Resonance Raman Spectra of Individual Excitonically Coupled

Article pubs.acs.org/JPCC

Resonance Raman Spectra of Individual Excitonically Coupled Chromophore Aggregates Christopher C. Rich and Jeanne L. McHale* Department of Chemistry, Washington State University, P.O. Box 644630, Pullman, Washington 99164-4630, United States S Supporting Information *

ABSTRACT: We report resonance Raman spectra of individual porphyrin nanotubular aggregates of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) deposited on glass. Using a novel internal/external standard method, we show that absolute Raman cross sections of low-frequency vibrational modes are greatly enhanced by J-band excitation. We report single-aggregate resonance Raman spectra obtained without surface enhancement. Variations in the relative intensities of low- and high-frequency Raman modes of different aggregates and images of the resonance light scattering in epiillumination reveal variations in aggregate structure and allow the possible correlation between Raman intensity and coherence to be explored. Polarized Raman spectra of individual aggregates confirm that the J-band is a composite of two closely spaced vibronically coupled transitions polarized parallel and perpendicular to the long axis of the aggregate, in accordance with our structural model of a hierarchical helical nanotube. The evolution of the Raman spectrum of a single aggregate during laser heating reveals the role of water in the assembly of structural subunits. Our experimental results provide insight into the concept of aggregation-enhanced Raman scattering. flattened when deposited on a surface. In the present work, we show that a combination of excitonic coupling, which serves to concentrate the resonance Raman enhancement into a narrow wavelength range, and self-assembly, which concentrates the molecules within the focal volume, enables the determination of Raman spectra of single TSPP aggregates and small bundles thereof, without the use of surface enhancement via noble metals. We report variations in the Raman spectra of individual aggregates that are suggestive of structural heterogeneity and perhaps aggregate-to-aggregate variations in coherent coupling. Polarized single-aggregate Raman measurements reveal a Jband which is incompletely polarized along the long axis of the flattened nanotube, consistent with our previous model of a helical nanotube built up from cyclic N-mers. We also present a novel internal/external standard approach for determining the Raman cross sections of the aggregate and monomer, enabling us to make a quantitative assessment of the significance of aggregation-enhanced Raman scattering, or AERS. As proposed by Akins15 and explored in a number of publications from his group16−18 and others,19−21 excitonic coupling has the potential to lead to enhancements in the resonance Raman intensity. In addition to reporting the absolute Raman cross sections of the solution-phase monomer and aggregate at similar detunings, we use resonance light scattering and resonance Raman spectra of individual

1. INTRODUCTION Excitonic coupling of strongly allowed electronic transitions in chromophore aggregates results in a number of interesting optoelectronic properties, such as excited state delocalization, enhanced light harvesting, and efficient energy transfer. These properties depend strongly on the aggregate supramolecular structure and on static and dynamic disorder within the assembly, which limit the coherence number, Ncoh, the number of molecules over which the exciton is delocalized.1−3 While images from scanning probe4−7 and electron microscopy8,9 experiments reveal the gross morphology of molecular aggregates, determination of their internal structure, for example by X-ray diffraction methods,10 is challenging, and thus intermolecular configurations are often inferred from perturbations to the optical spectra. The contribution of structural heterogeneity in an ensemble of aggregates complicates the attempt to use optical spectra to determine intermolecular couplings, which dictate shifts in the optical spectra, and coherence numbers, which influence spectral linewidths. Single-aggregate Raman spectroscopy provides a means to obtain information about internal structure and the nature of the resonant electronic transition without complications from aggregate heterogeneity. We have recently reported the results of scanning probe microscopy and polarized resonance Raman spectroscopy studies of the excitonically coupled aggregate of meso-tetra(4sulfonatophenyl)porphyrin (TSPP).4,11−14 Raman spectra and images are consistent with a hierarchical structural model in which cyclic N-mers assemble into a helical nanotube which is © 2013 American Chemical Society

Received: April 25, 2013 Revised: April 30, 2013 Published: May 1, 2013 10856

dx.doi.org/10.1021/jp404109u | J. Phys. Chem. C 2013, 117, 10856−10865

The Journal of Physical Chemistry C

Article

Figure 1. Schematic of confocal “internal/external” standard method for measuring absolute Raman cross sections.

aggregates of TSPP on glass to explore the possible correlation of exciton coherence and Raman intensities.

σR =

dσR,2/d Ω

=

The absolute Raman cross section of the sample is then determined by dividing the calculated cross section by k. σR,corr =

8π (1 + 2ρ) dσR 3(1 + ρ)k d Ω

(4)

To ensure a consistent k value for the sample measurement and the calibration measurement, the sample is measured first so that the beam focus is the same in the calibration measurement. The k values were checked by measuring the absolute Raman cross section of another reference with a known cross section using the previously determined k value. The known Raman cross section of the 918 cm−1 mode of acetonitrile from ref 23 was used to determine the value of k, using the 800 cm−1 of cyclohexane as the standard. This resulted in an experimental value of σR = 134.8 × 10−30 cm2/molecule for the 932 cm−1 of the perchlorate ion, compared to the literature value of 136.9 × 10−30 cm2/molecule. This check was performed for all laser excitations to ensure the validity of the k values. 2.3. Single Aggregate Resonance Raman Spectroscopy. TSPP aggregates were first drop cast onto glass coverslips and then allowed to dry. Spectra and images were measured using an Olympus IX70 inverted confocal microscope with an Olympus 100× oil immersion objective (see Figure 2). Images were visualized using a thermoelectrically cooled Andor CCD camera. Raman spectra were dispersed with an Acton SpectraPro i2300 single monochromator and collected with a thermoelectrically cooled CCD camera. For experiments in which we wanted to negate the polarization dependence of the aggregate Raman spectrum, a quarter wave plate was used to transform the linearly polarized incident light into circularly polarized light. Polarized spectra were recorded by using a halfwave plate to rotate the polarization of the incident light so that

I1n12C2 I2n22C1

(2)

where σR is the absolute resonance Raman cross section and ρ is the depolarization ratio of the Raman mode in question. Determination of absolute resonance Raman cross sections is a precise measurement and requires calibration. To take into account the differences in light collection between each lens, one must measure the absolute cross section of a standard with a reported cross section and compare it to a known literature value. A correction factor k is then determined by taking the ratio of the measured cross section to the literature value cross section. σR,expt k= σR,lit (3)

2. EXPERIMENTAL SECTION 2.1. Materials. meso-Tetra(4-sulfonatophenyl)porphyrin in its diacid form was purchased from Frontier Scientific. Aqueous solutions of TSPP aggregates were prepared by combining aqueous solutions of TSPP diacid and hydrochloric acid (HCl) so that the concentration of HCl was 0.75 M and the concentration of TSPP was 50 μM for the quantitative resonance Raman experiments. Samples for single aggregate spectroscopy were prepared from solutions containing 5 μM TSPP and 0.75 M HCl which were drop-cast onto glass coverslips and allowed to dry. Diacid monomer samples were prepared by combining aqueous solutions of the TSPP diacid with hydrochloric acid so that the concentration of HCl was 0.001 M and the concentration of TSPP was 50 μM. 2.2. Quantitative Resonance Raman Spectroscopy. Absolute Raman cross sections of the aggregate and the monomer in solution, computed on a per-molecule basis, were determined using an internal/external standard method, shown in Figure 1, developed in our lab. Resonance Raman spectra of the aqueous aggregates or monomers and a transparent reference (either neat cyclohexane or acetonitrile, with reported cross-section data from refs 22 and 23) were measured simultaneously using 488, 514.5, and 454.5 nm light from an Ar ion laser as shown in Figure 1. The scattered light was dispersed with a SPEX Triplemate triple monochromator and detected with a liquid nitrogen cooled charge-coupled device (CCD) camera. Polarized spectra were collected using a Melles-Griot polarizer for polarization selection followed by a Thorlabs DPU-25 depolarizer for scrambling to avoid polarization bias. Cross section measurements and calculations were performed using OriginPro 8 and Mathcad. Absolute resonance Raman cross sections were calculated by first determining the differential Raman cross section of the sample using the following equation dσR,1/d Ω

8π (1 + 2ρ) dσR 3(1 + ρ) d Ω

(1)

dσR,m/dΩ is the differential Raman cross section of a Raman band indexed by m (m = 1,2 is the sample and reference, respectively); Im is the intensity of the Raman mode defined by the peak area; nm is the index of refraction; and Cm is the concentration in moles of monomer per liter. The absolute resonance Raman cross section is then derived from the differential Raman cross section by 10857



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Figure 2. Schematic of the Raman microscopy setup, specifically for polarized Raman experiments.

it was either parallel or perpendicular to the long axis of an individual aggregate. A polarizer followed by a scrambler was used to select and then randomize the polarization of scattered light for which the polarization was either parallel or perpendicular to that of the incident light. The excitation source for both the ensemble and single aggregate experiments was an argon ion laser. For single aggregate experiments, the laser power at the sample was 0.75 μW for all Raman measurements except for the laser heating experiments where the laser power was 0.75 mW. 2.4. Resonance Light Scattering Microscopy. Resonance light scattering images were obtained using the microscope setup described above except that the incident beam was first diffused through frosted glass and passed through a lens before entering the 100× objective. This provided a broadened, diffuse laser spot to epi-illuminate a large area of the imaged sample. The scattered light was passed through a 500 nm long pass filter for laser light rejection. Images were collected with a thermoelectrically cooled Andor CCD camera.

Figure 3. Absorption spectra of the 50 μM TSPP diacid monomer in 0.001 M HCl (black) and the aggregate prepared from 50 μM TSPP in 0.75 M HCl (red). (Inset) 3D model of a fully protonated TSPP diacid molecule. Vertical lines in the spectrum mark the wavelengths used for resonance Raman spectroscopy of the aggregate (488 and 514.5 nm) and the monomer diacid (454.5 nm) in solution.

internal/external standard method. Depolarization ratios for the prominent modes of the TSPP aggregate, which have been previously reported in ref 12, show dispersion at excitation wavelengths across the J-band, with values that deviate from what would be expected for resonance via a single nondegenerate excited state. These results provided initial evidence that the J-band consists of two closely spaced excitonic transitions. Table 1 shows that compared to the data at 514.5 nm the Raman cross sections for resonance with the J-band are greatly enhanced, by as much as 3 orders of magnitude in the case of the 243 cm−1 mode. Table 2 shows the absolute Raman cross sections of the prominent modes of the monomer at an excitation wavelength of 454.5 nm. It is apparent from the comparison of these cross sections to those for the aggregate at a similar detuning (514.5 nm) that there is no particular enhancement of the Raman cross sections attributable to aggregation. In fact, at an excitation wavelength that is in both cases about 1000 cm−1 to the red of the absorption maximum, the cross sections for the aggregate Raman modes are considerably smaller than those of the monomer. This observation seems to dispute observations of aggregation-enhanced Raman spectroscopy. However, as reported in refs 15 and 18, the Raman intensities of the low-frequency modes of the aggregate are dramatically enhanced, by nearly 3 orders of magnitude as shown here, when the excitation wavelength is close to the absorption maximum. The low-frequency modes of the TSPP diacid monomer at 234 and 316 cm−1 are reasonably assigned to outof-plane vibrations involving motion of the pyrrolic hydrogens, sometimes referred to as “ruffling” and “doming” modes, respectively.24 Excitonically coupled chromophores often show large enhancements of low-frequency modes in their resonance Raman spectra.5,15,21,25−28 We have previously shown12 that the 243 and 316 cm−1 vibrations of the TSPP aggregate exhibit strong exciton−phonon coupling. Their A-term enhancement in the resonance Raman spectrum is expected since these outof-plane vibrations perturb the interchromophore separation which in turn modulates the coherent coupling that leads to

3. RESULTS 3.1. Condensed-Phase Absolute Raman Cross Sections. To investigate the possibility of aggregation-enhanced Raman scattering, we wished to compare the resonance Raman spectra of the solution-phase monomer diacid and aggregate, in each case using an excitation wavelength coincident with the maximum in the absorption spectrum, i.e., the Soret band or Jband, respectively. However, experimental limitations did not permit us to excite the Raman spectrum of the monomer at 434 nm, the maximum in the monomer Soret band. Thus, in addition to measuring the Raman spectrum of the aggregate at 488 nm, coincident with the J-band maximum, we obtained the resonance Raman spectra of the aggregate and monomer at similar detunings. For reference, Figure 3 shows the absorption spectra of the TSPP diacid monomer and aggregate and vertical lines indicating the excitation wavelengths used in our resonance Raman spectroscopy measurements. Figure 4 shows the background subtracted ensemble resonance Raman spectra of TSPP aggregates in aqueous solution with 0.75 M HCl, excited at 488 nm, which is resonant with the sharp J-band of the aggregate, and at 514.5 nm, which is 930 cm−1 to the red of the absorption maximum of the Jband. The inset shows the resonance Raman spectrum of the diacid monomer excited at 454.5 nm, which is 1039 cm−1 to the red of the 434 nm absorption maximum of the Soret band. Table 1 lists the absolute resonance Raman cross sections per porphyrin molecule of each of the seven prominent modes of the aggregate, determined using the 10858



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the two low-frequency modes, the 243 cm−1 mode is apparently more perturbed by either the environment or isotopic substitution and more strongly enhanced as the excitation wavelength is tuned to the excitonic transition. In contrast, the high-frequency modes are only enhanced by about 1 order of magnitude on going from 514.5 to 488 nm because they are less strongly coupled to the exciton than vibrations which perturb the interchromophore separation. It is likely that an overall, nonspecific increase in Raman cross section on aggregation derives from the concentration of the total Raman intensity into a more narrow range of excitation wavelengths in the case of the aggregate. To the extent that the absorption band of the aggregate is exchange narrowed by coherent coupling, this enhancement on aggregation perhaps represents a correlation between exciton coherence and resonance Raman intensity. 3.2. Single Aggregate Resonance Raman Spectroscopy and Resonance Light-Scattering (RLS) Images. Typically, it is quite difficult to produce single-molecule Raman spectra without the use of surface enhancement (a notable exception being the unenhanced Raman spectra of single carbon nanotubes29). Even with surface or resonance enhancement, imaging molecules with fluorescence or Raman microscopy is a challenge since the molecules are typically smaller than the diffraction limit. As shown above, aggregates of TSPP, however, have rather large absolute resonance Raman cross sections, on the order of 10−22 cm2/molecule with 488 nm excitation. For comparison, cross sections of the Raman modes of rhodamine 6G, a popular molecule for singlemolecule surface-enhanced Raman experiments,30 are on the order of 10−23 cm2/molecule at an excitation wavelength that coincides with the maximum in the optical absorption spectrum.31 In addition, the close packing of the porphyrins in the nanotubular aggregates facilitates that observation of Raman spectra of dispersed individual aggregates. While the TSPP nanotubes are only ∼20 nm in diameter (∼34 nm wide and 4 nm high when flattened on the surface),4,11 they can be several micrometers long and are thus visible in optical microscopy. Figure 5 shows the resonance Raman spectrum of 10 aggregate individuals with an image of a typical filament-like individual. Rod-like structures observed by optical microscopy may be separate aggregates or small bundles as observed in atomic force microscopy (AFM) images (see Figure S1 in Supporting Information). However, since their width dimension is too small to be seen via optical microscopy, whether they are single aggregates or bundles cannot be readily distinguished. The spectra in Figure 5 were obtained with circularly polarized excitation to eliminate polarization bias resulting from aggregates with different orientations, such that the intensity might be proportional to the number of aggregates in the focal volume. However, depending on the aggregate

Figure 4. Resonance Raman spectrum of aggregates prepared from 50 μM TSPP in 0.75 M HCl with cyclohexane as the intensity standard excited with 488 nm wavelength light (black) and 514.5 nm wavelength light (red). The inset shows the resonance Raman spectrum of the 50 μM TSPP diacid monomer in 0.001 M HCl with acetonitrile as the intensity standard excited with 454.5 nm excitation wavelength (blue). The spectra have been background subtracted, and asterisks mark solvent Raman bands.

delocalization of the excited electronic state. In agreement with previous work,4 the 243 cm−1 mode of the aggregate is blueshifted by 9 cm−1 compared to its value in the monomer diacid, while the 316 cm−1 mode of the diacid is not shifted appreciably on aggregation. We have also previously shown that the 243 cm−1 mode of the aggregate prepared in HCl/H2O (referred to as TSPP-h in that work) shifts to 239 cm−1 in DCl/ D2O solution (the TSPP-d aggregate), while the 316 cm−1 is negligibly red-shifted in the deuterated environment.12 Thus, of

Table 1. Absolute Resonance Raman Cross Sections and Corresponding Depolarization Ratios (ρ) of Prominent Modes of TSPP Aggregates Excited with 488 and 514.5 nm Excitation Wavelength Raman shift, cm‑1 Raman cross section, 10−23 cm2/molecule (488 nm) Raman cross section, 10−23 cm2/molecule (514.5 nm)

243 26.2, ρ = 0.45 0.0241, ρ = 0.51

316 19.6, ρ = 0.44 0.0508, ρ = 0.47

698

983

1013

1228

5.78, ρ = 0.41

6.37, ρ = 0.48

2.86, ρ = 0.45

6.84, ρ = 0.53

0.0578, ρ = 0.51

0.184, ρ = 0.38

0.0799, ρ = 0.51

0.169, ρ = 0.36

10859

1534 12.0, ρ = 0.61 0.983, ρ = 0.40



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Table 2. Absolute Resonance Raman Cross Sections and Corresponding Depolarization Ratios (ρ) of Prominent Modes of TSPP Diacid Monomers Excited with 454.5 nm Excitation Wavelength Raman shift, cm−1

234

316

1234

1540

Raman cross section, 10−23 cm2/molecule

0.245, ρ = 0.14

0.354, ρ = 0.17

0.653, ρ = 0.14

1.45, ρ = 0.55

Figure 5. Single TSPP aggregate resonance Raman spectra obtained from 10 different aggregates (left) and a false color epi-illuminated microscopy image of an aggregate excited with 488 nm light (top right). In the bottom right is a false color optical microscope image of an individual TSPP aggregate. The bright spot in this image is the excitation laser spot.

measured, the peak height of the 243 cm−1 mode varies somewhat with respect to that of the 316 cm−1 mode. Additionally the relative intensities of the high-frequency modes (698−1534 cm−1) to the low-frequency modes (243 cm−1 and 316 cm−1) vary significantly among different samples. Figure 6 shows that both the overall intensities and the relative intensities of various Raman modes, determined from the background-subtracted peak areas, are different for the spectra of the 10 sampled nanotubes shown in Figure 5. Figure 6a shows the intensities of the two low-frequency modes and the 1534 cm−1 mode for the 10 different aggregates expressed as histograms. If coherence or structural heterogeneity played no role in the Raman intensity, then we would expect the Raman intensities of each mode to vary in proportion to the number of bundled tubes. Figure 6b shows the intensities of these three modes compared to their values in sample 9, which exhibits the lowest Raman intensities of the 10 samples. It is apparent that the relative intensities of the three modes vary greatly from one aggregate to another. Of the three modes considered in Figure 6, only the 316 cm−1 mode displays intensities which vary by approximately integral multiples, as shown in Figure 6b. Figure 6c shows a histogram of the relative intensities of the three modes. While the relative intensities of the two lowest frequency modes are fairly constant from one aggregate to another (±10%), there is a larger variation in the relative intensity of either low-frequency mode to that of the

1534 cm−1 mode, more than ±20%. We conclude that while there are indications in Figure 6 that the number of bundled nanotubes probably varies for the 10 different probed regions local variations in aggregate structure and coherence may also contribute to different Raman intensities from different spots. The epi-illuminated image of a TSPP aggregate excited with 488 nm wavelength light in Figure 5 reflects the intensity of resonance light scattering (RLS) along the length of the nanotube. Variations in the RLS intensity arise from heterogeneity in the coherence number, with hot spots reflecting regions of enhanced coherence. To explore the effects of coherence versus bundling, we examined the Raman spectra from different spots along the length of a given nanotube. Figure 7 shows the RLS image of an individual aggregate and the Raman spectra obtained at the five spots indicated by the red circles in this image. Attempts were made to observe Raman spectra from hot spots located in the RLS image; however, the stage translation of the microscope requires manual adjustments, making precise aiming of the laser focus experimentally difficult. The laser spot diameter, as observed by optical images (see Figure 5), is approximately 500 nm. Nevertheless, it is apparent from Figure 7 that there are considerable variations in the Raman spectra obtained at these five different spots. The intensities of the seven prominent modes of each Raman spectrum are shown in the histogram in 10860



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Figure 7. (Left) Resonance light-scattering image of a TSPP aggregate (with 488 nm wavelength excitation) showing the places on the aggregate where Raman spectra (right) were measured.

Figure 6. (a) Single-aggregate resonance Raman intensities of 243 cm−1 (black), 316 cm−1 (red), and 1534 cm−1 (blue) modes measured at different aggregates; (b) data in (a) normalized to the intensity of the corresponding peaks in sample 9; (c) intensity ratios of the three Raman modes. The sample numbers correspond to the numbered spectra in Figure 5. The green lines in (b) address the approximate integer variance in the Raman peak intensity of the 316 cm−1 mode between samples.

Figure 8 as well as the intensity ratios of the two low-frequency modes and the 1534 cm−1 mode with respect to one another. The large intensities of the Raman modes for spot 1 appear to correlate with the overlap of an observed coherence hot spot, and the diminished intensities for spots 3 and 4 correspond with the observed diminished coherence in those spots. This would seem to suggest that in spots where effective coherence is large the Raman intensities, particularly of the low-frequency modes, will be strong. However, the large Raman intensities of spots 2 and 5, where there is no coherence hot spot, are inconsistent with this statement. Furthermore, the Raman intensity ratios of the low-frequency modes to 1534 cm−1 mode

Figure 8. Histograms of (top) the Raman intensities of the seven prominent modes at each spot on the aggregate in Figure 7 excited with 488 nm wavelength laser and (bottom) corresponding intensity ratios.

in Figure 8 do not show any particular trend between spots. In fact, spot 3, which exhibits low coherence in the RLS image, exhibits rather strong low-frequency modes with respect to the high-frequency mode. While it is possible that increases in 10861



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Figure 9. (Left) Resonance Raman spectra of the TSPP aggregate in Figure 7 measured at spot 1 with 0.75 mW of 488 nm wavelength laser light at the moment of exposure (black) and 92 s later (red). (Right) The time profile of the intensities of the 243, 316, and 1534 cm−1 modes over the course of the “high-power” exposure.

Raman intensity in spots with apparently weak coherence may be due to measurements of bundles of aggregate nanotubes, larger variation in the Raman intensities would be expected unless the number of nanotubes in a bundle was very large. However, AFM images reveal that the aggregates primarily form small bundles of nanotubes (see Figure S1 in Supporting Information), as suggested as well by the data in Figure 6. Our results do not support a clear correlation of coherence and Raman intensity enhancement. Rather, the variations in Raman intensity along the length of the nanotube probably reflect structural heterogeneity. Since water-mediated hydrogen bonds are hypothesized to maintain the nanotubular structure of TSPP aggregates,12 it is crucial to ascertain the effect on the coherence and the Raman intensities as water is forcibly driven away. To do this we have measured RLS images and Raman spectra of spot 1 of the aggregate in Figure 7 while exposing it to a 0.75 mW power 488 nm wavelength laser, rather than 0.75 μW as was used for typical single aggregate Raman experiments. Figure 9 shows the Raman spectra of the aggregates at the beginning and at the end of a 92 s exposure to this “high power” laser and the intensities of the 243, 316, and 1534 cm−1 modes measured every 0.5 s over the course of the exposure. It is evident that over this laser heating period the Raman mode intensities undergo decay but level off at certain intensities which may be indicative of a loss of resonance with the J-band due to disaggregation of the nanotubes into the diacid monomer, the hypothesized hierarchical subunit, or perhaps smaller assemblies of the circular N-mer. With weaker laser exposure used in previous experiments the intensity of Raman modes does not change with time. It is apparent that laser heating results in a great change in relative Raman intensities without a significant shift in the frequencies of the modes. Thus laser heating perturbs the self-assembly but does not result in degradation of the porphyrin molecules. Note also that the low-frequency vibration at 243 cm−1 would have shifted to 234 cm−1 if the laser heating had resulted in the formation of monomers. This suggests that the removal of water during laser heating leads instead to a subunit of the hierarchical aggregate, such as the cyclic N-mers that are presumed to be held together by electrostatic forces. In Figure 10 we compare the intensity ratios of the resonance Raman modes of a single aggregate excited with gentle laser

Figure 10. Histogram of the intensity ratios of the 243 cm−1:316 cm−1 modes (black), 316 cm−1:1534 cm−1 modes (blue), and 243 cm−1:1534 cm−1 (red) with gentle laser exposure (1) and high power laser exposure at t = 0 s (2) and t = 92 s (3) at spot 1 on the aggregate shown in Figure 7.

exposure (1) to those observed at the beginning (2) and end (3) of the laser heating experiment. After the initial rapid decay in the intensity of the Raman modes, there is no significant change in the relative intensities during the laser heating. Furthermore, the intensity ratios after laser heating differ from those obtained with gentler laser exposure, in that the relative intensities of the low-frequency modes are diminished by laser heating. This is consistent with the conclusion that laser heating results in partial disaggregation, which also results in loss of resonance enhancement and lower overall Raman intensities. Additionally, Figure 11 shows the RLS image of the aggregate before and after laser heating showing the vanishing coherence after the high power laser exposure. We suggest that these results reflect the important role of water in the assembly of the aggregate and its coherence, in accord with our previous conclusions from a study of the aggregate in deuterated environment: D2O/DCl.12 Figure 12 shows the results of using vertically (V) and horizontally (H) polarized incident light, with respect to the 10862



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spectroscopy. These off-diagonal components of α indicate vibronic coupling of the z- and x-polarized J-band excited states, which contribute to the observed depolarization ratio dispersion and relative intensity changes of the ensemble resonance Raman spectrum of the aggregate when probed with wavelengths spanning the J-band.12 While the intensities of the VH and HV spectra are nearly equivalent for the 243 cm−1 mode, the VH and HV intensities are different for the 316 and 1534 cm−1 modes. It is possible that similar magnitudes of αzx and αxz reflect increased delocalization for the 243 cm−1 mode, in accord with its stronger dependence on nanotube environment compared to the 316 cm−1 mode. Figure 11. RLS images of the aggregate in Figure 7 before (left) and after (right) laser heating at spot 1.

4. DISCUSSION Our measurement of the resonance Raman spectra of different individual aggregates and different spots on the same aggregate reveals variations in the absolute and relative intensities of the prominent modes. The changes in the relative intensity of the low-frequency modes compared to the high-frequency modes may suggest environmental perturbations to the aggregate and the degree of disassembly of the nanotube into constituent circular N-mers or porphyrin monomers. In ref 12, we reported that water serves a role in both the aggregate structure as well as in excitonic coupling and exciton−phonon coupling as shown by the large changes in the relative intensities of the lowfrequency modes and the intensity of the resonance light scattering for aggregates prepared in water and HCl compared to those prepared in D2O and DCl. In the same vein of thought the varying intensity ratios of the high- and low-frequency modes seen in this work may reflect heterogeneity among the aggregates in terms of exciton−phonon coupling depending on the dryness of the local environment of the aggregate and the degree to which the aggregates have collapsed due to the absence of water. The observed hot spots along the epiilluminated image of the aggregate may be an indication of localized areas of increased coherence which are reported by increased resonance light scattering intensity. It is also possible that these hotspots may be overlapping nanotubes, which may result in an increase in the resonance light scattering signal. The AFM images in Figure S1 (Supporting Information) show breaks in some of the aggregates which could limit coherence, potentially accounting for places where the RLS is weak. Since the resonance Raman spectrum in the laser heating experiment shows considerably more intensity from high-frequency modes than low-frequency modes compared to the gentler laser power experiments, we suggest that Raman spectra from aggregates which show relatively more intense high-frequency modes may indicate partial disaggregation. The lack of a frequency shift for the 243 cm−1 mode during laser heating suggests that removal of water does not result in porphyrin monomers but rather a structural subunit of the hierarchical assembly. The absolute Raman cross sections measured here are, to our knowledge, the first instance of quantitative Raman intensity comparison for an aggregate and its monomer. Comparison of the Raman cross sections of the diacid monomer and aggregate under similar detuning from resonance seems to suggest no aggregate-enhanced Raman scattering exists here. The dramatic enhancement of the low-frequency modes on resonance with the J-band absorption maximum is consistent with previous observations15,17,18 and might suggest some relation of intensities of low-frequency modes to the effective coherence of the aggregate. However, the lack of a clear correlation between intensities in the RLS image and relative intensities of

Figure 12. Polarized Raman spectra of an individual TSPP aggregate. V and H refer to the vertical and horizontal polarization, respectively, of the incident excitation (first letter) and scattered light (second letter) with respect to the orientation of the nanotube long axis as illustrated in the inset.

nanotube long axis, to excite an aggregate. As shown in the figure, the scattered light in each case was detected with polarization either parallel (VV and HH) or perpendicular (VH and HV) to the incident light polarization. The polarization ratios (VV:HV:VH:HH) for the 243, 316, and 1534 cm−1 modes are 5.4:2.4:2.4:1, 7.6:3.4:2.4:1, and 6.0:2.6:2.0:1, respectively. Referring to the long axis of the aggregate as the z direction, the VV spectrum selects for the zz component of the polarizability tensor and is the most intense of the four polarized spectra. However, the HH spectrum is also significant demonstrating the xx component of the polarizability, where x is perpendicular to the long axis of the aggregate and parallel to the surface. We presume for an intact helical nanotube aggregate that the x and y components of the J-band transition are degenerate. However, this is not the case for aggregates deposited on a surface since the nanotubes are usually flattened.11 A nonzero ratio of αxx to αzz is in accord with our previously determined depolarization ratios for the aggregate in solution. The values of (αxx/αzz)2 obtained here are in the range 0.13−0.18, in agreement with our previous study of TSPP aggregates deposited on Au(111).11 However, the nonzero intensities for the VH and HV polarizations reveal off-diagonal components of the polarizability αxz and αzx, which could not be determined in previous work using ensemble 10863



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aggregate at similar detuning. Polarized Raman spectra of an individual porphyrin nanotube proves the multicomponent nature of the J-band transition and reveals strong coupling of the parallel and perpendicular polarized J-band excitons. Though the finer details of the internal structure of the aggregate remain to be fully understood, our results support the idea that TSPP aggregates imaged in scanning probe microscopy are collapsed helical nanotubes for which the Jband is split into two closely spaced components. The standard cartoon of a J-aggregate as a staircase of planar porphyrin molecules is inconsistent with our Raman polarization data and with numerous previously reported scanning probe images. Combining the present results on individual TSPP aggregates with the information gathered from ensemble measurements and with theory that properly describes the excitonic coupling will lead to better understanding of light harvesting in these self-assembled nanostructures.

low-frequency Raman modes argues against this. This lack of correlation, despite clear connection of low-frequency mode intensity to self-assembly, may be a consequence of the effect of exciton−phonon coupling. Increased exciton−phonon coupling, which results in more intensity in resonance Raman, also results in diminished coherence.32 In ref 12, we compared the deuterated and protiated versions of the TSPP aggregate and found a larger RLS signal and weaker intensity of low-frequency modes from the former. Considering the much lower absolute Raman cross section of the low-frequency modes of the aggregate compared to those of the monomer when both are detuned from resonance, it is difficult to make an argument that Raman intensities are enhanced solely due to assembly. Rather, enhancement of Raman modes which are relevant to assembly (i.e., the low-frequency modes) occurs only when the excitation wavelength is resonant with the excitonic transition, as stated by Akins.15 The polarization ratios of the single aggregate resonance Raman spectra of TSPP aggregates reflect the multicomponent nature of the J-band transition as well as the hierarchical structure of the aggregate. This incomplete polarization of the J-band has been observed here and in other studies including ensemble resonance Raman spectra of the aggregate on Au(111),11 in resonance Raman data of the aqueous aggregate excited at wavelengths spanning the J-band,12 and in flowinduced linear dichroism.24,33 Partial polarization along the short axis must be attributed to a J-band which has more than one component. We argue that the J-band consists of two excitonic components as expected for a helical aggregate: one is a transition polarized parallel to the nanotube long axis and the other a doubly degenerate transition polarized along the nanotube short axis.

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ASSOCIATED CONTENT

S Supporting Information *

Figure showing the AFM image of TSPP aggregates deposited on mica. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the National Science Foundation (CHE1149013) is gratefully acknowledged.

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5. CONCLUSION As aforementioned, the goal of this work was to provide insight into the structural and electronic properties of the hierarchical TSPP aggregates without the hindrance of aggregate heterogeneity found in ensemble spectroscopic methods. In this Article, we have reported for the first time single aggregate resonance Raman spectra showing variations in the relative intensity of Raman modes reflecting the heterogeneity of the aggregate structure and of exciton−phonon coupling. A range of relatively large coherence numbers have been previously reported for the TSPP aggregate;14 for example, a value of about 500 was derived from Stark spectroscopy.34 Our findings show that this coherence varies within and among different aggregates, based on RLS images; however, these variations are not closely correlated with resonance Raman intensities. We speculate that variations in the relative intensity of low frequency arise from the role of water in the structure of the assembly and its influence on exciton−phonon coupling. The concept of aggregation-enhanced Raman scattering is valid only on resonance with the excitonic transition. Large enhancement of low-frequency modes when the excitation wavelength is tuned to resonance with the J-band absorption maximum is presumed to result from the intermolecular nature of the displacements of these modes and their expected coupling to the delocalized electronic transition. However, it is important to recognize that resonance with the excitonic state causes this enhancement of vibrational modes which are relevant to aggregate formation (i.e., out-of-plane distortions), rather than the assembly of the aggregate itself, as shown in the comparison of the resonant Raman cross sections of the monomer and

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Resonance Raman Spectra of Individual Excitonically Coupled

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