Thiophene-Based Oligomers Interacting with Silver ... - ACS Publications

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Thiophene-Based Oligomers Interacting with Silver Surfaces and the Role of a Condensed Benzene Ring A. Guarnaccio,*,† M. D’Auria,‡ R. Racioppi,‡ G. Mattioli,§ A. Amore Bonapasta,§ A. De Bonis,‡ R. Teghil,‡ K. C. Prince,∥ R. G. Acres,∥,⊥ and A. Santagata† †

CNR-ISM, UOS Tito Scalo, C/da S. Loja, 85050 Tito Scalo, Potenza, Italy Science Department, Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy § CNR-ISM UOS Montelibretti, via Salaria Km 29,300, C.P. 10, I-00015 Monterotondo Stazione, Roma, Italy ∥ Elettra Sincrotrone Trieste, Basovizza-Area Science Park, 34149 Trieste, Italy ‡

S Supporting Information *

ABSTRACT: A comparative study of DTBT (1,3-di(thiophen-2-yl)benzo[c]thiophene), Br−DTBT−Br (1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene), and a model 3T (2,2′:5′,2-terthiophene) oligothiophenes has been performed by investigating the interactions occurring between these compounds and both polycrystalline and single-crystal Ag surfaces. As a main result we report that the interaction of 3T with the Ag surfaces is followed by a fast, low-temperature polymerization. On the contrary, DTBT-like compounds, less reactive at low temperature, undergo thermally activated dissociation processes which hinder the possibility of a surface-induced polymerization of such compounds as a suitable synthetic strategy. Our results are supported by Raman and surface-enhanced Raman spectroscopy (SERS) measurements of the molecules in contact with a polycrystalline Ag substrate, showing the fingerprint of polythiophene chains in the case of 3T and a characteristic S−S stretching signal compatible with ring-opening processes in the case of DTBT-like compounds, not detected for 3T. Moreover, high-resolution X-ray photoelectron spectroscopy (XPS) measurements, probing carbon 1s and sulfur 2p core levels of the same molecules deposited onto a Ag(110) single crystal, confirm the suggestion of a ring-opening process by a C−S bond cleavage induced by both the interaction with silver and thermal treatments. Near-edge X-ray absorption fine structure (NEXAFS) carbon K-edge measurements complete the information, showing a common tendency of the molecules to lay flat on the Ag surface. Finally, density functional theory (DFT) calculations have been carried out to investigate the interaction and molecular conformation/ orientation at the 3T and DTBT/Ag(110) interfaces. Theoretical findings support the results of measurements and suggest that a fine interplay between structural and electronic modifications of the benzo-fused rings of DTBT-like compounds in contact with the Ag surface is responsible for the peculiar reactivity of such molecules.

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INTRODUCTION The study of the adsorption patterns, reactivity, and electronic structure of organic S-containing oligomers interacting with metal surfaces plays an important role in modern surface science and can foster new technological applications.1 It is well known that the interface between metal and organic components is of the utmost importance in determining the overall properties of such systems.2,3 In particular, thiophenebased compounds are considered as interesting starting materials for the preparation of organic conducting polymers.4 These compounds are characterized by long-chain conjugated backbones and are currently used as functional components in electronic devices.5,6 In this framework, recent synthetic strategies focus on the polymerization of oligomers confined and promoted by metal surfaces. Thus far, ultra-high vacuum (UHV) deposition of these building blocks has already been © XXXX American Chemical Society

used to obtain ultrathin polymer layers that exhibit 2D ordered structures.7 One of the most common ways to perform this kind of synthesis is the Ullmann-type reaction involving halogenated thiophene containing molecules in contact with metallic substrates.8 The metal surface acts as a catalyst in the cleavage of C−halogen bonds and stabilizes reaction intermediates characterized by new C−C bonds. Successful attempts of such reaction strategy have been reported in the case of different transition metals such as copper,9 gold,10 and silver.11 We report here our findings regarding the stability and the potential reactivity of thiophene-containing oligomers, namely, 1,3-di(thiophen-2-yl)benzo[c]thiophene (DTBT) and its diReceived: September 7, 2015 Revised: November 17, 2015

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DOI: 10.1021/acs.jpcc.5b08733 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

thiophene (Br−DTBT−Br) compounds were synthesized following recently published procedures.16,17 Raman and SERS Spectroscopy. Micro-Raman and SERS measurements were carried out in backscattering configuration by a HORIBA LabRam 800 HR apparatus, equipped with an edge filter, two gratings (600 lines/mm and 1800 lines/mm), and an Olympus microscope with 10×, 50×, and 100× objectives. Excitation was performed with 632.8 nm radiation from a He−Ne laser source. The laser power was varied by neutral density filters in order to avoid sample degradation. The laser spot size impinging on the sample surface was about 5 μm in diameter when the 100× microscope objective was used. The spectrometer was connected to a Peltier-cooled CCD detector. A spectral resolution of about 4 cm−1 was obtained by the holographic grating with 600 lines/mm. Pure bulk powder samples were used for Raman characterization. The substrates used for SERS spectra were prepared depositing Ag nanoparticles (NPs) on Si(100) substrates by pulsed laser ablation producing an Ag nanostructured film active as SERS substrate.18 The following instrumental and experimental conditions were used: Nd:glass laser (Twinkle, Light Conversion) 250 fs, 527 nm, 10 Hz; energy ≈ 3 mJ; laser spot size 2 × 10−4 cm2; pressure 10−4 Pa; ablation time 20 min; distance of the silicon wafer from the target 2 cm. The different oligothiophene thin film samples for SERS were prepared by immersion of the Ag nanostructured film in a 10−3 M ethanol solution for 2 h and subsequent washing by immersion in pure ethanol for 1 h. Synchrotron XPS and NEXAFS Spectroscopy. XPS and NEXAFS spectra were collected at the Materials Science Beamline (MSB) at the ELETTRA Synchrotron (Trieste, Italy). The beamline was equipped with a SPECS Phoibos 150 electron energy analyzer. S(2p), C(1s), and Ag(3d) spectra were collected in normal emission (NE) geometry (60° incidence/0° emission) using photon energy hν = 260, 30, and 430 eV, respectively, with a corresponding combined resolution (beamline + analyzer) of 0.33 eV. The base pressure of the analysis chamber was 2 × 10−10 mbar. Single-crystal Ag surfaces were cleaned by repeated cycles of Ar+ ion sputtering and annealing, and the cleanliness and surface structure were monitored by XPS and LEED until contaminants (typically C compounds) were below the detection limit. Once a clean substrate was prepared it was transferred into the attached UHV preparation chamber (base pressure 5 × 10−9 mbar or better) where the molecules were evaporated using a custombuilt Knudsen cell-type evaporator. The evaporation chamber pressures were 4.4 × 10−8 mbar for 3T, 7.3 × 10−8 mbar for DTBT, and 7.0 × 10−8 mbar for Br−DTBT−Br using an the evaporation rate of 0.2 °C/min. Following deposition, the sample was transferred back into the analysis chamber, and reference spectra for the as-deposited molecules were collected before flashing the surface to remove weakly bound multilayers. NEXAFS was performed at the C Kedge using the carbon KVV Auger yields at normal incidence (NI, θ = 90°) and grazing incidence (GI, θ = 10°), where θ represents the angle between the propagation vector of the photon beam and the Ag(110) surface plane (see upper part of Figure S4 of Supporting Information). The raw XPS data were normalized to the intensity of the photon beam. The organic film thickness evaluations were performed using Hill’s equation assuming a homogeneous thin film.19 The thickness was calculated from the ratio of the peak area of photoelectron signals coming from the substrate overlaid with

brominated analogue 1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene (Br−DTBT−Br), prepared by using synthetic procedures discussed elsewhere,12−15 interacting with Ag surfaces. Our results have been compared with those obtained in the case of the commercially available 2,2′:5′,2-terthiophene (3T) oligothiophene. 3T is a well-understood model compound which can facilitate by comparison the assignment of spectroscopic features related to the benzo-fused systems. The molecular structures of the investigated compounds are sketched in Figure 1.

Figure 1. Molecular structures of 1,3-di(thiophen-2-yl)benzo[c]thiophene (DTBT), 1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene (Br−DTBT−Br), and 2,2′:5′,2-terthiophene (3T) oligothiophenes.

The presence of a simplified sequence such as that offered by 3T is of particular help in the case of the spectral features of the molecules interacting with the two kinds of Ag surface taken into account in our investigation: (i) a surface-enhanced Raman spectroscopy SERS-active nanostructured Ag film and (ii) a single-crystal Ag(110) surface. The latter substrate has been considered, in particular, because a UHV deposition of brominated oligothiophene bricks onto the Ag(110) surface followed by thermal annealing has been reported to effectively promote the Ullman polymerization of similar thiophene derivatives.8 We employed a series of different characterization techniques to shed light on the interaction of these compounds with the Ag surfaces: (i) Raman and SERS spectroscopies; (ii) highresolution synchrotron XPS of carbon 1s and sulfur 2p core levels related to molecules deposited under various experimental conditions; (iii) synchrotron-based NEXAFS measurements. Moreover, the measurements have been supported by DFT-based ab initio calculations. Our results show that the Ag surface is far from being inert, not only with respect to the 3T model compound but also especially for benzo-fused thiophene-based oligomers. In particular, such DTBT-like compounds undergo unexpected reactive processes leading to molecular dissociation. The joint experimental−theoretical approach used here is therefore able to provide insight into the reactivity and stability of DTBT-like oligothiophenes in contact with Ag surfaces, where the role of the condensed benzene ring can strongly affect their behavior and therefore their use and applications.

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EXPERIMENTAL AND THEORETICAL METHODS Oligothiophenes. The 2,2′:5′,2-terthiophene (3T) 99% was purchased from Sigma-Aldrich and used without further purification, while the 1,3-di(thiophen-2-yl)benzo[c]thiophene (DTBT) and the 1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]B


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The Journal of Physical Chemistry C the thin film and the bare substrate using the inelastic mean free path (IMFP). IMFP was evaluated using the QUASES IMFP TPP2M software (Code written by Sven Tougaard, 2000− 2010, Quases-Tougaard Inc.) using the Tanuma, Pawell, and Penn’s formula20 in which the material characteristics were considered. In our case the well-known parameters related to 3T were taken into account, and these were used, to a first approximation, for DTBT too using a IMPN value of 5.56 Å. The thickness d of the organic layer is expressed in the following and has been based on Ag 3d5/2 peak area fitted by a Gaussian deconvolution: d = −ln(A0/A) × IMFP × sin θ, where IMFP = 5.56 Å, Θ = 90° is the angle between the metal surface and the analyzer, A0 = bare Ag 3d5/2 gauss-fitted area, and A = Ag 3d5/2 gauss-fitted area related to the metal substrate covered by organic layer. The standard deviation associated with the evaluation has been calculated using the following equation: σ(d) = ((σA0/A0) + (σA/A)). Density Functional Theory (DFT) Methods. Two parallel series of ab initio calculations have been performed to elucidate the properties of the investigated oligothiophenes, both isolated and in contact with the Ag surface. A first batch of DFT calculations of the properties of isolated molecules has been carried out by using the ORCA suite of programs21 in a localized-basis-set framework. In detail, the Kohn−Sham orbitals have been expanded on a def2-TZVPP Gaussian-type basis set.22,23 The same basis has been also used as an auxiliary basis set for Coulomb fitting. The molecular geometry has been fully optimized at the B3LYP level of theory,24,25 also including dispersion forces calculated by using the DFT-D3 approach26 with the Becke−Johnson damping.27−29 Raman spectra of isolated molecules have been also calculated by using the same level of theory. A second batch of DFT calculations of the properties of the 3T and DTBT molecules in contact with the Ag(110) surface have been performed by using the Quantum ESPRESSO suite of programs30 in a plane-wave pseudopotential framework. Total energies have been calculated by using ultrasoft pseudopotentials.31 Satisfactorily converged results have been achieved by using cutoffs of 40 Ry on the plane waves and of 320 Ry on the electronic density. The interaction of oligothiophene molecules with the Ag surface have been investigated by using periodically repeated supercells modeled by adding 13 Å of empty space to a 17 Å × 18 Å surface slab formed by five layers of bulk Ag parallel to the (110) crystal plane. Geometry optimization procedures were performed by fully relaxing the positions of all the atoms in the supercell, except for those belonging to the bottom layer of the Ag slab. A 2 × 2 × 1 k-point mesh has been used to sample the Brillouin zone. Particular care has been given to the simulation of dispersive interactions. In this regard, the exchange-correlation functional has been constructed by adding an ab initio nonlocal van der Waals correlation contribution32,33 to the semilocal gradient-corrected PBE functional.34

Figure 2. Comparison between experimental and calculated Raman spectra related to (a) 3T, (b) DTBT, and (c) Br−DTBT−Br pure bulk compounds. The luminescence contribution has been subtracted from the measured resonance Raman spectra. Numeric labels refer to the peak assignments reported in Table 1.

The spectra reported in Figure 2 show a good agreement between experimental and calculated Raman features. Among all signals, the two most intense vibrational (CC stretching) modes play a key role in the characterization and study of each molecule’s reactivity with the silver surface. Indeed, the experimentally observed bands at 1533 cm−1 (3T), 1508 + 1530 cm−1 (DTBT), and 1490 cm−1 (Br−DTBT−Br) are dominated by antisymmetric CC stretching modes. On the grounds of the calculated Raman lines reported in Table 1, no distinctions between signals have been detected experimentally for the predicted antisymmetric CC stretching modes related to central and side thiophene rings. Furthermore, the 1463, 1449 (shoulder at 1431.81 cm−1), and 1433 cm−1 bands are dominated by symmetric CC stretching modes, while the 1428, 1348, and 1354 cm−1 bands are related to single C−C stretching motions, respectively for 3T,35 DTBT, and Br− DTBT−Br oligothiophenes. These latter bands also contain a contribution from C−S stretching, as reported elesewhere.36

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RESULTS AND DISCUSSION Raman and SERS Spectra. Raman and SERS spectroscopies permit the investigation of the vibrational properties of both the pure bulk materials and those of the compounds deposited on a Ag SERS active substrate, with the latter ones prepared in submonolayer coverages. For the purposes of this study, the discussion about the assignment of the Raman and SERS bands will be reduced to some key features. The experimental and calculated Raman spectra of 3T, DTBT, and Br−DTBT−Br are presented in Figure 2a−c. C


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Table 1. Summary of the Main Experimental Raman Vibration Modes and Assignments Related to 3T, DTBT, and Br−DTBT− Br Molecules Raman shift (cm−1) 3T

DTBT

labels

exp

calcd

1 2 3 4

698 1056 1428 1463

672 1073 1393 1524

5

1533

1604

exp

1348 1432(shoulder) 1449 1508 1530

Br−DTBT−Br calcd 591 1083 1381

exp

calcd

assignments

1058 1354 1433

601 1093 1383 1494

C−S stretching in-plane C−H sym. bend. (side and central thiophenes) C−C stretching CC sym. stretching

1504 1594

CC antisym. str. (side and central thiophenes) 1594

The experimental bands at 1056 and 1354 cm−1 are attributed to in-plane symmetric C−H bending motions of the central and side thiophene rings deformations, respectively, for 3T and Br− DTBT−Br. While the experimental 698 cm−1 peaks correspond to C−S stretching (see Table 1), evident only in the case of 3T molecule and, because of the background noise, not appreciable for both the benzo-fused systems studied here. In summary, five main peaks have been experimentally determined for the model compound as reported in Table 1, in agreement with literature data already reported for thiophene (T)37 and 3T.35 Assignment of vibrational modes was also achieved for the two benzofused systems, both characterized by a blue shift of Raman vibrational modes compared with 3T. SERS measurements of the three investigated molecules deposited on a nanostructured Ag substrate are shown in Figure 3 in order to better characterize the different reactivity of the three systems with respect to the silver surface.

influencing the electronic structure of the starting material in the obtained Ag composite system. Indeed, after excitation of the oligothiophenes in the film substrates, excitonic states are populated depending on the relative molecular orientation and intermolecular coupling occurring on Ag nanostructured films. The deactivation of such states can occur via radiative or nonradiative decay. The evident SERS luminescence quenching of molecules indicates that nonradiative relaxation processes of excitonic states are dominant in the films (e.g., internal conversion, exciton−exciton annihilation, formation of triplet excitons). Moreover, this luminescence quenching can be also consistent with an oligomerization process leading to a variation of the molecular conformation and electronic state of the resulting polymerized system. In fact, a change in shape and relative intensities of the vibrational features for the adsorbed molecules with respect to the bulk samples have been clearly detected. The main spectral assignments are the C−S stretching modes at 684, 689, and 689 cm−1 and the C−H bending modes at 1053, 1052, and 1059 cm−1 for 3T, DTBT, and Br−DTBT−Br, respectively. The most pronounced changes in the SERS spectrum have been detected for the 3T molecule for which there is (i) a decrease in the ratio between the symmetric and the antisymmetric CC stretching modes and (ii) a pronounced red shift of the band related to the CC antisymmetric stretching mode from 1533 (Raman) to 1528 cm−1 (SERS) observed. It is well known that the main spectral fingerprint for the length n of a thiophene chain nT, corresponding to the polymerization degree, concerns the ratio between the symmetric and the antisymmetric CC stretching modes.38−40 Furthermore, the latter mode red shifts, broadens, and decreases in intensity as the chain length increases. From SERS data related to 3T, these criteria seem to be satisfied in the experimental conditions adopted here. Such a broadening may originate from the conformational disorder of the adsorbed molecules and/or from the presence of different conjugation lengths caused by a distribution of longer oligomer chains or polymers. As reported elsewhere,41 the proposed mechanism is related to a photoinduced polymerization under laser irradiation during SERS acquisition and/or catalyzed by the Ag surface, depending on the organic layer thickness deposited on it. It follows that, in general, the SERS 3T spectrum resembles that of a polythiophene sample.35 With respect to the possible molecular orientations of 3T oligomers and polymers on Ag surface, we can state that the amplifications of the in-plane ring motions in the 600−1200 cm−1 spectral region observed in SERS spectra give the indication that the 3T molecules are arranged with a certain tilt angle with respect to the Ag nanostructured surface,42 as it will be discussed in the next sections.

Figure 3. Experimental SERS spectra related to 3T, DTBT, and Br− DTBT−Br deposited on nanostructured Ag films. Numeric labels refer to the peak assignments reported in Table 2.

Looking at the differences between Raman (Figure 2a−c) and SERS (Figure 3) spectra, a clear change in the signal features can be observed. In particular, a luminescence band’s blue shift (Figure S1) has been observed in Raman spectra for benzo[c]thiophene-containing compounds rather than 3T. At the same time, a consistent decrease in the photoluminescence background has been observed in SERS spectra for all three oligothiophenes. This last observation can be related to molecular rearrangements occurring on the Ag film surface D


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The results outlined so far suggest that for 3T oligomers a polymerization reaction catalyzed by the Ag surface occurs as the principal process, as opposed to the benzo-fused oligothiophene systems (DTBT and Br−DTBT−Br), for which a ring-opening reaction, favored by the Ag surface, can occur. Synchrotron High-Resolution XPS and NEXAFS Measurements. The following analysis has been performed on DTBT, Br−DTBT−Br, and 3T oligothiophenes deposited from pure powders onto a Ag(110) single-crystal surface by UHV evaporation as described in the Experimental and Theoretical Methods section. By monitoring the evolution of the XPS S(2p) and C(1s) core-level spectra during subsequent flashing and/or annealing treatments at temperatures ranging between 100 and 140 °C, it has been possible to evaluate the stability and the reactivity of each oligothiophene/Ag(110) system. The experimental deposition and treatment conditions and the corresponding thickness (d) of the resulting organic film determined by attenuation of the Ag XPS signal are reported in Table 3. The S(2p) and C(1s) core-level spectra acquired after each treatment are shown in Figures 4 and 5 for 3T, DTBT, and Br− DTBT−Br, respectively. The peak assignments related to each spectrum are reported in Tables 4 and 5 for XPS S(2p) and C(1s) spectra, respectively. Figure 4a presents the 3T spectra showing that immediately after the deposition (10 min@80 °C) only one S(2p) doublet with a S(2p)3/2 binding energy of 164.9 eV is basically present in the spectrum. Such a doublet has been assigned to bulk material probably polymerized onto the Ag(110) surface after the deposition of monomeric 3T molecules. This confirms the fact that a fast polymerization process can occur immediately after the UHV deposition of 3T at a temperature of 80 °C. After the first annealing (5 min@100 °C) the S(2p) signals visibly shift in energy, indicating a strong difference in electron density. A shifted S(2p)3/2 signal is detected at the lower binding energy of 164.5 eV, which is assigned to the sulfur atoms still bonded to carbon in the intact 3T system although located at the interface rather than in the bulk. This behavior can be reasonably attributed to the Pauli repulsion due to charge interaction at the interface between the planar molecules and the metal surface. This finding is in good agreement with the available literature in which a value of 164.5 eV has been reported for sulfur signals of intact thiophene adsorbed onto the Ag(111) surface.44 After increasing the Ag substrate temperature (annealing for 5 min@140 °C) further desorption of multilayer molecules results in the formation of an adsorbed monolayer 3T (d = 3.22 ± 0.09 Å), whose fingerprint is represented by the appearance, together with the sulfur atoms of the adsorbed molecule (Sads) of two new S(2p) doublets indicating a different chemical state

The SERS spectra related to DTBT and Br−DTBT−Br (Figure 3) show a very complex pattern of weak lines in the region 1100−1300 cm−1 and the CC stretching frequencies related to the benzo-fused systems which seem to be affected by adsorption on the silver surface. Comparing Raman (Table 1) and SERS (Table 2) data, it can be verified that the DTBT Table 2. Summary of the Main Experimental SERS Vibration Modes and Assignments Related to 3T, DTBT, and Br− DTBT−Br Molecules SERS shift (cm−1) labels 1 2 3 4 5

3T

DTBT

Br−DTBT−Br

assignments

684 1053 1447 1528

479 689 1052 1457 1504

473 689 1059 1448 1497

S−S stretching C−S stretching C−H bending CC sym. stretching CC antisym. stretching

displays a slight shift of the CC symmetric stretching from 1449 (Raman) to 1457 cm−1 (SERS) as well as from 1508/ 1530 (Raman) to 1504 cm −1 (SERS) for the CC antisymmetric stretching mode. For Br−DTBT−Br the same shift has been also observed related to the CC symmetric stretching from 1433 (Raman) to 1448 cm−1 (SERS). In general, the major bands in the CC stretching mode ranges appear as not well-resolved broad bands that are typical of polymeric films. These observations suggest that for 3T a polymerization reaction onto the nanostructured Ag surface can be reasonably considered, while for the benzo-fused systems this behavior has not been strictly observed, or otherwise, polymerization does not seem to be the principal process occurring at the silver surface. Although in Raman spectra the CC stretching modes of the starting bulk benzo-fused materials appear much more widened than in 3T, the further observed SERS enlargement is not comparable to previous Raman features as it happens for 3T. This behavior can be induced due to a disordered system caused most probably by the compound degradation rather than its polymerization. In agreement with this hypothesis, it follows that, the most important feature observed only for DTBT and Br−DTBT−Br benzo-fused systems can be related to the SERS signals occurring at 479 and 473 cm−1, respectively. This band which is reported at 523 cm−1 in ref 43 has been assigned to a S−S stretching mode. The assignment of such lines implies that these benzo-condensed molecules undergo a thiophene’s ring opening, which includes a C−S bond cleavage, followed by the formation of atomic sulfur aggregates on the Ag surface. This peculiar reactivity of benzo-fused systems can be, therefore, favored/catalyzed through contact with an Ag surface, also at room temperature.

Table 3. Experimental Conditions Related to Deposition and Further Treatments (annealings and flashing) at Different Temperatures for 3T, DTBT, and Br−DTBT−Br Moleculesa deposition/treatments conditions deposition: 10 min@80 °C annealing: 5 min@100 °C annealing: 5 min@120 °C annealing: 5 min@140 °C a

3T layer thickness, d ± σ(d) 6.00 3.65 3.40 3.22

± ± ± ±

0.10 0.09 0.09 0.09

DTBT layer thickness, d ± σ(d)

deposition/treatments conditions Å Å Å Å

deposition: 5 min@60 °C flash: 100 °C flash: 130 °C annealing: 5 min@140 °C

4.40 3.40 3.07 1.94

± ± ± ±

0.10 0.09 0.09 0.09

Å Å Å Å

deposition/treatments conditions

Br−DTBT−Br layer thickness, d ± σ(d)

deposition: 20 min@60 °C annealing: 10 min@120 °C annealing: 5 min@200 °C

3.77 ± 0.1 Å 2.05 ± 0.1 Å 1.85 ± 0.1 Å

After each treatment the organic layers’ thicknesses (d) with the associated standard deviation σ(d) have been evaluated and reported. E


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Figure 5. High-resolution synchrotron XPS C(1s) core-level spectra acquired at photon energy hν = 330 eV and related to (a) 3T, (b) DTBT, and (c) Br−DTBT−Br molecules deposited onto the Ag(110) surface after deposition and subsequent thermal treatments.

Figure 4. High-resolution synchrotron XPS S(2p) core-level spectra acquired at photon energy hν = 260 eV and related to (a) 3T, (b) DTBT, and (c) Br−DTBT−Br molecules deposited onto Ag(110) surface after deposition and subsequent thermal treatments.

Table 4. High-Resolution Synchrotron XPS S(2p) CoreLevel Peaks Related to S(2p) Spectra of 3T, DTBT, and Br−DTBT−Br Deposited onto Ag(110) Surface and Related Assignments

of the remaining organic layer. The new S(2p) doublets, characterized by different relative intensities (Figure 6), can be attributed to molecule degradation. The former one (Sthiolate) characterized by S(2p)3/2 signal at 161.9 eV can be related to species originating from sulfur in a partially broken thiophene fragment (e.g., thiolate C−S−Ag species),45while the latter one labeled Sat (S(2p)3/2 = 160.6 eV) can be related to chemisorbed sulfur on Ag(110) no longer covalently bonded to carbon atoms and probably assigned to atomic sulfur and/or Ag2S species46−50 originating from a dissociated oligothiophene system. The ratio between these doublets is larger for the adsorbed undissociated molecule than for the two species formed after the thermal treatments (92%, 2%, and 6%). It follows that 3T is a relatively stable compound when deposited onto the Ag(110) surface since, even after long thermal treatments at higher

S(2p) core-level peaks on Ag(110) 3T 164.9 164.5 161.9 160.6

DTBT eV eV eV eV

Br−DTBT−Br

164.1 eV

163.5 eV

160.9 eV

160.8 eV

peak

assignments

S(2p)3/2 S(2p)3/2 S(2p)3/2 S(2p)3/2

bulk molecule adsorbed molecule dissociated molecule

temperatures (5 min@140 °C), its dissociation degree remains quite low and constant. The other two thiophene-based systems are characterized by a completely different behavior. In fact, the DTBT and Br− DTBT−Br S(2p) spectra (Figure 4b and 4c) show two wellvisible S(2p) doublets soon after deposition of the molecules F


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assigned to sulfur no longer covalently bonded to carbon atoms (Sat), as already discussed in the case of 3T. The presence of sulfur species chemisorbed on the Ag surface supports the hypothesis of C−S bond breaking, already reported in the case of thiophene adsorbed on Pt(111).51 The dissociation degree of the DTBT molecule can be derived from the analysis of the Sads and Sat contributions, with the adsorbed undissociated molecule representing 58% of the deposited amount of DTBT, while the dissociated species become the principal product reaching 66% after annealing (5 min@140 °C). A similar behavior has been obtained for the Br−DTBT−Br molecule after deposition and further annealing treatments. Our hypothesis of C−S bond breaking related to the present benzo-fused oligothiophene systems is in agreement with what was already reported by Terzi et al.52 They investigated a different thiophene-based system (3,4-ethylenedioxythiophene) undergoing a strong ring stress−strain causing the same kind of molecular dissociation observed here when the molecule interacts with the Au(111) surface. In that case, the molecular species formed after C−S bond cleavage reaction was associated with a S(2p)3/2 peak at 161.1 eV.52 The C(1s) core-level spectra for the three systems (Figure 5a−c) exhibit relatively broad and asymmetric peaks. In particular, for the 3T molecule after deposition (10 min@80 °C) the C(1s) spectrum appears as an asymmetric peak shifted at higher BE than that corresponding to the DTBT peak in which, most probably, more than one molecular species is present. This asymmetry, together with the shift, confirms that a possible 3T polymerized film is present on the Ag(110) surface. The major components are located at binding energies (C(1s) 285.9 and 285.3 eV) usually associated with polythiophene films.53 After thermal treatments, the intensities of the C(1s) spectra start to decrease, indicating the progressive narrowing of the film thickness. At the same time, the C(1s) spectra are broadened and shifted toward lower BEs (C(1s) 285.0 eV), most probably indicating the lack of any preferential orientation of the remaining molecules.52 Regarding the C(1s) spectra related to DTBT-like compounds (Figure 5 b and 5c) we note that no peaks belonging to polymerized species seem to be present in the binding energy range already discussed. Immediately after the deposition, only the C(1s) peak (284.6 and 283.6 eV for DTBT and Br−DTBT−Br, respectively) related to undissociated molecules adsorbed onto the metal surface is present as the main signal. This is consistent with the aromatic sp2 carbon

Table 5. High-Resolution Synchrotron XPS C(1s) CoreLevel Peaks Related to C(1s) Spectra of 3T, DTBT. and Br− DTBT−Br Deposited onto Ag(110) Surface and Related Assignments C(1s) core-level peaks on Ag(110) 3T 285.9 eV 285.3 eV 285.0 eV

DTBT

Br−DTBT−Br

assignments bulk molecule

284.6 eV 284.5 eV 285.1 eV

283.6 eV 284.0 eV 284.3 eV

adsorbed molecule dissociated molecule

Figure 6. XPS S(2p) core-level (hν = 260 eV) spectra of 3T deposited onto the Ag(110) surface. Circles represent raw experimental data, dotted line represents fitted data, and labeled curves are fitted Voigt S(2p)3/2 components which are related to 3T annealed for 5 min@140 °C after the initial deposition (10 min@80 °C).

onto the Ag surface. Also, in this case the spectra fittings (Figure 7) reveal the presence of a doublet at higher binding energies (S(2p)3/2 164.1 and 163.5 eV for DTBT and Br− DTBT−Br, respectively), which has been assigned to sulfur atoms of the adsorbed molecule (Sads) located at the silver interface, while a red-shifted doublet (S(2p)3/2 160.9 and 160.8 eV for DTBT and Br−DTBT−Br, respectively) has been

Figure 7. XPS S(2p) core-level (hν = 260 eV) spectra of DTBT deposited on Ag(110). Circles represent raw experimental data, dotted line represents fitted data, and labeled curves are fitted Voigt S(2p)3/2 components related to (a) DTBT after deposition for 5 min@60 °C and (b) DTBT annealed for 5 min@140 °C after deposition. G


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The Journal of Physical Chemistry C backbone contributions observed for other thiophene derivatives.52,54−56 By subsequent annealing at higher temperatures, the starting spectra begin to broaden and highlight the presence of one more C(1s) component as a shoulder (284.5 and 285.1 eV for DTBT and 284.0 and 284.3 eV for Br−DTBT−Br) probably belonging to the species produced after C−S bond cleavage (Table 5). While the assignment of the main C(1s) peak is straightforward, the origin of this shoulder is not clear enough. This has been observed in previous works related to different thioaromatic compounds where it was alternatively assigned to carbon atoms bonded to sulfur atoms or to shakeup processes occurring in the aromatic system.57−63 These findings are once more compatible with the hypothesis of a fast polymerization of 3T deposited onto the Ag(110) surface as the principal process, whose products are present on the Ag surface immediately after 3T deposition. By subsequent annealing at higher temperatures (100−140 °C), the multilayer system desorbs, leading mainly to adsorbed molecules onto the Ag(110) surface, and the C−S bond cleavage can be considered as a secondary process occurring in negligible percentages after annealing at higher temperatures. On the contrary, no evidence of oligomers polymerization is detected even at 140 °C for DTBT and 200 °C for Br−DTBT−Br, while the C−S bond breakage occurs in a high fraction of the adsorbate, that is a quite different behavior than that discussed for 3T. An explanation of such behavior is connected to the presence of the benzo-condensed system which could contribute to force the geometry of both the DTBT and the Br−DTBT−Br compounds in a conformation having the benzo[c]thiophene unit out of the terminal thiophenes plane, thus weakening the C−S bond, as opposed to the 3T more planar one. The XPS study can also provide additional information about the physical-chemical aspects of the desorption process. As it has been previously reported, a progressive desorption of the condensed multilayers takes place by increasing the temperature (Table 3). The interaction with the substrate, more relevant for the first layer in contact with the surface when there are no more organic layer above, produces also a stronger perturbation of the C−S bond with an extended orbital overlap from the metal, influencing mainly the carbons bound to sulfurs. In this way, the C−S bonds are more likely to undergo cleavage and Ag−C bond formation possibly occurs (Figure S2).64 NEXAFS C K-edge measurements flanked by DFT calculations of interacting oligothiophene/Ag(110) systems have been performed. The NEXAFS experiment generally consists in the excitation of core electrons to unoccupied states, and in the case of well-defined orientation of the investigated systems, it provides valuable information when the spectra are collected as a function of the X-ray beam incidence angle. More specifically, such core-level excitation processes are governed by selection rules, like the change of quantum number of the angular momentum (Δl = ± 1) and the direction of the electric field vector E of the linear polarized synchrotron light. Therefore, besides information about the electronic structure of unoccupied states, NEXAFS can also provide indications about the orientations taken from the molecules when interacting with the metal surface. According to dipole selection rules, core excitations from carbon 1s to π* (σ*) orbitals are induced when the electric vector of the incident beam is perpendicular (parallel) to the molecular plane. Figure 8 shows measured C K-edge NEXAFS

Figure 8. Carbon K-edge NEXAFS spectra of (a) 3T and (b) DTBT deposited by UHV deposition onto the Ag(110) surface recorded at normal (90°) and grazing (10°) photon incidence angles. A model of the molecules interacting with the surface is also shown.

spectra of 3T and DTBT UHV deposited onto the Ag (110) surface, taken at two different angles of the incident beam, i.e., normal (90°) and grazing (10°) with respect to the surface. A first analysis of the spectra is based on the comparison between such measurements and spectra related to the similar kind of molecules available in literature as well as our calculated C K-edge NEXAFS spectra of the isolated 3T and DTBT molecules (Figure S3). The results of such analysis permit a first identification and assignment of NEXAFS peaks. All carbon K-edge assignments have been finally confirmed by further analysis of ab initio results focused on the C contribution to the unoccupied electronic states of 3T and DTBT interacting with the Ag surface. In the case of the 3T/Ag(110) system, the main peaks appearing in Figure 8a have been collected and assigned in Table 6. The following contributions can be identified: (i) π1* peak at 285.2 eV; (ii) π2*(CC and C−H) + σ* (C−S) peak at 288.4 and 287.0 eV, respectively, and (iii) σ* (C−C) peak at 293.0 eV. It can be observed at first sight that the π1* and σ* (C−C) peaks vary in an anticorrelated manner: the π1* feature H


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atoms or to spontaneous fragmentation. However, significant absorption energy values (1.4 eV for 3T and 1.6 eV for DTBT, with a spread of ∼0.1 eV when different adsorption sites are tested) have been calculated. The molecule surface interaction is due to the strong dispersion forces which characterize the interaction of large π-conjugated molecules with metal surfaces.67−69 This suggests that the molecules, not easily desorbed from the surface, are quite free to move at low coverage. The most stable configurations of the 3T/Ag(110) and DTBT/Ag(110) systems found by the present simulations are shown in Figure 9. In the case of 3T each thiophene unit is

Table 6. Carbon K-Edge NEXAFS Peaks of 3T/Ag(110) and DTBT/Ag(110) Systems with Related Assignments carbon K-edge spectra 3T 285.2 287.0 288.4 293.0

DTBT eV eV eV eV

284.6 285.8 287.2 288.3 293.0

eV eV eV eV eV

assignments

ref

πB* (CC) π1*(CC) σ* (C−S) π2* (CC and C−H) σ* (C−C)

65 66

increases passing from 90° to 10° incident light angles, while the σ* feature decreases. Such angular dependence can be expected for a quite ordered film presenting a planar πconjugated system, and it has been evidenced for both 3T and DTBT systems. The NEXAFS spectra of the DTBT/Ag(110) system (Figure 8b and corresponding data reported in Table 6) display a richer structure than 3T/Ag(110), in agreement with its more complex chemical properties. Actually, in addition to the two terminal thiophene units, DTBT also contains a benzo[c]thiophene central unit which is expected to show a characteristic fingerprint in the NEXAFS spectra. The main contributions identified in the DTBT C K-edge spectra shown in Figure 8b are (i) πB*(CC) at 284.6 eV, (ii) π1*(CC) at 285.8 eV, (iii) π2*(CC and C−H) + σ* (C−S) at 288.3 and 287.2 eV, respectively, and (iv) σ* (C−C) at 293.0 eV. The energy of all such lines matches well with the energy of the corresponding 3T peaks that are π1*, π2* + σ* (C−S), σ* (C−C), although only a slight shift toward higher photon energies is observed for DTBT signals. Consequently, these peaks were assigned mostly to the thiophene units. The remaining πB* peak has been related to the benzo-fused ring, in close agreement with the analysis of theoretical NEXAFS results discussed in the Supporting Information. In the case of DTBT, the angular variation is more difficult to discern compared with 3T; however, it can be determined that, for DTBT, the π1* peak behaves similarly to π1* of the 3T molecule, which suggests that a similar lying down orientation of thiophene subunit occurs. In the same way, the πB* signal behaves as the π1* peak, suggesting that also the benzo-fused subunit has still a reasonable tendency to lie down on the surface. In this regard, the following analysis of DFT results provide more details on the different configurations of as-deposited 3T and DTBT molecules interacting with the Ag surface. We finally note that structural and chemical changes could alter the NEXAFS spectra, such as the occurrence of degradation of the DTBT system on the silver surface through C−S bond cleavages. For this reason only C K-edge spectra related to as deposited organic films and without any thermal treatments have been shown here, even if some details of NEXAFS spectra acquired after annealing treatments are provided in the Supporting Information (see Figure S4). DFT Calculations. Ab initio calculations have been performed for 3T and DTBT molecules adsorbed on Ag(110) in order to support and complement the experimental results discussed above. We limit here the discussion to the most stable 3T/Ag and DTBT/Ag configurations suggested by DFT simulations. A complete report on all investigated configurations of the molecules interacting with the surface can be found in the Supporting Information (Figure S5). In general, both molecules do not show any spontaneous tendency to the formation of covalent bonds with the Ag

Figure 9. DFT simulations of 3T/Ag(110) and DTBT/Ag(110) systems: (a, d) top view and (b, c, e, f) side views.

tilted with respect to the adjacent one, forming a S−C−C−S dihedral angle of about 10°. In first approximations, since each thiophene unit is rather small the isolated 3T molecule can be basically considered planar as a whole, supporting also our experimental observations related to the retaining of the 3T molecular structure. DTBT is characterized instead by a different behavior of the central block. While the peripheral thiophene rings are still found substantially parallel to the surface, the central fused unit raises the benzene ring and forms a 12−15° angle with respect to the Ag(110) surface (confirmed also in the case of different adsorption configurations, shown in Figure S5). This is probably due to the fact that the benzene ring is less involved in the interfacial charge displacement shown in Figure 9 in the case of all thiophene rings retaining a certain degree of repulsion from the electronic density of the Ag surface. Further details of the molecule/surface interaction are provided by the complementary Δρ maps shown in Figure 10 and Lowdin charge analyses reported in Table 7. Figure 10 displays the charge displacement induced by the interaction of 3T and DTBT with the surface: the charge flows from blue I


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may also be extended to the Br−DTBT−Br/Ag(110) system, not investigated by the present simulations. Moreover, the calculated Kohn−Sham states of the 3T/Ag and DTBT/Ag systems have been projected on the basis of C(2p) states, and the components of projections parallel to the Ag surface (px, py orbitals) have been separated from the perpendicular ones (pz orbitals). As confirmed by the comparison of NEXAFS measurements with NEXAFS calculations in a simpler case,70 the local character of X-ray absorption is compatible with such a simplified analysis of our results. However, we stress the limits of such a comparison so that a description just made on the grounds of empty states contains neither core−hole effects, significant in particular in the case of bound states of molecules, nor oscillator strength information. Even if the direct comparison between NEXAFS and projected density of states (PDOS) cannot be quantitatively justified, there is a clear similarity between spectra of Figure 8 and plots of Figure 11. In particular, the parallel (π, red line) Figure 10. Difference density maps (Δρ) calculated for the 3T (left panels, top and side view) and DTBT (right panels, top and side view) interacting with the Ag (110) surface (electronic densities sampled at 0.001 e/au3): charge density flows from blue to red zones when the molecules interact with the Ag surface. Red arrows indicate the charge accumulation layer arising in the interfacial region, while blue ones indicate the π-conjugated systems of the molecules and, in particular, the S atoms (upper panels), mostly involved in charge depletion.

Table 7. Lowdin Chargesa isolated 3T 3T/Ag(110) isolated DTBT DTBT/Ag(110)

S charge

C charge

H charge

5.73 5.55 5.71 5.53

4.20 4.13 4.20 4.12

0.79 0.78 0.79 0.78

a

Average Lowdin atomic charges (valence electrons only) calculated in the case of C, S, and H atoms belonging to 3T and DTBT, isolated and adsorbed on the Ag surface. Figure 11. Density of states (DOS) of the 3T/Ag (A) and DTBT/Ag (B) interacting systems projected on a basis set formed by the molecules C(2p) orbitals and shown by using a 0.3 eV Gaussian broadening. The zero value corresponds to the Fermi energy of the interacting systems. Contributions ascribed to px and py orbitals, parallel to the surface, are shown as black plots, while those ascribed to pz orbitals, normal to the surface, are shown as red ones. The equilibrium geometries are also shown as insets to A and B (top and side views). The lowest energy unoccupied electronic states involving the two molecules interacting with the surface are labeled by arrows in the PDOS plots, and the corresponding |ψ|2 plots are shown in panels C−E (top and side views).

regions to red regions when the molecules are adsorbed on the Ag surface. Both patterns show a depletion of charge from the thiophene π-conjugated system of the molecules (blue arrows in the side-view panels of Figure 10), particularly involving the S atoms (blue arrows in top-view panels of the same figure), but barely extended to the fused benzene ring of DTBT. Such a charge rather than being transferred to the Ag surface is placed in an interfacial region between the molecules and the topmost layer of Ag atoms (red arrows in the same figure), as already discussed in detail in the case of different macrocyclic molecules interacting with the Ag surface.67 Such findings are fully supported by the Lowdin analyses reported in Table 7, which show a consistent depletion of charge from the S and, to a lesser extent, C atoms when the molecules are in contact with the surface. Even if no tendency to the spontaneous fragmentation or polymerization of the molecules can be reported on the grounds of the present DFT results, such a charge depletion involving the C and S atoms may lower the internal connectivity of the molecules and facilitate thermally activated processes. Moreover, the stronger electronic repulsion of the central DTBT unit with respect to the thiophene rings can be expected to favor thermally activated fragmentation rather than polymerization processes. Similar considerations

and perpendicular (σ, black line) polarization of NEXAFS is well reproduced by PDOS plots of Figure 11. The appearance of a single strong peak involving the C and D unoccupied states of 3T is supported by NEXAFS calculations of isolated molecules (Figure S4), which suggest the occurrence of an electronic transition connecting the core orbitals of C atoms belonging to the central thiophene unit to both LUMO and LUMO+1. On the contrary, both the E and F states give rise to intense transitions of core electrons, with bound states labeled as πB* and π1* correctly reproduced by calculations. J


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CONCLUSIONS We investigated the interaction and reactivity of three thiophene-based compounds: DTBT (1,3-di(thiophen-2-yl)benzo[c]thiophene), Br−DTBT−Br (1,3-bis(4,5-dibromothiophen-2-yl)benzo[c]thiophene), and 3T (2,2′:5′,2-terthiophene) with different kinds of Ag surfaces. The adsorption process of the DTBT-like compounds on both polycrystalline and singlecrystal surfaces triggers a peculiar reactivity pattern which is unexpected if compared to the reference 3T analogue. SERS results suggest that 3T undergoes a polymerization catalyzed by a nanostructured Ag substrate. On the contrary, the benzo-fused systems, DTBT and Br−DTBT−Br, adsorbed on the same substrate promote thermally activated fragmentation processes, as indicated by a new S−S stretching signal which is the fingerprint of the formation of atomic sulfur on the Ag surface. This first suggestion is confirmed by synchrotron XPS measurements performed on 3T and DTBT-like oligomers UHV deposited onto the Ag(110) surface. Such photoemission results have indicated that the two kinds of systems can effectively undergo the two different reaction mechanisms mentioned above when interacting with Ag surface. This is due to the particular and quite unexpected electronic properties of the benzo[c]thiophene moiety, inducing strain in the molecules in direct interaction with the surface. In order to gain further insight into the reactivity of these compounds triggered by the Ag surface, analysis of highresolution XPS spectra of the different molecule/Ag(110) systems at different coverages and temperatures has been of utmost importance. The thermal effect on the adsorbed molecular networks has been probed by subsequent flashing and annealing treatments at temperatures higher than those used for deposition. The results indicate that immediately after the deposition 3T shows S(2p) and C(1s) peaks compatible with a diffuse polymerization of oligothiophenes catalyzed by the Ag(110) surface. No similar indication is suggested by the same XPS measurements acquired in the case of DTBT-like compounds, where no peaks belonging to polymerized species are present. On the contrary, the photoemission spectra provide also evidence of the presence of sulfur atoms not belonging anymore to thiophene rings, thus suggesting a ringopening reaction favored by Ag surface as the dominant process after the oligomers deposition. By subsequent annealing at higher temperatures, the as-deposited systems show broadened C(1s) spectra characterized by the presence of new C(1s) components attributed to the species produced after the hypothesized C−S bond cleavage. Moreover, the NEXAFS spectra of the 3T/Ag(110) and DTBT/Ag(110) systems indicate that the molecules have a general tendency to lay flat on the Ag surface, with small deviations from planarity such as that attributed to the benzofused moiety of DTBT. The close similarity between NEXAFS spectra measured in the case of as-deposited molecule/surface systems and those calculated in the case of isolated molecules suggests that no extended dissociation process is started before thermal treatments of the 3T and especially DTBT system, making the experimental spectra useful for qualitative orientations evaluations. Finally, all experimental evidence discussed above is quite consistent with the results of DFT calculations. The 3T system is arranged in a quite planar conformation, supporting our experimental observations related also to the retaining of its

aromaticity. On the contrary, the DTBT organizes its terminal thiophene rings in a preferentially planar orientation, forcing the benzo[c]thiophene moiety in a lifted-up position. Such a molecular orientation strongly influences the properties of adsorbed DTBT, suggesting an explanation of its tendency to the C−S bond cleavage promoted by the interaction with the Ag surface. As a final remark, we note that the peculiar interaction of DTBT-like oligomers can be considered as a high hurdle, hindering polymerization processes based on Ullmann-type reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08733. Luminescence of the Raman spectra related to bulk molecules, XPS Ag(3d) spectra of 3T and DTBT adsorbed onto Ag(110) surface, theoretical carbon Kedge NEXAFS spectrum of isolated 3T and DTBT molecules, experimental carbon K-edge NEXAFS spectra of annealed DTBT after its deposition onto Ag(110) surface, configurations of 3T and DTBT adsorbed on the Ag surface investigated by ab initio calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 0971 427227. Present Address ⊥

Australian Synchrotron, 800 Blackburn Road, Clayton, 3168, Victoria, Australia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank ELETTRA for providing high-quality synchrotron light and merit-based beamtime and also the team of the Materials Science Beamline for their kind assistance. This paper draws on work undertaken as part of the projects CLaN-4SolarE (Combined Laser Nanotechnology for Solar Energy) and CLaN-4-SEnSe (Combined Laser Nanotechnology for Solar Energy and Sensors) cofinanced by the Operational Programmes ESF and ERDF Basilicata 2007−2013. G.M. and A.A.B. acknowledge financial support by CNR under the INFN-CNR Project EOS. The authors are glad to thank Dr. Lorenzo Avaldi who, with his encouragements, has supported this work from the beginning.

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