Laser-Induced Crystal Growth and Morphology Control of Benzopinacol Produced from Benzophenone in Ethanol/Water Mixed Solution Tetsuo Okutsu,* Kazuhiko Nakamura, Hiroshi Haneda, and Hiroshi Hiratsuka
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 1 113-115
Department of Chemistry, Gunma University, Kiryu Gunma 376-8515, Japan Received March 31, 2003;
Revised Manuscript Received July 21, 2003
ABSTRACT: Laser-induced precipitation and crystal growth were observed by an irradiation of benzophenone in an ethanol and water mixed solvent. The crystal was composed of benzopinacol produced from excited benzophenone through a benzophenone ketyl radical. Polyhedral, skeletal, and dendrite morphologies of benzopinacol were controlled by laser energy. This is the first example of a photochemical morphology control of a crystal. Introduction A growth form of crystal reflects a supersaturation in which the crystal grows. This results from anisotropic growth rates of surfaces.1,2 One of the most popular example is snow crystals. Many researchers have investigated the growth forms of snow crystals.3-5 The results were summarized as in the Kobayashi diagram.6 When the snow nucleus grows at low supersaturation, the hexagonal plate is formed. The dislocation mechanism is responsible for this morphology. Then it passes through an atmosphere at higher supersaturation, and tips of the hexagon dominantly grow because of a 2-D nucleation mechanism. At the highest supersaturation, six dendrites grow from the tip of the hexagonal plate. A labile factor such as the fluctuation of a vapor pressure around the branch is responsible for this morphology. We, here by a photochemical approach, demonstrate a precipitation and control of the morphology of the growth form observed in the snow crystal. There are a few reports on the precipitation of crystals by light. Garetz et al. reported nucleation from aqueous urea or aqueous glycine solutions when they are irradiated by a 1064 nm laser light.7,8 These phenomena take place photophysically; the Kerr effect is assumed to be responsible for these phenomena because neither urea nor glycine absorbs light at this wavelength. Recently, Garetz et al. also reported a control of a crystal structure by the nonphotochemical light-induced nucleation of aqueous glycine solutions.9 Photochemical light-induced nucleation in solution was reported by Tyndall10 and in the vapor phase was reported as laser snow.11 Precipitations by light were briefly summarized in the literature.12 Even though precipitation or condensation by light is reported, morphology control by light has not been reported. To study crystal morphology under the influence of light, the photochemistry of benzophenone in solution was selected. Benzophenone is the most representative organic aromatic carbonyl compound. Electronic natures of the excited states, relaxation dynamics from excited * Corresponding author. Tel: +81-277-30-1242. Fax: +81-277-301244. E-mail:
[email protected].
states, and reaction mechanisms from the excited states are well-elucidated. Benzophenone is known to produce benzopinacol upon irradiation. The precipitation of a benzopinacol crystal in organic solvent is reported to take place on a time scale of few days. In this paper, the supersaturation of the ethanol/water 50:50 (vol/vol) solution is controlled by the solubility submitted to various intensities of a pulsed laser. Experimental Procedures Benzophenone, benzopinacol, ethanol, and anthracene were purchased from Tokyo Kasei (Guaranteed Reagent). Acetoned6 (Wako Pure Chemical) was used as the solvent for NMR measurement. Benzophenone and benzopinacol were recrystallized from ethanol before use. A mixture of ethanol and deionized water was used as a solvent. The third harmonic of the Nd3+: YAG laser (Spectra Physics GCR-130, 30 ns pulse-1) at 355 nm guided through a quartz fiber was used as an excitation light source. The excitation laser energy was 0.8∼2.5 mJ cm-2 pulse-1. The repetition rate of the pulsed laser was 10 Hz through this experiment. The sample solution was dropped on a quartz flat cell with a depth of 0.2 mm and sealed by a quartz glass. Crystals were observed by a stereo microscope (Olympus SZX-9) and recorded by a digital camera (Nikon F-4500). A JEOL R-500FT NMR spectrometer was used for NMR measurement.
Results and Discussion Figure 1 shows micrographs of crystals obtained from saturated benzophenone in an ethanol/water (50:50 vol/ vol) solution by pulsed laser irradiation. There was no crystal in the solution before irradiation. Figure 1a-d shows the micrographs of the same area after 300, 600, 900, and 1200 shots of the laser irradiation, respectively. The photographs were taken at a pause between irradiation. An octahedral crystal appeared, as in Figure 1a, and grew during the irradiation. With further irradiation of 600 and 900 shots, several crystals also appeared and grew. These crystals did not grow without irradiation. Benzophenone is known to produce a benzophenone ketyl radical from its triplet state by a hydrogen abstraction from solvent. This benzophenone ketyl radical, then, produces benzopinacol (φ2CH(OH)CH(OH)φ2)
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Figure 1. Crystals of benzopinacol precipitated by pulsed laser irradiation at 300 (a), 600 (b), 900 (c), and 1200 (d) shots from saturated solution of benzophenone in ethanol/water (1:1 vol %) mixed solution.
Figure 2. Solubility of benzopinacol (0) and benzophenone (b) against the water content in ethanol.
through a dimerization reaction.13 The production quantum yield of benzopinacol can be roughly estimated to be 0.5. To confirm the components of the crystal, an NMR measurement was carried out for the sample that was filtered and washed from the ethanol/water mixed solvent. For the acetone-d6 solution of this crystal, chemical shifts due to the benzopinacol were observed; thus, the crystal is confirmed to be a photoproduct from excited benzophenone. The photochemistry of benzophenone in solution is well-studied among photochemists, but we seldom observe a precipitation and growth of the product crystal during the irradiation. The precipitation and growth mechanism is explained as follows. The crystal precipitation and growth were observed only in solvents that contain from 30 to 70% water in ethanol. This is because of the lower solubility of the benzopinacol than benzophenone. Figure 2 shows the solubility of benzopinacol and benzophenone against the water content in ethanol. The solubility of benzopinacol largely decreases with an increase in the water content. At 10% water content, precipitation did not take place. This is because the solubility of benzopinacol is high enough to prevent it
from precipitating. At 90% water content, precipitation also did not take place. This is because the solubility of the reactant benzophenone is too low to precipitate the benzopinacol crystals. The concentration of the product is estimated. As the absorbance of the sample cell is 0.02, 4.5% of the excitation photons are absorbed by the benzophenone solution. For example, since one shot of 1.3 mJ pulse-1 of 355 nm laser pulse includes 9 × 1013 photons, 2 × 1012 molecules of benzopinacol must be produced. The concentration of benzopinacol is determined to be 1 × 10-5 M in our experimental conditions. Since the solubility of benzopinacol is 2.6 × 10-4 M in 50:50 (ethanol/water, vol/vol), the solution becomes saturated after 260 shots of the laser pulse. Further irradiations make the solution supersaturated and therefore induce the precipitation of benzopinacol crystals. As benzopinacol does not absorb 355 nm light, the product crystal is not influenced by the excitation light. We, here, try to control morphology of the crystal by altering the supersaturation of the benzopinacol. The crystals shown in Figure 1 kept a distorted octahedral shape uniformity during growth. A dislocation mechanism at low supersaturation is responsible for this type of growth mechanism. The growth mechanism is expected to change because the supersaturation moves from the stable region to the unstable region when the laser intensity increased the laser fluence. Figure 3 shows the crystals obtained after 300 shots of irradiation with different laser fluence by changing the laser energy per pulse with a constant repetition rate. Figure 3a shows a crystal obtained at 1.3 mJ cm-2 pulse-1, which is the lowest laser fluence to produce benzopinacol crystals. This octahedral morphology indicates that the crystal grew at low supersaturation (dislocation mechanism). Figure 3b shows that in crystals obtained at 1.6
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ogy is skeletal grown at higher supersaturation in a stable region by a 2-D nucleation mechanism. Figure 3c shows the crystal obtained in 1.8 mJ cm-2 pulse-1. This crystal grew radiately with saw-tooth appearances on the branches. The morphology is dendrite grown at the highest supersaturation by a labile growth mechanism in an unstable region. These series of micrographs demonstrate that the morphology due to the growth form is able to be controlled by alternating the magnitude of the excitation energy. The control of the morphology in this system is similar to the morphology observed with snow flakes. In this paper, the relation between the supersaturation and the excitation laser fluence has not been investigated. To determine the concentration around the growing crystal, the interference method is generally used.14 But in this system, the concentration is too low to bring about a significant change of refractive index between the bulk and the mother liquor in the vicinity of the crystals. We are planning to determine the supersaturation by simultaneous measurements of transient absorption and detection of light scattering due to the nucleus. Acknowledgment. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (14540527). We thank Prof. Kiyotaka Sato at Hiroshima University for helpful discussions. References
Figure 3. Morphology of benzopinacol crystals obtained at 1.3 (a), 1.6 (b), and 1.8 mJ pulse-1.
mJ cm-2 pulse-1, the morphology results mainly from the dominant growth of the crystal tips. This morphol-
(1) Kuroda, T.; Irisawa, T.; Ookawa, A. J. Cryst. Growth 1977, 42, 41. (2) Kuroda, T.; Lacmann, R. J. Cryst. Growth 1982, 56, 189. (3) Nakaya, U.; Sato, I.; Sekido, Y. J. Fac. Sci. Hokkaido Univ., Ser. II 1938, 2, 1. (4) Kampe, H. J. A.; Weickmann, H. K.; Kelly, J. J. J. Metals 1951, 168, 8. (5) Mason, B. J.; Roy, Q. J. Meteorol. Sci. 1953, 79, 104. (6) Kobayashi, T. Philos. Mag. 1961, 6, 1363. (7) Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young, R. G.; Myerson, A. S. Phys. Rev. Lett. 1996, 77, 3475. (8) Zaccaro, J.; Matec, J.; Myerson, A. S.; Garetz, B. A. Cryst. Growth Des. 2001, 1, 5. (9) Garetz, B. A.; Matec, J.; Myerson, A. S. Phys. Rev. Lett. 2002, 89, 175501. (10) Tam, A.; Moe, G.; Happer, W. Phys. Rev. Lett. 1975, 35, 1630. (11) Tyndall, J. Philos. Mag. 1896, 37, 384. (12) Voss, D. Science 1996, 274, 1325. (13) Turro, N. J. Modern In Molecular Photochemistry; The Benjamin/Cummings Publishing: 1978; p 261. (14) Berg, W. G. Proc. R. Soc. (London) A 1938, 164, 79.
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