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Anchoring of a Carboxyl-Functionalized Norbornadiene Derivative to an Atomically-Defined Cobalt Oxide Surface Matthias Schwarz, Susanne Mohr, Tao Xu, Tibor Doepper, Cornelius Weiß, Katharina Civale, Andreas Hirsch, Andreas Görling, and Jörg Libuda J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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The Journal of Physical Chemistry

Anchoring of a Carboxyl-Functionalized Norbornadiene Derivative to an Atomically-Defined Cobalt Oxide Surface Matthias Schwarz1, Susanne Mohr1, Tao Xu1, Tibor Döpper2,Cornelius Weiß3, Katharina Civale3, Andreas Hirsch3, Andreas Görling2,4, Jörg Libuda1,4 * 1

Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg,

Egerlandstraße 3, D-91058 Erlangen, Germany 2

Lehrstuhl für Theoretische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg,

Egerlandstraße 3, D-91058 Erlangen, Germany 3

Lehrstuhl für Organische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestraße 42, D-91058 Erlangen, Germany 4

Erlangen Catalysis Resource Center and Interdisciplinary Center Interface Controlled

Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany

*corresponding author: [email protected]

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Abstract We have investigated the anchoring of the molecular energy carrier norbornadiene (NBD) to an atomically-defined oxide surface. To this end, we synthesized a carboxyl-functionalized NBD derivative, namely 1-(2'-norbornadienyl)pentanoic acid (NBDA), and deposited it by physical vapor deposition (PVD) under ultrahigh vacuum (UHV) conditions onto a well-ordered Co3O4(111) film grown on Ir(100). In addition, we performed a comparative growth study with benzoic acid (BA) under identical conditions which was used as a reference. The interaction and orientation of NBDA and BA with the oxide surface was monitored in-situ during film growth by isothermal time-resolved infrared reflection absorption spectroscopy (TR-IRAS), both below and above the multilayer desorption temperature. The thermal behavior and stability of the films was investigated by temperature programmed IRAS (TP-IRAS), with help of density functional (DF) calculations. BA binds to Co3O4(111) under formation of a symmetric chelating carboxylate with the molecular plane oriented nearly perpendicular to the surface. At low temperature (130 K), intact BA physisorbs in form of dimers on top of the saturated monolayer. Upon annealing to 155 K, a reordering transition is observed, in which BA in the multilayer adopts a more flat-lying orientation. The BA multilayer desorbs at 220 K, whereas the surface-anchored BA monolayer is stable up to 400 K. At higher temperature (400 – 550 K), desorption and decomposition is observed. Very similar to BA, NBDA binds to Co3O4(111) by formation of a symmetric chelating carboxylate. In the multilayer, which desorbs at 240 K, hydrogen-bonded NBDA dimers are formed. Upon PVD of NBDA at 300 K, only a surface anchored carboxylate is stable. The anchored NBDA film shows a characteristic restructuring behavior as a function of coverage. At low coverage the NBDA, adopts a flat-lying structure in which the norbornadiene unit interacts with the Co3O4 surface. With increasing coverage, the norbornadiene units detach from the oxide and the NBDA adopts an upright-standing orientation. Similar to BA, the anchored film is stable up to 400 K and decomposes in the temperature region between 400 and 550 K, leaving behind hydrocarbon residues on the oxide surface.


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1. Introduction The storage of solar energy in chemical compounds is one of the major challenges in the field of renewable energies.1 A promising candidate for molecular energy storage is the valence couple norbonadiene (NBD) and quadricyclane (QC). The latter is formed in a photochemical intramolecular cycloaddition from NBD (see Figure 1a). This system has been proposed to hold great potential for solar energy harvesting and storage already half a century ago.2-6 Recently, it has attracted renewed attention

7-13

also inspired by the idea that the energy release could not

only be triggered catalytically but also electrochemically, thereby enabling the development of a “energy storing solar cell”13. Controlling the energy release process, i.e. the cycloreversion from QC to NBD, is the major challenge regarding future applications of the NBD/QC system. Recently, we investigated this reaction both under electrochemical conditions13 and in a model catalytic study under UHV conditions.12 Among the limiting factors are side reactions which are difficult to control in free NBD. In this work, we follow a new approach which is based on the anchoring of NBD to a solid surface. Energy storage in anchored molecular species at such a “rechargeable hybrid interface” may come with two advantages. First, undesired intermolecular side reactions could be avoided; secondly, charge transfer between the NBD/QC and the electrode may be controlled in a better way. In the present study, we focus on the preparation and characterization of a well-defined NBD film anchored to an atomically ordered oxide surface. The availability of such a model system will open the possibility to explore the surface chemistry, electrochemistry and charge transfer of NBD/QC at interfaces in great detail. To link the NBD to the surface, we introduce a carboxylic acid group in form of the NBD derivative 1-(2’-norbornadienyl)pentanoic acid (NBDA) (see Figure 1b). Carboxylic acids are among the most common linkers for oxide surfaces under ambient conditions

14-16

and a substantial amount of work has been performed under UHV

conditions to explore the anchoring reaction as well.17-25 As a model oxide surface, we use an atomically defined Co3O4(111) film grown on Ir(100).26-27 For the present study, this model oxide surface comes with a number of advantages. For instance, the atomic structure of the film has been established by IV-LEED (low energy electron diffraction) and scanning tunneling microscopy (STM).26 Moreover, the thickness of the oxide film can be varied over a large 3 ACS Paragon Plus Environment


range,27 which will permit controlling the electron transfer to the NBD. Finally, Co3O4 is stable within a large potential and pH window, which will permit future studies in electrochemical environments.28-29 In the present work, we focus on the characterization of the NBDA film on Co3O4(111 by timeresolved and temperature-programmed IRAS. As the NBDA shows a rather complex behavior as a function of coverage, we include a comparative study with benzoic acid (BA) performed under identical conditions. The latter data is used as a reference to assist the interpretation of the IR data. In particular, we explore the kinetics of film formation and the stability of the anchored films. This data will be key, when using the NBDA/Co3O4(111) as a model system to build up “energy storing hybrid interfaces” in future work.

2. Experimental All measurements were conducted in an UHV system (base pressure of 1.0×10-10 mbar) which was described in detail elsewhere.30 The UHV system consists of a preparation chamber for sample cleaning and preparation and a measurement chamber for the IRAS experiments. The latter is equipped with a FTIR spectrometer (Bruker VERTEX 80v) which is connected via differentially pumped KBr windows. Preparation of Co3O4(111)/Ir(100): The Co3O4(111) thin films were prepared by reactive deposition of cobalt in O2 atmosphere onto an Ir(100) single crystal substrate following the method previously described in literature,26 with slightly modified preparation parameters such as O2 partial pressure, annealing temperature and Co film thickness. First the Ir(100) single crystal (MaTeck) is cleaned by cycles of Ar+ sputtering (1.8 keV, 300 K, 1 hour; Linde, 99.999) and annealing (1400 K, 3 min) until a sharp LEED pattern (300 K) of the Ir(100)-(5×1) reconstructed surface was seen. In the second step, the Ir(100)-(5×1) surface was annealed at 1300 K in 5×10-8 mbar O2 (Linde, 99.999) for 3 min and, subsequently, cooled to room temperature in O2 atmosphere. This leads to formation of a Ir(100)-(2×1)O reconstructed surface as observable by LEED. On the Ir(100)-(2×1)O, Co is deposited at sample temperatures below 270 K using a commercial electron beam evaporator (Focus EFM3, 2 mm Co rod, Alfa Aesar 99,995%) in an oxygen atmosphere of 1.0 ×10-6 mbar. Co was evaporated for 18 min at a rate of 4 ACS Paragon Plus Environment

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2 Å/min as determined by a quartz micro-balance. After deposition, the film was annealed first in O2 atmosphere (1.0 ×10-6 mbar) at 520 K for 2 min and then in UHV at 690 K for 10 min. The films were checked by LEED and, qualitatively, by comparing the LEED I-V curves to previously published data in the literature.26 Deposition of BA: BA (Sigma-Aldrich, 99.9%) was deposited using a supersonic molecular beam (SSMB) source. The method was described in detail elsewhere 31. Briefly, Ar (Linde, 99.999%) flows through a BA reservoir kept at elevated temperature and, subsequently, the BA/Ar mixture is expanded through a nozzle into the vacuum to generate a molecular beam. The method allows well-controlled deposition of organic molecules at low rate avoiding any contamination of the UHV chamber. The deposition rate can be controlled by a shutter blocking the beam periodically or by the temperature of the BA reservoir. For the present experiments, the reservoir was heated up to around 470 K. No decomposition products were observed in the BA reservoir after the heating (as checked by transmission FTIR) or in the BA film deposited on the surface (see below). Using a continuous beam, the BA monolayer saturates at low sample temperature after approximately 5 min, allowing us to estimate the deposition rate to be approximately 0.2 ML/min. Synthesis of NBDA: 1-cyano-4-(2’-norbornadienyl)butane 1-bromo-4-(2’-norbornadienyl)butane (1,14 g; 6,58 mmol), which was synthesized according to literature-known procedure32, was dissolved in 2,50 ml ethylene glycol in a dry flask under nitrogen atmosphere. After the addition of dried potassium cyanide (0,523 g; 8,03 mmol) the reaction mixture was heated to 100°C for 22 hours. The solution was allowed to cool to room temperature and was diluted with water (50 ml) and extracted with diethylether (3 x 50 ml). The combined organic phases were washed with water and brine and dried over CaCl2 afterwards. The solvent was removed by rotary evaporation and the product which was purified by column chromatography (SiO2; hexane/ ethylacetate 9:1; Rf = 0,4) was received as a colorless oil with a yield of 75%. 1

H-NMR (400 MHz, CDCl3) δ (ppm) = 6.73 (d, 2H); 6.13 (s, 1H); 3.48 (m, 1H); 3.26 (m, 1H);

2.31 (t, 3J = 9.2 Hz, 2H); 2.23 (t, 3J = 8.8 Hz, 2H); 1.95 (m, 2H); 1.59 (m, 4H) 13

C-NMR (100 MHz, CDCl3) δ (ppm) = 157.4; 143.8; 142.2; 134.3; 119.7; 73.5; 53.3; 50.0;

30.4; 26.0; 24.8; 16.9 5 ACS Paragon Plus Environment


HRMS (EI): m/z for M = 12C12H14N1: found m/z = 172.1121; calculated m/z = 172.1119 1-(2'-norbornadienyl)pentanoic acid To a solution of 1-cyano-4-(2’-norbornadienyl)butane (0,422 g; 2,44 mmol) in ethylenglycol (1,5 ml) was added potassium hydroxide (0,278 g; 4,95 mmol) and the reaction mixture was heated to 160°C for 3 hours. After no further formation of ammonia could be observed, the solution was cooled to room temperature, diluted with water (50 ml) and acidified with 1M HCl to pH = 5. The aqueous phase was extracted with diethylether (3 x 50 ml) and the combined organic phases were washed with water (50 ml) and dried over CaCl2. Column chromatography (SiO2; ethylacetate; Rf = 0,6) gave the product as colorless oil with a yield of 60%. 1

H-NMR (400 MHz, CDCl3) δ (ppm) = 11.28 (br s, 1H); 6.74 (m, 2H); 6.12 (m, 1H); 3.47 (m,

1H); 3.25 (m, 1H); 2.33 (t, 3J = 7.5 Hz, 2H); 2.19 (m, 2H); 1.94 (m, 2H); 1.59 (m, 4H) 13

C-NMR (100 MHz, CDCl3) δ (ppm) = 180.0; 158.2; 143.8; 142.3; 133.7; 73.5; 53.4; 50.0;

33.8; 31.0; 26.6; 24.3 HRMS (EI): m/z for M = 12C12H16O2: found m/z = 192.1142; calculated m/z = 192.1145 Deposition of NBDA: Because of the small amounts of material available from synthesis, NBDA could not be deposited using the molecular beam method used for BA. Therefore, NBDA was evaporated from a home build evaporation source, consisting of a stainless steel reservoir that is connected to the UHV chamber via a full metal valve. A stainless steel tube guides the molecular flux to the sample. The valve and the line can be heated to prevent condensation. NBDA was filled into the reservoir using acetonitrile as solvent. Subsequently, the solvent was removed by pumping via a second high vacuum line. For evaporation, the NBDA was heated to 340 to 350 K, as controlled by an external water bath and thermostat. Prior to each experiment NBDA, was cleaned via pumping for at least 10 minutes over a separate high vacuum pumping line. For the NBDA uptake experiments, heating of the reservoir was started simultaneously with the acquisition of the IR spectra, leading to a lower deposition rate in the initial phase of the experiment. A constant deposition rate was reached after 10 min of deposition. Time-resolved and temperature-programmed IRAS experiments: All IRAS measurements were recorded with a liquid nitrogen cooled MCT detector at a resolution of 4 cm-1. The acquisition times per spectrum were 30 s for BA and 60 s for NBDA. For the isothermal adsorption 6 ACS Paragon Plus Environment

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experiments, reference spectra were recorded on the clean substrates immediately before the start of the deposition. For the TP-IRAS, two sets of experiments were performed. First multilayers of BA and NBDA were deposited at 130 K and IR spectra were continuously recorded while heating to 370 K at a rate of 2 K/min. The spectrum of the clean surface was used as a reference. For the second set of experiments, surface-bound monolayers prepared at 300 K were heated to 600 K at a rate of 2 K/min. For the latter data, we applied a data treatment procedure developed previously, which accounts for the temperature dependent changes in band intensity.21 All data is referred to the last spectrum of the temperature ramp, where we assume the surface to be more or less free of IR active species. Computational details: For identification of our IR signals we performed DFT calculations using the Turbomole program suite.33 We employed the exchange correlation functional of Perdew, Becke and Enzershof (PBE)

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in combination with the def2-TZVP basis set of Wiegand and

Ahlrichs 35 as well as the Grimme D3 dispersion correction36. The calculations utilized the RI-J37 and MARI-J38 approximations. The frequencies were calculated within the harmonic approximation.

3. Results and Discussion 3.1. IR Spectrum of 1-(2’-Norbornadienyl)pentanoic acid Before considering the thin film deposition experiments, we briefly discuss the IR spectra of the molecules used in this study. The IR spectrum of BA has been discussed in detail, previously.18, 25, 39-41

We provide an assignment of the IR bands in Table 1 together with the notation used in

this work and refer to the literature for details. To assign the IR bands of NBDA (Figure 2), we use the IR spectrum recorded after deposition of a multilayer (deposition time of 200 min) and assign the bands on the basis of DF calculations of the free molecule. For comparison we use both, the calculated spectrum of the free NBDA monomer and the one of the NBDA dimer to take into account the effect of hydrogen bond formation in the multilayer. A comparison of the experimental and the calculated spectra are shown in Figure 2. Similar to BA, the calculated spectrum of the NBDA dimer shows a better correspondence to the multilayer spectrum than the monomer, indicating the formation of H-bonded structures. The nature of the vibrational bands was carefully analyzed using the visualization tool QVibePlot (see 42). The bands were assigned on the basis of the calculated spectra and previous 7 ACS Paragon Plus Environment


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literature,12-13, 18, 25, 43-49 and the most prominent features and assignments are listed in Table 2. Briefly, the following spectral regions are of relevance for the discussion below. In the CH stretching region four characteristic features are observed at 2863, 2939, 2970 and 3065 cm-1, which are attributed to the symmetric CH stretching mode of the CH2 groups, νs(CH2), the antisymmetric CH stretching mode of the CH2 groups, νas(CH2), the CH stretching modes of the tertiary carbon C1 and C4 of the NBD unit, ν(CtH), and the CH stretching modes of the carbon in the doubly bonded C2,3,6 in the NBD unit, ν(=CH). The spectrum of the NBDA is dominated by the band of the C=O stretching mode ν(C=O) of the carboxylic acid unit at 1709 cm-1, which is typically broadened due to the formation of H bridges. DFT of free NBD (see 12), predicts the (very weak) symmetrically coupled C=C stretching mode, νs(C=C), and the (weak) antisymmetrically coupled C=C stretching mode, νas(C=C), of the NBD to be located at 1603 and 1561 cm-1, respectively. Indeed, we observe a weak band at 1569 cm-1 which we assign to νas(C=C). The broad band at 1450 cm-1 is assigned to the characteristic OH deformation mode of carboxylic acids, δ(OH), however, for NBDA it overlaps with the CH2 deformation modes, δ(CH2), of the alkyl chain and the apical CH2 of NBDA. The band at 1325 cm-1 is a characteristic deformation and stretching mode of the NBD unit, in which the C1,2,6 and C3,4,5 move antisymmetrically (see 12). At wavenumbers below the broad ν(C-O) band of the carboxylic acid at 1252 cm-1, several less characteristic CC stretching and deformation modes of the hydrocarbon framework are found. Among these, the most prominent feature is the out-of-plane CH deformation mode of the alkene, γ(CH), at 717 cm-1 which, in fact, is one of the most prominent modes in pure NBD.

3.2 Growth of Benzoic Acid on Co3O4(111)/Ir(100) at 130 K The growth of BA and NBDA thin films on Co3O4(111) at 130 K was studied by TR-IRAS during the deposition process. First, the adsorption of BA is discussed. Selected data is shown in Figure 3a (complete spectra are provided in the Supporting Information). In Figure 3b, a spectrum in the sub-monolayer region is compared to a spectrum at multilayer coverage. In Figure 3c selected peak areas are plotted as function of deposition time. In the initial phase of BA deposition (Figure 3a and b) a prominent peak is observed at around 1412 cm-1 shifting to 1423 cm-1 with increasing coverage. Based on previous work


18, 43-49

, the

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band is assigned to the OCO symmetric stretching mode of the surface anchored carboxylate, νs(OCO). This observation indicates deprotonation and the formation of benzoate species even at 130 K. The corresponding asymmetric OCO stretching mode, νas(OCO), which we would expected to appear 1550 cm-1 (see

18-19, 21

) is not observed, indicating the formation of a

symmetric carboxylate. In this adsorption geometry, νas(OCO) is polarized parallel to the surface and cannot be observed because of the metal surface selection rule (MSSR). The MSSR, which also holds for thin oxide films on metal supports, states that only those modes which have a dynamic dipole perpendicular to the surface can be probed by IRAS.50 Furthermore, it is likely that a chelating carboxylate is formed, although there is no direct proof for this assumption from the IR spectra.19-20 As the terminating Co2+ ions of Co3O4(111) have a distance of 5.7 Å, the carboxylate group (COO-) cannot bridge between two Co2+ ions. Note that the binding adsorption geometry is the energetically most favored one in most cases (see e.g. formic acid on ZnO(10ī0)24, rutile TiO2(110)51, anatase TiO2(110) 23, and on CeO2(111) 52 and benzoic acid on rutile TiO2(110) 25or on MgO(100) 18). Information on the molecular orientation of the benzene ring can be obtained from the out-ofplane CH deformation mode of the benzene ring, γoop(CH), at 714 cm-1. This band is polarized perpendicular to the aromatic ring and, taking into account the MSSR, is most intense for flatlying BA. The low intensity in comparison to the multilayer spectrum indicates strong tilting of the aromatic ring away from the surface. We conclude that the BA adsorbs on Co3O4(111) in the monolayer region in form of a strongly tilted chelating carboxylate. With increasing BA exposure, the intensity of the band at 714 cm-1 increases rapidly and the formation of several additional features is observed, with the most intense ones appearing at 1699, 1452, 1325, 1300, and 953 cm-1. The prominent band at 1699 cm-1 indicates the presence of intact BA (see Section 3.1) in the physisorbed multilayer. In the multilayer region, the γoop(CH) band is the dominating feature, indicating that the BA is oriented nearly parallel to the surface. In addition we observe a splitting of several bands, which has been associated with the formation of BA dimers, previously.18 A more detailed picture of the growth kinetics can be derived from the development of the band intensity with coverage, as shown in Figure 3c. Here three bands are shown, namely the ν(C=O) of the free acid at 1699 cm-1, the νs(OCO) band of the surface anchored carboxylate at 1423 cm-1, and the γoop(CH) mode at 717 cm-1. We can clearly 9 ACS Paragon Plus Environment


differentiate between monolayer formation in the first 5 minutes of deposition and multilayer growth at a later stage. In conclusion, we observe carboxylate formation for BA on Co3O4(111) even at 130 K. Our IRAS results suggest a strongly tilted orientation of the surface bound benzoate. Beyond the first monolayer, IRAS shows adsorption of molecular BA, which forms a multilayer film consisting of mainly flat-lying dimers.

3.3 Growth of 1-(2’-Norbornadienyl)pentanoic acid on Co3O4(111)/Ir(100) at 130 K Next, we investigated the adsorption of NBDA on Co3O4(111). The results are discussed in comparison to the simpler case of BA presented in Section 3.2. In Figure 3d, we show the evolution of the IR spectra as function of deposition time, acquired during evaporation of NBDA onto Co3O4(111) at 130 K. A comparison between a sub-monolayer and a multilayer spectrum is shown in Figure 3e. In contrast to BA, more information can be obtained from the CH stretching frequency region, which is, therefore, shown in Figure 3d and e. In Figure 3f, integrated areas of selected bands are plotted as function of deposition time. At low coverage, a prominent band at 1412 cm-1 dominates the spectra, while additional features are observed at 1700, 1450, 1300 and 700 cm-1. At higher coverage, the band at 1700 cm-1 grows rapidly and further weak features are observed at 1325, 1252 and 952 cm-1. In accordance with the BA data (see Section 3.2), the feature at 1412 cm-1 is attributed to the νs(OCO) mode of a surface bound carboxylate. The band position is identical to the one observed for BA. This observation suggests the bonding geometry of the carboxylate is similar for BA and NBDA, i.e. both species form a chelating carboxylate on Co3O4(111). The presence of the broad band between 1600 and 1700 cm-1 may be due to two effects. On the one hand, non-anchored, free NBDA may coexist with the anchored species giving rise to appearance of the ν(C=O) mode of the carboxylic acid. Secondly, traces of water may be coadsorbed on the surface. The later could not be avoided as NBDA could not be deposited by the cleaner molecular beam method due to the low amount of material which was available from the synthesis. At higher coverage, several new features are observed, which were assigned in Section 3.1 based on the DFT calculations and previous work 18, 43-49. Briefly, features in the CH stretching region are attributed νs(CH2)alkyl,NBD at 2863, to νas(CH2)

alkyl,NBD

at 2939 cm-1, to ν(CtH)NBD at 2970

cm-1, and to ν(=CH)NBD at 3065 cm-1. The ν(C=O) band of free NBDA is observed at 1709 cm-1 10 ACS Paragon Plus Environment

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and a very weak feature from νas(C=C) mode is observed at 1569 cm-1. The δ(OH) and ν(CH2) modes of free NBDA are observed at 1450 cm-1 and 1412 cm-1, with the latter overlapping with the νs(OCO) band in the monolayer. The characteristic antisymmetric NBD deformation and stretching band at 1325 cm-1 can be clearly identified, similar as the broad ν(C-O) mode at 1252 cm-1. At lower wavenumber, a number of weaker features appear, such as the out-of-plane deformation γ(CH)NBD at around 720 cm-1. In Figure 3f, we show the coverage dependent behavior of selected bands, namely the ν(C=O) band of the free carboxylic acid at 1709 cm-1, the δ(CH2) and δ(OH) band at 1450 cm-1 and the νs(OCO) and δ(CH2) band at 1412 cm-1. The thermal evaporator used for NBDA yields a less constant deposition rate than achieved with the molecular beam doser for BA. Still we can identify the monolayer growth regime at deposition times of up to 40 min in which mainly surface-anchored NBDA is formed. At larger deposition times, we observe multilayer growth of hydrogen-bonded NBDA. Hydrogen bonding is indicated by the position and width of the ν(C=O) band and by the appearance of the γ(OH) mode at 952 cm-1.18,

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In contrast to BA,

however, no preferred molecular orientation could be observed in the multilayer region. In conclusion, we find that the NBDA deprotonates on Co3O4(111) at 130 K and anchors to the surface by forming a chelating carboxylate. The coverage dependent transition between the anchored and the free NBDA is less well defined as for BA. Beyond the first monolayer, NBDA forms a hydrogen-bonded multilayer film without preferred molecular orientation.

3.4 Thermal Behavior of Multilayer Films of Benzoic Acid and 1-(2’-Norbornadienyl)pentanoic acid To probe the thermal behavior of BA and NBDA multilayers we performed TP-IRAS experiments. The IR spectrum of the clean Co3O4(111) film was used as a reference. After deposition of the films at 130 K, IR spectra were recorded continuously while heating the sample with a rate of 2 K/min to 370 K. For both molecules, selected data is displayed in Figure 4a and 4d, respectively (the complete set of IR data is provided in the Supporting Information). Spectra at characteristic temperatures are shown in Figure 4b and 4e. The development of selected peak intensities as a function of temperature is shown in Figure 4c and 4f (see Experimental Section).



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For BA, we observe strong changes in the intensity and shape of several modes in the temperature range between 155 and 175 K. An increase in intensity is accompanied by a narrowing of the peaks and the formation of characteristic doublets. We attribute these changes to a thermally induced reordering of the multilayer film. Similar crystallization processes have been observed for other multilayer films of organic molecules in UHV

19, 53-55

, including BA

multilayers on MgO(100)18. The narrowing and increasing intensity of specific bands is indicative of an increased ordering in the film. The splitting of the bands and the increase of the γ(OH) band at 952 cm-1 indicate dimer formation during the crystallization process. Note that the intensity increase in the range of the νs(OCO) band is not due to additional chemisorption but due to an increase in intensity of the δ(CH) and δ(OH) bands which overlap in this regime. In comparison to our previous study on of BA on MgO(100), some minor differences are identified regarding the temperature regime of the transition which we attribute to the different thicknesses of the BA multilayers in both experiments.18 The most important difference is associated with the orientation of BA as indicated by the γoop(CH) band. Whereas our experiments show an increase in intensity for γoop(CH), i.e. a transition to a more flat-lying orientation, the opposite was observed for BA films on MgO.18 Possibly, these differences may be associated with the different thicknesses of the BA films. Above 220 K most features vanish. The decreasing intensity indicates desorption of the BA multilayer. The most prominent feature remaining after multilayer desorption is the νs(OCO) band indicating surface bound benzoate. Most of the smaller features can also be related to the benzoate monolayer and are very similar to those observed in the isothermal uptake experiments (see Section 3.5). The remaining differences are related to the limited baseline stability in the TP-IRAS experiment. The behavior of the monolayer will be discussed in Section 3.6. Next we turn to the temperature-dependent behavior of the NBDA multilayer. In general, the behavior upon annealing is similar to the one found for BA. In the temperature region between 140 K and 180 K, we observe changes in intensity and peak shape for several bands but no major peak shifts. Around 180 K, narrowing of some features is observed. The integrated intensities of the ν(C=O) band of the free acid at 1700 cm-1, the νs(OCO) band of the carboxylate at 1420 cm-1 (containing contributions from δ(CH2) and δ(OH)) and the γ(OH) band at 952 cm-1 are displayed in Figure 4f. The latter two bands show a slight increase between 140 and 180 K, whereas the ν(C=O) band decreases in intensity. In part, the latter effect may be associated with desorption of 12 ACS Paragon Plus Environment

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coadsorbed water. The deformation vibrations of water at 1670 cm-1 overlap with the C=O stretching range. The narrowing of the bands and the increasing γ(OH) band above 140 K indicate some degree of reordering in the NBDA film associated with the formation of dimers. It is possible that the reordering is triggered by desorption of water. A similar phenomenon has previously been observed for PA multilayers.18-19 In comparison to the BA, however, the reordering phenomena in the NBDA multilayer are much less pronounced. At 240 K, all characteristic features of molecular NBDA disappear indicating desorption of the multilayer. Only the νs(OCO) band remains, showing that only the NBDA which is anchored as a carboxylate in the first monolayer remains adsorbed on the surface. The resulting carboxylate bands are nearly identical to those found in the isothermal uptake experiments (see Section 3.5). The remaining differences are related to the limited baseline stability in the TP-IRAS experiment. 3.5 Formation of Anchored Films of Benzoic Acid and 1-(2’-Norbornadienyl)pentanoic acid on Co3O4(111)/Ir(100) at 300 K To prepare anchored monolayers of BA and NBDA on Co3O4(111), we have performed isothermal deposition experiments at 300 K, i.e. at a temperature above the threshold of multilayer desorption. Growth of the anchored films was monitored in situ by TR-IRAS. Selected data for BA is shown in Figure 5a to c (the complete data is provided in the Supporting Information). BA was dosed for 65 min while IR spectra were recorded continuously. The corresponding spectra are plotted Figure 5a as function of deposition time. A comparison of spectra at low and high coverage is shown in Figure 5b. Figure 5c shows the integrated intensity of the νs(OCO) band as a function of deposition time. The spectra at 300 K are dominated by the νs(OCO) band at 1412 cm-1. Several weaker bands are observed at 1603, 1531, 1448, 1026, and 715 cm-1, which are all characteristic for BA. For a detailed assignment we refer to Table 1 and to literature

18, 25, 39-41

. The band νs(OCO) at 1412 cm-1 indicates anchoring of BA in form of a

symmetric chelating carboxylate. Noteworthy, the γoop(CH) band of the aromatic ring at 715 cm-1 is very weak, indicating a strongly tilted adsorption geometry. Interestingly, neither the band positons not the relative intensities change with deposition time (Figure 5b). This observation indicates that the local adsorption geometry is independent of coverage. A possible explanation for this behavior involves the formation of densely packed island under these conditions. The 13 ACS Paragon Plus Environment


anchored BA monolayer saturates after a deposition time of approximately 45 min, corresponding to a total exposure of approximately 8 monolayer equivalents. Next we turn to the growth behavior of NBDA on Co3O4(111) at 300 K. NBDA was dosed over a duration of 120 min while IR spectra were recorded continuously. The IR spectra as function of time are shown in Figure 5d. Selected spectra at low and high coverage are compared in Figure 5e. Integrated areas of selected bands are shown in Figure 5f. Specifically, we show the intensities of the νs(OCO) band at 1428 cm-1 and two new bands at 1657 cm-1 and 945 cm-1. The coverage dependent behavior for the NBDA is markedly different from that of BA and clearly more complex. Similar as for the BA, the spectra are dominated by the νs(OCO) band. However, the shape of this band changes as a function of coverage. Specifically, two features can be identified at low coverage. In addition, two new bands appear at low exposure at 945 and 1657 cm-1, while in the same coverage range the high frequency CH stretching bands at 2970 to 2980 and 3060 cm-1 are missing. With increasing NBD exposure the new features disappear again, while several bands appear which are characteristic for the NBD group, namely the γ(CH) band at 720 cm-1 and the high frequency CH bands at 2870, 2942, 2969, 2984 and 3068 cm-1. These changes indicate a coverage-dependent change in the adsorption geometry of anchored NBDA. The change in the shape of the νs(OCO) band together with an increased intensity in the spectral range of the νas(OCO) band at 1560 cm-1, which is IR forbidden for a symmetric carboxylate (see Figure 4a and Section 3.2), suggests the formation of a distorted (nonsymmetric) carboxylate at low coverage. The dramatic change in the NBD part of the CH stretching region suggests the NBD unit interacts with the surface at low coverage. This hypothesis is corroborated by the observation that a new band is observed in the C=C stretching region at 1657 cm-1. The second new band at 945 cm-1 may also arise from a chemical interaction of the NBD unit with the Co3O4(111), possibly via the terminating Co2+ ions. We conclude that intermittently a flat-lying intermediate is formed in which the NBD is in contact with the Co3O4 surface. This conclusion can be rationalized by the fact that, contrary to BA, the alkyl chain provides sufficient structural flexibility to enable surface contact at low coverage. With increasing exposure, the spectral features of the low coverage intermediate disappear and new features are observed which are characteristic for the free NBD unit. Therefore, we suggest that with increasing coverage a phase transition occurs from a flat-lying to and upright standing phase, in which the NBD unit is detaching from the surface. Such phase transitions are regularly 14 ACS Paragon Plus Environment

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observed in self assembled monolayers 56-57 and also have been observed for organic films grown in UHV on oxide surfaces20.

3.6 Thermal Behavior of Monolayer Films of Benzoic Acid and 1-(2’-Norbornadienyl)pentanoic acid on Co3O4(111)/Ir(100) To probe the thermal stability of the BA and NBDA monolayer films prepared at 300 K, we performed TP-IRAS experiments up to 600 K. As a reference, we used the last IR spectrum of the temperature ramp at 600 K. After preparation of the films the samples were heated linearly with a rate of 2 K/min while acquiring IR spectra (see Experimental Section). Selected data is shown in Figure 6 (the complete set of data is provided in the Supporting Information). In Figure 6a, we show the development of the BA spectra with temperature. The integrated intensity of the νs(OCO) and ν(C=C)ring bands of BA is plotted as function of temperature in Figure 6b. For the anchored BA monolayer, we observe that the film is essentially stable up to a temperature of 400 K. Above 400 K the intensity of the νs(OCO) band decreases and disappears nearly completely at 550 K. Noteworthy, the ν(CC)ring band shows a markedly different temperature dependence. It increases in intensity between 400 and 520 K and, subsequently, decreases. At 600 K, all IR bands have disappeared, suggesting that the benzoate has been removed from the surface. The different intensity development of the bands suggests that the BA film undergoes partial decomposition above 400 K. In Figure 6c the results are shown for an equivalent experiment using NBDA. The corresponding intensity in the νs(OCO) region is plotted in Figure 6d. Similar as for the BA, the band intensities remain largely constant up to 400 K. This finding suggests that the anchored NBDA film is stable up to this temperature. This conclusion is supported by the fact that the characteristic NBDA bands in the ν(CH) region also remain unchanged. At temperatures above 400 K all NBDA related bands decrease in intensity until they disappear at 550 K. Noteworthy, a number of positive bands appear in the IR spectrum at temperatures above 550 K. However, the characteristic bands of the flat-lying NBDA species at 945 and 1657 cm-1 (see above) are not observed. Therefore, we attribute the new features to decomposition products of NBDA. Specifically, a number of broad ν(CH) bands between around 2850 and 2950 cm-1 are observed, which indicate the formation of hydrocarbon residues. 15 ACS Paragon Plus Environment


4. Conclusion We have studied the adsorption, growth and stability of BA and NBDA deposited by PVD under UHV conditions onto well-ordered Co3O4(111) films on Ir(100). The interfacial reactions and ordering phenomena were probed as a function of coverage and temperature by time-resolved and temperature-programmed IRAS, with the help of calculated IR spectra from DFT. Specifically, we could observe the following: (1) BA readily deprotonates on Co3O4(111) already at 130 K and binds to the surface forming a symmetric chelating benzoate which is strongly tilted with respect to the surface. The same bonding geometry was found for deposition at 300 K. The bonding geometry and molecular orientation is independent of the coverage suggesting the formation of dense benzoate islands. (2) BA multilayer films grown at low temperature (130 K) on top of the anchored monolayer consist of hydrogen bonded flat-lying BA dimers. The BA multilayer shows a crystallization transition in the temperature range between 155 and 175 K and desorbs at 220 K, leaving behind a monolayer of surface-anchored benzoate. (3) The surface-anchored benzoate monolayer is stable up to temperatures of 400 K. In the temperature range between 400 K and 550 K, the film undergoes decomposition and desorption. (4) Similar to BA, NBDA deprotonates on Co3O4(111) at 130 K, forming a symmetric chelating carboxylate. For growth at 300 K, a markedly different behavior is observed. At low coverage, a flat-lying anchored phase is formed, in which the NBD unit interacts with the Co3O4(111) surface. With increasing coverage, the NBD units of the anchored molecules detach from the surface and adopt an upright standing orientation. (5) NBDA multilayer films grown at low temperature (130 K) show a less clear distinction between monolayer and multilayer growth, a less well-defined ordering behavior and no preferential orientation in the multilayer. The NBDA multilayer desorbs at 240 K.


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(6) The surface-anchored high coverage NBDA monolayer film on Co3O4(111) prepared at 300 K is stable up to temperatures of 400 K. In the temperature range between 400 K and 550 K the film decomposes, leaving behind a variety of hydrocarbon fragments. Based on the present results, we conclude that is possible to prepare dense and well-defined films of surface-anchored NBDA by PVD on Co3O4(111) at 300 K. Most likely, the method can be used to prepare high quality films on other oxide surfaces as well. In future work, such films will allow us to perform fundamental model studies of the energy storage and release processes in the norbornadiene/quadricyclane system at atomically defined interfaces.



Acknowledgements 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” within the framework of the Excellence Initiative. T. Xu acknowledges a PhD scholarship from China Scholarship Council (CSC). The authors also acknowledge additional support by the Deutsche Forschungsgemeinschaft within the DACH Project “COMCAT”.

Supporting Information TR-IRAS of BA uptake on Co3O4(111) at 130 K (Figure S1) TP IRAS of BA on Co3O4(111) from 130 K to 370 K (Figure S2) TR-IRAS of BA uptake on Co3O4(111) at 300 K (Figure S3) Comparison of IRAS spectra of saturated benzoic acid monolayers on Co3O4(111) by TP-IRAS and isothermal deposition. (Figure S4) TP IRAS of BA on Co3O4(111) from 300 K to 600 K (Figure S5) TR-IRAS of NBDA uptake on Co3O4(111) at 130 K (Figure S6) TP IRAS of NBDA on Co3O4(111) from 130 K to 370 K (Figure S7) TR-IRAS of NBDA uptake on Co3O4(111) at 300 K (Figure S8) Comparison of IRAS spectra of saturated 1-(2'-norbornadienyl)pentanoic acid monolayers on Co3O4(111) by TP-IRAS and isothermal deposition. (Figure S9) TP IRAS of NBDA on Co3O4(111) from 300 K to 600 K (Figure S10)


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Band position (cm-1)

Assignment

714

γ(CH)ring

953

γ(OH) (+ ν(CO))

1028, 1072, 1128, 1178

ν(CC), δ(CH)

1300, 1324

ν(CO), v(CC), δ(OH)

split, dimer formation

1423, 1452

δ(OH), ν(CO), v(CC)


1495

δ(CH), ν(CC)ring

1585, 1605

ν(CC)ring, δ(CH)

1699

ν(C=O)

2800 to 3100

ν(CH)

Comment


weak, complex structure

Table 1: Vibrational bands and assignments for BA multilayer (ν: stretching, γ: out of plane bending, δ: in plane bending).



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Band position (cm-1)

Assignment

Comment

717

γ(CH)NBD, γ(CH)alkyl

characteristic for NBD

735, 782, 816, 846, 873, 903

ν(CC), δ(CC), γ(CH)alkyl, NBD

953

γ(OH) (+ ν(CO))

1061

ν(CC) + δ(CC)

1252

ν(CO) (+ v(CC), δ(OH))

broad

1325

δ(CH) NBD + ν(CC)NBD

characteristic for NBD

1412

δ(CH2)

1450

δ(OH), δ(CH2)

broad

1567

ν(C=C)NBD

very weak

1709

ν(C=O)

broad

2863

νs(CH2)alkyl, NBD

2939

νas(CH2)alkyl, NBD

2970

ν(Ct-H)NBD

3065

ν(=C-H)NBD

Table 2: Vibrational bands and assignments for NBDA multilayer (ν: stretching, γ: out of plane bending, δ: in plane bending).


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Figure 1: (a) Energy storage in the valence couple norbonadiene (NBD) and qudricyclane (QC); (b) molecular structure of 1-(2'-norbornadienyl)pentanoic acid (NBDA).



Figure 2: Comparison of the gas phase spectra of NBDA monomer (red) and dimer (blue) calculated by DFT to the experimental multilayer spectrum of NBDA (black) on Co3O4(111)/Ir(100). The calculated spectra are plotted using Lorentzian smoothing with a full width at half maximum (FWHM) of 5 cm-1.


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Figure 3: (a) IR spectra recorded during deposition (60 min) of BA on Co3O4(111)/Ir(100) at 130 K; (b) Comparison of a sub-monolayer spectrum of BA on Co3O4(111)/Ir(100) (deposition time 4 min) and a multilayer spectrum (deposition time 60 min); (c) Integrated area for the BA bands at 714 (γoop(CH), violet, with guide line to the eye), 1423 (νs(O-C-O), black), 1699 cm-1 (ν(C=O,), red) as function of deposition time; (d) IR spectra recorded during deposition (200 min) of NBDA on Co3O4(111)/Ir(100) at 125 K; (e) Comparison of a sub-monolayer spectrum of NBDA (deposition time 30 min) and a multilayer spectrum (deposition time 200 min); (f) Integrated area of the NBDA bands at 1412 (νs(O-C-O), black), 1450 (δ(CH)alkyl, pink), 1709 cm1

(ν(C=O), red) as function of deposition time.



Figure 4: (a) TP-IRAS after deposition of a BA multilayer at 130 K; (b) Selected IR spectra for BA on Co3O4(111)/Ir(100): Multilayer spectrum at 130 K, multilayer after reordering (200 K), and after multilayer desorption (300 K) (see text for details); color code of the characteristic bands: νs(O-C-O) - black; ν(C=O) - red; γoop(CH) - violet; (c) Integrated peak area for BA at 714 (γoop(CH), violet), 1423 (νs(O-C-O), black), and 1699 cm-1 (ν(C=O), red) as function of temperature; (d) TP-IRAS after deposition of a NBDA multilayer at 130 K; (e) Selected IR spectra of NBDA on Co3O4(111)/Ir(100): Multilayer spectrum at 130 K, after restructuring and water desorption (200 K), and after multilayer desorption (300 K). Color codes of the characteristic bands: νs(O-C-O) - black; ν(C=O) - red; γ(OH)dimer – blue; e) Integrated areas of NBDA bands at 954 (γ(OH)dimer, blue), 1430 (νs(O-C-O), black) and 1709 cm-1 (ν(C=O), red).


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Figure 5: (a) IR spectra recorded during deposition of BA onto Co3O4(111)/Ir(100) at 300 K; (b) Spectrum in the sub-monolayer region and after saturation of the monolayer; (c) Integrated area of the band at 1412 (νs(O-C-O), black) and schematic representation of the film structure; (d) IR spectra recorded during deposition of NBDA on Co3O4(111)/Ir(100) at 300 K; (e) Spectrum in the sub-monolayer region and after saturation of the monolayer; f) Integrated area of the bands at 1428 (νs(O-C-O), black), 1657 (ν(C=C), blue), 945 cm-1 (green) and schematic representation of the adsorption geometry in the NBDA film. 29 ACS Paragon Plus Environment


Figure 6: (a) TP-IRAS of a BA monolayer film prepared on Co3O4(111)/Ir(100) at 300 K; (c) Integrated area of the BA bands at 1412 (νs(O-C-O), black) and 1603 cm-1 (ν(C=C), red); (c) TPIRAS of a NBDA monolayer film on Co3O4(111)/Ir(100) prepared at 300 K; (d) Integrated area of the carboxylate band at 1430 cm-1 (νs(O-C-O), black).


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