Raman Spectroscopic Studies of Terthiophenes for Molecular

Dec 21, 2006 - In contrast to terthiophene. (3T), the bisthiolterthiophene (T3) and biselenol-terthiophene (Se3) molecules were stable against photoin...
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J. Phys. Chem. B 2006, 110, 25671-25677

25671

Raman Spectroscopic Studies of Terthiophenes for Molecular Electronics Fredrik Svedberg,* Yury Alaverdyan, Patrik Johansson, and Mikael Ka1 ll Department of Applied Physics, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden ReceiVed: April 27, 2006

The effect of thiol and selenol functionalization on the vibrational spectra and photochemical stability of terthiophene based molecular wires was investigated using surface-enhanced Raman scattering (SERS). The molecules were found to exhibit markedly different properties at the silver surface of the SERS substrate, despite having almost identical Raman spectra in solution and in the solid state. In contrast to terthiophene (3T), the bisthiolterthiophene (T3) and biselenol-terthiophene (Se3) molecules were stable against photoinduced structural changes when adsorbed to the metal surface at low concentrations. This indicates that the strong bonds to the silver surface, via S or Se terminal atoms, leads to a rapid decay of photoexcited states. Comparison with ab initio calculations shows that both T3 and Se3 bind with only one of the functional groups to the Ag surface.

1. Introduction In the development of molecular electronic (ME) devices, one of the principal difficulties is the formation of electrical contact between the conducting molecules and the macroscopic electrodes.1 The prototypical molecular wire is a molecule with highly delocalized electron orbitals, which permit electrons to flow freely along the length of the wire.2 Hence, molecules with conjugated π orbitals and with thiol functional groups acting as molecular scale “alligator clips” have the desired properties for ME applications. Significant theoretical and experimental progress over the past few years has led to a deeper understanding of electron transport properties of single molecules.3 Due to their electro-optical properties, thiophene oligomers (nT) and polythiophene (PT) have turned out to be of great interest for ME applications, for example, as light-emitting diodes and fieldeffect transistors.4 The oligothiophenes are particularly interesting because of their well-defined chemical structure. This class of molecules has therefore been intensively investigated by optical and electrical means in order to reach a rational interpretation of their vibrational fingerprints and transport properties. Conductance measurements of metal-molecule-metal junctions typically involve a self-assembled monolayer (SAM) of the molecular species adsorbed on a lithographically prepared structure or on metal nanoparticles. Electrical contact to a single or a small number of molecules can then be made by scanning tunneling microscopy (STM) or by using mechanically controlled break (MCB) junctions. The vibrational spectrum of terthiophene (3T) has been determined by both IR and Raman spectroscopy5-8 as well as through surface-enhanced Raman scattering (SERS).9-12 SERS is a version of classical Raman scattering based on the adsorption of the molecules of interest to nanostructured noble metal surfaces,13 which means that it is highly suitable for characterization of metal-molecule interactions. The surface-enhancement phenomenon originates in enhanced electromagnetic fields due to surface plasmon excitations, and the enhancement is particularly strong in nanoscopic junctions between metal surfaces.14,15 This means that SERS can, in principle, probe the particular sites that are * Corresponding author. E-mail: [email protected].

responsible for molecular conduction in ME applications. The high sensitivity, sometimes reaching the single molecule detection limit,14 makes SERS a powerful vibrational spectroscopic technique for detection and analysis of molecules that are attached to metal nanostructures, as in ME devices. Recently, thiol and selenol terminated terthiophene molecules, T3 and Se3, respectively, were synthesized and characterized by Bourgoin and co-workers.16,17 Using STM and MCB junctions, the electronic coupling efficiency of T3 and Se3 molecules embedded in dodecanethiol (DT) matrices were compared. Adsorption and transport properties of the terthiophene molecules, as well as of the DT SAMs, were also investigated. It was concluded that the molecular groups containing Se provided better electronic coupling than S when adsorbed to a gold surface. These results were further confirmed by theoretical studies and ultraviolet photoelectron spectroscopy (UPS).18 It is now evident that films of 3T undergo photopolymerization and photodecomposition on silver,11,12,19 which motivates us to question how T3 and Se3 behave in the adsorbed state during illumination. Photochemical stability is clearly a required property for any ME device. We consider if they polymerize upon adsorption, and if they can serve as reliable conducting molecular wires, by presenting a spectroscopic analysis of T3 and Se3 using SERS methodology and ab initio calculations. Ordinary Raman measurements of the compounds were also performed, both in solution and in the solid phase. In order to study the binding of the molecules to small metal structures, we utilized SERS active colloidal silver nanoparticles. Noble metal nanoparticles exhibit unique optical properties, and for subwavelength silver particles, the localized surface plasmon resonance (LSPR) frequency lies in the optical range.20 The LSPR induces a very strong electromagnetic near-field close to the particle, a phenomenon that produces the SERS effect.13 The approach of using colloidal particles for ME investigations has recently been utilized in single molecule conduction measurements.21 2. Experimental Methods The T3 (2,5′′-bis(acetylthio)-5,2′5′,2′′-terthienyl) and Se3 (2,5′′-bis(acetylseleno)-5,2′5′,2′′-terthienyl) molecules, see Fig-

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Figure 1. Molecular structure of the thiophene oligomer 3T and its thiol and selenol terminated derivatives, T3 and Se3, respectively.

ure 1, were synthesized from 3T (2,2′:5′,2′′-terthiophene) following Miller’s method,22 as described in ref 17. The acetyl groups (COCH3) are needed to protect the reactive thiol and selenol functions and to increase the solubility of the molecules. Removing the acetyl protecting groups of T3 and Se3 by addition of dethylaminoethanol (DEAE) during the assembly process for the SERS samples did not alter their spectroscopic properties. Clearly, the protective groups did not prevent the thiols or selenols from adsorbing spontanously on the metal surface, in agreement with reports for other thiolated aromatic oligomers.23 For all of the spectra presented herein, the molecules were therefore used without addition of any deprotecting agent. The Raman measurements were performed in a backscattering configuration using an optical microscope coupled to a single grating spectrometer (Renishaw, Ramascope 2000) equipped with a charge-coupled device (CCD) detector and appropriate notch filters. An argon ion laser operated at 514.5 nm provided the excitation light. The surface-enhanced and solid-state Raman measurements were performed with a laser power of 10 µW (∼1 kW/cm2) at the sample in order to minimize photoinduced and thermal processes. Solid-state Raman spectra were obtained from crystallites (roughly 10 microns in size), while measurements in solution were performed by dissolving the analytes in chloroform (CHCl3). Complementary optical absorption measurements were performed using a UV-vis-NIR spectrophotometer (Varian 500 Scan). The concentrations of the studied compounds in CHCl3 ranged from 10-4 down to 10-7 M. The silver nanoparticles were made by the standard protocol in ref 24, that is, through reduction of Ag+ ions in aqueous solution using sodium citrate as a reducing agent. Inspection in a scanning electron microscope showed that the colloid consisted of roughly spherical particles with an average diameter of 40 nm, in accordance with earlier reports.24-26 SERS active substrates were prepared by immobilizing the dispersed colloidal silver particles on APTMS-coated glass surfaces, following ref 27. The SERS substrates were then rinsed thoroughly in Milli-Q water and dried, after which a droplet of the analyte dissolved in CHCl3 was applied. After the chloroform had evaporated, SERS measurements were made in aqueous solution on small clusters of aggregated particles, using a water immersion objective in order to reduce oxidation and laser illumination effects. As an aid to the spectral assignments, we performed ab initio calculations of the ground-state molecular structures and vibrational spectra. The structures were geometry optimized by density functional theory (DFT) calculations using the hybrid functional B3LYP28,29 and the standard 6-31G* basis set. For Ag, the LanL2DZ pseudo-potential basis set was used.30

Figure 2. Absorption spectra of the studied compounds: 3T (dotted line), T3 (dashed line), and Se3 (solid line). The inset shows the molar extinction around the excitation wavelength.

Minimum energy structures were then identified by second derivative calculations. The vibrational frequencies, together with IR and Raman intensities, were computed numerically. All obtained frequencies were scaled by a factor of 0.9614 in order to correct for systematic errors.31 To obtain energies, wavelengths, and oscillator strengths for the ground- to excited-state transitions, time-dependent DFT (TD-DFT) calculations were performed. All calculations were made using the program package Gaussian 03.32 3. Results UV-vis spectra are presented in Figure 2. The absorption maxima of the substituted terthiophenes are very similar and red-shifted roughly 20 nm with respect to the nonsubstituted molecule. This shift is related to the substituent’s effect on the electronic structure of the chromophoric conjugated chain in terthiophene. From our calculations, we find that the transitions were HOMO-LUMO dominated and the calculated excitation energies, wavelengths, and oscillator strengths for 3T, T3, and Se3 were found to be 3.34 eV (372 nm, f ) 0.79), 3.17 eV (392 nm, f ) 1.02), and 3.12 eV (397 nm, f ) 1.08), respectively. All other transitions have oscillator strengths (f) lower than 0.005. Although the calculated excitation wavelengths are red-shifted ∼20 nm compared to those observed experimentally, the relative shifts are the same as those in the experimental spectra. This indicates that the acetyl protecting groups do not alter the chromophoric properties of Se3 and T3. As shown in the inset of Figure 2, T3 and Se3 possess almost the same absorption in the region of the excitation wavelength. Although the Raman excitation wavelength (514.5 nm) is rather far from the absorption maxima of all three compounds, a molecular resonance Raman amplification is likely in the case of T3 and Se3. Preresonance Raman can generally be observed when having molar extinction coefficients of the order  ≈ 5-10,33 that is, much higher than those for 3T but in the same range as the T3 and Se3 values at 514.5 nm. The ordinary Raman spectra indeed contained a fluorescent background, which was quenched in the subsequent SERS measurements. In Figure 3, we show Raman spectra of 3T, T3, and Se3 in the solid phase and in solution. Whereas the Raman spectroscopic properties of 3T have been investigated experimentally and theoretically over the years,5-12 there are, to the best of our knowledge, no previous Raman studies of T3 and Se3. As

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Figure 3. Raman spectra of the studied compounds in CHCl3 solution and in the solid state, together with calculated spectra for comparison. To the right, we show the corresponding optimized molecular structures, calculated at the B3LYP/6-31G* level.

TABLE 1: Raman Shifts (cm-1) for the Most Intense Raman Lines of 3T, T3, and Se3 in the Solid Phase (solid), in Solvent (solv), and in Theory (calcd) 3T

T3

Se3

solid

solv

calcd

solid

solv

calcd

solid

solv

calcd

assignment

1055 1426 1461 1530

1050 1429 1464 1533

1044 1431 1454 1528

992 1054 1426 1453 1518

997 1056 1426 1460 1528

952 1047 1431 1447 1524

968 1052 1422 1452 1518

967 1055 1422 1458 1528

926 1046 1426 1446 1521

CsS/Se stretch in-plane CsH bend CsC stretch CdC symmetric stretch CdC antisymmetric stretch

reported before in the case of 3T, the total number of Raman lines observed experimentally is rather low compared to what is expected from group theory due to a large vibrational degeneracy. A summary of the most intense modes, an in-plane CsH bend at 1055 cm-1, a CsC stretch at 1425 cm-1, and a pair of symmetric and antisymmetric CdC stretch modes at 1460 and 1530 cm-1, respectively, is presented in Table 1. The presence of the thiol and selenol functional groups affects the last two modes with small shifts (5-10 cm-1) toward lower frequencies, whereas the first mode is shifted (∼5 cm-1) to higher frequencies in solution. The weak modes around 950990 cm-1 observed for T3 and Se3 are assigned to the CsS and CsSe vibration between the outer thiophene rings and the functional end groups. The spectra in Figure 3 also exhibit a set of weaker bands, such as the CsSsC ring deformations around 670-710 cm-1 and the CsC inter-ring stretching motion at 1220 cm-1. For spectra collected in solution, there are only minor differences in the band positions and relative intensities between the three analytes. Hence, any changes in the following SERS spectra cannot be attributed to solvent effects. In the calculation of the molecular structures and vibrational spectra, we excluded the acetyl protecting groups for T3 and Se3. However, as is clear from Figure 3 and Table 1, the calculated Raman spectra are in excellent agreement with the experimental results, indicating that the terthiophene backbone is the main contributor to the observed Raman signals. From their very similar ordinary Raman spectra, one may expect that the three terthiophene structures would also exhibit similar SERS properties. This is, however, not the case. Turning first to the unfunctionalized compound, that is, 3T, we show in

a separate publication19 spectral time series where the first SERS spectrum in the series is very similar to the ordinary terthiophene Raman spectra shown in Figure 3. However, from there on, the spectral development depends on the molecular coverage at the silver surface. For low coverage, the spectral evolution is highly erratic, and any resemblance to the original 3T spectrum is lost with time. The final end products and the transient species in this process are difficult to specify from the spectral characteristics but most likely include monomeric thiophene units as well as unspecific “amorphous carbon”-like structures.19 We interpret this effect as a photoinduced decomposition of the terthiophene unit. For high molar concentrations, one instead finds a well-defined end product, namely, polythiophene (PT). As described in ref 19, this process is also photoinduced. This kind of polymerization effect has also been observed in the previous SERS studies of 3T,11,12 where it was interpreted as an adsorption-induced process. We note that the experiments in ref 19 were performed with very low laser powers, of the order of 1 kW/cm2, and in aqueous solution, that is, in a medium with relatively high thermal conductivity. This strongly indicates that the primary light-induced processes are photochemical in nature and that thermal effects due to laser heating are insignificant. Surprisingly, and in stark contrast to the 3T case, the T3 and Se3 molecules were found to be essentially stable over long time scales. Additionally, SERS measurements using different analyte concentrations, ranging from 10-4 down to 10-7 M (corresponding to more than monolayer coverage down to well below monolayer coverage), gave essentially identical spectra. This is also in contrast to the 3T case, where the thickness of

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Figure 4. Time series SERS spectra of T3 (a) and Se3 (b). The concentration of the molecules was 10-7 M, and spectra were recorded every minute for 10 s during 20 min.

the molecular film was crucial for the outcome of the subsequent photoinduced changes. Figure 4 shows SERS spectral series in some more detail for the thiol and selenol functionalized terthiophene structures during the first 20 min directly after the excitation light first illuminates the sample. The experimental conditions were the same as those for 3T in ref 19 (i.e., 10-7 M and an intensity of ∼1 kW/cm2). As shown in parts a and b of Figure 4 for T3 and Se3, respectively, the behavior of the molecules is essentially similar and there are no signs of any transient effects. Additionally, no photoinduced polymerization effects could be observed at high molecular concentrations (10-4 M) as in the case of 3T; see insets in Figure 5. The spectral evidence that T3 and Se3 are maintaining their chemical structure is verified by comparison with spectra of PT (and adsorbed 3T). For T3 and Se3, the antisymmetric CdC stretching mode (at 1525 cm-1) is present, which is not the case for PT, where it is strongly reduced and shifted to lower wavenumbers close to the symmetric stretch. In addition, the CsC stretching mode at 1410 cm-1 is not observed in the PT spectra.11,12,19 In a previous study of other aromatic molecules, we showed that the important factor in the photoinduced changes was the bare Ag surface contribution.34 With thiophenol precoated Ag particles, the photochemical conversion of tyrosine to amorphous carbon was completely inhibited. Similarly,

Svedberg et al.

Figure 5. Integrated spectral intensity, with background subtracted, versus time for T3 (a) and Se3 (b), respectively. The lower traces, diamonds, are from the spectral time series collected at low concentrations (10-7 M) and displayed in Figure 4. The upper traces, circles, are from the spectral series shown in the insets, for the higher concentrations (10-4 M). The decrease in signal intensity is well described by a single exponential I ) A + B exp(-t/C) decay, as shown by the curve fit (solid lines). The dashed lines in parts a and b are fits to a spectral time series for an intermediate concentration (10-6 M).

polymerization of 3T have been reported to be inhibited by coating the Ag surface with alkanethiol SAMs.12 Hence, the observed differences in the SERS spectra of 3T and its derivatives must be due to differences in the photoinduced molecular interaction with the Ag surface due to the fact that Se3 and T3 covalently bind to the Ag surface via the functional groups. From Figures 4 and 5, it is clear that the relative intensities and peak positions were constant both in time and for different analyte concentrations. However, it is also clear that the Raman signal decreases in intensity over time for the higher concentrations. In Figure 5, we show the integrated Raman intensity with background subtracted versus time. Increasing the molar concentration by a factor of 103 only results in a Raman signal initially about 5 times higher than the lower concentration 10-7 M. This shows that we are close to saturation coverage at 10-7 M. The initially large intensity for the higher concentration immediately starts to decrease toward the level of the 10-7 M measurements, whose intensity does not change over time. The decrease of the signal was found to be faster for Se3 than for

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Figure 7. Calculated and experimental SERS spectra for T3 (a) and Se3 (b). The experimental spectra are the sum of the spectral time series obtained at the 10-7 M concentrations.

Figure 6. SERS spectral changes as a function of irradiation time. The spectral series show the evolution during the first 3 min (integration time 1 s) for T3 and Se3 in parts a and b, respectively. The concentration of the molecules was 10-5 M. The colors indicate Raman intensity.

T3; see Figure 5, where we also show results for one intermediate concentration. The intensity decrease rate seems to be concentration dependent and falls off with increasing concentration. To resolve the early time stages, the spectral development was followed with higher time resolution for the first 3 min upon irradiation. The initial evolution for T3 and Se3 is shown in parts a and b of Figure 6, respectively, which shows that stable terthiophene spectra are immediately established. In the case of T3, some temporal spectral fluctuations are resolved. Although these irregularities are minor, the differences between T3 and Se3 were reproduced for different concentrations, indicating that Se3 is more stable at the metal surface. Still, in comparison with 3T,19 both T3 and Se3 show very high stability. We now turn to the spectral changes when going from ordinary Raman to SERS. From the results in Figures 3 and 4, the mode most strongly affected by the presence of the Ag surface is the CsC stretch, which increased considerably in intensity and softened ∼15 cm-1 to ∼1410 cm-1 in SERS. In the case of the other intense bands, related to the CsC and CdC modes, there were only minor shifts in positions. The Cs C inter-ring stretch at 1220 cm-1, which is hardly observed in the ordinary Raman spectra, is now clearly visible. The weaker CsS and CsSe modes where shifted ∼20 cm-1 to 950 and 970 cm-1, respectively. The SERS spectra of T3 and Se3 are clearly very similar, as was the case for the molecules in solution

or the solid state, although the overall frequency decrease of the Raman active modes was larger for T3 compared to Se3. The most important spectral information should be obtained directly via the terminal atoms, from the SsAg and SesAg stretch modes. However, these low-frequency modes, predicted by the calculations to be around 200 cm-1, are not seen in the SERS spectra due to their relative low intensity in a region where the spectra is dominated by the AgsCl bonds formed in the preparation of the Ag colloid. Instead, spectral information can be obtained from the CsS and CsSe modes, which indirectly depend on the interaction with the surface when forming SsAg and SesAg bonds, respectively. Looking more closely at these modes, there is, however, no significant difference in the behavior between the CsSe and CsS modes upon adsorption to Ag. This is clearly seen in Figure 7, where we compare the solid-state and the SERS spectra in the actual region. The lower intensity of the CsSe band coincides with a slightly higher intensity of the 1050 cm-1 CsH bending motion, a property preserved when going from the solid state to molecules on the surface. In connection to these SERS results, we performed additional DFT calculations that simulated the metal-molecule interaction by connecting the T3 and Se3 molecules with either one or two Ag ions through the thiol and selenol groups. This simple approach to form a metal-molecule complex has previously been shown to yield good agreement with experiments.26,34 As before, the acetyl groups were not included in the calculations. Compared to the free molecules, the calculated HOMO-LUMO transitions for the one-Ag and two-Ag models are substantially red-shifted, to about 700 and 800 nm, respectively. A comparison between the experimental SERS spectra and the different simulations is shown in Figure 7 and Table 2. The observed SERS spectra are well described by the T3-Ag and Se3-Ag complexes, since the positions of the ring vibrations are more correct, in contrast to the Ag-T3-Ag and Ag-Se3-Ag complexes; see Table 2. Also, the one-Ag model accounts for the spectral features not seen for the free molecules with correct relative intensities; see Figure 7. The positions for the modes directly influenced by the presence of the end groups should differ from those of the real situation, as in the case of the S/Ses C bonds and the inter-ring stretching modes. The ring vibrations are isolated from the coupling groups; however, the relative intensities of the ring vibrations are not predicted by any of the models. One explanation for this could be that the splitting of

25676 J. Phys. Chem. B, Vol. 110, No. 51, 2006 TABLE 2: Raman Shifts (cm-1) for the Most Intense Bands for the Experimental SERS and Calculated Metal-Molecule Complexes Bisthiol-terthiophene (T3) SERS

Ag-T3

Ag-T3-Ag

assignment

971 1054 1220 1409 1447 1522

938 1048 1203 1403 1442 1514

938 1049 1201 1392 1439 1502

CsS stretching in-plane CsH bend inter-ring CsC stretch CsC stretch CdC symmetric stretch CdC antisymmetric stretch

SERS

Ag-Se3

Ag-Se3-Ag

assignment

950 1050 1220 1413 1449 1525

915 1046 1202 1407 1443 1515

916 1046 1201 1401 1440 1507

CsS stretching in-plane CsH bend inter-ring CsC stretch CsC stretch CdC symmetric stretch CdC antisymmetric stretch

Biselenol-terthiophene (Se3)

the degenerate modes is smaller in the real situation and that they add up to one strong peak in the real spectra. Looking closely at the calculated spectra in Figure 7, one can resolve two antisymmetric CdC stretches close in positions. This type of small splitting of the degenerate states also explains the broad peaks in that region. 4. Discussion The self-assembly properties of T3 and Se3 are not wellknown, except for the case of adsorption in a dodecanethiol matrix.16-18 However, self-assembly on Au of R-functionalized terthiophenes, containing disulfide and alkanethiol anchoring groups, shows that the molecules coordinate to the surface exclusively via the linking groups upon formation of the Au-S bonds, while aligning the terthiophene unit almost perpendicular to the surface.35 A number of other conjugated oligomers that have R,ω-dithiol and R,ω-dithioacetyl end groups have also been reported to form SAMs on Au surfaces, in which one thiol group binds to Au while the second thiol moiety projects upward.23 Hence, similar properties are expected in the case of T3 and Se3. However, the chemistry of adsorption on flat Au substrates could, in principle, be different from that on silver particles covered by citrate and chloride (remaining from the preparation of the colloids). Still, formation of alkanethiol SAMs has been demonstrated on colloidal Au particles stabilized in the presence of citrate and chloride remains.36 In addition, the self-assembly properties studied in ref 23 were reported to be the same on silver as on gold. Supported by our calculations, which indicate metal-molecule formation only at one of the end groups, it seems probable the functionalized terthiophene molecules bind to the surface via one of the linkers in an orientation normal to the surface. Further details on the adsorption geometry are beyond the scope of this investigation. One of the most puzzling findings in this study is the bleaching phenomenon seen in Figures 4-6. Although bleaching is in itself not surprising for chromophores like T3 and Se3, it is difficult to understand why the final SERS intensity after bleaching should be independent of the initial molecular concentration; see Figure 5. As discussed in the seminal paper by Nitzan and Brus,37 photochemistry on nanoparticles can broadly be divided into two cases: photochemical reactions that occur on a short or a long time scale compared to the relaxation time within the photoexcited state, respectively. In the first case, photochemistry can be enhanced due to surface-enhanced

Svedberg et al. absorption, which scales as the electromagnetic field enhancement factor (M) squared. In the second case, one also needs to consider surface-enhanced decay from the photoexcited state, which scales with the factor Md2. As shown in ref 38, the ratio (M/Md),2 which is proportional to the probability to find the molecule in a photoexcited state, is always equal to or less than unity, even for electromagnetic hot spots in gaps between nanoparticles. Hence, a slow photochemical reaction that proceeds from a photoexcited state should in principle not be surface-enhanced. In the present case, we have no indications that T3 or Se3 are photolabile in solution or in the solid phase. Moreover, we find no signs of photoinduced decomposition, which should result in amorphous carbon spectral features34 at the high molecular concentrations where the bleaching effect is most prominent. This indicates that the bleaching process is not due to a photochemical reaction in the traditional sense, that is, one that leads to a change of the molecular structure and the associated vibrational features. We also note that the spectral characteristics of the bleached and the stable components of the T3 and Se3 spectra in Figure 5 do not differ, which strongly indicates that we only measure molecules that are attached to the Ag surfaces. However, the small differences in initial SERS intensity for the different concentrations, that is, a factor of 10-15 increase in initial intensity for a factor of 1000 increase in concentration, clearly show that the metal surfaces are more or less saturated with molecules at the higher concentrations. One possible explanation for the observed behavior could then be that the additional molecules that adsorb at high concentrations are so weakly attached to the metal surface that they simply photodetach upon Raman excitation and then go into solution. Photodetachment from the metal surface can be expected to be a fast process and therefore subject to surface enhancement. The stable component of the SERS spectrum would then correspond to contributions from molecules that are firmly chemisorbed to specific favorable sites, perhaps near crystal defects. If the density of such sites is low, one would expect a constant, that is, saturated, SERS intensity at even lower molecular concentrations, something that could explain why the final SERS intensity after bleaching is independent of the initial molecular concentration. However, a clear proof of this hypothesis requires further study, in particular measurements at even lower concentrations than what was used here. The question whether selenol functions should be used as alternatives to the more common thiol functions in the context of molecular electronics is interesting. In the case of aromatic molecules functionalized with thiol and selenol, it has been demonstrated that benzenethiolate possesses greater adsorptivity and stability than benzeneselenolate on Ag.39 Benzenethiolate has also been predicted to be more conductive than benzeneselenoate on Au.40 For alkanethiolate and alkaneselenolate, on the other hand, the Au-Se bond was found to be stronger and more covalent than the Au-S bond on Au nanoparticles.41 Recently, it was reported that S was a more conductive headgoup than Se for alkyl chains on Au.42 Hence, a stronger bond does not necessarily mean a higher conductance; instead, the critical parameter for conductance seems to be the band alignment between the metal and the molecule.40,42,43 In the case of the functionalized terthiophenes, Se3 and T3, it has been reported that the Se group provides better electronic coupling than S to Au.16-18 The spectroscopic study presented here shows small differences between T3 and Se3 in terms of the spectral stability. These differences indicate a better adsorptivity and stability for Se3. Hence, in the case of terthiophenes, it seems that the Se functionalization provides both higher conductivity and better

Studies of Terthiophenes for Molecular Electronics adsorptivity, which indicates that Se3 is a good choice for ME applications. 5. Conclusions In summary, we have presented a Raman analysis of the effect of thiol and selenol substitution on the vibrational spectra of terthiophene. Surface-enhanced Raman spectroscopy together with ab initio calculations allowed us to determine the distinctly different behavior of the molecules at a metal surface (Ag). We found that the presence of thiol or selenol end groups inhibits any structural damage during irradiation when the molecules are adsorbed to the Ag surface at low concentration. This is in contrast to the unfunctionalized analog 3T, which photodecomposes or photopolymerizes rapidly, irrespectively of the irradiation level. Furthermore, the SERS spectra show that the molecules form covalent bonds to the Ag surface with only one of the anchoring groups. In addition, small differences in the spectral development during illumination between T3 and Se3 point to a better adsorptivity for the selenol function on Ag. The work presented herein shows that SERS is a suitable tool for studies of molecular electronic devices, allowing for novel investigations of molecular linking groups and photochemical stability. In the future, the combination of ultrasensitive SERS spectroscopy and simultaneous (single molecule) electrical transport could increase the understanding of vibrational and electronic characteristics of molecular electronics even further, and hence benefit the development of actual devices. Acknowledgment. We acknowledge Sergey Kubatkin, Mohammad Kabir, and Jean-Philippe Bourgoin for kindly providing the T3 and Se3 molecules through the NANOMOL project. This work was supported with financial support from the the Swedish Research Council and with computer resources from SNAC. References and Notes (1) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378. (3) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (4) Fichou, D., Ed. Handbook of Oligo- and Polythiophenes; WileyVCH: Weinheim, Germany, 1999. (5) Akimoto, M.; Furukawa, Y.; Takeuchi, H.; Harada, I. Synth. Met. 1986, 15, 353. (6) Furukawa, Y.; Akimoto, M.; Harada, I. Synth. Met. 1987, 18, 151. (7) Zerbi, G.; Chierichetti, B.; Inga¨na¨s, O. J. Chem. Phys. 1991, 94, 4637. (8) Louarn, G.; Buisson, J. P.; Lefrant, S.; Fichou, D. J. Phys. Chem. 1995, 99, 11399. (9) Sarkar, U. K.; Pal, A. J.; Chakrabarti, S.; Misra, T. N. Chem. Phys. Lett. 1992, 190, 59. (10) Sarkar, U. K.; Chakrabarti, S.; Misra, T. N. Chem. Phys. Lett. 1992, 200, 55. (11) Compagnini, G.; Bonis, A. D.; Cataliotti, R. S.; Marletta, G. Phys. Chem. Chem. Phys. 2000, 2, 5298. (12) Compagnini, G.; Bonis, A. D.; Cataliotti, R. S. Mater. Sci. Eng., C 2001, 15, 37. (13) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783.

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