Discovery of Black Dye Crystal Structure Polymorphs: Implications for

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Discovery of Black Dye Crystal Structure Polymorphs: Implications for Dye Conformational Variation in Dye-Sensitized Solar Cells Jacqueline M. Cole,*,†,‡ Kian Sing Low,† and Yun Gong† †

Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom Argonne National Laboratory, 9700 S Cass Avenue, Argonne, Illinois 60439, United States

‡

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S Supporting Information *

ABSTRACT: We present the discovery of a new crystal structure polymorph (1) and pseudopolymorph (2) of the Black Dye, one of the world’s leading dyes for dye-sensitized solar cells, DSSCs (10.4% device performance efficiency). This reveals that Black Dye molecules can adopt multiple lowenergy conformers. This is significant since it challenges existing models of the Black Dye···TiO2 adsorption process that renders a DSSC working electrode; these have assumed a single molecular conformation that refers to the previously reported Black Dye crystal structure (3). The marked structural differences observed between 1, 2, and 3 make the need for modeling multiple conformations more acute. Additionally, the ordered form of the Black Dye (1) provides a more appropriate depiction of its anionic structure, especially regarding its anchoring group and NCS bonding descriptions. The tendency toward NCS ligand isomerism, evidenced via the disordered form 2, has consequences for electron injection and electron recombination in Black Dye embedded DSSC devices. Dyes 2 and 3 differ primarily by the absence or presence of a solvent of crystallization, respectively; solvent environment effects on the dye are thereby elucidated. This discovery of multiple Black Dye conformers from diffraction, with atomic-level definition, complements recently reported nanoscopic evidence for multiple dye conformations existing at a dye···TiO2 interface, for a chemically similar DSSC dye; those results emanated from imaging and spectroscopy, but were unresolved at the submolecular level. Taken together, these findings lead to the general notion that multiple dye conformations should be explicitly considered when modeling dye···TiO2 interfaces in DSSCs, at least for ruthenium-based dye complexes. KEYWORDS: multiconformation, structure, dye-sensitized solar cell, black dye, polymorph, dye···TiO2

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INTRODUCTION The trithiocyanato(4,4′,4″-tricarboxy-2,2′:6′,2′-terpyridine) ruthenium(II) complex, more commonly known as Black Dye (BD), was the first dye to challenge the original world-leading N719 dye1,2 in dye-sensitized solar cell (DSSC) performance.3 Black Dye was also found to enhance the DSSC spectral response in the red and infrared (IR) region of the electromagnetic spectrum, where the most efficient dyes are lacking;4 it exhibits an effective incident-photon-to-conversionefficiency (IPCE) that covers the visible spectrum and extends to 920 nm.3 This superior spectral gain provides a high overall conversion efficiency of 10.4%,3 compared to 10% in N719.2 We have now reached an era wherein research findings regularly break the world-record DSSC performance, with the current record now set at 14.7%.5 Yet, the new dyes associated with these discoveries are not being determined systematically, which would be preferable from a molecular engineering perspective. In order to shift the status quo of intuitive dye discovery to one of a rational molecular design process, we need to gain a far better molecular understanding of how the key DSSC device components function. To this end, case studies on well-known DSSC dyes are invaluable for developing © 2015 American Chemical Society

the molecular guidelines required to establish systematic DSSC dye structure and function relationships. The manner by which a dye adsorbs onto the TiO2 nanoparticle surfaces of the DSSC working electrode dictates the associated device performance. On the one hand, the “anchor” part of the dye molecule, which binds it to the TiO2 surface, governs the electron injection properties of a DSSC.6 Dyes that feature carboxylate anions as anchoring groups tend to show the most superior DSSC characteristics.7 Carboxylic acid groups can also bind to TiO2, albeit less efficiently owing to their typical monodentate binding modes in contrast to the bidentate options available to carboxylate ions.8 Despite the exploration of many alternative anchoring groups,9 carboxylate anchors still far outstrip them in DSSC performance. Black Dye is generally modeled with the tacit assumption that its chemical structure hosts one carboxylate and two carboxylic acid groups. However, its previously reported crystal structure3,10 could not confirm this since its hydrogen atoms were not resolvable. Received: August 10, 2015 Accepted: November 24, 2015 Published: November 24, 2015 27646

DOI: 10.1021/acsami.5b07364 ACS Appl. Mater. Interfaces 2015, 7, 27646−27653

Research Article

ACS Applied Materials & Interfaces

orientation with the aid of corroboratory findings from photoelectron spectroscopy. Meanwhile, one recent scanning tunneling microscopy and spectroscopy (STM/STS) report17 experimentally evidenced, for the first time, coexisting multiple conformations of the N719 dye anion on TiO2. The imaging part of that study was able to distinguish dye monomers from its dimers, but not molecular features at the resolution needed to discern one molecular conformation from another. One therefore needs a way to probe dye conformations at the atomic scale in order to complement these state-of-the-art developments in materials characterization; while also allowing for the possibility of coexisting multiple conformations. This work will show that the use of single-crystal X-ray diffraction presents a way forward, as introduced via the following argument. It has long been held that the crystal structure of a material represents one of the lowest, if not the lowest, conformational energies of a molecule;18 this structure is indicative of conformational preferences in various states of matter.19 Furthermore, lowenergy, near-degenerate, molecular conformations can manifest in the form of polymorphic crystal structures. One can therefore intuit that if a dye exhibits crystalline polymorphs, it is statistically more likely to offer a range of low-energy conformational options for a dye molecule to adsorb onto TiO2. That said, carboxylato-pyridyl ruthenium-based dyes are renowned for their crystallization difficulties. As shown by Grätzel and co-workers,3,10 until now it has only been possible to realize the solvated crystal structure of the Black Dye, with models of modest statistical quality (R1 = 0.0759;3 R1 = 0.097810). Solvated crystal structures are also statistically more likely to present with conformational variation.20 Yet, because this has been the only crystal structure available, to date, it has nonetheless been used widely as the starting geometry for computational studies that model the Black Dye adsorption process onto TiO2. This article reports two new nonsolvated polymorphs of Black Dye, crystal structures 1 (Figure 2) and 2 (Figure 3 a−d); 2 is also a pseudopolymorph of the previously reported solvated Black Dye crystal structure, hereafter denoted 3 (Figure 3e,f). We show that this multiconformational variety of the Black Dye anion not only presents a more comprehensive and diverse set of structural models, to benefit modeling efforts on Black Dye; it also has a fundamental impact on how one should view the Black Dye structure. In particular, we find that 1 has the ability to resolve the previously assumed carboxylate ion and reveal the true bonding nature of the NCS groups, thereby forming a complete anionic structure of Black Dye for the first time. Meanwhile, the disorder of the NCS substitutents manifesting in 2 indicates a tendency toward ligand isomerism. These structural findings present distinct consequences upon the interpretation of electron injection and electron recombination processes in Black Dye incorporated DSSC devices. In more general terms, we conclude from this study that an explicit multiconformational analysis of dyes is needed in the modeling of DSSCs, in order to properly understand their photovoltaic function.

On the other hand, the molecular orientation of a dye controls the nature and extent of electron recombination processes in a DSSC device (Figure 1). To some extent, this is

Figure 1. (a) Dye molecule anchoring to a TiO2 surface subject to its 3-D conformational flexibility, with binding restrictions via its most common bidentate carboxylate. (b) Dye oriented such that its electronically or sterically prominent substituents shield the TiO2 from electrolyte-based electron recombination. (c) Dye oriented such that bulky substituents protect the anchoring group from desorption, thereby promoting electron injection. (d) Dependence of dye orientation on electrolyte-to-TiO2 electron recombination processes, according to 3-D dye shape.

related to the type of dye···TiO2 anchoring in force, since the binding mode will somewhat constrain the dye orientation upon the TiO2 surface (Figure 1a). However, the intrinsic molecular conformation of a dye manifesting at this dye···TiO2 interface is another important consideration. In particular, the electronic or steric characteristics of the molecular dye component which extrudes from the TiO2 surface will determine if the dye can shield the TiO2 surface from electrolyte attack (Figure 1b). In cases where dye conformers exhibit bulky chemical substituents that surround the anchoring group (Figure 1c), this structural formation will tend to stabilize the dye···TiO2 binding, thereby obviating recombination effects from TiO2-to-dye electron back transfer. The overall three-dimensional (3-D) shape of a given dye conformer will also dictate the lateral spacing between dye molecules across the TiO2 surface; small interdye spacings are best to preclude unwanted molecules or ions reaching the TiO2 (Figure 1d). The preferred molecular conformation of a dye on a TiO2 surface is generally modeled according to its lowest-energy electronic structure. By virtue of the dye···TiO2 working electrode being a buried interface within a DSSC device, its molecular structure cannot be observed directly, at least not while in its device operational state. Currently available materials characterization techniques for studying DSSC buried interfaces are generally limited to ex situ methods.9 Within that scope, average dye orientations have been inferred from a few isolated X-ray reflectometry11,12 and near-edge X-ray absorption spectroscopy (NEXAFS)13 studies. A few in situ studies, employing infrared spectroscopy14 or atomic force microscopy,15 have nonetheless inferred average dye orientations with help from theoretical calculations, while another in situ NEXAFS16 study draws qualitative conclusions about dye

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EXPERIMENTAL METHODS

Crystals of 1 and 2 emanated from slow solvent evaporation in methanol and methanol:acetonitrile (50:50 by volume), respectively. Each crystal was mounted onto a Rigaku Saturn 724+ CCD 27647


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diffractometer with data collected and integrated using CrystalClear2.0.21 The crystal structures were solved and refined using direct methods via Shelx software.22 Data for 1 were collected on a laboratory X-ray source. The I19 beamline at the UK synchrotron, Diamond Light Source, was used to obtain crystal data for 2 given that the sample diffracted weakly and manifested noticeable diffuse scattering; structure solution revealed the cause of this to be significant disorder, present throughout the molecule but especially in the two NBu4+ cations that accompany each Ru-based anionic complex. Because this anion represents the functional aspects of Black Dye for DSSC applications, its resolution was enhanced at the expense of its counterion. This was achieved by applying the SQUEEZE routine23,24 in PLATON25 to 1 and 2. This calculated, and thence removed, the structure factor contribution of the NBu4+ counterions from the overall structural model, by treating these cations as if they represented continuous volumes of electron density. The removal of this cationic obscuration led to a much more accurate structural model of the Black Dye anion in both 1 and 2 [c.f. R1 = 0.0552 versus 0.0957 (1) and R1 = 0.0784 versus 0.01151 (2) for post- and pre-SQUEEZE corrected models, respectively]. These statistics for 2 are comparable to the best previously reported model of its pseudopolymorph, 3; that is, the solvated crystal structure where R1 = 0.0754;3 while those of 1 represent the most well-defined model of the Black Dye anion to date. 1 is also a new polymorph of Black Dye, crystallizing in space group, P21/n, rather than in Cc (c.f. 2 and 3).

Figure 2. Ordered crystal structure of the Black Dye anion, 1. Image created using the software package Olex2, version 1.2.6.27

Figure 3. a,b (Left) The axial NCS disorder in anion 1 of the asymmetric unit of 2; major (86%) and minor (14%) components are shown above and below. c,d (Middle) The axial and equatorial NCS disorder in anion 2 of the asymmetric unit of 2; major (77% axial; 60% equatorial) and minor (23% axial; 40% equatorial) components are displayed above and below. e,f (Right) Anion 1 and anion 2 of the previously determined structure of 3.9 Arrows highlight disordered groups. Image created using the software package, Olex2, version 1.2.6.27 27648


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RESULTS AND DISCUSSION Fully Defined Structure of the Black Dye Anion. The structure of 1 is ordered and bears two NtBu+ cations per anion in its asymmetric unit (Z′ = 1), as opposed to the four cations per two anions in that of 2 and 3 (Z′ = 2). The anion is therefore doubly charged, and assuming a RuII center, the ligands need to accommodate quadruple negative charge to afford overall neutrality. One of these negative charges is borne by a carboxylate group that is para-substituted to the middle pyridyl group of Black Dye, as shown in Figure 2. Its anionic nature was primarily evidenced by its two C−O bonds whose lengths are similar (C17−O3 = 1.251(4) Å; C17−O4 = 1.238(4) Å) and characteristic of a conjugated C··· O bond distance.26 Contrast this with the markedly disparate C−O bond lengths of its neighboring carboxylic acid groups, denoting oxygens from hydroxyl and keto groups, respectively: C16−O1 = 1.334(4) Å; C16−O2 = 1.225(4) Å; C18−O6 = 1.285(4) Å; C18−O5 = 1.200(4) Å. Hydrogen atoms were located directly from the Fourier difference map of the residual electron density for these hydroxyl groups. Secondary evidence for the carboxylate anion is the short Ru1−N2 separation, relative to all other Ru−N coordination c.f. Ru1−N1: 2.080(3) Å; Ru1−N3: 2.059(3) Å; Ru1−N4: 2.044(3) Å; Ru1−N5: 2.081(3) Å; Ru1−N6: 2.043(3) Å; this reflects the greater electronic “pull” that a RuII ion imparts on a carboxylato-pyridyl group wherein a negative charge is delocalized. Three of these Ru−N coordination bonds are associated with thiocyanate ligands which all present with a linear geometry (NC̅ S = 177.4(3)−178.9(3)°). The bonding geometry in all three NCS groups is identical within experimental error, and reflects a predominant NC−S− resonance structure, judging from its CN and C−S typified bond lengths: C19−N4: 1.161(4) Å; C20−N5: 1.145(4) Å; C21−N6: 1.163(5) Å; C19−S1: 1.636(4) Å; C20−S2: 1.668(4) Å; C21−S3: 1.656(4) Å. As such, the Ru−N coordination must be delivered via dative bonding through the N lone pair. Meanwhile, the NCS− groups collectively provide the remaining 3- contribution needed to afford overall charge neutrality. It is worth noting that this NC−S− representation for all of the NCS groups in the Black Dye is different to its common depiction in chemical schematics, which presumably derives from that suggested by the two previous reports of its structure.3,10 Those studies displayed the NCS bonding depiction in at least two of their NCS groups, based on suggestive remarks that one NCS group located trans to this NC5H4COO− group could bear a distinct and anionic NCS structure. Nonetheless, those reports do share our premise that the middle pyridyl group bears the carboxylate anion, although this report is the first to definitively confirm its presence, as well as locate the hydrogens on the carboxylate groups in the vicinal pyridyl ligands. Implications of Structure 1 for Black Dye DSSC Function and Associated Computational Modeling. The fully defined anionic structure of the Black Dye featured in 1 stands to improve our understanding about its strong photovoltaic performance. As previously suspected,10 Black Dye appears to undergo preferential dye···TiO2 adsorption via the carboxylate anion in its middle pyridyl group, judging from its best possible anchoring options.8 This anchoring point will be protected from surrounding ions by the bulky nature of its two flanking pyridyl-carboxylic acid groups. This will cause the

NCS groups of Black Dye to be oriented away from the TiO2 surface. As such, their triple negative charges will form a strong electronic shield against electrolyte ion···TiO2 contact, which would generate unwanted electron recombination and dye aggregation and thus reduce the possible number of functional dye molecules. Overall, this would seem to promote monolayer adsorption tendencies that insulate well against dark current.28 Our explicit characterization of the anionic charge distribution of 1 also has distinct implications for computational modeling of Black Dye. This especially pertains to studies where calculations simulate charge-dependent problems such as dye-to-TiO2 charge injection,29 dye···TiO2 adsorption30 or dye···electrolyte ion complexation.31,32 However, modeling Black Dye with explicit NCS− and selective COO− charge representation stands to influence all theoretical studies conducted in solution. In general terms, 1 offers a much improved starting model structure for computations of Black Dye. It should be remembered that all of the implications stated above are predicated upon the premise that Black Dye anions will approach the TiO2 surface bearing a statistically preferred conformation that corresponds to the crystal structure of 1. This seems to be a reasonable assumption on the basis of energy considerations,18,19 as discussed earlier. So, while the dye···TiO2 adsorption process is highly dynamic, it would nonetheless seem statistically logical that low-energy conformers would feature more prevalently. Naturally, other similarly low-energy conformers would likely manifest at this surface, where they exist. Indeed, 1 is herein reported as a polymorph of 2 which, in turn, is a pseudopolymorph of the previously reported crystal structure of 3. The strength of our predication, therefore, depends somewhat on how similar the anionic constituent of 1 is to its polymorphic structures; the greater the similarity, the greater the corroboration of this predication. Expressed another way, to what extent are the salient anionic structural features of 1 common to other low-energy conformers? Disordered Molecular Conformations of the Black Dye. While the crystal structures of 2 and 3 are of statistically lower quality than 1, overarching similarities between these polymorphic structures could be probed. Crystal structure 2 presents as a racemic twin. In common with its pseudopolymorph 3, it crystallizes in space group Cc, exhibiting two symmetry-independent anions and four cations per asymmetric unit (Z′ = 2); that is, 2 displays the same 1:2 anion/cation ratio as that seen in 1 and 3. While all non-H atoms could be located in the NtBu groups, these cations appear somewhat labile, as judged by their particularly large atomic displacement parameters (Supporting Information). Having applied the SQUEEZE routine, many atoms in the resulting anionic structure of 2 were also found to exhibit large displacement parameters. Such atomic displacement parameters precluded the possibility of distinguishing carboxylate from carboxylic acid groups via C−O bond length analysis. Thus, the carboxylate assignment in 2 was made according to that evidenced in 1 and supposed in 3. However, the largest atomic displacement parameters across the entire anion, by far, pertain to the terminal S portions of several NCS groups which deviate from their usual linear arrangement (Figure 3). In crystal structure 2, one of the two axial NCS groups is disordered, while the opposing NCS ligand is ordered. This disorder manifests slightly differently in each of the two anions 27649


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bimodal, and so percentile characteristics are used for comparison; c.f. RuNC min, lower quartile and mean (N = 77 CSD observations): 149.34, 163.15, and 170.73°. All disordered NCS ligands subtend a RuNC angle less than the lower quartile, except one which is less than the mean. Three RuNC angles in 2 are less than all previous reports of such geometry, the lowest (128.2°), being more than 21° smaller than the most acute RuNC angle ever reported.35 Tendencies Toward Linkage Isomerism. The potential scientific implications of this exceptional angular geometry are somewhat stark. They reveal that Black Dye anions can exist in a stable, low-energy structural conformation within the solid state, where NCS ligands are heavily distorted, to the extent that their Ru-NCS coordination geometry can approach that required to transition into one of its linkage isomers. While such isomerism does not actually manifest in the crystal structure of 2, a tendency toward this end is clear from these heavily distorted NCS orientations. Indeed, the labile nature of NCS ligands is well-known given its ability to act as an ambidentate ligand. For example, cases where their lability can even lead to thiocyanate dissociation, have motivated the development of an entire range of thiocyanate-free rutheniumbased sensitizers.36 The possibility of solution-based NCS linkage isomerism in this family of ruthenium-based sensitizers has been studied previously. In 2001, Nazeeruddin and Grätzel claimed to have isolated several types of NCS linkage isomers in the Black Dye using pH titration.37 A corresponding time-dependent density functional theory study38 corroborated this work, by showing that the UV/vis spectra generated by these experimental results could be reproduced via theoretical structures of various Black Dye linkage isomers. Such agreement between theory and experiment was only achieved when solvent effects were included in the calculations; analogous in vacuo calculations did not concur with experiment. Solvent Considerations. This phase-specific discord shows that the Black Dye is sufficiently sensitive to its immediate molecular environment that it controls the likelihood of NCS groups to reorient within its structure. From a purely solution-based standpoint, this could be regarded as a pH-related phenomenon given that the solution-based linkage isomers were isolated via pH titration and the coordination of NCS ligands to ruthenium will clearly be influenced by the extent to which the carboxylic acid groups in Black Dye are deprotonated. Furthermore, solvatochromism effects are a common occurrence in molecular dyes, and they often play an important role in tuning their spectral properties.39,40 From a solid-state perspective, the molecular environment of a dye residing in a crystal lattice can also be solvent dependent: where polymorphs occur, the choice of crystallizing solvent frequently dictates their distinct molecular environments;20 where solvents of crystallization exist within a crystal lattice, the molecular environment of the dye differs from its pseudopolymorphic counterpart that lacks a solvent. The fact that 2 is a pseudopolymorph of 3 bears testament to a solvent influence on NCS linkage isomerism within the solid-state. Crystals of 2 and 3 emanated from different solvent crystallization conditions, using a 1:1 mixture of methanol: acetonitrile or dimethyl sulfoxide (DMSO) solvents, respectively; that is, solutions of 2 and 3 featured highly disparate pKa values. Crystal structure 3 hosts DMSO as a solvent of crystallization while 2 presents a void in this region of the crystal lattice. NCS groups happen to be oriented such that they extrude into this

residing in the asymmetric unit (Figure 3 a,b and c,d). In each case, the ligands are bent to varying degrees. The twocomponent disorder in the axial NCS ligand in one of the anions (1) could be resolved, displaying NCS angles of 157.6 or 122.4° for the major (86%; Figure 3a) and minor (14%; Figure 3b) components, respectively. The disorder in the other anion (2; Figure 3 c,d) could only be resolved for S, where the major component (Figure 3c) presented with 77% occupancy, and NCS angles subtended 172.2° or 154.8°. Contrast these angles with those of the ordered NCS groups situated in the opposing axial position of anions 1 and 2: 174.9° and 171.6°, respectively. These bent NCS ligands render a corresponding distortion in the N-bound coordination to the RuII ion, where RuNC angles are 128.2° or 155.2° for each resolved disordered component of anion 1 (Figure 3 a,b) and 144.1° for the partially resolved disorder in anion 2 (Figure 3 c,d). This contrasts with the linear coordination (175.0 or 173.8°) of the ordered NCS ligands to the RuII ion (Figure 2). The equatorial NCS group also displays disorder. This could only be partially resolved in one of its two anions within the asymmetric unit; as manifest by the 60:40 positional split of its S in anion 2, yielding respective NCS angles of 143.1 or 169.2° (Figure 3 c,d). The equatorial NCS group in anion 1 is also bent (157.2°), although the residual electron density around its S atom was insufficient to model any such disorder. Nonetheless, its N-bound coordination subtends a RuNC angle of 157.4°, a much greater distortion than the disordered equatorial NCS ligand, whose RuNC angle is near linear (175.1°). A visual comparison of crystal structure 2 (Figure 3a−d) with that of 3 (Figure 3e,f) evidences a similar presentation of NCS disorder in 3. However, a quantitative comparison between 2 and 3 was not possible owing to insufficient accuracy by which the molecular geometry of 3 could be determined from its crystallographic data. Indeed, the elucidation of the anionic component of 2, via the use of the SQUEEZE routine,23,24 is what makes possible the analysis of 2 herein. Remarkable Extents of NCS Bending. The substantially bent nature of these NCS ligands, and the large extent to which their N-bound coordination to ruthenium deviates from a linear arrangement, are somewhat exceptional. This statement is qualified by considering these results in light of all 77 previously reported Ru-NCS geometries that exist in the Cambridge Structural Database (CSD)33 (see Supporting Information for statistical survey). The extent to which these NCS groups are bent is unprecedented; compare NCS min, median, mean (N = 77 CSD observations): 161.57, 178.43, and 177.69°; given median ≈ mean, nearly all NCS groups are effectively linear. Indeed, this minimum refers to one crystal structure whose NCS sulfur is also disordered,34 and it presents as a statistical outlier; its removal would impose a minimum NCS bend statistic of 170.60°. Notwithstanding the NCS angles of the ordered axial ligands in 2, all such angles except for one are less than this minimum statistic. One could argue that there may be a small systematic error on the geometries of the partially resolved disordered NCS groups. However, it is one of the fully resolved disordered components that exhibits the most acute NCS bend (Figure 3b), while its disordered counterpart (Figure 3a) carries an NCS angle that is entirely in line with its disordered thiocyanate partners in 2. The substantially nonlinear coordination of NCS groups to ruthenium in 2 is qualified using statistics from the same CSD structural entries. This statistical distribution of RuNC angles is 27650


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energy barrier associated with one NCS group undergoing linkage isomerism.38 However, the fact that these polymorphs can be accommodated within a crystal lattice evidence their low energy barriers to interconversion in relative terms. The finding that the two crystallographically independent anions of 2 differ in energy by exactly three multiples of 0.17 eV is potentially interesting: if one assumes that the crystal field forces are similar in both crystallographically independent anions, it would be seem reasonable to consider that these forces normalize out; as such, the 0.51 eV difference between anion 1 and anion 2 of crystal structure 2 could be considered as somewhat akin to the solution-based environment, where 0.17 eV is needed to move each of the three NCS groups to a new ligand configuration.38 Since van der Waals interactions will presumably dominate these crystal field forces, once summed over all directions, given the 3-D bulky nature of the molecular structure, this assumption could be valid. So, this energy match is perhaps not a mere coincidence. More generally, the existence of crystal structure polymorphs shows that multiple low-energy conformations of Black Dye must be present in solution since these polymorphs were formed via solvent crystallization. Complementary Evidence for Multiple Conformations in DSSC Dyes. All of our findings imply that the Black Dye presents at the dye···TiO2 interface in a range of molecular conformations, the nature of which depends upon the intricate interplay between various low-energy barriers associated with various tendencies toward NCS linkage isomerism. This is a significant finding, particularly in light of the fact that there is currently a dearth in materials characterization techniques that can probe the dye structure at such buried interfaces. Even ex situ structural studies that probe the exposed dye on a TiO2 surface, rather than at the buried interface itself, remain challenging for state-of-the-art surface-science techniques. The fact that the first substantial evidence for multiple molecular dye conformations residing at this dye···TiO2 interface was only reported last year, via the aforementioned ex situ STM/STS study,17 bears testament to the extent of this challenge. Interestingly, that pioneering study featured the anion of the archetypal ruthenium-based dye, N719, which is very closely related to the Black Dye. This combined imaging and spectroscopy study evidenced five energetically distinct molecular configurations of the N719 anion when adsorbed to the (101) surface of TiO2. Supporting STM/STS simulations indicated that a dimer was among these configurations, while the experimental STM data were consistent with simulated images featuring the frequently modeled bidentate anchoring mode. Limitations in atomic resolution precluded the STM images from identifying more subtle intramolecular features associated with different molecular conformations, such as the ligand distortions demonstrated in the Black Dye via this study. It would therefore be prudent to combine efforts in these types of experimental and computational imaging studies of the dye··· TiO2 nanostructure with molecular-scale investigations such as those featured herein. To this end, the newly discovered molecular conformations of 1 and 2 stand to aid the nanoscopic understanding of its associated dye···TiO2 interface. In more general terms, the development of a more multiscalar approach to the study of buried interfaces will facilitate attempts in molecular engineering to create better functional dyes for DSSC applications.

void region. In effect, this void thus affords a solid-state “reaction cavity” within which NCS groups can reorient. However, its volume does not seem sufficiently large enough to entirely accommodate solid-state linkage isomerism, in contrast to these type of reaction cavities that feature in ruthenium complexes bearing other ligands.41 That said, while it is not stated in the report of 3, its structure does also evidence some puckering of NCS groups; though not to the extent featured in 2. It is worth noting that X-ray diffraction data for 3 were collected at a higher temperature (243 K) than that of 2 (150 K), so this lower level of NCS ligand distortion in 3 cannot be attributed to thermal effects. NCS linkage isomerism, or tendencies toward this end, can therefore occur in solution- or solid-state molecular conformations of the Black Dye, respectively, when presenting in certain molecular environments. The dye···TiO2 interfacial structure of a DSSC working electrode reflects essentially a buffering of solution and solid-state phases. So, it is reasonable to project that NCS linkage isomerism could readily occur at the working electrode. Whether NCS groups in the Black Dye undergo actual linkage isomerism at this dye···TiO2 interface, or just distort to the extent revealed by 2, there are significant implications for DSSC function. On one hand, significant bending of NCS ligands will affect the ability of Black Dye to sterically block the electrolyte from the TiO2 surface or to undergo dye··· electrolyte complexation.31,32 On the other hand, bonding changes associated with NCS linkage isomerism will likely stabilize the electron “hole” in Black Dye that is created after the DSSC electron injection process.38 Conformational Energies. Solution-based computational work38 indicates that linkage isomerism of just one of the NCS ligands in the Black Dye bears an energy cost of only 0.17 eV, relative to its undistorted, fully Ru-NCS coordinated structure, which is naturally the most energetically stable configuration of Black Dye. The energy barrier between the ordered and NCS-distorted molecular conformations of Black Dye in the crystal structure, 1 and 2 respectively, were calculated herein for comparison. This employed density functional theory (DFT) on the isolated anionic structures of 1 and 2 (major disordered components only) via Gaussian09,42 using the molecular geometries of the crystal structures 1 and 2 for calculations; angular constraints were imposed in 2 to generate the observed NCS disorder. Note that the use of the SQUEEZE routine,23,24 which removes the cations in the crystal structure determination process, precluded a conventional solid-state DFT study. The 3-21g* basis set and B3LYP functional43 were employed for H, C, N, O, S, and Ti atoms, while the Los Alamos relativistic effective core potential (ECP) plus Hay−Wadt double-ζ basis set (LanL2DZ ECP)44 was used for Ru atoms. The final energy difference between 1 and 2 (major disordered anions 1 and 2) were found to be 1.36 and 1.87 eV respectively, with 1 being lower in energy than 2, thus revealing that 1 is the more stable polymorph. Given that the primary difference in molecular conformation between these two polymorphs is the heavy distortion of two NCS ligands in 2 compared with the ordered structure, 1, this energy difference can thus be regarded as primarily emanating from this distortion. Given that crystal field forces will influence significantly the molecular structures of these polymorphs, these solid-state energy differences are naturally larger than the solution-state 27651



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ASSOCIATED CONTENT

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REFERENCES

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Our discovery of two new polymorphs of the Black Dye has fundamental consequences upon its modeling for DSSC applications. It is clear that multiple conformations of the Black Dye anion exist. Our projection that the Black Dye··· TiO2 interfacial structure of the DSSC working electrode also contains multiple dye conformations appears unambiguous, given that this atomic-scale study is corroborated by recent nanoscopic imaging of multiple conformations of a closely related dye on a TiO2 surface. The substantial structural differences between 1, 2, and 3 emphasize that this conformational variety needs to be explicitly considered; only then can the associated DSSC function be properly understood at the molecular level. The discovery of 1 provides a fully defined ordered structure of the Black Dye anion. Substantial disorder manifests in the NCS groups of 2, indicating that low-energy molecular conformations of Black Dye have a tendency toward NCS linkage isomerization. The translation of this solid-state information to that of the Black Dye···TiO2 interface appears sound, given supporting evidence of linkage isomerization in previous solution-based studies, and bearing in mind that we are dealing with a solution···solid (dye···TiO2) interface. Overall, our results provide better defined and more comprehensive structural models of Black Dye. This stands to enable more informed experimental and computational molecular studies on this dye and related dyes. In a wider sense, we advocate that multiple dye conformations should be explicitly considered in the molecular design and engineering of dyes for DSSC applications.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07364. Histograms that summarize the bond geometry survey of NCS groups in all published ruthenium-based crystal structures via the Cambridge Structural Database. (PDF) Crystallographic Information File for 1. (CIF) Crystallographic Information File for 2. (CIF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel: +44 (0)1223 337470. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.M.C. is grateful to the 1851 Royal Commission of the Great Exhibition for the 2014 Design Fellowship and to Argonne National Laboratory, IL, where work done was supported by DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-299 06CH11357. K.S.L. acknowledges the EPSRC for a Doctoral Training Grant (EP/P504120/1). Y.G. thanks the Cambridge Trusts for a PhD scholarship. All authors thank Dr. Sarah Barnett from beamline I19 at Diamond Light Source, United Kingdom, for collecting the data for 2 via the mail-in synchrotron access facility. 27652


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Discovery of Black Dye Crystal Structure Polymorphs: Implications for

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