6. Thin Solid Films and Surface Enhanced Vibrational Spectra of THIO-BIS(n-PROPYLIMIDO)PERYLENE
Alicia Kam, Ricardo Aroca*1
Material and Surface Science Group,
Department of Chemistry and Biochemistry
University of Windsor,
Windsor, ONT N9B 3P4,
James Duff, Carl P. Tripp*2
Xerox Researc Centre of Canada
2660 Speakman Drive,
Mississauga ONT L5K 2L1,
*1 Corresponding author
*2Present address: Dept of Chemistry & Laboratory for Surface Science & Technology, 5764 Sawyer Research Center, University of Maine, Orono, ME 04469-5764
The vibrational and surface-enhanced vibrational spectra of a new dye, thio-bis(n-propylimido)perylene (Thio-PTCDPr) are reported. Langmuir monolayers were formed at the air-water interface and Langmuir-Blodgett (LB) monolayers were fabricated on dielectric and metal substrates. Thin solid films of Thio-PTCDPr material were also made by vacuum evaporation. Nanometric films were studied using transmission and reflection-absorption infrared spectroscopy (RAIRS) in order to determine the long range molecular organisation in the film. RAIRS was also used to follow the effect of post-deposition thermal annealing on the structure and morphology of the evaporated films. Surface-enhanced Raman scattering studies of LB and evaporated films on silver were carried out to identify the interaction of the sulfur-containing perylene pigment with the metal substrate. Surface-enhanced infrared (SEIR) was obtained for an evaporated film on silver and tin islands films. The required vibrational work was aided with ab initio quantum chemical computations of the infrared and Raman intensities and wavenumbers.
Perylene dyes, and in particular perylene-3,4,9,10-tetracarboxylic diimides (PTCD) [see molecular structure below] are of industrial importance because of their low cost of fabrication while exhibiting a wide range of potentially useful properties. Their semiconducting properties and large extinction coefficients in the visible together with their chemical inertness, thermal stability and stability against ionizing radiation make them prime candidates for a variety of applications. In particular, PTCD materials are at the centre of a research effort intended to improve their n-type electron transporting properties by introducing chemical groups into the main PTCD backbone . These materials may be integrated into photovoltaic cells or electroluminescent displays . They have also been incorporated into heterojunction devices and they could be useful in chemical sensor technology.
The fundamental research that leads to our understanding of the underlying processes in optoelectronic applications includes the formation of aggregates and spatial organisation, the formation of excitons and the charge separation processes. There are a large number of publications in this particular field, some of the related work can be found in references [3-5]. The second aspect of the study of these classes of organic materials includes the physical modelling and the measurement of the properties of the metal-organic interfaces [5,6]. In our collaborative research with the Xerox Research Centre of Canada we have concentrated on the spectroscopic characterization of new PTCD materials [see Ref. 7 and references therein]. In the present report we describe the spectroscopic characterization of a new PTCD material, the successful formation of a floating monolayer and the fabrication of thin solid films. Surface-enhanced Raman scattering (SERS) and Surface-enhanced resonance Raman scattering (SERRS) spectra of LB monolayers and of vacuum evaporated have been obtained and are discussed. SERRS imaging is also reported. Infrared reflection-absorption spectroscopy was used to study the long range organisation in thin solid films and the organic–metal interactions of a new PTCD material containing a sulfur atom attached to the chromophore core.
Thio-bis(n-propylimido)perylene. The starting tetracarboxylic anhydride for the Thio-bis(n-propylimido)perylene derivative was prepared by sulphonation of the perylene-3,4,9,10-tetracarboxylic acid dianhydride with chlorosulfonic acid. Subsequent deoxygenation and dehydration resulted in the synthesis of perylo-thiophene-3,4,9,10-tetracarboxylic dianhydride. Condensation with propyl amine in appropriate solvent yields thio-bis(n-propylimido)perylene .
Vacuum Deposition. The substrates used were transparent and precleaned Borosilicate slides (Baxter Cat. M6145). They were cleaned by rubbing with absolute ethanol, and subsequent drying under a continuous flow of dry nitrogen gas. Metal deposition was performed in a Balzers vacuum system evaporator, equipped with an Edwards E2M2 rotary vacuum pump that functioned as a precursor to the Edwards diffusion pump. Silver shots (Aldrich 20,436-6) were thermally evaporated from a cupped tungsten boat, using a Balzers BSV 080 glow discharge/evaporation unit. The background pressure was nominally 10-6torr, as measured by a Balzers IKR 020 cold cathode gauge.
On a substrate preheated to 200 ºC, silver was deposited at a rate of 0.5 nm/s to a total mass thickness of 100 nm. This deposition rate was allowed to stabilise before the shutter was opened. Film thickness and deposition rate was monitored using a quartz crystal oscillator. The bulk density of silver employed was 10.5 g/cm3, the tooling factor: 105%, and the Z-ratio, 0.529. For surface-enhanced infrared experiments the metal films and the Thio-PTCDPr films were evaporated onto ZnS substrates in sequence.
The perylene derivative, thio-bis(n-propylimido)perylene was deposited onto continuous silver films (mass thickness, 100 nm), in turn supported on a glass substrate. The evaporation system used was of the same set-up aforementioned, with the exception that the substrate was not heated. The perylene pigment was evaporated from tantulum boats at room temperature. The deposition rate for the pigment was 0.2nm/s to a mass thickness of 20nm. They used the quartz crystal microbalance (QCM) technique to measure the amount of deposit by weight, and then converted it to the thickness of deposit, based on the assumption that deposited film is formed layer by layer. In a similar manner to the metal deposition, the rate for the organic deposition was allowed to stabilise prior to opening the shutter. The bulk density employed for this pigment was 1g/cm3, the tooling factor: 92%, and the Z-ratio, 1.
Langmuir Blodgett film measurements
The surface pressure-area isotherm of Thio-PTCDPr was recorded by spreading a fixed and known number of molecules onto the aqueous subphase and compressing the trough area at 19.5 cm2/min. The spreading solvent was 90% HPLC grade chloroform and 10% spectroscopic grade trifluoroacetic acid from Aldrich.. Milli-Q purified water with a measured resistivity of 18.2 MW /cm was used as the subphase. The preparation of floating monolayers at the air-water interface was successful and reproducible isotherms were recorded at 15 oC where the area/molecule extrapolated to zero surface pressure was found to be ca. 0.5 nm2. LB films were transferred to a variety of substrates in order to explore the surface enhanced vibrational phenomena. The LB monolayers of Thio-PTCDPr were prepared at 15oC through z-deposition in a Lauda Langmuir film balance Film. Transfer pressure was 25 mN/m and transfer ratios were all near unity.
Spectroscopy The transmission spectrum of the perylene pigment dispersed in potassium bromide, was recorded on a Bomem DA3 FTIR spectrometer. Infra-Red spectra were routinely recorded in the region of 400-4000 cm-1 with 1cm-1resolution. The reflection absorption infra-red spectra of the Thio-PTCDPr pigment was recorded on an in-house modified Bomem 110-E equipped with a Judson mid range (650 cm-1) 1mm x 1mm MCT detector. The entire system was purged and maintained under a closed cell of dry air. The sample holder was adapted to include four concave mirrors that focused the IR beam onto the substrate at an 80 degree angle of incidence . The substrate holder consisted of a flat Aluminium heating element, which was thermally controlled by a 100V variastat. The temperature of the substrate was monitored using a thermocouple. Annealing of the samples during the RAIRS measurements were performed in situ on the heating plate.
The FT-Raman spectra of the Thio-PTCDPr were recorded with a Bruker RFS100. Surface-enhanced Raman scattering and mapping were obtained with a Renishaw Research Raman Microscope System RM2000 equipped with a computer controlled 3-axis encoded (XYZ) motorized stage with a minimum step of 0.1 m m. The RM2000 uses a Leica microscope (DMLM series). The spectrum is measured using a Peltier cooled (-70 oC) CCD array. The high sensitivty of the detector and the high throughput of the instrument permits the use of very low powered lasers. Typically, 100 microW of the 514.5 nm argon ion laser line at the sample was used. The spectrograph is equipped with a 1200 g/mm grating and additional angle-tuned bandpass filter optics. Raman spectra were recorded at room temperature with ca. 4 cm-1 resolution.
Model Calculations Geometry optimisation and vibrational calculations performed to help the assignment were executed using Gaussian at RHF/3-21Glevel of theory.
Results and Discussion
The absorption spectra of PTCD derivatives in solution are essentially identical and they correspond to the electronic spectrum of the aromatic core . The absorption spectrum of Thio-PTCDPr in solution shown in Figure 1 is not an exception in spite of the presence of a sulfur atom attached to the perylene chromophore. The vibronic structure shows equally spaced maxima with a constant separation of ca. 1416 cm-1. The absorption spectra of the solid (pellet) and that of the evaporated film (see Figure 1) are more sensitive to the presence of substituent groups in the chromophore . The differences between the spectra of the bulk and that of the thin solid films speak to the fact that the absorption spectra of these materials were strongly dependent on the morphology of the solid. The absorption spectrum of the bulk (pellet) is broad and extends into the red with considerable absorption at 600 nm. The evaporated film shows a strong absorption with maxima similar to the solution spectrum in wavelength but with different intensities. The result is an indication of the higher degree of crystallinity observed in the evaporated film. The emission spectra of the solid is characteristic of excimer formation observed in PTCD materials [3,7] with a broad unresolved red shifted emission band.
Figure 1. Electronic absorption and emission spectra of Thio-PTCDPr. The intensity is in arbitrary units. The emission spectrum was obtained using the 514.5 nm excitation laser line.
Thio-PTCDPr contains 51 atoms and 147 fundamental normal modes of vibration. The discussion can be reduced to characteristic vibrational modes that serve as probes for analytical characterization and applications to molecular organisation studies and molecule-metal surface interactions. The assignment of observed vibrational bands is mainly reduced to the fundamental vibrations of the chromophore. The vibrational modes of the propyl moiety are well known and do not require a separate discussion. The local symmetry of the chromophore (planar moiety) allows the separation of normal modes according to the direction of their dynamic dipoles, helping the assignment of infrared active vibrations. For molecular orientation determination the most relevant normal modes are the in-plane carbonyl stretching vibrations, ring stretching vibrations and the out-of plane C-H wagging modes. The assignment of characteristic vibrational wavenumbers observed in the infrared spectra and the computed wavenumbers and intensities are given in Table 1.
|706||30||737 w||C-H wag|
|737||9||742 m||741 w||735 m||C-H wag|
|804||12||810 m||807 w||803 m||C-H wag|
|826||67||837 w||Ring def|
|839||34||846 w||Ring def|
|867||39||859 w||851 vw||C-H wag|
|869||29||888 vw||886 vw||C-H wag|
|1009||26||1046 vw||C-H wag|
|1088||11||1078 m||1078 m||1073 w||C-H bend|
|1109||27||1108 vw||1109 w||Ring str|
|1133||250||1143 vw||C-H bend|
|1146||68||1150 w||1146 w||Ring str|
|1168||108||1177 w||1178 vw||C-H bend|
|1232||131||1239 m||1238 m||1233m||C-H bend|
|1272||65||1248 m||C-H bend|
|1294||1060||1303 sh||1302 w||Ring str|
|1312||27||1315 s||1315 m||1308 m||C-N str|
|1330||183||1338 w||1337 w||1334 w||Ring str|
|1340||306||1346 w||1354 w||Ring str|
|1369||122||1350 w||C-N str|
|1381||155||1377 w||1379 w||1373 w||C-H bend|
|1401||253||1397 w||1396 vw||C-H bend|
|1425||7||1426 m||1420 m||Ring str|
|1442||4||1433 s||1433 w||Ring str|
|1463||5||1460 w||1456 w||Ring str|
|1574||215||1560 m||1560 w||1556 w||Ring str|
|1588||323||1597 s||1596 m||1589 s||C=C str|
|1675||758||1660 vs||1663 vs||1651 vs||C=O str|
|1713||760||1695 vs||1697 vs||1687 vs||C=O str|
Table 1. Calculated and observed IR wavenumbers and relative intensities.
* Scaling factor of 0.9 employed.
Vibrational assignments were helped by animation of the atomic displacements obtained from the solution of the vibrational problem in Gaussian at RHF/3-21Glevel of theory. The middle infrared spectrum and the calculated wavenumbers are illustrated in Figure 2. Considering that the calculated intensities are for the isolated molecule, and the experiment corresponds to a solid dispersion in KBr, it can be seen that the calculated IR intensities follow the general pattern of the observed spectrum. Similar observations can be made for the calculated Raman intensities and the most intense observed bands.
Figure 2. Calculated and observed infrared spectrum of Thio-PTCDPr in a KBr pellet. Intensity units are arbitrary.
The Raman wavenumbers and intensities are listed in Table 2. Clearly, quantum computations are now an essential part of the most common analytical tools used for materials characterization. The infrared spectrum is characterised by the PTCD vibrations: carbonyl modes at 1697 cm-1 and 1662 cm-1, ring stretching modes at 1597 cm-1 and 1560 cm-1, out-of-plane C-H wagging modes at 742 cm-1 and 810 cm-1.. The term out-of-plane is used here to indicate the local planar structure of the chromophore. There are also a number of intense infrared bands that can be assigned to “in-plane” C-H bending modes and ring stretching vibrations in the 1000-1450 cm-1 spectral region. The far-infrared spectrum of Thio-PTCDPr was also recorded and the agreement between calculated and observed wavenumbers is illustrated in Table 3. The vibrational modes active in the far-infrared region of the spectrum are highly coupled with important contributions from skeletal deformation, ring deformations and ring torsions.
Table 3. Far-infrared wavenumbers
The FT-Raman spectrum of thio-bis-(propylimido)perylene contain a group of medium intensity bands at 1695 cm-1, 1661 cm-1 (carbonyl stretching vibrations), 1621 cm-1, 1579 cm-1, 1555 cm-1 and 1538 cm-1 (ring stretching vibrations). The strongest band in the FT-Raman spectrum is observed at 1397 cm-1 and can be assigned to a perylene ring stretching vibration. The FT-Raman and the Raman spectrum of a Thio-PTCDPr powder excited at 780 nm are shown in Figure 3. The Raman spectrum simulated using calculated Raman intensities is also given in Figure 3 for comparison. It can be seen that the calculated spectrum gives a general pattern of relative intensities which is in fairly good agreement with the observed spectra of the solid and hence is a good guide for the interpretation of the observed spectra. The FT-Raman of the bulk (at 1064 nm and 780 nm) should be considered the reference spectra of the dye material. The typical chromophore fundamental modes are observed in the film spectrum at 1380 cm-1 and 1394 cm-1; a very strong pair of ring stretches at 1571 cm-1 and a shoulder at 1580 cm-1, followed by a very strong ring stretching-C-H with bending mode contribution at 1291 cm-1 [11-14]. The characteristic vibrational frequencies are given in Table 2.
Figure 3. Calculated and observed Raman spectra of Thio-PTCDPr. FT-Raman excited at 1064 nm and Raman spectrum of the bulk recorded with the 780 nm. The y-axis is in arbitray intensity units.
Surface-enhanced infrared and Surface-enhanced-Raman scattering and imaging
SEIR Surface-enhanced vibrational spectroscopy (SEVS) comprises the study of molecular vibrations of molecules adsorbed on surfaces that can enhance the absorption and the emission of electromagnetic radiation. SEVS comprises two complementary techniques: surface-enhanced infrared and surface-enhanced Raman scattering spectroscopies. The first report by Hartstein et al.  showed that silver films could be used for the observation of SEIR. According to the electromagnetic mechanism used in the interpretation and evaluation of the enhancement factor on rough surfaces the enhancement activity of the substrate depends on the shape and size of surface protrusions and the dielectric functionof the material in the spectral region of interest. Silver and gold have been commonly used in SEIR experiments [16,17]. Recently, we have added Sn island films as substrate for SEIR . Here, we present the results of SEIR experiments on Sn and Ag island films. The Sn was evaporated under a base pressure of 4 x 10-6 mbar while the temperature of substrates was held at 80 oC. The Sn evaporation rate was maintained at 0.5 nm/s, and film thickness was monitored using an XTC inficon quartz crystal oscillator. SEIR was achieved for both Sn and Ag substrates. The transmission SEIR spectra of Thio-PTCDPr on Ag islands and Sn islands (18 nm thickness) fabricated onto a ZnS substrate are shown in Figure 4. The spectrum of Thio-PTCDPr in a KBr pellet (middle spectrum) is also included in Figure 4 for direct comparison. The spectrum of the solid dispersed in a KBr matrix provides the absorptions due to infrared resonances with a random spatial distribution of molecules and aggregates and hence should closely follow the spatial averages directly obtained from the character tables of the point group symmetry. Notably, the observed relative intensities in the SEIR spectra of Thio-PTCDPr on Sn and Ag are very close to these of the solid matrices. However, as expected they are slightly different from those observed in RAIRS of thin solid films of the neat material on smooth metal surfaces. The actual enhancement factors were very modest, about 10 for silver and no more than 5 for Sn. The wavenumbers for the observed bands have been included in Table 1.
Figure 4. Surface-enhanced infrared of Thio-PTCDPr on silver island (top), reference spectrum of KBr pellet (middle) and SEIR on 18 nm Sn island film (bottom).
SERRS and SERRS line scanning Langmuir-Blodgett (LB) monolayers and evaporated films were fabricated onto silver island films in order to obtain the surface-enhanced vibrational spectra. The surface-enhanced resonance Raman scattering of a single LB monolayer on silver is shown in Figure 5 for two excitation laser lines. To facilitate transfer and ensure an homogeneous cover of the silver islands (6 nm mass thickness of silver on glass), mixed LB monolayers were fabricated using a 1:50 ratio of Thio-PTCDPr and arachidic acid ( CH3(CH2)18COOH ) (AA). The addition of fatty acids makes the monolayer more flexible, facilitating transfer to solid substrates. The formation of the Ag-S bond at the metal-organic interface was a possibility; similar to the formation of the Ag-S bond in thiolate complexes that has been reported by several groups [19-21]. A bond characteristic of the Ag-S bond in silver thiolate complexes is observed within the 150-250 cm-1 range . The Ag-S bond stretch found in the SERS spectrum of benzenethiol in silver sol is reported at 240 cm-1 . The Ag-S stretching vibration observed in the SERS spectrum of benzyl phenyl sulfide is at 215 cm-1 . Ulman et al.  have studied the Raman spectra of alkanethiolate monolayers on silver and gold. The work was carried out using about 2 mW of the 633 nm laser radiation. Unfortunately, the authors do not show the SERS spectra below 800 cm-1. The SERRS spectra shown in Figure 5 do not provide evidence of chemisorption. The SEIR spectrum is limited to the middle infrared and hence again does not provide any direct evidence of chemisorption. However, the out-of-plane modes are observed with some relative intensity. The latter intensity of out-of-plane modes should be minimal for a chemisorbed configuration forming a Ag-S bond. Since the SEIR enhancement factor is modest, the contribution from physisorbed layers to the spectrum is significant in the case of evaporated films. This is in contrast with SERS where the first layer may dominate the spectrum.
Figure 5. Surface-enhanced resonance Raman scattering of a single LB monolayer film deposited on a silver islands excited with 0.5 mW of the 633 nm and the 514.5 nm laser lines. Inset: plasmon absorption of a 6 nm mass thickness silver film.
Line scanning of SERRS spectra for a vacuum evaporated film of Thio-PTCDPr onto silver was recorded using 1 m m intervals with an illumination of ca. 1µm2. [ If we consider that the laser point is circular, the area is 10-12 m2] The sample was prepared by evaporating 10 nm mass thickness of Thio-PTCDPr onto 6 nm silver island film. The SERRS spectra for the evaporated film and that of the mixed LB monolayer given in Figure 5 are practically identical. The results for a line of 40 points are shown in Figure 6. It can be seen that apparently, there is a homogeneous SERRS activity throughout the line on the surface. The laser power at the sample was 250 µW. Photo-decomposition is observed with the 514.5 laser line when the power at the sample is higher than 2 mW i.e., 2×109Wm-2 .
Figure 6. Line scanning of the SERRS spectra of Thio-PTCDPr forming a 10 nm mass thickness evaporated film onto silver islands.
Thin solid films, organisation and molecular orientation
Molecular orientation of a film on a metal surface may be interpreted from the specular and refection absorption infrared spectroscopy. The intensity of a vibrational band is dependent on the change of the dynamic dipole moment, the electromagnetic field vector and the square of the cosine of the angle between the dynamic dipole moment and the electromagnetic field vector. In specular infrared reflection -RAIRS- the electric field is p-polarised. The dynamic dipole moments of those frequencies parallel to the p-polarised field display enhanced absorbance, while the absorbance of those with moments perpendicular to the field are suppressed. Given the assignment of symmetry species the orientation of the molecule on a metal surface may be deduced. The RAIRS spectra of Thio-PTCDPr are shown in Figure 7. Analysis of spectra indicates that the C-H wagging bands at 742 cm-1 and 808 cm-1 are observed with minimal relative intensity compared to reference spectrum of the isotropic sample (Figure 2). Furthermore, the relative intensity of the C=O bands have remained constant, thereby confirming the presence of an “edge-on” molecular orientation.
Figure 7. Infrared reflection-absorption of an evaporated Thio-PTCDPr film on a reflecting silver film. The first spectrum recorded at room temperature is followed by spectra of the annealead sample starting at 50 oC and further annealed at 100, 150 and 200 0C.
The thermal annealing of bis-PTCDPr has been recently reported . The main conclusions from the bis-PTCDPr work were that thermal annealing can induce a change in molecular orientation and may in fact produce a phase transition. The RAIRS spectra of the Thio-PTCDPr dye annealed at 50, 100, 150 and 200 oC are shown in Figures 7. The bottom spectrum was obtained at room temperature and is included as a reference. A comparative study of the untreated and annealed films of Thio-PTCDPr indicates no apparent change on annealing. The film morphology was obtained by AFM before and after annealing. Hence the morphological studies of the films under thermal annealing agree the spectral trends in sharing no change. It must therefore be concluded that thermal annealing does not induce phase changes in Thio-PTCDPr films. The results indicate that the smooth silver substrates, induce a molecular orientation for upper layers that does not change significantly under thermal annealing. However, annealing studies in which a layer of octadecyltrichlorosiliane (OTS) was first deposited on silver followed by Thio-PTCDPr indicates that reorientation does then occur in a manner similar to that of bis-PTCDPr. These results seem to suggest that the orientation could be driven by the formation of a Ag-S bond. However, this assumption is not supported by the surface-enhanced data.
Langmuir-Blodgett and vacuum evaporated films of a new material Thio-PTCDPr have been fabricated onto metal and dielectric substrates. The vibrational infrared and Raman spectra including computer aided assignment of fundamentals are reported. The surface-enhanced vibrational spectra, SEIR and SERRS were obtained for vacuum evaporated films on metal island films and SERRS of LB monolayers were also recorded. Line scanning of SERRS on silver islands reveal a fairly homogeneous SERRS activity at 1 micron intervals. Using RAIRS, it was found that solid films formed on smooth silver surfaces are not affected (structural change and molecular orientation) by thermal annealing as has been previously observed for other PTCD materials.
NSERC of Canada is gratefully acknowledged for its financial support.
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