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Direct Measurement of the Absolute Orientation of N3 Dye at Gold and Titanium Dioxide Surfaces with Heterodyne Detected Vibrational SFG Spectroscopy Christopher C Rich, Max A Mattson, and Amber T. Krummel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12649 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

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The Journal of Physical Chemistry

Direct Measurement of the Absolute Orientation of N3 Dye at Gold and Titanium Dioxide Surfaces with Heterodyne Detected Vibrational SFG Spectroscopy Christopher C. Rich, Max A. Mattson, and Amber T. Krummel* Chemistry Department, Colorado State University, Fort Collins, Colorado 80523-1872, United States *[email protected] Abstract Light harvesting dyes in dye sensitized solar cells (DSSCs) must be designed not only to effectively harvest visible light but also maintain an adsorption geometry at the solvent/TiO2 interface which encourages electron injection. Electron injection is encouraged when the dye is adsorbed to the TiO2 surface such that the LUMO of the dye is spatially near the surface. Furthermore, deleterious recombination pathways between the surface and dye are suppressed if the HOMO of the dye is spatially well separated from the surface. Thus measuring the configuration of dyes at these interfaces is important for understanding why some dyes perform better than others as well as providing insight into designing more ideal dyes. In this article, we investigate the adsorption geometry of N3 dye on gold and TiO2 using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG). Incorporating heterodyne detection into our VSFG experiment provides both enhanced SFG signal but also provides phase sensitivity which enables the measurement of the absolute orientation of molecules at interfaces. On gold, we find that N3 adsorbs to the surface by binding through one of its isothiocyanate ligands at a 36° tilt angle from the surface normal. The other isothiocyanate ligand exhibits a tilt angle of 82° and thus does not interact with the interface as strongly. Conversely, on TiO2, we

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find that N3 adsorbs to the surface through three carboxylic acid groups with both isothiocyanate ligands facing away from the surface at a 180° tilt angle from the surface normal. This adsorption geometry of N3 on the TiO2 is arranged such that its LUMO, which resides primarily on the bipyridine ligands, is positioned near the surface while the HOMO, which resides primarily on the isothiocyanate ligands, is oriented far away from the surface. This study presents the first HD-VSFG spectra of N3 on nanoparticulate TiO2 and on gold. Introduction The study of dye-nanoparticle interactions has been at the center of a wide variety of research topics such as surface-enhanced Raman spectroscopy (SERS),1-5 environmental remediation,6-9 and, as will be discussed here, photovoltaics. In pursuit of photovoltaic technology that is both efficient and cost-effective, a great deal of research has been directed toward understanding and improving dye-sensitized solar cells (DSSCs)10-24. Unlike commercial silicon based solar cells, DSSCs separate the light harvesting and charge transport components, which are typically a molecular dye and a thin film of titanium dioxide (TiO2) nanoparticles, respectively. In these systems the light harvesting pigment is excited by visible photons and injects its excited electron into the thin film which transports the charge to the cathode. The circuit is completed when the charge returns to the anode which then reduces a dissolved electrolyte which in turn reduces the oxidized dye. The chemistry at the dye/electron transport medium interface is rather complex in these devices with many components contributing to both interfacial electron transport and recombination pathways. In this study we will focus primarily on the light-harvesting dye and in particular its adsorption geometry. A variety of sensitizing pigments have been explored in pursuit of improving DSSCs including organometallics,25-27 organic dyes,28-32 natural pigments,33-35 and more recently


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perovskites.36 A variety of characteristics are important for high performing dyes: 1) they can adsorb to the metal oxide surface; 2) they efficiently absorb in the visible range of the solar spectrum; 3) they have high quantum efficiency for radiative relaxation from the excited electronic state; 4) the rate of electron injection from the excited state to the metal oxide conduction band is faster than other recombination or relaxation processes; 5) they interact favorably with the solvent and electrolyte environment at the interface such that the dye does not aggregate or degrade over long periods of use. Thus, dyes with exceptional light harvesting ability can perform poorly if they do not contain functional moieties which satisfy these necessary characteristics. It follows then that dyes which adsorb to metal oxide surfaces in a configuration that facilitates efficient and expedient charge injection, and thus LUMOconduction band electronic overlap, will perform best in DSSCs. Measuring the interfacial properties of sensitizing dyes is therefore paramount to discovering the molecular structures which are beneficial to both light harvesting and charge transport.

Figure 1. Structure of N3. There are many dyes to consider but we will focus now on N3 dye (see Figure 1), one of the best performing sensitizing dyes for DSSCs. Studies of the interfacial properties of N3 at



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metal oxide surfaces have utilized a variety of spectroscopic,26,27,37-42 electrochemical,43 and computational22,25,44-46 techniques. Interpretation of the adsorption geometry of N3 dye from these studies is quite varied. Experimental studies of N3 on TiO2 utilizing attenuated total reflection – Fourier transform infrared (ATR-FTIR) spectroscopy,40 confocal Raman microscopy,27 and isotherm studies43,47 have proposed possible adsorption schemes, however, the results were not entirely conclusive. This ambiguity in characterizing the adsorption geometry of N3 on TiO2 arises from the complexity of the interface. The heterogeneous nature of the TiO2 thin films, the influence of the solvent environment, and contributions from N3 molecules not at the interface makes the measurement of dye adsorption difficult. In order to truly extract the interfacial properties of these dyes an interface selective technique is required. Thus to determine the interfacial geometry of N3 we have utilized heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy. SFG spectroscopy is an important tool for the investigation of molecular phenomena at interfaces.48-58 In SFG spectroscopy, two radiation fields impinge on a sample which then emits a field whose frequency is the sum of the frequencies of the incident fields. In vibrational SFG (VSFG) spectroscopy, the input fields are typically a mid-IR laser pulse, which probes the IRallowed transition of an interfacial molecular vibration, and a visible laser pulse, which probes the Raman-allowed transition to produce the SFG field. A few studies have been conducted examining nanoparticulate TiO2 interfaces. Wang and coworkers have examined a variety of small molecules on anatase nanoparticle surfaces against different solvent environments.59-61 In Ref. 31, VSFG was used to probe the adsorption geometry of porphyrin dyes on anatase TiO2 nanoparticle films against different solvent environments. VSFG has also been implemented to measure the orientation of the rhenium based carbon dioxide reduction catalyst on titanium


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dioxide surfaces as well.62 However, while one can determine the tilt angle of the dipole moment of a molecular vibration with conventional VSFG, it is difficult to determine in which direction the dipole points along that tilt angle. Thus we incorporated heterodyne detection into our VSFG experiment to determine if the ligands associated with N3 are oriented towards or away from the TiO2 surface. Recently heterodyne detection has been implemented into SFG spectroscopy to improve the signal as well as obtain phase and absolute orientation information from spectra.63-69 HDSFG is implemented via spectral interferometry of the SFG signal with a local oscillator field, which typically originates from the strong nonresonant SFG of a substrate such as gallium arsenide (GaAs), quartz, or gold. Fourier filtering of these interferograms allows one to extract the real and imaginary part of the second order susceptibility, 𝜒 (!) , rather than only its magnitude. This result not only provides phase information, but can determine how the transition dipole moment is oriented and in which direction it points. Thus absolute interfacial orientation can be determined with HD-VSFG spectroscopy, making it a potentially powerful tool in understanding the link between DSSC performance and the adsorption geometry of the sensitizing dye. The configuration of the N3 ligands with respect to the surface has important implications on its electronic structure. The LUMO of N3 is localized primarily on the dicarboxyilic acid-bipyridine groups while the HOMO is localized on the isothiocyanate groups and ruthenium center.19,22,38,44 For a strong performing sensitizer in a DSSC, we would expect an adsorption geometry which orients the isothiocyanate groups away from the TiO2 surface and involves at least two or three carboxylic acid groups binding to the surface. This would inhibit



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recombination between the conduction band of TiO2 and the HOMO of N3 and encourage electron injection from the LUMO to the TiO2. In this work we implemented HD-VSFG spectroscopy to investigate the absolute adsorption geometry of N3 dye on gold and on a nanoparticulate anatase TiO2 thin film. At these two interfaces we expected markedly different adsorption geometries with the isothiocyanate groups preferentially binding to gold and the carboxylic acid groups binding preferentially to TiO2. Comparison of the spectra at these interfaces, combined with polarization analysis, have provided ample and conclusive evidence related to the absolute orientation of N3. From the adsorption geometry determined for N3 on TiO2 we gain insight into why this dye performs so well in DSSCs and further into what molecular designs will perform best in these systems. To our knowledge, this is the first HD-VSFG study of N3 on both gold and TiO2. Experimental Materials. N3 dye (cis-Bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II)) was purchased from Sigma-Aldrich. 5 nm anatase titanium dioxide (TiO2) nanoparticles were purchased from US Research Nanomaterials. Unprotected gold mirrors from Thorlabs were used as our gold surface. Thin films of TiO2 nanoparticles were prepared by the following spin coating procedure. 15 µL of an 8 g/L suspension of the nanoparticles in water was dropped on an unprotected gold mirror and spun for 30 s at 2000 rpm followed by 3 s at 3000 rpm. This procedure was repeated 3 times to produce a thin transparent film. Films were then placed face down on a hot plate at 300 °C for 2 hours to stabilize the film. The film was cooled in air for 15 minutes and then rinsed with deionized water. The TiO2 thin film and bare gold substrates were sensitized with N3 dye by soaking in a 0.1 mM solution of N3 in ethanol and allowing the substrates to sit for 20 hours (overnight). The sensitized substrates were removed from the


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sensitizing solution and rinsed with deionized water and ethanol to remove unbound dye and aggregates. The samples were then dried under nitrogen gas flow. Heterodyne-Detected Vibrational Sum Frequency Generation Spectrometer. A schematic of our spectrometer is shown in Supporting Information in Figure S1. To generate our mid-IR and visible pulses for HD-VSFG we use a Ti:sapphire based regenerative amplifier (Wyvern 1000, KM Labs) which outputs ~45 fs pulses at a 1 kHz repetition rate centered at 790 nm. Using a beamsplitter, 80% of this output is sent to an optical parametric amplifier (OPA, TOPAS Prime, Light Conversion) and the produced signal and idler are collinearly sent into a silver gallium sulfide (AgGaS2) crystal for difference frequency generation to produce mid-IR pulses centered at 4.76 µm. The remaining fraction is used as the visible light which is sent through an interference filter in order to frequency narrow the pulses to a FWHM of 0.5 nm (10 cm-1). The mid-IR and visible pulses are overlapped in space and time at the sample interface. The mid-IR and visible pulses are focused into the sample at 60° and 50° angles, respectively, relative to the surface normal. The two fields impinging on molecules at the interface produce sum frequency generation emission. The SFG emission and the reflected mid-IR and visible pulses are refocused by a curved mirror onto a GaAs crystal, with the VSFG emission passing through a 3 mm thick fused silica window to apply a ~4.5 ps delay. The mid-IR and visible fields impinging in space and time on the GaAs crystal produce a nonresonant SFG emission that is used as the local oscillator. The collinear local oscillator and SFG signal from the sample are dispersed in a spectrometer (iHR550, Horiba) with a 1200 lines/mm grating and are detected by a thermoelectrically cooled CCD camera. Further experimental detail is provided in Supporting Information.



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While the phase sensitivity of heterodyne detection allows us to determine the “up” vs. “down” pointing of the transition dipole moment, it does not directly measure the tilt angle the dipole moment makes with respect to the surface normal. Thus to best capture the absolute adsorption geometry of N3, we measure the HD-VSFG spectra of our samples in two different polarization schemes. The polarization schemes used in our experiments are represented by three letter acronyms with the first, second, and third letters denoting the polarizations of the collected VSFG signal, the visible field, and the mid-IR field, respectively. The polarization schemes used in this study were PPP and SSP, where P and S denote fields polarized parallel and perpendicular to the plane of incidence, respectively. Polarization of the visible pulse was controlled with a half wave plate and wire grid polarizer and the collected VSFG signal polarization was selected with a wire grid polarizer. The polarization of these fields with respect to the incident surface is demonstrated in Figure S2. Spectra collected on the CCD camera are then Fourier filtered and normalized to a reference to extract only the spectral features associated with the sample in question. Fourier Filtering and Normalization. Figure S3 outlines the Fourier filtering procedure to produce the HD-VSFG spectrum of N3 dye on gold in the PPP polarization scheme. The collected SFG spectrum appears as Fig. S3a and is the equivalent to the absolute square of the total field: 𝐸!"! (𝜔)

!

= 𝐸!"# 𝜔 𝑒 !"# + 𝐸!" (𝜔) !

= 𝐸!"# (𝜔) + 𝐸!"

!

!

∗ ∗ + 𝐸!"# 𝜔 𝐸!" 𝜔 𝑒 !"# + 𝐸!"# (𝜔)𝐸!" (𝜔)𝑒 !!"#

Equation 1


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where 𝑇 is the time delay between the local oscillator and SFG signal. The third and fourth terms in equation 1 are related to the interference fringes Figure S3a. To Fourier filter, the raw spectrum is Fourier transformed into the time domain (see Fig. S3b). A filter function is applied to select only the feature at time 𝑇 which are equivalent to the third term in equation 1. The filtered feature is then Fourier transformed back to the frequency domain to produce the spectral interferogram of the signal (see Fig. S3c). The same procedure is then performed for a reference sample, which in these experiments was an unprotected gold mirror with no sample (the blue spectra in Fig S3). Since the reference spectrum also contains the local oscillator field, and gold produces a surface plasmon derived SFG emission, one can produce the SFG spectrum of the sample by dividing the sample spectral interferogram by that of the reference: (!)

∗ 𝜒!"# 𝐸!"# 𝜔 𝐸!" 𝜔 𝐸!"# 𝜔 = ∝ ∗ (!) 𝐸!"# 𝜔 𝐸!" 𝜔 𝐸!"# 𝜔 𝜒!"#

Equation 2 To assure consistent phase relation between spectra, the spectral interferograms of the (!)

(!)

real 𝜒!"# are phased to the real 𝜒!"# of a bare gold reference.24,64 As all of our samples are mounted on gold substrates, all phase measurements are performed relative to that of the surface (!)

plasmon SFG of gold. We assume that the phase of 𝜒!"# for a bare gold substrate is !

approximately ! , however, the phase of the SFG emission of gold has been shown to be both wavelength dependent and adsorbate dependent which can result in phase deviation.70-74 Thus, we acknowledge that while we can negate some of the additional local field effects due to the gold surface plasmon with our phasing procedure, we cannot fully characterize the role of the adsorbate and its contribution to the phase of the gold SFG emission. However, as will be shown



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in the results section, this deviation in the phase will likely not be significant enough to severely distort the lineshape and more importantly change the sign of our spectra. Results

Figure 2. HDSFG spectra of N3 dye on gold. The black and red traces are the real and imaginary part of 𝜒 (!) for the PPP polarization scheme. The blue and green traces are the real and imaginary part of 𝜒 (!) for the SSP polarization scheme. Figure 2 shows the HD-VSFG spectra of the ν(CN) stretching mode from the isothiocyanate groups of the N3 dye on a gold surface. The spectra collected on the gold surface, as well as those collected on the TiO2 surface, were measured with PPP and SSP polarization schemes. An advantage of heterodyne detection is that it provides spectra that relate directly to


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!

the second order susceptibility, 𝜒 (!) , rather than its magnitude, 𝜒 (!) , as is the case in homodyne detected experiments. As a result, we obtain complex spectra which include the real and the imaginary parts of 𝜒 (!) which is critical for acquiring information regarding absolute orientation. Much like the refractive index, we can express the second order susceptibility as: 𝜒 (!) = 𝑅𝑒 𝜒

!

+ 𝑖𝐼𝑚(𝜒

!

) Equation 3

The real component, 𝑅𝑒 𝜒

!

, has a dispersive shape and refers to the refractive component of

the susceptibility. The imaginary component, 𝐼𝑚(𝜒

!

), refers to the absorptive or emissive

component of the susceptibility and appears as a peak as in conventional spectroscopy techniques like Raman spectroscopy and linear IR spectroscopy. The sign of both 𝑅𝑒 𝜒

!

and 𝐼𝑚(𝜒

!

) can report on the absolute orientation of the

transition dipole moment of the molecular vibration, specifically on whether it points “up” or “down” with respect to the surface.65,68. However, in order to extract absolute orientation of this dipole and the corresponding molecular orientation, we relate the emitted signal produced by the sample to molecular scale properties. In order to do this, we precisely characterize the field at the interface which produces the SFG emission. Calculating the field at the interface requires knowledge of the refractive indices of the input and emitted beams of the media which form the interface, the incident and exiting angles of the input fields, and their polarization. We then express the susceptibility of an experiment with PPP and SSP polarization schemes with respect to components of the lab frame 2nd order susceptibility tensor:



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! ! 𝜒!!! ∝ 𝐴!!,!" Ω!" , 𝛾!" 𝐴!!,!"# Ω!"# , 𝛾!"# 𝐴!!,!" Ω!" , 𝛾!" 𝜒!!" ! + 𝐴!!,!" Ω!" , 𝛾!" 𝐴!!,!"# Ω!"# , 𝛾!"# 𝐴!!,!" Ω!" , 𝛾!" 𝜒!"! ! + 𝐴!!,!" Ω!" , 𝛾!" 𝐴!!,!"# Ω!"# , 𝛾!"# 𝐴!!,!" Ω!" , 𝛾!" 𝜒!"" ! + 𝐴!!,!" Ω!" , 𝛾!" 𝐴!!,!"# Ω!"# , 𝛾!"# 𝐴!!,!" Ω!" , 𝛾!" 𝜒!!!

Equation 4 ! ! 𝜒!!" ∝ 𝐴!!,!" Ω!" , 𝛾!" 𝐴!!,!"# Ω!"# , 𝛾!"# 𝐴!!,!" Ω!" , 𝛾!" 𝜒!!"

Equation 5 Components of the lab frame susceptibility tensor are indexed by lab frame Cartesian coordinates, where the X and Y axes lie parallel to the surface and the Z axis lies along the surface normal. In our experiments we assume azimuthally isotropic interfaces which means that within our focal volume the X and Y axes are effectively indistinguishable. 𝐴 Ω, 𝛾 is a prefactor which includes a Fresnel coefficient, 𝐿, associated with the interface and relates the input and output fields to the field generated at the interface along a particular lab frame Cartesian axis. This prefactor also accounts for the angle of incidence of each field and its polarization: 𝐴!! Ω, 𝛾 = 𝐿!! 𝛾 cos Ω cos(𝛾) 𝐴!! Ω, 𝛾 = 𝐿!! 𝛾 sin Ω 𝐴!! Ω, 𝛾 = 𝐿!! 𝛾 cos Ω sin 𝛾 Equations 6 a-c where 𝛾 is the angle of incidence of the field with respect to the surface normal and Ω is the angle of the polarization of the field with respect to the incident plane of the experiment (Ω = 0∘


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for P and Ω = 90∘ for S). The lab frame susceptibility can then be related to the orientational average of the corresponding molecular frame property, the molecular hyperpolarizability, 𝛽: 𝜒 (!) = 𝑁 𝛽 𝑓(𝜔!" − 𝜔!"# ) Equation 7 where N is the number of molecules in the focal volume and the brackets, … , denote an orientational average. 𝑓(𝜔!" − 𝜔!"# ) is a lineshape function which is dependent on 𝜔!"# , the resonant vibrational frequency, and 𝜔!" , is the frequency of the IR field. Like the lab frame susceptibility, the hyperpolarizability is also a third rank tensor and is given by: 𝛽!!" = 𝛼!" 𝜇! Equation 8 In equation 8, 𝑖, 𝑗, 𝑘 index the molecular Cartesian coordinates, 𝛼!" is the 𝑖𝑗-component of the transition polarizability and 𝜇! is the 𝑘th component of the transition dipole moment. In our models we utilized Kubo lineshapes to best describe spectral features. In this formalism, spectral lineshapes are determined by: 𝐼 𝜔 =

1 2𝜋

!

𝜑(𝑡)𝑒 !!"# 𝑑𝑡

!!

Equation 9 𝜑(𝑡) is a lineshape function given by: 𝛥! 1 𝜑 𝑡 = exp − 𝑡 − 1 − 𝑒 !!" 𝛬 𝛬



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Equation 10 𝛥 is the amplitude of the perturbations to the resonant frequency and 𝛬 is the rate at which these perturbations relax, the inverse of which is the pure dephasing time. In this study, the target molecular vibration is the ν(CN) stretch of the isothiocyanate (SCN) groups. For N3, the normal mode associated with this vibrations is a symmetric stretch in which both CN bonds stretch in phase. Since the symmetry of N3 can be characterized by the C2 point group, group theory can be applied to determine that the nonzero elements of the hyperpolarizability tensor will be 𝛽!!! , 𝛽!!" , 𝛽!!" , and 𝛽!"# = 𝛽!"# . Using Euler rotation matrices we can then relate these hyperpolarizability tensor elements to elements of the 2nd order susceptibility tensor as follows: 1 (!) 𝜒!!" = 𝛽!!" (cos ! 𝜃 cos ! 𝜙 2 1 + cos 𝜃 sin! 𝜙) + 𝛽!!" (cos ! 𝜃 sin! 𝜙 2 1 + cos 𝜃 cos ! 𝜙) + 𝛽!!! (cos 𝜃 − cos ! 𝜃) + 𝛽!"# cos 𝜙 sin 𝜙 (cos ! 𝜃 − cos 𝜃) 2 1 (!) (!) 𝜒!"! = 𝜒!"" = 𝛽!!" cos ! 𝜙 (cos ! 𝜃 2 1 − cos 𝜃) + 𝛽!!" sin! 𝜙 (cos ! 𝜃 2 1 − cos 𝜃) + 𝛽!!! (cos 𝜃 − cos ! 𝜃) + 𝛽!"# cos 𝜙 sin 𝜙 (cos ! 𝜃 − cos 𝜃) 2 (!)

𝜒!!! = 𝛽!!" cos ! 𝜙 (cos 𝜃 − cos ! 𝜃) + 𝛽!!" sin! 𝜙 cos 𝜃 − cos ! 𝜃 + 𝛽!!! cos ! 𝜃 + 𝛽!"# sin 2𝜙 (cos 𝜃 − cos ! 𝜃)


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Equations 11 a-c 𝜃 and 𝜙 are tilt and twist angles, respectively. The tilt angle describes the angle between the molecule frame z-axis, defined by the molecular principal rotation axis, and the lab frame Z-axis, defined by the surface normal, and the twist angle describes rotation about the molecule frame zaxis. One would use equations 11 a-c to model orientation of N3 at the interface as long as it maintained its C2 symmetry. However, at the opposite extreme, chemically binding to the surface may change this symmetry. As a result the vibrational mode may localize to one CN bond rather than include both, particularly if one SCN group binds to the surface and the other does not. In this case it would be favorable to treat each SCN group as a linear species and the hyperpolarizability tensor would have only three nonzero elements: 𝛽!!! and 𝛽!!" = 𝛽!!" . As described previously, we can relate these tensor elements to the components of the 2nd order susceptibility as follows: 1 ! 𝜒!!" = 𝛽!!! cos 𝜃 1 + 𝑟 + cos ! 𝜃 1 − 𝑟 2 1 ! ! 𝜒!"! = 𝜒!"" = 𝛽!!! cos 𝜃 − cos ! 𝜃 2

1−𝑟

! 𝜒!!! = 𝛽!!! cos 𝜃 𝑟 + cos ! 𝜃 1 − 𝑟

Equations 12 a-c where 𝑟 =

!!!" !!!!

and 𝜃 is the tilt angle of the transition dipole moment from the surface normal.

Lastly, in interpreting the spectra presented here we need to incorporate the influence of electronic resonance. The SFG emission is centered at 676 nm, which is preresonant to a strong



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and broad metal-to-ligand charge transfer (MLCT) transition centered at 540 nm which has a long tail that extends into the near infrared region (see Fig. S4 in Supporting Information). As a result, we achieve, albeit weak, resonance enhancement, which increases the magnitude of 𝛽!!! substantially as the electronic transition dipole moment lies along the molecule’s z-axis. Thus, we can assume, and will show, that the other tensor components are negligibly small. As such equations 11 a-c effectively reduce to equations 12 a-c under resonance conditions and can be applied to the ν(CN) stretch regardless of the symmetry of N3 at the interface. It should be noted that when nearing resonance with an electronic state, the polarizability component of the hyperpolarizability becomes complex, as shown by the equation for Albrecht’s A-term used in resonance Raman spectroscopy, and when fully resonant becomes completely imaginary. This results in a phase shift of the signal as the sum frequency approaches double resonance, which can lead to lineshape changes in the imaginary and real components of 𝜒 (!) . This could hinder interpretation of the measured spectra but here we are not so susceptible to these problems. First, the sum of our frequencies, while preresonant, is still rather far from resonance with the MLCT transition and so a change in lineshape of our spectra is not significant enough to be observable in our data. To prove this we have measured the expected phase contribution from electronic resonance from the UV-Vis absorption spectrum of N3 in ethanol (see Supporting Information). At 676 nm the phase contribution from electronic resonance will (!)

be ±15°. In figure 3, we have applied a +15° and -15° phase shift to the imaginary 𝜒!!! spectra of N3 on gold and observe very small changes to the lineshape of the spectral features. We also show this is true for N3 on TiO2 in the Supporting Information in Figure S6. This minimal lineshape change additionally demonstrates that any additional phase contributions due to local field changes as the result of chemically bound adsorbates on gold will not likely change the


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lineshape enough to perturb conclusions derived from our spectra. Thus, even in the presence of a phase shift, we can determine the direction in which our transition dipole points and, using the ratio of the magnitudes of our PPP to SSP spectra, we can determine the tilt angle of transition dipole.

Figure 3. 𝐼𝑚(𝜒

!

) spectra of N3 on gold without (black) and with a +15° phase shift (red) and

with a -15° phase shift (blue). Using the relationships expressed in equations 12 a-c in equations 4 and 5, we have fit our experimental spectra to determine the tilt angle of the transition dipole moment and determine the orientation of our molecule at these interfaces. The only variable parameters in our fit are the relative value of 𝛽!!! , 𝑟, and 𝜃 for each mode in a given spectrum. The number of



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modes, the resonant vibrational frequency of each mode, and the ratio of 𝛥 to 𝛬 are determined by the collected spectra and are adjusted between calculations in order to improve the fit. For !

spectra of N3 on gold and TiO2 we find that ! ≫ 1 indicating that a Gaussian fit best describes the measured spectral lineshapes.


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Figure 4. HD-VSFG spectra with Kubo lineshape fits for two modes for N3 dye on gold in a) PPP and b) SSP polarization schemes. Red traces correspond to real 𝜒 (!) spectra and blue traces correspond to imaginary 𝜒 (!) spectra. Figures 4a and 4b show the experimental PPP and SSP HD-VSFG spectra of the N3 dye on gold with our fitted model. Our model fits two modes in our spectra centered at 2131 cm-1 and 2185 cm-1. The higher frequency mode is twice as broad as the lower frequency mode with a full width half maxima (FWHM) of 60 cm-1 and 38 cm-1, respectively. For our fits of the hyperpolarizability, we note that since these are normalized spectra that these are not absolute quantities but relative fits. We thus report the ratio, 𝑟 =

!!!" !!!!

, for each mode. For the low

frequency mode 𝑟 = 0.0122 while for the high frequency mode 𝑟 = −0.0870. As aforementioned, we expect the 𝛽!!" = 𝛽!!" to be negligibly small in the case of resonance enhancement. Furthermore, the larger magnitude of 𝑟 for the high frequency mode suggests comparatively smaller 𝛽!!! for the high frequency mode compared to the low frequency mode. An increase in this component may arise as a result in an increase in the polarizability or the transition dipole moment for the ν(CN) associated with the low frequency mode. With the transition dipole moment of the ν(CN) stretch on the SCN group pointing from the carbon to the nitrogen, the fitted tilt angle of the low frequency mode is 36°±5° and the fitted tilt angle of the high frequency mode is 82°±1°. We can conclude that on the gold surface, N3 binds to gold through the isothiocyanate group associated with the low frequency vibrational mode. As predicted we observe stark differences in the spectra of N3 on TiO2 in comparison to N3 on gold. Figures 5a and 5b show the experimental PPP and SSP HD-VSFG spectra of the N3 dye on the nanoparticulate anatase TiO2 thin film with our fitted model. The sign of the


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imaginary spectra is opposite of that for N3 on gold suggesting that the dipole moment has flipped to point in the opposite direction. This model fits one mode to our spectra centered at 2125 cm-1 with a FWHM of 34 cm-1. This suggests that this vibration behaves like the symmetric ν(CN) normal mode rather than two distinct vibrations localized on each CN bond as is the case with N3 on gold. For this mode 𝑟 = −0.00420 indicating that 𝛽!!! is significantly larger than 𝛽!!" = 𝛽!!" due to resonance enhancement. The fitted tilt angle of the mode is 180°±8°, indicating that the isothiocyanate groups are oriented away from the surface and the bipyridine groups oriented towards the surface.



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Figure 5. HD-VSFG spectra of N3 dye on a thin film of 5 nm anatase TiO2 nanoparticles with Kubo lineshape fits for one mode in a) PPP and b) SSP polarization schemes. Red traces correspond to real 𝜒 (!) spectra and blue traces correspond to imaginary 𝜒 (!) spectra. Black traces correspond to real 𝜒 (!) spectra fits and green traces correspond to imaginary 𝜒 (!) spectra fits. Figures 6a, 6b, and 6c show simulated 𝜒 (!) intensities for PPP and SSP polarization schemes for the three modes addressed as a function of the tilt angle. The simulations show a strong dependence of 𝜒 (!) on the tilt angle, particularly when the dipole moment is not aligned with surface normal. It is important to note that this dependency varies with changes to the interface, as the relationship between 𝜒 (!) and the tilt angle for N3 at the air/gold interface, as shown in figures 6a and 6b, differs substantially from that at the air/TiO2 interface, as shown in figure 6c. The strong influence of tilt angle leads to high certainty in the fitted tilt angles from our measured spectra, demonstrating the power of incorporating both heterodyne detection and polarization considerations into VSFG spectroscopy.



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Figure 6. Simulated PPP (black) and SSP (red) 𝜒 (!) intensities for a) the low frequency mode of N3 on gold, b) the high frequency mode of N3 on gold, and c) the normal mode of N3 on TiO2 as a function of tilt angle, θ. Discussion Utilizing HD-VSFG spectroscopy we are able to obtain a great wealth of information regarding the orientation of N3 on gold and anatase TiO2. Without applying a fit, the observed opposite sign of the imaginary spectra of the ν(CN) stretch for the dye at each interface corresponds to opposite orientation of the transition dipole moment. The ability to ascertain this information by a first approximation demonstrates the power of HD-VSFG spectroscopy as an interfacial technique. Applying the fits shown in Figures 4 and 5 reveal specific orientational information on the ν(CN) stretch from which one can determine more specific structural geometry of the dye at the gold and TiO2 surfaces. For N3 in the condensed phase and in the absence of an interface, symmetric and antisymmetric ν(CN) stretches are expected, though generally the frequency spacing between these modes are observed to be too small to distinguish in IR and Raman spectra. The spectra of N3 on gold show that upon binding to the surface, the ν(CN) peak can be represented by two features: a larger low frequency mode and a weaker high frequency mode. Adsorption to the surface would predictably break the approximate C2 symmetry of the molecule and would potentially lead to separate vibrational modes for each ν(CN) stretch. Our fits suggest that the lower frequency mode, whose dipole makes a 36° angle with respect to the surface normal, is likely associated with an SCN group which is bound to the gold surface through the sulfur atom. The high frequency mode, whose dipole makes an 82° angle with respect to the surface normal, is likely associated with an unbound SCN group. Based on the tilt angles obtained from the fit of



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the spectra we produce a model of the N3 dye at the surface shown in Figure 7a. In this configuration, it follows that one or two of the carboxylate groups are near or at the surface and potentially bind to it as proposed by Britton and coworkers.75 We also note relatively larger oscillator strength, through the relatively larger 𝛽!!! , of the low frequency mode compared to that of the high frequency mode. This is likely due to increased magnitude of either the transition dipole moment or polarizability as a result of binding to the gold surface. The substantial 54 cm-1 red shift of the low frequency mode compared to that of the high frequency mode suggests an increase in the reduced mass of the vibration of the lower frequency mode due to its bonding to the heavy gold surface. The high frequency mode is also twice as broad as the low frequency mode. As we are conducting frequency resolved HD-VSFG our ability to discern a specific reason for the substantial broadening of the unbound mode is limited. One reason may be that the unbound CN vibration has substantially shorter vibrational dephasing time than that of the bound mode, the latter of which may be elongated by its interaction with the gold surface. The unbound SCN group may also have greater diversity in its solvation than the bound SCN group. The difference in the linewidth of the bound CN vibration and the unbound CN vibration leaves an intriguing open question for more time-resolved spectroscopies to answer.


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Figure 7. Schema for adsorption geometry of N3 dye on a) gold and b) TiO2 based on HDVSFG spectra. On the geometry optimized N3 molecules, white = hydrogen, gray = carbon, red = oxygen, blue = nitrogen, and yellow = sulfur, and teal = ruthenium. The orange arrows represent the transition dipole moment vectors associated with the CN bond on each isothiocyanate groups. In b, the violet arrow represents the transition dipole moment vector for the normal mode associated with the symmetric stretch of both CN bonds.



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It must be put forward in this discussion that consideration has been made that the low and high frequency modes observed in the spectra of N3 on gold could be either in-phase and out-of-phase vibrations of the two ν(CN) stretches or possibly coupling between the two stretches which may be close in energy. As adsorption to the surface breaks the approximate C2 symmetry of the molecule, however, the ν(CN) vibrational modes are more likely to be defined as independent stretching vibrations from each isothiocyanate group rather than symmetric and antisymmetric modes. The weak Raman contribution of the antisymmetric vibration and its predicted lack of resonance enhancement also eliminates the possibility of this contribution. Further the red shift observed in the VSFG spectrum of the high frequency mode from the low frequency mode is much larger than that between the symmetric and antisymmetric stretch. In regards to coupling, as the isothiocyanate moieties in N3 are oriented nearly 90° from one another, transition dipole coupling would be predicted to be small and has been shown to be small in 2D IR measurements of metal cyanide complexes.76,77 The substantial shift between the low and high frequency contributions in our spectra seem to eliminate the possibility that coupling is involved. Thus it is more likely that the spectral features observed are from the independent vibrations of the two CN bonds. As previously stated, many studies have been conducted to attempt to measure or calculate the absolute orientation of the N3 dye, as well as other photosensitizers, at the TiO2 interface. A variety of approximate orientations have been postulated but there is a lack of consensus as to which adsorption geometry is more probable. The reason for this ambiguity is in part due to the difficulty in studying this interface. The heterogeneous nature of doctor-blade deposited thin films of the anatase nanoparticles leads to a highly scattering sample which can make SFG detection difficult. By using more tightly packed and optically transparent films, we


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are able to reduce light scattering. Additionally, heterodyne detection enhances the signal since the measured spectral interferogram is much stronger than the homodyne detected signal of the sample alone. This increased sensitivity allows us to measure SFG emission which was previously too weak to measure without heterodyne detection. HD-VSFG spectra of N3 on TiO2 show that the ν(CN) stretch does not fit two nondegenerate modes on TiO2 as it does on gold. This observation suggests that the symmetry of N3 is not substantially broken when bound to TiO2 which implies that the ν(CN) stretch is best described as a symmetric stretch normal mode which includes both CN bonds, as shown in Fig. 6b. The fit of this peak corresponds to a tilt angle of the ν(CN) transition dipole that is 180° which implies that the isothiocyanate groups are oriented away from the TiO2 thin film surface. In this configuration, it follows that two carboxylic acid groups from separate bipyridine groups are close enough to the surface to bind to it as shown in Figure 7b. This adsorption geometry agrees with recent calculations in Refs. 44 and 45 and for the model proposed in Refs. 43 and 46 which argued that this conformation fit the spacing of titanium atoms on the TiO2 surface. Uncertainty in the tilt angle, ±8°, although small might suggest some further tilt from the surface normal. However the lack of SSP signal suggest that tilt angles less than 170° are not likely. The measured configuration of the N3 dye on TiO2 fits well into the context of the importance of adsorption geometry of dyes in DSSCs. The HOMO of N3 resides primarily on the isothiocyanate groups while the LUMO resides primarily on the bipyridine moieties. With the isothiocyanate groups oriented away from the surface, this provides the ideal adsorption geometry for optimal overlap of the LUMO and the TiO2 conduction band. Furthermore, the spatial orientation of the HOMO away from the surface provides an optimal pathway for electron injection into the TiO2 surface, with the electron moving predominantly towards the surface



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when the dye is electronically excited. The molecular structure of N3 thus provides both spatial separation of the HOMO and LUMO but also allows for ideal adsorption geometry for charge injection which in part explains why it is one of the most efficient sensitizing dyes in DSSCs. Additionally, this may explain why the best performing DSSCs using N3 typically use nitrile containing solvents for the electrolyte component as these solvents would interact strongly with isothiocyanate groups and likely draw them away from the surface and promote the optimal adsorption geometry. One seemingly unusual result from these measurements is that the ν(CN) mode at the TiO2 interface is significantly red shifted from both bound and unbound modes at the gold interface. The more massive gold atoms should be expected to reduce the vibrational frequency of the ν(CN) modes, especially for the feature that is bound to the surface as expected by the relation, 𝜈 ∝

! !

,which shows that the vibrational frequency, 𝜈, is proportional to the square root

of the ratio of the force constant of the vibration, 𝑘, to the reduced mass of the vibration, 𝜇. The red shift of the vibrational mode at the TiO2 interface compared to those at the gold interface must then result from the strengthening of the bond when bound to gold. Strong interactions with the gold surface for both the bound and unbound SCN groups would result in primarily triple bond character for both CN bonds which would increase the vibrational frequency. However in the case of N3 on TiO2 the SCN groups are oriented away from the surface and are unbound which results in more doubly bond character in the CN bonds and decreased vibrational frequency closer to that of N3 dye in condensed phase. Conclusions


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In summary, we have presented here the first HD-VSFG spectra of N3 dye on gold and nanoparticulate TiO2 thin film surfaces revealing the ability to measure the absolute orientation of the dye at theses interfaces. Spectra of the dye on gold reveal both bound and unbound isothiocyanate moieties at the gold surface and may suggest some electronic interaction which increases the bond strength of the CN bond. SFG spectra of N3 on the TiO2 thin film confirms the SCN groups are 1) not bound to the surface and 2) oriented such that two carboxylate groups chemically bind to the surface. Furthermore the measured adsorption geometry of N3 on TiO2 spatially separates the HOMO that is localized on the isothiocyanate groups away from the surface and brings the majority of the LUMO that is localized on the bipyridine groups close to the surface. This provides an ideal orientation for electron injection into TiO2 and potentially justifies why nitrile containing solvents work well in DSSCs which use N3. However, a full understanding of these sorts of solvent interactions on adsorption geometry has yet to be experimentally observed but could be accomplished with HD-VSFG. Understanding adsorption geometries at the interface in DSSCs can give us insight into designing dyes which can improve DSSC efficiency. Finally, we have shown that for complex and heterogeneous dye-nanoparticle interfaces, such as those relevant to DSSCs, HD-VSFG is an immensely powerful tool that can provide absolute orientation information of molecules at interfaces. Supporting Information. Detailed description of HD-VSFG spectrometer, description of Fourier filtering and normalization procedure, and determination of phase contribution due to electronic resonance. Figures of HD-VSFG spectrometer diagram (Figure S1), illustration of field polarizations (Figure S2), spectra corresponding to Fourier filtering procedure (Figure S3), UV-vis absorption spectrum of N3 dye (Figure S4), spectra of the real and imaginary refractive (!)

index and phase of N3 in ethanol (Figure S5), and spectra of the imaginary 𝜒!!! of N3 on TiO2



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with +15° and -15° phase shifts (Figure S6). This material is available free of charge at http://pubs.acs.org. Acknowledgements The authors would like to extend their gratitude to British Petroleum Exploration Operating Company Ltd. for funding this work. Dr. Christopher C. Rich would like to thank Prof. John T. Fourkas for helpful discussions associated with the development of the HD-VSFG spectrometer used in this work and Prof. Bruce A. Parkinson for helpful discussions associated with sample preparation. References

1. Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering. Science 1997, 275, 1102-1106.

2. Qian, X.; Peng, X.-H.; Ansari, D. O.; Yin-Goen, Q.; Chen, G. Z.; Shin, D. M.; Yang, L.; Young, A. N.; Wang, M. D.; Nie, S. In Vivo Tumor Targeting and Spectroscopic Detection with Surface-Enhanced Raman Nanoparticle Tags. Nature Biotechnology 2008, 26, 83-90.

3. Doering, W. E.; Nie, S. Spectroscopic Tags Using Dye-Embedded Nanoparticles and SurfaceEnhanced Raman Scattering Anal. Chem. 2003, 75, 6171-6176.

4. Franzen, S.; Folmer, J. C. W.; Glomm, W. R.; O’Neal, R. Optical Properties of Dye Molecules Adsorbed on Single Gold and Silver Nanoparticles. J. Phys. Chem. A 2002, 106, 6533-6540.


Page 33 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5. Kometani, N.; Tsubonishi, M.; Fujita, T.; Asami, K.; Yonezawa, Y. Preparation and Optical Absorption Spectra of Dye-Coated Au, Ag, and Au/Ag Colloidal Nanoparticles in Aqueous Solutions and in Alternate Assemblies. Langmuir 2001, 17, 578-580.

6. Salehi, R.; Arami, M.; Mahmoodi, N. M.; Bahrami, H.; Khorramfar, S. Novel Biocompatible Composite (Chitosan-Zinc Oxide Nanoparticle): Preparation, Characterization and Dye Adsorption Properties. Colloids and Surfaces B: Biointerfaces 2010, 80, 86-93.

7. Geng, Z.; Lin, Y.; Yu, X.; Shen, Q.; Ma, L.; Li, Z; Pan, N.; Wang, X. Highly Efficient Dye Adsorption and Removal: A Functional Hybrid of Reduced Graphene Oxide-Fe3O4 Nanoparticles as an Easily Regenerative Adsorbent. J. Mater. Chem. 2012, 22, 3527-3535.

8. Afkhami, A.; Moosavi, R. Adsorptive Removal of Congo Red, a Carcinogenic Textile Dye, from Aqueous Solutions by Maghemite Nanoparticles. J. Hazard. Mater. 2010, 174, 398-403.

9. Inbaraj, B. S.; Chen, B. H. Dye Adsorption Characteristics of Magnetite Nanoparticles Coated with a Biopolymer Poly(γ-glutamic acid). Bioresour. Technol. 2011, 102, 8868-8876.

10. Hagfeldt, A.; Grätzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49–68.

11. Grätzel, M. Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells. Inorganic Chemistry 2005, 44, 6841–6851.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 51

12. Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover,V.; Fisher, C.-H.; Grätzel, M Acid-Base Equilibria of (2,2’Bipyridyl-4,4’-dicarboxylic acid)ruthenium(II) Complexes and the Effect of Protonation on Charge-Transfer Sensitization of Nanocrystalline Titania. Inorganic Chemistry 1999, 38, 6298– 6305.

13. Benkö, G.; Kallioinen, J.; Myllyperkiӧ, P.; Trif, F.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundstrӧm, V. Interligand Electron Transfer Determines Triplet Excited State Electron Injection in RuN3-Sensitized TiO2 Films. J. Phys. Chem. B 2004, 108, 2862–2867.

14. Zhang, D.; Downing, J. A.; Knorr, F. J.; McHale, J. L. Room-Temperature Preparation of Nanocrystalline TiO2 Films and the Influence of Surface Properties on Dye-Sensitized Solar Energy Conversion. J. Phys. Chem. B 2006, 110, 21890-21898.

15. Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. Parameters Influencing the Efficiency of Electron Injection in Dye-Sensitized Solar Cells. J. Amer. Chem. Soc. 2009, 131, 4808–4818.

16. Knorr, F. J.; Mercado, C. C.; McHale, J. L. Trap-State Distributions and Carrier Transport in Pure and Mixed-Phase TiO2: Influence of Contacting Solvent and Interphasial Electron Transfer. J. Phys. Chem. C 2008, 112, 12786–12794.


Page 35 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17. Listorti, A.; O’Regan, B.; Durrant, J. R. Electron Transfer Dynamics in Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3381–3399.

18. Paoprasert, P.; Laaser, J. E.; Xiong, W.; Franking, R. A.; Hamers, R. J.; Zanni, M. T.; Schmidt, J. R.; Gopalan, P. Bridge-Dependent Interfacial Electron Transfer from RheniumBipyridine Complexes to TiO2 Nanocrystalline Thin Films. J. Phys. Chem. C 2010, 114, 9898– 9907.

19. Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Grätzel, M. Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers. J. Amer. Chem. Soc. 2005, 127, 16835–16847.

20. Benkö, G.; Kallioinen, J.; Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundström, V. Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States. J. Amer. Chem. Soc. 2002, 124, 489–493.

21. Kallioinen, J.; Benkö, G.; Myllyperkiӧ, P.; Khriachtchev, L.; Skårman, B.; Wallenberg, R.; Tuomikoski, M.; Korppi-Tommola, J.; Sundström, V.; Yartsev, A. P. Photoinduced Ultrafast Dynamics of Ru(dcbpy)2(NCS)2-Sensitized Nanocrystalline TiO2 Films: The Influence of Sample Preparation and Experimental Conditions. J. Phys. Chem. B 2004, 108, 6365–6373.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 51

22. De Angelis, F.; Fantacci, S.; Selloni, A. Alignment of the Dye’s Molecular Levels with the TiO2 Band Edges in Dye-Sensitized Solar Cells: A DFT-TDDFT Study. Nanotechnology 2008, 19, 424002.

23. Wang, H.; Li, J.; Gong, F.; Zhou, G.; Wang, Z.-S. Ionic Conductor with High Conductivity as Single-Component Electrolyte for Efficient Solid-State Dye-Sensitized Solar Cells. J. Amer. Chem. Soc. 2013, 135, 12627–12633.

24. Xiong, W.; Laaser, J. E.; Paoprasert, P.; Franking, R. A.; Hamers, R. J.; Gopalan, P.; Zanni, M. T. Transient 2D IR Spectroscopy of Charge Injection in Dye-Sensitized Nanocrystalline Thin Films. J. Amer. Chem. Soc. 2009, 131, 18040–18041.

25. Martsinovich, N.; Ambrosio, F.; Troisi, A. Adsorption and electron injection of the N3 metal-organic dye on the TiO2 rutile (110) surface. Phys. Chem. Chem. Phys. 2012, 14, 1666816676.

26. Pérez León, C.; Kador, L.; Peng, B.; Thelakkat, M. Characterization of the Adsorption of Rubpy Dyes on Mesoporous TiO2 Films with UV-Vis, Raman, and FTIR Spectroscopies. J. Phys. Chem. B 2006, 110, 8723–8730.

27. Lee, K. E.; Gomez, M. A.; Elouatik, S.; Demopoulos, G. P. Further Understanding of the Adsorption Mechanism of N719 Sensitizer on Anatase TiO2 Films for DSSC Applications Using Vibrational Spectroscopy and Confocal Raman Imaging. Langmuir 2010, 26, 9575–9583.


Page 37 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28. Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers. Nature Chemistry 2014, 6, 242–247.

29. Hoffman, D. P.; Lee, O. P.; Millstone, J. E.; Chen, M. S.; Su, T. A.; Creelman, M.; Fréchet, J. M. J.; Mathies, R. A. Electron Transfer Dynamics of Triphenylamine Dyes Bound to TiO2 Nanoparticles from Femtosecond Stimulated Raman Spectroscopy. J. Phys. Chem. C 2013, 117, 6990–6997.

30. Varnavski, O.; Abeyasinghe, N.; Aragó, J.; Serrano-Pérez, J. J.; Ortí, E.; López Navarrete, J. T.; Takimiya, K.; Casanova, D.; Casado, J.; Goodson, T. High Yield Ultrafast Intermolecular Singlet Exciton Fission in a Quinoidal Bithiophene. J. Phys. Chem. Lett. 2015, 6, 1375–1384.

31. Ye, S.; Kathiravan, A.; Hayashi, H.; Tong, Y.; Infahsaeng, Y.; Chabera, P.; Pascher, T.; Yartsev, A. P.; Isoda, S.; Imahori, H., et al. Role of Adsorption Structures of Zn-Porphyrin on TiO2 in Dye-Sensitized Solar Cells Studied by Sum Frequency Generation Vibrational Spectroscopy and Ultrafast Spectroscopy. J. Phys. Chem. C 2013, 117, 6066–6080.

32. Rais, D.; Toman, P.; Černý, J.; Menšík, M.; Pfleger, J. Roles of Octabutoxy Substitution and J-Aggregation in Stabilization of the Excited State in Nickel Phthalocyanine. J. Phys. Chem A 2014, 118, 5419–5426.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 51

33. Sandquist, C.; McHale, J. L. Improved Efficiency of Betanin-Based Dye-Sensitized Solar Cells. J. Photochem. Photobiol. A 2011, 221, 90–97.

34. Calogero, G.; Yum, J.-H.; Sinopoli, A.; Di Marco, G.; Grätzel, M.; Nazeeruddin, M. K. Anthocyanins and Betalains as Light-Harvesting Pigments for Dye-Sensitized Solar Cells. Solar Energy 2012, 86, 1563–1575.

35. Calogero, G.; Di Marco, G.; Cazzanti, S.; Caramori, S.; Argazzi, R.; Di Carlo, A.; Bignozzi, C. A. Efficient Dye-Sensitized Solar Cells Using Red Turnip and Purple Wild Sicilian Prickly Pear Fruits. International Journal of Molecular Sciences 2010, 11, 254–267.

36. Jeon, N. J.; Lee, J.; Noh, J. H.; Nazeeruddin, M. K.; Grätzel, M.; Seok, S. I. Efficient Inorganic-Organic Hybrid Perovskite Solar Cells Based on Pyrene Arylamine Derivatives as Hole-Transporting Materials. J. Amer. Chem. Soc. 2013, 135, 19087–19090.

37. Siefermann, K. R.; Pemmaraju, C. D.; Neppl, S.; Shavorskiy, A.; Cordones, A. A.; VuraWeis, J.; Slaughter, D. S.; Sturm, F. P.; Weise, F.; Bluhm, H., et al. Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface. J. Phys. Chem. Lett. 2014, 5, 2753– 2759.


Page 39 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38. Persson, P.; Lundqvist, M. J. Calculated Structural and Electronic Interactions of the Ruthenium Dye N3 with a Titanium Dioxide Nanocrystal. J. Phys. Chem. B 2005, 109, 11918– 11924.

39. Shoute, L. C. T.; Loppnow, G. R. Excited-State Metal-to-Ligand Charge Transfer Dynamics of a Ruthenium(II) Dye in Solution and Adsorbed on TiO2 Nanoparticles from Resonance Raman Spectroscopy. J. Amer. Chem. Soc. 2003, 125, 15636–15646.

40. Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell. J. Phys. Chem. B 2003, 107, 8981–8987.

41. Nomoto, T.; Fujio, K.; Sasahara, A.; Okajima, H.; Koide, N.; Katayama, H.; Onishi, H. Phonon Mode of TiO2 Coupled with the Electron Transfer from N3 dye. J. Chem. Phys. 2013, 138, 224704.

42. Pollard, J. A.; Zhang, D.; Downing, J. A.; Knorr, F. J.; McHale, J. L. Solvent Effects on Interfacial Electron Transfer from Ru(4,4’-dicarboxylic acid-2,2’-bipyridine)2(NCS)2 to Nanoparticulate TiO2: Spectroscopy and Solar Photoconversion. J. Phys. Chem. A 2005, 109, 11443–11452.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 51

43. Lu, Y.; Choi, D.; Nelson, J.; Yang, O.-B.; Parkinson, B. A. Adsorption, Desorption, and Sensitization of Low-Index Anatase and Rutile Surfaces by the Ruthenium Complex Dye N3. J. Electrochem. Soc. 2006, 153, E131-E137.

44. De Angelis, F.; Fantacci, S.; Selloni, A.; Nazeeruddin, M. K.; Grätzel, M. First-Principles Modeling of the Adsorption Geometry and Electronic Structure of Ru(II) Dyes on Extended TiO2 Substrates for Dye-Sensitized Solar Cell Applications. J. Phys. Chem. C 2010, 114, 6054– 6061.

45. De Angelis, F.; Fantacci, S.; Selloni, A.; Nazeeruddin, M. K.; Grätzel, M. Time-Dependent Density Functional Theory Investigations on the Excited States of Ru(II)-Dye-Sensitized TiO2 Nanoparticles: The Role of Sensitizer Protonation. J. Amer. Chem. Soc. 2007, 129, 14156– 14157.

46. Fillinger, A.; Soltz, D.; Parkinson, B. A. Dye Sensitization of Natural Anatase Crystals with a Ruthenium-Based Dye. J. Electrochem. Soc. 2002, 149, A1146-A1156.

47. Fillinger, A.; Parkinson, B. A. The Adsorption Behavior of a Ruthenium-Based Sensitizing Dye to Nanocrystalline TiO2 Coverage Effects on the External and Internal Sensitization Quantum Yields. J. Electrochem. Soc. 1999, 146, 4559-4564.

48. Shen, Y. R. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation. Nature 1989, 337, 519-525.


Page 41 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49. Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Mapping Molecular Orientation and Conformation at Interfaces by Surface Nonlinear Optics. Phys. Rev. B 1999, 59, 12632.

50. Aliaga, C.; Baldelli, S. A Sum Frequency Generation Study of the Room-Temperature Ionic Liquid-Titanium Dioxide Interface. J. Phys. Chem. C 2008, 112, 3064–3072.

51. Brandes, E.; Karageorgiev, P.; Viswanath, P.; Motschmann, H. Breaking the Symmetry of Ions at the Air-Water Interface. J. Phys. Chem. C 2014, 118, 26629–26633.

52. Curtis, A. D.; Reynolds, S. B.; Calchera, A. R.; Patterson, J. E. Understanding the Role of Nonresonant Sum-Frequency Generation from Polystyrene Thin Films. J. Phys. Chem. Lett. 2010, 1, 2435–2439.

53. Fourkas, J. T.; Walker, R. A.; Can, S. Z.; Gershgoren, E. Effects of Reorientation in Vibrational Sum-Frequency Spectroscopy. J. Phys. Chem. C 2007, 111, 8902–8915.

54. Fu, L.; Wang, Z.; Psciuk, B. T.; Xiao, D.; Batista, V. S.; Yan, E. C. Y. Characterization of Parallel β-Sheets at Interfaces of by Chiral Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2015, 6, 1310–1315.

55. Ji, N.; Ostroverkhov, V.; Belkin, M.; Shiu, Y.-J.; Shen, Y.-R. Toward Chiral Sum-Frequency Spectroscopy. J. Amer. Chem. Soc. 2006, 128, 8845–8848.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 51

56. Laaser, J. E.; Zanni, M. T. Extracting Structural Information from the Polarization Dependence of One- and Two-Dimensional Sum Frequency Generation Spectra. J. Phys. Chem. A 2013, 117, 5875–5890.

57. Li, B.; Lu, X.; Han, X.; Wu, F.-G.; Myers, J. N.; Chen, Z. Interfacial Fresnel Coefficients and Molecular Structures of Model Cell Membranes: From a Lipid Monolayer to a Lipid Bilayer. J. Phys. Chem. C 2014, 118, 28631–28639.

58. Mifflin, A. L.; Velarde, L.; Ho, J.; Psciuk, B. T.; Negre, C. F. A.; Ebben, C. J.; Upshur, M. A.; Lu, Z.; Strick, B. L.; Thomson, R. J., et al. Accurate Line Shapes from Sub-1 cm-1 Resolution Sum Frequency Generation Vibrational Spectroscopy of α-Pinene at Room Temperature. J. Phys. Chem A 2015, 119, 1292–1302.

59. Wang, C.; Groenzin, H.; Shultz, M. J. Molecular Species on Nanoparticulate Anatase TiO2 Film Detected by Sum Frequency Generation: Trace Hydrocarbons and Hydroxyl Groups. Langmuir 2003, 19, 7330–7334.

60. Wang, C.; Groenzin, H.; Shultz, M. J. Surface Characterization of Nanoscale TiO2 Film by Sum Frequency Generation Using Methanol as a Molecular Probe. J. Phys. Chem. B 2004, 108, 265–272.


Page 43 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


61. Wang, C.; Groenzin, H.; Shultz, M. J. Comparative Study of Acetic Acid, Methanol, and Water Adsorbed on Anatase TiO2 Probed by Sum Frequency Generation Spectroscopy. J. Amer. Chem. Soc. 2005, 127, 9736–9744.

62. Anfuso, C. L.; Snoeberger, R. C. III; Ricks, A. M.; Liu, W.; Xiao, D.; Batista, V. S.; Lian, T. Covalent Attachment of a Rhenium Bipyridal CO2 Reduction Catalyst to Rutile TiO2. J. Amer. Chem. Soc. 2011, 133, 6922-6925.

63. Nihonyanagi, S.; Mondal, J. A.; Yamaguchi, S.; Tahara, T. Structure and Dynamics of Interfacial Water Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation. Ann. Rev. Phys. Chem. 2013, 64, 579–603.

64. Stiopkin, I. V.; Jayathilake, H. D.; Bordenyuk, A. N.; Benderskii, A. V. Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy. J. Amer. Chem. Soc. 2008, 130, 2271– 2275.

65. Yamaguchi, S.; Tahara, T. Heterodyne-Detected Electronic Sum Frequency Generation: “Up” Versus “down” Alignment of Interfacial Molecules. J. Chem. Phys. 2008, 129, 101102.

66. Okuno, M.; Ishibashi, T. Heterodyne-Detected Achiral and Chiral Vibrational Sum Frequency Generation of Proteins at Air/Water Interface. J. Phys. Chem. C 2015, 119, 9947– 9954.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 51

67. Nihonyanagi, S.; Yamaguchi, S.; Tahara, T. Direct Evidence for Orientational Flip-Flop of Water Molecules at Charged Interfaces: A Heterodyne-Detected Vibrational Sum Frequency Generation Study. J. Chem. Phys. 2009, 130, 204704.

68. Laaser, J. E.; Xiong, W.; Zanni, M. T. Time-Domain SFG Spectroscopy Using Mid-IR Pulse Shaping: Practical and Intrinsic Advantages. J. Phys. Chem. B 2011, 115, 2536-2546.

69. Pool, R. E.; Versluis, J.; Backus, E. H. G.; Bonn, M. Comparative Study of Direct and PhaseSpecific Vibrational Sum-Frequency Generation Spectroscopy: Advantages and Limitations. J. Phys. Chem. B 2011, 115, 15362-15369.

70. Ward, R. N.; Davies, P. B.; Bain, C. D. Orientation of Surfactants Adsorbed on a Hydrophobic Surface. J. Phys. Chem. 1993, 97, 7141-7143.

71. Buck, M.; Eisert, F.; Grunze, M.; Trager, F. Second-Order Nonlinear Susceptibilities of Surfaces. Appl. Phys. A.: Mater. Sci. Process. 1995, 60, 1-12.

72. Dreesen, L.; Humbert, C.; Celebi, M.; Lemaine, J. J.; Mani, A. A.; Thiry, P. A.; Peremans, A. Influence of the Metal Electronic Properties on the Sum-Frequency Generation Spectra of Dodecanethiol Self-Assembled Monolayers on Pt(111), Ag(111), and Au(111) Single Crystals. Appl. Phys. B.: Laser Opt. 2002, 74, 621-625.


Page 45 of 51

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


73. Covert, P. A.; Hore, D. K. Assessing the Gold Standard: the Complex Vibrational Nonlinear Susceptibility of Metals. J. Phys. Chem. C. 2015, 119, 271-276.

74. Le Rille, A.; Tadjeddine, A. In Situ Visible-Infrared Sum and Difference Frequency Generation at the Electrochemical Interface. J. Electroanal. Chem. 1999, 467, 238-248.

75. Britton, A. J.; Weston, M.; Taylor, J. B.; Rienzo, A.; Mayor, L. C.; O’Shea, J. N. Charge Transfer Interactions of a Ru(II) Dye Complex and Related Ligand Molecules Adsorbed on Au(111). J. Chem. Phys. 2011, 135, 164702.

76. Slenkamp, K. M.; Lynch, M. S.; Van Kuiken, B. E.; Brookes, J. F.; Bannan, C. C.; Daifuku, S. L.; Khalil, M. Investigating Vibrational Anharmonic Couplings in Cyanide-Bridged Transition Metal Mixed Valence Complexes using Two-Dimensional Infrared Spectroscopy. J. Chem. Phys. 2014, 140, 084505.

77. Courtney, T. L.; Fox, Z. W.; Estergreen, L.; Khalil, M. Measuring Coherently Coupled Intramolecular Vibrational and Charge-Transfer Dynamics with Two-Dimensional VibrationalElectronic Spectroscopy. J. Phys. Chem. Lett. 2015, 6, 1286–1292.



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