Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline

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Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly-Resonant Sum Frequency Spectroscopy Sanghamitra Sengupta, Leander Bromley, and Luis Velarde J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12166 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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The Journal of Physical Chemistry

Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly-Resonant Sum Frequency Spectroscopy Sanghamitra Sengupta, Leander Bromley III, and Luis Velarde*. Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY 14260-3000 USA

Abstract

Broadband Doubly-Resonant Sum Frequency Generation (DR-SFG) spectroscopy was used to probe selectively the vibrational signatures of a series of chalcogenorhodamine dyes tethered to TiO2 either as monomers or as H-aggregates. This selectivity was achieved by virtue of the vibronic coupling associated with the unique electronic transitions for the different states. It was found that clear spectral differences and polarization dependences were observed contingent on whether the outgoing SFG wavelength was in resonance with either the monomer or H-aggregate electronic excited states. Our results also indicate that orientation and proximity of the xanthylium core to the semiconducting surface play an important role on this vibrational-electronic coupling and may affect the character of the charge-transfer complex. Our DR-SFG results also provide important clues into the anchoring motifs for carboxylic and phosphonic acid groups.

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INTRODUCTION Metal-free conjugated dyes are attractive organic optoelectronic materials and photosensitizers.1-4 Interfacial structure at the organic-inorganic interface plays a vital role in enabling the efficient integration of such dyes into devices. Therefore, electrode-organic and dielectric-organic interfaces are crucial to device performance as the nature of these interfaces exerts a key influence on many optical and electronic properties, such as light harvesting, energy transfer, and carrier injection, blocking and recombination.1, 5-6 Particularly, understanding the interrelation between the nuclear and electronic degrees of freedom at the chromophore-semiconductor interface stands as a profound challenge in the design of organic solar-to-electrical and solar-to-chemical energy conversion systems.7 For instance, electronvibrational couplings in surface-bound chromophores has gained renewed interest as researchers have shown that these couplings can play a critical role in excited state energy and charge transfer.8-10 Furthermore, interfacial organic chromophores typically have strong lateral interactions among adsorbed moieties that lead to spontaneous self-aggregation at the surface. The aggregates may exhibit distinct changes in their absorption profiles compared to the monomeric species. Three different aggregation patterns of the dyes have been proposed: red-shifted J-aggregates, blue-shifted H-aggregates, and both red- and blue-shifted herringbone aggregates.2 Under some conditions, the emerging aggregate bands can advantageously widen the typically narrow light harvesting spectral region of the monomeric organic dyes, such as it has been demonstrated for various cyanine dyes with both H- and J- aggregates.11-15 In some other cases, however, aggregation may results in motional and exchange-narrowing of the absorption spectrum followed by a characteristic sharpening of the absorption band. This somewhat unfortunate narrowing effect is most characteristic for Jaggregates with static disorder and/or fast fluctuations.16-19 However, the influence of aggregation on the overall conversion efficiencies is currently challenging to predict. Aggregation can increase the dye loading and, hence, the amount of absorbed light. It may also inhibit parasitic recombination reactions between the injected electrons and the oxidized species in the electrolyte solution by acting as an effective blocking layer.20-21 Several reports have postulated the controlled and selective aggregation of structure-tailored dyes as an attractive approach for achieving high electron injection


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yields into the TiO2 conduction band.11,

13-14, 22-25

However, dye aggregation is also considered an

undesirable phenomenon in organic solar cells and a detrimental process to the cell efficiency by virtue of intermolecular excited state quenching.2,

26-28

This discrepancy is clearly suggestive of a

delicate correlation between interfacial structure and function. Hence, a detailed molecular-level characterization of the dynamic interrelationship between the electronic and structural properties of aggregates at the semiconductor interface is of great practical interest for tailoring molecular design to function-specific strategies. In spite of numerous theoretical and experimental efforts, many aspects of the interfacial morphology and charge transfer mechanism remain elusive for interfacial aggregates. The atomistic description of organic dye aggregation in connection to injection yields is vastly incomplete. A significant barrier underlying the rational design of efficient and reliable aggregated photosensitizing films lies precisely in our limited knowledge of the structural and electronic properties of such interfaces at a molecular/atomistic level. It is therefore a pressing challenge to find ways to measure directly the influence of aggregation on the interfacial molecular structure and coupling energetics in order to assist the systematic design of photosensitizers with controlled surface aggregation. Vibrational sum frequency generation (VSFG) spectroscopy is a second-order coherent optical probe capable of providing a wealth of molecular-level information with high surface selectivity.29-32 Shultz and co-workers, employed VSFG spectroscopy to identify methanol and methoxy adsorbates on nanoparticulate anatase TiO2 thin films.33 This work was expanded recently by Ren and coworkers.34 Ye at al. studied the orientation and adsorbed geometry of a Zn-porphyrin dye on nanocrystalline titania.35 These previous studies demonstrate that SFG can be effectively used to identify species on the surface of nonporous titania films and determine their orientation. In this work, we therefore use infrared-visible VSFG spectroscopy to elucidate the interfacial properties of a series of selenium-containing chalcogenorhodamine dyes tethered to nanocrystalline titania via carboxylate and phosphonate groups. These particular systems have shown promising results for applications in dye sensitized solar cells (DSSCs) and photocatalytic production of solar hydrogen.1314, 24-25

In such studies, H-aggregation on the titania surface was correlated with a substantial

enhancement in the electron injection rates and quantum yields. The geometrical configuration of the


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surface aggregates and anchoring motifs on the semiconductor surface were previously suggested as the underlying source for the increased performance and stability, raising important questions regarding the relation of interfacial electronic couplings with conformational and tethering changes induced during the aggregation process. This contribution addresses several fundamental aspects of these queries by selectively probing coexisting monomeric and aggregated molecular domains on a series of thin films by making use of doubly-resonant sum frequency generation (DR-SFG) spectroscopy, a two-dimensional variation of VSFG spectroscopy capable of simultaneously interrogating the electronic and vibrational transitions of surface species.36-37 VSFG is conventionally performed by means of an off-resonance visible pulse of fixed frequency (ωVIS) and a tunable or broadband IR beam (ωIR) overlapped spatially and temporally at the sample to generate an optical signal at the sum frequency (ωSFG = ωIR + ωVIS). When ωIR matches a vibrational resonance of the surface, the output SFG intensity exhibits resonant enhancement yielding a surface-specific vibrational spectrum. Raschke et al. demonstrated that when ωVIS is tuned such that the resulting sum frequency ωSFG is also in resonance with an interfacial electronic transition (see Fig. 1), the SFG signal is further enhanced for vibrational modes which are coupled to the excited electronic state, much in a way as resonant Raman scattering.37 With the capability of tuning both the incident ωIR and ωVIS frequencies, a two-dimensional spectrum can be obtained, making it a powerful, yet under-utilized, surface probe for studying interfacial excited electronic states which are coupled to specific vibrational modes.38 While DR-SFG is often thought of as a tool for further increasing the sensitivity of VSFG measurements, the substantial signal enhancement obtained under doubly-resonant conditions offers an additional species selectivity to SFG spectroscopy by discriminating the doubly-resonant modes against the much weaker contributions which are not coupled to the electronic excitation (i.e. not doubly-enhanced).39-41 This makes DR-SFG a particularly attractive tool for investigating mixed interfaces comprising adsorbates with distinct electronic absorption bands. Based on prevalent exciton models,42-43 stacked molecular aggregates of a dye have characteristic absorption bands with respect to that of the monomer (typically blue-shifted for H-type aggregation and red-shifted for J-type aggregates42,

44

). We

therefore carried out DR-SFG measurements designed to probe selectively the electronic and vibrational contributions from co-existing H-aggregated and monomeric surface states of the


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Figure 1. (Left) Schematic representation of a VSFG measurement on a donor-π-bridge-acceptor system. The anchored dye-nanocrystalline TiO2 complex is irradiated by co-propagating infrared (IR) and visible (VIS) beams overlapped spatially and temporally and the generated SFG signal goes to our detection system. (Right) Representation of a typical doubly resonant infrared-visible sum frequency generation process between electronic ground (S0) and excited state (S1).

chalcogenorhodamine dyes on titania. We demonstrate that while their absorption bands, as measured by UV-VIS spectroscopy, may have substantial overlap, clear changes are observed in the SFG vibrational bands as ωVIS is tuned over the light-harvesting region of the film. These observations provide unique spectroscopic insights into the structural and vibronic properties of interfacial dye aggregation and corresponding charge transfer complexes.

EXPERIMENTAL Sample preparation. Three classes of rhodamine derivatives were studied (Figure 2) in order to study the effects of orientation, aggregation state, and anchoring properties of the dyes on the titania surface. The basic rhodamine structure was modified by the Detty group to produce 3,6bis(dimethylamino)-9-(2-thienyl-5-carboxy)

selenoxanthylium

bis(dimethylamino)-9-(3-thienyl-2-carboxy)selenoxanthylium bis(dimethylamino)-9-(2-thienyl-5-phosphono)

dye dye

selenoxanthylium

hexafluorophosphate salts, according to literature procedures.13-14,

(dye

1),

(dye

2),

dye

(dye

24-25

and

3,6-

and

3,6-

3),

all

Optimized molecular

geometries at the DFT level using the B3LYP hybrid functional revealed that the smaller 9-(2thienyl) substituent has roughly a 5 kcal mol-1 barrier to coplanarity in the 3,6bis(dimethylamino)chalcogenoxanthylium system.45 X-ray crystallographic studies have shown that


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Figure 2. Molecular Structures of dye 1, dye 2 and dye 3 (from left to right) respectively.

the 2-thienyl group can be indeed coplanar, enabling the dyes to adopt conformations that allowed for H-aggregation.46 Nanocrystalline TiO2 films were deposited onto glass slides, as described previously.13-14,

24, 47

TiO2 films were 4.1 ± 0.9 µm thick and consisted of anatase TiO2 particles with average diameters of 36 ± 6 nm.47 TiO2 films were functionalized by immersion in concentrated (0.3−2 mM) solutions of the dyes in CH2Cl2 (for carboxylic acid-functionalized dyes 1 and 2) or CH3OH (for phosphonic acid, dye 3), rinsed with the solvent and dried with Ar gas. Extinction coefficients for the dyes in acidified acetonitrile were used to calculate surface coverages. The surface coverages (dyes per projected surface area), were on the order of 10-7 mol cm-2, a typical value in DSSC’s around saturation coverage.24 UV/vis absorption spectra were obtained with an Agilent 8453 diode array spectrophotometer. DR-SFG measurements. The DR-SFG experiments as implemented here, are based on our previously described broadband SFG system.48-49 In brief, a regeneratively amplified Ti:Sapphire laser (Legend Elite HE+, Coherent Inc.) produces 6 mJ/pulse at 800 nm with a typical pulse duration of ca. 32 fs and 1 kHz repetition rate is split threefold. Of the amplifier output, 3 mJ are used to pump an optical parametric amplifier (OPA) (TOPAS Prime, Light Conversion) followed by a noncollinear difference frequency generator (NDFG) unit to generate broadband (~400 cm-1 FWHM) tuneable mid-IR pulses in an AgGaS2 crystal. A beam with c.a. 1.7 mJ first passes through a second harmonic bandwidth compressor (SHBC, Light Conversion) where narrowband (5 ps, c.a. 7 cm-1) 400 nm pulses are generated and used to pump a picosecond OPA (TOPAS-400-WL, Light Conversion) to generate continuously tuneable narrowband upconverting pulses in the visible range. The third beam with ~0.83 mJ is reserved for a separate experiment. The visible and IR beams get overlapped temporally and spatially at the sample surface with incident angles of 45° and 70°,


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respectively. The IR beam is focused by a 10 cm focal length BaF2 plano-convex lens (ISP Optics) to a spot size of c.a. 150-200 µm. The visible beam is mildly focused with a 40 cm BK7 plano-convex lens to a spot size of ca. 500 µm. The visible and IR powers on sample for this experiment are held fixed at 0.15 mW and 4 mW, respectively. The VIS energy was kept as low as possible to minimize photodegradation. Due to the double-resonance enhancement, the obtained signal-to-noise ratios at this unusually low VIS power were outstanding. No damage to the sample was observed at this energy levels (see Figure S1). Both the visible and the IR beam pass through polarization selective optics before reaching the sample. The resultant SFG beam is collimated by a visible achromatic doublet lens with 150 mm focal length (Newport Corp.) and angle tuned short-pass filters are used to remove unwanted incoming light before the detection optics. A polarizer on the SFG signal path controls the input polarization of SFG signal to the spectrograph and an achromatic half-waveplate is used immediately prior to the polarizer in order to select the desired polarization. The SFG signal is focused at the spectrograph (Acton, 500 mm focal length, 1200 g/mm grating blazed at 500 nm) entrance slit by an f-number matched achromatic aspherical lens (Newport). We take advantage of multiplex signal detection50 with a liquid nitrogen cooled CCD camera (PYL100BRX, Princeton Instruments) which is kept at 120 °C throughout the experiment. Acquisitions had integration times of 1 to 5 minutes. Each spectrum is background subtracted and normalized by acquisition time and with respect to the broad SFG signal of a bare gold film.51 The 1601.35 cm-1 line of a polystyrene film was used for calibration. Both the normalization and calibration procedures act as our “internal standard” for varying ωVIS as illustrated in Figure 3. The SFG signal of a bare gold film is relatively flat as a function of ωSFG in the 540-650 nm range and starts to increase gently below 530 nm, peaking at 475 nm, which corresponds to the s–d transitions on the Au surface.52 In order to avoid skewing of the normalized SFG intensities by the wavelength-dependent response of gold standard, we limit our analysis to signals with ωSFG in the 540-635 nm range. The mid-IR path and the sample are continuously purged with dry air to alleviate atmospheric absorption of the mid-IR beam. The acquisition time while mapping the visible beam for all the dyes was 3 min. per selected wavelength. The temporal overlap for each of the scan wavelengths had to be slightly re-optimized using the Au film while mapping the visible, this perhaps due to small changes in the optical path length for each


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Figure 3. (a) IR energy profile obtained from the SFG response of a gold film under ppp polarization without (black lines) and with a polystyrene film in the IR optical path (shaded contours) obtained with (a) 550 nm and (c) 660 nm visible excitation wavelengths. The (*) indicates the polystyrene film absorption line used for calibrating the SFG spectra according to ωSFG = ωIR + ωVIS. This procedure was carried out at each VIS wavelength as shown in (b) and (d) corresponding to the raw spectra in (a) and (c), respectively.

visible wavelength. The spatial displacement of the VIS beam was negligible while scanning through the wavelength range used in this work. The VIS beam size was purposely larger than the IR spot size to mitigate potential problems regarding beam displacement while tuning the wavelength.

RESULTS Selective probing of coexisting monomeric and aggregated states. The absorbance spectra of nanocrystalline titania films functionalized with the dyes in Figure 2, shows a band ca. 606 nm attributed to monomeric species, as well as the appearance of a second band at higher energies (see for example Figure 4-1c). There is considerable evidence that dye 1 undergoes substantial H-type aggregation on the TiO2 surface due to face-to-face π-stacking interactions.13-14, 24-25 These aggregates are implicated in the appearance of the high energy band at ca. 560 nm. Thus, monolayers of dye 1 consisted of mixtures of H-aggregated and non-aggregated dyes. Such mixtures exhibited broadened


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absorption spectra and increased light-harvesting efficiencies relative to pure monolayers of Haggregated or non-aggregated dyes. Dye 2 was designed to be electronically similar to dye 1. However, due to the different position of the anchoring carboxylate group, the xanthylium core is expected to be constrained to the plane of the semiconductor and orthogonal to the pendant 9-(3thienyl-2-carboxy) substituent upon chemisorption. Consequently, dye 2 would be sterically hindered from face-to-face (or head-to-tail) orientations and should bind to the titania surface in essentially amorphous monolayers. As seen Figure 4-2c, the blue-shifted band attributed to the presence of aggregates is indeed significantly diminished.13-14, 24-25 Raschke at al. showed that for Rhodamine 6G on silica, a double-resonance enhancement was observed for the xanthene skeleton vibrational modes when ωSFG matched the S0–S1 electronic transition. The strong coupling of these vibrational modes with the electronic transition is an indication that the transition is dominated by the π–π* excitation of the large xanthene group of the molecule.37 In contrast, the coexistence of H-aggregated and non-aggregated dye 1 species on TiO2 surfaces enable us to potentially probe selectively into the lower energy band of non-aggregated species or the higher-energy band of H-aggregated dyes during the doubly-resonant SFG experiments. We will now examine how the vibrational SFG signals respond to up-conversion in these two regions. Figure 4 shows the broadband DR-SFG spectra for films of dye 1 and dye 2 in the ssp and ppp polarization combinations (with the indexing corresponding to the polarization of the SFG, VIS, and IR fields, respectively) using two different narrowband VIS wavelengths. The center frequency of the broad-bandwidth IR pulse was fixed at ca. 1500 cm-1 and spanned the 1300 to 1800 cm-1 region. This coverage is adequate for most of the chalcogenoxanthylinium skeletal motions. The two visible wavelengths, experimentally determined as 609 nm (16,420 cm-1) and 701 nm (14,265 cm-1) were chosen such that ωSFG is within the range of the H-aggregated and non-aggregated absorption bands in the UV/Vis spectrum, respectively (Figure 4-1c). For dye 2, the corresponding visible wavelengths were determined as 609 nm (16,420 cm-1) and 664 nm (15,060 cm-1). Clear differences in the SFG vibrational signatures are evident in the data presented in Figure 4. We reproducibly obtained distinctive vibrational spectra when each film was illuminated (on the


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Figure 4. (1a) and (1b) show representative DR-SFG spectra in ssp (red) and ppp (blue) polarization combinations at 609 and 701 nm visible excitation, respectively, for a titania film loaded with a dye 1 monolayer. (2a) and (2b) are ssp (red) and ppp (blue) DR-SFG spectra obtained at 609 and 664 nm visible excitation, respectively, for a titania film loaded with a dye 2 monolayer. 1(c) and 2(c) display the UV/Vis absorption spectra for dye 1 and dye 2 films, respectively. The orange and the yellow bars illustrate the range of SFG wavelengths accessible by the sum of the narrowband upconverting frequency and the broadband pulse. As it can be seen, the SFG wavelength falls (selectively) on each major contribution band in the absorption spectra.

same spot) with the two different upconverting VIS wavelengths. We regard this result as a clear indication that doubly-resonant SFG can selectively probe different species in a mixed monolayer as long as they have different vibronic transitions. The two species here are attributed to the monomeric or aggregated domains of the film, with the selectivity imparted in the electronic excitation step. In addition, a remarkably different spectrum was obtained for dye 2 films, indicating that conformation at the surface plays an important role in the DR-SFG lineshapes. Below, we attempt to describe the most apparent spectral differences seen in Figure 4 based on the following description of the DRSFG process. For any medium in which the constituent particles possess a sufficiently random orientation, the medium can be considered to be isotropic, the SFG signal can therefore only originate from the surface of the medium. The signal intensity is dictated by the following proportionality:32, 53-54

∝

(1)


10

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where is the non-linear susceptibility calculated as:

= ∑ 〈 〉 + ,

(2)

R corresponds to the Euler rotation matrices, the angular brackets represent an average over molecular orientations, Ns is the molecular surface density for the species under study, and βijk is the

microscopic first-hyperpolarizability, and , describes the non-resonant contributions. Assuming that both electronic and vibrational states are near-resonance with the SFG and IR frequencies (i.e. DR-SFG), this hyperpolarizability for the qth vibrational mode may be determined, under the Born−Oppenheimer approximation as:37, 55 '(,)|+, |-,./0-,.+1 (,!2

!

−#SFG ; #VIS , #IR = &∑-;(,. 3

45678 9 3SFG 9 :45678

< ∙ 3

'(,!|+> |(,)/

7?678 93IR 9:7?678

(3)

where µ is the electric-dipole operator. The vibrational transition from ground state |@, 0/ to |@, B/ occurs with a transition frequency ωgq-g0 and Γgq-g0 denotes the damping constant for this transition. Here |C, D/ represents a vibronic molecular state where n labels the electronic excitation and b the

vibrational excitation at frequency #-.9() and corresponding damping constant Γ-.9() . The first

term in brackets is related to the resonance enhanced polarizability and a parameter indicative of electron-vibration coupling. The second term is related to the IR transition dipole moment. Equation (3) can be further simplified to:56 !

−#SFG ; #VIS , #IR = 3 (-

where KL M ((

N

74

FG,1 H

IJ

77

∙+>

7?678 93IR 9:7?678

(4)

is the resonance-enhanced anti-Stokes Raman polarizability of the nth excited state

and O is the vibrational transition dipole integral for the qth mode in the electronic ground state. From this it can be seen that a DR-SFG active mode needs to be IR and resonant-Raman active. The )resonance-enhanced anti-Stokes Raman polarizability KL M

N

can be split into multiple terms

through first-order vibronic coupling theory as:57-59 L = P + Q


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where the polarizability is expressed in terms of the A and B terms described in the Albretch theory.57 Full details of the derivation are given elsewhere.55,

57-59

This explicitly shows the

contributions of resonance enhancement and vibronic coupling in DR-SFG. The P term contains only symmetric components of the Raman tensor and the Q term contains a symmetrical component and an antisymmetric part. In order to obtain good agreement between simulations and experimental results, both Franck-Condon (Albretch A–term) and Herzberg-Teller terms (Albretch B-term) are needed.55 Recent work showed that non-Condon effects should be considered for the quantitative analysis of DR-SFG. For instance in Rhodamine 6G (R6G), the two contributions can have comparable intensities in the spectrum depending on the orientation of R6G at the surface. It was shown that when the xanthene ring is perpendicular to the silica surface (binding through one of

the ethyl amine groups) biases Herzberg-Teller modes while disfavouring the generally stronger Franck-Condon modes.55 Based on Equation (4), the vibrational spectra obtained under DR-SFG can be fitted by:40, 60

= ∑-,!

NR,5,? ∙ NSTSUVW,4? 3? 93IR 9:?

+ ,

(5)

with: PXYXZ[,-! =

Q-! #- − #SFG − \Γ-

where PXYXZ[],-! and P^.,! are the oscillator strengths relevant to the nth electronic absorption of the adsorbate and the oscillator strength of the qth SFG active mode of the adsorbate, respectively, and PXYXZ[],-! is approximated as a Lorentzian centered at #- with amplitude Q-! . The form of these equations describe resonant transitions by Lorentzian profiles, thus neglecting inhomogeneous broadening. Even though such a line profile seems too simplistic, it has been a satisfying approximation for most purposes based on our spectral resolution.61-63 With this in mind, we now proceed to discuss the most notable vibrational peaks in the spectra shown in Figure 4 (see the Supporting Information for depictions of these normal mode vibrations). It is important to mention that the visible pulse energy was kept at 0.15 mJ at the sample (150


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nJ/pulse). At these pulse energy levels, the doubly-enhanced peaks (i.e., modes with the largest Franck–Condon displacements in the excited state) dominate the spectra since the electronic resonance effect is known to enhance the VSFG contributions by a factor of 103 to 105.37, 58 Dye 1 has been shown to form monolayers of aggregates where each molecule is bonded to the surface, as opposed to forming “deck-of-cards” aggregates where the aggregated network extends perpendicularly away from the surface or in chiral networks that may generate a “bulk” response. As a test we performed chiral-selective experiments under psp polarization and saw no SFG signal (see Figure S1).54, 64-65 The clean blank titania film at this power levels also shows no SFG response both in the CC stretch region and in the CH stretch region (see Figure S2).

The 1620 cm-1 mode and evidence for lateral coupling. This mode is assigned here as the symmetric ring C=C stretch of the selenoxanthylium moiety. This is based on our density functional theory (DFT) calculations (B3LYP/Def2TVPZ level of theory using the Gaussian09 software package66) as well as reported Raman results on methylene blue (MB), a dye possessing a very similar chemical structure to the bis(dimethylamino)xanthylinium core.67-68 This is also in agreement with the vibrational analysis of R6G where the symmetric CC stretch contribution of the xanthene group was found at 1657 cm-1.37, 55 For dye 1, it is apparent that this peak increases in intensity for the aggregated region when compared to the monomer under ssp polarization. According to Equations 1-4, the changes in intensity can indicate the following: varying strength of the electron-vibration coupling, number density of aggregated or non-aggregated species, and changes in orientation. It has been seen in some porphyrin aggregates, that molecular distortion due to intermolecular interactions upon aggregation can result in changes in the dimensionless displacement (Huang-Rhys factor) for particular vibrational modes. This can alter the magnitude of the Franck-Condon overlaps and consequently change the intensity.69


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A blue shift of 9 cm-1 is noticeable in the aggregated region when compared to the non-aggregated spectra according to our fits (see Figure 5). In addition our curve fitting show that the peak narrows by 3.7 cm-1.70-72 While this narrowing effect is smaller than our spectral resolution (~7 cm-1), it is fully reproducible and it is a motivation for higher resolution DR-SFG studies.73-74 The measurements were done on the same day using the same exact experimental conditions. It is possible that the narrowing effects are more pronounced but we are limited by our spectral resolution. The features described here resemble the observations by Cho, Hess and Bonn for coverage-dependent investigations of CO on Ru(001). It was determined that as a result of increased intermolecular interaction with increasing coverage (due to the closer proximity of CO molecules on the surface), pronounced frequency blue shifts of the vibrational resonances were observed as well as a narrowing of the spectral peak. These experimental results were interpreted within the framework of localized vibrations, where the coupling between molecules, with the CO stretching motions at high coverages, mainly of a dipole-dipole nature.75 One one hand, intermolecular interactions can shift the positions of features in vibrational spectra, and one can still think of the spectra as arising from individual molecules in a different dielectric environment, with possibility of excitation

Figure 5. Close up on the 1620 cm-1 peak (ssp) obtained at 609 nm (top) and 701 nm (bottom) visible excitation for a film of dye-1. The dotted lines show the blue shift in the peak position as described in the text. The solid lines are the results from the fitting procedure described in the text.


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hopping among neighbouring molecules. However, in a vibrational exciton, a vibrational excitation is shared by multiple oscillators, rather than being localized on an individual oscillator. The sharing of the excitation is the result of coupling among the vibrational moieties, typically through a transition dipole–transition dipole mechanism.76 The study of delocalized vibrational excitations (or vibrational excitons) in SFG is relatively unexplored, but it may play an important role in understanding aggregated states. While the blue shift is highly pronounced in our case, it is not as large as in the case of CO;75, 77 this is likely because the CO possesses a larger transition dipole moment. Another interesting aspect that one can obtain from Figure 4 is that the ratio of the ssp vs ppp intensity is different in the monomeric vs aggregated states. For instance, the calculated Issp/Ippp values for the 1620 cm-1 band in monomeric region (visible excitation at 701 nm) is 1.56±0.03 and for the aggregated region (visible excitation in at 609 nm) is 1.70±0.04. The Issp/Ippp ratio is usually correlated to the tilt angle of the transition dipole and the surface normal.53 We can argue that the transition dipoles for the monomer and aggregated species are oriented differently. Note that the electronic transition moment for the monomer is perpendicular to the molecular long axis of the selenoxanthylium core (i.e. pointing from the thienyl ring to the selenium atom), and is parallel to the vibrational transition dipole of the symmetric C-C stretch of the core. The detailed orientation calculations under doubly-resonant conditions are outside the scope of the work presented here as coupling also depends upon changes to elements of the resonance Raman tensor, particularly in the Albrecht B-term contributions affecting the intensity ratio between the ssp and ppp contributions. In this light, we only can predict that aggregation causes reorientation of the transition dipole with respect to the surface. The polarization technique, under electronically off-resonance conditions, has been used by several groups to obtain the orientation and anchoring structure of several dyes on TiO2. Lian and co-workers78-79 and Calabrese et al.80 have used SFG spectroscopy to determine the adsorption geometry of a model rhenium tricarbonyl catalytic complex on single-crystal TiO2 (rutile) surface and indicate bidentate or tridentate binding linkage of the carboxylate groups to the TiO2 surface with an upright molecular orientation that leaves the rhenium atom exposed for maximum reductive capacity. Ye and co-workers determined that Zn-porphyrin attached to the TiO2 surface by a


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carboxylic group had an appreciable tilt angle, and this angle was dependent on the specifics of the porphyrin as well as properties of the sensitization solvent and duration of the sensitization.35 Rich et al. recently determined that the N3 dye on TiO2 adsorbs to the surface through three carboxylic acid groups with both isothiocyanate ligands facing away from the surface.81 Contrary to dye 1, our DR-SFG experiments show that the 1620 cm-1 peak is not the one with the strongest response for films of dye 2. A substantial enhancement was obtained for a mode appearing at c.a. 1398 cm-1. Such drastic intensity difference for this peak in films of dye 2 compared to the film with dye 1, was not observed in the ATR-FTIR spectrum (see Figure S4) and it is therefore unique to the DR-SFG measurements. We will now discuss this intriguing observation. The 1398 cm-1 mode and differences in the charge-transfer complex. This mode may appear to originate from the carboxylate anchoring group as an IR band in this proximity range has been reported by others.82 However, this assignment was not supported by the persistence of this mode when the carboxy group was replaced by the phosphono group (dye 3) as it will be discussed later. Several reports describe the detection of a 1390 cm-1 Raman band in methylene blue films and attributed it primarily to the stretching of the C―N bonds directly linked with the methyl groups of the dye.67-68 Our B3LYP calculations also support that there is a mode in this frequency range that shows large symmetric displacement of the C―N bonds (with some coupling to the C―H bending

Figure 6. The SFG spectra (ssp) for dye-1 (green) and dye-2 (black) films at 609 nm visible excitation normalized to the corresponding 1620 cm-1 peak intensity.


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modes), a more likely assignment for this mode. This is also supported by the assignment of the 1377 cm-1 band in 4-dimethylaminobenzonitrile (DMABN) to the phenyl ring-N stretch83 and a 1388 cm-1 band in thionine-coated gold electrodes assigned as well to the CN symmetric stretch.84 The 1535 cm-1 mode. The precise origin of this band is not entirely clear at present. This band can, however, be tentatively assigned by the results of our DFT frequency calculations and by the report by Roy et al. on the surface enhanced resonance Raman spectroscopy (SERRS) of methylene blue adsorbed on gold colloidal nanoparticles.68 In essence, this mode can be assigned to the ν(CN) stretching coupled to a CH3 scissoring vibration associated with the externally attached methyl groups of the molecule. This band is considerably weaker for dye 1. As the nitrogen atoms are directly linked with the methyl groups of the molecule, the large enhancement of this band for dye 2, compared to dye 1, suggests the involvement of either one or both of these nitrogen atoms in the adsorption process. The involvement of these nitrogen atoms is further supported by the enhancement of the other band centred at ~1390 cm-1 for dye 2. The band is as described above, considered to represent the ν(CN) stretching vibrations. Another possible assignment for this band, based on the DFT calculations, is the asymmetric stretch of the thienyl ring which is predicted to appear in this range. However, in the ssp polarization combination, only modes that have transition dipole moments with a component perpendicular to the surface appear in the observed SFG spectra, as only a single component of the nonlinear

susceptibility __` is probed in this combination (where the subscript refers to the orientational alignment of the sum frequency signal, visible pump, and IR pump beams, respectively). As the IR component resides solely in the z axis, only transition dipoles with a component in this orientation will be probed. This is typically true for symmetric vibrations. On the other hand, spectra in the ppp

combination contain contributions from several susceptibility components, specifically ``` , aa` ,

a`a , and `aa . IR-active vibrations in both the z and the x axes are therefore capable of giving rise to an SFG signal in the ppp beam polarization, i.e., this polarization probes vibrational resonances with transition dipole moments oriented both parallel and perpendicular to the surface. Often times the ppp polarization combination is more sensitive to asymmetric vibrations.32 As seen in Figure 4, the


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1535 cm-1 mode is most intense in the ssp polarization, an indication that this is likely a symmetric vibration. The low frequency (

Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline

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