Illuminating Excitonic Structure in Ion-Dependent Porphyrin

Article pubs.acs.org/JPCC

Illuminating Excitonic Structure in Ion-Dependent Porphyrin Aggregates with Solution Phase and Single-Particle Resonance Raman Spectroscopy Christopher W. Leishman and Jeanne L. McHale* Department of Chemistry, Washington State University, Box 644630, Pullman, Washington 99164, United States S Supporting Information *

ABSTRACT: Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) prepared in aqueous LiCl, NaCl, KCl, CsCl, and HCl were studied with solution phase and single-particle resonance Raman (RR) spectroscopy. Solution phase excitation profiles are sharply peaked at 488.0 nm excitation for samples in HCl, LiCl, and CsCl, which show narrow J-bands in the corresponding absorption spectra, but more gently peaked for those induced by NaCl or KCl, which show broader absorption J-bands. The former three samples also exhibit larger low to high frequency mode intensity ratios with excitation near the J-band peak and smaller depolarization ratios compared to the latter two samples. Polarized spectra of individual aggregates correlate with the solution phase results, exhibiting an increase in intensities involving either incident or scattered light polarized transversely to the aggregate long axes in conditions shown previously to induce bundling of nanotubes or greater disorder. These results, along with previous absorption, resonance light scattering (RLS), and imaging data, suggest that excitonic coupling across nanotubular components in bundled aggregates leads to spectral broadening. This is attributed to increased spectral density of allowed excitonic transitions, particularly those polarized transversely to the aggregate length. Disorder leads to deviation of excitonic transition polarizations from pure axial and transverse directions, resulting in greater transverse relative to axial polarization but smaller excitonic coherence as measured by RLS intensity. These results suggest that environment-induced morphological variations can affect the energies, polarizations, and spatial structure of excitons in dye aggregates.

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INTRODUCTION Tetrasulfonatophenylporphyrin (TSPP) aggregates1−4 have yielded valuable insights into systems that exploit excitonic coherence for light harvesting. In the structurally analogous nanotubular bacteriochlorophyll complexes of green sulfur bacteria, environment-induced heterogeneity leads to a broadened absorption spectrum and creates an energy gradient, enabling highly efficient energy conversion.5−7 There are tradeoffs between increased energy transfer and energy loss rates with coherent delocalization in photosynthetic light harvesting, such that maximum coherence does not necessarily yield optimal efficiency.8 In artificial excitonic light harvesting systems, tuning the size and location of ordered and disordered regions for exciton generation and diffusion and charge separation is crucial to maximizing device performance.9 Attempts to use aggregates in dye-sensitized solar cells (DSSCs) have yet to take full advantage of excitonic coherence to obtain performance better than or even comparable to that of monomeric sensitizers. 10,11 To design devices that advantageously exploit the characteristics of natural light harvesting, we need to understand how morphology and disorder affect excitonic energy transport. We previously showed spectroscopic and imaging evidence from salt-induced © XXXX American Chemical Society

TSPP aggregates that coupling between subunits of nanotube bundles and associated structural disorder influences excitonic structure.12 Here we provide more detailed evidence for that hypothesis by examining these same samples with resonance Raman spectroscopy. TSPP aggregates have been most extensively studied in aqueous HCl (≥0.1 M), which serves as a reference sample for our studies. Below pH ≈ 3 two of the peripheral sulfonate groups of the TSPP diacid dianion monomer (Figure 1a) gain protons, inducing self-assembly via ion pairing of the remaining anionic sulfonates with cationic porphyrin cores. The doubly degenerate monomer Soret band splits into a sharp, red-shifted J-band and a broader, blue-shifted H-band.2−4,13 Strong resonance light scattering (RLS) intensity, indicative of coherent excitonic delocalization, is observed in the J-band region.14−19 AFM, STM, TEM, and X-ray scattering consistently indicate a nanotubular morphology, with variable length (a few hundred nm to several μm), uniform diameter (∼15−20 nm), and wall width (∼1.5−2 nm).20−29 Linear dichroism Received: January 26, 2016 Revised: April 6, 2016

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DOI: 10.1021/acs.jpcc.6b00867 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

The low frequency 245 and 316 cm−1 modes show strong RR enhancement in TSPP aggregates compared to the monomer, sharply dependent on close resonance with the J-band peak.22−24,38−40 Aggregation in DCl/D2O solution led to a lower ratio of these low frequency mode RR intensities to those of high frequency modes, indicating smaller displacement of low frequency modes in the excited state than in HCl/H2O.22 A slightly broader absorption J-band and greater RLS intensity indicated greater excitonic coupling of J-band components in deuterated conditions. These results highlighted how composite band structure confounds the 1/√NC dependence of J band width on coherence number (NC) expected of a noncomposite band41,42 and pointed to an integral structural role for water. Decreased ratios of RR intensities of low to high frequency modes for individual aggregates compared to the solution phase, and further decreases with laser heating, indicated coherence reduction by drying of deposited aggregates.40 This was attributed to decoupling of hierarchical subunitswhich themselves remained intactas residual water was driven off. RR intensity with incident and scattered light both polarized in the aggregate axial direction (designated hereafter as VV) was much greater than with both polarizations perpendicular to the axis (HH), suggesting that the axially polarized excitonic transitions are stronger, consistent with helical nanotube models. The component spectra with differing incident and scattered radiation polarizations (VH and HV) were similar for low frequency modes but significantly different for high frequency modes, suggesting vibronic coupling effects (explored further in this article). Taking our investigation of aqueous environment effects a step further, we recently presented absorption and RLS spectra alongside TEM and AFM images of aggregates formed in a series of aqueous alkali chloride solutions.12 Salts induce aggregation by attenuating the repulsive interactions between anionic groups of TSPP diacid dianion monomers (Figure 1a).3,13,20,38,43−47 We observed that nanotube bundling correlates with a broader J-band in the absorption spectrum. Coherence as measured by RLS was smaller compared to HClinduced aggregates in one of these cases (KCl-induced), which showed greater visual evidence of disorder but was larger in another (NaCl-induced), with a more orderly appearance, suggesting that activation of more excitonic states via coupling between neighboring nanotubes, not merely inhomogeneity, contributes to broadened absorption bands. Moreover, a slightly narrower absorption band and slightly smaller RLS intensity was observed for CsCl-induced aggregates compared to HCl-induced aggregates, and vice versa for LiCl-induced aggregates. The RLS data thus indicated that the absorption line width of the J-band does not follow a simple 1/√NC trend. In light of the imaging experiments, the J-band excitonic structure therefore appears to depend on the aggregationinducing salt through morphological variations. In this article, we present RR data for this series of saltinduced TSPP aggregates in solution and as deposited single aggregates, further exploring the connections between counterion species, morphology, and excitonic structure. As with HClinduced aggregates, relative excitation profiles and depolarization ratio dispersion in the solution phase reveal multiple transitions of different polarizations comprising the J-band, not resolved in the absorption spectra. These results corroborate the hypothesis12 that orderly bundling activates excitonic transitions polarized perpendicularly to the aggregate long axis. Polarized single aggregate RR data further indicate how

Figure 1. (a) TSPP diacid dianion monomer. (b) A nanotube composed of 26 helical chains (“26-start”), with detail of local lattice structure. The red and blue arrows represent the transition dipole moments parallel and perpendicular to the nanotube surface, respectively. (c) Extinction spectra of 50 μM TSPP diacid dianion monomer (dashed line) and aggregate in 0.75 M aqueous HCl and alkali chloride solutions. Inset: J-band detail, indicating laser excitation wavelengths used for resonance Raman (RR) experiments.

(LD) of oriented aggregates indicates that the J- band is primarily polarized parallel to the aggregate long axis and the H-band perpendicular.3,27,30 Chirality is evident from circular dichroism (CD), suggestive of helical architecture.3,31,32 In previous work, we proposed a hierarchical helical nanotube model of HCl-induced TSPP aggregates with cyclic subunits, based on STM images.22−24 This structure results in a J-band composed of two major closely spaced excitonic transitions, one polarized in the tube axis direction and the other perpendicular to it, consistent with resonance Raman (RR) depolarization ratio dispersion. Other studies have supported a packing geometry of multiple helical sulfonate-tocore chains, as depicted in Figure 1b.21−23,28 Multihelical architecture has also been deduced for cyanine dye aggregates and correlated with similar spectral properties.34−37 B


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10 mW. The signal was collected in confocal backscattering geometry. Vertically and horizontally polarized components of the scattered light were selected with a polarizer, followed by a depolarizer (Thorlabs DPU-25) to prevent monochromator grating bias, and a holographic notch filter for each laser line. The wavenumber scale was calibrated to known Raman spectral peaks of cyclohexane (384.1, 426.3, 801.3, 1028.3, 1157.6, 1266.4, and 1444.4 cm−1). A polarized and a depolarized spectrum were acquired for each sample at each excitation wavelength. A spline fit was subtracted to remove the fluorescence and other background components from each of these spectra. Each of the selected peaks was integrated over the same range in each pair of polarized/depolarized spectra, and the sum of these was divided by the sum of integrated polarized and depolarized intensities of the Raman line at ∼1537 cm−1 in the same spectrum to derive relative total intensities. The depolarization ratio of each mode was found by dividing the depolarized by the polarized integrated intensity. All data processing and calculations were performed in Origin Pro 8.6. Single Aggregate Resonance Raman Spectroscopy. A 20 μL aliquot of each 5 μM TSPP aggregate sample was deposited on a borosilicate glass coverslip, allowed to stand for 5 min, and then spun dry for 1 min. This method prevented the formation of a salt coating which would otherwise obscure the aggregates. Each deposited aggregate sample was placed on the stage of an Olympus inverted confocal microscope with a 100× oil-immersion objective. The vertically polarized 488.0 nm output of the argon ion laser was adjusted in power with a variable neutral density filter to ∼1 μW and focused onto the sample. Individual aggregates were selected for variety in size and other attributes. A “coffee ring” region formed by solution drying prior to spinning the sample was avoided so that the aggregates studied would be minimally altered by the drying process. Each selected specimen was aligned so that its longer dimension coincided with the polarization direction of the laser. The polarization was switched between this direction (defined as “V” for vertical) and the perpendicular direction (defined as “H” for horizontal) with a half-wave plate up-beam of the entry port. The signal from the microscope side port was collected as in the solution phase measurements. After aligning the laser on each specimen, the power was increased to 10 μW and data were collected for 10 s, closing the shutter between each acquisition. For each specimen, a spectrum excited with each polarization and scattered with each polarization was collected, giving the four-component spectra VV, VH, HV, and HH. The first letter represents the polarization of the incident light and the second that of the scattered light. The power with V and with H incident polarization was measured at the microscope turret top and output port (in the latter case with a mirror placed at the turret top), and a correction value for each component spectrum was computed. This was required to account for the different transmission and reflection efficiencies of the half-wave plate and the beamsplitters in the microscope. Spectra of cyclohexane were collected in the same configuration on the microscope, and depolarization ratios of 0.75 for the nontotally symmetric modes were calculated to within a few percent error, validating the correction factors. The data were adjusted according to the correction factors, and ratios of integrated intensities were calculated in a similar fashion as for solution phase data after subtracting the fluorescence background. Relative intensities of polarization components versus the VV polarized component were taken

structural hierarchy and internal disorder influence excitonic properties. In particular, the “diagonal” intensities (VV and HH) directly probe the relative strengths of differently polarized transition dipole moments in the aggregate coordinate frame: greater relative HH intensity in wider aggregate specimens is consistent with activation of the transversely polarized transitions. The “off-diagonal” intensities probe the degree to which transition dipole moment polarizations are skewed away from perfect alignment with the aggregate coordinate axes and from exact right angles to each other, thus giving a measure of effects of disorder. The larger dif ference of VH and HV intensities in wider specimens, and particularly in those samples that showed greater structural disorder in AFM and TEM images, indicates greater vibronic coupling between excitonic states of differing polarization. In our recent publication,12 we discussed the effects of counterions and solvation water on aggregate morphology and optical spectra in terms of the Hofmeister series and related concepts.48−55 In the present work, we use RR to illuminate further details of excitonic structure that arise from these influences of the solution environment. As will be shown, the present RR observations and our previous absorption and RLS data arise from the same morphological features and thus the same influences of the aggregation-inducing salts.

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EXPERIMENTAL METHODS Sample Preparation. Stock solutions were prepared in 18 MΩ Millipore water. 100 μM aqueous solutions were prepared from the dihydrochloride salt of 5,10,15,20-tetrakis(4sulfonatophenyl)porphyrin (Frontier Scientific Inc., Logan, UT). 1.5 M aqueous HCl was prepared from concentrated HCl solution. 1.5 M aqueous LiCl, NaCl, KCl, and CsCl solutions were prepared using oven-dried salts of ≥99% purity from various suppliers. Stock solutions other than 1.5 M HCl were adjusted to pH ≈ 3 with 1 M HCl. For absorption and solution phase RR spectroscopy, 50 μM aqueous TSPP aggregate solutions in the presence of 0.75 M HCl or salt were made by mixing the appropriate 1.5 M salt or acid and 100 μM diacid monomer solutions in equal portions. 50 μM TSPP diacid monomer solution was also prepared by dilution for absorption measurements. For single aggregate RR spectroscopy, a portion of TSPP diacid stock solution was first diluted to 10 μM, which was mixed 1:1 with the appropriate 1.5 M salt or acid solution to give 5 μM TSPP aggregate/0.75 M salt or acid sample solution. Each sample pH was adjusted to about 2.7 (except for samples containing 0.75 M HCl) with 1 M HCl(aq) and left in the dark for 3−5 days for consistent equilibration of the aggregation process. Extinction Spectroscopy. Extinction spectra were acquired on a Shimadzu spectrophotometer over the range 350− 750 nm, using 50 μM solutions of diacid monomer and aggregate in the presence of 0.75 M HCl, LiCl, NaCl, KCl, or CsCl in 1 mm path length quartz cuvettes. Solution Phase Resonance Raman Spectroscopy. Each sample was placed in a 1 cm path length quartz cuvette. Solutions were continuously magnetically stirred during signal optimization and data acquisition. A Lexel argon ion (Ar+) laser with vertically polarized output was used to excite each sample at 476.5, 488.0, 496.5, and 514.5 nm. A 90%:10% transmission:reflection beamsplitter cube was employed along with a 10× microscope objective to direct and focus the beam onto the sample. Incident power at the sample was approximately 8− C


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The Journal of Physical Chemistry C rather than depolarization ratios, and sums of all four component intensities were used to calculate total relative intensities vs the 1537 cm−1 mode. White light images were acquired in transmission using a thermoelectrically cooled Andor Clara CCD camera with the shutter to the laser port closed. Images of each aggregate with the laser spot were also acquired after collection of spectra. Because of a mismatch in the ideal focus for imaging and spectroscopy, these images are not displayed here, but the pixel coordinates of the laser spot were used to locate the sites of excitation indicated in the corresponding images shown here. UHV-LT STM. Scanning tunneling microscopy experiments were performed at the Environmental Molecular Sciences Laboratory of Pacific Northwest National Laboratory, Richland, WA. Epitaxially grown Au(111) on mica substrates were prepared by methods described elsewhere,56 H2 flameannealed, and stored in a vacuum desiccator prior to use. A 10 μL aliquot of 5 μM TSPP in 0.75 M LiCl was deposited on the gold substrate and placed directly in the load lock chamber of the Omicron UHV-LT STM. Roughing and turbo pumps were immediately engaged, and all visible liquid was removed in less than 15 s. Images were acquired under UHV conditions (∼3 × 10−11 Torr) at 80 K using an electrochemically etched tungsten tip in constant current mode at 2.0 V and 10 pA. Flattening, noise reduction, and height profiling were performed in SPIP 6.3.6.

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RESULTS Solution Phase Resonance Raman Spectra, Excitation Profiles, and Depolarization Ratios. Total (polarized plus depolarized) RR spectra excited at 488.0 nm, scaled relative to the maximum of the 1537 cm−1 mode (Figure 2), clearly show

Figure 3. RR data for 245 and 1230 cm−1 modes from 50 μM TSPP in 0.75 M aqueous HCl, NaCl, CsCl, KCl, and LiCl. Top two figures: integrated relative intensities vs the ∼1537 cm−1 mode Bottom two figures: depolarization ratios. Dashed lines indicate ρ values for resonance with a noncomposite electronic transition, as discussed in the text.

Figure 2. Total RR spectra excited at 488.0 nm, scaled to the 1537 cm−1 intensity maximum, of 50 μM TSPP in aqueous 0.75 M HCl, LiCl, NaCl, KCl, and CsCl. The left side shows the two prominent low frequency modes; the right side shows the high frequency region at 5× magnification in intensity.

J-band were used for excitation, as shown in Figure 1c. We present data for one low frequency (245 cm−1) and one high frequency (1230 cm−1) mode here, with additional data in Figures S1A and S1B. For each sample, excitation profiles of these two modes relative to the 1537 cm−1 mode intensity are schematically shown in the upper two bar graphs of Figure 3, and depolarization ratios (ρ) of these modes are shown in the lower two bar graphs. RR enhancement of a totally symmetric mode via one singly or doubly degenerate excited electronic state would have led to depolarization ratios of 1/3 or 1/8, respectively, independent of excitation frequency. These are indicated on the depolarization ratio figures for reference as well as the value of 3/4 expected for a nontotally symmetric mode. In all samples, the dispersion (frequency dependence) of

the most prominent similarities and differences among the samples. The relative intensities of the two main low frequency modes (245 and 316 cm−1) are highest for HCl-induced aggregates, followed closely by LiCl- and CsCl-induced aggregates. In contrast, KCl- and especially NaCl-induced aggregates have somewhat lower relative intensities in these modes. On the other hand, differences among samples in relative intensities for the high frequency modes are very small. The low frequency modes thus exhibit greater sensitivity to differences in sample conditions. Detailed relative intensity and depolarization data for each sample are displayed in Figure 3. Four laser lines spanning the D


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The Journal of Physical Chemistry C ρ is strong evidence for enhancement via simultaneous resonance with multiple states of differing polarization, consistent with the helical nanotube model. Intensity profiles and depolarization ratios for several additional modes can be found in Figures S1A and S1B. For the most part, the 316 cm−1 mode behaves similarly to the 245 cm−1 mode, while the higher frequency modes behave similarly to the 1230 cm−1 mode. The comments on each sample that follow are headed with the salt (or HCl) present in the solution. HCl. The peak intensity of the 245 cm−1 mode at 488.0 nm excitation (near the J-band absorption maximum) is greater relative to high frequency modes than in most of the saltinduced samples. The relative intensity of the 1230 cm−1 mode, on the other hand, increases toward the blue, with the largest values occurring for 476.5 nm excitation. The peak depolarization ratio values occur with 488.0 nm excitation, but the 1230 cm−1 mode shows a notable increase in ρ going from 496.5 to 514.5 nm excitation. Generally higher ρ values indicate more interference of different excitonic states than in either LiCl- or CsCl-induced samples, but perhaps less than in KCl-induced and comparable to NaCl-induced samples. NaCl. Here again the 245 cm−1 relative intensity peaks with 488.0 nm excitation, and the 1230 cm−1 relative intensity increases toward the blue. The sharpness of the peak in excitation profiles is less pronounced for NaCl-induced aggregate samples than the others, except KCl-induced aggregates. A greater uniformity of mode intensities in any given RR spectrum accompanies the gentler peak of excitation profiles. Both of these effects correlate with a broader J-band in the absorption spectrum. Significant dispersion in ρ is seen for both 245 and 1230 cm−1, indicating resonance with multiple electronic transitions. Values of ρ never approach 1/8 as excitation is detuned from resonance with the J-band maximum, so at none of these excitation wavelengths is the laser line primarily resonant with doubly degenerate transitions. CsCl. The excitation profile of the 245 cm−1 mode peaks more sharply near the J-band absorption maximum than in any of the other samples. The 1230 cm−1 mode excitation profile is similar to that observed for the other samples. Values of ρ again reach maxima with 488.0 nm excitation but have generally smaller values compared to the other samples except LiClinduced aggregates. This may be due to less constructive interference, likely due to a greater difference in the relative strengths of transitions with different polarizations. Most of these values are significantly above 1/3, again consistent with simultaneous resonance with at least two excitonic transitions of different energies and polarizations. KCl. Relative intensity profiles are similar to those for NaClinduced aggregates, although the peak at 488.0 nm excitation for the 245 cm−1 mode is even less pronounced. Intensities within each RR spectrum are also slightly more uniform, as can be seen by comparing the relative intensities for several modes at the same excitation (both Figure 3 and Figure S1A) We noted that similar features correspond to a broader absorption J-band in the case of NaCl-induced aggregates. However, the absorption J-band is narrower for KCl-induced aggregates than for NaCl-induced aggregates (though broader than for the remaining three samples). We reported lower RLS intensity for KCl- than NaCl-induced aggregates in our recent publication, indicative of lower overall excitonic coherence.12 Less sharp excitation profiles for KCl- compared to NaCl-induced aggregates could be connected to relatively lower coherence more than to broadening of the absorption spectrum.

In KCl-induced aggregates, we observe generally greater values of ρ, except with 514.5 nm excitation, as well as higher maxima at 488.0 nm excitation, compared to NaCl-induced aggregates. The J-band in both absorption and RLS spectra were both weaker on the red side for KCl- than for NaClinduced aggregates. Thus, lower values of ρ with excitation on the red side of the J-band for KCl-induced aggregates could be due to less simultaneous excitation of multiple excitons in that region. LiCl. The 245 cm−1 mode relative intensity here again peaks sharply with 488.0 nm excitation, though a bit less so than in the CsCl-induced case. The profiles, especially of the modes shown in Figure S1A, are slightly blue-shifted compared to those for aggregates induced by the other salts and more similar to those for HCl-induced aggregates. Values of ρ are smaller compared to NaCl- and KCl-induced samples and even slightly lower than in the CsCl-induced case. The peaking of depolarization ratios for the 245 cm−1 mode is less sharp than in any of the other cases. Also in contrast to the other samples, most of the high frequency modes do not show peak depolarization ratios in the center of the J-band (Figure S1B). These results suggest less interference among excitonic transitions as compared to those in other samples. General observations on the sharpness of excitation profiles are further illustrated by the relative standard deviations (RSD) in total scattering intensities for each mode, taken over the four excitation wavelengths (Table 1). Here the low frequency Table 1. Relative Standard Deviations of Relative RR Intensities vs 1537 cm−1 over the Four Spectra at Different Excitation Wavelengths for Each Major Mode and Sample Solution mode (cm−1)

NaCl

KCl

CsCl

LiCl

HCl

245 316 700 986 1015 1230

1.0 0.89 0.35 0.25 0.14 0.42

0.87 0.86 0.42 0.26 0.03 0.29

1.4 1.3 0.54 0.48 0.42 0.28

1.2 1.0 0.61 0.35 0.23 0.39

1.1 0.98 0.73 0.47 0.17 0.43

modes have the greatest RSD values in aggregates induced by CsCl, followed by LiCl, HCl, NaCl, and KCl. The pattern becomes less regular going to higher frequency modes, but LiCl- and HCl-induced aggregates still tend to show larger RSD in the intensities than NaCl- and KCl-induced aggregates. Average depolarization ratios for each mode over the four laser lines (Table 2) show a pattern of smallest to largest ρ for aggregates induced by LiCl, CsCl, HCl, NaCl, and finally KCl, with just a few exceptions. Smaller depolarization ratios Table 2. Mean Depolarization Ratios for Each Major Mode and Sample Solution over the Four Spectra at Different Excitation Wavelengths

E

mode (cm−1)

NaCl

KCl

CsCl

LiCl

HCl

245 316 700 986 1015 1230 1537

0.48 0.47 0.51 0.45 0.48 0.44 0.47

0.50 0.49 0.55 0.51 0.50 0.54 0.51

0.41 0.37 0.48 0.41 0.47 0.42 0.44

0.40 0.40 0.46 0.41 0.43 0.40 0.43

0.43 0.42 0.50 0.44 0.49 0.50 0.47


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Figure 4. Left column: optical images of two specimens from each of the five samples studied. The cyan circles indicate the location of laser illumination. The upper/leftmost image shows the polarization directions designated V and H, which is the same relative to all image frames. On the right are RR data excited at 488.0 nm for the ten samples shown in the images, for the 245 and 1230 cm−1 modes. Upper right: relative total intensity vs 1537 cm−1. Center and lower right: relative intensities (vs VV) corresponding to the four polarization combinations.

collapsed nanotubes in width, or larger, and structures of significantly smaller apparent width, which may be bundles or individual nanotubes. The polarization directions are designated “V” along the longest dimension of the aggregate specimen and “H” perpendicular to this, as indicated on the first optical image in Figure 4 (top left). The polarization directions are the same relative to the image frame for all specimens shown. The following comments on each set of specimens are headed with the salt (or HCl) present in the solution from which aggregates were deposited and the figure number. HCl. In the narrower specimen (HCl I), relatively small differences are seen between off-diagonal HV and VH intensities, particularly for the low frequency modes. This is consistent with results shown in our group’s previous publication.22 HCl II, a composite structure, shows larger intensities of spectra involving H polarization and greater differences between HV and VH intensities for the low frequency modes. Of the remaining samples, these results are most similar to those for the LiCl-induced aggregates, while the effect of bundling of small numbers of tubes is similar to that observed for CsCl-induced aggregates. NaCl. NaCl I is likely narrower than the limit of resolution, while NaCl II is wider. While the VV spectrum for both has the largest intensity among the four polarization combinations for both 245 and 1230 cm−1, greater intensities involving at least one H polarization are seen in NaCl II than in NaCl I. At least

(tending closer to the single transition value of 1/3) could be connected to smaller coupling between excitonic transitions of differing polarization, possibly due to one transition being markedly less allowed, corresponding to narrower J-bands as observed for the LiCl- and CsCl-induced samples. Polarization-Dependent Resonance Raman Spectra of Individual Aggregates. We examined several aggregate specimens deposited from each of the 5 μM TSPP aggregate samples, presented in Figure 4. Each sample was searched for specimens with a variety of apparent widths. A narrower specimen from each sample is designated with Roman numeral I. For HCl and CsCl, the specimen II spectra are taken at an apparent point of overlap of a small number of subunits, while for the remaining samples specimen II appears to be a more continuous, wide structure. Full spectra for each specimen I (with the fluorescence background) and relative intensity data for several additional modes are shown in Figures S2 and S3, respectively. Specimens that appeared to have undergone bleaching at the laser spot during exposure were excluded, and those presented here show data typical of a given solution condition and apparent size. From microscopy studies we expect component nanotubes to have a collapsed width no greater than ∼35 nm, including possible salt coverage.12 The apparent widths in the optical images are somewhat larger than they would be with exact focus, as a slight defocusing was found to improve visibility of aggregates against the background. We can only distinguish between bundles of several tens of F


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The Journal of Physical Chemistry C for the 245 cm−1 mode in NaCl I, relative intensities of the two “off-diagonal” components (HV and VH) differ significantly. In NaCl II, a clear difference between HV and VH intensities is also seen for the 1230 cm−1 mode. Comparing the two specimens, the wider NaCl II shows significantly larger HH and VH intensities in the 245 cm−1 mode, with the HV intensity larger particularly in 1230 cm−1 and other high frequency modes. The total intensities relative to the 1537 cm−1 mode are smaller in the wider specimen, especially in the 245 cm−1 mode. CsCl. The two “specimens” in this case are taken at two different points on the same aggregate. CsCl I may correspond to an isolated, single nanotube section, while at CsCl II there is some appearance of a composite structure. At site I, the VV spectrum is profoundly more enhanced than the other spectra. There is also a large difference between the off-diagonal HV and VH intensities. This section is noticeably curved, and although care was taken to position the laser spot over a nearly straight segment, imperfect alignment between the aggregate frame and the lab frame polarizations could affect the data somewhat. In contrast, at site II, all of the intensities involving H polarization are larger, especially HV and HH, which increase by a factor of 1.5−6 depending on the mode. The relative intensity of the 245 cm−1 mode versus 1537 cm−1 is also less at site II than at site I, though the difference is less than observed between the specimens from NaCl solution. KCl. The specimen KCl I is similar in width to NaCl I but has a knotted appearance, which may be due to microcrystalline KCl deposits. The VV spectrum is again the most intense. The HV spectrum is more intense than in the NaCl-induced specimens, especially compared to NaCl I. Off-diagonal component intensities differ significantly for both 245 and 1230 cm−1. In contrast with NaCl II, HV is larger compared to HH and VH across the whole spectrum. This is even more prominent for KCl II where the HV spectrum is 1.5 times or more as intense as the VV spectrum over the whole range, while the HH and VH spectra also show greater intensity compared to KCl I. These effects probably are partly due to curving of nanotubes inside the laser spot, but they also likely indicate misalignment at contact points between component nanotubes. LiCl. These specimens have spectra somewhat similar to those of CsCl-induced aggregates, especially in the very large intensity of the VV spectrum compared to the other components. The most notable distinction is a smaller difference between the off-diagonal component intensities because of the greater intensity of the HV spectrum. Greater intensities of non-VV spectra, greater uniformity of low and high frequency mode relative intensities vs 1537 cm−1, and slightly larger differences between HV and VH intensities are all observed in the data for the wider specimen LiCl II. Specimens induced by all salts show a large decrease in the fluorescence background relative to solution, consistent with a decrease in dephasing due to removal of solvent, but this decrease is less pronounced in CsCl- and LiCl-induced aggregates than the others (Figure S3). Several recurring patterns among aggregates appear to correlate with width and apparent orderliness. First, wider aggregates tend to give significantly larger intensities in components involving H polarization, suggesting some mechanism related to bundling that selectively boosts transversely polarized transitions. Second, wider aggregates show more uniform enhancement of low and high frequency modes than narrower aggregates. We speculate that low frequency modes are more resonant with

axially versus transversely polarized excitonic transitions and that the latter transitions are relatively stronger in wider aggregates. Finally, for aggregates that appear more orderly or straight, and to a lesser extent for narrower aggregates, smaller differences between the VH and HV spectra are observed, indicating less vibronic coupling between excitonic transitions of differing polarizations. The structural origins of these effects, the relations between RR spectra of individual aggregates, and the corresponding solution phase RR data, as well as to absorption and RLS spectra, are fleshed out below. Scanning Tunneling Microscopy of LiCl-Induced Aggregates. UHV STM images of LiCl-induced aggregates shown in Figure 5 and Figure S4 contrast with our previously

Figure 5. Top: constant current (+2.0 V, 10 pA) STM image of an aggregate deposited on Au(111) from 5 μM TSPP in 0.75 M LiCl(aq). Bottom: height profiles, averaged over the regions enclosed in dashed lines, at the locations as numbered and colored in the image.

reported AFM data12 (where typical heights were 10−20 nm). The experimental conditions here preclude the formation of an adsorbed water layer, the likely presence of which complicated the comparison of LiCl-induced aggregates to those induced by the other salts (which did not show evidence of adsorbed water) in the AFM experiments. The cross-sectional profiles in Figure 5 were taken as averages over 15 lines enclosed in the dashed lines, centered at the dotted lines. These show respectively a maximum height of about 2 nm (profile 1) and 2.4 nm (profile 2), consistent with a single monomer thickness. Profile 1 depicts a region which appears to comprise four subsections, consistent with subunits ∼16−17 nm in width. It seems likely that the substructures represent open-edged structures rather than complete individual nanotubes. The G


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The Journal of Physical Chemistry C first profile in Figure S4 may represent a single unclosed nanotube or two single layers with partial overlap. The second profile exhibits a nearly rectilinear shape, further suggesting a single, flat layer rather than a collapsed tube. In addition to the contribution of adsorbed water to the measured thickness in the AFM experiments, the aggregates may also have consisted of multiple layers of porphyrin sheets. The STM images shown here were taken from a peripheral region of the sample deposit because much of the sample deposit area was covered in an insulating layer of salt and so may mainly have aggregates on the small side of the size distribution present in solution. However, nearly identical excitonic band positions, along with tight constraints between packing geometry and the coupling strength determining those peak positions, suggest similar packing geometry in all samples of the present series.27,33 These images therefore suggest an architecture for LiCl-induced aggregates based on layering or bundling of nearly flat sheets, consistent with open ribbons or partial nanotubes.

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DISCUSSION We begin with some theoretical aspects of excitons in a helical, nanotubular aggregate, constructed as a stack of N1 rings of N2 monomers each, with an angle γ of rotation between successive rings.27,36,57 We index the monomer location by n = (n1, n2), where n1 = 1, ..., N1 is the ring index and n2 = 1, ..., N2 is the helix index. The monomer Soret band is doubly degenerate, with p = x, y designating the polarization components in the monomer frame. We take one of the transition dipole moments of each monomer to be oriented roughly normal to the tube surface and the other roughly parallel to it. The latter is tilted at angle θ with respect to the tube axis. The indices n and p describe a site-excitation basis |np⟩. In the long nanotube limit (N1 ≫ N2) and with no disorder, we can transform to a basis |kp⟩ in reciprocal space, where k = (k1, k2) is the excitonic wave vector. For each k, a 2 × 2 Hamiltonian can be diagonalized to give two eigenstates (α = 1, 2): 1 |kα⟩ = N =

1 N

∑

Figure 6. (a) Disorder-free nanotube excitonic states, with corresponding transition dipole moments. Top: axially polarized k = 0 state. Second row: transversely polarized states (real-valued linear combinations of k = ±1 states). (b) Excitonic coupling across multiple nanotubes in a bundle. (c) An excitonic state with skewed transition dipole moment polarization.

c(pα)[

∑ exp[−i(k1φ1n1 + k 2φ2n2)]|np⟩]

p

n

∑ c(pα)|kp⟩ (1)

p

corresponding to α = 1, 2: one representing the J-band and the other the H-band. Focusing just on the J-band region, the absorption line spectrum can be written per ref 30:

where N = N1N2 is the total number of monomers and φi = 2π/ Ni is the relative phase between successive values of ni (i = 1, 2). Excitonic transition dipole moments are written analogously as μ kα =

1 = N 1 N

∑ p

c(pα)[

∑ exp[−i(k1φ1n1 + k 2φ2n2)]μnp ]

A(ω) =

p=x ,y

n

Nμ2 {cos2 θp[δ(ωL − ω0)] 3

+ sin 2 θp[δ(ωL − ω±(γ / φ1,1))]}

∑ c(pα)μkp p

∑

(3)

where μ = |μnp| is the monomer transition dipole moment magnitude, ωL is the incident light frequency, and ω0, ω±(γ/φ1,1) are the transition frequencies to the k = 0, k = ±(γ/φ1,1) excitons, respectively. This expression shows that due to the tilt angles θx,y of the monomer transition dipole moments with respect to the nanotube axis the J-band resolves into a nondegenerate and a doubly degenerate transition, polarized perpendicularly to each other in the aggregate frame. In reality, transitions to higher k states are allowed due to finite nanotube lengths. Bundling or disorder can further redistribute intensity to these transitions. Even in a perfectly

(2)

Transitions to the k2 = 0 exciton and to the doubly degenerate pair of excitons k2 = ±1 are allowed for light polarized parallel and perpendicular to the tube axis, respectively. For long tubes (N1 ≫ N2), k1 = 0 when k2 = 0 and k1 = ±γ/φ1 when k2 = ±1. All monomer wave functions are in phase in the k = 0 exciton, while in the k = ±(γ/φ1, 1) excitons, the phase varies through one cycle of 2π around the tube circumference. These states and corresponding transition dipole moments are depicted, along with the definition of the aggregate coordinate frame, in Figure 6a. There are two sets of these transitions, H


Article

The Journal of Physical Chemistry C ordered nanotube bundle (Figure 6b), spreading excitons over neighboring subunits can increase the associated intensity. Because of the geometrical relationship between neighboring bundles, from excitonic coupling strength equations4,22 we would predict additional small shifts in J-band components: a red-shift for the H-polarized transitions (transition dipole moments along the direction of bundling) and a blue-shift for the V-polarized transitions (transition dipole moments perpendicular to the direction of bundling). Orientational disorder in the molecular packing or misaligned contact between nanotubes in bundles can result in skewed polarizations, neither orthogonal to each other nor fully aligned with the aggregate frame (Figure 6c). Some basic theoretical considerations of RR spectroscopy can aid in the interpretation of our results. We consider the excitonic wave functions to include all modes of molecular vibration. Let i, n, and f designate the initial, intermediate, and final vibronic states of the aggregate, respectively (including all normal modes and the excitonic index k), and μρ and μσ the dipole operator components for two Cartesian polarizations in the aggregate frame (i.e., ρσ = VV, HH, VH, or HV). We also define the ground to intermediate state transition frequency ωin, the incident light frequency ωL, and the exciton inverse lifetime Γk. In the aggregate frame, the transition polarizability between the ground state i and final state f (for incident and scattered light polarizations ρ and σ, respectively) is then written as a Kramers−Heisenberg−Dirac (KHD) sum over intermediate states: (αρσ )if =

1 ℏ

∑ n

⟨i|μρ |n⟩⟨n|μσ |f ⟩ ωin − ωL − i Γk

ρ=

5Σ1 + 3Σ2 10Σ0 + 4Σ2

(5)

Σ is proportional to Tr|(αρσ)if | = |2(αHH)if + (αVV)if | , and Σ2 depends on sums of pairs of off-diagonal elements ((αHV)if and (αVH)if) and differences between pairs of diagonal elements ((αHH)if and (αVV)if). Σ1 depends on differences between offdiagonal elements, as were found in the single aggregate spectra. This quantity vanishes in the absence of vibronic coupling, as explained below. For a totally symmetric mode, resonance with a nondegenerate electronic transition corresponds to a transition polarizability with one nonzero element, on the diagonal, giving ρ = 1/3, whereas resonance with a doubly degenerate electronic transition yields two equal nonzero transition polarizability elements, both diagonal, giving ρ = 1/8. We would get these values, respectively, if we could excite spectra at resonance only with the nondegenerate, axially polarized transition or only with the doubly degenerate, transversely polarized transition. In the present case, simultaneous resonance with two closely spaced axially and transversely polarized electronic (excitonic) transitions leads to a total polarizability which is the sum of contributions from two excitonic states, as in eq 4. In adding these and taking the square modulus, interferences occur that result in depolarization ratio dispersion. We observe exactly such dispersion in the ρ values in all samples. In addition, marked differences in typical ρ values among samples, where larger typical values occur with bundled aggregates (Table 2), suggest a greater density of active, closely spaced transitions of different polarizations in these instances, giving rise to stronger interference effects. Single aggregate specimens with larger off-diagonal intensity differences IHV − IVH also show larger depolarization ratios in the corresponding solution phase data, consistent with larger values of Σ1 relative to Σ0. This consistency suggests similarity of aggregates in solution and as deposited, with some caveats described in the Supporting Information. To analyze the role of vibronic coupling, especially in the offdiagonal component intensities, we employ the Albrecht A and 0 B term formalism.58 Let μk,l be the zero-order (Condon approximation) transition dipole moments and Ek,l the energies for states k and l, respectively. We use a to index the normal mode and Qa for the corresponding normal coordinate. Hakl = ⟨k|(∂Ĥ /∂Qa)0|l⟩ is the linear vibronic coupling matrix element giving the first-order energy correction for dependence on Qa. The Herzberg−Teller expansion of the transition dipole moment to first order is 0

(ax) (tr) = αρσ + αρσ

(4)

This expression describes how the transition dipole moment magnitudes, Cartesian components resulting from their orientations, and detuning ωin − ωL between excitation and transition frequencies determine the VV, HH, VH, and HV intensities in the aggregate frame. The second equality in this expression emphasizes that the transition polarizability can be viewed as the sum of contributions from axially and transversely and α(tr) polarized excitonic transitions α(ax) ρσ ρσ , which have different frequency dependence due to different energies ωin. The polarized spectral intensities of individual aggregates are proportional to the square moduli of the polarizabilities, Iρσ ∝ (tr) 2 |α(ax) ρσ + αρσ | . For ρ = σ, if we have μV ≫ μH for the axially (V) and transversely (H) polarized transitions, the VV intensity will be much greater than HH, as seen in the narrower aggregate specimens. This occurs due to tilt angles θx,y ≲ 40° and the smaller overlap integrals between the aggregate ground state and the k = ±1 exctions than the k = 0 exciton. Larger HH intensities observed for broader aggregate specimens (particularly NaCl- and KCl-induced) point to larger transversely polarized transition dipole moments, a shift of these transitions into closer resonance with 488.0 nm, or both, resulting from bundling. The depolarization ratio ρ (not to be confused with the polarization index in eq 4) for solution phase resonance Raman spectra is related to the aggregate frame transition polarizability elements by averaging over random orientations of the aggregates relative to the lab frame. In terms of rotational invariants Σ0 , Σ 1 , and Σ 2 (details in the Supporting Information), the depolarization ratio is

2

μ k = μ k0 +

∑ a,l≠k

μ l0 Hakl El − Ek

2

Qa

(6)

The A term for ρ-polarized scattered light excited by σpolarized incident light is Aρσ , if = =

1 ℏ

(ax) Aρσ , if

∑ (μρ0 )k (μσ0 )k n,k

+

(tr) Aρσ , if

⟨vi|vn⟩⟨vn|vf ⟩ ωin − ωL − i Γk (7)

Here vi, vn, and vf designate the (multimodal) vibrational wave functions, and (μ0ρ,σ)k,l are the ρ- and σ-polarized components of the Condon approximation transition dipole moments. As in eq 4, we can view the A term part of the polarizability as a sum of contributions from distinct axially and transversely polarized I


Article

The Journal of Physical Chemistry C transitions. The A term gives the portion of totally symmetric mode activity that does not require vibronic coupling. The diagonal component intensities, and differences in them among aggregate specimens, are due at least in part to A term activity. VV intensity could decrease relative to HH when disorder shifts some of the corresponding transitions’ polarization into the transverse direction. Such a disorder-induced skewing of polarizations contributes some amplitude to the off-diagonal AVH,if (= AHV,if) components. Without this disorder, either (μ0ρ)k or (μ0σ)k would be zero for each k so each term in eq 7 for the off-diagonal components would vanish. A term activity is insufficient to give the off-diagonal terms unequal intensities, as eq 7 is symmetric with respect to exchange of polarization indices ρ and σ. To further investigate the unequal off-diagonal intensities, we consider linear vibronic coupling through dependence of transition dipole moments on normal coordinate Qa. Using the terms for mode a from the sum in eq 6 a (Bρσ )if =

1 ℏ ×

∑ n , k, l k≠l

loosening of the selection rules that forbid transitions in perfectly ordered, individual nanotubes to excitonic states other than those with k = 0, ±1. Orientational disorder can also contribute to strengthening the relative intensities of HH, HV, and VH spectra versus VV because it skews transition dipole moment polarizations away from alignment with the aggregate frame. The effects of counterions on aggregate morphology were discussed extensively in our previous publication. In brief, we expect the cosmotropic cations Na+ and Li+, which tend to strongly order solvation water, to pair weakly with the chaotropic peripheral sulfonate groups of the TSPP molecules and to stabilize the porphyrin−porphyrin interactions. For NaCl-induced aggregates, this weak affinity of Na+ for sulfonates increases the rigidity of nanotubes and encourages bundling, while Li+ leads to more water on the surface of the aggregates. While closed nanotube formation is disfavored, local packing apparently does not become more disordered. In contrast, stronger association is expected between chaotropic Cs+ or K+ and sulfonate groups. While Cs+ disfavors bundling, K+ allows bundling, though in a much less orderly fashion than with Na+. These effects on the material structure determine the excitonic energy level structure as well as the extent and shape of coherently coupled regions, leading to spectral variations. The RR data here provide finer detail about the same morphology−spectroscopy relationships suggested by absorption and RLS spectroscopy. NaCl-induced aggregates exemplify the effects of bundling with limited disorder. The absorption J-band is greatly broadened compared to HCl-induced aggregates. The total RLS intensity reported in our prior publication is larger, indicating larger characteristic coherence numbers, but the peak intensity is smaller, suggesting the larger apparent coherence is due mainly to more allowed transitions to higher energy excitons and perhaps larger transition dipole moments in the transverse direction. Relative RR excitation profiles in solution are less sharply peaked near 488.0 nm excitation than with HClinduced aggregates. For individual aggregates, we observe greater HH intensity and greater difference of off-diagonal intensities IHV − IVH, effects that increase with apparent bundle size, especially for low-frequency modes. Both of these results from the RR data corroborate the attribution of increased absorption bandwidth concurrent with increased RLS intensity to the activation of transversely polarized excitons due to close packing of individual nanotubes. Aggregates formed in CsCl solution primarily demonstrate the effects of orientational disorder. Individual nanotubes and small bundles with few components, which appear curved (likely due to looser molecular packing), were observed in images. The J-band absorption bandwidth was slightly smaller and its integrated RLS intensity slightly lower than for HClinduced aggregates. In solution, the relative RR intensities peak sharply with 488.0 nm excitation, similarly to HCl-induced aggregates, and the depolarization ratios are generally smaller than for NaCl-, KCl-, or HCl-induced aggregates. In the single aggregate data of a putative individual nanotube, the VH, HV, and VH intensities relative to VV are smaller than for the other samples, indicating weaker transversely polarized transition dipole moments. This also suggests that the smaller RLSmeasured coherence is likely primarily due to greater disorder within individual nanotubes as compared to HCl-induced aggregates.

Hkla El − Ek

(μρ0 )k (μσ0 )l ⟨vi|vn⟩⟨vn|Q a|vf ⟩ + (μσ0 )k (μρ0 )l ⟨vi|Q a|vn⟩⟨vn|vf ⟩ ωin − ωL − i Γk

(8)

B terms allow nontotally symmetric mode activity and contribute additional totally symmetric mode intensity through coupling between pairs of states k and l. For each k, l pair of intermediate states involved in eq 8, each product of zero-order transition dipole moments occurs once with each combination of overlaps and the resonance denominator for each k or l. The pairing of these factors is switched with exchange of polarization indices ρ and σ, so B terms lead to an asymmetric transition polarizability tensor. This in turn results in unequal values of the HV and VH intensities. Because IHV − IVH is larger both in the presence of bundling and with greater structural disorder, both of these effects appear to enable a greater degree of vibronic coupling, as observed in all aggregate specimens except perhaps the apparently individual nanotube of specimen HCl I. Linear and circular dichroism of the J-band in HCl induced aggregates suggest that the weaker, transversely polarized transition is centered on the blue side and the stronger, axially polarized transition on the red side.27,30−32 With bundling, a small red-shift from weak J-band-like coupling of the former and a small blue-shift from weak H-band-like coupling of the latter between constituent nanotubes, would decrease the energy gap El − Ek in the denominator of eq 8. This would increase the amplitudes of both the HV and VH polarized B terms. A larger number of active excitonic transitions could further contribute to this effect. Even if the energy order of the two polarizations in the J-band is the reverse, as suggested by our prior RLS depolarization data,22 boosting the transversely polarized transition dipole moments and activating transitions to more k states would amplify the effects of vibronic coupling.

■

CONCLUSIONS The RR spectra presented here demonstrate several effects of counterion-dependent morphological variations on the excitonic states of porphyrin aggregates. We have shown evidence that bundling can lead to excitons delocalized over multiple component nanotubes, leading to a broader, more complexly structured J-band. The broadening could arise through J



■

With KCl-induced aggregates, we observe combined effects of bundling and orientational disorder. Deposited aggregates are mainly large bundles, evidenced in images by more curving and twisting. A broadened absorption J-band was observed for the corresponding solution, though less so than with NaClinduced aggregates, while J-band integrated RLS intensity was lower than for all other solutions. The solution phase and RR data show trends similar to those observed for NaCl-induced aggregates. Increased relative intensity of the HH component in individual aggregate RR spectra indicates a boost in the relative strength of transversely compared to axially polarized excitonic transitions. The even greater intensity difference IHV − IVH compared to NaCl-induced aggregates could be due to skewing of transition polarizations at misaligned, coupled contact regions. The greater prominence of these effects could indicate greater disorder, apparent in the twisted bundle images, which would both decrease VV intensity and increase the effect of vibronic coupling. For LiCl-induced aggregates, a typical thickness consistent with a single porphyrin layer was found in STM data (Figure 5 and Figure S4); thus, they appear more ribbon-like than tubular. These aggregates exhibit a J-band with a slight increase in both the absorption bandwidth and integrated RLS intensity. RR excitation profiles of these solutions peak with similar sharpness at 488.0 nm excitation to those of both HCl- and CsCl-induced aggregates, but depolarization ratios are smaller compared to all other samples. Lower non-VV intensities are observed for single aggregates, but these are again larger for wider than for narrower specimens. Because of noncyclic geometry in these aggregates, some activity of excitonic transitions higher in energy than the lowest transversely polarized exciton could be allowed. Because of the small amount of bundling observed, larger coherence in this case likely arises mainly from larger orderly domains, rather than through coupling across subunits as is thought to occur with NaCl-induced aggregates. Vibronic coupling appears to play a greater role in samples with more disorder and with larger bundles. While we have suggested some reasons why the HH, VH, and HV components are preferentially enhanced relative to the VV component, these observations are difficult to fully explain based only on a simple, qualitative model of helical excitons. More detailed numerical modeling based on Frenkel exciton theory, in progress, may help illuminate these elusive issues and possibly bolster the analysis here. RR spectroscopy, along with absorption and RLS spectroscopy and imaging studies, has been shown here to elucidate the excitonic structure of self-assembled aggregates and its connection to morphological effects induced by variations in the solution environment. Further application of this methodology will be shown in a future publication, with examples of morphological variations distinctly affecting absorption, RLS and RR spectra, in aggregates induced by HCl in ethanol. The ability to make these connections between material morphology and disorder, on the one hand, and the energetic and geometric properties of excitons, on the other, allows us to connect solution processing conditions to the size, shape, and location of ordered, excitonically coupled domains, the control of which is crucial for successful use of aggregates in solar energy applications. These insights may also inform our understanding of excitonic light harvesting more generally, whether in natural photosynthetic systems or in devices employing excitonic processes for solar energy conversion.

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00867. Resonance Raman data for additional vibrational modes for aggregates induced by all solution conditions; additional STM data for an aggregate induced by LiCl; mathematical details on the relationshiop between using spectral intensities and polarizability tensor components in estimating depolarization ratios (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 509-335-4063 (J.L.M.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The support of the National Science Foundation through Grants CHE 1149013 and DMR 1305592 is gratefully acknowledged. A portion of the research (STM) was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. For the STM experiments, we thank Dr. Igor Lyubinetsky for assistance in experiment planning and Drs. Arjun Dahal and Rentao Mu for sample mounting and instrument operation.

■

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