Hierarchal Light-Harvesting Aggregates and Their Potential for Solar

Perspective pubs.acs.org/JPCL

Hierarchal Light-Harvesting Aggregates and Their Potential for Solar Energy Applications Jeanne L. McHale Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, United States ABSTRACT: The tunable optical properties of self-assembled chromophores are exploited by photosynthetic organisms to optimize their ability to harvest a broad range of the solar spectrum. Similarly, the efficiency of solar photovoltaic and photoelectrochemical devices depends strongly on the coincidence of the absorption spectrum of the photoactive components with the spectrum of the sun. While the possibility of borrowing ideas about light-harvesting aggregates from nature in order to improve the efficiency of solar energy conversion is quite attractive, progress to date is hindered by incomplete understanding of aggregate internal structure and its relation to excitonic states. In this Perspective, we describe our recent work on the hierarchal structure of self-assembled porphyrin aggregates that are similar to light-harvesting complexes of photosynthetic bacteria. We address the question of whether aggregation can be beneficial to dye-sensitized solar energy conversion and present promising results for a solar cell based on an abundant plant pigment that displays signatures of aggregation when adsorbed on TiO2.

D

windling fossil fuel reserves and the dire consequences of climate change are presently motivating intense interest in economical and efficient solar energy conversion. While there is more than enough solar energy striking the earth to power the planet, the challenge is to capture and utilize this energy with devices based on cheap, earth-abundant materials and sustainable manufacturing processes. The TiO2-based dye-sensitized solar cell (DSSC)1 is often cited as a viable economical alternative to costly silicon-based solar cells. However, the maximum theoretical energy conversion efficiency of about 30% (the Shockley−Queisser limit for a single-junction solar cell) has never been achieved with a DSSC, owing in part to poor utilization of longer wavelengths of solar energy. In addition, frequent reliance on synthetic metal− organic sensitizing dyes mitigates the environmental and economic advantages of the DSSC, spurring our interest in the use of natural plant pigments as sensitizers.2,3 There is an essential dilemma in trying to extend the response of the sensitizing dye to longer wavelengths. The sensitizing dye, D, must absorb a photon and undergo a transition to an excited state D* for which the redox potential (D*/D+) is more negative than that of the semiconductor substrate in order to inject an electron into TiO2. Regeneration of the dye in its original oxidation state requires a redox mediator with a redox potential more negative than that of the D/D+ couple. The long-wavelength response of the dye could be improved by a modification that lowers the excited-state energy, but this would decrease the driving force for electron injection. On the other hand, raising the groundstate energy of the dye on an electrochemical scale could make dye regeneration unfavorable. Dye aggregation can broaden the absorption spectrum, which might be favorable for extending light harvesting in the DSSC. Unfortunately, aggregation may be accompanied by enhanced excited-state decay, which competes with electron injection into the semiconductor.4 Aggregation is therefore generally found to be © 2012 American Chemical Society

Dye aggregation can broaden the absorption spectrum, which might be favorable for extending light harvesting in the DSSC. detrimental to photoconversion efficiencies, and efforts are made to avoid it using coadsorbents and structural modifications.5−7 There are, however, cases in the literature where aggregation was found to be beneficial to solar photoconversion performance.8−11 Perhaps the varying effects of aggregation on the efficiency of solar devices stem from the existence of different kinds of aggregates, for example, strongly excitonically coupled assemblies characterized by delocalized excited states versus more weakly coupled, heterogeneous aggregates. The former may show exchange-narrowed absorption bands, while more disordered aggregates exhibit broadened absorption spectra. However, while dye-sensitized solar energy conversion is often likened to photosynthesis, the comparison fails when one considers that in the DSSC, the same molecule that does the job of light harvesting also performs the primary step of electron injection. In contrast, an amazing 99% of the chlorophyll molecules of plants are devoted to light harvesting, while only 1% participate in electron transfer. If we were to borrow more heavily from nature, then perhaps dye aggregates would be advantageously applied as energy relays that, like the light-harvesting complexes of photosynthetic organisms, funnel energy to the redox-active molecules. There have been some Received: January 16, 2012 Accepted: February 9, 2012 Published: February 9, 2012 587

dx.doi.org/10.1021/jz3000678 | J. Phys. Chem. Lett. 2012, 3, 587−597

The Journal of Physical Chemistry Letters

Perspective

Here, μge is the magnitude of the transition dipole moment, û1,2 is a unit vector in the direction of this dipole moment for molecule 1 or 2, r is the distance between the molecular centers, and r̂ is a unit vector in the direction of aggregation. When the coupling V12 is positive, the aggregate electronic transition should be blue-shifted from that of the monomer (in so-called H aggregates). For example, as shown in Figure 1, this happens when planar chromophores are stacked like a staircase with their transition dipole moments aligned perpendicular to the direction of aggregation. In contrast, V12 is negative, and red-shifted transitions result when these moments make an angle of θ < 54.7° with respect to the aggregation direction, as in J aggregates. These predictions may be erroneous, however, owing to the presence of site shifts22 and the failure of the point-dipole approximation,23 but in porphyrins, the nomenclature can be potentially quite misleading. The strong Soret band is doubly degenerate with two equal and orthogonal inplane transition dipoles; thus, a single aggregate structure can give rise to both a blue-shifted and a red-shifted transition, which will be referred to here as the H band and the J band, respectively. An additional concern is that porphryins may be considerably nonplanar, which precludes the formation of the sort of linear staircase array shown in Figure 1, often used to depict a J aggregate. For the diacid forms of tetraphenylporphyrin derivatives of interest here, protonation of the porphyrin core results in steric hindrance of the pyrrolic N−H groups. This leads to nonplanarity (ruffling, saddling, etc.) as large as 30°, while at the same time, the phenyl groups at the meso positions tend to rotate to become more nearly coplanar with the mean plane of the macrocycle.24−26 Even more interesting to consider is the possibility that porphyrin flexibility permits the conformation to change upon formation of a LHA. These subtleties are evidenced by our surprising results from the study of LHAs of meso-(tetracarboxyphenyl)porphyrin, TCPP,27,28 which forms an aggregate in acidic aqueous solution for which the optical properties and structure depend on the counterion. In acidic ethanol, the H2TCPP2+ monomer exhibits a Soret band at 440 nm. As shown in Figure 2, the LHA of TCPP in aqueous HCl, however, displays a blue-shifted Soret band at 417 nm, while in aqueous HNO3 the Soret band is split into both blue- and red-shifted components at 406 and 467 nm. At the pH ∼ 1 at which the LHA forms, the diacid H2TCPP2+ bears a net positive charge owing to the protonation of the porphyrin core. In ref 27., we speculated that in the presence of Cl− counterions the mean porphyrin planes are stacked like plates, which should result in an allowed blue-shifted transition. With NO3− counterions, however, a slipped near-neighbor alignment of the porphyrin planes, such as that sketched in Figure 1, should lead to a splitting of the Soret band into H and J components, both of which are allowed. Despite the simplicity of the TDC model on which these ideas were based, subsequent structural studies using atomic force microscopy (AFM) seem to provide some support for these speculations.28 As shown in Figure 2, we found that the aggregates of TCPP deposited from aqueous HCl were imaged as nanodonuts and spirals with heights of 2 to 4 nm, some larger, and with variable diameters ranging from hundreds of nanometers to several micrometers across. Deposited from aqueous HNO3, on the other hand, the TCPP aggregates are observed to be nanorods about 3 to 4 nm in height and tens of nanometers in width. For the former case, the large radii of curvature observed are indeed consistent with near neighbor arrangements in which one porphyrin is stacked on top the other with a negligible angle

If we were to borrow more heavily from nature, then perhaps dye aggregates would be advantageously applied as energy relays that, like the light-harvesting complexes of photosynthetic organisms, funnel energy to the redox-active molecules. efforts to exploit biomimetic supramolecular assemblies for dye-sensitized solar energy conversion.12−14 Better understanding of the relationship between structural and electronic properties of chromophore aggregates would further the goal of exploiting them to enhance solar photoconversion efficiencies. The light-harvesting aggregates (LHAs) of photosynthetic bacteria provide excellent examples of the structural richness of chromophore assemblies in nature. In purple photosynthetic bacteria, X-ray crystallography reveals LHAs to be ring-shaped assemblies of bacteriochlorophylls associated with accessory pigments and a protein scaffold.15−17 The internal structure of rod-shaped chlorosomes, the LHAs of green photosynthetic bacteria, is less certain,18−20 being not amenable to X-ray diffraction. Though there is evidence for certain kinds of intermolecular forces (hydrogen bonding and metal−ligand coordination) that influence the nearneighbor arrangements in chlorosomes,21 the challenge is to understand how these interactions lead to the overall size and shape of the assembly. In what follows, it is shown how similar questions arise in studies of LHAs of synthetic porphyrins, close cousins to the chlorophyll and bacteriochlorophyll workhorses of photosynthesis. Counterion-Dependent Porphyrin Aggregate: TCPP. Aggregation of porphyrins, particularly in aqueous solution, is well-known and often first surmised on the basis of perturbations to the optical spectrum. We are interested here in noncovalent assemblies in which strong electronic interactions result in perturbed, delocalized excited electronic states despite only weak perturbations to the ground electronic state. Simple theoretical approaches to excitonic transitions in LHAs, based on perturbation via coupling of transition dipole moments, enable the basis for blue-shifted versus red-shifted electronic transitions to be understood, as shown in Figure 1 for a LHA in

Figure 1. Arrangement of transition dipoles leading to red-shifted (J band) and blue-shifted (H band) transitions in a linear staircase LHA .

which molecular planes are arranged as in a staircase. The transition dipole coupling (TDC) V12 between molecules 1 and 2 in the aggregate decides the sign of the spectral shift. V12 =

μ2ge r3

[u1̂ ·u2̂ − 3(u1̂ ·r )( ̂ u2̂ ·r )] ̂

(1) 588



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Figure 2. Optical spectra (left) and AFM images (right) of H2TCPP2+ aggregates with (a) NO3− and (b) Cl− counterions. Adapted from refs 27 and 28.

between their mean molecular planes. However, the question remains as to why a variety of radii are observed, with some nanostructures forming closed rings and others spirals. The observation of a nanorod in the presence of the nitrate counterion might seem to provide evidence for a linear staircase structure, but the heights are on the order of twice the dimension of the porphyrin. Further, the nonplanarity of H2TCPP2+ is incompatible with the formation of straight nanorods that derive from a model like that of Figure 1. At the time of these earlier studies, we did not appreciate the importance of the porphyrin shape in dictating the structure of the LHA. The next section summarizes results for a related porphyrin derivative, TSPP, for which additional details on the internal structure were uncovered. It will be seen that these results shed further light on the structural intricacies of the TCPP LHAs. Hierarchal Structure of the TSPP Aggregate. The aggregation of meso-tetra(para-sulfonatophenyl)porphyrin (TSPP) in acidic aqueous solution has been widely studied.29−31 As in the case of TCPP, aggregation is preceded by the formation of the diacid from the free base, as shown in Scheme 1. In contrast to TCPP,

Figure 3. Optical absorption spectra of TSPP free base (FB, blue) and diacid (DA, green) monomers and the LHA (red), the latter showing the H and J bands at 420 and 490 nm, respectively. The inset shows the changes to the Q bands on an expanded scale.

about the spectral perturbations that ultimately provide insight into the structure and prove the staircase cartoon to be invalid. For one, the increased intensity of the Q bands upon going from the free base to the diacid, and more so upon going from the diacid to the LHA (see inset to Figure 3), suggest symmetry-lowering distortions that increase the intensity of the Q bands at the expense of the Soret (or “B”) band. Ruffling and saddling distortions of the porphyrin core have the capacity to cause this B−Q intensity borrowing.34 Using scanning tunneling microscopy (STM), we have obtained experimental confirmation of the saddled geometry of the TSPP diacid monomer adsorbed on graphite.35 In addition, there is strong overlap of the blue-shifted (H band) component of the LHA at 420 nm with the residual monomer peak at 434 nm, despite the larger red vshift of the J band to 490 nm. Also, the integrated intensity of the H band is much less than that of the J band, and experiments show that the latter exhibits a larger coherence number Nc,36 which is the number of molecules over which the excited state is delocalized. If the simple linear staircase model were correct and the transition moments parallel and perpendicular to the direction of aggregation were equal, the Soret band would be split into equal intensity H and J bands.

Scheme 1. Conversion of TSPP Free Base to Diacid by Protonation of the Porphyrin Core

the strong acidity of the sulfonic acid group results in a negatively charged zwitterion upon formation of the diacid at a pH below about 5, with protons serving as counterions. As the pH is lowered further, or ionic strength is increased, the diacid H2TSPP2− self-assembles into LHAs, which, upon deposition, are imaged as nanorods by AFM.32,33 The splitting of the Soret band into H and J components upon aggregation, as seen in Figure 3, tends to support a conventional J aggregate picture, and indeed, variations on the linear staircase cartoon are ubiquitous in the literature. However, there are several puzzles 589



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Further, the larger red shift of the latter compared to the blue shift of the former, when based on the original monomer absorption energy, suggests a site shift to the red, which may correlate to the increased intensity of the Q band, that is, the conformation of the porphyrin in the LHA differs from that of the monomer precursor. According to our resonance light scattering experiments,36 the Q band does not undergo significant excitonic coupling, and we have speculated that vibronic borrowing results in the enhanced Q band intensity of the LHA, permitted in the lowered site symmetry. Thus, optical spectra of the TSPP LHA do not support a simple linear aggregate as is often depicted. Though such a model is expected to optimize the electrostatic interaction between the positively charged porphyrin core and the negatively charged sulfonato groups, early light-scattering data suggested the presence of structure on several different length scales,37,38 possibly fractal in nature as in other porphyrin aggregates.39 Micali et al.37 interpreted their dynamic light-scattering data in terms of large (1−1.5 μm), medium (100−200 nm), and small (3−6 nm) aggregates, the latter having an aggregation number between 6 and 32. Others have noted that the TSPP aggregate exhibits a hierarchal structure,40 but the path from the near-neighbor interactions, on which eq 1 is based, to the overall mesoscale morphology has not been straightforward.

Figure 4. (a) AFM (b) STM images of TSPP LHA deposited from aqueous HCl on Au(111). High-resolution STM images (c−e) show the 6 nm disks that comprise the flattened nanotubes and the effects of repeated probing. An intact nanotube with a split side is seen in (f). Scale bars are 250 nm in (a) and (b), 30 nm in (c−e), and 20 nm in (f). Adapted from refs 36 and 43.

twinning and bundling also observed. (Apparent nanorod widths from AFM are somewhat larger.) However, high-resolution STM images of some nanorods reveal them to be composed of 6 nm diameter disks, about 2 nm in thickness. These disks are disaggregated by repeated probing with the tip (Figure 4c−e) and are seen to litter the surface in some images. Occasionally, as shown in Figure 4f, intact nanotubes are seen. Taken with the fact that the imaged nanorod thicknesses are twice the shell thickness of the nanotubes reported in refs 41 and 42, it is apparent that the TSPP LHAs are actually nanotubes that are flattened on the substrates used for AFM and STM. RRS provides insight into the symmetry and degeneracy of the resonant excited electronic states, which are in turn related to the physical structure of the LHA. Figure 5 shows RR spectra of the

The splitting of the Soret band into H and J components upon aggregation tends to support a conventional J aggregate picture, and indeed, variations in the linear staircase cartoon are ubiquitous in the literature. However, there are several puzzles about the spectral perturbations that ultimately provide insight into the structure and prove the staircase cartoon to be invalid. As in the case of the TCPP aggregate, the ∼4 nm height of the putative TSPP nanorods imaged in AFM and their consistent widths of about 30 nm belie the simple staircase model. Rotomskis et al.33 proposed a model for the TSPP LHA in which a single strand of the linear staircase curls around to form a tube that is then flattened on the substrate and imaged as a rod. X-ray scattering of the solutionphase aggregate was interpreted in terms of a tubular structure in which the porphyrin planes are arranged perpendicular to the long axis of the tube,41 which would result in a tube with a ∼2 nm thick shell. This is in agreement with recent results from cryoelectron microscopy that found intact tubes with a 2 nm shell thickness.42 In collaboration with the group of Ursula Mazur, we have recently used STM and resonance Raman spectroscopy (RRS) to investigate the internal structure of the TSPP LHA.35,36,43,44 As seen in Figure 4, the results were quite surprising. Our own AFM and low-resolution STM images confirm the nanorod structures observed in previous studies, having consistent heights and widths of about 4 and 25−27 nm, respectively, with

Figure 5. Resonance Raman spectra excited at 488 nm of the TSPP LHA. (a) Polarized and depolarized spectra in aqueous HCl, (b) SS and SP spectra of the LHA deposited on Au(111), and (c) lowfrequency modes of the LHA on Au(111) for PP, SS, PS, and SP polarizations. Adapted from refs 36 and 43.

TSPP LHA in solution and on the same substrate used for STM imaging, Au(111), obtained upon resonance with the J band. The spectra are dominated by the low-frequency vibrations at ∼240 and 315 cm−1, often referred to as the ruffling and doming modes, respectively, though their precise assignments are uncertain. Depolarization ratios ρ = Idep/Ipol represent the ratio of the intensity of scattered light with polarization perpendicular to that of the incident radiation (Idep, the depolarized spectrum) to that which is parallel (Ipol, the polarized spectrum). For totally 590



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understand these results, we turn next to our model of the internal structure of this aggregate. Any model for the TSPP LHA must account for the occurrence of the constituent 6 nm disks, their 2 nm thickness, and the consistent widths of isolated nanotubes, which are ∼26 nm wide according to STM. Figure 7 explains the model that has emerged from our spectroscopic and imaging studies. We propose that the 6 nm disks imaged in STM are cyclic N-mers (Figure 7b) driven to assemble by electrostatic interactions between protonated pyrroles and −SO3− groups of neighboring molecules. These Nmers are reminiscent of the BChl 850 rings of photosynthetic bacteria. In the simple model, as we presented in ref 43, a closed ring of N molecules results when the nonplanarity α is an integral divisor of 2π. As a working hypothesis, N is taken to be 16, consistent with a reasonable nonplanar distortion of 21° and with the formation of 6 nm diameter disks. If no further assembly took place, these putative 16-mers would exhibit a coherence number of 16, and the Soret band would be split into a doubly degenerate J band polarized in the plane of the disk and a perpendicularly polarized nondegenerate H band. To form a nanotube, the disks are first pictured to assemble into a hexagonal sheet, as depicted in Figure 7e, which shows the J band transition moments polarized in the plane of the sheet. This sheet is then rolled along a circumference vector for which the length (54 nm) is consistent with twice the width of the flattened tubes and results in a nanotube radius of 8.6 nm. The result is the helical nanotube sketched in Figure 7f, where the green beads represent porphyrins. The fixed angle of 140° between the planes of adjacent disks in the helix determines the fixed radius of the nanotubes and is perhaps a consequence of hydrogen bonding between hydrated sulfonato groups on adjacent disks. The thickness of the nanotube shell, about 2 nm, corresponds to the size of the porphyrin and is consistent with data from cryoelectron microscopy42 and smallangle X-ray scattering,41 as well as with the ∼4 nm height of the flattened nanotubes when imaged in both AFM and STM.43 This model captures the structural features revealed in our images, but unfortunately, molecular-scale structural information is obscured not just by limited resolution but also by the coherent coupling among the monomers within the N-mers and by further weaker coherent coupling among the N-mers. Because STM images conductivity rather than topography and the conductivity is mediated by the LUMO of the porphyrin,44 which is delocalized over the aggregate, individual disks are imaged only when the forces holding them together in the nanotube are perturbed. Further, we do not observe a cavity in the imaged disks possibly because of the contribution of water, which appears to play a major role in the aggregation of TSPP.46,47 We posit that the hierarchal structure of TSPP is tied to the large variations in reported coherence numbers, which range from about 1127 to as large as 500.48 The helical nanotube model is well-aligned with our RRS results for J band excitation. Using the simple TDC model, the ∼3400 cm−1 splitting between the H and J bands results mostly from the strong excitonic coupling within the N-mer, that is, the geometry of the putative 16-mer supports a splitting of a few thousand cm−1 according to eq 1. Upon assembly into the nanotube, further excitonic coupling among the N-mers results in a splitting of their J band transitions into longitudinal and transverse helical excitons.49 The former is a nondegenerate transition giving rise to the zz component of the polarizability tensor, while the transversely polarized doubly degenerate transition results in αxx = αyy, accounting for our polarized RRS results. The transition dipole moment of the N-mer is amplified by

symmetric modes, which are the most strongly enhanced in RRS, values of ρ are expected to be 1/3 for resonance via a single nondegenerate excited electronic state and 1/8 for resonance via a doubly degenerate state. (These rules apply to randomly oriented molecules.) If the Soret band were merely split into perpendicularly polarized H and J bands, as expected for the simple model of Figure 1, we should have seen depolarization ratios equal to 1/3. However, we found ρ to be on the order of 0.5 for the ruffling and doming modes when the excitation wavelength was 488 nm, resonant with the J band. Further, as shown in Figure 6, depolarization ratios of Raman modes are a strong function of excitation wavelength.

Figure 6. Depolarization ratios of Raman modes of the TPPS LHA in aqueous HCl at various excitation wavelengths spanning the J band. The curves are a guide for the eye. Reproduced from ref 45 by permission of The Royal Society of Chemistry.

In the case of the LHA on the gold surface, the RR spectra were obtained for four combinations of the incident and scattered light, SS, PP, SP, and PS, where the first letter indicates the incident and the second the scattered light polarization, which is either perpendicular (S) or parallel (P) to the scattering plane. In this case, surface selection rules enable the determination of the relative magnitudes of the three components of the diagonal polarizability tensor. As discussed in ref 36, the polarized RRS data for both the solution-phase and gold-deposited LHA are consistent with a polarizability tensor with axial symmetry, αxx = αyy ≠ αzz, where the z direction is the long axis of the nanotube. The symmetry of the Raman tensor reflects the directions of the transition moments of the resonant states. Thus, the RRS data suggest that the J band consists of two closely spaced transitions, one of which is doubly degenerate and polarized perpendicular to the nanotube axis and the other a nondegenerate transition polarized along the axis. As explained below, this is consistent with the formation of a helical nanotube. Further evidence for the composite nature of the J band derives from our observation of depolarization ratio dispersion for excitation wavelengths within the J band, as shown in Figure 6.45 Dependence of ρ on the excitation wavelength is a signature of resonance via more than one excited electronic state. Further, as reported in ref 45 and shown below in Figure 8, the relative Raman intensities also vary strongly with excitation wavelength, also indicative of resonance via overlapping electronic transitions. To 591



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Figure 7. Formation of a helical nanotube from cyclic N-mers. The monomer diacid (a) with Soret band transition moments μge,x and μge,y is assembled into a cyclic N-mer (b). The μge,x transition moments aligned parallel to the plane of the N-mer lead to a doubly degenerate, red-shifted J band (c), while the μge,y moments perpendicular to the plane give rise to a nondegenerate H band (d) of the N-mer. The N-mers are envisioned to stack in a two-dimensional hexagonal array (e) with a thickness of ∼2 nm. The sheet (e) is then rolled up along the circumference vector C to form the nanotube depicted in (f), where z is the long axis and the green beads represent porphyrin molecules. The resulting helical nanotube (f) can be compared to the STM image shown in (g). Adapted from refs 43 and 45, the latter by permission of the Royal Society of Chemistry.

Figure 8. RR spectra of the TSPP-h and TSPP-d aggregates in HCl/H2O and DCl/D2O solutions, respectively, obtained at six excitation wavelengths spanning the J band. Adapted from ref 45 by permission of The Royal Society of Chemistry.

a factor of N1/2 over that of the monomer, but because the N-mers are more widely separated in the nanotube than are adjacent monomers in the N-mer, the expected value of V12 for inter-N-mer coupling is reduced by an order of magnitude. The result is a splitting of a few nanometers between the longitudinal and transverse components of the nanotube J band, leading to interference effects in RRS data for excitation wavelengths spanning this band. As shown in Figure 8, these interference effects manifest themselves as relative Raman intensities that vary with excitation wavelength. Like the depolarization ratio dispersion shown in Figure 6, this is a signature of overlapping resonant excited states. If the J band were associated with a single excited

electronic state, depolarization ratios and relative intensities of Raman modes would not vary greatly with excitation wavelength. In addition, we observe a background assigned to J band fluorescence, which lends intensity to overlapping Raman modes. In addition to accounting for our own Raman data, the model of Figure 7 also addresses an array of previously puzzling literature data on the TSPP LHA. The observed slightly blurred isosbestic point, for example, in spectra taken as a function of pH,27 ionic strength,29 or concentration,36 is understood because this spectrum is largely decided by the coupling within the N-mer. Also, the observation that the J band transition moment is incompletely aligned in flow-induced linear dichroism 592



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Figure 9. Absorption spectra (a), 488 nm resonance Raman spectra (b), and RLS spectra (c) of TSPP LHA in HCl/H2O (black) and DCl/D2O. Adapted from ref 45 by permission of The Royal Society of Chemistry.

experiments30,42 and other poled samples50 is a natural consequence of the overlapping, orthogonally polarized excited states, rather than incomplete alignment of the nanotubes, as has been assumed. The helical aggregate model can account for the observation of circular dichroism in vortex-stirred solutions.51,52 Reported coherence numbers might vary from a minimum corresponding to the size of the presumed cyclic N-mer to larger values, dependent on preparation, that reflect structural order on a larger scale. If the apparent exchange narrowing of the J band is used, where it is assumed that the width is reduced by a factor of Nc1/2, the estimated value of Nc will surely be too small because the J band is split into two unresolved components. Estimates of Nc of 1127 and 1631 have been obtained this way, while much larger numbers are obtained from Stark effect measurements of the excited-state polarizability.48 To pursue the speculation that water-mediated hydrogen bonds are responsible for the assembly of the N-mers into helical nanotubes, we investigated the TSPP aggregate in D2O with aggregation promoted by DCl (TSPP-d) in comparison to the aggregate in HCl/H2O (TSPP-h).45 In the DCl/D2O environment, the four protons in the core of the macrocycle are replaced by deuterons, as evidenced by red shifts in Raman modes involving these atoms. AFM reveals that the structure of the TSPP LHA deposited from DCl/D2O is negligibly different from that obtained from HCl/H2O; however, spectroscopic data do depend on isotopic substitution. Figure 9 summarizes some of our key results. Upon careful comparison of the absorption spectra (Figure 9a), that of TSPP-d shows a J band with a slightly larger half-width (408 cm−1) compared to that of TSPP-h (343 cm−1). Though isotopic substitution has only a small effect on the relative Raman intensities of the monomer diacid, the J band resonance Raman spectra of TSPP-h and TSPP-d (Figure 8 and Figure 9b) are very different. Specifically, the low-frequency ruffling and doming modes are more intense relative to higher-frequency Raman modes for TSPP-h than those of TSPP-d, indicating relatively more excited-state reorganization along these out-of-plane modes in TSPP-h. Thus, despite the similar ground-state structures of the TSPP-h and TSPP-d aggregates, their excited-state structures and dynamics, which influence RRS intensities, are very different. Also interesting, the resonance light-scattering spectrum (RLS), as shown in Figure 9c, is twice as intense for TSPP-d as that for TSPP-h, indicating that deuteration increases the coherence number of the excited electronic state by a factor of about 1.4. RLS signals are evidence of delocalized excited electronic states; therefore, we are challenged to understand why the TSPP LHA

should exhibit more delocalization in the deuterated than in the protiated environment. The value of Nc increases with the strength of the transition dipole coupling V12.53 These RLS and RRS data for TSPP-d and TSPP-h are understood within our structural model if the coupling among cyclic N-mers in the nanotubes is greater for the deuterated LHA, resulting in greater splitting of the unresolved longitudinal and transverse excitons and hence an increase in the apparent width of the J band. It is possible that deuteration increases the strength of the hydrogen bonds that are presumed to hold the cyclic N-mers together in the nanotube. Dependence of the coherence number upon isotopic substitution might also be a consequence of exciton−phonon coupling,54,55 which tends to decrease Nc and becomes more important for vibrational modes that are more strongly coupled to the transition.52 These more strongly coupled modes are the ones that are more intense in the RR spectrum. The apparently larger dimensionless displacements (RRS intensities) for the lowfrequency modes of TSPP-h compared to those of TSPP-d might translate into reduced coherence of the former as a result of increased exciton−phonon coupling. To our knowledge, this is the first report of an isotope effect on excitonic coupling. It has been concluded that intermolecular hydrogen bonds contribute to excitonic pathways in chlorosomes,19 and perhaps, they do so as well in the TSPP LHA. Plant Pigments for Dye-Sensitized Solar Energy Conversion: Betanin. In dye-sensitized solar energy conversion, the use of natural plant pigments in place of synthetic dyes would greatly reduce manufacturing costs. Chlorophyll and anthocyanins are two examples of plant-based sensitizers that have been tried, but reported energy conversion efficiencies are less than 1%.56−59 We have recently investigated a class of plant pigments known as betalains (Scheme 2) for their potential as dye sensitizers2,3 and have reported the largest energy conversion efficiency (2.7%) obtained to date with an unmodified natural dye sensitizer. The betalains are a family of compounds found in plants of the order Caryophyllales, having a similar biological role as the anthocyanins, namely, to serve as photoprotectors and antioxidants. Two classes of betalains are found, as shown in Scheme 2, the reddish-purple betacyanins and the yellow betaxanthin pigments. We extracted and purified an example of the former, betanin, from red beet roots and used it as the sensitizer in a conventional dye-sensitized solar cell.3 The results were remarkable in that extremely good currents were obtained, as high as 14 mA/cm2 under 100 mW/cm2 illumination approximating AM 1.5 radiation. However, maximum 593



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example, double bond formation, occurring in a single step. Alternatively, a one-electron oxidation product of betanin could have a redox potential more negative than that of the TiO2 conduction band edge. This would be similar to the currentdoubling mechanism observed during photo-oxidation of alcohols by band gap excited TiO2.60,61 Another possibility is that this is a case of intermolecular singlet fission, the production of two triplet states from a single excited singlet state.62,63 This sort of singlet fission takes place in synthetic62 and natural64 carotenoid assemblies but has not yet been realized in a solar cell. The viability of each of these explanations depends on certain energetic constraints, for example, the redox potential of betanin on TiO2 in its excited singlet and triplet states and their relative energies. Future electrochemical and spectroscopic studies of betanin on TiO2 are planned to address these speculations. Also in progress is work to determine the mechanism for spectral broadening of betanin upon adsorption on TiO2. LHAs of synthetic porphyrin dyes are excellent model systems for understanding how the molecular structure determines the morphology and properties of the assembly. It has been proposed that the flexibility of chlorophyll and bacteriochlorophyll molecules has a biological role.65 For example, different degrees of distortion for the two BChl b molecules of the special pair of the R. viridis reaction center and resulting differences in the electronic structure may be responsible for unidirectional electron transfer.66 Studies of synthetic chlorosome analogues show that minor changes in the molecular structure influence the morphology on a micrometer length scale.67 We suspect that the counterion-dependent TCPP LHAs discovered in our lab27,28 assume different shapes because of the effect of the counterion on the nonplanarity of the porphyrin.24 In the presence of Cl− counterions, the appearance of wheels and spirals with different radii may reflect a distribution of molecular conformations. The large site shift and spectral perturbations in the case of the TSPP LHA may be hinting at a change in conformation upon aggregation, beyond that which accompanies the formation of the diacid monomer precursor. This presumed change in molecular geometry may be tied to the loss in intensity of the H band and its weaker intensity compared to the J band. Efforts to model the line shapes of the TSPP H and J bands including vibrations are currently underway and will shed light on this question.

Scheme 2. (a) Generic Structure of a Betacyanin, where R1 = β-D-glucose and R2 = H for betanin, and (b) Indicaxanthin, an Example of a Betaxanthin

photovoltages were rather low, suggestive of recombination-limited performance.

In dye-sensitized solar energy conversion, the use of natural plant pigments in place of synthetic dyes would greatly reduce manufacturing costs. Figure 10a shows the absorption spectrum of the betanin in solution and adsorbed on TiO2. The increased breadth of the spectrum of the adsorbed dye is indeed favorable to performance, as shown by the wavelength-dependent incident photon-to-current conversion efficiencies, IPCE(λ), shown in Figure 10b. The spectral changes of the TiO2-adsorbed dye could be the result of aggregation or could just as well stem from heterogeneous dye adsorption made possible by the existence of three carboxylic acid anchoring groups. In any case, these spectral changes are apparently not detrimental to IPCE(λ), the shape of which overlaps the absorption spectrum quite well, as shown in the inset to Figure 10b. In fact, the maximum IPCE approaches unity, despite being uncorrected for losses from reflection by the conductive glass and absorption by the electrolyte. Though much more data is needed, our results are tantalizing evidence that betanin is capable of injecting more than one electron per incident photon. The basis for this apparent multielectron injection is currently under investigation, and one can speculate on several different explanations for this unexpected phenomenon. Though mechanistically improbable, we cannot rule out the occurrence of two-electron, two-proton redox chemistry, for

Figure 10. (a) Absorption spectrum of betanin in aqueous solution (red) and adsorbed on a nanocrystalline TiO2 film (black). (b) IPCE(λ) for three different betanin-sensitized DSSCs. The inset compares the IPCE(λ) to the scaled absorption spectrum of betanin on TiO2. Reprinted from ref 3, copyright 2011, with permission from Elsevier. 594



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Christopher Rich and Candy Mercado, and colleagues Benjamin Friesen, K. W. Hipps, and Ursula Mazur.

Concerning the influence of aggregation on dye-sensitized solar energy conversion, it does not appear possible to say that it is always bad or always good. Literature suggests that the formation of amorphous aggregates on TiO2 introduces detrimental quenching of the excited electronic state or perhaps decreases electronic coupling to the semiconductor.68 In contrast, a study of a series of chalcogenoxanthylium dyes as sensitizers concluded that H aggregation improves IPCE, perhaps by enhancing electron injection to TiO2.9 More reminiscent of photosynthesis would be DSSCs that use LHAs to direct energy to the interface where charge separation takes place. Balaban et al.12,69 have endeavored to prepare such biomimetic DSSCs sensitized by Zn porphyrins that mimic chlorosomal bacteriochlorophylls. Rather small yields of chargeseparated states were attributed to residual monomer and inefficient orientation of the porphyrin self-assemblies. One of the challenges of using chromophore assemblies in DSSCs based on nanoporous TiO2 is that pore size and surface effects can restrict or perturb aggregation.70 If using LHAs as energy relays, the exciton diffusion length is critical, and this too is dependent on the ordering within the assembly.71 Porphyrin LHAs such as that of TSPP may be able to address some of these problems by virtue of their hierarchal structure. While the strongly coupled putative cyclic N-mers bear some resemblance to the cyclic light-harvesting complexes of purple photosynthetic bacteria, the helical nanotubes formed from them are more akin to the rod-shaped chlorosomal LHAs found in green photosynthetic bacteria. Evidence that the N-mer units are held together by water-mediated hydrogen bonds inspires efforts to control the solvent environment and the texture of nanostructured TiO2 in order to optimize exciton diffusion and the alignment of LHAs. Future experiments of this nature are planned. We have presented the first studies of the betalain plant pigments as dye sensitizers and though results are preliminary, the spectral changes upon adsorption on TiO2 may evidence aggregation. These spectral changes greatly extend the wavelength range of light harvesting, and good photon-tocurrent conversion yields are obtained across this range. Ongoing studies are pursuing the question of aggregation and attempting to understand the molecular details of the interfacial electron transfer and our observed high IPCE values.

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(1) O’Regan, B.; Grätzel, M. A. Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Titanium Dioxide Films. Nature 1991, 353, 737−40. (2) Zhang, D.; Lanier, S.; Downing, J. A.; Avent, J.; Lum, J.; McHale, J. L. Betalain Pigments for Dye-Sensitized Solar Cells. J. Photochem. Photobiol., A 2008, 195, 72−80. (3) Sandquist, C.; McHale, J. L. Improved Efficiency of BetaninBased Dye-Sensitized Solar Cells. J. Photochem. Photobiol., A 2011, 221, 90−97. (4) Kay, A.; Grätzel, M. Artificial Photosynthesis. 1. Photosensitization of Titania Solar Cells with Chlorophyll Derivatives and Related Natural Porphyrins. J. Phys. Chem. 1993, 97, 6272−6277. (5) Taty, S.; Haque, S. A.; O’Regan, B.; Durrant, J. R.; Verhees, W. J. H.; Kroon, J. M.; Vidal-Ferran, A.; Gaviña, P.; Palomares, E. Kinetic Competition in Liquid Electrolyte and Solid-State Cyanine Dye Sensitized Solar Cell. J. Mater. Chem. 2007, 17, 3037−3044. (6) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V. Controlling Dye (Merocyanine-540) Aggregation on Nanostructured TiO2 Films. An Organized Assembly Approach for Enhancing the Efficiency of Phtotosensitiztion. J. Phys. Chem. B 1999, 103, 4693− 4700. (7) Wang, Z. S.; Cui, Y.; Dan-oh, V.; Kasada, C.; Shinpo, A.; Hara, K. Thiophene-Functionalized Coumarin Dye for Efficient Dye-Sensitized Solar Cells; Electron Lifetime Improved by Coadsorption of Deoxycholic Acid. J. Phys. Chem. C 2007, 111, 7224−7230. (8) Ehret, A.; Stuhl, L.; Spitler, M. T. Spectral Sensitization of TiO2 Nanocrystalline Electrodes with Aggregated Cyanine Dyes. J. Phys. Chem. B 2001, 105, 9960−9965. (9) Mann, J. R.; Gannon, M. K.; Fitzgibbon, T. C.; Detty, M. R.; Watson, D. F. Optimizing the Photocurrent Efficiency of DyeSensitized Solar Cells through the Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline Titania Films. J. Phys. Chem. C 2008, 112, 13057−13061. (10) Kawasaki, M; Aoyama, S. High Efficiency Photocurrent Generation by Two-Dimensional Mixed J-Aggregates of Cyanine Dyes. Chem. Commun. 2004, 988−989. (11) Mulhern, K. R.; Detty, M. R.; Watson, D. F. AggregationInduced Increase of the Quantum Yield of Electron Injection from Chaclgogenorhodamine Dyes to TiO2. J. Phys. Chem. C 2011, 115, 6010−6018. (12) Marek, P. L.; Hahn, H.; Balaban, T. S. On the Way to Biomimetic Dye Aggregate Solar Cells. Energy Environ. Sci. 2011, 4, 2366−2378. (13) Modesto-Lopez, L. B.; Thimsen, E. J.; Collins, A. M.; Blankenship, R. E.; Biswas, P. Electrospray-Assisted Characterization and Deposition of Chlorosomes to Fabricate a Biomimetic LightHarvesting Device. Energy Environ. Sci. 2010, 3, 216−220. (14) Marek, P. L.; Sieger, H.; Scherer, T.; Hahn, H.; Balaban, T. S. Self-Assembled Chromophores within Mesoporous Nanocrystalline TiO2: Towards Biomimetic Solar Cells. J. Nanosci. Nanotechnol. 2009, 9, 3708−3713. (15) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Structure of the Protein Subunits in the Photosynthetic Reaction Center of Rhodopseudomonas Viridis at 3 Å Resolution. Nature 1985, 318, 618−624. (16) Allen, J. P.; Geher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C. Structure of the Reaction Center from Rhodobacter Sphaeroides R-26: The Protein Subunits. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 6162− 6166. (17) Cherezov, V.; Clogston, J.; Papiz, M. Z.; Caffrey, M. Room to Move: Crystallizing Membrane Proteines in Swollen Lipidic Mesophases. J. Mol. Biol. 2006, 357, 1605−1618. (18) Linnanto, J. M.; Korppi-Tommola, J. E. I. Investigation on Chlorosomal Antenna Geometries: Tube, Lamellar, And Spiral-Type Self-Aggregates. Photosynth. Res. 2008, 96, 227−245.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. Biography Jeanne L. McHale (http://physical.chem.wsu.edu/faculty/mchale) earned a B.S. in chemistry from Wright State University in 1975 and a Ph.D. in physical chemistry from the University of Utah in 1979. She is professor of chemistry and materials science at Washington State University, where she focuses her long-standing interest in photoinduced electron transfer on the problem of achieving sustainable and economical solar energy conversion.

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REFERENCES

ACKNOWLEDGMENTS

The support of the National Science Foundation through Grant CHE 0848511 is gratefully acknowledged. I am pleased to acknowledge the contributions of former students Stephanie Doan, Myong Yong Choi, and Cody Sandquist, present students 595



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