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Organic Salts Suppress Aggregation and Enhance Hyperpolarizability of a -Twisted ChromophoreAlexander J.-T. Lou,[a] Stefania Righetto,[b] Elena Cariati,[b] and Tobin J. Marks*[a]

Abstract: Twisted intramolecular charge transfer (TICT) chromophores exhibit extraordinary hyperpolarizabilities, , and are therefore promising for electro-optic technologies. Nevertheless, centrosymmetric aggregate formation severely diminishes in concentrated solutions or in polymer matrices. Here we report the remarkable effects of organic salts on the linear and nonlinear optical response of a benzimidazolium-based high- TICT chromophore, B2TMC-2. Addition of Bu4P+Brˉ to B2TMC-2 solutions in CHCl3 induces a halochromic blue-shift, primarily reflecting interactions between Bu4P+Brˉ and the B2TMC-2 cationic portion. DC electric-field induced second-harmonic generation (EFISH) measurements on B2TMC-2 + Bu4P+Brˉ solutions reveal a large = -22,160 10-48 esu - unprecedented for any chromophore with such a broad optical transparency window. Moreover, Bu4P+Brˉ is shown to mitigate B2TMC-2 aggregation, thereby preserving high in concentrated solutions. This phenomenon should be applicable to many other NLO chromophores.

Electro-optic (E-O) materials are of great interest for applications in telecommunications, image processing, and sensing.[1] The E-O response of organic materials depends on, among other factors, the hyperpolarizability () of the individual molecular constituents. Therefore, intense investigations have focused on designing suitable chromophores with high values and low optical absorbance at telecommunication wave-lengths.[2] Our research focuses on the design and characterization of twisted π-system intramolecular charge transfer (TICT) chromophores,[3] composed of donor and acceptor moieties connected by a twisted bi-aryl fragment.[4] Their remarkable values arise from a large ground state dipole moment () and careful balancing of aromatic/zwitterionic and quinoidal/neutral electronic structure contributions, which are primarily dictated by the magnitude of the bi-aryl torsion (Figure 1, top).[5] However, while TICT chromophores exhibit some of the highest measured values in dilute solution, translating this exceptional performance to device-ready materials has proven challenging;[6]

the large that typically accompanies high also drives the formation of centrosymmetric aggregates for which necessarily approaches zero. Thus, the full potential of TICT chromophores

Figure 1. Top: Aromatic and quinoidal resonance forms of B2TMC-2 with torsion angle labelled. Bottom: TICT chromophore B2TMC-2 (with labelled aromatic protons) and organic salts Bu4P+Brˉ and Bu4N+Brˉ.

in concentrated solutions or in host-guest polymer/chromophore films is not realized.[7] Previously, polar solvents such as DMF were shown to suppress TICT aggregation, but also to increase the HOMO/LUMO energy gap, resulting in unacceptably lowered .[4, 8] Only recently we reported a new benzimidazolium-based chromophore B2TMC-2 (Figure 1, bottom), which combines reduced aggregation and good performance ( ~ -20,730 10-

48 esu) in polar solvents such as DMF. [9] This development motivates the investigation described here.

In this contribution, we seek to suppress TICT aggregation with organic salt additives (Figure 1), which could in principle modify the effective solvent polarity[10] and introduce new coulombic interactions that compete with electrostatically driven aggregation.[6f, 11] Tetra-n-butylphosphonium bromide (Bu4P+Brˉ) and tetra-n-butylammonium bromide (Bu4N+Brˉ) are selected here as model salts due to their good chemical and thermal stability, compatibility with organic solvents and polymers, and minimal visible region optical absorption. It will be seen that the presence of these salts, in addition to significantly increasing the B2TMC-2 in concentrated solutions, effect changes in in the linear optical absorbance, 1H NMR chemical shifts, and solution vibrational frequencies of B2TMC-2.

The linear optical absorbance spectrum of B2TMC-2 in CHCl3 solution (Figure 2A) is characterized by an intense charge transfer (CT) absorption at CT = 425 nm and a strong sub-fragment transition at ~320-340 nm. In 1.0 M Bu4P+Brˉ/CHCl3, CT blue-shifts to 391 nm, which is similar to that in acetone solution. The analogous ammonium salt yields similar results; in 1.0 M Bu4N+Brˉ/CHCl3, B2TMC-2 exhibits CT = 395 nm (Figure S2). Note that changes in CT are purely a result of salt-induced effects rather than aggregation, as confirmed by the absence of

[a] A. J.-T. Lou, Prof. T. J. MarksDepartment of ChemistryNorthwestern University2145 Sheridan Rd., Evanston IL, 60208, USAE-mail: t-marks@northwestern.edu

[b] Dr. E. Cariati, S. RighettoDipartimento di Chimica and Unità di Ricerca Università di Milano and INSTM di MilanoVia Golgi 19, I-20133 Milano, Italy

Supporting information for this article is given via a link at the end of the document.

COMMUNICATION spectral changes upon diluting B2TMC-2 from 10-4 to 10-6 M in pure CHCl3 (Figure S3).

Figure 2. (A) Linear optical absorbance spectra of a 1.0 10-4 M CHCl3

solution of B2TMC-2 with varied Bu4P+Brˉ concentrations; (B) Aromatic region 1H-NMR chemical shifts of B2TMC-2 in CDCl3 solutions as a function of the indicated Bu4P+Brˉ concentrations (labelling according to Figure 1), with the 1H-NMR chemical shifts in DMSO shown for reference; (C) Trends in benzimidazolium acceptor fragment 1H-NMR chemical shifts and CT as a function of added Bu4P+Brˉ concentration indicate selective solvation of B2TMC-2 by Bu4P+Brˉ. Labelled concentrations in (A) and (B) refer to the amount of Bu4P+Brˉ in each respective solution.

To determine whether these salt effects persist in a glassy polymer matrix as might be employed in an organic E-O modulator,[1a, 2b] host-guest films were fabricated from a poly(methylmethacrylate) host polymer (PMMA) cast with ~15 wt % B2TMC-2 and Bu4P+Brˉ ranging from ~2 - 30 wt %. The ~13 nm hypsochromic shift from the neat PMMA/B2TMC-2 film (CT = 398 nm) to a film with 30 % Bu4P+Brˉ (CT = 385 nm) is somewhat smaller than in solution, but clearly indicates that Bu4P+Brˉ/B2TMC-2 interactions persist in the film (Figure S1). This result demonstrates one of the principal attractions of these salt additives; unlike a volatile organic solvent additive, Bu4P+Brˉ can be used to tune the properties of a polymeric host matrix with no risk of evaporation under normal use conditions.

The twisted bridge in B2TMC-2 (Figure 1) serves to decouple the respective chromophore fragments, so that changes in the donor fragment 1H NMR chemical shifts (Hc) do not strongly impact the acceptor resonances (Ha and Hb), and vice versa. This segmentation permits individual assessment of salt/donor and salt/acceptor effects. A solution containing ~0.7 M Bu4P+Brˉ exhibits a ~0.07 ppm upfield shift of Hc versus the pure B2TMC-2/CHCl3 solution, consistent with marginally increased electron density on the donor fragment (Figure 2B). Simultaneously, the acceptor Ha shifts 0.26 ppm downfield in the same solution, corresponding to increased electron deficiency at the acceptor fragment. A parallel experiment using Bu4N+Brˉ (Figure S5) yields essentially identical trends. In both cases, the absence of multiple sets of signals rules out the formation of static covalent salt/chromophore bonds on the NMR timescale. FT-IR vibrational spectroscopy of B2TMC-2 solutions in pure CHCl3, and with 0.05, 0.1, and 0.5 M Bu4P+Brˉ, reveals a ~3 cm-1

decrease in the energy of the cyano group stretching mode energy, v(CN), from pure CHCl3 to 0.5 M Bu4P+Brˉ (Figure S4). This small shift corresponds to slightly increased electron density on the carbanion.

The observed spectroscopic changes indicate that the Bu4P+Brˉ and Bu4N+Brˉ/B2TMC-2 systems exhibit negative halochromism, which is defined as the change in CT resulting from specific, coulombic interactions between a salt and a chromophore (loose ion-pair formation), in the absence of covalent bond formation.[12] In B2TMC-2, such interactions would raise the ionization potential of the donor fragment (RC(CN)2ˉ) or lower the electron affinity of the acceptor fragment (Bz+), accounting for the observed negative shift in CT.[12] The large magnitude of the aforementioned salt-induced changes in Ha

relative to Hc are consistent with a dominant specific coulombic interaction between Bz+ and Brˉ. In the present system, there are four available ion pairings (Figure 3), for which the ion pair distances generally reflect the strength of the interaction. The ion pairing distances of representative diffraction-derived single crystal structures (Figures S6, S7) decrease in the order of Bu4N+ˑˑˑˉ(CN)2CR (~4.7-5.6 Å)[13] > Bz+ˑˑˑˉ(CN)2CR of neighboring B2TMC-2 molecules (~4.5 Å)[9] > Bu4P+ˑˑˑBrˉ (~4.1-4.4 Å)[14] > Bz+ˑˑˑBrˉ (~3.8-4.1 Å),[15] supporting a scenario in which Bz+ˑˑˑBrˉ interactions dominate and the shift in CT primarily results from the decreased electron affinity of the benzimidazolium fragment. In general, the sterically encumbered Bu4P+ n-butyl groups likely limit the electrostatic interactions with both Brˉ and RC(CN)2ˉ, consistent with the small observed shifts in v(CN) noted above.[12, 16] This relatively weak Bu4P+ˑˑˑBrˉ ion-pairing likely also increases the availability of Brˉ to interact with the benzimidazolium fragment (see Section S7), resulting in a large change in Ha and CT.

Halochromism is often accompanied by preferential solvation,[17] meaning in the present case that the B2TMC-2 solvation shell has a disproportionally large amount of Bu4P+Brˉ

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Figure 3. Single crystal structures showing ion pairing interactions in (A) B2TMC-2 dimers and (B) Bu4P+Brˉ. Proposed interactions of B2TMC-2 in the Bu4P+Brˉ/B2TMC-2 system: (C) Bu4P+-donor based interaction and (D). Brˉ-acceptor based interaction.

versus the bulk solution composition. In the present systems, this is manifested as large spectroscopic changes at low salt concentrations, which begin to approach a limiting value at < 20 mol % Bu4P+Brˉ (Figures 2C and S6), presumably as the salt saturates the B2TMC-2 solvent shell. As such, further addition of Bu4P+Brˉ is unlikely to result in significant spectroscopic changes.

Measurements of μβvec, the product of the μ and the vector part of the molecular first-order hyperpolarizability βvec tensor in the μ direction, were performed by the solution-phase DC electric-field induced second-harmonic generation (EFISH) method at 1907 nm, which provides direct information on the intrinsic molecular nonlinear optical (NLO) response via eq 1. Here, μβ/5kT is the dipolar orientational contribution, and γ(−2ω;ω,ω,0), the third-order term at frequency ω of the incident light, is the electronic contribution to γEFISH, which should be negligible for this type of molecule.[18]

γEFISH = (μβ/5kT) + γ(−2ω; ω, ω, 0) (1)

To isolate the monomeric B2TMC-2 response in various environments and the impact of Bu4P+Brˉ on aggregation, EFISH measurements were performed on B2TMC-2 in five CHCl3

solutions having varied concentrations of Bu4P+Brˉ: A (10-4 M, ~0.04 mg/mL Bu4P+Brˉ), B (0.05 M, ~20 mg/mL), C (0.1 M, ~40 mg/mL), D (0.5 M, ~200 mg/mL), and pure CHCl3 (Figure 4). All measurements were performed in anhydrous solvent to ensure that the ion pairs were not separated by the applied electric field, and control experiments indicate that salt itself has a negligible contribution to the measured vec (Table S1). It is well established that the inverse relationship between concentration and vec in TICT chromophores is a result of chromophore aggregation,[4] so the most dilute measurements are taken to be representative of monomeric vec. Solution B (5.0 10-5 M) exhibits vec = -923 10-30 esu, indicating that B2TMC-2 maintains a large monomeric vec in the presence of Bu4P+Brˉ as compared to that in pure DMF (-828 10-30 esu) or DCM (-1056 10-30 esu).

The effects of aggregation are qualitatively evidenced by the decrease in vec with increasing B2TMC-2 concentration (Figure 4). In Bu4P+Brˉ containing solutions B-D, this characteristic diminution shifts to higher B2TMC-2 concentrations, thereby extending the high vec threshold by at least 5 versus pure CHCl3 solutions. The 10-4 M B2TMC-2 solutions offer a striking comparison; vec of solutions B and D, ~ -20,000 10-48 esu, far exceeds that of the heavily aggregated CHCl3 (-2,160 10-48 esu) and DCM (-9,660 10-48 esu) solutions. Note that even solution A (-13,000 10-48 esu) exhibits major enhancement, despite the low Bu4P+Brˉ concentration (10-

4 M).

Figure 4. EFISH derived vec as a function of B2TMC-2 concentration in DCM and at the indicated Bu4P+Brˉ concentrations in CHCl3: A (10-4 M), B (0.05 M); C (0.1 M); D (0.5 M). Lines are drawn to guide the eye.

It is known that vec is related to the CT energy, transition oscillator strength and change in state dipole moment, all of which are reflected in the linear absorbance.[1a] However, while bulk salt-induced changes in these parameters must certainly influence vec, they cannot reasonably account for the observed dramatic increases. Therefore, the increased response in solutions A-D must arise predominantly from the influence of Bu4P+Brˉ on B2TMC-2 aggregation tendencies. The proposed mechanism of disaggregation relates to the competition of dipolar and coulombic chromophore-chromophore interactions with coulombic salt-chromophore interactions. In salt-free, moderately polar solvents, the dominant stabilizing interaction of

COMMUNICATION TICT chromophores is the formation of centrosymmetric dimers, as supported by spectroscopic and crystallographic data. [4, 9] The addition of Bu4P+Brˉ introduces a new set of stabilizing interactions, evidenced by strong halochromic shifts, which are in direct competition with the coulombic donor-acceptor attraction. Similarly to polyzwitterion/salt systems, this type of coulombic interaction also screens the dipole-dipole attractions that contribute to aggregation.[19] This suppression of aggregate forming chromophore-chromophore interactions prevents the formation of significant centrosymmetric structures, leading to enhanced vec. Furthermore, preferential solvation causes disaggregating effects to manifest at very low salt concentrations, as seen in solution A. The addition of more Bu4P+Brˉ increases vec, although the similarity in response of solutions B-D presumably reflects the saturation of Bu4P+Brˉ in the solvation shell or the limit on interaction strength of this particular salt - chromophore pairing.

In summary, we have shown that Bu4P+Brˉ can significantly increase the solution phase vec of the -twisted B2TMC-2 chromophore by introducing coulombic interactions which compete with B2TMC-2 aggregation. Bu4P+Brˉ appears to function similarly in host-guest polymer films and therefore should be applicable to E-O devices. Reducing the steric bulk of the cation, using an alkali or alkaline earth metal cation and organic anion, or employing multiply charged ions may further enhance the coulombic interaction strengths, perhaps increasing NLO response. In addition to suppressing aggregation, salt additives provide a means to blue-shift CT, which could widen the operating transparency of organic E-O modulators and possibly suppress photochemical damage.[20] This exploration of salt modifiers to polymer and solvent environment serves as proof-of-concept which could doubtless be expanded to a greater variety of salts and chromophores. Ongoing investigations are focused on the application of these salt effects in E-O materials.

Experimental Section

For experimental methods, please see the Supporting Information.

Acknowledgements

This work was supported by AFOSR MURI grant FA9550_14_1_0040. A. J.-T. L thanks NDSEG for a Graduate Research Fellowship.

Keywords: aggregation • electrostatic interactions • halochromism • hyperpolarizability • nonlinear optics

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Table of Contents

Add a pinch of salt: Tetrabutylphosphonium bromide causes halochromic shifts and increases the hyperpolarizability of twisted -system chromophore B2TMC-2.

Alexander J.-T. Lou, Stefania Righetto, Elena Cariati, and Tobin J. Marks*

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Organic Salts Suppress Aggregation and Enhance Hyperpolarizability of a -Twisted Chromophore