Highly Nonlinear Optical Configurationally Locked Triene Crystals

May 22, 2008 - Jongtaek Kim , Ji-Youn Seo , Mojca Jazbinsek , Seong-Ji Kwon , Eun-Young Choi , Jung-In Seo , Hoseop Yun , Yoon Sup Lee , Peter Günter...
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7846

J. Phys. Chem. C 2008, 112, 7846–7852

Highly Nonlinear Optical Configurationally Locked Triene Crystals Based on 3,5-Dimethyl-2-cyclohexen-1-one† Seong-Ji Kwon,‡ O-Pil Kwon,*,‡,§ Jung-In Seo,| Mojca Jazbinsek,‡ Lukas Mutter,‡ Volker Gramlich,⊥ Yoon-Sup Lee,| Hoseop Yun,# and Peter Gu¨nter*,‡ Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland, Department of Molecular Science and Technology, Ajou UniVersity, Suwon 443-749, Korea, Department of Chemistry and School of Molecular Science (BK 21), Korea AdVanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea, Laboratory of Crystallography, ETH Zurich, CH-8093 Zurich, Switzerland, and DiVision of Energy Systems Research and Department of Chemistry, Ajou UniVersity, Suwon 443-749, Korea ReceiVed: NoVember 29, 2007; ReVised Manuscript ReceiVed: January 8, 2008

New organic nonlinear optical configurationally locked polyene (CLP) crystals based on 3,5-dimethyl-2cyclohexen-1-one have been designed and their supramolecular organization investigated. Acentric single crystals 2-(5-methyl-3-(4-(pyrrolidin-1-yl)styryl)cyclohex-2-enylidene)malononitrile (MH2) of large sizes with a maximal side length of up to 1 cm have been grown from acetonitrile solution. The acentric MH2 crystals present the monoclinic space-group symmetry Cc and exhibit a large macroscopic nonlinearity with a similar powder second-harmonic generation efficiency at 1.9 µm as the well-studied DAST (N,N-dimethylaminoN′-methylstilbazolium p-toluenesulfonate), which is about seven times larger than that of analogous CLP crystals studied previously. The microscopic and macroscopic nonlinearities are also investigated theoretically using quantum chemical calculations. Introduction Organic nonlinear optical materials combine large optical nonlinearities with low dielectric constants and are therefore very attractive for high-speed photonic applications.1–3 For second-order nonlinear optical applications such as terahertz (THz) generation4 and electro-optics,5 a noncentrosymmetric arrangement of highly polar π-conjugated molecules is required. Substantial progress has been recently achieved in poledpolymer systems, reaching extremely high optical nonlinearities with electro-optic coefficients above 300 pm/V at 1.5 µm and allowing for over 100 GHz-bandwidth active photonic structures.1 Beside poled polymers, single crystalline materials are also attractive since they offer a superior thermal and photochemical stability. The arrangement of molecules in a crystalline solid with a desired orientation is a challenging topic for fundamental and applied research. The understandings of the supramolecular organization and of noncovalent intermolecular interactions with various conformations of a chemically identical molecule are main aims of crystal engineering in order to predict the crystal structures and to achieve the desired physical properties. To achieve an acentric molecular orientation in crystals, several approaches such as the introduction of molecular asymmetry, chirality, nonrod-shaped cores, Coulomb interactions, octupolar shape, cocrystallization2,3 and molecular host frameworks6 have been investigated. Recently, configurationally locked polyene (CLP) crystals7–9 have shown to be very promising for achieving a noncentrosymmetric molecular orientation in the crystalline state with large macroscopic nonlin† Part of the “Larry Dalton Festschrift”. * Corresponding authors. E-mail: (P.G.) [email protected]. ‡ Nonlinear Optics Laboratory. § Department of Molecular Science and Technology, Ajou University. | Korea Advanced Institute of Science and Technology (KAIST). ⊥ Laboratory of Crystallography, ETH Zurich. # Division of Energy Systems Research and Department of Chemistry, Ajou University.

SCHEME 1: The Chemical Structures of Configurationally Locked Polyene (CLP) Chromophores Studied Previously (a) and Synthesized Recently (b)

earity and for applying various melt- or solution-based crystal growth techniques. A few examples of acentric CLP crystals studied previously are shown in Scheme 1a. However, the macroscopic nonlinearity of the CLP crystals is still far from the theoretical limits because of a nonoptimal crystalline packing. Therefore, chemical modifications of the CLP chromophores are required to optimize their orientation by intermolecular interactions in the crystalline state. The main supramolecular interactions of acentric CLP crystals studied previously are hydrogen bonds of -CN · · · HC- or -CN · · · HO- groups.7–9 The crystal structures of the CLP crystals are strongly dependent on their methyl substituents on

10.1021/jp711300w CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

Triene Crystals Based on 3,5-Dimethyl-2-cyclohexen-1-one TABLE 1: Physical, Chemical, and Structural Data of CLP Crystals: λmax is the Wavelength of the Maximum Absorption in Chloroform Solution, Tm the Melting Temperature, and Ti is the Weight-Loss Temperaturea

MH1 MH2b MH3 MM1 MM2 HH3

λmax (nm)

Tm (°C)

Ti (°C)

space group

powder SHG

503 518 502 502 521 506

196 190 218 235 178 211

325 290 333 293 290 300

Cc P21 P21 P21

0 7.1 0 1.0 0.6 2.0

this work this work this work ref 7 ref 8 ref 9

a Powder SHG efficiency was measured at a fundamental wavelength of 1.9 µm relative to that of MM1 powder. b Acentric nonhydrated form.

the cyclohexene ring; for example, in MM17 and MM28 crystals the equitorial and axial methyl groups on the cyclohexene ring are main hydrogen bond donor sites, wheras in HH39 crystals without the methyl substituents, the cyclohexene ring does not involve supramolecular interactions. Although H-C- groups of methyl group on the cyclohexene ring are relatively weak hydrogen bond donors,10 they associate important hydrogen bond network with CN groups in CLP crystals.7,8 In this work, we investigate experimentally and theoretically a new series of CLP crystals obtained by introducing only one methyl substituent on the cyclohexene ring (see Scheme 1b), which is designed to change the crystalline packing by modifying intermolecular hydrogen bonds. Acentric single CLP crystals, 2-(5-methyl-3-(4-(pyrrolidin-1-yl)styryl)cyclohex-2enylidene)malononitrile (MH2), exhibit a large macroscopic nonlinearity with about 3 orders of magnitude greater powder second-harmonic generation efficiency than that of urea, and seven times larger than that of analogous MM1 crystals. The microscopic and macroscopic nonlinearities of the CLP chromophores are also analyzed theoretically by using the timedependent density functional theory (TD-DFT) and finite field (FF) methods. Experimental Section General Characterization. UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 9 spectrometer. The thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere using Perkin-Elmer TGA-7 and DSC-7 spectrometer, respectively (10 °C/min scan rate). The results of the chemical and physical characteristics of the investigated chromophores are summarized in Table 1. 1H and 13C NMR spectra were recorded on a Varian 300 MHz NMR spectrometer. The chemical shifts are reported in ppm (δ) relative to (CH3)4Si. Synthesis. All chemicals were from commercial suppliers, mainly from Aldrich, and used as received unless otherwise noted. The MH chromophores were synthesized by consecutive Knoevenagel condensations. The intermediate 2-(3,5-dimethylcyclohex-2-enylidene)malononitrile was prepared by the condensation of 3,5-dimethyl-2-cyclohexen-1-one with malononitrile using ammonium acetate and was then used for the second condensation having the corresponding aldehyde with electron donor groups. Synthesis of 2-(3,5-Dimethylcyclohex-2-enylidene)malononitrile. 3,5-Dimethyl-2-cyclohexen-1-one (25 g, 0.2 mol) and malononitrile (13.3 g, 0.2 mol) were dissolved in benzene (150 mL). Ammonium acetate (3.10 g, 4 mmol) and acetic acid (4.15 mL, 7 mmol) were added to this solution, and the mixture was

J. Phys. Chem. C, Vol. 112, No. 21, 2008 7847 refluxed for 36 h at 80 °C under nitrogen atmosphere. The resulting mixture was concentrated by evaporation. The concentrate was dissolved in dichloromethane and then extracted with water. The organic residue was purified by column chromatography (ethyl acetate/hexane ) 1:1). After recrystallization in hexane, we obtained an ivory solid. Yield: 75%. 1H NMR (CDCl3, δ): 1.10 (3H, d, -CH3), 2.04 (3H, s, -CH3), 2.16-2.26 (2H, 2 × d, J ) 11.4 Hz, -CH2-), 2.32-2.43 (2H, 2 × d, J ) 12.6 Hz, -CH2-), 2.87-2.94 (1H, 2 × d, J ) 3 Hz, -CH-), 6.60 (1H, s, -C)CH-). 13C NMR (CDCl3,): 170.67, 161.18, 121.36, 113.12, 112.37, 39.82, 37.06, 29.02, 25.30, 20.81. Synthesis of MH Chromophores. 4-(Dimethylamino)benzaldehyde for MH1 was purchased from Aldrich. The other aldehydes for MH2 and MH3 were synthesized according to literature.11 The prepared intermediate, 2-(3,5-dimethylcyclohex2-enylidene)malononitrile was mixed with an equimolar amount of the corresponding aldehyde in ethanol. Piperidine was dropped into the mixture, which was stirred for 24 h at room temperature. We obtained a crystalline solid from the mixture by filtration. The MH materials for all experiments were purified by recrystallization in methylenechloride/methanol solution for several times. 2-(3-(4-(Dimethylamino)styryl)-5-methylcyclohex-2-enylidene)malononitrile (MH1). 1H NMR (CDCl3, δ): 1.00-1.17 (3H, 2 × d, -CH3), 2.09-2.18 (1H, m, -CH-), 2.21-2.49 (2H, m, -CH2-), 2.79-2.98 (2H, m, -CH2-), 3.0 (6H, s, N-CH3), 6.62-6.70 (2H, d, J ) 8.7 Hz, Ar-H), 6.73 (1H, s, -C)CH-), 6.76-6.81 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.02-7.04 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.11-7.43 (2H, d, J ) 8.7 Hz, Ar-H). 13C NMR (CDCl3, δ): 169.71, 156.67, 138.24, 129.38, 123.86, 123.49, 122.16,114.22, 113.46, 112.01, 74.89, 40.25, 37.53, 33.63, 28.88, 21.15. Elementary analysis (%) Calcd for C20H21N3: C, 79.17; H, 6.98; N, 13.85. Found: C, 79.00; H, 7.02; N, 13.96. 2-(5-Methyl-3-(4-(pyrrolidin-1-yl)styryl)cyclohex-2-enylidene)malononitrile (MH2). 1H NMR (CDCl3, δ): 1.00-1.17 (3H, 2 × d, -CH3), 2.01-2.06 (4H, m, -CH2-), 2.09-2.19 (1H, m, -CH-), 2.21-2.51 (2H, m, -CH2-), 2.80-2.97 (2H, m, -CH2-), 3.31-3.38 (4H, m, N-CH2-), 6.47-6.56 (2H, d, J ) 8.7 Hz, Ar-H), 6.72 (1H, s, -C)CH-), 6.74-6.79 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.02-7.07 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.11-7.42 (2H, d, J ) 8.7 Hz, Ar-H). 13C NMR (CDCl3, δ): 169.60, 156.80, 148.95, 138.61, 129.55, 123.17, 122.87, 121.80, 114.30, 113.53, 118.89, 74.35, 47.64, 37.49, 33.61, 28.85, 25.53, 21.11. Elementary analysis (%) Calcd for C22H23N3: C, 80.21; H, 7.04; N, 12.75. Found: C, 79.99; H, 7.08; N, 12.70. 2-(3-(4-(2-(Hydroxymetyl)pyrrolidin-1-yl)styryl)-5-methylcyclohex-2-enylidene)malononitrile (MH3). 1H NMR (CDCl3, δ): 1.00-1.17 (3H, 2 × d, -CH3), 2.00-2.08 (4H, m, -CH2-), 2.12-2.18 (1H, m, -CH-), 2.22-2.51 (2H, m, -CH2-), 2.79-2.97 (2H, m, -CH2-), 3.32-3.97 (4H, 4 × m, NCH2-), 6.61-6.69 (2H, d, J ) 8.7 Hz, Ar-H), 6.72 (1H, s, -C)CH-), 6.75-6.80 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.01-7.06 (1H, d, J ) 15.9 Hz, -CH)CH-), 7.11-7.42 (2H, d, J ) 8.7 Hz, Ar-H). 13C NMR (CDCl3, δ): 169.71, 156.65, 149.00, 138.20, 129.50, 123.83, 123.74, 122.18, 114.22, 113.46, 112.56, 74.84, 63.31, 60.08, 49.07, 37.52, 33.64, 28.88, 28.65, 23.57, 21.15. Elementary analysis (%) Calcd for C23H25N3O: C, 76.85; H, 7.01; N, 11.69; O, 4.45. Found: C, 76.57; H, 7.08; N, 11.62; O, 4.73. X-ray Crystal Structure Analysis. Single crystal X-ray diffraction experiments at 294 K were carried out on a single crystal X-ray diffractometer equipped with a CCD detector

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TABLE 2: Results of the FF Method Calculated with the Optimized (OPT) and Experimental (EXP) Molecular Structuresa (µg() -µz)) Rxx Rxy Ryy Rxz Ryz Rzz βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz βz βmax θ

MM1 (OPT)

MM1 (EXP)

MH2 (OPT)

MH2 (EXP)

14.01 239.26 18.10 469.40 -15.97 -188.83 1046.43 -0.27 -0.11 -0.77 -10.45 -1.00 2.58 31.94 -6.36 -80.45 166.45 197.39 224.4 24.5

13.72 219.78 12.59 438.97 5.05 -194.67 960.48 -0.05 -0.03 -0.10 -13.65 -0.93 0.59 35.86 -1.88 -82.32 164.51 199.44 226.7 25.5

14.84 240.19 10.97 482.06 -6.17 -201.06 1132.19 -0.18 0.16 -0.02 -8.94 -1.31 0.95 31.28 -3.48 -86.19 192.40 222.37 249.5 22.6

14.79 219.91 3.86 436.04 5.14 -184.38 1054.10 -0.25 -0.03 0.35 -8.51 -0.89 -0.50 28.78 0.27 -81.83 194.58 222.46 245.8 21.9

a Dipole moments µg (D), static polarizability Rij (×10-25 esu), the zero-frequency hyperpolarizability tensor βijk (×10-30 esu), the vector component βz along the dipole moment direction of the hyperpolarizability tensor βijk (×10-30 esu), the first-order hyperpolarizability βmax (×10-30 esu), and the angle θ (deg) between the dipole moment µg and the main direction of the first-order hyperpolarizability βmax. The z-direction is the direction of the ground-state dipole moment µg.

TABLE 3: Results of TD-DFT Calculation from the Optimized (OPT) and Experimental (EXP) Molecular Structures major contribution λmax (nm) Emax[ICT] (eV) fos µge (D) ∆µge (D) β0 (10-30 esu)

MM1 (OPT)

MM1 (EXP)

MH2 (OPT)

MH2 (EXP)

HOMO-1 f LUMO HOMO f LUMO 468 2.65 1.30 11.40 17.61 190.49

HOMO f LUMO

HOMO-1 f LUMO HOMO f LUMO 473 2.62 1.39 11.84 17.41 207.79

HOMO f LUMO

463 2.68 1.20 10.85 19.58 188.05

(Xcalibur PX, Oxford Diffraction) with 65 mm sample-detector distance.7–9 Data reduction and numerical absorption correction were performed using the software package CrysAlis.12 The crystal structures were solved by direct methods and the full data sets refined on F2, employing the programs SHELXS-97 and SHELXL-97.13 To analyze the structure of the cyclohexene ring in acentric MH2 crystals in more detail, single crystal structures were determined again at low temperature (T ) 150 K). Of the 8855 reflections collected in the θ range 2.0-27.5° using an ω scan on a Rigaku R-axis Rapid S diffractometer, 3757 were unique reflections (Rint ) 0.015, completeness ) 99.9%). The structure was solved and refined against F2 using SHELX-97,13 324 variables, wR2 ) 0.0925, R1 ) 0.0327 (3478 reflections having Fo2 > 2σ(Fo2)), GOF ) 1.082, and max/min residual electron density 0.175/-0.200 eÅ-3. Acentric MH2 Crystal (294 K). C22H23N3, Mr ) 329.43, monoclinic, space group Cc, a ) 12.960 (3) Å, b ) 14.970 (3) Å, c ) 9.5840 (19) Å, R ) 90°, β ) 90.30 (3)°, γ ) 90°, V ) 1859.4 (6), Z ) 4, T ) 294 (2) K, CCDC 636314. Acentric MH2 Crystal (150 K). C22H23N3, Mr ) 329.43, monoclinic, space group Cc, a ) 12.8213 (5) Å, b ) 14.9556 (5) Å, c ) 9.4805 (3) Å, R ) 90°, β ) 90.198 (1)°, γ ) 90°, V ) 1817.9 (1), Z ) 4, T ) 150.0 (5) K, CCDC 672171. Centrosymmetric-Hydrated MH2 Crystal (294 K). C22H23N3(H2O), Mr ) 1335.75, tetragonal, space group P42(1)c, a ) 16.651 (2) Å, b ) 16.651 (2) Å, c ) 14.709 (3) Å, R ) 90°, β ) 90°, γ ) 90°, V ) 4078.2 (12), Z ) 2, T ) 294 (2) K, CCDC 638874.

462 2.68 1.33 11.43 19.00 201.53

Computational Details. All the calculations were performed with the Gaussian 0314 Program. The molecular geometries were fully optimized with no restrictions by using the hybrid functional B3LYP15 with the 6-311+G(d) basis set. These minimizations have been performed until the rms residual force was lower than 1 × 10-5 au (tight threshold in Gaussian 03). Vibrational frequencies were calculated for optimized molecular structures (OPT) to verify that no negative frequencies were present for local and global minimum structures on the potential energy surface. The experimental molecular structures (EXP) of MM1 and MH2 in the acentric crystal structures were also considered using single point energy calculations. Both optimized (OPT) and experimental (EXP) molecular structures were analyzed by FF and TD-DFT methods. Groundand excited-state dipole moments were calculated by using the self-consistent field (SCF) and the one-particle configuration interaction (CI) densities, respectively. For the FF procedure, an electric field step of 0.001 au was used. All DFT calculations have been performed with an Ultrafine integration grid [pruned (99 590) grid]. Using the FF method, the hyperpolarizability tensor βijk was calculated in the molecular system xyz, which is here defined so that the direction of the ground-state dipole moment µg points along its z direction (µg ) -µz). The components of the hyperpolarizability tensor βijk in this system are given in Table 2. We have also determined the main direction and the main value βmax of the first-order hyperpolarizability, which are given in Table 2 as well.

Triene Crystals Based on 3,5-Dimethyl-2-cyclohexen-1-one

Figure 1. As-grown acentric MH2 crystals obtained by the slow evaporation method in acetonitrile solution.

TABLE 4: Summary of Crystallographic Data for Acentric and Hydrated MH2 Crystals Acentric MH2

Hydrated MH2

100% C22H23N3 329.43 monoclinic Cc 12.960(3) 14.970(3) 9.5840(19) 90 90.30(3) 90 1859.4(6) 4

0% C88H92N12 (H2O) 1335.75 tetragonal P42(1)c 16.651(2) 16.651(2) 14.709(3) 90 90 90 4078.2(12) 2

powder SHGa formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg.) β (deg.) γ (deg.) V (Å3) Z

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Figure 2. Molecular structure of the MH2 chromophore in the acentric monoclinic Cc crystal structure as determined by X-ray diffraction experiment at 294 K. Hydrogen atoms are omitted for clarity. The cyclohexene ring is disordered with a statistical distribution of 50% carbon atoms C3 pointing up and 50% pointing down. X-ray measurements at 150 K reveal a disorder of C2 atoms, and eventually a slight disorder of C7 atoms within the thermal ellipsoids indicated in this figure.

a Powder SHG measured at a fundamental wavelength of 1.9 µm relative to that of DAST powder.

Although all 27 components of the hyperpolarizability tensor βijk can be evaluated by the FF method, by electric-field-induced second-harmonic generation (EFISH) experiments only the vector component along the dipole moment µg direction is measured. The vector part βz of the hyperpolarizability tensor βijk is calculated as16

βz ) βzzz + βxxz + βyyz

(1)

and is given in Table 2. For the TD-DFT calculation, the two-level model was used,17 in which the static first hyperpolarizability β0 is given by

β0 )

3∆µge(µge)2 2(Emax)2

Figure 3. (a) Crystal-packing diagram of MH2 chromophores in the acentric monoclinic Cc crystal structure projected along the lattice vector [10-1]. The solid and dotted vectors present the directions of the maximum first hyperpolarizability βmax and the dipole moment µ of two MH2 molecules as determined by finite-field calculations, respectively. (b) MH2 and MM1 molecules are superimposed with respect to the polar crystalline axis to point out the difference in the crystalline packing.

(2)

where µge is the transition dipole moment between the excited and the ground state, ∆µge is the dipole moment change, and Emax is the energy of the maximal charge transfer absorption.18 The results of TD-DFT calculation from the optimized (OPT) and experimental (EXP) molecular structures are given in Table 3. Results and Discussion New CLP Chromophores. The chemical structures of newly designed CLP chromophores are shown in Scheme 1b. “MH” stands for the methyl and hydrogen substituents on the non-πconjugated part in the cyclohexene ring, while “MM” and “HH” stand for two methyl and two hydrogen substituents, respectively. For introducing only one methyl group on the cyclohexene ring, 3,5-dimethyl-2-cyclohexen-1-one was used as a starting material for the synthesis of the MH series instead of 3,5,5-trimethyl-2-cyclohexen-1-one for the MM series and 3methyl-2-cyclohexen-1-one for the HH series. As electron donor groups, we chose dimethylamino, pyrrolidino, and pyrrolinol groups because previously studied CLP chromophores with

these groups, typically MM and HH series, exhibit acentric crystal structures.7–9 Microscopic and Macroscopic Nonlinearities. To determine molecular nonlinearities of the newly designed CLP chromophores, quantum chemical calculations were performed. The molecular geometries were fully optimized with no restrictions by using the hybrid functional B3LYP15 with the 6-311+G(d) basis set. The molecular structures were analyzed by FF and TD-DFT methods. The resulting hyperpolarizabilities are given in Tables 2 and 3. The hyperpolarizability βz of the MM1 chromophore was previously determined by EFISH measurements at the wavelength 1907 nm, giving a large value of 1100 × 10-40 m4 V-1 (260 × 10-30 esu).7 The newly synthesized MH2 chromophore exhibits slightly higher molecular nonlinearity than MM1 according to quantum-chemical calculations as listed in Table 2 due to different donor groups. Moreover, the wavelength of maximum absorption λmax of the MH chromophores in solution is similar to analogous MM and HH chromophores having same electron donor groups as listed in Table 1. Therefore, according to the nonlinearity-transparency tradeoff2 and the results of quantum chemical calculations, the large microscopic nonlinearity of CLP derivatives is not affected

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Figure 4. Schematic illustration of the intermolecular interactions in the acentric MH2 crystals. They form a three-dimensional network by C≡N · · · H-C hydrogen bonds, which are indicated by dotted lines. Figure 6. Crystal-packing diagrams of the centrosymmetric hydrated MH2 crystals having tetragonal space-group symmetry P42(1)c projected along the crystallographic c-axis (a) and b-axis (b).

Figure 5. Molecular structures of the MH2 chromophore in the centrosymmetric hydrated MH2 crystals having tetragonal space-group symmetry P42(1)c. Hydrogen atoms are omitted for clarity. The upper and lower envelope conformers are related with the conformation of the cyclohexene ring and form an ordered structure with four upper and four lower envelope conformers in the unit cell (see Figure 6).

by the chemical modification on the non-π-conjugated part in the cyclohexene ring. For screening the macroscopic second-order optical nonlinearity of newly developed CLP chromophores, the Kurtz and Perry powder test19 was performed at a fundamental wavelength of 1.9 µm. We compared the second harmonic generation (SHG) signal with the signal generated by the MM1 crystalline powder, which possesses 2 orders of magnitude greater SHG efficiency than urea.7 Among three MH crystals, MH2 exhibits very strong SHG signal that is about seven times larger than that of MM1 (see Table 1). The powder SHG efficiency of MH2 is similar as that of DAST (N,N-dimethylamino-N′-methylstilbazolium p-toluenesulfonate) crystals,20 which is the state-of-the-art organic nonlinear optical crystal salt with the second-order susceptibility of χ(2)(–2ω,ω,ω) ) 2020 pm/V at 1.3 µm and 420 pm/V at 1.9 µm.21 Thermal Properties. The thermal properties of CLP crystals were investigated by TGA and DSC under nitrogen atmosphere (10 °C/min scan rate). The thermal weight-loss temperature Ti was estimated as the temperature at the intercept of the leading edge of the weight loss by the baseline of the TGA scans. The CLP chromophores exhibit very high weight-loss temperatures Ti of at least over 290 °C, which involves sublimation and/or decomposition. Like the MM and HH derivatives, they also

exhibit a large difference between the thermal weight-loss temperature Ti and the melting temperature Tm (peak position in DSC scan). The thermal stability of acentric MH2 crystals (Ti ) 290 °C, Tm ) 190 °C) is much higher than that of DAST crystals (Ti ) Tm ) 256 °C), which is an advantage for applying a melt-based crystal growth that cannot be applied to DAST. Moreover, bulk single crystals of MH2 were easily grown by a slow evaporation technique in acetonitrile solution. Typical asgrown MH2 crystals of about 1 cm in length are shown in Figure 1. Therefore, MH2 crystals are very attractive for nonlinear optical applications due to their large macroscopic nonlinearity, the large crystal size, and their high thermal stability. Single Crystal Structures. To understand the supramolecular organization of the MH2 molecules, the single crystal structures of the noncentrosymmetric MH2 crystals and the centrosymmetric hydrated MH2 crystals were determined by X-ray diffraction analysis. The noncentrosymmetric MH2 crystals have monoclinic space-group symmetry Cc, while the hydrated MH2 crystals have centrosymmetric tetragonal space-group symmetry P42(1)c, which is a rare space group for organic crystals22 (see Table 4). The molecular structures and crystal packing diagrams of the MH2 chromophores are shown in Figures 2, 3, and 4 for the noncentrosymmetric phase and in Figures 5, 6, and 7 for the centrosymmetric hydrated phase. The hydrated phase undergoes an irreversible phase transition9 to the noncentrosymmetric monoclinic Cc crystal structure by heat treatment that eliminates water. This is confirmed by powder X-ray diffraction patterns and DSC curves before and after heat treatment (see Figure 8). Cyclohexene has usually a half-chair conformation in which four carbon atoms are coplanar but one carbon is above and one next to it below the plane.23 However, in the centrosymmetric MH2 phase the cyclohexene ring has upper or lower envelope conformation in which five carbon atoms are coplanar and one carbon is above or below the plane (see Figure 5). In acentric MH2 crystals, the cyclohexene ring has disordered structure (see Figure 2) in contrast to the ordered structure of

Triene Crystals Based on 3,5-Dimethyl-2-cyclohexen-1-one

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Figure 7. Schematic illustration of the intermolecular interactions of the hydrated MH2 crystals in the lower conformer layers as shown in Figure 6. Both lower and upper conformer layers exhibit same intermolecular interactions. The hydrogen bonds of C≡N · · · H-O (water) and C-H · · · O-H (water) are indicated by dotted and solid lines, respectively.

Figure 8. The irreversible phase transition of centrosymmetric hydrated MH2 crystals to the noncentrosymmetric monoclinic Cc crystal structure at the phase transition temperature Ttr (100 ( 2 °C): (a) DSC (10 °C/min scan rate) and (b) powder X-ray diffraction patterns before and after dehydration by the heat treatment at 110 °C for 10 min. They are compared with untreated acentric crystals.

the cyclohexene ring in centrosymmetric hydrated MH2 crystals (see Figures 5 and 6). Despite of disordered C3 carbons in acentric MH2 crystals, the substituent methyl group (labeled C7) connected with C3 has a nearly similar position equatorially as shown in Figure 2. The C7 methyl group is an important hydrogen bond donor site in the crystalline structure (see Figure

4). Therefore, the disordered structure of the cyclohexene ring does not disturb the crystal growth, leading to a good crystalline quality of the acentric MH2 crystals. Like in CLP crystals studied previously, the CN groups are hydrogen bond acceptor sites. The main supramolecular interactions of the acentric MH2 crystals are weak hydrogen bonds of C≡N · · · H-C with distances of 2.67∼3.34 Å as illustrated in Figure 4. Microscopic and Macroscopic Nonlinearities in the Crystallographic System. The molecular structure of MH2 chromospheres in the monoclinic Cc crystal structure is shown in Figure 2. The π-conjugated bridge of MH2 molecules in the acentric crystal structure is nearly planar and similar to optimized molecules in the gas phase as determined by quantum chemical calculations, which results in efficient π-electron delocalization. Therefore, the optimized molecules, calculated by the Gaussian 03 program, and EXP molecules, determined by the X-ray diffraction analysis, show similar microscopic molecular nonlinearities, as determined by the quantum chemical calculations (see Table 2). One interesting feature of the acentric MH2 crystals are the directions of the maximum first hyperpolarizability βmax and the dipole moment µ. Because of the strong tendency of dipole-dipole aggregation of the molecules with a relatively high dipole moment,24 the dipole moments µ of the MH2 molecular pair are almost antiparallel in the crystalline state as shown in Figure 3a. However, because the angle between the directions of the maximum first hyperpolarizability βmax and the dipole moment µ is about 22° (see Table 2), the direction of the maximum first hyperpolarizability βmax is not antiparallel and MH2 crystals therefore exhibit a large macroscopic secondorder nonlinearity. Designing molecules with different directions of the maximum first-order hyperpolarizability βmax and the dipole moment µ may therefore prove as a useful technique to achieve a desired arrangement of highly polar molecules in the crystalline state resulting in a large macroscopic nonlinearity. The reason for the about seven times higher SHG efficiency of the MH2 crystals compared to the MM1 crystals can be related to the ordering of the chromophores in the crystalline lattice. The relation between the macroscopic χ(2) coefficients

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and the microscopic nonlinearities β can be estimated using the oriented-gas model25 giving, for example, for second-harmonic generation (2) eff χijk (-2ω, ω, ω) ) N fi2ωfjωfkω βijk (-2ω, ω, ω),

(3)

where N is the number of molecules per unit volume. In this model, the intermolecular interactions are neglected except for ω eff the local field corrections fi . The effective βijk coefficients can be calculated from the hyperpolarizability tensor components βmnp as eff βijk )

1 n(g)

n(g) 3

∑ ∑ cos(θims)

s cos(θjn )cos(θskp) βmnp

(4)

s mnp

where n(g) is the number of equivalent positions in the unit cell, s denotes a site in the unit cell, and θsim is the angle between the Cartesian axis i and the molecular axis m. As shown in Figure 3b, the long axis of the MH2 molecules, which is approximately along the direction of βmax, makes a smaller angle with the polar axis than for MM1. The maximal component of eff (2) the effective hyperpolarizability tensor βijk ∝ χijk is by a factor of about 2.5 larger in MH2 compared to MM1 (see the Supporting Information). The orientation of the chromophores in case of MH2 is also close to the optimal orientation for phaseeff matching.25 Therefore, larger components of βijk and possibility for phase matching lead to a considerably higher SHG efficiency of MH2 crystals. Conclusions We investigated experimentally and theoretically new organic nonlinear optical CLP crystals based on 3,5-dimethyl-2-cyclohexen-1-one. Acentric single MH2 crystals of large sizes with a maximal side length of up to 1 cm exhibit large macroscopic nonlinearity at 1.9 µm that is similar as of the state-of-the-art DAST crystals. MH2 crystals are therefore very interesting for nonlinear optical applications. Furthermore, the thermal stability of MH2 crystals (with thermal weight-loss temperature of Ti ) 290 °C) is much higher than that of DAST crystals (Ti ) 256 °C), which is an advantage for applying a melt-based crystal growth that cannot be applied for DAST. Acknowledgment. This work has been partially supported by the Swiss National Science Foundation. The authors highly appreciate the pioneering role of Larry Dalton in the development and understanding of electro-optical polymers with extremely large optical nonlinearities. Supporting Information Available: Details of quantum chemical calculations and microscopic and macroscopic nonlinearities in the crystallographic system. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dalton, L. AdV. Polym. Sci. 2002, 158, 1. (b) Shi, Y. Q.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 119. (c) Pereverzev, Y. V.; Gunnerson, K. N.; Prezhdo, O. V.; Sullivan, P. A.; Liao, Y.; Olbricht, B. C.; Akelaitis, A. J. P.; Jen, A. K.-Y.; Dalton, L. R. J. Phys. Chem. C 2008, 112, 4355. (d) Dalton, L. R.; Sullivan, P. A.; Bale, D. H.; Bricht, B. C. Solid-State Electron. 2007, 51, 1263. (2) (a) Bosshard, Ch.; Bo¨sch, M.; Liakatas, I.; J¨ager, M.; Gu¨nter, P. In Nonlinear Optical Effects and Materials; Gu¨nter, P., Ed.; Springer-Verlag: Berlin, 2000; Chapter 3. (b) Bosshard, Ch.; Sutter, K.; Preˆtre, Ph.; Hulliger, J.; Flo¨rsheimer, M.; Kaatz, P.; Gu¨nter, P. Organic Nonlinear Optical Materials; AdVances in Nonlinear Optics; Gordon and Breach Science Publishers: Langhorne, PA, 1995; Vol. 1. (3) (a) Chemla, D. S.; Zyss, J. In Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press: New York, 1987; Vol. 1. (b) Marder,

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