Forcing Aggregation of Cyanine Dyes with Salts: A Fine Line between

Jul 30, 2014 - It is uncommon to read about cyanine dyes in the literature and not have their aggregation discussed. They are of high interest conside...
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Forcing Aggregation of Cyanine Dyes with Salts: A Fine Line between Dimers and Higher Ordered Aggregates Sara M. Mooi, Samantha N. Keller, and Belinda Heyne* Chemistry Department, University of Calgary, 2500 University Drive NW, Calgary T2N 1N4, AB, Canada S Supporting Information *

ABSTRACT: It is uncommon to read about cyanine dyes in the literature and not have their aggregation discussed. They are of high interest considering their propensity to undergo self-organization in aqueous solution, leading to interesting photophysical properties resulting from the formation of their dimers and higher ordered aggregates. Currently, the study of their aggregation is in high demand due to their diverse application range including dye-sensitized solar cells. However, their aggregation in high salt solutions is under studied, and the effect on aggregation in congruence with high ionic strength is often overlooked. In a previous study, our group established the role of specific ion effects and in particular the necessity of matching water affinity to induce aggregation of a cationic cyanine dye, thiazole orange. In order to advance the understanding of this topic, we present in this article the diverse aggregation of cyanine dyes, as a single monovalent salt can cause different aggregation responses in a variety of these dyes. We established via absorption spectroscopy combined with chemometric analyses that the inherent monomer−dimer equilibrium of a dye depends on its geometry. More interestingly, experimental data coupled with DFT calculations reveal that not only the geometry of a dye but also its charge location plays a role in the aggregate morphology formed by the interaction of a cationic cyanine dye and an anion. It is thought that contact ion pair formation and effective charge screening generated within that ion pair are responsible for aggregates with a greater order.

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INTRODUCTION Cyanine dyes have a rich experimental and industrial history, and as a result whole textbooks have been devoted to characterizing and describing the vast catalogue of this dyes’ family.1,2 Undeniably, cyanine dyes have been exploited due to their light absorbing properties, which results from a polymethine backbone that contains an extended delocalized π-system. Because of this particular geometry, they have highly sensitive photophysics allowing the effects of variable solvents,3 temperature,3,4 and ionic strength5 to be characterized. Furthermore, as cyanine dyes undergo self-organization due to their inherent structure,6 they have been found to be an invaluable tool used to study molecular organizationa topic that is in the forefront of many disciplines including proteomics,7 optics,8 and photovoltaics.9 Like in many systems that undergo self-organization, van der Waals, London dispersion forces, and hydrophobic interactions dominate the attractive force between charged cyanine dye molecules in aqueous solution, whereas Coulombic interactions predominate the repulsive forces.10,11 The sum of these forces dictates the inherent organization of charged cyanine dyes in solution, where often a simple equilibrium exists between the monomeric and dimeric forms at micromolar concentrations.12 In addition, under the right environmental conditions, higher ordered aggregates, classified as either H- or J-aggregates, have also been observed.3,13−15 Dictated by the geometry of the molecules within the aggregate, H- and J-aggregates can be identified easily by spectroscopic means. According to Kasha, © 2014 American Chemical Society

the interaction of neighboring transition dipoles of tightly packed molecules generates a splitting of the excited state into new states referred to as exciton levels that are shared between all the molecules within the aggregate.16 Depending on how the molecules are ordered within the aggregate, either the higher or lower exciton transition is allowed. For molecules stacked one on top of one another, classified as H-aggregates, the higher exciton is allowed, thus resulting in a hyposchromic shifted absorption peak.14,16 The lower exciton is allowed for molecules packed end-to-end, depicted as J-aggregates, and results in a bathochromic shifted absorption.14,16 Classifying the degree of aggregation is imperative when working with cyanine dyes such as in the case of dye sensitized solar cells, where aggregation has been known to affect one-electron-oxidation potentials of cyanine dye molecules in an aggregate structure.17 In addition, by knowing the different types of aggregate morphology, one can start to probe how aggregation of molecules can be promoted or discouraged. The formation of highly ordered cyanine dye aggregates has been inhibited in the past by working at high temperatures18 or by using binding agents such as DNA.5,19,20 Conversely, aggregation beyond dimers has been shown to be promoted by the addition of certain agents such as macrocycles,13,14 and additionally, altering the ionic strength has recently been shown to be a Received: June 3, 2014 Revised: July 20, 2014 Published: July 30, 2014 9654

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negligible influence on the experiments. In order to assess the effect of the salts on the spectroscopic properties of these dyes, inorganic salts were added via consecutive additions, and an absorption spectrum was recorded after each addition. Absorption spectra were measured using a Varian Cary-50 single beam spectrophotometer. The solutions were placed in a semi-micro disposable cuvette of 0.45 cm width 1 cm path length (Plastibrand), and absorption spectra were normalized to the absorption maxima for each dye. The inorganic salts that were chosen to study were sodium acetate (NaCH3CO2, molecular biology grade), sodium chloride (NaCl, 99.0%), sodium bromide (NaBr, 99.0%), sodium iodide (NaI, 99.5%), sodium tetrafluoroborate (NaBF4, 99.0%), and sodium perchlorate (NaClO4, 99.0%). The salts were purchased from Sigma-Aldrich and were used as received. Stock solutions (1 M) for each salt were prepared in Milli-Q water (18.2 MΩ·cm at 20 °C). The salts were added up to a final concentration of 250 mM for DEQTC and 125 mM for THIA and PIC. Density functional theory (DFT) calculations were carried out using the Gaussian 09 program27 for the three cyanine dyes (DEQTC, THIA, and PIC) as bare cations as well as in ion pairs with the I−, BF4−, and ClO4− anions. All structures were optimized at the B3LYP level of theory with the 6-31+g(d) basis set (iodine atoms were treated with LANL2DZ), and solvation in water was simulated using the polarizable continuum model (PCM). Frequency calculations were performed to verify that the stationary points were minima on the potential energy surface. The monomer−dimer equilibrium constants and thermodynamic parameters (standard enthalpy and entropy changes) for DEQTC, TO+, and THIA were evaluated in water via a chemometric method allowing for the simultaneous analysis of whole absorption spectra measured at various temperatures. Absorption spectra were carried out in a 1 cm × 1 cm plastic cuvette in order to minimize adsorption of the cyanine dyes on the cuvette’s walls. Measurements were performed on a Varian Cary-50 single beam spectrophotometer at temperature ranging from 278.15 to 348.15 K, by 5 K increments. Absorption spectra were corrected for thermal expansion of water (see Supporting Information). Data were then analyzed using the DATAN V5.0 software (multid) developed by Kubista.28−30 The principles of this chemometric method have been presented in detail in several other papers.28−30 Briefly, this method is based on Van’t Hoff relationship, where the standard enthalpy change (ΔH0) is assumed to remain constant for the dyes under investigation within the temperature range used.

simple and advantageous method to induce aggregation of cyanine dyes.15 Our interest lies in the aggregation of cyanine dyes in high ionic strength solution, as recently higher ordered aggregates of thiazole orange (TO+) have been shown to form in high salt conditions when the aggregate is more energetically stable than the separate cyanine dye molecules in solution.15 This condition arises when a contact ion pair forms between the cationic cyanine dye and its counterion, and effective charge shielding is present between the ions within the ion pair.15 However, for a contact ion pair to form, the dye and the counterion must have similar free hydration energies or water matching affinities.15,21 Therefore, it has become clear that it is not simply an increase in ionic strength that drives the aggregation of cyanine dyes, but more specifically it is the size and property of the ion used to create the phenomenon.15 Trends of cyanine dye aggregation have arisen in the literature, and it has primarily been found that the structure of the dye dictates the types of aggregates formed.11,22,23 For instance, alone in solution at micromolar concentrations, 1,1′diethyl-2,2′-cyanine iodide (PIC) exists primarily in its monomeric form,24 yet it can easily be persuaded to form Jaggregates when its concentration is increased to millimolar25 or with the addition of macrocycles.13 On the other hand, a micromolar concentration solution of TO+ contains both monomeric and dimeric species and under the right conditions H-aggregates have been observed.14 The inclination for Haggregates to form over J-aggregates and vice versa in solution is hypothesized to be due to the geometry of the dye in question. Generally, planar dyes, such as TO+, tend to form Haggregates and nonplanar dyes, such as PIC, are prone to form J-aggregates.25,26 The question then arises whether this trend is preserved when salts are used to induce aggregation of a variety of cyanine dyes. We present in this article detailed observations describing the effect on dye aggregation when the structure of the dye changes and when variations of counterion sizes are used. The effect of salt on the aggregation of three cationic cyanine dyes1,3′diethyl-4,2′-quinolylthiacyanine (DEQTC), 3,3′-diethylthiacyanine iodide (THIA), and PICis presented in this article. We will show that the ability of a cyanine dye to form a contact ion pair in solution with a counterion is crucial for structured aggregate formation. In addition via sophisticated chemometric analyses and DFT calculations, we will establish that the geometry of a dye and its charge location play a role in its inherent monomer−dimer equilibrium, which therefore affects its propensity to aggregate, and the aggregate morphology in the presence of certain salts.

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RESULTS AND DISCUSSION DEQTC, THIA, and PIC are prime candidates to examine molecular aggregation as they can be easily studied using UV− vis absorption spectroscopy (Figure 1). Although they are all cyanine dyes, PIC absorbs at the lowest energy, with an absorption maxima at 523 nm, due to the increased conjugation within the molecule compared to DEQTC and THIA as seen in their molecular representations in Figure 1. With the exception of DEQTC, PIC and THIA have been extensively studied in the literature and are thought to be purely monomeric in aqueous solution.4,6,23,31 Therefore, these dyes were initially selected to allow us to study the effect of salts on cyanine dyes that do not entertain inherent monomer−dimer equilibrium. In addition, the literature suggests that they form different aggregate structures, where THIA molecules aggregate with sodium dodecyl sulfate in the H formation,31 and PIC forms Jaggregates at high concentrations25 and with cucurbit[n]uril hosts13 or H-aggregates at low temperatures.24 However, to properly understand how the salts affect aggregation, we first need to discuss each dye separately and their inherent aggregation tendencies alone in aqueous solutions upon a change in concentration.

EXPERIMENTAL SECTION

Thiazole orange (TO+, ultrapure, Anaspec), 1,3′-diethyl-4,2′-quinolylthiacyanine iodide (DEQTC, ≥98%, BioChemika), 3,3′-diethylthiacyanine iodide (THIA, ∼97%, Sigma-Aldrich), and 1,1′-diethyl-2,2′cynaine iodide (PIC, 97%, Sigma-Aldrich) were used as received without further purification. Stock solution of TO+ (10 mM), DEQTC (20 mM), THIA (1 mM), and PIC (1 mM) were prepared by dissolving the dyes initially into DMSO (Sigma), a commonly used solvent for the dissolution of cyanine dyes. The stock solutions were kept for a maximum of 2 months at 4 °C in the dark. Samples for the titration experiments were initially prepared by directly diluting the stock solution of the desired dye into a cuvette containing Milli-Q water (18.2 MΩ·cm at 20 °C) to a concentration of 20 μM for DEQTC and 10 μM for THIA and PIC. The concentration of DMSO in the final solution was 0.1% and is assumed to have a 9655

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DATAN and assuming that ΔH0 remains constant within the temperature range used under our experimental conditions, we were able to evaluate the temperature dependence of Keq for DEQTC (see Figure S2). The value found at 20 °C is reported in Table 1, where Keq for TO+ obtained under the same experimental conditions is shown for comparison. According to our data, DEQTC presents a smaller Keq than the one measured for TO+, suggesting DEQTC is less inclined at forming dimers in aqueous solution. This result can be attributed to the steric hindrance of the ethyl groups on DEQTC and provides basic evidence that the monomer−dimer equilibrium for a dye is in part a function of the dye’s structure.

M+ + M+ ⇄ (M+)2 Keq =

(1)

[(M+)2 ] [(M+)]2

(2)

Using the estimated value for Keq, the molar extinction coefficients of the monomeric and dimeric form of DEQTC, with their maximum absorption located respectively at 501 and 477 nm can be mathematically assessed using a method that has been previously described in the literature15,32 and is briefly explained in the Supporting Information (see Table 1 and Figure S3). As listed in Table 1, an εm value at 501 nm of 69 500 M−1 cm−1 was extracted from our data analysis, which is slightly higher than εm of 63 000 M−1 cm−1 reported for TO+ 5 or the value of 60 500 M−1 cm−1 found under our data analysis. This result is surprising since the two molecules have the same chromophore, and by consequence, an identical extinction coefficient would have been expected. However, this trend is also observed in methanol (see Figure S4) where the cyanine dyes are believed to be present as monomer, suggesting thus a difference in purity between the two molecules. On the contrary, THIA and PIC, the two other cyanine dyes under consideration, have been reported in the past to be present only in the monomeric form at low dye concentrations in aqueous environment.4,6,24,31 The absorption spectra of THIA and PIC both contain a main absorption band at 421 and 523 nm, respectively, along with a second peak at 404 and 490 nm, respectively (Figure 1). This second band has been assigned previously to a vibrational mode of the molecules4,24,31 and is commonly observed in other cyanine dyes.4,22 As expected for monomeric species, the absorption maxima of THIA and PIC seem to obey Beer−Lambert law in water at low dye concentrations. Thus, their molar extinction coefficients could be extracted from a linear regression analysis of a Beer− Lambert plot and were determined in water to be 73 000 M−1 cm−1 at 421 nm for THIA and 66 000 M−1 cm−1 at 523 nm for PIC (see Figure S5). These values are close to the previously reported values for THIA of 81 300 M−1 cm−1 at 424 nm in

Figure 1. (a) Molecular structure representation of THIA, DEQTC, PIC, and TO+ with their respective counterions. (b) Normalized absorption spectra of THIA (10 μM, triangles), DEQTC (20 μM, circles), PIC (10 μM, squares), and TO+ (20 μM, dashed line) in aqueous solution.

DEQTC is structurally very similar to thiazole orange (TO+),19 where the only difference resides in the presence of ethyl groups on the nitrogen moieties (Figure 1). Given the structure similarities, it is unsurprising that DEQTC shows similar properties to TO+. In aqueous solution, DEQTC exists in equilibrium between its monomeric and dimeric forms (see Figure S1 in the Supporting Information). At a 20 μM concentration, the monomer and dimer can be observed using absorption spectroscopy as they have characteristic peaks at 501 and 477 nm respectively, which are reminiscent of the one observed for TO+ (Figure 1).15 A simple equilibrium equation can be written to express this most basic form of aggregation (eq 1), where M+ corresponds to the monomer and (M+)2 represents the dimer. In addition, the equilibrium constant (Keq), dictating the inclination of this dimerization to occur, can be expressed as in eq 2. By analyzing simultaneously the absorption spectra of DEQTC measured at different temperatures with the software

Table 1. Equilibrium Constant at 20 °C Evaluated with the DATAN Software and Molar Extinction Coefficients for the Monomeric (εm) and Dimeric (εd) Form of DEQTC and TO+ in Aqueous Solution TO+

DEQTC εma 477 nm 501 nm Keqb (M−1)

(M−1 cm−1)

47000 ± 500 69500 ± 4000 (1.7 ± 0.3) × 104

(M−1 cm−1)

εm (M−1 cm−1)

εda (M−1 cm−1)

120000 ± 2000 96000 ± 1500

39000 ± 400 60500 ± 500 (2.5 ± 0.2) × 104

126500 ± 1500 100000 ± 2000

εda

a

a

Values obtained by assuming a simple monomer−dimer equilibrium (see Supporting Information). bValue obtained by chemometric method using the DATAN software (see Supporting Information). 9656

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75% ethanol31 and 60 000 M−1 cm−1 for PIC in water at 520 nm.33 Surprisingly, while the absorption spectrum of THIA seems to vary linearly with respect to its concentration (see Figure S5) and the molecule has been suggested to be monomeric,6,31 further spectroscopic investigations clearly indicate that THIA exists as a mixture of monomer and dimer in aqueous solution (see Figure S6). While this unexpected result is certainly not affecting the outcome of studies conducted in the past involving THIA, it is of the utmost importance in our current investigation. Attempts to evaluate Keq for THIA via the chemometric method described earlier were carried out by measuring the absorption spectrum of THIA at different temperatures. However, the results were deemed unsatisfactory, as the program could not converge to a coherent solution (see the Supporting Information). This is probably due to the combination of a low value for Keq and an important overlap between the absorption spectra of the monomer and dimer. In order to ensure the existence of PIC as a monomeric molecule at low concentration in water, similar spectroscopic investigations were performed and confirmed the monomeric nature of the dye (see Figure S6). Crystal structures34 and DFT calculations25 provide insight into understanding this result, as PIC is fairly twisted compared to both DEQTC and THIA (see Figure S8), minimizing thus the attractive forces between PIC molecules necessary to induce dimerization.10,11 Therefore, for the discussion presented in this article, the only dye that is considered purely monomeric in aqueous solution under the concentrations studied is PIC. The effects of the addition of salts on cyanine dye absorption spectra are discussed in two sections: first the smaller monovalent salts (with their corresponding bare radii) NaCH3CO2 (0.159 nm35), NaCl (0.181 nm36), and NaBr (0.196 nm36) will be presented followed by the effects of the larger salts NaI (0.216 nm35), NaBF4 (0.230 nm36), and NaClO4 (0.236 nm35). While in a previous study we have shown the counterion of the monovalent salt used had no effect on the aggregation of TO+,15 the cation is kept constant herein, as sodium, so that results due to the six salts can be attributed to the anion. The addition of small monovalent salts to a solution of DEQTC resulted in a decrease in the absorption intensity of both the monomer and dimer absorption peaks at 501 and 477 nm (Figure 2a). As seen previously for TO+ and other dyes, this decrease can be attributed to a change in the monomer−dimer equilibrium.15 Changes to the monomer−dimer equilibrium can be monitored by simply observing changes in the equilibrium constant.37 Therefore, in order to detect the effect of added salt on the equilibrium constant, the equilibrium constant at each salt concentration was calculated, as the ionic strength of the solution was changed. The equilibrium constant of DEQTC was plotted as a function of salt concentration (see Figure 2b and derivation in the Supporting Information). It is apparent that with any of the three salts the equilibrium increases in all cases. This is a direct indication that the monomer−dimer equilibrium shifts to the right (eq 1) to form more dimer. This result is not surprising as salt induced aggregation has been previously reported in the literature for other small molecules.15,21 However, what is particularly striking is that the equilibrium constant does not change by the same amount for the three salts, and therefore dimerization is not purely

Figure 2. (a) Normalized absorption spectrum of DEQTC (20 μM) in aqueous solution (black line) in the presence of 250 mM NaCH3CO2 (circles), NaCl (squares), and NaBr (triangles). (b) Mathematically determined Keq for DEQTC in aqueous solution plotted as a function of added salt concentration of NaCH3CO2 (circles), NaCl (squares), and NaBr (triangles).

dependent on the increased ionic strength. This is also observed visually in Figure 2, as at a final concentration of 250 mM there is a different decrease in the absorption intensity depending on the salt used. When comparing the change in the equilibrium constant, the change is largest for NaBr and smallest for NaCH3CO2 (see Figure 2) where the percentage of monomer, initially at 69% without any salt, decreases to 61% with NaCH3CO2, 55% with NaCl, and 50% with NaBr (see the Supporting Information). This result is reminiscent of the one found in the past for TO+,15 and the increased dimerization of the DEQTC in aqueous solution is thus a function of the type of salt used to increase the ionic strength. Interestingly, the addition of NaCH3CO2, NaCl, or NaBr to a solution of THIA results in a decrease in absorption intensity of the main monomeric absorption peak at 421 nm and a slight increase in the absorption peak at 404 nm (see Figure S10). The change in the absorption peak at 404 nm versus the monomer peak at 421 nm can be monitored as the salt concentration increases (Figure 3). For all three salts, NaCH3CO2, NaCl, and NaBr, as the salt concentration increases, this ratio also increases. Similarly to what was observed for DEQTC, the change in the ratio is greatest for NaBr and the least for NaCH3CO2, and as suggested earlier this result can be considered indicative of dimer formation.15 9657

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Figure 3. Absorbance of THIA (10 μM) at 404 nm over the absorbance at 421 nm plotted as a function of increasing salt concentration for NaCH3CO2 (circle), NaCl (squares), and NaBr (triangles).

Interestingly, when a dye exists solely as a monomer in aqueous solution, no change is witnessed when small monovalent ions are added to the solution, as seen for PIC (see Figure S11). It is then of interest to try larger monovalent ions as we have shown in a previous study that the size of the anion plays a role in the aggregation of dyes. The addition of I−, BF4−, and ClO4− to a solution of DEQTC results in a decrease in the absorption intensity for the monomer and dimer (see Figure 4a and Figure S12). But, there is a corresponding increase in absorption regions around 425 and 600 nm, characteristic of an increase in a higher ordered aggregate concentration,15,16 which was not observed for the small monovalent anions. Interestingly, a similar trend is observed for the addition of I−, BF4−, and NaClO4− to a solution of THIA as there is both a decrease in the monomer absorption intensity and an increase in absorption both red- and blue-shifted compared to the monomer peak (Figure 4b and Figure S13). For DEQTC and THIA, the addition of the larger salts encourages aggregate formation. Similarly, as in the case for small anions, the size of the salt seems to be playing a role in the aggregate formation as each salt causes a different degree of change in the dyes absorption spectra (see Figure 4 and Figures S12 and S13). For both DEQTC and THIA the largest decrease in absorption intensity occurs when NaClO4 is added, and the smallest decrease is for NaI. Remarkably, the magnitude of changes observed in absorption intensity correlates well to the relative size of the anion given that ClO4− is larger than BF4− which is larger than I−. Although it appears from our results that the size of the anion plays a role in the aggregation of cationic cyanine dyes, in order to understand this phenomenon we must look further as the size of an ion is associated with its free hydration energy. Recently, the theory of water matching affinity38,39 has described that the aggregation of a cyanine dye is reliant on the ability of a dye and a counterion to form a contact ion pair.15 Before discussing the theory behind the formation of contact ion pair, it is pertinent to first mention the necessity of the contact ion pair for aggregation. As mentioned previously, intermolecular forces dictate aggregation of cyanine dyes, and primarily aggregation is hindered by the repulsive effect of electrostatic interactions between nearby molecules.10,11 In many examples in the literature, effective charge screening by additives has shown to induce cyanine dye aggregation, as these

Figure 4. Evolution of the normalized absorption spectrum of (a) 20 μM DEQTC in water with additions of NaClO4 added up to a final concentration of 250 mM and (b) 10 μM THIA in water with the additions of NaClO4 added up to a concentration of 125 mM.

repulsive Coulombic forces are reduced, and in some cases this is due to the formation of contact ion pairs.15,31 The theory of matching water affinities describes the ability and predisposition of contact ion pair formation by taking into account the free energy of hydration of the ions. This theory dictates that the driving force of the formation of an ion pair, containing one anion and one cation, is dependent on the hydration energies of the ions in solution and is not purely an electrostatic effect.39 This free hydration energy or matching water affinity is the energy that represents the ions ability to associate water molecules within its hydration sphere, which relates to the size of the hydration sphere. Water molecules within the hydration sphere of a small ion are associated closely with the ion due to the high point charge localized on the ions surface. This results in strong interactions between the ion and the water molecules in the hydration sphere and by consequence in a highly exothermic free hydration energy. Alternatively, for large ions, the hydration sphere is weaker due to a more dispersed point charge, thus resulting in less exothermic free hydration energies as water molecules are more weakly associated with the ion.39 The theory of matching water affinity states that only ions with similar free hydration energies will form contact ion pairs. For two oppositely charged ions that both have similar free hydration energies the formation of an ion pair is spontaneous.39 Briefly, when an ion has a weakly associated 9658

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resides out of the molecular plane. In contrast, the tetrahedral BF4− and ClO4− anions fit closer into the region of high positive electrostatic potential on the THIA cation localized on the polymethine backbone (Figure 5a and Figures S8 and S15).

hydration sphere, the energy needed to disrupt this hydration sphere is minimal as the ion−water interactions are weak. For these weakly hydrated ions, the spontaneity for the formation of a contact ion pair is driven out of the stronger water−water interactions of the bulk phase, occurring when water molecules are released from the hydration sphere compared to that of weaker ion−water interactions of the individual hydrated ions. Likewise, for small ions with tightly held hydration spheres the ion−ion interactions formed between the two hard ions is stronger that the ion−water interactions of the solvated ions.39 Water matching affinity could account for aggregation of dye molecules as the spontaneous formation of contact ion pairs could be factor of ion size effects. For both DEQTC and THIA we believe that I−, BF4−, and ClO4− have similar matching water affinities and contact ion pairs are able to form between these dye molecules and the added anions. This hypothesis is supported by our spectroscopic data (see Figure 4 and Figures S12 and S13), as contact ion pairs would exhibit similar absorption characteristics as monomeric species albeit a decrease in intensity due to reduced oscillator strength.40−42 On the contrary, we believe for the small monovalent anions, CH3CO2−, Cl−, and Br−, higher ordered aggregates do not form because the small anions and the large cationic dye do not have proper water matching affinities. As mentioned in the Introduction, characteristic absorption peaks for H- and J-aggregate appear blue- and red-shifted, respectively, compared to the monomer absorption peak, and their appearance occurs consecutively with a substantial decrease in the monomer absorption. Evidence of aggregate formation has thus been seen with the addition of large monovalent salts to solutions of both DEQTC and THIA as increase absorption intensity in the H and J regions appears (Figure 4 and and Figures S12 and S13). However, the presence of sharp H- and J-aggregate absorption peaks is characteristic of extended molecular excitons present when molecules are arranged in close proximity to one another and only a distinct H-aggregate peak is observed at around 375 nm for THIA when ClO4− is the counterion (Figure 4b).23,43 This is specific for THIA with ClO4− as only broad peaks are observed for additions of the other large anions to solutions of THIA and DEQTC. Two possibilities, which are not mutually exclusive, could account for poor molecular stacking in aggregates formed between I− and BF4− with THIA and I−, BF4−, and ClO4− with DEQTC. The first is steric hindrance; small molecules with larger substituents, such as the ethyl groups located on the nitrogen moieties on THIA and DEQTC (see Figure 1a), have shown increased monomer−monomer space distance within an aggregate.23,33,43 The second reason could be a poor charge screening of the dye by the counterion. It is possible, that the contact ion pair formed between THIA and I− or BF4− does not result in a complete neutral ion pair, and thus Coulombic factors still play a role in dye−dye repulsion. It is possible that for THIA, ClO4− is able to adequately screen the positive charge of THIA so that enough charge screening occurs, thus causing a more neutral and therefore hydrophobic contact ion pair and subsequently forming an organized H-aggregate. The DFT calculations of the ion pairs also support the observation of the organized H-aggregates in the ion pair between THIA and ClO4−. As shown in Figure S15, the ion pair of THIA with I− prefers a geometry where the ethyl groups are turned in the same direction, while the spherical I− anion

Figure 5. Optimized structures at the B3LYP/6-31+g(d) level in water simulated using PCM of ion pairs of (a) THIA with ClO4− and (b) DEQTC with ClO4−.

This allows for an overall flatter geometry, which would have a higher likelihood of forming ordered aggregates. On the contrary, as shown in Figure 5b, the ion pair between DEQTC and ClO4− is twisted and much more out of plane than its THIA counterpart due to the favorable interaction of ClO4− with the quinoline ring of DEQTC. Through molecular simulations, we were able to observe a larger positive charge density on the quinoline ring for DETQC that is absent for THIA, whose charge is located centrally within the molecule (see Figure S8). This result is supported by the literature where an increased basicity of the quinoline ring compared to the thiazole moiety is suggested for asymmetric cyanine dyes.44,45 Therefore, our molecular simulations suggest that although DEQTC and ClO4− may easily form an ion pair, this ion pair may have a difficult time forming ordered aggregates due to its twisted structure. As mentioned previously in the article, PIC does not dimerize in the presence of small monovalent salts as compared to DEQTC and THIA. It thus pertinent to question whether PIC will form higher ordered aggregates with the large monovalent salts or will remain unaffected by their presence due to its monomeric nature. With the smallest of the large monovalent anions, I−, there is only a slight increase in the red region of PIC absorption spectrum (see Figure S14). This effect is considered negligible, as it only exists at the highest concentration of I− and demonstrates a limited interaction between PIC and I−. On the other hand, as the size of the anion increases to BF4− and ClO4−, there is a significant decrease in the absorption intensity of PIC (Figure 6 and Figure S14). We suspect that this decrease is a result of a contact ion pair formed between PIC and BF4− or ClO4−. As shown previously, efficient charge screening can exist between ions in the contact ion pair, and if this situation is met, aggregation of the seemingly neutral ion pair will result.15 Although we believe a contact ion pair forms between PIC and BF4−, no evidence of dye aggregation is present in the absorption spectra (see Figure S14), which is 9659

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formation was observed. However, the monomer−dimer equilibria of these dyes were altered. On the other hand, addition of these anions to a PIC solution did not induce changes in the dye absorption spectrum, suggesting lack of dimerization. From these results we hypothesize that contact ion pair formation is not achieved between any of the cyanine dyes studied and the small monovalent anions. This is likely due to noncomplementary water matching affinities between the cyanine dyes and the anions. However, once matching water affinities are achieved for DEQTC, THIA, and PIC, aggregate formation is observed. We propose that aggregation of cyanine dyes in the presence of salts is due to the formation of a seemingly neutral contact ion pair when the condition of water matching affinity is met between the dye and its counterion. Our results also suggest that the more planar a contact ion pair is, the more structured the aggregates will be. This hypothesis is strongly supported by molecular simulations. Specifically, the more structured Haggregates present for THIA with ClO4− over the aggregates of DEQTC with ClO4− seems to be a reflection of the planarity of the ion pair and the cyanine dye, where more planar dyes have the ability to form ordered aggregates over less planar dyes. Our results demonstrate that although the aggregation of dyes appears to be unique for each cyanine dye, knowledge of dyes inherent monomer−dimer equilibrium and geometry are the keys for predicting aggregation extent and structure.

Figure 6. Evolution of the normalized absorption spectrum of a 10 μM aqueous solution of PIC with the additions of NaClO4 added up to a final concentration of 125 mM.

likely due to the lack of charge screening within the ion pair. However, when the size of the anion is increased to ClO4−, aggregates of the ion pair, albeit poorly structured, are observed around 600 nm (Figure 6). The absence of structure within the aggregates is thought to be due to poor molecular packing as a result of low charge screening between ions in the ion pair. This result is supported by DFT calculations, as ion pairs appear to be quite twisted in solution despite a close fit of the anion into the region of high positive electrostatic potential on the PIC cation localized on the polymethine backbone (see Figure S17). We believe that the lack of planarity of the contact ion pair limits the formation of ordered aggregates as seen with DEQTC and ClO4−. Interestingly, these results are reminiscent of the one reported by Daltrozzo et al.,46 where the authors suggests a competition between PIC crystallization and Jaggregate formation at low dye concentration. The authors found this competition to be dependent on the nature of PIC counteranion. The lack of planarity of the contact ion pair formed between PIC and BF4− or ClO4− reported by DFT calculation (see Figure S17) could lead to the crystallization phenomenon suggested by Daltrozzo et al.46 Crystallization of the contact ion pair could certainly explain the decrease observed in the main absorption peak of PIC and the increase in the red region around 600 nm (Figure 6).

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ASSOCIATED CONTENT

* Supporting Information S

Temperature study of DEQTC, THIA, and TO+, molar extinction coefficients for DEQTC, THIA, and PIC, evaluation of the percentage of monomer in solution, Cartesian coordinates of minimized structures from DFT calculations, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Computational resources were provided by Compute Canada and WestGrid.

CONCLUSION As seen in a previous publication, the aggregation of the asymmetric cyanine dye, thiazole orange (TO+), could be tuned by varying the size of the counterions added in solution.15 Interestingly, this idea can be extended to other cyanine dyes, where aggregation was observed by the simple addition of certain monovalent salts. In order to understand this phenomenon, we first explored the photophysics of three cyanine dyes: DEQTC, THIA, and PIC. By comparing, the equilibrium constants obtained via chemometric methods of TO+ with its ethyl-substituted version, DEQTC, we found that dimerization of DEQTC was reduced compared to TO+. This result is the first indication that aggregation is linked to dye geometry, matching a theory that had previously been suggested in the literature. We also provide evidence that THIA is not purely monomeric in aqueous solution, and the only dye that can be considered as monomer in this study is PIC. Upon the addition of small monovalent anions (CH3CO2−, Cl−, and Br‑) to solutions of DEQTC and THIA, no aggregate

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REFERENCES

(1) Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.; Behera, G. B. Cyanines during the 1990s: A review. Chem. Rev. 2000, 100 (6), 1973−2011. (2) Strekowski, L. Heterocycylic Polymethine Dyes; Springer: Berlin, 2008; Vol. 14. (3) Renge, I.; Wild, U. P. Solvent, temperature, and excitonic effects in the optical spectra of pseudoisocyanine monomer and J-aggregates. J. Phys. Chem. A 1997, 101 (43), 7977−7988. (4) Gulen, D.; Ozcelik, S. Absorption spectrum of monomeric pseudoisocyanine: A new perspective and its implications for formation and spectral response of J-aggregates in solution and in thin films. J. Lumin. 2008, 128 (5-6), 834−837. (5) Nygren, J.; Svanvik, N.; Kubista, M. The interactions between the fluorescent dye thiazole orange and DNA. Biopolymers 1998, 46, 39− 51.

9660

dx.doi.org/10.1021/la502124b | Langmuir 2014, 30, 9654−9662

Langmuir

Article

(6) Ghosh, J. K.; Mandal, A. K.; Pal, M. K. Energy transfer from thiacyanine to acridine orange facilitated by DNA. Spectrochim. Acta, Part A 1999, 55, 1877−1886. (7) Loose, M.; Kruse, K.; Schwille, P. Protein self-organization: Lessons from the Min system. Annu. Rev. Biophys. 2011, 40, 315−336. (8) Wakamatsu, T.; Toyoshima, S.; Saito, K. Optical reflection response of dye-aggregate films in the absorption bands. J. Opt. Soc. Am. B 2006, 23 (9), 1859−1866. (9) Kang, S. J.; Ahn, S.; Kim, J. B.; Schenck, C.; Hiszpanski, A. M.; Oh, S.; Schiros, T.; Loo, Y. L.; Nuckolls, C. Using self-organization to control morphology in molecular photovoltaics. J. Am. Chem. Soc. 2013, 135, 2207−2212. (10) Birkan, B.; Gulen, D.; Ozcelik, S. Controlled formation of the two-dimensional TTBC J-aggregates in aqueous solutiono. J. Phys. Chem. B 2006, 110, 10805−10813. (11) Stadler, A. L.; Renikuntla, B. R.; Yaron, D.; Fang, A. S.; Armitage, B. A. Subtituent effects on the assembly of helical cyanine dye aggregates in the minor groove of a DNA template. Langmuir 2011, 27 (4), 1472−1479. (12) West, W.; Pearce, S. The dimeric state of cyanine dyes. J. Phys. Chem. 1965, 69 (6), 1894−1903. (13) Gadde, S.; Batchelor, E. K.; Weiss, J. P.; Ling, Y. H.; Kaifer, A. E. Control of H- and J-aggregate formation via host-guest complexation using cucurbituril hosts. J. Am. Chem. Soc. 2008, 130 (50), 17114− 17119. (14) Lau, V.; Heyne, B. Calix[4]arene sulfonate as a template for forming fluorescent thiazole orange H-aggregates. Chem. Commun. 2010, 46 (20), 3595−3597. (15) Mooi, S. M.; Heyne, B. Size does matter: how to control organization of organic dyes in aqueous environment using specific ion effects. Langmuir 2012, 28, 16524−16530. (16) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Exicton model in molecular spectroscopy. Pure Appl. Chem. 1965, 11 (3-4), 371−392. (17) Lenhard, J. R.; Hein, B. R. Effects of J-aggregation on the redox levels of a cyanine dye. J. Phys. Chem. 1996, 100, 17287−17296. (18) Behera, G. B.; Behera, P. K.; Mishira, B. K. Cyanine dyes: self aggregation and behaviour in sufactants. A Review. J. Surf. Sci. Technol. 2007, 23 (1-2), 1−31. (19) Lee, L. G.; Chen, C. H.; Chiu, L. A. Thiazole orange - a new dye for reticulocyte analysis. Cytometry 1986, 7 (6), 508−517. (20) Van Bockstaele, D. R.; Peetermans, M. E. 1,3′-Diethyl-4,2′quinolylthiacyanine iodide as a “thiazole orange” analogue for nuclei acid staining. Cytometry 1989, 10, 214−216. (21) Struganova, I. A.; Hazell, M.; Gaitor, J.; McNally-Carr, D.; Zivanovic, S. Influence of inorganic salts and bases on the J-band in the absorption spectra of water solutions of 1,1′-diethyl-2,2′-cyanine iodide. J. Phys. Chem. A 2003, 107 (15), 2650−2656. (22) Shklyarevskiy, I. O.; Christianen, P. C. M.; Aret, E.; Meekes, H.; Vlieg, E.; Deroover, G.; Callant, P.; van Meervelt, L.; Maan, J. C. Determination of the molecular arrangement inside cyanine dye aggregates by magnetic orientation. J. Phys. Chem. B 2004, 108, 16386−16391. (23) Didraga, C.; Pugzlys, A.; Hania, P. R.; von Berlepsch, H.; Duppen, K.; Knoester, J. Structure, spectroscopy, and microscopic model of tubular carbocyanine dye aggreagates. J. Phys. Chem. B 2004, 108, 14975−14985. (24) Cooper, W. Multiplet structures of aggregated states in 1,1′diethyl-2,2′-cyanine dye. Chem. Phys. Lett. 1970, 7 (1), 73−77. (25) von Berlepsch, H.; Bottcher, C.; Dahne, L. Structure of Jaggregates of pseudoisocyanine dye in aqueous solution. J. Phys. Chem. B 2000, 104 (37), 8792−8799. (26) Shank, N. I.; Pham, H. H.; Waggoner, A. S.; Armitage, B. Twisted cyanines: A non-planar fluorogenic dye with superior photostability and its use in a protein-based fluoromodule. J. Am. Chem. Soc. 2012, 135, 242−251. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;

Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Wallingford, CT, 2009. (28) Elbergali, A.; Nygren, J.; Kubista, M. An automated procedure to predict the number of components in spectroscopic data. Anal. Chim. Acta 1999, 379 (1-2), 143−158. (29) Kubista, M.; Sjoeback, R.; Albinsson, B. Determination of equilibrium constants by chemometric analysis of spectroscopic data. Anal. Chem. 1993, 65 (8), 994−998. (30) Kubista, M.; Sjoeback, R.; Nygren, J. Quantitative spectral analysis of multicomponent equilibria. Anal. Chim. Acta 1995, 302 (1), 121−125. (31) Mandal, A. K.; Pal, M. K. Spectral analysis of complexes of the dye, 3,3′-diethyl thiacyanine and the anionic surfactant, SDS by the principal component analysis method. Spectrochim. Acta, Part A 1999, 55 (7-8), 1347−1358. (32) Spencer, W.; Sutter, J. R. Kinetic study of the monomer-dimer equilibrium of methylene blue in aqueous solution. J. Phys. Chem. 1979, 83, 1573−1576. (33) Harada, T.; Kurihara, M.; Kuroda, R.; Moriyama, H. On-off switching of the novel thermochromic chiroptical behavior of pseudoisocyanine drive by association/dissociation. Chem. Lett. 2013, 41, 1442−1444. (34) Dammeier, B.; Hoppe, W. Crystal and molecular structure of N,N′-diethylpseudoisocyanin chloride. Acta Crystallogr., Sect. B 1971, 27, 2364−2370. (35) Marcus, Y. Ion Solvation; John Wiley & Sons Limited: Chichester, 1985. (36) Li, H. H.; Imai, Y.; Yamanaka, M.; Hayami, Y.; Takiue, T.; Matsubara, H.; Aratono, M. Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface. J. Colloid Interface Sci. 2011, 359, 5. (37) Ghasemi, J. B.; Shiri, F.; Deligeorgiev, T. G.; Kubista, M. Thermodynamic characterization of the dimerization equlibrium of newly synthesized polymethine cyanine dyes. J. Serb. Chem. Soc. 2008, 73 (10), 1011−1019. (38) Collins, K. D. Why continuum electrostatics theories cannot explain biological structure, polyelectrolytes or ionic strength effects in ion-protein interactions. Biophys. Chem. 2012, 167, 33−49. (39) Collins, K. D.; Neilson, G. W.; Enderby, J. E. Ions in water: Characterizing the forces that control chemical processes and biological structure. Biophys. Chem. 2007, 128, 95−104. (40) Lamere, J. F.; Saffon, N.; Dos Santos, I.; Fery-Forgues, S. Aggregation-induced emission enhancement in organic ion pairs. Langmuir 2010, 26 (12), 10210−10217. (41) Laursen, B. W.; Reynisson, J.; Mikkelsen, K. V.; Bechgaard, K.; Harrit, N. 2,6,10-Tris(dialkylamino)trioxatriangulenium salts: a new promising fluorophore. Ion-pair formation and aggregationo in nonpolar solvents. Photochem. Photobiol. Sci. 2005, 4, 568−576. (42) Westerlund, F.; Elm, J.; Lykkebo, J.; Carlsson, N.; Thyrhaug, E.; Akerman, B.; Soresen, T. J.; Mikkelsen, K. V.; Laursen, B. W. Direct probing of ion pair formation using a symmetric trianulenium dye. Photochem. Photobiol. Sci. 2011, 10 (1963-1973), 1963−1973. (43) von Berlepsch, H.; Kirsten, S.; Bottcher, C. Effects of alcohols on the J-aggregation of a carbocyanine dye. Langmuir 2002, 18, 7699− 7705. (44) Brooker, L. G. S.; Sklar, A. L.; Cressman, H. W. J.; Keyes, G. H.; Smith, L. A.; Sprague, R. H.; Van Lare, E.; Van Zandt, G.; White, F. L.; Williams, W. W. Color and constitution. VII. Interpretation of 9661

dx.doi.org/10.1021/la502124b | Langmuir 2014, 30, 9654−9662

Langmuir

Article

absorptions of dyes containing heterocyclic nuclei of different basicities. J. Am. Chem. Soc. 1945, 67, 1875−1889. (45) Brooker, L. G. S.; White, F. L.; Sprague, R. H. Color and constitution. IX. Absorption of cyanines derived from 3-methylisoquinoline; a rule relating basicity and absorption in symmetrical cyanines. J. Am. Chem. Soc. 1951, 73, 1087−1093. (46) Daltrozzo, E.; Scheibe, G.; Gschwind, K.; Haimerl, F. Structure of the J-aggregates of pseudoisocyanine. Photogr. Sci. Eng. 1974, 18 (4), 441−50.

9662

dx.doi.org/10.1021/la502124b | Langmuir 2014, 30, 9654−9662