Effects of Counterions with Multiple Charges on the Linear and

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Effects of Counterions with Multiple Charges on the Linear and Nonlinear Optical Properties of Polymethine Salts Zhong’an Li,† Hyeongeu Kim,‡ San-Hui Chi,‡ Joel M. Hales,‡ Sei-Hum Jang,† Joseph W. Perry,*,‡ and Alex K.-Y. Jen*,† †

Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

‡

S Supporting Information *

ABSTRACT: Counterions with multiple charges have been used to organize polymethines into multichromophore salt complexes. The intramolecular Coulombic interactions between multiply charged counterions and polymethines can play a significant role in modifying intermolecular interactions (i.e., aggregation). Here, we report a detailed study of the linear and nonlinear optical properties of such complexes with dications and hexacations over a large range of molecular concentrations. Our results have demonstrated that, despite strong intramolecular interactions, the preorganization of chromophores into the multichromophore salt constructs with multiple charges can provide a steric repulsive effect that can mitigate intermolecular interactions in the solid state. This results in a more efficient translation between microscopic and macroscopic optical properties for highly polarizable polymethines, which is essential for all optical signal switching.

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INTRODUCTION High-speed all-optical signal processing (AOSP) requires nonlinear optical (NLO) materials in waveguide devices with good optical quality and very large macroscopic third-order nonlinearities (χ(3)).1−3 Among various third-order NLO organic materials, interest in using polymethine dyes as material candidates for AOSP has recently grown rapidly.4−6 One of the most attractive features of polymethines as NLO materials is their very large magnitude of the molecular thirdorder polarizability (γ), because of the minimal bond-length alternation (BLA) between adjacent C−C bonds in the polymethine backbone, thereby giving rise to a large transition dipole moment and a small energy gap between the ground state and first excited state. In addition, polymethines possess a large energy gap between the lowest vibronically allowed twophoton absorption (2PA) and the first allowed 2PA band, providing a wide optical transparency window for both linear and nonlinear optical absorption at near-infrared (NIR) wavelengths.4−9 To enable AOSP applications, one prerequisite is that the NLO chromophores should be processable as high-opticalquality thin films with high loading density. However, it is wellknown that polymethines are highly polarizable ionic compounds that are susceptible to strong intermolecular and/ or intramolecular interactions in the solid state, because of aggregation10,11 or ion paring.12 If such interactions are not controlled, undesirable charge localization and/or distortion of the molecular symmetry could result in a dramatic reduction of χ(3). In addition, these interactions can produce shifting and © 2016 American Chemical Society

broadening of both the linear and 2PA bands, which may reduce the width of the optical transparency window and, thus, the 2PA figure of merit (FOM = |Reχ(3)/Imχ(3)|). Therefore, effective translation of the desirable optical properties from the molecular level to high-number-density solid-state materials is a major challenge for the development of polymethines as highperformance third-order NLO materials.4,10,13−15 The relative spatial distribution and nature of counterions have been shown to significantly affect the electronic structure of polymethines and their molecular geometries, as well as their linear and NLO properties.12,16−19 Dähne et al. have demonstrated that the distortions of polymethine electronic structures in crystals are predominantly driven by the Coulombic interactions between cationic dyes and anionic counterions.16 Quantum chemical calculations on selected polymethine-counterion complexes by Gieseking et al. have clearly shown that the alteration of the average BLAand, consequently, the NLO properties of polymethinesare mainly due to the strong electrostatic interactions between polymethine and counterion.18,19 Moreover, our recent work showed that the counterionic structure has a profound effect on the optical quality of guest−host blend films of anionic tricyanofuran (TCF)-terminated polymethines with very high loading densities (∼50 wt %).20 To improve material properties Received: February 12, 2016 Revised: April 9, 2016 Published: April 11, 2016 3115

DOI: 10.1021/acs.chemmater.6b00641 Chem. Mater. 2016, 28, 3115−3121

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Chemistry of Materials

or PM−CI interactions (i.e., Coulombic). Furthermore, these interactions could be either intramolecular or intermolecular in nature, depending on the concentration, which controls the distance between adjacent polymethine complexes. The normalized linear absorption spectra of the three salts in chloroform (CHCl3) solutions at dilute concentrations (i.e., 12 μM) are shown in Figures 2a−c, and a summary of their optical properties on a per-chromophore basis is given in Table 1, as well as Figure S1 in the Supporting Information. It is noted here that all molar concentrations are reported as per chromophore in each salt complex consistently in this study. At this low concentration, only intramolecular interactions are expected to play a role. All three complexes show the characteristic features of highly polarizable and symmetric polymethines; the sharp (0,0)-absorption bands (at λmax ≈ 906 nm) with very large molar extinction coefficient and a small vibronic shoulder (ca. 780 nm).20,21 The spectra of the multiply charged complexes, C2 and C6, exhibit negligible changes to the (0, 0)-electronic transitions, compared to the singly charged C1, as evidenced by the similarity in the peak wavelengths and band widths. This suggests that PM−PM interactions are negligible, which is not surprising, given the relatively large distances between adjacent chromophores in each salt complex (afforded by the planar geometry in C2 and the interdigitated bulky counterions in C6). However, C6 shows a stronger relative absorbance of the vibronic shoulder, when compared to C1 or C2 (see Figure 2d). This suggests that stronger PM−CI interactions occur in the hexacationic construct, which constrains the positions of the chromophores, with respect to the counterions. This spatially constrained geometry does not provide as much freedom for the solvent to effectively dissociate the polymethine and its counterion, thereby giving rise to a higher local charge density and making them more susceptible to ionpairing effects. This is consistent with previous reports,12,18 which have shown that such ion-pairing and charge-localization effects lead to an increase in BLA. Furthermore, as shown in Table 1, the extinction coefficient (ε), oscillator strength ( f), and transition dipole moment of the ground state to lowestlying excited state (Mge) are also reduced as the number of counterions is increased. This again suggests a higher degree of charge localization and indicates that the colligation of multiply charged counterions can lead to stronger PM−CI intramolecular interactions. Note that the absorption spectra of dilute solutions of C2 in toluene and C6 in a solvent mixture of toluene:CHCl3 (9:1, V/ V) exhibit features that deviate from those of highly delocalized polymethines,20,21 while the spectrum of C1 in toluene is relatively unchanged, compared to that in CHCl3 solution (see Figure S2 in the Supporting Information). As the nonpolar solvent is more likely to induce stronger association of ion pairs, this again suggests stronger electrostatic interactions in C2 and C6.12 These observations further support the hypothesis that PM−CI interactions are playing the primary role in the charge localization of these multiply cationic systems in dilute concentrations. The same linear optical properties were investigated at higher concentrations (i.e., 5 mM) in order to observe the impacts of intermolecular interactions. In Figure 2d, the relative absorbance of the vibronic shoulder of C1 and C2 shows a significant increase as the concentration increases from 10−5 to 10−3 M, while such an increase is relatively gradual for C6 over the same range of concentrations. Furthermore, as shown in

for device applications, it is critical to further understand the impact of counterions. Here, we report on the use of multiply charged counterions with different spatial arrangements for TCF-polymethines and their impact on the linear and NLO properties over a range of concentrations that spans 5 orders of magnitude. A variety of intramolecular and intermolecular interactions play significant roles over this concentration range; however, a polymethine− hexacationic complex is found to be successful at resisting intermolecular interactions at high concentrations such as those found in neat films.

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RESULTS AND DISCUSSION As shown in Figure 1, dicationic 1,4-bis-[(triphenylphosphonium)methyl] benzene and hexacationic hexakis-[4-

Figure 1. Chemical structure of tricyanofuran (TCF)−polymethine salts.

(triphenylphosphonium)-methyloxybenzyl] cyclotriphosphazene were designed to form two new polymethine salts with multiple charges, containing C2 and C6, respectively, while C1 with a monocationic counterion was reported in our earlier work.20 The synthetic details of C2 and C6 are provided in the Supporting Information. Compared to C1 and other previously reported TCF polymethines with singly charged counterions,21 the position of cations in C2 and C6 is spatially prearranged and, because of Coulomb interactions, polymethines could associate dynamically with the charges on the cations with a spatial distribution that would facilitate cation−anion interactions and minimize repulsion energies between the anionic polymethines. Consequently, the polymethine−polymethine (PM−PM) aggregation and polymethine−counterion (PM− CI) interactions within the multiply charged polymethine salts are partially controlled. Furthermore, the spatial distribution of polymethines around cations with multiple charges could have a significant impact on the solid-state molecular packing. For example, in salt C2, the planarity of the phenyl ring coupled with the para-substitution of the phosphonium groups could facilitate aggregation through π−π stacking interactions or Coulombic interactions at high concentration. On the other hand, in salt C6, the polymethines could arrange themselves, relative to the hexacations, in a roughly spherelike volume, because of the particular molecular geometry of the hexa-substituted cyclotriphosphazene,22 which could impart an amount of steric repulsion. Therefore, the hexa-cationic complex could be resistant to the same types of interactions present for C2 at elevated concentrations. By investigating the linear and NLO properties of the polymethine complexes in solution and high-number-density films, the dominant molecular interactions can be gauged over a large range of concentrations. These should consist of either PM−PM interactions (i.e., aggregation through π−π stacking) 3116


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concentration. Therefore, perhaps counterintuitively, these results suggest that the stronger ion-pairing interactions within each C6 complex unit may, through steric repulsion, shield it from interacting with neighboring complexes. The proposed shielding effect is again indicated in Figure 2d, where the increase of relative absorbance of the vibronic shoulder begins at a lower onset concentration of ∼5 μM for C1 and C2, compared to a higher onset concentration of ∼20 μM for C6. While both PM−PM and PM-CI intermolecular interactions may play a role at this concentration, the latter is probably more significant. This is consistent with the observed spectral changes, which are likely due to charge localization and the stronger distance dependence of the London dispersion force (∝ R−6) that governs PM−PM interactions. The changes in the electronic structures of C1 and C2 and the proposed steric repulsive effect in C6 at increased molecular concentration are observed as well in the 1H NMR experiments. As shown in Figures 3a−c, the pair of sharp doublets assigned to the vinylic protons (labeled as i and j) become more difficult to identify, because of line-broadening, and even disappear completely at dilute concentration (∼1.0 mM). According to reports in the literature,21,23 this line-broadening phenomena may be due to the formation of different conformational isomers (i.e., anti-type and syn-type conformations) of TCF−polymethines, as a result of the constrained rotation of TCF end-groups. We have collected the 1H NMR spectra of C1 at variable temperatures from 246 K to 323 K (Figure S4 in the Supporting Information), which, however, demonstrated that the line-broadening phenomena of sharp doublets is temperature-independent, suggesting that the changes in splitting pattern of the vinylic proton peaks observed are not due to conformational dynamics. On the other hand, as the concentration increases, the “hidden” proton peaks appear gradually for all three salts, suggesting that the changes in splitting pattern described above are concentration-dependent, and may come from enhanced intermolecular interactions, including both PM−PM and PM− PI interactions. Such interactions may restrain the rotation of TCF end-groups and, consequently, force a specific geometry of TCF−polymethines (i.e., syn-type conformation). Moreover, by comparing their 1H NMR spectra at the same concentration of ∼10 mM shown in Figures 3a−c, we note that C6 has virtually no evidence of this peak, relative to C1 and C2. When the concentration reaches ∼30 mM, the “hidden” sharp doublets in the 1H NMR spectrum of C6 appear completely. This observation supports the hypothesis of limiting intermolecular interactions through the use of counterions with multiple charges. In addition, we also have examined the effect of the solvent polarity on geometry change through 1H NMR experiments. By adding benzene-d6 into chloroform-d solutions, two new wellresolved vinylic proton signals, marked as i′ and j′, are observed in the cosolvent spectra of C2 and C6 (Figure 3d) compared to those of pristine chloroform-d solutions at the same concentration (∼10 mM), while the spectrum of C1 is relatively unchanged. This could be attributed to the fact that multicationic systems are more susceptible to ion-pairing effect, and such effect becomes more pronounced when adding nonpolar solvent such as benzene into more polar chloroform solution, which results in more significant charge localization and consequent symmetry breaking. Moreover, as benzene is the nonsolvent for dyes, the formation of random aggregation may also play a role in inducing charge localization.

Figure 2. Normalized ultraviolet−visible−near-infrared (UV-vis-NIR) spectra of (a) C1, (b) C2, and (c) C6 in CHCl3 solutions at concentrations of 12 μM (black) and 5 mM (red). (d) Relative absorbance of vibronic shoulder absorption (805 nm) at concentrations over a range of concentrations (7 × 10−7− 5 × 10−3 M). Absorption spectra of C1, C2, and C6 were normalized to the absorption maximum to compare the spectral evolution with increasing concentrations. The molar concentrations are represented as per chromophore in each salt complex.

Table 1, the oscillator strengths, and transition dipole moments of C1 and C2 at this elevated concentration are notably smaller than for the dilute concentration, suggesting that the electronic structures of C1 and C2 are strongly affected by intermolecular interactions. In contrast, C6 shows a more modest change, somewhat remaining impervious to increases in molecular 3117


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Table 1. Comparison of Relative Oscillator Strengths ( f) and Extinction Coefficients Per Chromophore of Polymethine Complexes in Dilute (12 μM) and Concentrated (5 mM) CHCl3 Solutions Dilute Solution

dye

relative oscillator strength, f/ f(dil.C1)

extinction coefficient, εa (× 105 M−1cm−1)

C1 C2 C6

1 0.84 0.54

3.3 2.8 1.3

Concentrated Solution

transition dipole moment of the ground state to lowest-lying excited state, Mgeb (D)

relative oscillator strength, f/ f(dil.C1)

extinction coefficient, εa (× 105 M−1cm−1)

transition dipole moment of the ground state to lowest-lying excited state, Mgeb (D)

17.0 15.6 12.8

0.88 0.72 0.52

2.0 1.6 0.97

16.0 14.4 12.3

Extinction coefficients. bObtained from main absorption band using Mge = 0.09854[∫ (ε/ν) dν]0.5, where ε is given in units of M−1 cm−1, ν in cm−1, and the transition moments are expressed in Debye (D). The molar concentrations are represented in per chromophore in the salt complexes.

a

Figure 3. 1H NMR spectra of (a) C1, (b) C2, and (c) C6 dissolved in chloroform-d solutions with varying molar concentrations. Also shown is (d) the 1H NMR spectra of C1, C2, and C6 in chloroform-d/benzene-d6 mixed solutions at the same concentration of ∼10 mM. Note that, for C1 and C2, the volume ratio between chloroform-d and benzene-d6 is 1:1, whereas, for C6, it is 2:1. The molar concentrations are represented in per chromophore in the salt complexes.

solution), PM−PM intermolecular interactions may be more pronounced. It is likely that a distribution of conformational geometries of the aggregates would exist, giving rise to an overall band-broadening of the spectrum, as opposed to the well-defined and spectrally shifted peaks that are indicative of Jor H-aggregates. Interestingly, the absorption spectra of the C1 and C2 films are broader than that of C6. This is consistent with the results for the concentrated solutions discussed above, that is, the use of counterions with multiple charges could provide an approach for modulating intercomplex interactions, and potentially improving the macroscopic material properties for use in AOSP.4 In the next paragraphs, the NLO properties of C1, C2, and C6 in solutions with different concentrations and in neat films are discussed in detail.

The steric repulsive effect in C6 that seemingly limits intercomplex interactions at high concentrations is further supported by observations in neat films, where there is an ∼240-fold increase in molecular concentration beyond the concentrated solutions described above. The absorption spectra of thin neat films are shown in Figure 4. Compared to the solution spectra, the film spectra are found to exhibit an ∼30 nm red-shift in absorption maxima and significantly broadened absorption bands, clearly indicating strong intermolecular interactions. This band broadening could also indicate strong PM−CI interactions resulting from charge localization and the associated increase in contribution from the vibronic shoulders (similar to the case for the concentrated solutions). However, because of the smaller intermolecular distances in films (roughly six times smaller than those for the concentrated 3118


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involved and combined result of Re(γ) and Im(γ) dispersion.27 When the excitation wavelength is near-resonant with the vibronically allowed 2PA, as in this study, a small modification in electronic structure may result in a relatively small change in Im(γ) dispersion, while the change in Re(γ) dispersion can be significant, with regard to both magnitude and sign. The macroscopic third-order nonlinearities of neat films (with thicknesses of 1.2−1.6 μm) at 1.55 μm are summarized in Table 3. The observed values of Re(χ(3)) for all three salts are Table 3. Macroscopic Third-Order NLO Properties of Polymethine Complexes Measured in Neat Films with Thicknesses of 1.2−1.6 μm, Using the Femtosecond Pulsed Z-Scan Method at 1.55 μm Figure 4. Normalized UV-vis-NIR spectra of C1, C2, and C6 as neat thin films (80 nm thick).

Observed

The real and imaginary components of γ, Re(γ) and Im(γ), of the polymethine salt complexes were determined in CHCl3 solutions using the femtosecond pulsed Z-scan method at 1.55 μm.5 The results are summarized in Table 2, along with calculated values of Re(γ), using a simplified two-level sumover-states (SOS) expression for the nonlinear polarizability.4,5,24 All the salts in relatively dilute solutions (∼0.5 mM chromophore) exhibit typical NLO properties for TCF heptamethines at 1.55 μm:20,25 large negative values of Re(γ) and modest values of Im(γ), resulting from the close proximity of the excitation wavelength to the lowest vibronically allowed 2PA band. Compared to C1 and C2, C6 shows a reduced Re(γ) consistent with the results from the previous concentration-dependent solution studies (see Figures 2d and 3) that revealed the impacts of charge localization due to, predominately, intramolecular PM−CI interactions. The experimentally determined values of Re(γ) are significantly larger than the values obtained from the SOS calculations. We attribute this to the dispersion enhancement of Re(γ) caused by the presence of the 2PA band,4 which is not taken into account in the simplified SOS expression.26 When the concentration of the solution is increased 10-fold to 5 mM, the magnitude of Re(γ) for all the complexes shows a significant reduction, which is attributed to the enhanced charge localization associated with the intermolecular PM−CI interactions, as discussed above. Given the positive impacts of the steric repulsion observed for C6 when discussing the linear optical properties at 5 mM, the reduction in the Re(γ) and 2PA FOM values should be expected to be less dramatic, when compared to C1 or C2. However, this trend is not observed and may simply be too small of an effect to be observed, given the experimental errors. Alternatively, there may be a more

Extrapolated

dye

Re(χ(3))a (× 10−11 esu)

figure of merit, FOM

Re(χ(3))b (× 10−11 esu)

figure of merit, FOM

C1 C2 C6

−2.3 −3.4 −3.6

1.4 1.8 1.8

−6.2 −5.2 −5.1

1.6 1.6 1.7

Error bars were estimated to be ±13% for Re(χ(3)) and ±18% for | Reχ(3)/Imχ(3)|. bExtrapolated neat values were determined using the following equation: χ(3) = γ × N × L4, where the values of γ determined in 5 mM solutions (Table 2) were used. N is the density of polymethines in the film (assuming a film density of 1.1 g/cm3) and L is the Lorentz field factor, assuming film refractive indexes of 1.61, 1.64, 1.72 for C1, C2, and C6, respectively.

a

comparable to the best values reported using other types of polymethine-based films.10,13,20 The measured values of χ(3) and 2PA-FOM are also in reasonable agreement with the extrapolated χ(3) and 2PA-FOM values (see footnote b in Table 3), suggesting that these chromophores apparently retain their microscopic NLO response, despite the ∼240× increase in concentration after the incorporation into high-number-density films. Despite the observed differences in the neat film absorption spectra, compared to the concentrated solutions, the impact of the increased intermolecular interactions does not have a deleterious impact on the NLO properties. It is worth noting that, among all of the complexes, C6 exhibits the highest magnitude of Re(χ(3)), i.e., ∼3.6 × 10−11 esu, which is a factor of ∼3 larger than that of silicon (∼1.4 × 10−11 esu).28 The results for the neat film of C6 show the strongest correlation between experimental and extrapolated NLO values. This again highlights the impact of the steric shielding, in terms of mitigating the intercomplex interactions in the solid state.

Table 2. Molecular Third-Order Polarizabilities of Polymethine Complexes Measured in Dilute and Concentrated CHCl3 Solutions Using the Femtosecond Pulsed Z-Scan Method at 1.55 μm dyes/concentrationa C1 C2 C6

0.5 5.0 0.5 5.0 0.5 5.0

mM mM mM mM mM mM

Re(γ)b (10−32 esu)

Im(γ)b (10−33 esu)

|Re(γ)/Im(γ)|b

calc Re(γ)c (10−32 esu)

−3.4 −1.6 −2.8 −1.2 −2.4 −1.0

9.0 9.7 6.6 7.5 5.7 6.0

3.8 1.6 4.2 1.6 4.2 1.7

−1.4 −1.0 −1.0 −0.67 −0.42 −0.32

The concentrations and the NLO parameters are represented in per chromophore in each salt complex. bErrors were estimated to be ±8% for Re(γ) and 11% for |Re(γ)/Im(γ)|. cFor calculating Re(γ) at 0.5 mM and 5 mM, the Mge values in Table 1 at 12 μM and 5 mM, respectively, are used. a

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(6) Hales, J. M.; Zheng, S.; Barlow, S.; Marder, S. R.; Perry, J. W. Bisdioxaborine Polymethines with Large Third-Order Nonlinearities for All-Optical Signal Processing. J. Am. Chem. Soc. 2006, 128, 11362− 11263. (7) Gieseking, R. L.; Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Design of Polymethine Dyes for All-Optical Switching Applications: Guidance from Theoretical and Computational Studies. Adv. Mater. 2014, 26, 68−84. (8) Marder, S. R.; Gorman, C. B.; Meyers, F.; Perry, J. W.; Bourhill, G.; Brédas, J.-L.; Pierce, B. M. A Unified Description of Linear and Nonlinear Polarization in Organic Polymethine Dyes. Science 1994, 265, 632−635. (9) Lu, D.; Chen, G.; Perry, J. W.; Goddard, W. A., III. Valence-Bond Charge-Transfer Model for Nonlinear Optical Properties of ChargeTransfer Organic Molecules. J. Am. Chem. Soc. 1994, 116, 10679− 10685. (10) Scarpaci, A.; Nantalaksakul, A.; Hales, J. M.; Matichak, J. D.; Barlow, S.; Rumi, M.; Perry, J. W.; Marder, S. R. Effects of Dendronization on the Linear and Third-Order Nonlinear Optical Properties of Bis(thiopyrylium) Polymethine Dyes in Solution and the Solid State. Chem. Mater. 2012, 24, 1606−1618. (11) Li, Z.; Mukhopadhyay, S.; Jang, S.-H.; Brédas, J.-L.; Jen, A. K.-Y. Supramolecular Assembly of Complementary Cyanine Salt JAggregates. J. Am. Chem. Soc. 2015, 137, 11920−11923. (12) Bouit, P.-A.; Aronica, C.; Toupet, L.; Le Guennic, B.; Andraud, C.; Maury, O. Continuous Symmetry Breaking Induced by Ion Pairing Effect in Heptamethine Cyanine Dyes: Beyond the Cyanine Limit. J. Am. Chem. Soc. 2010, 132, 4328−4335. (13) Barlow, S.; Brédas, J.-L.; Getmanenko, Y. A.; Gieseking, R. L.; Hales, J. M.; Kim, H.; Marder, S. R.; Perry, J. W.; Risko, C.; Zhang, Y. Polymethine materials with solid-state third-order optical susceptibilities suitable for all-optical signal-processing applications. Mater. Horiz. 2014, 1, 577−581. (14) Gieseking, R. L.; Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Bréd as, J.-L. Effect of Bulky Substituents on Thiopyrylium Polymethine Aggregation in the Solid State: A Theoretical Evaluation of the Implications for All-Optical Switching Applications. Chem. Mater. 2014, 26, 6439−6447. (15) Li, Z.; Ensley, T. R.; Hu, H.; Zhang, Y.; Jang, S.-H.; Marder, S. R.; Hagan, D. J.; Van Stryland, E. W.; Jen, A. K.-Y. Conjugated Polycyanines: A New Class of Materials with Large Third-Order Optical Nonlinearities. Adv. Opt. Mater. 2015, 3, 900−906. (16) Dähne, L.; Reck, G. Deformation of Polymethine Structures by Intermolecular Interactions. Angew. Chem., Int. Ed. Engl. 1995, 34, 690−692. (17) Bianco, A.; Del Zoppo, M.; Zerbi, G. Molecules with enhanced negative third order vibrational polarizabilities: polymethine dyes and their vibrational spectra. Synth. Met. 2001, 125, 81−91. (18) Mukhopadhyay, S.; Risko, C.; Marder, S. R.; Brédas, J.-L. Polymethine dyes for all-optical switching applications: a quantumchemical characterization of counter-ion and aggregation effects on the third-order nonlinear optical response. Chem. Sci. 2012, 3, 3103−3112. (19) Gieseking, R. L.; Mukhopadhyay, S.; Shiring, S. B.; Risko, C.; Brédas, J.-L. Impact of Bulk Aggregation on the Electronic Structure of Streptocyanines: Implications for the Solid-State Nonlinear Optical Properties and All-Optical Switching Applications. J. Phys. Chem. C 2014, 118, 23575−23585. (20) Li, Z.; Liu, Y.; Kim, H.; Hales, J. M.; Jang, S.-H.; Luo, J.; BaehrJones, T.; Hochberg, M.; Marder, S. R.; Perry, J. W.; Jen, A. K-Y. Highoptical-quality blends of anionic polymethine salts and polycarbonate with enhanced third-order non-linearities for silicon-organic hybrid devices. Adv. Mater. 2012, 24, OP326−OP330. (21) Bouit, P.-A.; Di Piazza, E.; Rigaut, S.; Le Guennic, B.; Aronica, C.; Toupet, L.; Andraud, C.; Maury, O. Stable Near-Infrared Anionic Polymethine Dyes: Structure, Photophysical, and Redox Properties. Org. Lett. 2008, 10, 4159−4162. (22) Inoue, K.; Itaya, T. Synthesis and Functionality of Cyclophosphazene-Based Polymers. Bull. Chem. Soc. Jpn. 2001, 74, 1381− 1395.

CONCLUSIONS In summary, we have systematically studied the linear and nonlinear optical properties of anionic tricyanofuran (TCF)− heptamethine-based salt complexes with phosphonium-based monocationic, dicationic, and hexacationic counterions. The optical properties of the monocationic and dicationic complexes are strongly susceptible to intermolecular interactions at high number density. While the hexacationic complex is not immune to these interactions, it is affected, but to a much smaller degree. Counterintuitively, this is a result of strong intramolecular polymethine−counterion interactions, which are largely controlled by the preorganization of the multiply charged counterionic phosphonium groups. This counterionic construct with multiple charges provides a steric repulsive effect that can mitigate the intercomplex interactions in the solid state, resulting in a more efficient translation between microscopic and macroscopic optical properties for highly polarizable polymethines, which is essential for high-speed alloptical signal processing (AOSP).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00641. Synthetic details and characterization data, UV-vis-NIR spectra, preparation of films and Z-scan method, NLO properties at 1.3 μm, and NMR data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. W. Perry). *E-mail: [email protected] (A. K.-Y. Jen). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the DARPA ZOE Program (No. W31P4Q-09-1-0012), and the AFOSR MURI (No. FA955010-1-0558). A.K.-Y.J. thanks the Boeing−Johnson Foundation for its support.

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REFERENCES

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