DOI: 10.1021/cg900882h
Crystal Growth and Morphology Control of OH1 Organic Electrooptic Crystals
2010, Vol. 10 1552–1558
Seong-Ji Kwon,*,† Mojca Jazbinsek,† O-Pil Kwon,†,‡ and Peter G€ unter† †
Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland, and Department of Molecular Science and Technology, Ajou University, 443-749 Suwon, Korea
‡
Received July 28, 2009; Revised Manuscript Received January 23, 2010
ABSTRACT: We report on the growth of large-size phenolic configurationally locked polyene OH1 (2-(3-(4-hydroxystyryl)-5,5dimethylcyclohex-2-enylidene)malononitrile) organic bulk crystals. OH1 has been recently identified as very promising for electrooptic and THz-wave applications, with several advantages compared to the benchmark ionic salts such as DAST (40 -dimethylaminoN-methyl-4-stilbazolium tosylate) and DSTMS (4-N,N-dimethylamino-40 -N0 -methyl-stilbazolium 2,4,6-trimethylbenzene sulfonate). We investigate the thermodynamic equilibrium conditions, the solubility, and the metastable zone of OH1/methanol system. The intermolecular hydrogen-bond interactions between the OH1 and methanol molecules impede the main supramolecular interactions between the OH1 molecules, which results in a strong delay in nucleation and provides a very large metastable-zone width. We furthermore studied the crystal morphology and the solvent effect by analyzing the relationship between the morphology and the molecular orientation at the crystal surface as well as the growth direction. The growth kinetics of specific OH1 crystal faces for different growth methods (slow evaporation or supercooling) has been studied. In the OH1/methanol system with a large metastable-zone width, we can grow large OH1 crystals and control their morphology. Thinner or thicker crystals of about 15 12 1 mm3 and 18 17 4 mm3, respectively, with good optical quality can be grown in roughly 2 weeks time.
Introduction Organic nonlinear optical (NLO) materials are interesting for electro-optic and frequency conversion applications due to their large nonlinear optical susceptibilities and ultrafast response times.1 For example, organic NLO single crystals are very promising for terahertz (THz) wave generation and detection with a broad bandwidth, high efficiency, and high signal-to-noise ratios.2-4 Their high potential for these applications encourages the development of novel organic NLO crystals and the optimization of their crystallization process. At the current stage, the organic salt DAST (40 -dimethylamino-N-methyl-4-stilbazolium tosylate)5 is the most widely used organic crystal for THz wave generation and detection3,4 because of its very large nonlinear (2) (λ/2, λ,λ) = 420 pm/V at λ = 1.9 optical susceptibility with χ111 μm wavelength and an electro-optic coefficient of r111 = 77 pm/ V at λ = 800 nm.6 This has motivated several research groups to investigate the growth properties to achieve high-quality and large-size crystals of DAST, which is still very challenging.7 With DAST, a high THz-wave generation efficiency by both optical rectification or difference frequency generation can be achieved in a wide frequency range due to the possibility of velocity-matching; however, the generation of THz waves is limited at around ∼1.1 THz due to the high THz absorption in this range. A new stilbazolium salt crystal DSTMS (4-N,Ndimethylamino-40 -N0 -methyl-stilbazolium 2,4,6-trimethylbenzene sulfonate) has been recently developed as a derivative of DAST, with similar second order susceptibilities and much better growth characteristics than DAST.8 The THz wave absorption is in DSTMS greatly reduced with respect to DAST; however, the efficiency of THz wave generation at around 1 THz is still limited, attributed to a transverse optical phonon of stilbazolium salts in this frequency range.8b *To whom correspondence should be addressed. E-mail: nlo@ phys.ethz.ch. pubs.acs.org/crystal
Published on Web 03/03/2010
The organic nonionic molecular crystal OH1 (2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene)malononitrile) is a novel nonlinear optical crystal with superior properties compared to those of stilbazolium salts DAST and DSTMS. The OH1 molecule and its crystal structure were first reported by Lemke and Kolev at al.,9 while its potential for the applications in electro-optics and in THz generation and detection has been only recently discovered.10,11 The OH1 molecules consist of a π-conjugated hexatriene bridge between a phenolic electron donor and a dicyanomethylidene electron acceptor. In contrast to strong Coulomb forces that present main supramolecular interactions in organic ionic salts such as DAST and DSTMS, OH1 crystals are based on hydrogenbond interactions.10 OH1 chromophores arrange in the crystalline state with orthorhombic structure (space group symmetry Pna21 and point group symmetry mm2). Because of the high symmetry of OH1 crystals, the dielectric axes coincide with the crystallographic axes, which is very favorable to the crystal preparation for optical applications. The OH1 crystals show an electro-optic figure of merit of n33r333 = 2070 ( 80 pm/V and 970 ( 100 pm/V at λ = 632.8 and 785 nm, respectively,10b which is among the highest in organic crystals.1 Bulk crystals of OH1 with side lengths of up to 1 cm were obtained by slow evaporation in methanol solution.10a It is also an advantage that OH1 crystals are not dissolved in water and that there is no hydrated form as, e.g., is the case with DAST. THz generation by optical rectification of 160 fs pulses in OH1 shows a continuous spectrum from 0.1 to 3 THz, whereas DAST has a strong absorption band and therefore shows a gap in the spectrum at 1.1 THz.10a,c For integrated electro-optic applications such as electro-optic modulators, single crystalline thin films in the range of 0.2-10 μm are required. We successfully obtained single crystalline thin films of OH1 grown directly on glass substrates with large area and high optical quality by the ELSSI method (evaporation-induced r 2010 American Chemical Society
Article
Crystal Growth & Design, Vol. 10, No. 4, 2010
1553
Table 1. Solubility in Different Solvents (g of OH1/100g of Solvent) solvent temperature 25 °C 40 °C
acetone
acetonitrile
ethanol
methanol
methylene chloride
25.9 41.8
4.32 5.3
2.34 2.5
2.15 3.4
1.4 1.9
local supersaturation with surface interactions),11a which were used to demonstrate high-efficiency electro-optic phase modulation in OH1 channel waveguides.11b However, despite its high potential for various applications, the growth process of bulk OH1 crystals has not yet been investigated in detail. For the reliable, optimized, and reproducible single crystal growth from solution, details on the solubility, nucleation, and their temperature variations are required. We, therefore, study the solubility of OH1 in different solvents (acetone, acetonitrile, ethanol, methanol, and methylene chloride), and for the selected solvent (methanol), we more precisely determined the solubility and metastable boundaries as a function of temperature. We describe the natural morphology of OH1 crystals considering the interactions between the methanol solvent and the crystal surface. By analyzing the influence of the methanol molecules on the morphology, we can control the growth kinetics and crystal morphology for different growth methods. Experimental Section Solubility Studies. OH1 crystalline powders were dissolved by stirring the powder in 100 g of the solvent at the respective temperature over 24 h. Then, the solutions were filtrated to remove the solutes that had not dissolved. The solvents were evaporated completely and dried. The weight of the remaining solute was measured to determine the solubility. Metastable-Zone Width. All of the solutions were prepared in 21 mL of methanol. The amount of the solute was estimated in accordance with the solubility curve. They were dissolved at different temperatures. The solutions were overheated by 5 °C and filtrated by a paper. We kept the solutions at their equilibrium temperature for 2 h. The solutions were then cooled from the equilibrium temperature until first visible nuclei were observed (the cooling rate was 2 °C/h). If the nucleation in the OH1/methanol solution did not appear until room temperature, we waited for 20 h considering the time taken for the attainment of critical nuclei. Afterward, the cooling continued. Crystal Growth. The saturated solutions were prepared in accordance with the solubility curve for a desired temperature. All of the saturated solutions were overheated and then filtrated by paper. The seeds were placed on the bottom of a glass container without or with a holder, on which the seeds were mounted by silicone glue. In the slow-evaporation method, the saturated solutions with a seed were kept at room temperature and an evaporation rate of 1-2 mL/ day. In the supercooling method, the saturated solutions were cooled with a linear cooling rate of 1 °C/day from 42 to 28 °C.
Results and Discussion Solubility and Phase Diagram. The solubility of OH1 in various solvents has been measured at two different temperatures of 25 and 40 °C, i.e., covering the temperature range commonly used for the solution growth of organic NLO crystals. The results are listed in Table 1. OH1 molecules are very well dissolved in acetone and also in acetonitrile, much better than in most of the other solvents. The solubility in various solvents reflects the intermolecular interactions between the solvent and solute molecules, including hydrogen bonds, dipole-dipole interactions, and van der Waals forces. The phenolic group of OH1 molecules
Figure 1. Solubility (9) and metastable-zone boundary (0) measured for OH1 in methanol. The solid line is according to eq 1, and the dotted line is to guide the eye. The shadow region is the metastable zone.
is likely to mainly contribute to the difference in solubility. The hydrogen on the phenolic group of OH1 can interact with the lone electron pair of the solvent molecules, i.e., ketonyl oxygen (OdC
