New Thiolated Nitrophenylhydrazone Crystals for Nonlinear Optics

Article pubs.acs.org/crystal

New Thiolated Nitrophenylhydrazone Crystals for Nonlinear Optics Ji-Youn Seo,† Seung-Heon Lee,† Mojca Jazbinsek,‡ Hoseop Yun,§ Jong-Taek Kim,∥ Yoon Sup Lee,∥ In-Hyung Baek,§ Fabian Rotermund,§ and O-Pil Kwon*,† †

Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea Rainbow Photonics AG, CH-8048 Zurich, Switzerland § Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea ∥ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡

ABSTRACT: We investigate new 4-nitophenylhydrazone crystals with thiolated electron donors for nonlinear optical applications. The methylthiolated (SM) electron donor and the biphenyl sulfane (SB) electron donor, which promotes a herringbone packing motif, are incorporated with an acentric nonlinear optical core, nitrophenylhydrazone (NPH), which promotes a λ-shaped packing motif in the crystalline state. SM-NPH (4-(methylthio)benzaldehyde4-nitrophenylhydrazone) crystals show head-to-tail hydrogen-bonded chains, which outweigh the usual λ-shaped packing of the nitrophenylhydrazone group. SB-NPH crystals (4-(phenylthio)benzaldehyde-4-nitrophenylhydrazone) show simultaneous λ-shaped packing, based on the hydrogen bonds of nitrophenylhydrazone with the nitro electron acceptor group, and the herringboneshaped packing of biphenyl sulfane groups. Polar SM-NPH chains pack in a centrosymmetric crystal structure, while SB-NPH crystals exhibit a large macroscopic second-order nonlinearity resulting from an acentric molecular ordering of two rotational isomers. In addition, SB-NPH crystals undergo an irreversible phase transition before melting, with the second phase being also acentric with a strong second harmonic generation efficiency.

1. INTRODUCTION During the last three decades, nonlinear optical properties of organic crystals have been significantly improved with various chemical modifications of ionic and nonionic crystals.1,2 Many efforts are focused on finding acentric core structures, which lead to acentric molecular ordering in the crystalline state and result in macroscopic second-order nonlinear optical responses.1,2 The successful examples with large macroscopic nonlinearities are, on the one hand, ionic stilbazolium crystals based on Coulomb interactions3−6 and, on the other hand, nonionic crystals, such as configurationally locked polyene (CLP) crystals7,8 and nitrophenylhydrazone (NPH) crystals,9−13 which are based on molecular asymmetry and hydrogen bonds. The NPH core structures most often form strong hydrogen bonds between the nitro group and the hydrazone group, which leads to a λ-shaped packing of the constituting molecules in the crystalline state.9−12 For example, T-NPH crystals ((thiophene-2-carbaldehyde)4-nitrophenylhydrazone; see Figure 1) form λ-shaped packing of 4-nitrophenylhydrazone with strong hydrogen bonds between N−H···O−N groups, as shown in Figure 2a.11 Recently, the electron donating characteristics of the methylthiolated (SM) group and the biphenyl sulfane (SB) group have been reported in a series of π-conjugated polyene derivatives.14 Both methylthiolated and biphenyl sulfane groups show an efficient electron donating ability. In addition, the biphenyl sulfane (SB) group promotes a herringbone packing motif based on strong π−π interactions between the neighboring phenyl groups, as shown in Figure 2b.14 © 2011 American Chemical Society

Figure 1. Chemical structures of the investigated nitrophenylhydrazone derivatives.

Here, we investigate new 4-nitrophenylhydrazone crystals with thiolated electron donors for nonlinear optical applications. The biphenyl sulfane (SB) electron donor, which is promoting a herringbone packing motif, and for comparison the methylthiolated (SM) electron donor are incorporated with the acentric nonlinear optical core, with nitrophenylhydrazone (NPH) promoting a λ-shaped packing motif. 9−12 SM-NPH Received: August 25, 2011 Revised: November 28, 2011 Published: November 30, 2011 313

dx.doi.org/10.1021/cg201115s | Cryst. Growth Des. 2012, 12, 313−319

Crystal Growth & Design

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(%): Calcd: C 65.31, H 4.33, N 12.03, S 9.18. Found: C 64.97, H 4.46, N 11.80, S 8.81. 2.2. Crystal Growth. For the growth of single crystals of thiolated nitrophenylhydrazone derivatives, the rapid cooling method was used. SM-NPH or SB-NPH synthesized compounds were mixed with acetonitrile solvent in vials, which were kept in an oven at a constant temperature of 40 °C during one week for saturation. The amount of the nitrophenylhydrazone material was chosen large enough to be above saturation in the acetonitrile solution. The resulting SM-NPH and SB-NPH solutions were filtered for removing insoluble materials and kept for a while in an oven at a temperature of 40 °C. The oven was then rapidly cooled down to room temperature, and single crystals were obtained. The grown SM-NPH and SB-NPH crystals are yellowish plates and orange needles, respectively. 2.3. X-ray Crystal Structure Analysis. SM-NPH. C14H13N3O2S, Mr = 287.34, monoclinic, space group P21/n, a = 6.5050(5) Å, b = 7.8622(4) Å, c = 27.1096(13) Å, β = 94.290(2)°, V = 1382.6 (1) Å3, Z = 4, T = 290(1) K, μ(Mo Kα) = 0.24 mm−1. Of 13171 reflections collected in the θ range 3.0−27.5° using ω scans on a Rigaku R-axis Rapid S diffractometer, 3169 were unique reflections (Rint = 0.023, completeness = 99.9%). The structure was solved and refined against F2 using SHELX97,15 233 variables, wR2 = 0.132, R1 = 0.040 (1981 reflections having Fo2 > 2σ(Fo2)), GOF = 1.14, and max/min residual electron density 0.20/−0.31 eÅ−3. CCDC-824805. SB-NPH. C19H15N3O2S, Mr = 349.41, monoclinic, space group P21, a = 5.5734(8) Å, b = 28.393(4) Å, c = 11.0183(17) Å, β = 94.397(4)°, V = 1738.5(4) Å3, Z = 4, T = 290(1) K, μ(Mo Kα) = 0.20 mm−1. Of 13639 reflections collected in the θ range 3.4−27.5° using ω scans on a Rigaku R-axis Rapid S diffractometer, 7779 were unique reflections (Rint = 0.049, completeness = 99.5%). The structure was solved and refined against F2 using SHELX97,15 451 variables, wR2 = 0.193, R1 = 0.072 (3490 reflections having Fo2 > 2σ(Fo2)), GOF = 0.98, Flack x parameter = 0.5(1), and max/min residual electron density 0.29/−0.18 eÅ−3. CCDC-824806.

Figure 2. Examples of (a) the λ-shaped packing of 4-nitrophenylhydrazone with strong hydrogen bonds between N−H···O−N groups in thiophene-based T-NPH crystals reported in ref 11 and of (b) the herringbone packing of the biphenyl sulfane group in πconjugated polyene SB1 (2-(5,5-dimethyl-3-(4-(phenylthio)styryl)cyclohex-2-enylidene)malononitrile) crystals reported in ref 14. The molecular structure excepting the biphenyl sulfane group of SB1 is omitted for clarity.

(4-(methylthio)benzaldehyde-4-nitrophenylhydrazone) crystals show head-to-tail hydrogen-bonded chains based on the introduced methylthiolated donor, which outweighs the usual λ-shaped packing of the nitrophenylhydrazone group. SB-NPH crystals (4-(phenylthio)benzaldehyde-4-nitrophenylhydrazone) show simultaneous λ-shaped packing of nitrophenylhydrazone with the nitro electron acceptor group and the herringboneshaped packing of the biphenyl sulfane group. The grown SBNPH crystals exhibit a large macroscopic nonlinearity with 2 orders of magnitude higher second harmonic generation (SHG) efficiency than that of urea at the fundamental wavelength of 1.9 μm.

3. RESULTS AND DISCUSSION 3.1. Design of New NPH Crystals. Figure 1 shows the chemical structures of the 4-nitrophenylhydrazone (NPH) derivatives. The T-NPH crystals introduced in ref 11 are thiophenebased and lead to a typical (nonherringbone) λ-shaped packing, as shown in Figure 2a. In order to introduce new thiolated electron donors into the NPH derivatives promoting a λ-shaped packing motif, a donor inducing a nonherringbone packing motif, 4-(methylthio)benzaldehyde, and another donor inducing a herringbone packing motif, 4-(phenylthio)benzaldehyde, are used for the synthesis of SM-NPH and SB-NPH, respectively. The NPH derivatives are synthesized by condensation reaction of the corresponding thiolated benzaldehyde14 with 4-nitrophenylhydrazine according to the literature.11 To investigate single crystal structures and physical properties of crystals, including nonlinear optical properties, single crystals of SM-NPH and SB-NPH have been grown by a rapid cooling method from saturated acetonitrile solutions at 40 °C. Yellowish platelike SM-NPH and orange needle SB-NPH crystals have been obtained. SM-NPH and SB-NPH exhibit a centrosymmetric monoclinic P21/n structure and a noncentrosymmetric monoclinic P21 structure, respectively, as determined by single-crystal X-ray analysis. Figure 3 shows absorption and emission spectra of SM-NPH and SB-NPH in acetone solution. The SM-NPH and SB-NPH chromophores show a similar wavelength of maximal absorption λmax and wavelength of maximal emission λem: λmax = 408 nm and λem = 523 nm for SM-NPH and λmax = 401 nm and λem = 524 nm for SB-NPH. This means that the electronic transition properties of these chromophores in solution are not considerably affected by the introduced change on the thiolated

2. EXPERIMENTAL SECTION 2.1. Synthesis. The new thiolated nitrophenylhydrazone derivatives, SM-NPH and SB-NPH, were synthesized by a condensation of the corresponding thiolated benzaldehyde14 with 4-nitrophenylhydrazine.11 1-(4-(Methylthio)benzylidene)-2-(4-nitrophenyl)hydrazine (SMNPH). Yield: 30%. 1H NMR (CDCl3): 2.51 (s, 3H, −SCH3), 7.09 (d, J = 9.2 Hz, 2H, C6H4), 7.24 (d, J = 8.4 Hz, 2H, C6H4), 7.58 (d, J = 8.4 Hz, 2H, C6H4), 7.75 (s, 1H, HC=N-), 8.02 (s, 1H, NH), 8.16 (d, J = 9.2 Hz, 2H, C6H4). 13C NMR (CDCl3): 149.5, 141.0, 130.9, 127.2, 126.4, 126.3, 111.9, 94.6, 15.8. Elemental Analysis for C14H13N3O2S (%): Calcd: C 58.52, H 4.56, N 14.62, S 11.16. Found: C 58.51, H 4.58, N 14.53, S 11.13. 1-(4-Nitrophenyl)-2-(4-(phenylthio)benzylidene)hydrazine (SBNPH). Yield: 45%. 1H NMR (CDCl3): 7.1 (d, J = 8.8 Hz, 2H, C6H4), 7.26 (d, J = 8.0 Hz, 2H, C6H4), 7.32 (d, J = 8.0 Hz, 2H, C6H4), 7.40 (d, J = 8.0 Hz, 2H, C6H4), 7.56 (d, J = 8.4 Hz, 2H, C6H4), 7.74 (s, 1H, HCN−), 8.05 (s, 1H, NH), 8.45 (d, J = 8.4 Hz, 2H, C6H4). 13 C NMR (CDCl3): 149.7, 140.6, 138.9, 136.1, 132.4, 130.0, 129.6, 129.6, 128.0, 127.5, 126.3, 111.9. Elemental Analysis for C19H15N3O2S 314



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The detailed characteristics of this phase transition are discussed in section 3.5. 3.2. Microscopic Optical Nonlinearities. Figure 5 shows molecular conformations of thiolated NPH chromophores in

Figure 3. Absorption and emission spectra of SM-NPH and SB-NPH in acetone solution.

electron donor part. Also T-NPH exhibited a similar λmax = 405 nm, as measured in acetonitrile solution in ref 11. We have investigated the thermal stability by differential scanning calorimetry (DSC) with a scan rate of 10 °C/min. As shown in Figure 4a, both thiolated NPH crystals exhibit a similar thermal

Figure 5. Top and side views of the molecular structure of SM-NPH, SB-NPH(S), and SB-NPH(D) molecules in the crystalline state. The solid and the dotted vectors present the directions of the dipole moment μ and the maximum first hyperpolarizability βmax of the molecules as determined by finite-field calculations, respectively.

SM-NPH and SB-NPH crystals. While SM-NPH crystals consist of one conformer, SB-NPH crystals consist of two conformers, SB-NPH (S) and SB-NPH (D), which are rotational isomers of phenylthiol groups.14 Since the change of the molecular conformation is often accompanied by a large variation of the amplitude and the main direction of the first hyperpolarizability,10a,11,13,16 the characteristics of microscopic optical nonlinearity of new thiolated NPH derivatives have been calculated by quantum chemical calculation17 with finitefield density functional theory (FF-DFT) using a B3LYP/ 6-311+G(d) hybrid functional/basis set.18 The experimental (EXP) molecular conformations, determined by the X-ray crystal structure analysis have been used in the calculations.17 We have also calculated the value and direction of βmax, the maximum value of the FF hyperpolarizability tensor in the main charge-transfer direction.17 The results are listed in Table 1.

Figure 4. (a) DSC curves of SM-NPH and SB-NPH crystalline powders. (b) Irreversible phase transition of SB-NPH crystalline powders.

stability with the decomposition temperature Td of over 240 °C, which is here defined by the initial exothermic point, as well as the similar melting temperature Tm = 189 °C for SM-NPH and Tm = 193 °C for SB-NPH, defined by the peak position. It is interesting to note that SB-NPH exhibits a phase transition before melting at about 148 °C, which is defined by the initial endothermic point. 315



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4-nitrophenylhydrazone part, which is mainly contributing to the molecular optical nonlinearity. On the other hand, the conformers in the T-NPH crystal have different conformations in the 4-nitrophenylhydrazone part, leading to considerably different βmax.11 The rotational isomerization of the biphenyl sulfane group in SB-NPH does not strongly influence the molecular optical nonlinearity, but it can affect the molecular ordering in the crystalline state and the resulting macroscopic optical nonlinearity. 3.3. Single Crystal Structure. The thiolated NPH crystals exhibit different space groups: SM-NPH crystals have a centrosymmetric crystal structure with the monoclinic P21/n symmetry, and SB-NPH crystals have acentric crystal structure with the monoclinic P21 symmetry, with the latter one being particularly interesting because it leads to a macroscopic second-order nonlinear optical response. Figures 6 and 7 show crystal packing diagrams of SM-NPH and SB-NPH crystals, respectively. In contrast to most conventional NPH crystals having a λ-shaped packing based on the strong hydrogen bond between N−H···O−N groups on the 4-nitrophenylhydrazone group,9−12 SM-NPH crystals having a methylthiolated group do not show a λ-shaped packing, as shown in Figure 6. The main supramolecular interactions in SM-NPH crystals are hydrogen bonds between S−CH3···O−N groups with a H···O distance of 2.536 Å, which form head-to-tail hydrogen-bonded polar chains. These head-to-tail interactions due to the introduced methylthiolated group are interesting, since they overweigh the usual λ-shaped packing.9−12 This has been previously observed in DH-NPH (dihydroxybenzaldehyde−4-nitrophenylhydrazone) crystals, which however have a stronger hydrogen bond donor, phenolic group.13 The head-to-tail hydrogenbonded polar chains of SM-NPH molecules are linked in an antiparallel way by other supramolecular interactions, e.g. hydrogen bonds between the N−H···S− groups with a H···S distance of 3.054 Å. Figure 7 shows a crystal packing diagram of SB-NPH crystals. The SB-NPH(S) conformers are presented by the ball-and-stick model and the SB-NPH(D) conformers by the wire model. Due to the nitrophenylhydrazone group promoting a λ-shaped packing,9−12 one of the important supramolecular interactions in SB-NPH crystals is strong hydrogen bonds forming a λshape: SB-NPH molecules are linked with intermolecular hydrogen bonds of (SB-NPH(S)) N−O···H−N (SB-NPH(D)) with O···H distance of 2.154 Å, presented by thick dotted lines, and (SB-NPH(D)) N−O···H−N (SB-NPH(S)) with O···H distance of 2.188 Å, presented by thin dotted lines. Due to the herringbone packing motif induced by the biphenyl sulfane electron donor group,14 the λ-shaped packing is slightly

Table 1. Results of the Finite-Field (FF) Method: Dipole Moments μg (D), the Zero-Frequency Hyperpolarizability Tensor Elements βijk (×10−30 esu), the First Hyperpolarizability βmax (×10−30 esu), and the Angle θ (deg) between the Dipole Moments μg and the Main Direction of the First-Order Hyperpolarizability βmax

μg = (−μz) βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz βmax θ

T-NPH (OPT)11

SM-NPH (EXP)

SB-NPH(S) (EXP)

SB-NPH(D) (EXP)

9.07 0.00 0.52 0.00 0.28 −0.77 0.00 0.51 0.00 −12.43 49.72 54.0 12.9

10.52 −0.08 −0.46 1.05 11.11 −0.52 0.69 14.91 −0.53 28.90 65.75 87.1 25

8.31 −0.13 −0.52 1.90 9.68 −0.95 1.18 15.82 −0.01 27.85 47.76 73.9 30

6.55 0.37 0.54 −5.95 17.15 1.62 −5.44 15.75 −11.33 23.76 51.88 77.2 31

The EXP molecules of SM-NPH, SB-NPH(S), and SBNPH(D) conformers in the crystalline state exhibit a relatively large molecular optical nonlinearity with the first hyperpolarizability βmax of 87.1 × 10−30 esu, 73.9 × 10−30 esu, and 77.2 × 10−30 esu, respectively. Although the thiolated NPH chromophores display the wavelengths of maximal absorption λmax similar to those of thiophene-based T-NPH chromophores in solution as discussed above, SM-NPH and SB-NPH conformers possess a larger first hyperpolarizability βmax than that of T-NPH molecules (27.8 × 10−30 esu and 40.8 × 10−30 esu for EXP molecules and 54.0 × 10−30 esu for the optimized (OPT) molecule).11 Therefore, the newly introduced thiolated electron donor groups exhibit a high electron donating ability. The solid and the dotted vectors in Figure 5 present the directions of the dipole moment μ and the maximum first hyperpolarizability βmax (the main direction of the chargetransfer) for the corresponding EXP molecular conformations, as determined by the above calculations. Although the rotation angles of the phenylthiolated group on the biphenyl sulfane parts occurring in two rotational conformers, SB-NPH(S) and SB-NPH(D), are very different, they show a very similar direction and amplitude of the maximum first hyperpolarizability βmax (see Figure 5 and Table 1). In SB-NPH crystals, the rotational isomerization on biphenyl sulfane parts only influences slightly the characteristics of the microscopic optical nonlinearity, and the two rotamers show a small difference in their properties. This is attributed to the fact that SB-NPH(S) and SB-NPH(D) conformers have similar conformations of the

Figure 6. Crystal packing of SM-NPH crystals projected along the crystallographic a-axis. 316



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Figure 7. Crystal packing of SB-NPH crystals projected along the crystallographic a-axis: SB-NPH(S) are presented with a ball-and-stick model, and SB-NPH(D) molecules with a wire model. The arrows present the directions of the main charge transfer βmax of the molecules and its relative size projected along the bc crystallographic plane.

Table 2. Componentsa of the Effective Hyperpolarizability eff Tensor βijk (10−30 esu) of SB-NPH Crystals

modified and the biphenyl sulfane groups form the expected herringbone packing as shown in Figure 7. An interesting aspect of SB-NPH crystals is that the molecular ordering looks almost centrosymmetric when one ignores the existence of two different conformers. However, due to the high flexibility of both the NPH group and the biphenyl sulfane electron donor part, the two conformers pointing in opposite directions lead to a nonvanishing macroscopic crystal polarity, as well as the second-order optical nonlinearity and other physical properties only existing in acentric crystals. In thiophene-based T-NPH crystals, which also consist of two different conformers, the two conformers are not antiparallel and canceling each other, as here their main effective charge transfer direction is perpendicular to each other.11 The details in the effective macroscopic second-order nonlinear optical properties are discussed in the following section. 3.4. Macroscopic Optical Nonlinearities in the Crystalline State. The macroscopic optical nonlinearities of SB-NPH crystals having an acentric P21 space group are examined by powder second harmonic generation (SHG) measurement19 and FF-DFT quantum chemical calculations considering the exact molecular ordering in the crystalline state.17 In a powder SHG test at a nonresonant fundamental wavelength of 1.9 μm, SB-NPH crystals exhibit a large macroscopic nonlinearity with a slightly higher SHG efficiency compared with the case of the thiophene-based T-NPH crystals, which exhibit a 2 orders of magnitude higher SHG efficiency than urea.11 In SB-NPH crystals, the largest calculated diagonal and off-diagonal effective hyperpolarizability eff , obtained by averaging over all molecules in tensor elements βijk 17 eff eff the unit cell, are β222 = −18.9 × 10−30 esu and β112 = 10.4 × −30 10 esu, respectively. The details are listed in Table 2. These nonzero values are obtained despite the contributions of the two conformers pointing to the opposite directions in the unit cell because the SB-NPH(D) molecules pack with a higher order parameter compared to the SB-NPH(S) molecules: θp ≈ −28° and order parameter cos3(θp) = −0.68 for SB-NPH(D)

sum of contribution of SB-NPH(S) and (D) contribution of SB-NPH(S) contribution of SB-NPH(D)

eff β112

eff β222

eff β332

10.4 28.4 −18.2

−18.9 18.9 −56.8

−4.2 0.8 1.3

a

These components are given in the Cartesian system x1x2x3 with eff = 0 for x2 parallel to the crystallographic symmetry axis b, in which β123 eff = each contribution. Note that the diagonal component β222,crystal eff eff + β222,D )/2, while for nondiagonal components a rotation of the (β222,S system around x2 = b is also involved.

and θp ≈ 49° and cos3(θp) = 0.29 for SB-NPH(S), where θp is the angle between the polar axis of the crystal (the symmetry axis b) and the main direction of the hyperpolarizability βmax of each conformer (see Figure 7). If only SB-NPH(S) conformers or SB-NPH(D) conformers had contributed to the optical eff would nonlinearity, the effective hyperpolarizability tensor βijk have been higher than that in both contributions, as listed in Table 2. Nonetheless, the calculated value of the largest diagonal and off-diagonal effective hyperpolarizability tensor eff eff of SB-NPH is higher than that of T-NPH (β111 = element βijk 11 −30 −30 eff 12.1 × 10 esu and β223 = −7.6 × 10 esu), which agrees well with the measured SHG efficiency. 3.5. Phase Transition. In SB-NPH crystals, a phase transition at about 148 °C before melting is observed in DSC measurements, as shown in Figure 4a. To confirm that it is either a reversible or irreversible phase transition, sequential DSC scans with a scan rate of 10 °C/min are performed as shown in Figure 4b. After heating up to 173 °C and cooling down to room temperature, which are the first heating and the first cooling scans, respectively, in the second heating scan, the phase transition is absent. This confirms that the phase transition of SB-NPH crystals is irreversible. We denote the SB-NPH crystals before and after the phase transition as “low-temperature phase SB-NPH(L)” and “high-temperature phase SB-NPH(H)”, respectively. 317



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that of the SB-NPH(L) crystals corresponding to the shorter hydrogen bond. The later absorption peak of 3273 cm−1 in SB-NPH(H) is slightly shifted to a higher wavenumber compared to the case of SB-NPH(L) (2.188 Å). These results indicate that SB-NPH(H) crystals most probably exhibit similar main supramolecular interactions with strong hydrogen bonds between the hydrazone (=N−N−H) and NO2 groups, i.e. those leading to λ-shaped packing, and the strength and the distance of one hydrogen bond is similar and that of the other is weaker and longer compared to those in SB-NPH(L) crystals.

During the phase transition, single SB-NPH(L) crystals change to polycrystalline SB-NPH(H) powders, for which we could not perform a reliable X-ray single crystal structure analysis. In order to characterize the details of the hightemperature phase, we prepared the SB-NPH(H) crystalline powder by isothermal heat treatment of SB-NPH(L) single crystals at 180 °C for 90 min and, after cooling, measured the powder X-ray diffraction pattern, powder SHG efficiency, and infrared (IR) absorption spectra. Figure 8a shows the powder

4. CONCLUSIONS We have investigated new 4-nitophenylhydrazone crystals with thiolated electron donors for nonlinear optical applications. The biphenyl sulfane electron donors promoting a herringbone packing motif and methylthioled electron donors are incorporated with an acentric nonlinear optical core, nitrophenylhydrazone (NPH), promoting a λ-shaped packing motif. SM-NPH crystals show head-to-tail hydrogen-bonded chains, which outweigh the usual λ-shaped packing of the nitrophenylgroup. SB-NPH crystals show simultaneous λ-shaped packing of nitrophenylhydrazone with the nitro electron acceptor group and the herringbone-shaped packing of the biphenyl sulfane group. The SB-NPH crystals exhibit a large macroscopic second-order nonlinearity with acentric molecular ordering of two rotational isomers. In addition, SB-NPH crystals undergo an irreversible phase transition before melting, with the second phase also being acentric, with a strong second harmonic generation efficiency.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS This work has been supported by the Basic Science Research Program (2011-0004065) and the Priority Research Centers Program (2011-0022978) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (MEST). I.-H.B. and F.R. have been supported by the NRF (2011-0017494), funded by MEST. The work at KAIST was in part supported by the NRF (2011-0001213) and KISTI (KSC-2011-C2-18).

Figure 8. (a) Powder X-ray diffraction patterns and (b) infrared absorption spectra of SB-NPH polymorphs. The SB-NPH(H) crystalline powders were prepared by heat treatment of SB-NPH(L) single crystals at 180 °C for 90 min.

X-ray diffraction patterns of SB-NPH polymorphs before and after phase transitions. The high-temperature phase SB-NPH(H) shows a distinguishable X-ray diffraction pattern compared to the low-temperature phase SB-NPH(L). In a powder SHG test at a fundamental wavelength of 1234 nm, both SB-NPH(H) and SB-NPH(L) crystalline powders show a similar SHG efficiency. Therefore, as is the case for SB-NPH(L), high-temperature phase SB-NPH(H) crystals also possess an acentric crystal structure. Figure 8b shows the IR absorption spectra of SB-NPH polymorphs. SB-NPH(H) crystals show similar IR absorption spectra except in the region from 3200 to 3300 cm−1, which in SB-NPH(L) crystals corresponds to the N−H stretching vibration on the hydrazone (=N−N−H) group, which forms strong hydrogen bonds with NO2 groups. SB-NPH(L) crystals show two N−H stretching vibration peaks of 3249 and 3263 cm−1, which are related with two intermolecular hydrogen bonds, (SB-NPH(S)) N−O···H−N (SB-NPH(D)), with a O···H distance of 2.154 Å, and (SB-NPH(D)) N−O···H−N (SB-NPH(S)), with a O···H distance of 2.188 Å, respectively. SB-NPH(H) crystals also reveal two absorption peaks with small shifts, 3249 and 3273 cm−1; the former is identical with

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