Self-Assembled Organic Nanocrystals with Strong Nonlinear Optical

Oct 8, 2015 - The diffractogram was indexed and refined by the Pawley method(33) using Materials Studio software(34) (see Supporting Information for d...
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Self-assembled Organic Nanocrystals with Strong Nonlinear Optical Response Shaked Rosenne, Eran Grinvald, Elijah Shirman, Lior Neeman, Sounak Dutta, Omri Bar-Elli, Regev BenZvi, Eitan Oksenberg, Petr Milko, Vyacheslav Kalchenko, Haim Weissman, Dan Oron, and Boris Rybtchinski Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b02010 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Self-assembled Organic Nanocrystals with Strong Nonlinear Optical Response Shaked Rosenne,† Eran Grinvald,‡ Elijah Shirman,†⊥ Lior Neeman,‡ Sounak Dutta,† Omri Bar-Elli,‡ Regev Ben-Zvi,║ Eitan Oksenberg,║ Petr Milko,± Vyacheslav Kalchenko,§ Haim Weissman,† Dan Oron,‡* and Boris Rybtchinski†* Departments of †Organic Chemistry, ‡Physics of Complex Systems, ║Materials and Interfaces, ±Chemical Research Support and §Veterinary Resources, Weizmann Institute of Science, Rehovot, Israel ⊥Current Address: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

Abstract Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.

Keywords NLO organic crystals, SHG, self-assembly, hyper-Rayleigh scattering, nanocrystals, perylene diimides. Nonlinear optical (NLO) materials are important in laser technology and in electrooptical systems. Second Harmonic Generation (SHG) is the most explored and widely utilized NLO process, in which two photons of the irradiating light are coherently converted into a single photon having a doubled frequency.1 Inorganic crystals are the most common SHG materials used in commercial NLO applications.2,3 Inorganic NLO components are often expensive due to demanding crystallization processes and other difficulties associated with their fabrication. Organic NLO materials are considerably cheaper and allow tuning of their properties. Although less robust than their inorganic counterparts, organic NLO arrays exhibit attractive optical properties and can be sufficiently stable.4,5 A common strategy to create organic SHG systems is to use conjugated compounds with electronically donating and accepting chemical moieties, resulting in highly polar molecules having high molecular hyperpolarizability.6,7,8 Yet, in order to construct a bulk SHG material, the molecular building blocks must be assembled into a non-centrosymmetric macroscale array.9,10 This has been achieved by fabricating 1 ACS Paragon Plus Environment

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poled polymers, employing polymer film hosts, in which the guest chromophores are aligned by an external electric field to give poled systems.11,12 However, such induced alignment decays over time and/or at increased temperatures or laser intensities. Other approaches to NLO material fabrication include thin film deposition on surfaces13,14,15 and creation of non-centrosymmetric organic SHG crystals.16–20 The rational design of the latter is extremely challenging,21 since polar organic compounds tend to form centrosymmetric crystals or crystals where the molecular dipoles interact unfavorably, resulting in a loss of SHG activity.22,23 Furthermore, when SHG activity is achieved, relatively large insoluble crystals are often formed, precluding solution-based processing. Thus, reports of chemically stable, synthetically accessible and easily processable organic NLO materials are scarce.24–27 In this contribution, we describe a new family of SHG-active organic nano- and microcrystals based on readily available mono-substituted perylenediimide (PDI) derivatives. The crystals are easily fabricated via self-assembly in aqueous solutions, remain in the solution phase, have tunable sizes, and exhibit second order susceptibilities comparable to those of benchmark organic crystals such as urea28 and ammonium dihydrogen phosphate (ADP) crystals.29 Taking advantage of the facile self-assembly of PDI-based amphiphiles in aqueous media,30,31 we chose to study a family of simple mono-substituted PDIs (Scheme 1 1-3, see Supporting Information for synthesis and characterization). They have a rather small molecular dipole of 2.5-4.4 D (see Supporting Information), yet we hypothesized that the self-assembly may orient molecular dipoles due to the specific solvation in aqueous media, eventually leading to polar crystals.

O

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NO2

NH2

OH

O

O

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O

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O

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Scheme 1. Molecular structure of the compounds.

Compounds 1-3 exhibit a strong tendency to assemble into nano- and microcrystals under various conditions by simply mixing an organic solution of the compound with water (Figures 1, 2 and S9). SEM and TEM imaging of 1 assembled in water/THF (85/15, v/v) mixture reveal faceted crystalline needles that occasionally bundle, forming 2 ACS Paragon Plus Environment

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larger aggregates (Figure 1A, B and S5). These needles are typically 500±150 nm thick and their length is 30±10 µm. They tend to precipitate out of solution after several days of aging. High magnification TEM images (Figures 1C and S5) reveal a crystalline order with spacings of 1.56-1.61 nm and 1.20 nm (depending on the crystals’ orientation toward the electron beam). Synchrotron powder X-ray diffraction (XRD) measurements also indicate crystallinity (Figure 3). Importantly, SHG microscopy reveals that the nanocrystals have significant SHG ability (Figure 1D). This is in line with a notion that the magnitude of a ground state molecular dipole may not be directly linked to the second order susceptibility, while the specific orientation of the dipoles in the crystal is of primary importance.32 Simple variations in assembly conditions enable tuning the dimensions of the nanocrystals such that they remain stable in solution for extended periods of time. Thus, the crystals of 1 grown in water/ethanol (85/15, v/v) solution are typically 40±3 nm thick and their length exceeds 10 µm; when grown in water/ethanol (65/35, v/v) mixtures they are typically 100±30 nm thick with lengths exceeding 15 µm, as revealed by TEM imaging (Figure S7). The crystals grown in the water/ethanol solutions show almost no precipitation after weeks of aging, and exhibit strong SHG activity as revealed by SHG microscopy (Figure S8). Notably, the crystals are transparent at the fundamental frequencies used, and are not strongly absorbing at the SHG frequency (Figure S3).

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Figure 1. Crystals of 1 aged in water/THF (85/15, v/v) solution at concentration of 1×10-4 M. A) TEM image of crystals of 1. B) SEM image of crystals of 1. C) TEM image of 1. Inset shows FFT corresponding to 1.61 nm spacing, corresponding to the (0,1,1) plane of the crystal. D) SHG microscopy image of 1. Excitation at 800 nm, detection at 394-405 nm.

In order to further investigate the SHG properties of crystals based on monosubstituted PDIs, we investigated two other derivatives bearing small bay-area substituents - amino and nitro groups (compounds 2 and 3, respectively). Compound 2 was assembled in water/THF (85/15, v/v) solution to form faceted crystals that are 600±200 nm thick, with lengths exceeding 100 µm (Figure 2). As in the case of 1 in water/THF (85/15, v/v) mixture, these crystals precipitate after several days of aging. XRD measurements confirm their crystallinity (Figure S6), and SHG microscopy – their strong SHG properties (Figure 2D). Similarly, compound 3 was assembled in water/THF (70/30 and 60/40, v/v) solutions to form crystals, which are SHG-active (Figure S9). Interestingly, in compound 3, the molecular dipole is oriented in opposite direction to the

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ones in 1 and 2 (table S2). Thus, the non-centrosymmetric packing appears to be dominating aqueous assembly of mono-substituted PDIs.

Figure 2. Crystals of 2 aged in water/THF (85/15, v/v) solution at concentration of 1×10-4 M. A and B) SEM images of crystals of 2 aged for 8 days. C) cryo-TEM image of 2 aged for 9.5 hours. Insets: magnification of marked region shows FFT fitting of 1.61 nm spacing. Scale bar is 10nm. D) SHG microscopy image of 2 in water/THF (85/15, v/v). Excitation at 800 nm, detection at 394-405 nm.

In order to probe further the structure of the crystals, a high resolution powder X-ray diffraction (XRD) study on needle-like crystals of 1 grown in water/THF (85/15, v/v) was performed (Figure 3 and Supporting Information; we note that the fine needle-like crystals of 1-3 were not suitable for single crystal X-ray measurements). The diffractogram was indexed and refined by the Pawley method33 using Materials Studio software34 (see Supporting information for details). The unit cell is found to be 5 ACS Paragon Plus Environment

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orthorhombic, polar class mm2 (Pna21), with a=19.4127(5), b=14.5227(7), c=24.1810(7). The diffraction peaks assigned to the (0,1,1) and (0,2,0) planes correspond to the spacings observed in TEM images, suggesting that the needle growth is along the a axis of the unit cell (Figures 1, 3 and S5).

Figure 3: Synchrotron powder X-ray diffractogram of crystals of 1 (from 1×10-4 M water/THF (85/15, v/v): Experimental (dotted red), calculated (blue), and difference (black) profiles of Pawley refined diffractions; expected peak positions based on unit cell assignment are shown in green. The d spacings corresponding to the (0,1,1) and (0,2,0) planes are d=15.14 Å and d=12.09 Å respectively.

We quantified the second order susceptibility of the solution-phase crystals based on 1 and 2 using a Hyper Rayleigh scattering (HRS) technique. 28,35–37 The HRS method enables to characterize the average hyperpolarizability utilizing the solvent HRS signal as reference. From the extracted value of it is possible to infer the bulk equivalent value of the nonlinear susceptibility tensor 𝜒 (2) as described by the reduced representation dijk.38,39 Notably, HRS measurements were performed on the solution of crystals (rather than on the molecular precursor) and thus reflect the nonlinear optical properties of the crystallized phase. We conducted HRS measurements for three different crystalline systems at irradiating wavelength of 800 nm: 1 in water/THF (85/15, v/v), 1 in water/ethanol (85/15, v/v), and 2 in water/THF (85/15, v/v). For these systems, we assumed a single dominant effective d element (see Supporting information for details). For the crystals of 1 in water/THF 𝑒𝑒𝑒 (85/15, v/v) solution we obtain a value of 𝑑 𝑧𝑧𝑧 = 0.7pm/V, for the crystals of 1 in 𝑒𝑒𝑒 water/ethanol (85/15, v/v)- 𝑑𝑧𝑧𝑧 = 0.3 pm/V, and for the crystals of 2 in water/THF 𝑒𝑒𝑓 (85/15, v/v)- 𝑑𝑧𝑧𝑧 = 0.015 pm/V (some variations were noticed due to different size distributions in each sample). It should be noted that the analysis of HRS experiments assumes that the samples are fully crystalline, neglecting the possible presence of defects or multiple crystalline domains. Due to the dependence of HRS intensity on crystal size, 6 ACS Paragon Plus Environment

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and the fact that the crystals measured are larger than the focal spot of the irradiating laser, the reported values represent a lower bound of the nonlinear susceptibility values. These values are comparable with that of common nonlinear crystals such as potassium dihydrogen phosphate (KDP) with 𝑑𝑧𝑧𝑧 = 0.43 pm/V28 or urea with 𝑑𝑥𝑥𝑥 = 1.4 pm/V.21,40 To obtain further insight into details of the nanocrystals nonlinear susceptibility tensor, we performed single-crystal SHG measurements. Figure 4 depicts SHG responses from two crystals of 1 grown from water/THF (85/15, v/v) solution. Raster scans (Figure 4A and 4C) of the SHG signal along the crystals reveals variations in signal intensities that are due to possible differences in crystal thickness and due to some inhomogeneity of domains within the crystal. Figures 4B and 4D depict the SHG signal as a function of the irradiating laser’s polarization, providing an insight into the ratios of the different elements that compose the second order susceptibility tensor. The observed SHG response has a dominant two-lobe structure, where the lobes are oriented along the long axis of the crystal. The polarization dependence plot (Figure 4B and 4D) also indicates that the measured area is crystalline and relatively uniform.26 Notably, the dominant signal is polarized perpendicular to the crystal’s long axis. The measured polar plots differ slightly from crystal to crystal and sometimes from point to point on the same crystal. We attribute such observations to poly-crystallinity due to oriented attachment of individual crystalline domains upon crystal growth and fusion, which is also apparent in Figure 4C and to some extent in Figure 4A. Nevertheless, the feature of maximal measured intensity with polarization perpendicular to the excitation polarization recurs regardless of the particular illuminated position.

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Figure 4. Raster scans and polarization dependent measurements of crystals of 1 assembled from a water/THF (85/15, v/v) mixture (λex = 950 nm, intensity in a.u.). A) Raster scan of a crystal with the black circle indicates the point for which polarization dependent polar plot was measured. B) Polar plot of the SHG intensity at the location indicated in A as a function of the incoming beam polarization angle fitted with Z-Y’-Z” Euler angles set (104°, 40°, 117°), R2=0.972. D) Polar plot of the SHG intensity at the location indicated in C as a function of the incoming beam polarization angle fitted with Z-Y’-Z” Euler angles set (73°, 50°, 65°), R2=0.977. The red and blue curves in the polar plots are the intensity of the responses analyzed in the lab horizontal X (blue) and lab vertical Y (red) directions respectively as indicated by the red and blue axes. The dotted line indicates the experimental data, and the continuous line is the calculated fit. The dominant signal arises from incident polarization parallel to the crystal long axis and emitted at a polarization perpendicular to it.

The SHG polar plots presented in Fig. 4 were then used to extract the ratios of the nonlinear susceptibility tensor elements. The polarization dependent SHG signal collected at wavelength of 475 nm is on the slope of the absorption curve of the crystals thus the Kleinman symmetry is not obeyed and all 5 nonzero dijk elements of the mm2 group were used as fitting parameters along with the 3 Z-Y’-Z” intrinsic convention Euler 8 ACS Paragon Plus Environment

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angles representing the orientation of the crystal’s optical axes (X, Y, Z) in the lab frame. A full determination of the nonlinear susceptibility tensor from these measurements is not possible considering the equivalence of the X and Y axes of the mm2 class. We were, nevertheless, able to find a consistent fit of the measured data of two different crystals while using the same d matrix for both fits (dxxz=-0.30, dyyz=0.87, dzxx=0.41, dzyy=-1 and dzzz=1), as presented in Figure 4. The goodness of fit was judged by the least squares method with the best fits having R2>0.97. While HRS allowed us to estimate the d value compared to the solvent, single crystal measurements enable estimation of the d value via comparison to a known crystalline sample. We compared the signals measured from the crystals of 1 grown in water/THF (85/15, v/v) solution with that of a ZnO nanowire (see Supporting Information).41 This 𝑒𝑒𝑒 comparison yielded a value of 𝑑 𝑧𝑧𝑧 = 2pm/V which is in good agreement with the value obtained from the HRS measurements. The reported lower bound second order susceptibility values are lower than some organic NLO crystals.18,20 However, in contrast to most known organic NLO crystals, the origin of the observed NLO activity in these systems stems from the straightforward self-assembly process based on simple molecules with rather modest dipole moments, resulting in soluble SHG nanocrystals. In conclusion, a new family of nonlinear organic crystals based on mono-substituted PDIs is readily accessible by simply adding the organic solution of the compound to water. By choosing assembly conditions, under which the crystals remain soluble, one can apply solution based techniques for additional processing. The second order susceptibility is a result of an assembly mode leading to favorable orientation of small molecular dipoles within the nanocrystals, in contrast to common design schemes based on large molecular dipoles. Simplicity of the molecular systems and the assembly process, together with the possibility to generate soluble and tunable SHG nanocrystals may open new avenues in in the area of organic SHG materials. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. Acknowledgements. We thank Prof. Boaz Pokroy and Dr. Iryna Polishchuk for the help with XRD measurements, and Drs. Ronit Popovitz-Biro and Linda J.W. Shimon for valuable discussions. This work was supported by the Israel Science Foundation, Gerhardt M.J. Schmidt Minerva Center of Supramolecular Architectures, and the Helen and Martin Kimmel Center for Molecular Design, The Israeli centers of research excellence program, the Minerva foundation and the European Research Council project SINSLIM 258221. The TEM studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging (Weizmann Institute).

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Supporting Information Available: Compounds synthesis, self-assembly conditions, UV/Vis spectra, molecular dipole calculation, XRD spectra, Synchrotron Powder XRD data and analysis, TEM and SEM images, SHG microscopy, hyper Rayleigh scattering and polarization dependence data. This material is available free of charge via the Internet at http://pubs.acs.org.

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