Raman Spectroscopic Study of 1,4-Benzenedithiol Adsorbed on Silver

Ft-SERS studies on 1,3-thiazolidine-2-thione, 2,5-dimercapto-1,3,4-thiadiazole and 2-thiouracil adsorbed on chemically deposited silver films. V. T. J...
0 downloads 0 Views 697KB Size
Download PDF
10594

J. Phys. Chem. 1995,99, 10594-10599

Raman Spectroscopic Study of 1,4-Benzenedithiol Adsorbed on Silver Seong Ho Cho, Hyouk So0 Han,Du-Jeon Jang,* Kwan Kim,* and Myung So0 Kim* Department of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151 -742, Korea Received: December 22, 1994; In Final Form: March 30, 1995@

Surface-enhanced Raman scattering (SERS) of lY4-benzenedithiol (1,4-BDT) has been investigated in aqueous silver sol. The vibrational assignment of 1,4-BDT and its dianion species has been made by referring to that of 1,4-dichlorobenzene. From the SER spectrum, the 1,4-BDT molecule was found to chemisorb on the silver surface via Ag-S bonds after deprotonation, assuming a flat orientation with respect to the surface. The definitive orientation allowed us to test the validity of some SERS selection rules. Although the direct surface-benzene ring n interaction should result in a substantial increase of the bandwidths of ring modes, peak shifts alone seemed not to secure the presence of such an interaction. The tactics to classify the ring modes into the so-called in-plane and out-of-plane modes seemed also not pertinent in determining the orientation of benzene derivatives adsorbed on metal surfaces by virtue of SER spectra. A more unequivocal SERS selection is expected to be established in the near future since even the symmetry-based electromagnetic selection rule did not work in the present system.

Introduction When a molecule is adsorbed on the rough surface of some metals like Ag, Au, and Cu, its Raman scattering is tremendously enhanced. This phenomenon is called surface-enhanced Raman scattering (SERS).Ip3 Ever since its discovery, lots of experimental and theoretical investigations have been made to elucidate its exact n a t ~ r e . ~In - ~parallel, much efforts have also been directed to its application to numerous problems in scientific and applied fields.10-12Investigation of the molecular adsorption mechanism on the metal surface was one of the fields since only the chemisorbed molecules appeared to display significant surface enhan~ement.'~-'~ The major difficulty in using SERS as a surface vibrational spectroscopic tool arises from its very origin. It is widely accepted that the enhancement of the electromagnetic field near the rough surface and the surface resonance Raman effect are the main causes of SERS.4-9 For the first layer of the adsorbate, both mechanisms are thought to operate, but with varying degrees for different systems. However, the surface selection rules derived from the two models, which are only approximate and complicated, are usually contradictory to each other. Namely, no general rule is available at the moment to predict the surface orientation of an adsorbate reliably from its SER spectrum. This is in contrast with other surface vibrational spectroscopic means such as infrared and electron energy loss spectroscopies, for which the surface selection rules are well Nonetheless, SERS enjoys superior sensitivity and resolution to other vibrational means. Without a reliable theoretical guideline, we have been attempting to establish SER spectral correlation with the adsorption mechanism for a series of related compounds through a detailed analysis of peak shift and band broadening caused by surface adsorption. In that way, we could establish, for example, a qualitative spectral correlation that is applicable to the determination of the adsorption mechanism of aromatic nitriles on silver surface.Ig Invoking that organic thiol molecules form oriented monolayers on noble metals such as gold and ~ i l v e r , ' ~we , ' ~hope currently to establish similar SER spectral

* To whom all correspondence should be addressed.

@Abstractpublished in Advance ACS Abstracts, June 1, 1995.

0022-3654/95/2099- 10594$09.00/0

correlation for aromatic thiols. Despite numerous SERS investigations, more thorough SERS data seems, however, necessary to reach such a goal, especially for highly symmetric thiol molecules. In this respect, we have performed a SERS study of 1A-benzenedithiol (1,4-BDT) in aqueous silver sol. For a better SER spectral analysis, the vibrational mode assignment has been made for 1,4-BDT by referring to the ab initio calculated result on 1,Cdichlorobenzene.

Experimental Section The method of preparation of the aqueous silver sol has been reported previously.2o Methanolic solution of 1,Cbemenedithiol (1,4-BDT) was added to the silver sol to a final concentration M. After the color of the sol solution had changed of from yellow to dark green, poly(viny1pyrrolidone)(MW 40 OOO) was added up to 0.01% as a colloid stabilizer. When needed, the pH of the sol solution was adjusted with HCl or NaOH. Raman spectra were obtained using a Japan Spectroscopic Model R-300 Raman spectrophotometer; 514.5 nm radiation from an argon ion laser (Spectra-Physics Model 164-06) was used to excite the Raman spectra. A glass capillary tube was used as a sampling device and Raman scattering was observed with 90" geometry using a commercial photon-counting system. In a typical experiment, the laser power was 100 mW at the sample position and the spectral slit width was 5-10 cm-'. A heated capillary cell was employed when obtaining the ordinary Raman (OR) spectrum of 1,CBDT in liquid state. The OR spectrum of the dithiolate species was taken after dissolving 1,4-BDT in 6 M NaOH solution. Mid- and far-infrared spectra were obtained for 1,CBDT dispersed, respectively, in KBr and paraffin oil pellets using a Bruker Model IFS 113v Fourier transform IR spectrometer. 1,CBDT purchased from Tokyo Kasei was purified before use by vacuum sublimation and recrystallization. All other chemicals were reagent grade, and triply distilled water was used throughout.

Results and Discussion As to be discussed later, the peaks appearing in the SER spectrum of 1,4-BDT are observed to correlate very well with those in the corresponding OR spectrum. Then, to obtain 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99,No. 26, 1995 10595

SERS of 1,6Benzenedithiol Adsorbed on Silver

TABLE 1: Vibrational Assignment of 1,4-Dichlorobenzene"

m

s

I

Scherer (ref 23) 3070 1574 1169 1096 747 328 B1, 815 Bz, 934 687 298 B3g 3065 1574 1290 626 350 A, 951 410 B1, 3090 1477 1090 1015 550 Bzu 3090 1394 1221 1107 226 B3, 819 485 125 A,

YI

I

I 500

1000

1600

2800

3000

Raman Shift ( cm.')

Figure 1. Ordinary Raman spectra of (a) 1,4-benzenedithiol(1,4-BDT), (b) 1,4-dichlorobenzene, and (c) 1,4-benzenedithioIate. Spectra a and b were taken in the molten state, and spectrum c was taken after dissolving 1,4-BDT in 6 M NaOH solution.

information on the surface adsorption mechanism from the SER spectrum, it will be necessary to analyze minute spectral changes caused by the surface adsorption. In this respect, a correct vibrational assignment is a prerequisite. The OR spectrum of 1,4-BDT is shown in Figure la. In order to help assign the peaks of 1,4-BDT, the OR spectrum of 1,4dichlorobenzene has been taken by referring to the fact that the vibrational frequencies of mercaptans are generally very close to those of corresponding chlorides.z1 In fact, the peaks of 1,4BDT shown in Figure l a are observed to correlate very well with those of 1,4-dichlorobenzene, shown in Figure lb. Some minor mismatches can be attributed to either the SH grouprelated vibrations or the somewhat lowered symmetry in 1,4BDT than in 1,4-dichlobenzene. Namely, the extra peaks at 2555 and 910 cm-' in Figure l a can be assigned, respectively, to the S-H stretching and bending vibrations. For 1,4dichlorobenzene having Dzh symmetry, the ungerade vibrations are Raman inactive, but such kinds of vibrations can become Raman active for 1,4-BDT owing to the presence of two unsymmetrical SH groups, as the ring modes 16b (B3,) at 472 cm-' and 20a (B1,) at 530 cm-' in Figure la. As the two SH groups are deprotonated in basic medium resuming a D2h structure, neither the SH group vibrations nor the ungeradetype vibrations appear any longer in the OR spectrum, as can be seen in Figure IC. Another noticeable mismatch between the OR spectra of 1,4-BDT and 1,Cdichlorobenzene concerns the peaks appearing in the 1050-1 100 cm-I region. Two strong polarized bands appear at 1054 and 1090 cm-' in the former, while three polarized bands appear at 1064, 1080, and 1099 cm-I in the latter spectrum.

Green (ref 24) 3072 1574 1169 1096 747 328 815 934 687 298 3065 1574 1290 626 350 951 405 3078 1477 1090 1015 550 3087 1394 1220 1107 226 819 485 122

(2)c (8a,dp) (X-sens) (9a) (X-sens) (loa) (IR) (IR) (IR) (7b) (8b) (3) (6b) (X-sens) (17a) (16a) (20a) (19a) (X-sens) (18a) (X-sens) (20b) (19b) (14) (15) (X-sens) (11) (16b) (X-sens)

Varsanyi (ref 22) 3070 1574 1169 1092 747 326 934 687 306 3065 1290 626 350 951 407 3086 1477 1084 1015 550 3095 1394 1265 1107 226 815 485 114

(2) (8a) (9a) (1) (6a) (7a) (loa) (5) (4) (lob) (7b) (8b) (3) (6b) (9b) (17a) (16a) (13) (19a) (12) (18a) (20a) (20b) (19b) (14) (18b) (15) (17b) (16b) (11)

Gribov this (ref 25) calculatedb observed 3070 1630 1169 1096 747 328 815 934 687 298 3065 1574 1290 626 350 951 407 3090 1477 1115 1015 550 3090 1394 1290 1096 226 819 485 125

3073 1621 1202 1077 728 313 890 1057 747 299 3056 1593 1330 641 333 1056 425 3055 1505 1092 1023 501 3070 1407 1193 1117 202 902 520 102

(2) (8a) (9a) (1) (7a) (6a) (loa) (5) (4) (lob) (7b) (8b) (3) (6b) (9b) (17a) (16a) (13) (19a) (12) (Ma) (20a) (20b) (19b) (14) (18b) (15) (17b) (16b) (11)

3067 1630 1163 1099' 740 322 811

pd p p p p p dp

684 dp 291 dp 1569 1284 620 344

dp dp dp dp

3075 1476 1090 1013 547 3092 1393 1264 1106 821 485

Wavenumber in cm-]. Values scaled by 0.9. Wilson notation. X-sens: substituent sensitive. p, polarized; dp, depolarized. e Peaks near this band at 1064 and 1080 cm-' are also polarized. The resulting triplet is thus thought to occur from one fundamental and two overtone or combination bands in Fermi resonance with one another.

In spite of extensive s t u d i e ~ , 2 ~some - ~ ~ controversy still remains on the vibrational assignments for 1,4-dichlorobenzene. For example, the polarized bands at 1099 and 1163 cm-' in its OR spectrum (Figure lb) were assigned, respectively, to the ring modes 1 and 9a by Varsanyi,zz but to the ring mode 9a and an X-sensitive mode by Green.24 Similarly, the infrared active band appearing at 821 cm-' was assigned to 17b and 11, both belonging to B3,, by Varsanyi and by Green, respectively. Some conflicting symmetry classification is also noticeable in the literature. The band at 1569 cm-' in Figure l b assignable to ring mode 8b was claimed to belong to B3g symmetry by Gribov et aLz5 and to a composite of A, and B3, by Scherer and Evansz3and by but to A, by Varsanyi?z albeit the band was depolarized in the OR spectra of various p-xc6H4x compounds (X = CH3, F, C1, Br, and I). On the other hand, in contrast with the present study, earlier workers except Varsanyizz could identify only one peak around 1099 cm-' in the 1050-1100 cm-' region. Varsanyi reported the presence of two peaks, one appearing at 1092 cm-' and the other at 1084 cm-I. Although being polarized, the latter band was classified by Varsanyi to belong to Bl, symmetry. Further, Scherer and Evansz3 suggested the fundamental band around 1096 cm-' to be in Fermi resonance with a certain overtone or combination band. Hoping to resolve the discrepancy in the assignments of peak frequencies of 1,4-dichlorobenzene, we have performed an ab initio quantum mechanical calculation by using the Gaussian 92 program for Windowsz6 with a 6-31G basis set. After determining the equilibrium geometry, the fundamental vibrational frequencies were calculated. The results are listed in Table 1. The six possible A, fundamental frequencies are

Cho et al.

10596 J. Phys. Chem., Vol. 99, No. 26, 1995

TABLE 2: Spectral Data for 1,4-Benzenedithiol (1,4-BDTPblc SERS vibrational OR OR IR assignmentd dithiolate 1,CBDT 1,4-BDT 1,4-BDT 276 m 291(16) w 313 sh 10b 350 w 348(17.5) vs 6a 329 w 323 s 407 w 407 vw 400(25) vw 16a 16b 487 s 472 w 489(30.5) w 20a 535 m 530 w 530(27.3) w 635(15.3) vw 6b 626 m 633(4.6) w 4 695(10.4) w 687 w 7a 737(12) m 731(11.7) s 747 s 17b 815 vs 815 vw 811(35.8) w 902 m 910 m PW) 930 vw 5 1014 s 1009 vw 1008 sh 18a 1054 m 1059(14) vs 1066(37) vs 1' 1090 vs 1088(12) vs 1084(26) vs 1' 1116 vs 1117 sh 18b 9a 1178 m 1179(8.7) s 1181(10.8) s 1262 w 1256 vw 1257(15.7) vw 14 1295 w 1286(6) w 1285(13.4) vw 3 1394 s 1380 w 1383(26.7) vw 19b 19a 1476 vs 1470 vw 1463(14.6) w 1520(10) w 1545 sh 8b 1570 s 1565(8) vs 1560(19.2) vs 8a 2560 s 2555 m v(SH) vw 3051 vvw 2 3050 w 3044 m 3048 Values in parentheses are bandwidths a Wavenumber in cm-'. corrected for the instrumental effect. Intensities: vs (very strong), s (strong), m (medium), w (weak), vw (very weak), and sh (shoulder peak). Wilson notation. e One of the two bands arises from an overtone or a combination band such as 6a+7a, in Fermi resonance with the V I fundamental. The peak positions and bandwidths were obtained by resolving the overlapped band with the curve-fitting process. calculated to be 3073, 1621, 1202, 1077, 728, and 313 cm-I. One of the difficulties encountered in assigning the observed spectrum is that eight polarized bands appear distinctly in the OR spectrum (Figure lb), namely, at 3067, 1630, 1163, 1099, 1080, 1064,740, and 322 cm-I. This implies that two of the latter eight bands must be attributed to overtone and/or combination modes belonging to A, symmetry. Comparing the calculated and observed Ag frequencies, two of the three peaks at 1064, 1080, and 1099 cm-I in the observed spectrum seem to be attributed to overtone and/or combination bands. Otherwise, the remaining five frequencies are seen to be in fair agreement with the calculated frequencies. Considering that the above mentioned three peaks are closely spaced, they are supposed, furthermore, to be in Fermi resonance with one another. The other noticeable feature is that the polarized bands at 740 and 322 cm-I in Figure l b are calculated to be attributable to the ring 7a and 6a modes, respectively. This contrasts with the earlier assignments made by VarsanyiZ2that those two bands arise, respectively, from the ring 6a and 7a modes. As far as the depolarized bands are concerned, the observed bands can be assigned without difficulty by refemng to the calculated frequencies, as collectively summarized in Table 1. The earlier controversial assignments on 1,Cdichlorobenzene seem thus to be resolved. We could then readily assign the peaks of 1,4-BDT (Figure la) and its conjugate base, 1,4benzenedithiolate (Figure IC),by correlating with those of 1,4dichlorobenzene. The assignments made in that way are listed in Table 2. One noticeable difference between the OR spectra of 1,4-BDT and 1,4-dichlorobenzeneis that two strong polarized bands appear in the 1050-1100 cm-I region in the former spectrum, while three bands are observable in the latter spectrum. Even though the higher frequency band of the doublet is more intense in the 1,4-BDT spectrum, the opposite is observed in its conjugate dianion spectrum. It is apparent that

I

I

1

1

1

500

1oW

1500

2500

3000

Raman Shift ( cm" )

Figure 2. SER spectra of 1.4-benzenedithiol in aqueous silver sol at (a) pH = 7 and (b) pH = 1.

the frequency of the overtone or combination band contributing to the doublet is higher than that of the fundamental in the case of the dianion. The SER spectrum of 1,4-BDT in silver sol solution is shown in Figure 2a. As mentioned earlier, the peaks appearing in the SER spectrum correlate well with those in the OR spectra of 1,4-BDT and its dianion species. The SH group related vibrations are completely missing even in the SER spectrum obtained in a highly acidic (pH 1) medium, as shown in Figure 2b. This means not only that the two thiol protons are lost simultaneously upon the surface adsorption but also that the resulting dianion is in direct interaction with the silver atoms on the sol surface via the two sulfur atoms. The present observation agrees with our previous SER s t ~ d i e s ' ~ ,on ~~,~' various mono- and dithiol molecules. Complete absence of the SH group related vibrations also suggests that the SER spectrum originates only from the surface monolayer of 1,4-BDT. In fact, according to our previous investigation on the adsorption of 1-propanethiolon the surface of silver colloidal particle~,'~ 1-2 x M bulk concentration of 1,4-BDT is needed for the full monolayer coverage. At the bulk concentration of used here, it is highly unlikely that a multilayer of 1,4-BDT is formed, in agreement with the SER spectral feature. Adsorption via two atoms at para positions implies, in turn, that the molecular plane of the dianion lies flat on the silver surface. Although the SER spectrum makes, in general, a good match with the anion OR spectrum, some of the minor bands in the former spectrum cannot find their counterparts in the latter. Those bands seemed, however, to be correlated with either the minor bands in the OR spectrum or the major bands in the infrared spectrum of pure 1,4-BDT. Namely, nearly all of them can be assigned to belong to the ungerade vibrations of dianion

SERS of 1,4-Benzenedithiol Adsorbed on Silver

J. Phys. Chem., Vol. 99, No. 26, 1995 10597

species. In fact, appearance of Raman forbidden bands in the TABLE 3: Relative Enhancement Factors for the SER Bands of l,4-Benzenedithiol SER spectra is well-known. This is usually explained in terms of the symmetry reduction caused by the surface a d s o r p t i ~ n ~ ~ . ~symmetry ~ polarizability normal relative enhancement or the high-order SER effect related with the quadrupole type" tensor elementb mode' factor ( ~ E R ~ ~ O R ) ~ p~larizability.~~.~' When the dianion is adsorbed on silver with 2', 0 . 9 a flat orientation, the overall symmetry will be reduced from observed not observed 40 D2h to C2". Since all the normal modes are Raman active in 5.7 the latter symmetry, appearance of the ungerade vibrations can 1.3 be understood. The second mechanism usually operates when 1 the electric field gradient is very large in the vicinity of metal surface. A huge field gradient can be present when the surface not observed observed not observed observed plasmon resonance condition is matched. The symmetry types '0.78 of quadrupole polarizability tensor elements are the same as c0.90 those of hyperpolarizability elements. All the ungerade modes 0.95 can thus become Raman active via the second mechanism, again in agreement with the present observation. Although both mechanisms may operate at the same time, it is not certain which of the two is more responsible for the appearance of ungerade Symbols in parentheses are symmetry types corresponding to the modes in the present case. CzV point group. bSubscripts, i.e. x , y, and z, correspond to the conventional molecular axes. The x-axis lies perpendicular to the ring, With the molecular plane lying flat on the surface by being and the z-axis passes through the two sulfur atoms. See Table 2 for locked tightly via the two Ag-S bonds, there exists a possibility the vibrational assignment. Normalized to 1.0 for the 9a band at ca. of surface-ring n interactions. In order to examine such a 1180 cm-' in the SER and dianion OR spectra. See text. e Evaluated possibility, one may need an appropriate guideline. For benzene with the lower frequency component of the Fermi resonance doublet and its monosubstituted derivatives, red-shift and broadening at ca. 1060 cm-'. /Evaluated with the higher frequency component of of the ring breathing modes were taken, by Gao and W e a ~ e r , ~ , ~ the ~ Fermi resonance doublet at ca. 1085 cm-I. as evidence for the surface-ring n interaction. We have the validity of variously proposed SER selection rules. Acpreviously observed similar peak shift and band broadening in cording to the electromagnetic (EM) surface selection rule,4,7336 the SER spectra of some para-disubstituted benzene molrelative enhancements for normal modes can be classified into e c u l e ~ . ~For ~ ,example, ~ ~ , ~ ~the ring breathing mode of p-xylenethree groups in terms of the polarizability tensor elements. a,a'-dithiol displayed a 10 cm-' red-shift as well as noticeable Namely, normal modes associated with the tensor element a 3 3 band broadening upon adsorbing on the silver surface.21 In the are the most enhanced, modes associated with a13 or a 2 3 are SERS of 4-(methylthio)ben~onitrile,~~ however, neither the peak less enhanced, and modes associated with a l l , a12, or a 2 2 are shift nor the band broadening was observed. Referring to the the least enhanced. Here, the subscripts 1-3 refer to the report of Gao and one may simply conclude that orthogonal axes parallel (1 and 2) and perpendicular (3) to the the surface-ring n interaction is significant in the former case surface. To test the EM selection rule, the relative surface but insignificant in the latter. However, for a more firm enhancements have been evaluated for the normal modes of conclusion to be made for disubstituted benzenes in general, 1,4-BDT. Initially, the ring mode intensities were normalized the applicability of the above correlation should be checked to the intensity of the ring mode 9a at ca. 1180 cm-' in both further with a prototype molecule such as 1,4-BDT, which the OR (Figure IC) and SER spectra, and then the SER-to-OR definitely assumes a flat orientation. intensity ratios were computed for each peak regarding them Even though the ring breathing mode 1 appears distinctly in as the relative enhancement factors. Table 3 lists the intensity the SER spectra, its exact position and width cannot be ratios along with the relevant molecular polarizability elements. determined accurately due to the splitting caused by a Fermi The subscripts, i.e. x, y, and z, in Table 3 correspond to the resonance. The doublet peaks in the OR spectra (Figure la,c) conventional molecular axes. Namely, the x-axis lies perpenare merged into one broad feature in the SER spectrum (Figure dicular to the ring and the z-axis passes through the two sulfur 2a), indicating that both peaks have been substantially broadatoms. When the dianion lies flat on the surface, its x- and ened. In the SER spectrum taken in an acidic medium (Figure z-axes can be taken to coincide, respectively, with the axes 3 2b), those peaks are somewhat separated, but any significant and 1 of the surface plane. In that case, BI, modes should be red-shifts are hardly observable. Hence, the peak shift of the associated with the polarizability element ~ 2 3 Bzg , modes with ring breathing mode seems not to be a reliable indicator to judge ~ 1 3 and , B3g modes with a12. The A, modes could possess the the presence of a significant surface-ring n interaction. nonvanishing all, az2, andor a 3 3 elements. Then, the EM Nonetheless, the doublet in Figure 2b is substantially broader surface selection rule dictates that the Bzg modes have to be than the corresponding bands in the anion OR spectrum (Figure more enhanced than the B3g modes at least by 1 order of IC), as seen from the correct bandwidths shown in Table 2. As magnitude. However, the relative enhancements shown in Table a matter of fact, almost all the ring modes except the ring modes 3 do not display such a trend. Among the A, modes, the 6a 7a and 9a at 73 1 and 1181 cm-I, respectively, are significantly mode at ca. 350 cm-' is tremendously enhanced, while the ring broader in the SER spectrum than in the OR spectrum. Even mode 2 at ca. 3050 cm-' is suppressed. One may argue that though band broadening can be ascribed to the opening of a the former possesses a particularly large a, component, while new vibrational relaxation channel caused by surface adsorption, the latter does not, and takes these as evidence for the validity it is not certain yet why the extent of broadening is normal of the EM selection rule. It is to be noted, however, that the mode-dependent. It appears, however, that substantial broaden6a mode is substituent sensitive, containing significant contribuing of a large fraction of ring modes is indicative of significant tion from the CS motion. This can be evidenced from the OR surface-ring n interaction. spectra of 1,4-BDT and its conjugate dianion species. Namely, the 6a mode is very strong in 1,4-BDT but becomes very weak The present definitive geometry of adsorbed species, i.e. lying upon deprotonation. Hence, it may be conjectured that the flat on the silver surface, provides a rare opportunity to examine

-

--

10598 J. Phys. Chem., Vol. 99, No. 26, 1995

Cho et al.

analysis suggested further that substantial increase in the change in polarizability caused by the Ag-S interaction is more bandwidths of benzene ring modes is indicative of the presence responsible for the enhancement of the 6a mode rather than the of a surface-ring n interaction, albeit the detailed broadening EM selection rule. In the case of ring mode 2, it has been mechanism is not certain. In contrast, peak shifts of the ring widely recognized that the band appears with very weak modes seemed not to be a definitive indicator in judging the intensity or does not appear at all when the benzene ring lies presence of such an interaction. The definite planar orientation flat on the surface, while it appears distinctly when the ring lies perpendicular to the s u r f a ~ e One . ~ often ~ ~ encounters ~ ~ ~ ~ ~ ~ has ~ led us, on the other hand, to test the validity of several SERS selection rules proposed so far. Although a rigorous test in the l i t e r a t ~ r e ~a, loosely ~ ~ - ~ ~defined EM selection rule that could not be made on the resonance Raman selection rule, it for benzene derivatives the in-plane vibrations be enhanced more has been found clearly that the so-called in-plane/out-of-plane than the out-of-plane ones when the benzene ring is perpendichotomy as well as the more elaborate symmetry-based dicular to the surface, and the opposite occurs when the ring electromagnetic selection rule did not work in the present case. lies parallel to the surface. In the case of 1,4-BDT dianion, A, A more unequivocal selection rule is hoped to be established and B3g modes are in-plane vibrations, while B1, and B2, modes in the near future to explain the SERS of 1,CBDT. are out-of-plane vibrations. With its benzene ring lying flat on the surface, this model suggests the predominance of the outAcknowledgment. This work was supported by the S. N. of-plane vibrations in the SER spectrum. However, data in U. Daewoo Research Fund, 1995. K.K. and M.S.K. acknowlTable 3 do not support this model. Accordingly, the in-plane/ edge also the Korea Research Foundation, Korea Science and out-of-plane dichotomy seems not to work as a useful guideline Engineering Foundation, and the Ministry of Education, Rein determining the orientation of the benzene ring. public of Korea, for their support to purchase various instruThe other major surface enhancement mechanism is the ments. D.J. thanks the Molecular Science Research Center for resonance Raman scattering mediated by a charge transfer providing a Fellowship. p h e n ~ m e n o n . ~Howevet, -~ the model has not yet reached an extent for its associated selection rule to be systematically References and Notes applied. Otto and his c o - w o r k e r ~observed ~ ~ ~ ~ that for unsaturated hydrocarbons the CC stretching modes were more enhanced (1) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. than the CH stretching modes. The conspicuous enhancement 1974, 26, 163. of the CC stretching mode was explained in terms of the efficient (2) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (3) Wenning, U.; Pettinger, B.; Wetzel, H. Chem. Phys. Lett. 1980, electron-photon coupling caused by temporary electron transfer 70, 49. from silver into the normally unoccupied antibonding n* (4) Chang, R. K., Furtak, T. E., Eds.; Surface Enhanced Raman orbitals. On the other hand, the CH stretching motion was Scattering; Plenum; New York, 1982. (5) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. claimed hardly to couple with either n* or CJ*orbitals. An (6) Creighton, J. A. Spectroscopy of Surfaces; Clark, J. H., Hester, R. extremely weak CH symmetric stretching band near 3050 cm-' E., Eds.; John Wiley & Sons: New York, 1988; p 37. in the SERS of 1,CBDT could be explained on similar grounds. (7) Lombardi, J. R.; Birke, R. L.; Lu, T.; Xu, J. J. Chem. Phys. 1985, Nonetheless, it is intriguing that the 9a mode at 1181 cm-I, 848, 4174. (8) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: which is mostly CH stretching vibrations, is enhanced as much Condens. Matfer 1992, 4, 1143. as others in the SER spectrum of 1,CBDT. In addition, a (9) Persson, B. N. J. Chem. Phys. Left. 1981, 82, 561. preliminary study reveals that the CH stretching band gains (10) Shi, C.; Zhang, W.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990, 94, 4766. intensity in the SER spectrum of 1,ZBDT. These observations (1 1) Vo-Dinh, T. Chemical Analysis OfPolycyclic Aromatic Compounds; appear to be in contrast with what has been claimed by Otto Vo-Dinh, T., Ed.; Wiely; New York, 1989; Chapter 5. and his c o - ~ o r k e r s . ~On - ~the ~ other hand, the 6a and 7a modes, 1121 Evans. S . D.: Urankar. E.: Ulman. A.: Fems. N. J. Am. Chem. SOC. both of which contain a significant amount of CS stretching 1+1,ZZ3, 4121. (131 Joo. T. H.: Kim. K.: Kim. M. S. J. Phvs. Chem. 1986, 92. 5816. character,22are more conspicuously enhanced than others. In (14) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. SOC.1991,113,8284. analogy with the interpretation for the large enhancement of (15) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Suc. 1991,113,3629. the CF stretching mode in the SERS of c& proposed by (16) Fan, J.; Trenary, M. Langmuir 1994, 10, 3649. (17) Greenler, R. G. J. Chem. Phys. 1966.44, 310. Grabhorn and one might take this as evidence for the (18) Hoffman, F. M. Sui$ Sci. Rep. 1983, 3, 107. coupling of the CS stretch motion with either n* or CJ*orbitals. (19) Lee, E. A.; Yi, S. S.; Kim, K.; Kim, M. S . J. Mol. Strucf. 1993, However, it is to be noted that the relative intensities of CS 298, 47. stretching bands in the SER spectrum are comparable to those (20) Joo, T. H.; Kim, K.; Kim, M. S. Chem. Phys. Lett. 1984, 112, 65. (21) Lee, T. G.; Kim, K.; Kim, M. S. J. Phys. Chem. 1991, 95, 9950. in the OR spectrum of pure 1,4-BDT. This contrasts with what (22) Varsanyi, G. Assignmentsfor Vibrafional Specrra of Seven Hundred has been observed for the 1,CBDT dianion. Namely, the CS Benzene Derivative; Wiley; New York, 1974. stretching bands are substantially weak in the dianion OR (23) Scherer, J. R.; Evans, J. C. Spectrochim. Acta 1963, 19, 1739. (24) Green, J. H. S . Spectrochim. Acta 1973, 26A, 1503. spectrum. It may then be more reasonable to suppose that the (25) Gribov, L. A.; Davidova, I. E.; Decoret, C.; Royer, J. Spectrochim. decrease in the band intensities occurring by the loss of protons Acta 1993, 49A, 425. is restored in the SER spectrum through the Ag-S bond (26) Frisch, M. J.; Trucks, G . W.; Head-Gordon, M.; Gill, P. M. W.; formation. This may imply that the charge transfer SERS Wong, M. W.; Foresman, J. B.; Johnson, B. G . ;Schlegal, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. R.; Raghavachari, K.; Binkley, mechanism is not operating at the silver sol surface on which J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; 1,CBDT is adsorbed. Stewart J. J. P.; Pople, J. A. Gaussian 92, Revision 0.2; Gaussian Inc.: Pittsburgh, PA, 1992. In summary, the earlier controversial vibrational assignment (27) JOO, T. H.; Kim M. S.; Kim, K. J. Raman Spectrosc. 1987,18, 57. of 1,4-dichlorobenzene has been resolved by referring to the (28) Hallmark, V. H.; Campion, A. J. J. Chem. Phys. 1986, 84, 2933. ab initio quantum mechanical calculation. The assignment made (29) Erdheim, G. R.; Birke, R. L.; Lombardi, J. R. Chem. Phys. Lett. on 1,4-dichlorobenzene could be successfully applied to the 1980, 69, 495. (30) Moskovits, M.; Dilella, D. P.; Maynard, K. J. Langmuir 1988, 4, vibrational assignment of 1,4-BDT and its dianion species. From 67. the SER spectral analysis, 1,4-BDT was found to chemisorb (31) Polubotko, A. M. Phys. Lett. 1990, 146, 81. on the silver surface via Ag-S bonds after deprotonation, (32) Gao, X.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, assuming a flat orientation with respect to the surface. The 6858.

SERS of 1,4Benzenedithiol Adsorbed on Silver (33) Kwon, Y. J.; Lee, S. B.; Kim, K.; Kim, M. S. J. Mol. Stmcr. 1994, 318, 25. (34) Lee, H. M.; Kim, M. S.; Kim, K. Vib. Spectrosc. 1994, 6, 205. (35) Yi, S. S.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1993,24,213. (36) Creighton, J. A. Sur$ Sci. 1983, 124, 209. (37) Moskovits, M. J. Phys. Chem. 1981, 75, 3126. (38) Moskovits, M.: Suh, J. S . J. Phys. Chem. 1988, 92, 6327. (39) Park, S. H.; Kim, K.; Kim, M:S. J. Mol. Struct. 1993, 301, 57.

J. Phys. Chem., Vol. 99, No. 26, 1995 10599 (40) Takahashi, M.; Fujita, M.; Ho, M. Chem. Phys. Len. 1984, 109, 122. (41) Park,H. K.; Lee, S. B.; Kim, K.; Kim, M. S. J. Phys. Chem. 1990, 94, 7576. (42) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 1293. (43) Grabhom, H.; Otto, A. Vacuum 1990, 41, 473. JF943391A