Noncentrosymmetric Crystals with Marked Nonlinear Optical

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J. Phys. Chem. A 2010, 114, 5099–5103

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Noncentrosymmetric Crystals with Marked Nonlinear Optical Properties B. B. Ivanova* and M. Spiteller* Institut fu¨r Umweltforschung, UniVersita¨t Dortmund, Otto-Hahn-Strasse 6, 44221 Dortmund, Germany ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: March 10, 2010

The crystal structures and optical properties of the novel (1R,2R)-(-)-1,2-diammoniumocyclohexane squarate trihydrate (1) and bis(hydrogen squarate) (2) were elucidated and reported. Physical methods, single crystal X-ray diffraction, UV-vis-NIR, polarized IR spectroscopy of oriented colloids, mass spectrometry, and thermal methods were used. Quantum chemical DFT calculations were performed with a view to predict the optical and NLO properties of the systems (1) and (2). Both compounds are characterized with the noncentrosymmetric orthorhombic space groups C2221 (1) and P212121 (2), indicating nonlinear optical properties in the bulk. These organic salts are thermal stable up to 400 °C and transparent over the range from 270 to 1100 nm. Introduction Organic crystals are of great interest for the nonlinear optical (NLO) technologies in line with the tendency to replace the classical electronics with organic materials.1-6 The design and synthesis of new organic crystals with NLO properties both in solution and in bulk is the first step. The NLO properties in the bulk mainly depend on the symmetry of the organic crystals. If the crystals are noncentrosymmetric, then they show NLO properties of even and odd orders. That is why producing such crystals is the main goal in the design of new materials. The classical NLO organic materials such as p-nitroaniline and its derivatives are limited in their applicability because of their relatively poor efficiency transparency trade-off. It would therefore be desirable to develop new organic materials with improved NLO characteristics. The design of novel organic compounds with NLO properties and better efficiency-transparency trade off is of current interest. The strategy adopted, of fine tuning self assemblies consisting of polarizable cations in general and amino acids in particular and anions with organizational features, as for example, squaric acid, appeared particularly attractive. Squaric acid is a very strong dibasic acid.7,8 The squarate dianion, is a rigid and fully delocalized planar aromatic dianion capable of being a powerful acceptor of hydrogen bonds. By transfer of one proton, hydrogensquarate can be both a donor or an acceptor and could be ideal for controlling the organization and tuning its proton transfer capabilities.6,9,10 The salts of squaric acid with the chiral aliphatic counterions are especially attractive because of the expected transparent properties in the UV-vis-NIR region. Therefore, we report two novel organic salts, that is, (1R,2R)-(-)-1,2diammoniumocyclohexane squarate trihydrate (1) and bis(hydrogensquarate) (2), depicted in Scheme 1. Their crystal structures and physical properties are studied by means of methods such as single crystal X-ray diffraction, UV-vis-NIR, polarized IR-spectroscopy of oriented colloids, mass spectrometry, and thermal methods. * Corresponding authors. (B.B.I.) Tel: +49231 755 6970. E-mail: [email protected]. (M.S.) Tel: +49231 755 4080. E-mail: [email protected].

SCHEME 1: Chemical Diagram of (1) and (2) As Well As Structural Motifs (VII) and (Ia) of HSq- and Sq2Ionsa

a Squaric acid fragments are represented as a square and the hydroxyl positions by dots.8

Experimental Section Synthesis. Compound (1) was obtained by mixing of equimolar amount of squaric acid (0.114 g, Sigma) and 0.114 g (1R,2R)-(-)-1,2-diaminocyclohexane (Sigma-Aldrich) in 20 mL of solvent mixture methanol/water 1:1 under stirring for 30 min at 50 °C. The resulting colorless crystals (Figure 2a) were filtered off, washed with CH3OH, and dried on P2O5 at 298 K. Yield 91%. Found: C, 42.60; H, 7.73; N, 9.90. Calcd for C10H22O7N2: C, 42.55; H, 7.86; N, 9.92%. The most intensive signal in the mass spectra of (1) is the peak at m/z 57.51, corresponding to the singly charged [C6H16N2]2+ ion with an m/z value of 116.21. The thermal methods within the range of 300-500 K reveal a weight loss of 19.14% and an enthalpy effect of 30.32 kcal/mol at 137 °C, corresponding to solvent molecules being included in the crystal lattice of (1). Compound (2) is obtained in the same way as (1), by mixing an equimolar ratio of squaric acid and (1R,2R)-(-)-1,2-diaminocyclohexane (2:1). The resulting colorless crystals (Figure 2b) were filtered off, washed with CH3OH, and dried on P2O5 at 298 K. Yield 95%. Found: C, 49.18; H, 5.15; N, 8.15. Calcd for C14H18O8N2: C, 49.12; H, 5.30; N, 8.18%. The thermal methods within the range of 300-500K indicate absence of solvent of crystallization. Materials and Methods. Single Crystal X-ray Diffraction. The X-ray diffraction intensities were measured on an Bruker Smart X2S diffractometer using microsource Mo KR radiation and employing the ω scan mode. The data were corrected for Lorentz and Polarization effects. The structures in Figures 1 and 2 are presented by ORTEP 3.11 An absorption

10.1021/jp1002758  2010 American Chemical Society Published on Web 03/25/2010

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Figure 1. ORTEP diagrams of (a) (1) and (b) (2), respectively. Displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are drawn at arbitrary size.

Figure 2. Unit cell content and hydrogen bonding pattern in the structures (a) (1) and (b) (2). Photograph of the crystals of (1) and (2).

correction was based on multiple scanned reflections.12 The crystal structures were solved by direct methods using SHELXS-97.12,13 The crystal structures were refined by fullmatrix least-squares refinement against F2.14 Anisotropic displacement parameters were introduced for all nonhydrogen atoms. The hydrogen atoms attached to carbon were placed at calculated positions and refined, allowing them to ride on the parent carbon atom. The hydrogen atoms bound to nitrogen and the oxygen were constrained to the positions, which were confirmed from the difference map and refined with the appropriate riding model, with the exception of the amino and water hydrogen atoms. The experimental data are summarized in Table 1.

ConWentional and Linear-Polarized IR Spectroscopy. The IR-spectra were measured on a Thermo Nicolet 6700 FTIR spectrometer (4000-400 cm-1, 2 cm-1 resolution, 200 scans) equipped with a Specac wire-grid polarizer. Nonpolarized solid-state IR spectra were recorded using the KBr disk technique. The oriented samples were obtained as a suspension in a nematic liquid crystal (ZLI 1695, Merck).8 Electronic UV-Wis-NIS Spectra. The UV-vis-NIR spectra were recorded on an Evolution 300 spectrometer (Thermo Scientific) within the 190-1100 nm range. Mass Spectrometry. The analyses of the samples by ESI mass spectrometry were performed with a Thermo Finnigan surveyor LC-Pump. Compounds were separated on a Luna C18 column

Noncentrosymmetric Crystals

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TABLE 1: Crystallographic and Refinement Data for (1) and (2), Respectively empirical formula Mr crystal size crystal system space group T [K] λ [Å] a [Å] b [Å] c [Å] R ) β ) γ [deg] V [Å] Z µ [mm-1] Fcalcd [mg m-3] 2θ [deg] reflections collected unique reflections goodness-of-fit on F2 R1 [I > 2σ (I)]

(1)

(2)

C10H16N2O7 276.25 0.21 × 0.17 × 0.14 orthorhombic C2221 300(2) 0.71073 8.0606(7) 14.6983(16) 23.782(2) 90 2817.6(5) 8 0.111 1.303 25.05 8950 2220 1.309 0.0477

C14H18N2O8 342.30 0.42 × 0.32 × 0.22 orthorhombic P212121 300(2) 0.71073 7.5616(12) 13.630(3) 14.338(3) 90 1477.7(4) 4 0.128 1.539 25.15 14185 2194 0.960 0.0798

(150 × 2 mm, 4 µm particle size) from Phenomenex (Torrance, CA), using a gradient program. Elemental Analysis. The elemental analysis was carried out according to the standard procedures for C and H (as CO2 and H2O) and N (by the Dumas method). Thermal Methods. The thermogravimetric study was carried out using a Perkin-Elmer TGS2 instrument. The calorimetric measurements were performed on a DSC-2C Perkin-Elmer apparatus under argon. Computational Details. Quantum chemical calculations are performed with Gaussian 98 and Dalton 2.0 program packages.15,16 The result files are visualized by means of the GausView03.17 The geometry of isolated dication was optimized at density functional theory (DFT) using the 6-31++G** basis set. The DFT method employed is B3LYP, combing Backe’s threeparameter nonlocal exchange function with the correlation function of Lee, Yang, and Parr. Molecular geometry was fully optimized by the force gradient method using Bernys’ algorithm. For every structure, the stationary points found on the molecule potential energy hypersurfaces were characterized using standard analytical harmonic vibrational analysis. The vibrational spectrum was modificated using the empirical scaling factors 0.9614. The UV spectra in ethanol solution of (1) and (2), using the experimental geometries by the crystallographic experiment, are obtained by CIS/6-31++G** and TDDFT calculations at same basis set.18 In these cases, the second order of Møller-Plesset perturbation theory (MP2) and the polarized SBK basis set were used.20 The effect of geometry on the visible-UV energies was fairly small. In addition, polarized SBK MP2 is an efficient, medium-level correlated method that normally gives very accurate geometries at modest computational cost. For the calculation of the visible-UV energies, we used both all-electron 6-311+G(2d,p) bases21 and the polarized SBK bases and employed both HF- and DFT-type methods. In a few cases, we also have also utilized the large “correlation consistent” basis sets from Dunning’s group22 at the aug-cc-pVDZ and aug-ccpVTZ (augmented correlation-consistent polarized valence double and triple-ζ levels). In evaluating excitation energies, we used either the configuration interaction singles (CIS) method,23 the time-dependent Hartree-Fock method (TD HF),24 the CIS(D) method, or the time-dependent density functional method (TD DFT) method.25-27 The DFT studies have been

done mainly with the hybrid B3LYP potential.28 Analyses of these different methods for calculating excitation energies are given. Basically, CIS describes the excited state wave function at a level comparable to Hartree-Fock, using single excitations from the HF determinant. The TD HF method (also called the random phase approximation, RPA) includes some double excitations, giving a slightly correlated description of both ground and excited states, The CIS(D) method also incorporates some double-substitution corrections to the CIS energies in a size-consistent way. TD DFT includes additional electron correlation through the exchange correlation potential. To describe the species in aqueous solution, we use both an explicit supermolecule or microhydration approach in which several water molecules are coordinated to the solute at the optimized geometry of the supermolecule and a polarizable continuum approach. The geometries of all supermolecules in the present study were obtained by a similar approach but utilizing the polarized SBK basis set at the MP2 level. The geometries have been verified to be local energy minima by frequency analysis but are not necessarily the global minima. These species are in fact just simple approximations of the real hydrated species. We have not systematically studied the effect of varying the number of water molecules in the supermolecule, although we have employed a very large supermolecule. In general, better results are obtained for solutes in aqueous environments if the solute is immersed in a polarizable continuum. Most recent application is also a so-called “mixed” approach, employing both microhydration and a polarizable continuum. However, in many of the studies done so far with this approach, the number of water molecules used has been very small, usually only one or two coordinating to the chromophoric group of the molecule.29 We have tried to approach the problem in a fairly simple yet systematic manner for the present species, using several waters of hydration in the microhydrated species and then immersing this species in a polarizable continuum. We have primarily utilized the COSMO or CPCM version30 of the polarizable continuum model, although we have also tested the computationally less-stable isodensity polarized continuum model.31 The COSMO solvation approach has been applied both to bare anionic solutes and to the microhydrated species. For the very largest species considered, we have employed the ONIOM method,32 in which different parts of the supermolecule can be treated at different levels of accuracy, using different basis sets or even different quantum methods. Results and Discussion The compounds (1) and (2), that is, (1R,2R)-(-)-1,2-diammoniumocyclohexane squarate trihydrate and bis(hydrogensquarate), crystallize in the noncentrosymmetric orthrombic space groups C2221 (1) and P212121 (2), respectively. That directly indicates the nonlinear optical properties in the bulk. ORTEP 311 diagrams are given in Figure 1a,b. The Sq2- dianion in (1) is the VII structural motif (Scheme 1) of isolated squarate dianion.8 The HSq- anions form a new type of R chains, similar to the classical Ia-type (Scheme 1), via strong hydrogen bond interactions. In contrast with the self-assembly substructures of Ia, where the anions are coplanar oriented in the frame of the corresponding R chains, in (2), the planes of the HSq- by pairs are disposed at an angle of 55.4(9)°. The strong intermolecular hydrogen bonds of OH · · · O type are observed in (2) with lengths of 2.448 and 2.451 Å, respectively. Cations and anions in the structures (1) and (2) as well as the solvent molecules in the first case form 3D networks by means of the following

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Figure 3. IR spectra of (1) and (2) in the solid state, respectively.

interactions: (1) NH3+ · · · O (2.784, 2.828, 2.713, 2.815 Å), NH3+ · · · OH2 (2.823 Å), HOH · · · O (2.722, 2.860 Å), HOH · · · OH2 (2.790 Å) (Figure 2a) and (2) NH3+ · · · O (2.987, 2.774, 2.818, 2.795, 2.784, 2.742), OH · · · O (2.451, 2.2448 Å) (Figure 2b), respectively. The solid-state IR spectra of (1) and (2) in nematic host are depicted in Figure 3. Like other derivatives of H2Sq8, the corresponding IR spectra are characterized with the marked Fermi-Davydov (FD), Fermi resonance (FR), Davydov’s splitting effect (DS), and the “transparent window”, typical for Evan’s hole (EH) effect, within the whole 1700-400 cm-1 IR region. Compound (1) exhibits a relatively “pure” IR spectroscopic pattern with the highest frequency IR band at 3224 cm-1 of the νasNH3+ stretching vibration. In both cases, the corresponding νasNH3+ and νsNH3+ are observed as broad multicomponent band within 3200-2400 cm-1 as a result of the FD effect, which is in contrast with the corresponding theoretical IR bands (Figure 4).8 The calculated four frequencies of νasNH3+ given in Scheme 2 (the last one is degenerated) are observed as multicomponent bands in the solid-state IR-spectra, where the degeneration is perturbed as a result of the different type of intermolecular interactions in the crystals of (1) and (2), respectively. The corresponding maxima are with different polarization, according to the polarized IR measurements. The distortion of 2the D4h symmetry of the Sq is typical for (1). In (2), the C2V* symmetry of the HSq- leads to an observation of corresponding νsCdO and νCdC bands at 1808 and 1660 cm-1, respectively. The electronic transmission spectra of compounds (1) and (2) revealed transparency within the region 200-1100 nm (Figure 5). The UV bands within 200-260 nm correspond to perturbed n f π* transition of CdO with εν of ∼1000 L mol-1 cm-1. The obtained differences within 5-17 nm between the theoretical and experimental electronic spectra of (1) and (2) (Table 2) illustrate the successful applicability of the theoretical

Figure 4. Theoretical IR spectrum of the dication of (1R,2R)-(-)1,2-diaminocyclohexane.

SCHEME 2: Visualization of the Calculated Transition Moments (Brown) of the νasNH3+ Stretching Vibrations in the Dication of the (1R,2R)-(-)-1,2-Diaminocyclohexanea

a

Values are given in inverse centimeters.

polarizable continuum model approach. The corresponding HOMO-LUMO MOs gaps are depicted in Scheme 3, showing the energy differences between the ground and excited states.

Noncentrosymmetric Crystals

J. Phys. Chem. A, Vol. 114, No. 15, 2010 5103 Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (Fax: +44 1223 336 033; E-mail: [email protected]; http:// www.ccdc.cam.ac.uk). Supporting Information Available: Structural motifs of H2Sq, and its HSq- and Sq2- ions and HOMO and LUMO MOs gaps of (1) and (2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 5. Transmission UV-vis-NIR spectra of (1) and (2).

TABLE 2: Experimental and Theoretical Electronic Spectra of (1) and (2) Using the Theoretical Polarizable Continuum Model Utilizing the Polarized SBK Basis Set at the MP2 Level λ [nm] (1) theor (f)

a

190 (0.2341) 200 (0.3356)

(2) exptl

theor

exptl

205 210

190 (0.2355) 200 (0.3461) 220 (0.0871)

207 212 228

a f-factor, given information for the possibility of the electron transition.

SCHEME 3: HOMO and LUMO MOs Gaps of (1) and (2), Respectively, Obtained by TD-DFT Calculations at 6-31++G** Basis Set

Conclusions The novel derivatives of squaric acid with (1R,2R)-(-)-1,2diaminocyclohexane, that is, squarate trihydrate (1) and bis(hydrogensquarate) (2) salts, which were synthesized, isolated, and structurally characterized by single crystal X-ray diffraction, are crystallized in the noncentrosymmetric orthorhombic space groups C2221 (1) and P212121 (2), indicating NLO properties in the bulk. The physical properties are studied by UV-vis-NIR and polarized IR spectroscopy. These organic salts are thermal stable up to 400 °C and transparent in the region within 270-1100 nm. The theoretical calculations of the optical properties of the studied compounds additionally indicated their possible utility as new organic materials in optical and NLO technologies. Acknowledgment. B.I. wishes to thank the Alexander von Humboldt Foundation for the Fellowship and for the donation Bruker Smart X2S diffractometer. B.I. and M.S. thank the DAAD for a grant within the priority program “Stability Pact South-Eastern Europe”. M.S. wishes to thank the DFG for grand SP 255/21-1. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 761131 and 761132. Copies of this information may be obtained from the

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