Laser-Induced Morphology Control and Epitaxy of Dipara-anthracene

photoproduct on the mother anthracene substrate was observed. The a and b axes of the (001) surface of DPA are parallel to the b and a axes of the mon...
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Laser-Induced Morphology Control and Epitaxy of Dipara-anthracene Produced from the Photochemical Reaction of Anthracene Tetsuo Okutsu,* Kouji Isomura, Nobutoshi Kakinuma, Hiroaki Horiuchi, Masafumi Unno, Hideyuki Matsumoto, and Hiroshi Hiratsuka

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 461-465

Department of Chemistry, Gunma University, Kiryu 375-8515, Japan Received June 8, 2004;

Revised Manuscript Received October 21, 2004

ABSTRACT: Laser-induced crystal growth and morphology control were observed by irradiation of anthracene in saturated acetonitrile solution. The crystal was composed of dipara-anthracene (DPA), which is a product of intermolecular [4+4] photocycloaddition of anthracene. Polyhedral, skeletal, and dendrite morphologies were controlled by laser fluence. In a suspension system with anthracene microcrystals, laser-induced epitaxy of the photoproduct on the mother anthracene substrate was observed. The a and b axes of the (001) surface of DPA are parallel to the b and a axes of the monoclinic anthracene (001) surface. EpiCalc calculations indicated that almost perfect lattice matching exists between the (1 × 2) supercell dimension at an angle 90°, which is in good agreement with the experimental results. Epitaxy was formed by 2-D nucleation of DPA, which was produced in the solution phase and was brought about by diffusion. Introduction It is well-known that the growth form of a crystal reflects the supersaturation in which the crystal grows.1,2 This results from the anisotropic growth rates of surfaces. At low supersaturation, polyhedral crystals grow due to a dislocation mechanism. At higher supersaturation, skeletal crystals grow due to a 2-D nucleation mechanism. At the highest supersaturation, dendrite crystals result from labile factors. We have, in a previous report, demonstrated laser-induced morphology control of benzopinacol by a photochemical technique.3 Supersaturation of benzopinacol, which is produced by a photochemical reaction from benzophenone in solution, was altered by changing the excitation laser energy; polyhedral, skeletal, and dendric morphology of benzopinacol were controlled by laser fluence per pulse. There have been several reports concerning lightinduced nucleation or crystal growth in the gas phase or liquid phase.4-7 Our report was the first example of photochemical morphology control of a crystal. There remains a question of whether this technique is available for other systems or not. In this paper, we show that laser-induced morphology control is applicable to other systems. For this purpose, the photochemistry of an aromatic hydrocarbon is selected. Excited-state aromatics, except for benzene, are known to take place through cycloaddition with ground-state molecules.8 Anthracene is one of the most representative organic aromatic compounds whose electronic nature of the excited states and relaxation dynamics from excited states are well elucidated, and it is known to produce dipara-dianthracene (DPA) through [4+4] intermolecular cycloaddition reaction.9,10 We also observed laser-induced epitaxy of DPA on anthracene mother crystals suspended in saturated anthracene solution. Epitaxial growth of DPA by direct * To whom correspondence should be addressed. Tel: +81-277-301242. Fax: +81-277-30-1242. E-mail: [email protected].

irradiation of anthracene mother crystals under He atmosphere was already reported.11,12 The epitaxy was formed by photodimerization on the surface of the anthracene crystal. On the other hand, in a suspension system, epitaxy is formed by 2-D nucleation of DPA produced in the solution phase brought about by diffusion on the surface of the substrate. Experimental Procedures Anthracene was purchased from Tokyo Kasei (Guaranteed Reagent) and used after recrystallization from ethanol. The third harmonic of an Nd3+:YAG laser (Spectra Physics GCR130, 30 ns pulse-1) at 355 nm guided through a quartz fiber was used as an excitation light. The excitation laser fluence per pulse was 0.06-4.8 mJ cm-2 pulse-1. The repetition rate of the pulsed laser was 10 Hz. 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). X-ray crystallography of the photoproduct was measured as follows. A colorless prism crystal of C28H20 having approximate dimensions of 0.40 × 0.40 × 0.20 mm was mounted on a glass fiber, and measured by a Rigaku RAXIS V++ imaging plate area detector with graphite monochromated Mo-KR radiation (λ ) 0.71070 Å) at -160 ( 1 °C. The data were corrected for Lorentz and polarization effects. A correction for secondary extinction was applied (coefficient ) 330.279999). We have examined several smaller crystals; however, the analysis with this crystal gave the best result. We understand an appropriate absorption correction may be needed, but in this case, the µx value is 0.078 × 0.4 ) 0.031, and thus no correction seems to be necessary. The structure was solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined isotropically. The final cycle of full-matrix least-squares refinement on F2 was based on 1852 observed reflections and 168 variable parameters and converged (largest parameter shift was 0.00 times its esd) with unweighted and weighted agreement factors of R1 ) ∑ ||Fo| - |Fc||/∑ |Fo| ) 0.058, wR2 ) [∑(w(Fo2 - Fc2)2)/∑w(Fo2)2]1/2 ) 0.064. CCDC 252358 contains the crystallographic data for this paper. These data can be obtained online free of charge (or from the Cambridge Crystal-

10.1021/cg049816s CCC: $30.25 © 2005 American Chemical Society Published on Web 12/04/2004

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Figure 1. Crystals of photoproduct grown in solution by pulsed laser irradiation at 1200 (a) and 1800 (b) shots from a saturated solution of anthracene in acetonitrile solution obtained at a 0.8 mJ cm-2 pulse-1. lographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or [email protected]).

Results and Discussion Laser-Induced Crystal Growth and Morphology Control. Figure 1 shows a micrograph of a crystal obtained from saturated anthracene in acetonitrile solution taken after 1200 (a) and 1800 (b) shots of pulsed laser irradiation at 0.8 mJ cm-2 pulse-1. There were no crystals in solution before irradiation. A hexagonal crystal appeared as shown in Figure 1a and it grew with the irradiation. Thus, laser-induced crystal growth was observed in this system. To confirm the structure of the crystal, XRD measurements were carried out. The crystal was grown to ∼1 mm by laser irradiation with feeding anthracenesaturated solution, which was measured by X-ray diffraction. Figure 2 shows the molecular structure of the crystal. The crystal is DPA. Cell constants and an orientation matrix for data collection corresponded to a primitive orthorhombic cell with dimensions: a ) 7.944(1) Å, b ) 11.812(2) Å, c ) 18.376(3) Å, V ) 1724.3(5) Å3. For Z ) 4 and FW ) 356.47, the calculated density is 1.37 g/cm3. The systematic absences of 0kl: k * 2n, h0l: l * 2n, hk0: h * 2n uniquely determine the space group to be Pbca (No. 61). The data were collected to a maximum 2θ value of 66.0°. A total of 260 oscillation images were collected. A sweep of data was done using φ oscillations from 25.0 to 155.0° in 0.5° steps. The exposure rate was 480.0 [s/°]. The detector swing angle

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Figure 2. Molecular drawing of DPA. Thermal ellipsoids are shown in 50% probability. Selected bond lengths (Å) and angles (°): C(1)-C(2) 1.369(3), C(1)-C(6) 1.374(3), C(1)-C(8*) 1.491(3), C(2)-C(3) 1.372(3), C(3)-C(4) 1.374(3), C(4)-C(5) 1.381(3), C(5)-C(6) 1.361(3), C(6)-C(7) 1.497(3), C(7)-C(8) 1.599(3), C(7)-C(14*) 1.488(3) C(8)-C(1*) 1.491(3), C(8)-C(9) 1.494(3), C(9)-C(10) 1.366(3), C(9)-C(14) 1.389(3), C(10)-C(11) 1.364(3), C(11)-C(12) 1.372(3), C(12)-C(13) 1.380(3), C(13)-C(14) 1.370(3), C(14)-C(7*) 1.488(3). C(6)-C(1)-C(2) 119.7(2), C(8*)C(1)-C(2) 122.5(2), C(1)-C(2)-C(3) 119.7(2), C(8*)-C(1)-C(6) 117.7(2), C(1)-C(6)-C(5) 120.8(2), C(1)-C(6)-C(7) 117.2(2), C(1)-C(8*)-C(7*) 111.6(2), C(2)-C(3)-C(4) 120.7(2), C(3)C(4)-C(5) 119.3(2), C(4)-C(5)-C(6) 119.9(2), C(5)-C(6)-C(7) 122.0(2), C(6)-C(7)-C(8) 111.9(2), C(6)-C(7)-C(14*) 108.3(2), C(14*)-C(7)-C(8) 111.7(2), C(7)-C(8)-C(1*) 111.6(2), C(7)C(8)-C(9) 110.6(2), C(9)-C(8)-C(1*) 107.9(2), C(8)-C(9)C(10)-123.5(2), C(8)-C(9)-C(14) 116.9(2), C(14)-C(9)-C(10) 119.6(2), C(9)-C(10)-C(11) 120.4(2), C(9)-C(14)-C(13) 119.9(2), C(9)-C(14)-C(7*) 117.5(2), C(10)-C(11)-C(12) 120.5(2), C(11)C(12)-C(13) 119.6(2), C(12)-C(13)-C(14) 120.0(2), C(13)C(14)-C(7*) 122.5(2), C(14)-C(7*)-C(8*) 111.7(2).

was 0.22°. The crystal-to-detector distance was 100.38 mm. Readout was performed in the 0.100 mm pixel mode. Of the 12608 reflections that were collected, 2374 were unique (Rint ) 0.058); equivalent reflections were merged. The linear absorption coefficient, µ, for Mo-KR radiation is 0.8 cm-1. An empirical absorption correction was applied, which resulted in transmission factors ranging from 0.73 to 1.00. The standard deviation of an observation of unit weight was 1.53. The weighting scheme was based on counting statistics. Plots of ∑w(|Fo| - |Fc|) versus |Fo|2, reflection order in data collection, sin θ/λ and various classes of indices showed no unusual trends. The maximum and minimum peaks on the final difference Fourier map corresponded to 0.35 and -0.38 e-/Å3, respectively. All calculations were performed using the crystal structure crystallographic software package. The crystallographic analysis of DPA was reported in 1966, and a similar result was obtained.

Laser-Induced Morphology Control and Epitaxy

Figure 3. Morphology of DPA crystals obtained at 1.4 (a), 2.4 (b), 3.0 (c), and 4.8 mJ cm-2 pulse-1 (d) after 1200 shots of the irradiation.

However, we measured at low temperatures (-160 °C), and thus the lattice constant and structural parameters were defined more precisely.13 We then tried to control the morphology of the crystal by altering the supersaturation of DPA. The crystal shown in Figure 1 maintained a distorted hexagonal shape during growth. A dislocation mechanism is known to be responsible for this type of growth mechanism at low supersaturation. The growth mechanism is expected to change because the supersaturation moves from the stable region to the unstable region when the laser fluence was increased. Figure 3 shows the crystals obtained after 1200 shots of irradiation with different laser fluence by changing the laser energy per pulse. Figure 3a shows a crystal obtained at a 1.4 mJ cm-2 pulse-1; the morphology results mainly from the dominant growth of the crystal tips. This morphology is skeletal growth at higher supersaturation in a stable region by a 2-D nucleation mechanism. Figure 3b,c shows the crystal obtained at a 2.4 and a 3.0 mJ cm-2 pulse-1, respectively. These crystals grew with a sawtooth appearance on one branch. The morphology of the crystal is dendrite grown at the highest supersaturation by a labile growth mechanism in an unstable region. At a 4.8 mJ cm-2 pulse-1, many crystals appeared as very small fractions. Morphology was not dendrite or skeletal. This is because the supersaturation decreases because the highest laser fluence produces large numbers of nuclei in solution. These series of micrographs demonstrate that the morphology due to the growth form is able to be

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controlled by altering the excitation fluence. The control of the morphology in this system is similar to the morphology control in a benzophenone/benzopinacol system. Laser-Induced Epitaxy. We observed laser-induced epitaxy of DPA on an anthracene mother substrate suspended in an anthracene-saturated solution. Figure 4 shows the photographs of crystals during the course of laser irradiation. Figure 4a is an anthracene mother microcrystal before irradiation. Figure 4b shows the photograph recorded after 200 shots of the irradiation at 0.06 mJ cm-2 pulse-1. Several daughter crystals appeared on the top plane, and the mother crystal became smaller. Figure 4c is the photograph taken after 700 shots of pulsed irradiation. The mother crystal completely disappeared. Five hundred more shots of irradiation did not change the daughter crystals. The daughter crystals in Figure 4b are epitaxy on the mother substrate, because all the edges of the daughter crystals are parallel with each other. The parallelogram daughter crystal is surrounded by angles at 68° and 112°, respectively. The diamond shape whose ratio of diagonals is 7.94/11.81 (ratio between the a axis and b axis of DPA lattice constants) is surrounded by angles at 68° and 112°. Thus, the top plane of daughter crystals is assigned as (001). Anthracene crystals were reported to belong to a monoclinic system whose a and b axes are parallel to the top plane.14 Lattice constants are a ) 8.56 Å and b ) 6.04 Å, respectively.15,16 Top plane of the mother crystal is reported to be (001). These axes are illustrated by arrows in Figure 4a for the mother crystal and Figure 4b for the daughter crystal, respectively. The a axis of the mother crystal is parallel to the b axis of the daughter crystal and vice versa. To estimate lattice matching between mother and daughter crystals, a program EpiCalc17 was utilized. This software is available as a free download from the Web.18 According to recent literature,19 explanation of this software is referred to briefly. EpiCalc calculations are based on a simple analytic function that measures the degree of geometric “fit” between two chemically different surfaces. The program rotates the organic overlayer (b1, b2, β) with respect to other substrates (a1, a2, R) through a series of azimuthal angles θ. The degree of epitaxy is given as a dimensionless value V/V0, which ranges from 0 to 1. Commensurate surfaces (V/V0 ) 0) are those in which a perfect geometric match exists between every overlayer and the substrate lattice position at a given angle. A subset of geometrically matched lattice positions exists for coincident surfaces (V/V0 ∼ 0.5) at a given angle. Incommensurate surfaces do not

Figure 4. Laser-induced liquid-phase epitaxy of DPA grown on anthracene mother substrate. Before irradiation (a), and after 200 (b) and 700 (c) shots of the irradiation at 60 µJ cm-2 pulse-1.

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Figure 5. (a) EpiCalc calculation results of lattice matching between anthracene (001) and DPA (001) surfaces. (b) Schematic of a molecular overlayer with lattice parameters of anthracene and DPA. (1 × 2) supercell dimension of anthracene matches well with a DPA unit cell.

match at any angle (V/V0 ) 1). Therefore, the smaller the value of V/V0, the better the geometric match. Calculated results of surface matching by EpiCalc between two (001) surfaces are shown in Figure 5a. V/V0 shows the minimum value 0.02 at an angle 90°, which indicates almost perfect matching exists when the a and b axes of daughter crystals are parallel to the b and a axes of the mother crystal. This calculated result supports well the experimental results. The calculated results also indicate a (1 × 2) supercell dimension of the mother (001) unitcell matches well with the DPA (001) surface. A schematic geometric overlayer with lattice parameters is illustrated in Figure 5b. We must take into account the second polymorphic form of anthracene (triclinic form).20 The lattice constants are a ) 8.342 Å; b ) 5.89 Å; c ) 11.282 Å; R ) 123.34°; β ) 96.70°; γ ) 85.91°, respectively. We also calculated the surface matching between (001), (010), and (100) surfaces of triclinic anthracene, and the (001) surface of DPA. Results of V/V0 ) 0.06 with (001), V/V0 ) 0.07 with (010), and V/V0 ) 0.14 with (100) surfaces are obtained. These values are considered to match well with the (001) surface of DPA. However, the value (V/ V0 ) 0.02) with the monoclinic (001) surface is still the smallest value in the series of calculations. Thus, we conclude that the surface matches between the (1 × 2) supercell dimension of the mother (001) surface and (001) of DPA.

Figure 6. Experimental setup of the irradiation cell and laser incidence of the experiment (a) in Figure 4 and (b) the laser was not incident directly to the surface of the substrate. Surface of the substrate before irradiation (c) and the same area after 300 shots of irradiation (d) at 60 µJ cm-2 pulse-1.

Finally, we consider what happened in the suspension system under the influence of laser irradiation. First,

Laser-Induced Morphology Control and Epitaxy

why did the mother crystal disappear and how much did the temperature rise in the solution? Second, did direct incidence of the laser to the substrate bring about epitaxy, or did diffusion in solution of the photoproduct to the substrate surface cause epitaxy? Figure 6a shows the experimental setup of the cell used during the experiment of Figure 4. The depth of the sample cell is 200 µm, and the mother crystal sank to the bottom of the cell. The crystal is covered with anthracenesaturated solution. Since the average thickness of the mother crystal is ca. 50 µm, the mother crystal is covered with an anthracene-saturated solution layer of ca. 150 µm. An extinction coefficient of anthracene at 355 nm is 8000 mol-1 cm-1. The solubility is 0.012 M. This means that 99% of the incident light is absorbed within 45 µm from the solution surface and that the light does not reach directly to the surface of the substrate. The photochemical reaction should take place apart from the surface of the substrate. Anthracene in solution is consumed with the increase in irradiation, and the solution becomes undersaturated, which causes mother crystal dissolution into solution. The irradiation also brings about a temperature rise of the solution. The incident energy 60 µJ cm-2 pulse-1 causes a temperature rise of 0.02 K pulse-1 at most, if all the incident energy is converted into heat. 700 shots of irradiation give a temperature rise of 14 K by using the excitation volume of the solvent (1.5 × 10-2 cm3) and heat capacity of the solvent (100 J K-1 mol-1). A temperature rise of the solution also causes an increase in solubility. Dissolution enthalpy was determined to be 2.5 kJ mol-1 from a van’t Hoff plot. The solubility change by this temperature rise is estimated to be factor of 1.4, which cannot be negligible to allow dissolution of mother crystal. The dissolution of mother anthracene crystal is caused by both undersaturation of the solution and a temperature rise by light irradiation. Next, we will discuss the mechanism of the epitaxial growth. Direct incidence of UV irradiation to the surface of the substrate was known to cause epitaxy of DPA.11,12 In a suspension system, we can expect epitaxy without direct irradiation to the surface because DPA is dissolved in solution at near saturated concentration. We examined irradiation only to the solution part with a small filter, which cuts laser light above the mother crystal as illustrated in Figure 6b. Figure 6c shows the

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surface of the substrate before irradiation. Figure 6d shows the same area irradiated in the vicinity of the mother crystal after 300 shots of irradiation at 60 µJ cm-2 pulse-1. Several daughter crystals appeared on the substrate. This result indicates that diffusion of the photoproduct induces 2-D nucleation on the mother substrate to cause epitaxy. Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (417) (No. 15033213) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government. We thank Dr. Ste´phane Veesler at CRMCN-CNRS France and Prof. Kiyotaka Sato at Hiroshima University for helpful discussions. References (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) Okutsu, T.; Nakamura, K.; Haneda, H. and Hiratsuka, H. Cryst. Growth Des. 2004, 1, 113. (4) Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young R. G.; Myerson, A. S. Phys. Rev. Lett. 1996, 77, 3475. (5) Zaccaro, J.; Matec, J.; Myerson, A. S.; Garets, B. A. Cryst. Des. Growth 2001, 1, 5. (6) Tam, A.; Moe, G.; Happer, W. Phys. Rev. Lett. 1975, 35, 1630. (7) Tyndall, J. Philos. Mag. 1896, 37, 384. (8) Linebarger, C. Am. Chem. J. 1892, 14, 597. (9) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/ Cummings Publishing: San Francisco, Calfornia, 1978; p 456. (10) Hochstrasser, R. M.; Porter, G. Quart. Rev. 1960, 14, 146. (11) Bernard, J.; Madad, M.; Kottis, Ph. Chem. Phys. Lett. 1987, 73, 133. (12) Bernard, J.; Madad, M.; Kottis, Ph. Chem. Phys. 1987, 118, 211. (13) Ehrenberg, M., Acta Crystallogr. 1966, 20, 177. (14) Pfefer, G.; Boistelle, R., Trans. IChemE, A 1996, 74, 744. (15) Mason, R. Acta Crystallogr. 1964, 17, 532. (16) Brock, C. P.; Dunitz, J. D., Acta Crystallogr. Sect B Struct. Sci. 1990, 46, 795. (17) Hillier, A.; Ward, M. D. Phys. Rev. B 1996, 54, 14037. (18) http://www1.cems.umn.edu/research/ward/Software/Software.html. (19) Frincu, M. C.; Sharpe, R. E.; Swift, J. A. Cryst. Growth Des. 2004, 4, 223. (20) Marciniak, B.; Pavlyuk, V., Mol. Cryst. Liq. Cryst. Sci. Technol., Section A 2002, 373, 237.

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