Effect of Bulky Substituents on Thiopyrylium Polymethine Aggregation

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Effect of Bulky Substituents on Thiopyrylium Polymethine Aggregation in the Solid State: A Theoretical Evaluation of the Implications for All-Optical Switching Applications Rebecca L. Gieseking, Sukrit Mukhopadhyay, Chad Risko, Seth R. Marder, and Jean-Luc Bredas Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5028755 • Publication Date (Web): 21 Oct 2014 Downloaded from http://pubs.acs.org on October 28, 2014

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Chemistry of Materials

Effect of Bulky Substituents on Thiopyrylium Polymethine Aggregation in the Solid State: A Theoretical Evaluation of the Implications for All-Optical Switching Applications

Rebecca L. Gieseking, Sukrit Mukhopadhyay,‡ Chad Risko,† Seth R. Marder, and Jean-Luc Brédas #,*

School of Chemistry and Biochemistry and Center for Organic Materials for All-Optical Switching Georgia Institute of Technology Atlanta, Georgia 30332-0400

‡

Present address: The Dow Chemical Company, Midland, Michigan 48674 New permanent address: Department of Chemistry and Center for Applied Energy Research (CAER), University of Kentucky, Lexington, Kentucky 40506-0055, USA # New permanent address: Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology – KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia †

* Corresponding author: [email protected]

1 ACS Paragon Plus Environment


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Abstract

Polymethine dyes in dilute solutions display many of the optical properties required for alloptical switching applications. However, in thin films, aggregation and polymethine-counterion interactions can substantially modify their properties and limit their utility. Here, we examine the impact of a series of bulky substituents on the solid-state molecular packing of thiopyrylium polymethines, by using a theoretical approach combining molecular-dynamics simulations and quantum-chemical calculations. Importantly, it is found that the positions of the substituents near the center and/or ends of the dye determine the extent to which aggregation is reduced; in particular, substituents near the polymethine center primarily modify the type of aggregation that is observed, while substituents near the polymethine ends reduce aggregation and aid in maintaining solution-like properties in the solid state. Our theoretical study elucidates relationships between molecular structure and bulk optical properties and provides design guidelines for all-optical switching materials.


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1. Introduction

Understanding the structure-property relationships that underlie the molecular- and materialscale nonlinear optical (NLO) properties of organic π-conjugated materials has enabled the development of materials for many NLO applications, such as electro-optic modulation,1-4 twophoton imaging,5-7 or optical power limiting.8,9 However, developing materials for all-optical switching (AOS) applications at telecommunications wavelengths has remained a challenge.10,11 In all-optical switching, one beam of light is used to control a second beam of light through their interactions within a material with the requisite third-order nonlinear optical response. The NLO material must have: (i) a very large real part of the macroscopic third-order electric susceptibility χ(3), stemming from a very large real part of the third-order molecular polarizability γ, to maximize the modulation of the material refraction index; and (ii) a very small imaginary part of χ(3) (and γ) to minimize two-photon absorption losses; as a result, an appropriate figure-of-merit for AOS applications corresponds to the a ratio of the real and imaginary parts |Re(χ(3))/Im(χ(3))|, which is required in practice to be >> 4π.12

The inherent electronic and optical properties of polymethines endow them with favorable NLO properties for AOS.11,13,14 Symmetric polymethines typically have a large energetic spacing (energy window) between the first excited state, which is one-photon allowed, and the second excited state, which is two-photon allowed (Figure 1), with the two-photon state appearing at about 1.7 times the energy of the one-photon state.13,14 If the energy ħω of the incoming light is tuned such that 2ħω falls into the window between the first and second excited-state energies, it



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has been demonstrated that a large negative Re(γ) and small Im(γ) can be achieved at the molecular level.15

However, polymethines have a strong tendency to aggregate, which dramatically changes their linear and nonlinear optical properties.16-20 This is a problematic feature as realizing a large χ(3) in a molecular material requires that the material contain a high number density N of molecules with a very large molecular third-order polarizability γ. Our earlier studies have shown that, regardless of whether the aggregates form in H-aggregate, J-aggregate, or perpendicular geometries, aggregation closes the energetic window between the first and second excited states (Figure 1), which strongly decreases |Re(γ)| and increases Im(γ).21 Similarly, ion pairing between the charged polymethines and their counterions can cause the polymethine charge to localize on one end of the molecule, which also alters the optical properties.21-24 Thus, the message from earlier investigations is that, to achieve large |Re(χ(3))| and small Im(χ(3)) values, electronic interactions between polymethines and ion pairing between the polymethines and counterions must remain negligibly small at large concentrations.

Figure 1. Excited state energies of a prototypical polymethine and several aggregate structures (data taken from Ref. 21). 4 ACS Paragon Plus Environment

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A recent strategy proven successful to minimize polymethine aggregation is the addition of bulky substituents to various parts of the polymethine structure to prevent close contact among the polymethines. Using this approach, figures-of-merit (|Re(χ(3))/Im(χ(3))|) as large as 20 have been achieved in thin films of thiopyrylium polymethine dyes.25 However, one disadvantage of this approach is that the steric bulk of the added substituents substantially reduces the effective concentration of the NLO-active chromophores in neat films, as the substituents more than double the molar mass of the polymethine/counterion complex. If the bulky substituents can be selected strategically to prevent aggregation without adding excess steric bulk, it may be possible to achieve a larger polymethine concentration while still effectively eliminating aggregation to achieve a larger |Re(χ(3))|.

Understanding the structure-property relationships involved in polymethine aggregation requires a molecular-scale view of the aggregate geometries, which in many cases is not easily achievable through experimental approaches. We have recently introduced a computational methodology combining molecular-dynamics and electronic-structure approaches, which in the case of simple streptocyanine dyes has provided insight into the molecular-scale polymethine-counterion and polymethine-polymethine packings and the resulting impact on the electronic couplings among neighboring polymethines.26 Here, we apply this approach to a series of AOS-relevant thiopyrylium polymethines that have various patterns of bulky substitution: (i) on the polymethine end groups; (ii) near the molecular center; or (iii) a combination of the two ( Figure 2). Since the thiopyrylium polymethine dyes are positively charged, we consider as counterion the chemically soft, bulky tetraphenylborate BPh4- anion, similar to the experimentally-used counterion.25 Our goal is to understand the dependence of the aggregate



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structures and electronic properties on the substitution pattern. Given that aggregation dramatically affects the thin-film NLO properties, understanding and controlling aggregation are critical to develop materials with bulk NLO properties suitable for AOS applications.

Compound

R

R’

R”

1 2 3 4 5 6 7 8

-H -H -H -H t-butyl t-butyl t-butyl t-butyl

-H carbazole -H carbazole -H carbazole -H carbazole

2 -H cyclohexyl fluorene fluorene cyclohexyl cyclohexyl fluorene fluorene

Concentration (M) 1.83 1.15 1.40 1.00 1.14 0.87 0.99 0.78

Figure 2. Chemical structures and bulk concentrations of the polymethine dyes and BPh4counterion studied here. R denotes the substituents on the polymethine ends; R’, on the center front; and R”, on the center back.


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2. Theoretical Methodology

Atomistic molecular dynamics (MD) simulations were performed using the OPLS-AA force field27 in the GROMACS 4.5.4 package,28 which has previously shown good agreement with experiment for polymethine aggregates.26,29 Initial isolated polymethine and counterion geometries were obtained via geometry optimization using a density functional theory (DFT) approach employing the ωB97XD functional30,31 and cc-pVDZ basis set32 as implemented in the Gaussian 09 (Rev. B.01) suite of programs.33 We note that torsions about all C-C bonds in the polymethine backbone were restrained to fall within 10° of planarity during the MD simulations to prevent trans-cis isomerization during high-temperature annealing; although this isomerization can occur through a photoisomerization process,34 the energetic barrier to rotation is large in the ground state.35 The atomic charges used in the MD simulations were obtained from natural bond order (NBO) calculations of the isolated polymethines and counterion at the ωB97XD/cc-pVDZ level; using these charges in the simulations has previously led to good agreement with experimental crystal structures and the DFT-optimized polymethine-counterion structures of simpler polymethines.26 We note that only symmetric polymethine charge distributions are considered here as we have shown in our previous investigations that using symmetric or asymmetric (symmetry-broken) charge distributions on the dyes has only minor effect on the aggregate geometries;26 work to better understand the origin of symmetry-breaking in polymethines is ongoing.

The initial configurations for the MD simulations were constructed by randomly placing the polymethines

and

counterions

in

a

cubic

periodic


box;

the

total

number

of


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polymethine/counterion pairs was varied to maintain a total number of atoms on the order of 40,000 for all systems (the complete list of number of polymethines/counterion pairs per simulation, equilibrated system volume, and number of simulations is provided in the Supporting Information, SI). The energy was then minimized at constant volume; this was followed by an initial run of 10 ps at 50 K under the NVT ensemble using a time step of 0.5 fs to avoid atomic overlap. The simulation box was then equilibrated at 800 K under the NPT ensemble until the volume equilibrated and for several additional ns using the Berendsen barostat. Three configurations at 1 ns intervals were extracted from the simulations to obtain a series of independent thin-film-like (amorphous) geometries. These configurations were then equilibrated for 1 ns at 800 K using the Parrinello-Rahman barostat, cooled over 2 ns to 300 K, and simulated for 1 ns at 300 K. The final 1 ns of this simulation was considered for analysis. For all simulations, the Nose-Hoover thermostat and periodic boundary conditions were used with a time step of 1 fs. A spherical cutoff of 0.9 nm was taken for the summation of van der Waals interactions and the Ewald summation was used for Coulomb interactions. For all polymethines, the results were averaged over enough simulations to obtain a total of at least 1200 polymethinecounterion complexes. The bulk concentration of polymethines was computed as: [(# polymethines) / (average box volume at 300 K)] and then converted to molarity.

The analysis of the results regarding the polymethine- counterion and polymethine-polymethine geometries and interactions was conducted as in our earlier work.26 For the sake of completeness, the protocol for this analysis is reproduced in the SI.


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The electronic couplings between neighboring polymethines were then evaluated on the basis of the MD geometries. All polymethine pairs within a radial distance of 6 Å and a longitudinal offset distance of 12 Å (see SI) were extracted from five frames of each simulation at an interval of 250 ps. For each pair, the electronic coupling (transfer integral) was computed using the INDO Hamiltonian36 in the Mataga-Nishimoto parameterization.37 The electronic couplings between the HOMOs and LUMOs were considered. The electronic coupling values were counted in bins of 10 meV width and normalized by multiplying by the total volume of pairs considered and dividing by the bulk density of polymethine pairs.

3. Results and Discussion

To investigate the effect of the substitution pattern on polymethine aggregation, a series of thiopyrylium polymethines 1-8 were investigated, see Figure 2. These have varying substituents on three parts of the polymethine structure: (i) on the thiopyrylium end groups (R); (ii) on the center of the polymethine bridge in the “front” of the molecule (R’); and (iii) on the center “back” (R”). In each case, one bulky substituent and one less bulky alternative were considered. At this stage, it is useful to stress that our present focus is on the location of the bulky substituents and not on the substituent shape or size. We note that the carbazole (R’) and fluorene (R”) substituents are rigid and maintain large torsion angles relative to the polymethine backbone, which implies that they have a large projection above and below the polymethine π plane. Because of the steric bulk of the substituents, the concentration of the neat polymethine/counterion systems varies by a factor of 2.3 across the series. We underline that the series of molecules 1-8 has been chosen because their chemical structures are



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(nearly) identical to molecules that have recently been synthesized and characterized in terms of their linear and nonlinear optical behavior.25 We note that the optical and NLO characteristics of the neat films are comparable to those of the 50 wt% chromophore in amorphous polycarbonate, and neat films of some structurally similar compounds have shown large figures-of-merit for AOS.25

For each polymethine, the polymethine-counterion and polymethine-polymethine packing configurations and the subsequent impact on polymethine intermolecular electronic couplings were considered. In each of the following sections, we first discuss the limiting cases of the unsubstituted (1) and fully substituted (8) polymethines to highlight the extent to which bulky substituents can limit polymethine aggregation and then describe the specific impact of each substituent location by considering polymethines 2-7.

3.1. Polymethine-counterion interactions

Because ion pairing between the polymethine and counterion can induce symmetry-breaking, with the consequence that |Re(γ)| decreases and Im(γ) increases,21-24 we first consider the geometries of polymethine-counterion interactions. For each polymethine, the positions of all nearby counterions are considered.

For polymethine 1, the counterions have a broad range of positions with similar probabilities, as indicated by the ring of higher probability (darker color) around the polymethine in


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Figure 3. The counterion probability is slightly larger near the center of the polymethine with a small positive displacement along the polymethine short axis (the reason being that this structure allows a stronger electrostatic attraction between the polymethine and counterion because of the inherent charge alternation along the polymethine backbone26). The broad distribution of counterion probability found here is similar to that observed for the streptocyanine/BPh4system.26

Figure 3. Counterion probability distribution in bulk MD simulations of the complexes of polymethines 1 (top) and 8 (bottom) with BPh4-. The color scale corresponds to the probability of finding aggregates, with a probability of one corresponding to the average bulk density of polymethine-counterion pairs. The counterion probability in each 1 Å × 1 Å square is averaged over a depth ranging from –5 Å to +5 Å (left) and –12 Å to +12 Å (right). The superimposed polymethine images represent the orientation and scale of the molecule. 11 ACS Paragon Plus Environment


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For the fully substituted polymethine 8, the bulky substituents cause the counterion to sit much farther from the polymethine backbone than for 1, as shown by the comparison of the two plots in Figure 3. Indeed, most of the polymethine backbone is shielded from the counterion by the substituents. In particular, the counterion cannot approach the back of the polymethine backbone due to the blocking induced by the fluorene and t-butyl substituents; although there would appear in the figure to be some counterion probability in this area, it is due to counterions that are several angstroms above or below of the plane of the polymethine.

The most probable counterion position is near the sulfur atoms in the thiopyrylium rings; this is in fact the only area where the counterion is sterically able to be close to the positively charged polymethine core. Analysis of the radial distribution functions (RDFs, see SI) for the sulfurboron distances shows an increase in the onset from 4.4 Å in 1 to 4.8 Å in 8. It is useful to note that, although the probability of finding a counterion near the sulfur atom is much larger relative to the bulk counterion concentration for 8 than for 1 as seen in Figure 5, this difference is primarily due to the factor of 2.3 difference in polymethine concentrations (see Figure 2); when taking the concentration difference into account, the peak probability is larger by only a factor of 1.2 for polymethine 8 than for 1 in absolute terms.

When bulky substituents are added only to the center of the polymethine backbone (2-4), the counterion has expectedly a high probability of being near the thiopyrylium rings where there is no steric hindrance to ion pairing (


Figure 4). In contrast, in the polymethines with bulky end substituents (5-7), the counterion can only approach the terminal thiopyrylium rings in positions near the sulfur atoms as in the case of 8. Taking into account the variations in polymethine concentrations, the probabilities of finding the counterions near the sulfur atoms are larger by only a factor of 1.2-1.4 for polymethines 5-7 with respect to 1.

13


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The substitution pattern also influences the counterion probability distribution near the polymethine backbone. In some of the polymethines, a cyclohexyl ring is added on the back of the bridge (R”) to increase thermal and photo-stability.10 This ring also aids in partially shielding the counterion from interacting with the back of the polymethine. In polymethines 7-8, the tbutyl and fluorene substituents completely prevent the counterion from approaching the back of the polymethine.

In polymethines 5 and 7 with no substituents on the center front, the counterion is able to fit between the end t-butyl substituents and approach the front of the polymethine backbone, as evidenced by the dark areas just above the centers of the plots in

Figure 4. When normalizing to consider concentration differences, the absolute probability of finding a counterion in the areas with the largest counterion probabilities is about 1.5 times larger for 5 and 7 than it is for 1.

Since polymethine symmetry-breaking is related to the combined electrostatic interactions derived from all counterions and polymethines, it is challenging to directly evaluate the extent of symmetry breaking in these systems. However, since symmetry breaking is essentially induced by the electric field felt along the polymethine long axis, counterions positioned near one end of the polymethine can have a much more significant contribution to symmetry breaking than counterions either near the center of the polymethine or far from the polymethine core. In polymethines 1-4 without bulky substituents near the end groups, the large probability of counterions near the end groups underline the potential for significant symmetry breaking.


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Although the bulky substituents near the end groups in 5-8 slightly increase the counterion probability in a small area near the sulfur atoms, they greatly reduce the counterion probability in all other geometries near the thiopyrylium end groups. This suggests that these bulky substituents may be sufficient to reduce (but not necessarily entirely eliminate) symmetry breaking of the polymethines. Further efforts to understand the role of the counterion positions on polymethine symmetry breaking are currently in progress. 3.2. Polymethine-polymethine interactions

Since strong aggregation of polymethine dyes introduces low-lying two-photon excited states that very much decrease the figure-of-merit for all-optical switching,21 it is important to evaluate the aggregate geometries, to which we now turn with a focus on the relative positions and orientations of the molecular long axes in polymethine pairs. All polymethine pairs with intermolecular backbone-to-center distances (radial distances) less than 6 Å are considered. For each pair, the offset of the molecular centers along the long axis of one polymethine and the torsion angle between the long axes are examined. This analysis allows us to distinguish between perpendicular aggregates (large torsion angles) and parallel aggregates in either H-aggregate (small longitudinal offset) or J-aggregate (large offset) geometries. This geometric distinction between H-aggregation and J-aggregation is largely consistent with the experimentally observed spectral shifts of thin films of structurally similar polymethines.25 We note that, since this approach neglects the polymethine short-axis orientations, it provides an overview of the sterics of polymethine-polymethine interactions but disregards the details of the electronic interactions that determine to what extent the optical properties are affected (the next section will address this limitation).



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Polymethine 1 forms many aggregates, each dye having an average of 2.6 neighbors within the distance cutoffs used. The most probable pair geometries for 1 are H-aggregates with relatively small offsets of 1-4 Å, as shown by the darker region near the bottom left corner in

Figure 5.38 While H-aggregates are more common than J-aggregates or perpendicular aggregates by roughly a factor of four, it should be emphasized that a broad distribution of aggregate geometries is observed.

Figure 5. Probability distribution of aggregate geometries for the complexes of polymethines 1 and 8 with BPh4-. Two molecules are considered to aggregate when their intermolecular centerto-center distance is below 6 Å. The color scale corresponds to the probability of finding aggregates; a probability of one corresponds to the average bulk density of polymethine pairs. The offset is defined as the longitudinal displacement between the two polymethine centers along the long axis of the first polymethine backbone; the torsion angle is that between the long axes of the two polymethines.

Very importantly and in strong contrast to 1, polymethine 8 has almost no geometries in which the polymethines can aggregate such that the π-backbones are near each other, as indicated by the nearly white areas throughout most of the probability distribution plot on the right of Figure


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7. The probability of finding any pairs within a radial distance of 6 Å only becomes significant at offset distances greater than 11 Å. Within the cutoffs, each polymethine has an average of only 0.32 neighbors. The differences between the aggregate probabilities in these systems highlight that the bulky substituents eliminate nearly all polymethine aggregation by sterically hindering interactions between the polymethine cores.

The specific locations of the substituents greatly impact the polymethine aggregate structures. The addition of bulky substituents to the center of the polymethine (2-4; Figure 6) hinders H-aggregation, though stacking of the terminal thiopyrylium rings is still possible. Since the thiopyrylium rings can stack within a wide distribution of torsion angles, these structures range from J-aggregates with offsets > 9 Å to perpendicular aggregates with offsets on the order of 4-5 Å. A detailed analysis shows that some of the pairs at large torsion angles instead have interactions between one thiopyrylium ring of the first polymethine and the π-system of the carbazole or fluorene substituent of the second polymethine (representative images of the polymethine aggregates are provided in the SI). The presence of substituents both in the center front and the center back of the polymethine backbone hinders H-aggregation to a greater extent than having only one such substituent present.



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Figure 6. Probability distribution of aggregate geometries for the complexes of polymethines 2-7 with BPh4-. Two molecules are considered to aggregate when their intermolecular center-tocenter distance is below 6 Å. The color scale corresponds to the probability of finding aggregates; a probability of one corresponds to the average bulk density of polymethine pairs. The offset is defined as the longitudinal displacement between the two polymethine centers along the long axis of the first polymethine backbone; the torsion angle is that between the long axes of the two polymethines.

Interestingly, substituents on the terminal groups tend to reduce polymethine aggregation in all geometries. This is the case even in 5 where no bulky substituents are added to the center of the polymethine. However, while aggregation is substantially reduced, there are still more polymethine pairs in close proximity than in 8. Analysis of these pairs shows that 5 forms aggregates in both parallel and perpendicular geometries. In contrast, in 6 and 7, the polymethine pairs within the distance cutoffs typically have a significant tilt or short-axis offset between the π-systems, which reduces the stacking efficacy. This distinction between π-stacked and non-πstacked aggregate geometries points to the need for further analysis to fully understand the effect of substituents on the materials optical properties.


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3.3. Electronic couplings between polymethine molecules

We have shown earlier21,26 that electronic couplings between adjacent polymethines larger than ~10 meV alters the characteristics of the lowest-lying excited states and adversely affects the nonlinear optical properties relevant for AOS applications; see the SI for further information. Thus, it is important to evaluate the impact of substitution on the electronic couplings between the HOMO and LUMO levels of neighboring molecules (as these are the electronic levels essentially involved in the lowest excited states. Here, we focus on the HOMO electronic couplings (the LUMO couplings are similar and described in the SI). We recall that electronic couplings are a direct function of wavefunction overlap (and not of spatial overlap)39 and thus strongly depend on aggregate geometries.

As indicated in the previous section, polymethine 1 forms a substantial number of polymethine pairs in close proximity. The electronic-coupling calculations were performed for all of these polymethine pairs extracted at several time steps from the MD simulations; there occurs a mixture of stacked pairs and pairs in orientations with no significant electronic interaction between the dye molecules. Here, there are many polymethine pairs with substantial electronic couplings; on average, each polymethine has 1.4 neighbors with electronic couplings greater than 10 meV. The distribution of electronic couplings extends to very large values, significantly beyond the 100 meV limit chosen in Figure 7. The large number of polymethine pairs with strong electronic couplings points to large modifications of the linear and nonlinear optical properties upon aggregation, which is typical of traditional polymethines.16,18-21 H-aggregates tend to have the largest electronic couplings, with



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the electronic coupling generally decreasing as a function of increasing offset and torsion angle (the distributions of electronic couplings as a function of aggregate geometry is provided in the SI). Because of the nodal pattern in the orbitals, in pairs with nearly parallel long axes, the electronic coupling strongly fluctuates as the offset increases, as rationalized in our earlier work.39

Figure 7. Distribution of the absolute electronic couplings between adjacent dye molecules for polymethines 1 and 8. The number of pairs in each range is normalized relative to the bulk density of polymethine pairs as in the previous polymethine-polymethine analysis. For polymethine 1, some polymethine pairs (approximately 23 pairs per frame of 500 polymethines) have an electronic coupling > 100 meV.

In polymethine 8, there are substantially fewer polymethine pairs in close proximity, out of which only a small number have any significant electronic coupling: less than 3% of the polymethines have a neighbor with an electronic coupling > 10 meV. This marked reduction in the number of closely-packed polymethine pairs combined with the generally weak electronic couplings underline that polymethine 8 should largely retain dilute solution-like absorption


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spectra and NLO properties in the bulk, which is fully consistent with very recent experimental observations.25

In polymethines with bulky substituents exclusively in the molecular center, the number of polymethine pairs with electronic couplings greater than 100 meV is strongly reduced relative to 1, although there are still many pairs with electronic couplings > 10 meV ( Figure 8). This is consistent with the elimination of H-aggregates; we note that the electronic coupling in a well-aligned J-aggregate is substantially smaller than that in a well-aligned Haggregate, due to decreased wavefunction overlap of the π-orbitals.39 The large number of polymethine pairs with significant electronic couplings in J-aggregate geometries is consistent with a marked broadening/shift of the thin-film absorption spectra with respect to dilute solutions, as has been observed experimentally.25



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Figure 8. Distribution of the absolute electronic coupling between dye molecules for polymethines 2-7. The number of pairs in each range is normalized relative to the bulk density of polymethine pairs as in the previous polymethine-polymethine analysis.

Although the presence of substituents in the center of the polymethine backbone changes the bulk morphologies relative to 1, the effect is primarily one of changing what type of aggregation occurs. The polymethines and counterions still ion-pair in geometries where the counterions are near the polymethine end groups and J-aggregation is enhanced. While greater steric hindrance is needed to substantially reduce aggregation, a strategy of adding steric bulk only to the center of the polymethine structure should prove a viable strategy to selectively form J-aggregates in applications where controlled aggregation is desired.


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In contrast, bulky substituents on the end groups result in largely reduced electronic couplings between polymethines, as was seen for polymethine 8. Among these polymethines, 5 has somewhat more pairs with significant electronic couplings. This is consistent with the limited steric bulk and the number of pairs that appear to have significant stacking of the conjugated backbones, though the maximum electronic couplings remain low because the pairs in π-stacked geometries have relatively large intermolecular distances and/or large torsion angles. While polymethines 6 and 7 have substantially more pairs in close proximity than does 8, the number of pairs with significant electronic couplings is still small. These small electronic couplings are consistent with the moderate thin-film AOS figures-of-merit measured for polymethines with similar substitution patterns.25

Overall, our results indicate that while bulky substituents tend to reduce polymethine aggregation through steric hindrance, it is the location of the bulky substituents that plays a critical role in determining the extent of aggregation and the types of aggregates that form. In particular, the degree of electronic coupling is not directly correlated with the number of pairs in close proximity. This is especially apparent when comparing polymethines 4 and 5. These two polymethines have essentially the same number of neighbors per polymethine within the distance cutoffs (

Figure 9); however, polymethine 4 has nearly three times as many pairs with significant electronic couplings as polymethine 5. The large decrease in the number of polymethine pairs with significant electronic couplings in polymethines 5-7 suggests that increasing the steric bulk on all parts of the molecular structure (ends, center back, and center front) is not be a necessary



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condition to maintain solution-like linear and nonlinear optical properties in thin films, which is consistent with experimental observations.25

Figure 9. Distribution of the absolute electronic couplings between polymethines, normalized in terms of the average number (expectation value) of neighboring polymethines each polymethine will have within each electronic coupling range.


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4. Synopsis

Understanding polymethine aggregation is an essential step in developing polymethine-based materials with a large figure-of-merit for all-optical switching in thin-film devices. Both polymethine-counterion interactions and polymethine-polymethine interactions must be minimized to prevent symmetry-breaking of the polymethines and the appearance of low-lying two-photon excited states. The molecular-scale aggregate structures described here provide insight into why the addition of bulky substituents such as those depicted in Figure 2 has proven effective experimentally at maintaining solution-like linear and nonlinear optical properties in thin films. In particular, a proper choice of bulky substituents can result in efficient steric hindrance for both polymethine-counterion and polymethine-polymethine interactions, thereby nearly eliminating polymethine pairs with significant electronic couplings. However, it must be borne in mind that the presence of bulky substituents decreases the concentration of NLO-active chromophores in the thin films, thereby reducing the maximum achievable |Re(χ(3))|.

With regard to the polymethine-counterion interactions, we have found that while the substituents we have investigated tend to sterically reduce the interactions, the counterion can still appear close to the sulfur atoms of the thiopyrylium rings even in the presence of t-butyl end groups. Thus, such bulky substituents may not be sufficient to entirely eliminate symmetry breaking of the polymethines due to ion pairing.



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The number and locations of the substituents are seen to affect polymethine aggregation dramatically. The unsubstituted thiopyrylium polymethine forms aggregates in many geometries with substantial electronic couplings, particularly H-aggregates. Substituents in the center of the molecule tend to hinder H-aggregation and enhance J-aggregation, while substituents on the ends of the polymethine somewhat reduce but do not entirely prevent aggregation. When bulky substituents are present on both the molecular ends and center, aggregation is then almost entirely suppressed due to steric hindrance. Our analysis of the electronic-coupling results points out that it is possible to strategically select bulky substituents that increase the AOS figure-ofmerit with minimal excess bulk in order to maximize the chromophore concentration in thin films.

To conclude, our methodology allows a deeper understanding of how bulky substituents affect polymethine aggregation on the molecular scale as compared to what can be easily obtained through experimental studies. Thus, our theoretical approach provides a means of evaluating the effects of molecular structure on aggregation prior to synthesis, which will aid in providing further design guidelines for polymethine-based AOS materials.


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Acknowledgements

This work was supported by the AFOSR MURI program (FA9550-10-1-0558) within the Center for Organic Materials for All-Optical Switching (COMAS). We gratefully acknowledge stimulating discussions with Drs. S. Barlow, J.M. Hales, and J.W. Perry.

5. Supporting Information

Further details concerning the simulation parameters, polymethine-counterion geometries, polymethine pair geometries and electronic couplings, and molecular charges and force-field parameters are provided. This information is available free of charge via the Internet at http://pubs.acs.org.



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6. References (1) Kim, T.-D.; Kang, J.-W.; Luo, J.; Jang, S.-H.; Ka, J.-W.; Tucker, N.; Benedict, J. B.; Dalton, L.; Gray, T.; Overney, R. M.; Park, D. H.; Herman, W. N.; Jen, A. K. Y. J. Am. Chem. Soc. 2007, 129, 488. (2) Dalton, L. R.; Benight, S. J.; Johnson, L. E.; Knorr, D. B.; Kosilkin, I.; Eichinger, B. E.; Robinson, B. H.; Jen, A. K. Y.; Overney, R. M. Chem. Mater. 2011, 23, 430. (3) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem. Rev. 2010, 110, 25. (4) Kwon, O. P.; Kwon, S.-J.; Jazbinsek, M.; Seo, J.-Y.; Kim, J.-T.; Seo, J.-I.; Lee, Y. S.; Yun, H.; Günter, P. Chem. Mater. 2011, 23, 239. (5) Albota, M. A.; Beljonne, D.; Brédas, J. L.; Ehrlich, J. E.; Fu, J. Y.; Heikal, A. A.; Hess, S.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-Maughon, D.; Perry, J. W.; Röckel, H.; Rumi, M.; Subramaniam, G.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998, 281, 1653. (6) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Angew. Chem. Int. Ed. 2009, 48, 3244. (7) He, G. S.; Tan, L.-S.; Zheng, Q.; Prasad, P. N. Chem. Rev. 2008, 108, 1245. (8) Spangler, C. W. J. Mater. Chem. 1999, 9, 2013. (9) Hales, J. M.; Cozzuol, M.; Screen, T. E. O.; Anderson, H. L.; Perry, J. W. Opt. Express 2009, 17, 18478. (10) Hales, J. M.; Barlow, S.; Kim, H.; Mukhopadhyay, S.; Brédas, J.-L.; Perry, J. W.; Marder, S. R. Chem. Mater. 2014, 26, 549. (11) Gieseking, R. L.; Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Adv. Mater. 2014, 26, 68. (12) Stegeman, G. I.; Stolen, R. H. J. Opt. Soc. Am. B 1989, 6, 652. (13) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry, J. W.; Bourhill, G.; Brédas, J. L.; Pierce, B. M. Science 1994, 265, 632. (14) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bredas, J. L. J. Am. Chem. Soc. 1994, 116, 10703. (15) Hales, J. M.; Matichak, J.; Barlow, S.; Ohira, S.; Yesudas, K.; Bredas, J. L.; Perry, J. W.; Marder, S. R. Science 2010, 327, 1485. (16) Belfield, K. D.; Bondar, M. V.; Hernandez, F. E.; Przhonska, O. V.; Yao, S. Chem. Phys. 2006, 320, 118. (17) Jelley, E. E. Nature 1936, 138, 1009. (18) Baraldi, I.; Caselli, M.; Momicchioli, F.; Ponterini, G.; Vanossi, D. Chem. Phys. 2002, 275, 149. (19) von Berlepsch, H.; Böttcher, C.; Dähne, L. J. Phys. Chem. B 2000, 104, 8792. (20) Scarpaci, A.; Nantalaksakul, A.; Hales, J. M.; Matichak, J. D.; Barlow, S.; Rumi, M.; Perry, J. W.; Marder, S. R. Chem. Mater. 2012, 24, 1606. (21) Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Chem. Sci. 2012, 3, 3103. (22) Bouit, P.-A.; Aronica, C.; Toupet, L.; Guennic, B. L.; Andraud, C.; Maury, O. J. Am. Chem. Soc. 2010, 132, 4328. (23) Bamgbelu, A.; Wang, J.; Leszczynski, J. J. Phys. Chem. A 2010, 114, 3551. (24) Karaman, R.; Menger, F. M. J. Mol. Struct. THEOCHEM 2010, 959, 87. 28 ACS Paragon Plus Environment

Page 29 of 30

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(25) Barlow, S.; Brédas, J. L.; Getmanenko, Y. A.; Gieseking, R. L.; Hales, J. M.; Kieu, K.; Kim, H.; Marder, S. R.; Norwood, R. A.; Perry, J. W.; Peyghambarian, N.; Risko, C.; Shahin, S.; Zhang, Y. Mater. Horiz. 2014, DOI: 10.1039/c4mh00068d. (26) Gieseking, R. L.; Mukhopadhyay, S.; Shiring, S. B.; Risko, C.; Brédas, J.-L. J. Phys. Chem. C 2014, http://dx.doi.org/10.1021/jp507920j. (27) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem. Soc. 1996, 118, 11225. (28) Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. J. Chem. Theory Comput. 2008, 4, 435. (29) Das, S.; Bwambok, D.; El-Zahab, B.; Monk, J.; de Rooy, S. L.; Challa, S.; Li, M.; Hung, F. R.; Baker, G. A.; Warner, I. M. Langmuir 2010, 26, 12867. (30) Chai, J.-D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615. (31) Chai, J.-D.; Head-Gordon, M. J. Chem. Phys. 2008, 128, 084106. (32) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007. (33) Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2010. (34) Huang, Z.; Ji, D.; Xia, A.; Koberling, F.; Patting, M.; Erdmann, R. J. Am. Chem. Soc. 2005, 127, 8064. (35) Baraldi, I.; Momicchioli, F.; Ponterini, G.; Tatikolov, A. S.; Vanossi, D. Phys. Chem. Chem. Phys. 2003, 5, 979. (36) Ridley, J.; Zerner, M. Theor. Chim. Acta 1973, 32, 111. (37) Mataga, N.; Nichimoto, K. Z. Phys. Chem. 1957, 13, 140. (38) We note that in similar systems, aggregation is strongly dependent on the counterion size; see ref. 10. (39) Brédas, J. L.; Calbert, J. P.; da Silva Filho, D. A.; Cornil, J. Proc. Nat. Acad. Sci. 2002, 99, 5804.



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Table of Contents Graphic


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Effect of Bulky Substituents on Thiopyrylium Polymethine Aggregation

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