,4-PDI (see the peak at 850 cm-1 in Figure 1a). As mentioned previously, the ring 17b mode belongs to a kind of out-of-plane vibration. Recalling the well-known infrared surface selection rule33,34 that only the vibrational modes whose dipole moment derivatives have components normal to the metal substrates are exclusively active in the RAIR spectroscopy, all of the above observations dictate that 1,4-PDI is adsorbed on silver via only one NC group with a perpendicular orientation with respect to the silver surface. SEIR Spectrum of 1,4-PDI on Vacuum-Evaporated Thin Ag Film. It has been reported in the literature35-41 that surface-enhanced infrared (SEIR) spectra can be obtained for molecules deposited on vacuum-evaporated thin island films of noble metals such as gold and silver. Recently, we demonstrated that a film of silver colloids could also be used as an infrared substrate to quantify organic molecules with a nanogram detection limit.42 Osawa and Ikeda39 claimed that three different mechanisms act simultaneously in obtaining infrared spectra of molecules on thin metal island films: long-range electromagnetic enhancement through the excitation of collective electron resonance, short-range chemical enhancement associated with the change in vibrational polarizability of the molecule caused by chemical interaction with the metal surface, and the effect of the orientation of vibrational dipoles of the molecule with respect to the metal surface. Nonetheless, there is a dispute whether the usual infrared selection rule is applicable to the analysis of SEIR spectra.40,41 In this respect, we have also prepared a vacuum-evaporated thin Ag film on an IRtransparent CaF2 crystal and attempted to compare the SEIR spectrum of 1,4-PDI deposited on the thin Ag film with the RAIR spectrum on a thick Ag film. In Figure 1c is shown the SEIR spectrum of 1,4-PDI taken with a transmission mode. Unfortunately, the spectral region below 1000 cm-1 could not be obtained due to the absorption characteristics of CaF2. Otherwise, it is remarkable that the SEIR spectral feature is barely different from the RAIR spectral feature (see Figure 1b). Namely, as in the RAIR spectrum, we could identify four peaks at 1510, 1599, 2121, and 2176 cm-1 in the SEIR spectrum (32) (a) Yamamoto, Y. Coord. Chem. Rev. 1980, 32, 193. (b) Treichel, P. M. Adv. Organomet. Chem. 1973, 11, 121. (c) Singleton, E.; Oosthuizen, H. E. Adv. Organomet. Chem. 1983, 22, 130. (33) Korto¨m, G. Reflectnace Spectroscopy; Springer-Verlag: Heidelberg, 1969. (34) Dignam, M. J.; Fedyk, J. Appl. Spectrosc. Rev. 1978, 14, 249. (35) Merklin, G. T.; Griffiths, P. R. Langmuir 1997, 13, 6159. (36) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951. (37) Nishikawa, Y.; Fujiwara, K.; Shima, T. Appl. Spectrosc. 1990, 44, 691. (38) Nishikawa, Y.; Fujiwara, K.; Shima, T. Appl. Spectrosc. 1991, 45, 747. (39) Osawa, M.; Ikeda, M. J. Phys. Chem. 1991, 95, 9914. (40) Osawa, M.; Ataka, K.; Yoshii, K.; Nishikawa, Y. Appl. Spectrosc. 1993, 47, 1497. (41) Merklin, G. T.; Griffiths, P. R. J. Phys. Chem. B 1997, 101, 5810. (42) Kang, S. Y.; Jeon, I. C.; Kim, K. Appl. Spectrosc. 1998, 52, 278.
6872
Langmuir, Vol. 15, No. 20, 1999
(see Table 1); their peak positions as well as relative intensities are comparable to each other. It is intriguing that, even though the amount of 1,4-PDI deposited on the Ag-covered CaF2 substrate was more than a monolayer coverage limit, the SEIR spectral pattern hardly changed. However, this can be understood by recalling that the SEIR effect occurs prominently at the molecule/metal interface.40,41 From the similarity of the SEIR and RAIR spectra, we are forced to conclude that the usual infrared surface selection rule is also applicable to the SEIR spectroscopy. The SEIR spectrum seems thus to reflect the perpendicularly adsorbed 1,4-PDI on silver via only one of the lone-pair electrons of the two NC groups. DRIFT Spectrum of 1,4-PDI on Powdered Ag. Recently, we could obtain very high SNR infrared spectra of 4-cyanobenzoic acid and 4-nitrobenzoic acid adsorbed on fine silver particles by diffuse reflectance Fourier transform (DRIFT) spectroscopy.43 The DRIFT spectral patterns were little different from the RAIR spectral patterns taken for the same molecules on vacuumevaporated thick silver films. Thus, the usual surface selection rule seemed applicable even to the surfaces of fine metal particles. The latter aspect was further confirmed by comparing the DRIFT spectrum of stearic acid (STA) on powdered silver with the RAIR spectrum of STA on a vacuum-evaporated silver film.44 Furthermore, the silver powders used in the DRIFT study were also found to act as very efficient substrates for the occurrence of surface-enhanced Raman scattering (SERS) phenomena.45 On these grounds, we have also attempted to obtain the DRIFT spectrum of 1,4-PDI on powdered silver to confirm the conclusions made by the RAIR and SEIR spectroscopies. In Figure 1d is shown the DRIFT spectrum of 1,4-PDI self-assembled on 2-3.5-µm-size silver powders. In fact, silver powders with diameters greater than 5 µm appeared to be inappropriate as adsorbent probably due to their particle sizes being quite close to the wavelength of the light source,43 so 2-3.5-µm-size silver powders were exclusively used in the present work as the adsorbent of 1,4-PDI molecules. Although the spectral region around 1600 cm-1 is somewhat noisy due to the interference of water vapor rotational lines, four peaks are identified at 1510, 1599, 2119, and 2180 cm-1. These DRIFT peak positions are in fact hardly different from those in the RAIR and SEIR spectra. In the DRIFT spectrum, the band at 2180 cm-1 is observed, however, to be far more intense than its counterpart at 2119 cm-1. The relative intensities of the three bands at 1510, 1599, and 2119 cm-1 in the DRIFT spectrum are nonetheless comparable to those in the RAIR and SEIR spectra. Since differences in the relative peak intensities may be ascribed to the different sampling techniques, the appearance of the above four peaks can be ascribed once again to the perpendicularly adsorbed 1,4-PDI on the powdered silver surface. Considering that the usual infrared selection rule is applied even to powdered metal particles,43,44 it is not surprising that the very intense benzene ring 17b band of 1,4-PDI in a neat state is completely absent in the DRIFT spectrum. It is somewhat surprising that the shapes and relative intensities of the bands in the DRIFT spectra of 1,4-PDI on Ag and Au22 are considerably different albeit that the frequencies of the bands are very similar. Regarding this matter, one may speculate that the powdered silver used in this work is covered with oxides. From X-ray diffraction (43) Han, H. S.; Kim, C. H.; Kim, K. Appl. Spectrosc. 1998, 52, 1047. (44) Lee, S. J.; Kim, K. Vib. Spectrosc. 1998, 18, 187. (45) Han, S. W.; Han, H. S.; Kim, K. Submitted to Vib. Spectrosc. for publication.
Han et al.
and infrared spectroscopy, we could not identify any oxides on the powdered silver, however; in this respect, we plan in the near future to perform an extended X-ray absorption fine structure (EXAFS) study. Nonetheless, it should be informative to address the DRIFT spectral feature of 1,4PDI on Ag2O powders. When 1,4-PDI was self-assembled on the surfaces of Ag2O powder, a broad ν(NC) band was identified at ∼2180 cm-1. In contrast with the DRIFT spectrum on the powdered silver (Figure 1d), we could not identify, however, any peak around 2120 cm-1 in the DRIFT spectrum on the Ag2O powders. Separately, we have also recorded the RAIR spectrum of 1,4-PDI that has been self-assembled on a vacuum-evaporated thick gold film. The RAIR spectral feature on the gold film was observed to be quite different from the DRIFT spectral feature on the powdered gold reported by Robertson and Angelici.22 Instead, the RAIR spectral features on the gold film were almost the same as those on the silver film (Figure 1b). The present observation suggests that the difference in the shapes and relative intensities of the bands in the DRIFT spectra of 1,4-PDI on Ag and Au22 may have to be attributed to the difference in their surface morphology rather than to the difference in the nature of two metals, Au and Ag. SERS Spectrum of 1,4-PDI on Powdered Ag. Ever since the discovery of the SERS phenomenon two decades ago by Fleishmann et al.,46 SERS has become one of the most sensitive surface vibrational techniques. The sensitivity of SERS is remarkable, enabling routine investigation of adsorbates even at submonolayer coverages.47,48 SERS has, however, two noticeable disadvantages, namely, that its applicability is limited to a few metals and that its unequivocal surface selection rule has not been established yet. From the latter point of view, it may be highly desirable to combine the SERS technique with infrared spectroscopy for a better analysis of the adsorption behavior of molecules on SERS-active metal surfaces. The two most common substrates used for the SERS effect are electrochemically roughened electrodes and metal colloids.48 Many experimental variables are involved in the preparation of the electrode.49,50 Although the procedure for the preparation of metallic sols is simple, many factors such as time, temperature, concentration, and impurity can affect the aggregation of the colloidal particles, causing substantial intensity variation in SER spectra.47,48 Of course, SERS-active substrates can also be prepared by the vacuum thermal deposition of metals24,51 or the chemical reduction method.52,53 As mentioned above, we found recently that commercially, readily available powdered silver itself is a very efficient SERS-active substrate.45 Any special pre- and/or posttreatment seemed not to be required to obtain a high-quality SER spectrum (at least under a 514.5 nm excitation). More importantly, it was evidenced that the DRIFT and SER spectra could be simultaneously measured with these powders.45 Considering that the usual infrared selection rule is applicable (46) Fleischmann, H.; Weaver, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (47) Garrell, R. L. Anal. Chem. 1989, 61, 401A. (48) Chang, R. K., Furtak, T. E., Eds. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (49) Park, H.; Lee, S. B.; Kim, M. S.; Kim, K. Chem. Phys. 1992, 161, 265. (50) Brolo, A. G.; Irish, D. E.; Szymanski, G.; Lipkowski, J. Langmuir 1998, 14, 517. (51) Moskovits, M.; DiLella, D. P. J. Chem. Phys. 1980, 73, 6068. (52) Boo, D. W.; Oh, W. S.; Kim, M. S.; Kim, K. Chem. Phys. Lett. 1985, 120, 301. (53) Li, Y.-S.; Wang, Y. Appl. Spectrosc. 1992, 46, 142.
Adsorption of 1,4-PDI on Silver
Figure 2. (a) Ordinary Raman spectrum of neat 1,4-PDI. (b) Surface-enhanced Raman spectrum of 1,4-PDI self-assembled on 2-3.5-µm-size silver powders. See text.
even to the DRIFT spectra, the latter aspect must be very advantageous, specifically in the interpretation of SER spectra. In Figure 2a is shown the ordinary Raman (OR) spectrum of neat 1,4-PDI. The peak positions in Figure 2a are listed in Table 1. It can be confirmed from the table that the so-called exclusion rule is applied to the infrared and Raman spectra of 1,4-PDI; this illustrates that 1,4PDI has a D2h symmetry in a neat state. In Figure 2b is shown the SER spectrum of 1,4-PDI self-assembled on 2-3.5-µm-size silver particles; the sample is the same as that used in obtaining the DRIFT spectrum shown in Figure 1d. Considering the amount of 1,4-PDI molecules in the laser-sampling volume, the spectrum must be a SER spectrum. The positions of the major SER peaks are also summarized in Table 1. In fact, the observed peaks in the SER spectrum can be correlated with those in the OR spectrum except three SER peaks at 1326, 1392, and 1510 cm-1. The two SER peaks at 1392 and 1510 cm-1 seem to be assigned to the Raman-inactive benzene ring 19b and 19a modes, respectively. The appearance of such modes in the SER spectrum has to be attributed to the disruption of symmetry of 1,4-PDI caused by surface adsorption.54 On the other hand, the band at 1326 cm-1 seems to be assigned to the inherently very weak Ramanactive benzene ring 3 mode. The appearance of the ring 3 mode in the SER spectrum while being hardly detected in the OR spectrum may be ascribed to the enhancement of its Raman intensity upon adsorption of 1,4-PDI on silver (vide infra). (54) Devlin, J. P.; Consani, K. J. Phys. Chem. 1981, 85, 2597.
Langmuir, Vol. 15, No. 20, 1999 6873
Although an unequivocal SER selection rule is not available yet, it is well-documented that the presence of the ring C-H stretching band in a SER spectrum is indicative of a vertical (or at least a tilted) orientation of the benzene ring moiety on a silver substrate.55,56 In this respect, the distinct appearance of the ring C-H stretching band at 3072 cm-1 in the SER spectrum may be regarded as showing a perpendicular (or a tilted) orientation of 1,4PDI on silver. It has also been documented in the literature57 that when a benzene ring moiety interacts directly with a metal surface, the ring breathing mode has to be red-shifted by ∼10 cm-1 along with a substantial band broadening in SER spectra. Neither a substantial red-shift nor a significant band broadening was identified in Figure 2b, implying that the possibility of a direct ring π orbital to metal interaction must be quite low. These SER spectral features dictate that 1,4-PDI should adsorb on silver only via the lone-pair electrons of one of the terminal carbon atoms of the isocyanide groups, as is concluded from the RAIR, SEIR, and DRIFT spectral data. It is seen in the SER spectrum that the ν(NC) peak is substantially broader than its counterpart in the OR spectrum. The peak position of the ν(NC) band in the SER spectrum is 51 cm-1 higher than that in the OR spectrum. This is obviously due to the interaction of 1,4-PDI with the silver via the lone-pair electrons of the carbon atom. It would be worthwhile to recall at this juncture the SER spectral feature of 1,4-DCB in aqueous silver sol whereby the ν(CN) band of 1,4-DCB is also blue-shifted by adsorbing on the silver surface.21 In the previous study, this was explained as due to a disruption of conjugation rather than an end-on coordination via the nitrogen lone-pair electrons of one cyano group; namely, 1,4-DCB was concluded to adsorb on silver with a flat orientation. Unlikely in the case of 1,4-PDI, the amount of blue-shift of the ν(CN) mode for 1,4-DCB was in fact at best 14 cm-1 and the benzene ring breathing mode was substantially broadened in the SER spectrum of 1,4-DCB. In the latter spectrum, the ring CH stretching band was also hardly detected in contrast with the SER spectrum of 1,4-PDI. Nonetheless, it is intriguing that the ν(NC) mode of the uncoordinated isocyanide group is not clearly resolved in the SER spectrum of 1,4-PDI; as already discussed, the free isocyanide group is clearly identified in the DRIFT spectrum. However, we can notice that the ν(NC) band in Figure 2b is asymmetric toward the low-frequency region. This may be ascribed to the overlap of uncoordinated and coordinated NC stretching vibrations. A Gaussian-type deconvolution of the ν(NC) band in Figure 2b revealed that the band was composed of two bands centered at 2144 and 2178 cm-1; that the former frequency value is in fact greater than that observed in the DRIFT spectrum may arise from a Gaussian-type deconvolution. On the other hand, invoking the electromagnetic (EM) and chemical enhancement mechanisms,48,58 a vibrational mode that is far distant from the metal surface will appear weakly in the SER spectrum. Hence, the adsorption behavior of 1,4-PDI on silver is conjectured to be different from that of 1,4-DCB. According to the EM selection rule,58 albeit a qualitative one, vibrational modes whose polarizability tensor elements are perpendicular to a metal surface should be strongly enhanced in a SER spectrum, namely those corresponding to R′zz where z is along the surface normal. Vibrations derived from R′xz and R′yz should be the next (55) Suh, J. S.; Michaelian, K. H. J. Phys. Chem. 1987, 91, 598. (56) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1988, 92, 6327. (57) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (58) Moskovits, M. J. Chem. Phys. 1982, 77, 4408.
6874
Langmuir, Vol. 15, No. 20, 1999
most intense modes while those corresponding to R′xx, R′yy, and R′xy are less enhanced. On these grounds, with an upright orientation of 1,4-PDI on silver, the most intense bands in the SER spectrum will be derived from the Agtype modes. Bands belonging to the B2g and B3g symmetries will be the next most intense ones. In fact, as can be seen in Figure 2 and Table 1, the distinctly observed SER peaks are mostly attributable to the Ag bands. The distinct appearance of the ring 3 band, belonging to a B3g symmetry, at 1326 cm-1 in Figure 2b can be understood similarly on the basis of an upright orientation. If 1,4PDI molecules were lying flat on the silver surface, such a B3g-type mode would hardly be detected in the SER spectrum. However, these SER spectral patterns do not necessarily mean that the adsorbate should have a perfectly perpendicular orientation on the silver surface. From the SER spectral data alone, one can only deduce that 1,4-PDI does not take a flat orientation on the silver surface. In contrast, the infrared spectral data dictated that 1,4-PDI should have a nearly perpendicular orientation with respect to the silver surface. This implies that simultaneous measurement of infrared and Raman spectra is very valuable in elucidating the adsorption characteristics of molecules on metal surfaces. 4. Summary and Conclusion We have investigated the adsorption characteristics of 1,4-PDI on silver by the simultaneous use of infrared and Raman spectroscopies. For a reliable spectral analysis, we have also performed an ab initio vibrational frequency calculation. Regarding the infrared spectroscopic study, we have employed three different sampling methods, i.e., RAIR, SEIR, and DRIFT spectroscopies. The SEIR and DRIFT spectral features of 1,4-PDI assembled, respectively, on vacuum-evaporated thin Ag film and 2-µm-size Ag powders were both consonant with the RAIR spectral features on vacuum-evaporated thick Ag film, implying that the usual infrared surface selection rule was applicable even to SEIR and DRIFT spectroscopies. All three kinds of infrared spectral data clearly dictated that 1,4PDI should be adsorbed on silver via the carbon lone-pair electrons of one isocyanide group assuming a vertical
Han et al.
orientation with respect to the silver substrate. We could also obtain the SER spectrum of 1,4-PDI adsorbed on powdered silver that was used in the DRIFT spectroscopic study. Although an unequivocal SER selection rule has not yet been established, the same conclusion as that from the infrared data could be derived from the peak shift, band broadening, and the presence of the ring C-H stretching band in the SER spectrum. The observation made on the adsorption of 1,4-PDI on silver is in fact comparable to the earlier DRIFT study made on gold powders;22 1,4-PDI has also been reported to adsorb on gold via the lone-pair electrons of the carbon atom of only one of two isocyanide groups. For the latter case, however, the researchers could observe only the NC stretching peaks in contrast with the present work; in fact, we could identify several characteristic ring modes of 1,4-PDI along with the NC stretching bands in all infrared and Raman spectra, thereby allowing a firmer conclusion to be made about the adsorption characteristics of 1,4-PDI on silver. The adsorption characteristics of 1,4-PDI on silver observed in this work may thus be regarded as analogous to the bonding characteristics in organometallic complexes in which the carbon of the isocyanide ligand is usually bonded to one metal atom.32 Nonetheless, it is intriguing that, although 1,4-DCB, a geometric isomer of 1,4-PDI, is also usually coordinated to metal via the nitrogen lone-pair electrons of only one cyano group in organometallic complexes, the molecule is presumably adsorbed on the silver surface via the two CtN π systems.21 The origin of the different adsorption characteristics of 1,4-PDI and 1,4-DCB on silver is not certain. To resolve this difficulty, we plan to do ab initio molecular orbital calculations on a silver surface. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Center for Molecular Catalysis at Seoul National University (SNU) and by the Korea Research Foundation through the Research Institute for Basic Sciences at SNU. S.W.J. thanks the KOSEF for an Intern Research Fellowship. LA990396W
