Dye Self-Association Identified by Intermolecular Couplings between

Article pubs.acs.org/JPCC

Dye Self-Association Identified by Intermolecular Couplings between Vibrational Modes As Revealed by Infrared Spectroscopy, and Implications for Electron Injection Jennifer E. Laaser,†,§ Jeffrey R. Christianson,† Tracey A. Oudenhoven,† Yongho Joo,‡ Padma Gopalan,‡ J. R. Schmidt,† and Martin T. Zanni*,† †

Department of Chemistry, and ‡Department of Materials Science and Engineering, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: In this Article, we investigate the effects of binding geometry and intermolecular interactions in monolayers of a rhenium-based dye adsorbed to TiO2. We combine two-dimensional infrared (2D IR) spectroscopy of samples prepared with different dye loadings with density functional theory (DFT) calculations of dye binding energies and vibrational frequencies. Our 2D IR spectra reveal splitting of the CO symmetric stretch mode into two peaks of unequal intensity at high surface coverages, which persists even when samples are washed to remove unadsorbed aggregates. Our DFT calculations indicate that it is unlikely that dye binding geometries account for the shifts in peak frequency observed in our experimental spectra. Instead, we find that the shifts in vibrational frequency and 2D IR peak structure are consistent with coupling of dyes associated on the TiO2 surface. The relative peak intensities in our 1D and 2D spectra indicate different transition dipole strengths, also a signature of molecular coupling. We show that aggregation of dyes on the surface is energetically favorable. Adsorbate−adsorbate interactions may play an important role in defining surface structure and electronic properties of dyesensitized solar cells and related organic/inorganic interfaces. Infared spectroscopy is a good means to identify its occurrence, and to begin exploring its effects on phenomena like electron injection kinetics.

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INTRODUCTION Functionalized organic/inorganic interfaces play a key role in many promising materials systems. Such interfaces are at the heart of dye-sensitized solar cells, in which a light-absorbing sensitizer molecule transfers an electron across an organic/ inorganic interface to the semiconductor to which it is adsorbed.1,2 Similar structures are found in promising catalytic systems, such as those being investigated for water splitting3−5 and for CO2 reduction.6,7 Functionalizing semiconductor oxides may also improve electron transport properties even within the semiconductor via passivation of surface defect states.8 A wide range of experimental techniques have been used to study photoinduced charge transfer at these interfaces. The extent of light-induced charge transfer from sensitizer to semiconductor has been investigated via photocurrent measurements,9 and time-resolved surface photovoltage measurements have been used to monitor electron injection processes on time scales of nanoseconds or shorter, depending on the laser pulse width.10 Recently, a variety of ultrafast spectroscopies have brought new insight into the femtosecond to picosecond timescale kinetics of electron transfer from sensitizer molecules to the semiconductor surface.11−15 Many of these studies have explored the effects of molecular structure (such as the chemical structure of the linker and anchoring groups) and © 2014 American Chemical Society

surface identity (such as the type of semiconductor used or the crystal face to which the molecules are bound). For example, ultrafast electron-transfer studies have shown that carboxylic and phosphonic acid anchoring groups result in different electronic couplings to the surface, and that longer and unconjugated linkers effectively block electron transfer while shorter or conjugated linkers facilitate it.13,15 Yet interfacial heterogeneity and intermolecular interactions within the monolayer may play an equally important role in determining the interfacial electronic properties. In our previous work, we observed multiple subpopulations of a rhenium-based dye on TiO2, with different subpopulations exhibiting different electron-injection rates.16 Formation of dye multilayers or dye agglomeration has also been suggested to affect charge-transfer efficiencies,17−21 and dye orientations may also dictate electronic couplings and electron-transfer efficiencies.22 Recent reports have attempted to directly identify the conformation of sensitizers bound to single semiconductor− oxide interfaces through a combination of calculations and orientation-sensitive experimental measurements such as vibraReceived: December 18, 2013 Revised: February 24, 2014 Published: February 26, 2014 5854

dx.doi.org/10.1021/jp412402v | J. Phys. Chem. C 2014, 118, 5854−5861

The Journal of Physical Chemistry C

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tional sum frequency generation and near edge X-ray absorption fine spectroscopy.22−24 Metal carbonyl dyes are a useful system for investigating the factors controlling structure and dynamics in these systems because the dyes have strong spectroscopic signatures, and a variety of related structures are synthetically accessible. In this Article, we study a pair of mono- and dicarboxylate-linked Re dyes, denoted ReC and ReCC, respectively, which bind to TiO2 through COOH groups on a bipyridyl ligand. The dyes have three CO moieties on the rhenium center, which serve as vibrational reporter groups sensitive to the electronic state and molecular environment of the dye; the structures of the ReC and ReCC sensitizers are shown in Figure 1a and b.

Article

METHODS

Preparation of Dye-Sensitized TiO2 Films. Nanocrystalline TiO2 films were created by screen-printing a paste of 20 nm TiO2 nanoparticles (Solaronix) onto 1 mm thick sapphire substrates, using a procedure adapted from Ito et al.25 Three 3 μm layers were printed for a total film thickness of approximately 9 μm. The films were dried at 115 °C between each layer. After addition of the final layer, films were heated to 500 °C for 1 h to sinter the films and burn off organic binders. Films were then cooled to 80 °C and immersed in solutions of ReC or ReCC in acetonitrile for 18−20 h. Solutions were prepared as 0.5, 0.02, and 0.005 mM dye in 10 mL of anhydrous acetonitrile; the 0.005 mM solutions appeared completely exhausted after film sensitization. After the 20 h sensitization period, films were removed from the dye solution and soaked in fresh anhydrous acetonitrile for 20 min to help remove multilayers and weakly physisorbed dye aggregates, as is standard in spectroscopic studies of sensitized interfaces.14 Samples were then transferred to a sample cell and encased under hexane to minimize photo-oxidation during spectroscopic measurements.16 Two-Dimensional Infrared Spectroscopy. Our 2D IR experiment has been described in detail previously.16,26,27 Briefly, the output of a 2 kHz, 1 W Ti:sapphire regenerative amplifier producing 100 fs pulses at 800 nm was directed into an optical parametric amplifier (OPA) followed by difference frequency generation to generate 5 μJ pulses of mid-IR light at 5 μm. The majority of the output from the OPA was directed into a pulse shaper based on a germanium acousto-optic modulator, which generated pump pulse pairs with time delays and phases updated on a shot-to-shot basis.26,27 The output of the pulse shaper was focused with the remainder of the mid-IR light (the mid-IR probe) onto the sample using a pair of off-axis parabolic mirrors. The transmitted probe was then recollimated and directed into a polychromator, where it was dispersed on a 75 g/mm grating and detected on a 64-pixel mercury cadmium telluride (MCT) array. The pump delay was scanned from 0 to 5500 fs in 80 fs steps, with a rotating frame frequency of 1915 cm−1, and data from the array detector were Fouriertransformed along the pump axis to retrieve the full twodimensional spectrum. For each sample, a reference scan was taken at t2 = −20 ps to measure a pump scatter spectrum, which ensured that the probe came far enough before the pump to completely suppress third-order responses while minimizing errors due to stage translation and detector dynamic range. The reference scan was then subtracted from the spectrum at t2 = 0 ps to obtain a background-corrected spectrum. DFT Calculations of Dye Binding Energies and Vibrational Frequencies. All calculations were performed using the Vienna Ab-initio Simulation Package (VASP)28−31 within the Atomistic Simulation Environment (ASE).32 The TiO2 anatase (101) surface was constructed using experimental bulk anatase lattice parameters (a = 3.7845 Å, c = 9.5143 Å).33 The slab model consisted of two layers of Ti atoms and was a 4 × 2 supercell, containing 64 TiO2 units and spanning 20.48 Å and 15.14 Å in the x- and y-directions, respectively. A vacuum gap of 15 Å was employed in the z-direction (for a total length of 20.86 Å) to minimize interactions with periodic images. Geometry optimizations were done using the FIRE34 optimizer in ASE and were considered converged when the maximum atomic force was below 0.05 eV/Å for surface−dye systems or 0.01 eV/Å otherwise. The top half of the slab was allowed to

Figure 1. Structures of the (a) ReC and (b) ReCC dyes used in this study, and diagrams of the (c) monodentate nondissociative, (d) monodentate dissociative, (e) bidentate bridging dissociative, (f) bidentate bridging nondissociative, and (g) defect site binding configurations for a single COOH anchor.

In our previous work, we observed multiple subpopulations of these dyes on TiO2 nanocrystalline thin films, which we attributed to multiple conformations of the molecules on the surface. Here, we use a combination of two-dimensional infrared (2D IR) spectroscopy and density functional theory (DFT) calculations to explore the relative contributions of binding geometry and intermolecular interactions at the interface. We consider five different ways in which the dye COOH group can bind to the TiO2 surface: on the pristine anatase (101) surface, the COOH group may bind through one or both oxygen atoms (which we refer to as “monodentate” and “bidentate” binding, respectively), and its proton may either dissociate or remain bound to the carboxylic acid moiety. These binding modes are shown in Figure 1c−f. We also consider binding to oxygen vacancy defects in the anatase (101) surface, as shown in Figure 1g, and investigate contributions from aligned dimers and trimers of dyes on the surface. Our results show that binding geometry alone is inadequate to explain the coverage-dependent trends in our experimental spectra; our calculations instead suggest that intermolecular coupling is a more likely explanation for the trends observed in our experimental spectra, although defect sites may also play a role. Because intermolecular interactions may affect electron injection processes and interfacial electronic couplings, elucidating the extent of intermolecular couplings will provide significant new insight into the electronic properties of these interfaces. 5855



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Figure 2. 2D IR spectra for (a−c) the ReC dye and (d−f) the ReCC dye at high, medium, and low surface coverages. θ is a measure of surface coverage. Diagonal line segments indicate the nodal line, while a horizontal line at the frequency of the low-coverage peak is included to illustrate the coverage-dependent frequency shift.

DFT-D2 method, but all transition metals within the same row (3d or 4d) have the same parameters. Therefore, parameters for Re were taken to be the same as those parametrized for Au.42 This should give at least qualitatively meaningful results, especially because each Re atom is buried within a dye molecule by its ligands.

relax in both surface and surface−dye optimizations, whereas the bottom half of the slab was always fixed in bulk positions. Each single point along the optimization trajectory was calculated in VASP using the Perdew−Burke−Ernzerhof (PBE)35,36 GGA exchange-correlation functional with low precision and an energy cutoff of 300 eV. The interaction between core electrons and valence electrons (including the 3p electrons on Ti) was described using the projector augmented wave (PAW)37,38 approach. Because of the large supercell used, the Brillouin zone was sampled only at the gamma point. A dipole correction was employed in the z-direction. At the optimized geometry, single point calculations for obtaining binding energies and finite-difference frequency calculations using ASE were performed using normal precision and an energy cutoff of 400 eV. The frequency calculations were performed by displacement of only the C and O atoms within the carbonyl ligands, because inclusion of more atoms had an insignificant impact on the frequencies. While the frequencies were calculated for the full periodic system, calculation of the IR intensities for the periodic system through ASE was not practical due to numerical instabilities in calculating the transition dipole. Therefore, IR intensities were estimated from gas-phase calculations of the dyes in the same geometry as when adsorbed to the surface. While this completely neglects the surface, the carbonyl groups are wellseparated (7−10 Å) from the surface in every binding motif considered. Thus, the gas-phase calculations will capture the qualitative trends in intensity. The Gaussian 09 software package39 was used to carry out these gas-phase calculations using the Stuttgart/Dresden effective core potential40 and corresponding basis set for Re and the Dunning style correlation-consistent double-ζ (cc-pVDZ) basis set for all other atoms. For dimer and trimer aggregation energies, dispersion corrections according to Grimme’s DFT-D241 method were added to the total PBE energies. The DFT-D2 correction was implemented into a locally modified version of ASE. Parameters for 5d transition metals were not originally included in the

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RESULTS Experimental Results. In Figure 2, we present 2D IR spectra of the ReC and ReCC dyes at full and partial surface coverage, with surface coverages, θ, estimated by integrating the peak areas in the corresponding FTIR spectra (see the Supporting Information). The coverages thus obtained are consistent with those expected from the dye solution concentrations during sensitization. All six 2D IR spectra exhibit a main peak with a more inhomogeneous tail at lower frequencies, and the spectra for both dyes become more inhomogeneous at lower coverage. In the ReC samples at full monolayer coverage (θ = 1), the low-frequency tail appears as a distinct peak; in the lower-coverage ReC spectra and in all ReCC spectra, the low-frequency tail instead appears as a long smear along the diagonal. Both the ReC and the ReCC spectra exhibit a red shift in the primary peak frequency as the surface coverage decreases. In Figure 3, we compare diagonal cuts through the 2D IR spectra to FTIR spectra of the same samples. For the ReC dye at low coverage, the FTIR and 2D IR cut have the same peak position and identical linewidths. At high coverage, the 2D IR cut separates into two peaks approximately 12 cm−1 apart, with the high-frequency peak at 2036 cm−1 approximately 3 times as intense as the low-frequency peak at 2024 cm−1. The FTIR spectrum similarly develops a shoulder at lower frequency, leading to an asymmetric line shape, but the intensity difference is smaller than in the 2D IR cut. In the ReCC spectra, the FTIR spectrum is consistently broader than the cut through the 2D IR spectrum, even at low surface coverage, although the principal peak does get narrower, blue-shift, and develop a prominent low-frequency shoulder at full monolayer coverage. 5856



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Table 1. Binding Energies and Infrared Transitions of Various Monomer Binding Motifs with Various Rotations about the O−C−C−C Dihedrala binding mode MHa

M(H)

BB(H)

BBH D (H)

rotation (deg)

binding energy, incl. dispersion (eV)

ν̃symm (cm−1)

intensity (gas phase, D2 Å−2amu−1)

0 90 180 270 0 90 180 270 0 90 180 270 0 0

−1.25 −0.92 −1.28 −0.95 −1.08 −1.13 −1.08 −1.14 −0.95 −0.91 −1.11 −0.94 −0.40 −2.39

2013 2013 2015 2012 2006 2012 2011 2015 2018 2014 2019 2011 2026 2013

29 27 30 27 30 37 23 20 27 26 14 22 12 29

a

The angle (deg) given corresponds to the approximate dihedral. The most favorable energy for each binding motif is in bold.

frequencies for these types of dye aggregates are given in Table 2.

Figure 3. Comparison of the FTIR spectra to the corresponding diagonal cuts through the 2D IR spectra for the ReC and ReCC dyes at high, medium, and low surface coverages.

Table 2. Total Binding Energies, Aggregation Energies (eV, in Parentheses), and Infrared Transitions for Increasing Numbers of Dye Molecules in the MHa Binding Motif with an Approximate Dihedral Angle of 0°

The difference in relative intensity between the FTIR and 2D IR spectra indicates that the transition dipoles are different between the two modes,43 which is often characteristic of strong vibrational coupling,44 as we show below. Computational Results. In our previous work,16 we tentatively attributed the multiple peaks in our 2D IR spectra to different dye conformations or binding modes on the surface. To explore the extent to which binding mode and conformation affect infrared peak positions and intensities, we calculated vibrational responses for five different binding modes of the ReC dye to anatase (101). Vittadini et al.45 reported five favorable binding motifs for formic acid adsorption on anatase (101). One of them (MHb) involves the aliphatic hydrogen coordinating with a surface oxygen, and thus is not directly relevant to the present study. We consider the other four binding motifs plausible for the adsorption of Re1C on anatase (101). In addition, we also consider adsorption to an oxygen vacancy in the surface, as has been experimentally observed.46 These five binding motifs are shown schematically in Figure 1. The only significant conformational degree of freedom that ReC has is rotation of the bipyridine with respect to the carboxylic acid linking group. Thus, for each binding motif, we consider various rotations about the corresponding O−C−C− C dihedral angle, indicated in Figure 1a, and find local minima nearby. On the basis of the calculated binding energies (Table 1), calculated as BE = Esurface+adsorbate − Esurface − Eadsorbate, adsorption to any oxygen vacancies present in the sample is most favorable. To the extent that adsorption may be kinetically (rather than thermodynamically) controlled, or oxygen vacancies become saturated, any of MHa, M(H), or BB(H) binding motifs may be present. In addition, aggregation of two or three dyes on the surface into dimers or trimers (N = 2, 3) is favorable with respect to isolated dyes. Aggregation energies (Eaggregation = BEaggregate − N*BEmonomer) and vibrational

binding mode

binding energy (eV)

ν̃symm (cm−1)

intensity (gas phase, D2 Å−2 amu−1)

monomer dimer

−1.25 −2.86 (−0.35) −4.39 (−0.63)

2013 2031 2018 2038 2025 2020

29 43 2 50 3 11

trimer

For the lowest binding energy configurations, the frequency dependence on the MHa, M(H), and BB(H) binding motifs and rotation about the O−C−C−C dihedral angle (Table 1) is too small to account for the 12 cm−1 splitting measured in the high-coverage experimental spectra, and the gas-phase intensities differ by no more than a factor of 2. However, the calculated splitting of the symmetric carbonyl stretch frequencies of the aggregates is more significant, at 13 cm−1 for the dimer and 18 cm−1 for the trimer. In addition, the gasphase intensities of the dimer and trimer aggregates show clear bright and dark states corresponding to differing phases of each dye’s completely symmetric stretch (Figure 4 and Table 2). Because the observed aggregation arises from weak π−π interactions, we assume that these same features are qualitatively similar to aggregates of other energetically favorable binding motifs (such as M(H) and BB(H)).

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DISCUSSION In our previous work,16 we speculated that the multiple peaks observed in our 2D IR spectra of ReC on TiO2 originated from different binding conformations on the surface. In this work, we explored this possibility using coverage-dependent 2D IR measurements and electronic structure calculations. While our calculations show that some binding modes (especially the 5857



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Figure 4. Unit cell geometries (top) and calculated IR vibrational spectra in the region of the completely symmetric C−O stretches (bottom) for the optimized monomer (left), dimer (middle), and trimer (right). The arrows indicate the relative phase of the completely symmetric stretch of each dye molecule.

intensities in 2D IR than in FTIR,43 consistent with our spectra as shown in Figure 3. Finally, excitonically delocalized vibrational modes usually have narrower, more homogeneous lineshapes than localized modes,43,48 consistent with the nodal slopes in our spectra tending toward the vertical as surface coverage increases. Thus, we conclude that the peak splittings and frequency shifts observed in our 2D IR spectra are in fact indicative of intermolecular interactions and coupling between adjacent dye molecules on the surface. Our calculations reveal that aggregate formation on the surface is energetically favorable, and can indeed give rise to the peak patterns observed in our spectra. In particular, the dimer has two vibrational modes in the 2000−2050 cm−1 range, at 2031 and 2018 cm−1, with the intensity of the transition at 2031 cm−1 an order of magnitude stronger than that of the transition at 2018 cm−1. A similar trend is observed for the trimer, in which the high-frequency transition at 2038 cm−1 has the highest intensity, with lower-intensity transitions red-shifted by approximately 15 cm−1. A block-diagonalization of the Hessian (see the Supporting Information) reveals that these patterns arise due to a combination of geometric factors and vibrational coupling of the symmetric CO stretches between dyes. First, there is a blue-shift from the monomer (2013 cm−1) to all of the symmetric dimer and trimer modes due simply to the change in the local environment of each dye. In addition, the vibrational modes of the aggregate dyes couple together, giving rise to a splitting consisting of one high-intensity mode in which all of the symmetric stretches are in phase with each other, and one (dimer) or two (trimer) low-intensity modes in which at least one dye’s symmetric stretch is out of phase with the others (see Figure 4). Because the calculated dimer and trimer intensities were based on minimum energy configurations where the dipole moments of adjacent dyes are perfectly aligned, there is a much larger disparity in the intensities than observed experimentally, because thermal fluctuations lead to imperfect dipole alignment. Nevertheless, these calculations qualitatively agree with our experimental spectra, which show a higher-intensity peak at 2036 cm−1 and a lower-intensity peak 12 cm−1 lower, at 2024 cm−1. The effect is similar to H-aggregates of planar dyes in solution.49 Our calculations are thus consistent with the formation of dye

monodentate MHa mode) are energetically more favorable than others, all four of the nondefect binding modes included in our calculations are essentially competing for the same binding sites on the surface, and their distribution should not exhibit a strong coverage dependence unless entropic factors play a significant role. Additionally, our calculations reveal that the vibrational frequency shifts induced by different binding modes to anatase (101) are too small to account for the peak splitting and frequency shift observed in our spectra. In particular, the most favorable binding mode on the pristine anatase (101) surface is the monodentate nondissociative motif (MHa). Binding through the monodentate dissociative binding motif (M(H)) is only about 0.15 eV less favorable, and may be present on the surface, but three of the four rotation angles we calculated exhibited negligible frequency shifts relative to the monodentate non-dissociative mode (MHa), and the fourth was redshifted, which cannot explain the strong blue-shift seen in our high-coverage experimental spectra. The binding mode exhibiting the strongest blue-shift was the bidentate nondissociative binding mode (BBH), but its binding energy was more than 0.6 eV less favorable than both monodentate motifs and the bidentate dissociative motif (BB(H)). Finally, binding to defect sites (D(H)) was significantly more energetically favorable than any of the monodentate or bidentate binding modes on the pristine anatase (101) surface, but our calculations showed no significant frequency shift relative to the most favorable bindings to the pristine anatase surface. Because of known deficiencies of DFT in describing such defects, we confirmed that calculations performed within the DFT+U formalism47 do not qualitatively change these results. Thus, we conclude that while differences in binding mode and molecular geometry may contribute to spectral broadening, they are unlikely to account for the spectral shifts and splittings seen in our experimental spectra. However, frequency splittings such as those observed in our 2D IR spectra are also characteristic of molecular couplings. Additionally, coupling of two or more identical modes generally produces normal modes with unequal transition dipole moments. Because the intensity of 2D IR spectra scales as μ4 while FTIR scales as μ2, coupling also leads to more dramatic disparities in the normal mode peak 5858



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binding process is kinetically controlled. Alternatively, to bring both anchoring groups in line with binding sites on the surface, the doubly anchored dye may preferentially orient itself in configurations that promote alignment of the dye molecules and thus strong molecular couplings even at low coverage. Future dilution experiments using isotopically shifted vibrational modes will help disentangle this question. In our previous work on this system using transient 2D IR, we found that different peaks in the static 2D IR spectrum corresponded to different electron-transfer rates in the transient 2D IR measurements.16 In that work, we attributed these multiple peaks to different conformations of the dye on the surface. In light of our present results showing that the multiple peaks in the 2D IR spectrum are more properly attributed to molecular coupling and intermolecular interactions in the densely packed monolayer, it seems likely that these intermolecular interactions play a role in the injection processes as well. One possibility is that aggregation constrains or otherwise influences dye conformation and thus perhaps also affects injection kinetics. Additionally, molecules that interact strongly enough to be vibrationally coupled are almost certainly also electronically coupled, and the different electron injection rates may reflect different injection rates from different electronic normal modes of the system. Future theoretical work will investigate calculated injection rates as a function of dye aggregation and will hopefully shed light on the role of electronic coupling and defect states in determining electrontransfer rates and efficiencies in these systems.

aggregates and excitonically delocalized vibrational modes at high surface coverages. Our calculations also reveal that aggregation should be energetically favorable on the anatase (101) surface. The dimer and the trimer both have favorable aggregation energies, at −0.35 and −0.63 eV, respectively. The aggregation energy is dominated by dispersion and likely arises from π−π interactions in the bipyridyl rings. The aggregation energy for the trimer is significantly more favorable than that for the dimer. Only a small fraction (0.05 eV) of this difference is due to the higher coverage modeled by the trimer versus the dimer, as measured by gas-phase dispersion calculations. Therefore, aggregation of multiple dye molecules on the surface is energetically favorable. Although our calculations do not address multilayer formation, we believe multilayers are unlikely to contribute significantly to our spectra for several reasons. First, we wash our sensitized films in clean solvent after sensitization, which is a standard procedure for preparing dye-sensitized thin films,14 and should remove most loosely bound molecules that are not directly adsorbed to the surface. Additionally, we find that the optical density of the films does not keep increasing indefinitely even in high-concentration dye solutions, and that the limiting optical density is consistent with previous samples that were shown by X-ray photoelectron spectroscopy (XPS) to have a single monolayer coverage.15 This suggests that the dye does not continue to deposit on the surface after formation of the first strongly adsorbed monolayer. Because we only investigated binding energies and vibrational frequencies for dyes bound to the anatase (101) surface, it may also be possible that differential dye adsorption to other exposed crystal faces in our TiO2 nanoparticles contributes to the trends observed in our spectra,50 although were this the case we would expect to see similar effects in both the ReC and the ReCC dyes. An intriguing aspect of the calculated frequencies included in Table 2 is that the frequencies for the ν̃symm mode exhibit an overall blue-shift moving from the monomer to the dimer to the trimer. The monomer frequency, at 2013 cm−1, is 7 cm−1 lower than the lowest frequency of the trimer, at 2020 cm−1. In the experimental spectra, however, at low coverage the peak frequency is directly between the frequencies of the two peaks seen at higher coverage. This may indicate some aggregation of the dye molecules even at low surface coverage, consistent with our calculation suggesting that aggregation of the dyes on the surface is energetically favorable. However, the increased inhomogeneity of the 2D IR spectrum at low coverage (evidenced by the nodal slope parallel to the diagonal in Figure 2c) indicates that the partial monolayer is more disordered and less coupled at low coverage than at high coverage, even if the dye molecules may not be completely independent of one another. While our calculations only addressed binding of the singleanchor dye, we included the variant with two carboxylic acid anchors in our spectroscopic experiments to investigate how multiple attachment points affect the distribution of dyes on the surface. We found that while the spectra of the single-anchor dye collapse to a single peak in the low-coverage limit, in which the FTIR and 2D IR spectra are a near perfect match (see Figure 3e), the doubly anchored dye maintains significantly more inhomogeneous character and a strong red-shifted shoulder. The doubly anchored dye may be more disordered on the surface than the singly anchored dye, because multiple binding points may make dye binding favorable enough that the

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CONCLUSIONS Molecular structure at the organic−inorganic interface plays a critical role in interfacial functionality. In this work, we showed that intermolecular interactions in a densely packed monolayer also play a role. Comparison of FTIR and 2D IR spectra at varying surface coverages exhibited distinct frequency shifts and changes in transition dipole moment as surface coverage increased. Calculations of binding energies and vibrational frequencies for monodentate and bidentate dissociative and nondissociative binding motifs, as well as binding to oxygen defect sites, showed that binding motifs and molecular conformation alone are not enough to account for experimentally observed frequency shifts as a function of surface coverage. To explain the complex vibrational spectra of these systems, we must also take into account molecular aggregation and coupling on the surface, even at low surface coverages, which are consistent with the peak splittings and changes in transition dipole observed in our experimental spectra. Intermolecular interactions may also affect electrontransfer kinetics at organic−inorganic interfaces, and carefully controlling intermolecular spacings and arrangements may help optimize these contributions in devices relying on the electronic properties of these types of interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

Surface coverage calculation and details of partial Hessian analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 5859



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Present Address

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Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the support of the NSF through grants CHE1012380 and CHE-1266422. J.E.L. and T.A.O. were supported by the NSF Graduate Research Fellowship Program (DGE0718123 and DGE-1256259). J.R.C. acknowledges support and use of computational facilities from the University of Wisconsin Materials Research Science and Engineering Center (DMR1121288) and additional computational resources from National Science Foundation Grant CHE-0840494. J.R.S. is an Alfred P. Sloan Research Fellow. P.G. and Y.J. acknowledge funding from the UW-MRSEC (DMR-1121288).

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