Molecular Orientation and Structural Transformations in Phthalic

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Molecular Orientation and Structural Transformations in Phthalic Anhydride Thin Films on MgO(100)/Ag(100) Susanne Mohr,† Tao Xu,† Tibor Döpper,‡ Mathias Laurin,† Andreas Görling,‡,§ and Jörg Libuda*,†,§ †

Lehrstuhl für Physikalische Chemie II, ‡Lehrstuhl für Theoretische Chemie, and §Erlangen Catalysis Resource Center and Interdisciplinary Center Interface-Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany ABSTRACT: Structural control of organic thin films on dielectric substrates is the key to tailoring the physical properties of hybrid materials, for example, for application in solar energy conversion, molecular electronics, or catalysis. In this work, we investigate the molecular orientation of phthalic anhydride (PAA) films on atomically well-defined MgO(100) on Ag(100) using temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) in combination with density-functional theory (DFT). A robust procedure is presented to determine the orientation of the PAA molecules, which relies on the intensity ratios of vibrational bands only. We show that even at deposition temperatures of 110 K, the PAA multilayer grows with a specific molecular orientation; that is, the PAA molecular plane is preferentially aligned parallel with the MgO surface. No change of molecular orientation occurs up to a temperature of 145 K. Between 145 and 160 K, the film restructures adopting a nearly flat-lying molecular orientation. Between 170 and 205 K, the film undergoes a second structural transition to a crystalline phase. This transition is associated with a pronounced molecular reorientation. The molecules adopt a tilted orientation and, simultaneously, rotate around their C2 axes. The reorientation behavior suggests that the molecular orientation in the crystalline phase is controlled by the interaction with the MgO(100) substrate. At higher temperature, no further restructuring is observed until the PAA multilayer desorbs at temperatures above 230 K.

1. INTRODUCTION Thin films of organic molecules on dielectric substrates, in particular on oxides, are key building blocks in new technologies, for example, in molecular electronics,1,2 in solar energy conversion,3,4 in sensor technology,5,6 and in catalysis.7,8 The physical and chemical properties of such films critically depend on their structure. To understand structure formation and thin film growth from a fundamental point of view, two challenges have to be addressed: First, we have to develop a molecular-level understanding of adsorption and chemical bonding at the molecule/surface interface. Second, we need to understand the thin film growth processes beyond the first monolayer. There are many examples in the literature in which the growth of organic films on dielectric substrates has been explored, for example, by preparing self-assembled monolayers (SAMs) from solution (see, e.g., refs 9−11). In most of these cases, little is known on the atomic structure of the oxide surface itself. To understand these hybrid interfaces at the atomic level, it is essential, however, to characterize the structure and adsorption sites of the surface. This is possible using a surface science approach, starting from single crystal oxides12−16 or from ordered oxide films.17−21 To date, most surface science studies have addressed the growth of organic molecules on metallic substrates, mostly noble metals. For benzene on Ru(0001), for example, a complex crystallization process has been described,22 in which © XXXX American Chemical Society

an amorphous metastable phase crystallizes upon annealing on top of an ordered physisorbed and an ordered chemisorbed layer. Also, several growth studies of larger functional organic molecules, such as porphyrins or phthalocyanines, have been performed (see, e.g., refs 23−27). Very few studies have been carried out on ordered oxide surfaces so far.13,17,21,28−30 The reason is that, with few examples, the preparation and the characterization of atomically well-defined oxide surfaces are often more difficult and more demanding than that of metal surfaces. In addition, the formation of stable organic films on oxide surfaces typically requires other concepts, for example, the anchoring of the molecules via specific interaction and chemical bonds.13,31−33 This additional chemistry at the interface substantially complicates the situation, especially for larger molecules. In the present work, we make a first step to address this challenge. We start from an atomically well-defined MgO(100) surface, which is prepared as a thin film on a Ag(100) substrate.34−36 The use of a oxide thin film provides a number of advantages.12,18,37 For example, we can straightforwardly apply most surface science techniques including photoelectron spectroscopies, scanning tunneling microscopy, and, in the present work, infrared reflection absorption spectroscopy Received: April 21, 2015 Revised: June 17, 2015

A

DOI: 10.1021/acs.langmuir.5b01392 Langmuir XXXX, XXX, XXX−XXX

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helps to focus the PAA flux onto the sample. The vapor pressure of PAA permits evaporation already at room temperature. To ensure a constant vapor pressure and evaporation rate, the metal crucible was kept at a constant temperature of 296 K using a water bath and thermostat. To prevent condensation of PAA along the lines of the evaporator and inside the dosing tube, the steel parts were simultaneous heated during the deposition. Prior to each experiment, PAA was pumped for at least 10 min and thereby cleaned by sublimation. The MgO(100) films were prepared following the procedures previously reported in the literature.36 The Ag(100) crystal (MaTecK) was cleaned by several cycles of ion bombardment (Ar+, 0.8 keV, 20 min, room temperature) and subsequent heating to 650 K for 5 min. The quality of the surface was checked by low-energy electron diffraction (LEED). The epitaxial MgO(100) films were prepared by PVD of metallic Mg from a Mo crucible using a home-built thermal evaporator. The evaporation rate was calibrated using a quartz microbalance and, typically, was in the range from 1.00 to 1.25 Å/min. For the preparation of MgO, Mg was deposited in an O2 atmosphere of 4 × 10−7−1 × 10−6 mbar at a sample temperature of 450 K for 17 min. Subsequently, the film was annealed at 650 K for 10 min in O2atmosphere. For the present experiments, MgO films with a thickness of 20 Å were used. The quality of the film was checked by LEED. The well-ordered MgO films give rise to a sharp (1 × 1) pattern, with spot intensities that differ characteristically from the Ag substrate at specific electron energies, for example, at 92, 123, and 143 eV. A thin film of about 15 ML of phthalic anhydride was prepared on the MgO/Ag(100) substrate, while cooling the latter with liquid nitrogen to 114 K. During deposition of phthalic anhydride, the background pressure in the main chamber increased by about 2 orders of magnitude to approximately 6 × 10−8 mbar. After deposition, the sample is continuously heated with a heating rate of 1 K/min from 114 to 413 K, controlled by an automated temperature controller (written in house using LabView). Infrared reflection absorption spectra were recorded in the repeated measurement mode, choosing a scan time of 0.9 min (54 s) and a delay time between each spectrum of 60 s. Thus, every minute a spectrum was recorded. The multilayer covered surface was used as reference. Density-Functional Theory (DFT). To assign experimentally measured frequencies to specific vibrations of the molecule, we performed density-functional calculations of the phthalic anhydride molecule in vacuum using the VASP software.44−47 The exchangecorrelation functional of Perdew, Burke, and Ernzerhof (PBE)48 was employed in combination with a plane wave basis set using the projector-augmented wave (PAW) method.48 The van der Waals correction scheme from Grimme (D3) with Becke−Johnson damping49,50 was adopted. The molecule was calculated in a cubic supercell with a volume of 20 Å3 and a plane wave cutoff energy of 450 eV. A Methfessel−Paxton51 smearing of 0.1 eV was used. The geometry was optimized until all forces were smaller than 0.01 eV/Å. Vibrational frequencies were calculated within the harmonic approximation and visualized with QVibePlot. Visualization. The vibrational analyses from the DFT calculations are visualized using QVibePlot 1.6.0 (based on Open Babel52 compiled from source at commit 437fece). QVibePlot displays two-dimensional representations of molecular vibrations. The representations show the changes of the normal modes (stretching, angle, and torsion) and maintain information on the phase of the changes using colors. Increased amplitude is shown by increased line width for the stretching, increased radius for the angles, and increased curve length for the torsions. QVibePlot relies on Open Babel to open output files from a large number of DFT calculation packages without conversion. QVibePlot is a free and open-source software (see http://vibeplot. sourceforge.net and ref 53 for more information).

(IRAS). One of the advantageous features of IRAS is that it can provide information on the molecular orientation both in the monolayer and in the multilayer region. Starting from the well-defined MgO(100) surface, we study its interaction with linker molecules, that is, molecular entities that mediate specific interactions, under ultrahigh vacuum (UHV) conditions. We have chosen MgO(100) as a prototype of a simple, nonreconstructed, and insulating oxide surface. Among the most common linker groups for oxide surfaces are carboxylic acids.3,38,39 In this work, we investigate phthalic anhydride (PAA), which either can adsorb to the oxide surface directly or, after ring opening, can form carboxylates, similar to a carboxylic acid. In a first study, we have explored the interaction of PAA with ordered MgO(100), with defected MgO, and with hydroxylated MgO surfaces.40 We have shown that the PAA interacts weakly with the MgO(100) at low temperature (100 K). Time-dependent IRAS during PAA deposition allows for a clear distinction between the monolayer and multilayer regime. At higher temperature (300 K), both adsorption mechanisms are possible, molecular adsorption and ring opening. Indeed we have shown that mixed adsorbate layers are formed, which consist of both carboxylates and intact molecular PAA adopting a tilted adsorption geometry. It is known that the adsorption behavior of organic molecules on oxide surfaces can strongly depend on the surface roughness of the substrate (see, e.g., refs 41,42); therefore, we have performed adsorption experiments of PAA on surfaces with higher roughness at 300 K.40 In this Article, we go one step further and explore the molecular orientation of PAA multilayers on the MgO(100) substrate. Toward this aim, we perform temperatureprogrammed infrared reflection absorption spectroscopy (TPIRAS). We present a robust analysis procedure for the TP-IRAS data that allows us to determine the molecular orientation of PAA as a function of the temperature. Interestingly, we observe two distinct structural transitions in the PAA film, one transition to a structure consisting of flat lying molecules and, at higher temperature, a second transition to a crystalline phase consisting of tilted molecules. This behavior suggests that the molecular orientation in the film is controlled by the molecular orientation at the PAA/MgO(100) interface. The present study contributes to a knowledge basis for the next step, which will involve the integration of linker units like PAA into larger functional organic entities, for example, porphyrins. On the basis of the present data and analysis methods, the upcoming work will aim at controlling the structure formation of such large functional entities on atomically well-defined oxide surfaces.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS TP-IRAS. The IRAS experiments were performed in a UHV system described elsewhere (base pressure below 2 × 10−10 mbar).43 Briefly, the system combines facilities for molecular beam, a supersonic beam, and a home-built evaporator for dosing organic compounds. The system was equipped with a Fourier-transform infrared (FTIR) spectrometer (Bruker IFS 66v/S), two quadrupole mass spectrometers, two microbalances, a low-energy electron diffraction LEED optic, a sputter gun, and a transfer system. Phthalic anhydride (PAA) (Merck, for synthesis) was evaporated from a home-built source. The evaporator setup consists of a stainless steel crucible connected to two full metal valves. Via the first valve the crucible is evacuated and the PAA is cleaned by repeated evacuation and sublimation steps. The second valve is used to dose the PAA into the UHV chamber. A stainless steel tube pointing toward the sample

3. RESULTS AND DISCUSSION 3.1. Vibrational Spectrum of Phthalic Anhydride. As a starting point for the analysis, we summarize briefly some important aspects of the vibrational spectrum of PAA. The B


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Figure 1. Calculated vibrational spectrum of PAA in the gas phase (see text for details). The six most intense vibrational modes were visualized using the software QVibePlot (see ref 53). The insets graphically illustrate the nature of the vibrational mode. The arrows indicate the orientation of the dynamic dipole moment.

νas(CO) at 1786 cm−1. Both are polarized along the molecular axis 2. Finally, we have bands corresponding to vibrational modes with symmetry A1. The modes at 1084 and 1238 cm−1 are coupled vibrations involving the symmetric C− O stretching mode νs(C−O) and C−C stretching modes of the rings; the band at 1838 cm−1 corresponds to the symmetric CO stretching mode νs(CO). All three A1 modes are polarized along the molecular axis 1. For further discussion on the vibrational spectra of PAA, we refer to the literature.40,54,55 3.2. Temperature-Programmed Infrared Reflection Absorption Spectroscopy (TP-IRAS). For the TP-IRAS experiments, we prepared an ordered MgO(100) film on Ag(100) and, subsequently, deposited a multilayer film of PAA (approximately 15 monolayers) from a Knudsen cell at a surface temperature of 114 K. The spectra taken during adsorption and growth at 100 K have been discussed elsewhere,40 including the influence of the surface temperature, the PAA film thickness, and MgO defect structure. Here, we focus on the temperature-dependent behavior of the PAA multilayer film only. Immediately before starting the TP-IRAS experiment, the reference spectrum was recorded. During the experiment, the temperature was linearly increased at a heating rate of 1 K min−1, and IR spectra were continuously recorded at a rate 1 spectrum min−1 during the temperature ramp. For our analysis, we focus on the five most intense bands, which are found at 725−727, 916−921, 1262, 1776−1779, and 1856 cm−1. On the basis of a comparison with the calculated spectrum in section 3.1, these bands can be assigned to the out-of-plane C−H deformation mode γ(C−H), the antisymmetric C−O stretching mode νas(C−O), the symmetric C−O stretching mode ν s (C−O) (both coupling to C−C stretching modes), antisymmetric CO stretching mode νas(CO), and the symmetric CO stretching mode νs(CO), respectively. In Figure 2a−e, the development of these bands is shown as a

vibrational properties of PAA have been investigated in the crystalline phase,54 in matrix,55 and in form of adsorbates on metal surfaces.56 Up to now, studies on atomically well-defined oxide substrates have not been reported apart for our own work mentioned above.40 While we focused on the adsorption behavior of PAA in the monolayer region in a previous publication, the present study aims at the investigation of the multilayer. Figure 1 shows the calculated IR spectrum from DFT in the frequency range from 600 to 2000 cm−1, which is most relevant in the following. For easier comparison with the experiment, the normal modes were convoluted by Lorentz profiles with a full width at half-maximum (fwhm) of 5 cm−1. Here, we focus on the most intense bands at 694, 842, 1084, 1238, 1786, and 1838 cm−1 only. The nature, symmetry, and polarization of the six most intense bands are shown in the insets in Figure 1. Note the C2v symmetry of PAA simplifies the analysis, as the modes with the symmetry A1, B1, and B2 are polarized along the molecular axes 1, 3, and 2, respectively (we denote the molecular axes as 1, 2, and 3 and the axes along the surface directions as x, y, and z to avoid confusion). In addition, we have visualized the nature of the modes using the software QVibePlot.53 In these visualizations, torsions between 4 centers are represented as yellow and green bows, deformations between 3 centers are shown as blue and red arcs, and stretching movements between 2 centers are displayed as blue and red lines. For more information, we refer to the literature and documentation of QVibePlot.53 The normal mode at 694 cm−1 is easily identified as the outof-plane C−H deformation mode γ(C−H). It is the only mode of symmetry B1 that gives rise to a high intensity band, and it is polarized along axis 3. Two intense normal modes are visible that correspond to modes with symmetry B2. These are the antisymmetric C−O stretching mode at 842 cm−1 νas(C−O) and the very intense antisymmetric CO stretching mode C


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Figure 2. In the upper panels the developments of five different vibrational modes recorded during the TP-IRAS experiments are shown. Note that the reference spectrum was recorded after deposition of the PAA film and before the temperature ramp. The integrated intensity of the respective frequency range is plotted in the lower panels. The inset illustrates the nature of the mode and its polarization. (a) Development of the out-of-plane deformation mode C−H bending mode γ(C−H) at 725 cm−1. (b) Development of the antisymmetric C−O stretching mode νa (C−O) at 916 cm−1. (c) Development of the symmetric C−O stretching mode and C−C ring mode νs (C−O) at 1262 cm−1. (d) Development of the antisymmetric C O stretching mode νa (CO) at 1779 cm−1. (e) Development of the symmetric CO stretching mode νs (CO) at 1856 cm−1. D


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Langmuir function of temperature. Only every second spectrum is displayed for clarity, and spectra above 300 K are not shown here, as no further changes occurred. It is important to note that we recorded difference spectra with the as-grown multilayer film at 114 K as a reference. Therefore, positive difference peaks indicate a decrease in band intensity, and negative peaks correspond to an increase in band intensity. Complex peak shapes in the difference spectrum, for example, s-shaped peaks, arise from peak shifts. For the orientation analysis, all peak intensities were integrated. The results are displayed in the lower panels in Figure 2a−e. Note that the peak intensity at 114 K corresponds to the original PAA multilayer coverage. At 300 K multilayer desorption is complete, and the corresponding intensity is used as the baseline. Characteristic changes in intensity are observed for the five bands. The intensity decreases between 230 and 240 K for all bands, indicating desorption of the multilayer. Therefore, we can use the intensity values above 240 K as the zero-level for all bands. At temperatures below 230 K, changes in band intensity are not due to desorption but indicate changes of the molecular orientation. Interestingly, the intensity changes are very different for modes of different symmetry. More precisely, the behavior is similar for modes of identical symmetry, that is, for the two A1 and the two B2 modes. This behavior perfectly agrees with what is expected from the symmetry arguments discussed in section 3.1. Next, we use the intensity data in Figure 2 to determine the absolute molecular orientation in the PAA multilayer as a function of temperature. 3.3. Procedure for Determination of the Molecular Orientation. Vibrational spectroscopy is one of the standard methods to determine the molecular orientation in thin films.57,58 Different experimental methods including IRAS, sum frequency generation, or high-resolution electron loss spectroscopy have been used to measure molecular orientation in vacuum, in the presence of a gas phase and at solid/liquid interfaces.59,60 Different experimental procedures were applied, depending on the molecule and the experimental data that are available. Here, we describe a procedure that makes use of intensity ratio only and, therefore, is particularly well suited for temperature-programmed experiments, in which the morphology or film thickness may change. For the case of PAA, the application is particularly simple as the dynamic dipole moments μ⃗ i are aligned with the symmetry elements of the molecule (see section 3.1). The analysis procedure may, however, be straightforwardly adapted to more complex and less symmetric molecules if sufficient information on the polarization of the vibrational modes is available. As illustrated in Figure 3, we define three unit vectors along the principal axes of the molecule, denoted as ei⃗ with i = 1, 2, 3, and three unit vectors ek⃗ with k = x, y, z defining the orientation of the surface. Using the common definition, ez⃗ is the direction perpendicular to the surface and ex⃗ ,y are unit vectors parallel to the surface. As a result of the metal surface selection rule (MSSR) [see, e.g., ref 57], which also holds for thin oxide films on metal substrates, only the z-component of the dynamic dipole moments μ⃗i (with μ⃗i = μiei⃗ ) leads to absorption of IR radiation. This yields for the intensity I of a band along the molecular axis i: Ii = c |Eez⃗ μi ei⃗ |2 = cE2μi 2 | ez⃗ ei⃗ |2 = cE2μi 2 cos2 αz , i i = 1, 2, 3

Figure 3. (a) Orientation of the molecular axes and polarization of the vibrational modes. (b) Ratio of the normalized intensities of three differently polarized modes as derived from the experimental data in Figure 2. See text for details.

Here, E is the electric field strength, c is an experimental constant, and αz,i is the angle between the molecular axis i and the surface normal. To account for the different magnitude μi of the dynamic dipole moments of the vibrational modes, a reference spectrum is required, which can be a disordered multilayer, a transmission spectrum, or a gas-phase spectrum. Normalizing the intensities Ii in eq 1 to the reference Iref,i, we obtain the normalized intensities In,i: In, i =

Ii Iref, i

= c′ cos2 αz , i

with

i = 1, 2, 3 (2)

To eliminate the experimental constant c′ and compensate for time- and temperature-dependent changes, we use intensity ratios ri,j instead of the intensities In,i: ri , j =

In, i In, j

=

cos2 αz , i cos2 αz , j

with

(i , j) = (1, 2), (2, 3), (3, 1)

(3)

To find the best fit between the ideal intensity ratios ri,j and real intensity ratios rexp,i,j, we minimize the sum over squares of the relative deviations: R=

with

⎛ rexp, i , j − ri , j ⎞2 ⎟⎟ = min ri , j ⎠ (i , j) ⎝

∑ ⎜⎜

(i , j) = (1, 2), (2, 3), (3, 1)

(1) E

with (4) DOI: 10.1021/acs.langmuir.5b01392 Langmuir XXXX, XXX, XXX−XXX

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Langmuir The procedure is particularly simple for the case of three vibrational modes polarized along the principal axes of the molecule (see below). However, it can be straightforwardly extended to a larger number of bands or to lower symmetry molecules, provided that the orientation of the dynamic dipole moments is known, for example, from theoretical calculations. 3.4. Orientation, Thermal Behavior, and Phase Transformations of Phthalic Anhydride Thin Films on MgO(100). Next, we apply the above-described procedure to determine the molecular orientation of the PAA from the temperature-dependent IRAS data shown in section 3.2. As expected, the modes with identical symmetry show a very similar intensity behavior. To keep the analysis simple, we use only the three modes of different symmetry, which show the highest intensity in the multilayer spectra (see ref 40). These are the out-of-plane C−H bending mode γ(C−H) at 727 cm−1 (B1, polarization along axis 3), the symmetric C−O stretching mode νs(C−O) at 1262 cm−1 (A1, polarization along axis 1), and the antisymmetric CO stretching mode ν a(CO) at 1776 cm−1 (B2, polarization along axis 2). Next, a reference spectrum is required (see eq 2). In case of a random orientation, a multilayer spectrum would be a suitable reference. In the present case, we may expect, however, that even for PAA multilayers at low temperature, the PAA molecules may still adopt a preferential orientation. Therefore, we rather use the gas-phase spectrum for normalization (from ref 61, relative intensities γ(C−H):νs(C−O):νa(CO) ≈ 10:24:64). It should be noted that this procedure will introduce some systematic error because of coupling effects in the solid film. In the crystalline phase of PAA, for example, vibrational coupling is relatively complex due to the low-symmetry of the unit cell containing four PAA molecules.62 Because of the nearly parallel (or antiparallel) molecular orientation, the systematic error is, however, relatively small in the present case. The calculated intensity ratios after normalization are displayed in Figure 3. We identify six characteristic regions labeled (i)−(vi). In region (i) ranging up to 145 K, the intensity ratios do not change, indicating that the as-grown film structure remains frozen. In the temperature window from 145 to 205 K, pronounced changes of the intensity ratios indicate a sequence of restructuring steps of the film. The corresponding regions (ii)−(iv) will be discussed below. Qualitatively, high values of r3,1 and low values of r2,3 indicate that the PAA is preferentially oriented with the molecular plane parallel to the surface. In the temperature region (v) from 205 to 230 K, no further restructuring is observed. Above 230 K, the PAA multilayer desorbs and only the monolayer remains.40 The present TP-IRAS experiment does not provide sufficient sensitivity to determine the molecular orientation in region (vi). We refer to another publication for more details on the monolayer regime.40 To obtain a more quantitative picture, we apply the analysis procedure described in section 3.3 using the experimental data displayed in Figure 3. For each temperature, the R factor is calculated as defined in eq 4 for all possible molecular orientations. An example is shown in Figure 4a (T = 130 K) where R is plotted as a function of the fractions f1z and f 2z of a unit vector along molecular axes i = 1, 2 along the z direction (f 3z follows from the values f1z and f 2z):

Figure 4. (a) Polar plot of the R-factor as a function of the molecular orientation at a surface temperature of 130 K; and (b) molecular orientation as derived from the minimum of the R-factor as a function of temperature. Two reorientation transitions are clearly resolved. The yellow (1), red (2), and green (3) dots indicate the limiting molecular orientations as shown in the insets. The black triangle (4) corresponds to a random molecular orientation (magic angle).

fi = | ei⃗ ez⃗ | = i = 1, 2, 3

μi⃗ μi

ez⃗

with

∑ fi2

=1

and

i

(5)

The corresponding molecular orientations at the corner positions of the polar plot ( f iz = 1) are shown schematically in the insets. The minimum of R indicates the molecular orientation and can be determined with good precision. An estimate of the experimental error of the procedure can be obtained, if the same analysis is performed with other sets of vibrational modes. Using the A1 mode νs(CO) at 1856 cm−1 instead of the νs(C−O) mode at 1262 cm−1, for example, we obtain very similar results. The maximum angular deviation is between 1° and 3° for the three molecular axes (see below). This confirms the reliability of the procedure. F


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orientation in region (i), indicating that the structure remains frozen up to a temperature of 145 K. In the narrow temperature interval between 145 and 160 K, the film restructures adopting a nearly flat-lying orientation. Here, the average tilting angle of molecular axis 3 decreases to values below 25°, and the tilting angles of axes 1 and 2 increase to values between 70° and 75°. Note that these angles are macroscopic averages including rougher surface areas. This implies that the degree of orientation on well-defined MgO(100) facets will be even larger. The flat-lying phase is stable in a small temperature window (iii) from 160 to 170 K only. In the temperature interval (iv) between 170 and 205 K, we observe a second and very pronounced reorientation transition. The tilting angle of axis 3 increases dramatically to a value of 45°. Simultaneously, the molecule rotates around the axis 1, so that the tilting angle of axis 2 decreases to about 52°. We assign this transition to a crystallization transition in the film. In this process, the orientation of the crystallites that are formed will be controlled by their interaction with the MgO(100) surface. In our previous study, we have shown that the first monolayer of PAA adopts a different orientation at 100 and at 300 K, respectively.40 Experimentally, we were not able to pinpoint a precise transition temperature between these limiting cases because of the experimental difficulty to detect weak monolayer features in the presence of a growing multilayer. However, we could show that the orientation in the first monolayer is related to the interaction with the oxide. Briefly, we attributed the temperature-dependent change in orientation to a thermally activated linking to the support. At low temperature, PAA adsorbs molecularly and without structural transformation, adopting a preferentially parallel orientation. At room temperature (300 K), the molecule undergoes partial ring opening and binds to the surface via both surface carboxylates and intact PAA entities. As a result, the molecules adopt a tilted orientation.40 We suggest that the monolayer film forms the template for the growth of the crystalline multilayer film. Thereby, it controls the molecular orientation of the crystalline PAA film. At temperatures above 230 K, the PAA multilayer desorbs. The remaining monolayer cannot be detected in the present experiments, because of the relatively low background stability of the TP-IRAS experiments. We refer to our second publication in which corresponding isothermal experiments are discussed.40

In Figure 4b, the development of the molecular orientation is shown as a function of the temperature in the form of a 3D polar plot. The molecular orientation (1) corresponds to a flatlying PAA, while (2) and (3) represent upright standing PAA with the C2 axis parallel or perpendicular to the surface, respectively. Point (4) indicates the magic angle position, which would correspond to random molecular orientation. The relative fractions f iz of the dynamic dipole as a function of the temperature are displayed in Figure 5a. From these fractions, we can calculate the tilting angles between the molecular axes and the z direction, which are displayed in Figure 5b.

4. CONCLUSIONS We have investigated the molecular orientation of phthalic anhydride (PAA) films on atomically well-defined MgO(100) on Ag(100) using temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS). We propose a robust analysis procedure to extract the molecular orientation, which relies on the intensity ratios of the most intense vibrational bands. The nature and polarization of these bands are illustrated by density-functional (DFT) calculations. (1) Even at low deposition temperature around 110 K, the PAA multilayer grows with a preferential orientation. The PAA molecular plane shows some degree of alignment with the MgO(100) surface and the average tilting angle is determined to be 35°. No change of the molecular orientation occurs up to 145 K, indicating that the film structure remains frozen up to this temperature. (2) The PAA film shows a first structural transition in a temperature interval between 145 and 160 K. Here, the PAA

Figure 5. (a) Relative fractions along the surface normal z of the dynamic dipole moments of the three selected modes polarized along the axes 1, 2, and 3 as a function of temperature; and (b) angles between the molecular axes 1, 2, and 3 and the surface normal z as a function of temperature. The red and the black parts of the molecule indicate the anhydride group and the phenyl ring, respectively.

The experimental results in Figure 5b reveal a quite surprising temperature-dependent behavior of the PAA film. We find that even upon deposition at 100 K, the PAA adopts a preferential orientation with the molecular plane parallel to the surface. The average tilting angle with respect to the surface normal α3z is about 35°. There is no change of the molecular G


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Langmuir molecules adopt a nearly flat-lying orientation, with an average tilting angle of the molecular plane that decreases to values below 25°. The flat lying structure, however, is stable only in a narrow temperature window from 160 to 170 K. (3) In the temperature range between 170 and 205 K, the PAA undergoes a second structural transition. This transition is attributed to crystallization of the film and is associated with a pronounced reorientation of the PAA molecules. The latter adopt a strongly tilted orientation with the angle between the molecular plane and the surface increasing to a value of 45°. This reorientation occurs mainly via a molecular rotation around the C2 axis; that is, this axis retains its preferential orientation parallel to the MgO surface (angle between the C2 axis and surface normal: 70°). (4) The oriented crystalline film shows no further changes in orientation at temperatures above 205 K. At 230 K, the PAA multilayer desorbs from the MgO surface. The current study provides a detailed picture of the temperature-dependent molecular structure of an organic thin film and shows that the molecular orientation in the film is largely controlled by its interaction with the surface. The results will serve as a basis for forthcoming studies of larger functional molecules in which PAA or phthalic acid groups may serve as linker units.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 9131 8527308. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Research Unit FOR 1878 “funCOS − Functional Molecular Structures on Complex Oxide Surfaces”. Additional support is acknowledged from the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. T.X. gratefully acknowledges support from a Ph.D. grant of the China Scholarship Council (CSC). We also acknowledge support by the COST Action CM1104 “Reducible oxide chemistry, structure and functions”.

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Molecular Orientation and Structural Transformations in Phthalic

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