Anthracene Crystallization Induced by Single-Shot Femtosecond Laser Irradiation: Experimental Evidence for the Important Role of Bubbles Kazuhiko Nakamura,†,‡ Yoichiroh Hosokawa,*,‡,# and Hiroshi Masuhara*,†,‡,#
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 885-889
Graduate School of Frontier Biosciences, Osaka UniVersity, Suita, Osaka 565-0871, Japan, Department of Applied Physics and Center for AdVanced Science and InnoVation, Osaka UniVersity, Suita, Osaka, 565-0871, Japan, and CREST JST, Honcho, Kawaguchi, Saitama 332-0032, Japan ReceiVed September 20, 2006; ReVised Manuscript ReceiVed January 4, 2007
ABSTRACT: A mechanism of femtosecond laser-induced crystallization was investigated using a supersaturated solution of anthracene. When a single-shot femtosecond laser pulse with a pulse energy above 3.1 µJ/pulse was shot into a sufficiently supersaturated solution, crystallization of anthracene was induced immediately after irradiation at the vicinity of the laser focal point. The threshold energy of the crystallization (3.1 µJ/pulse) was in agreement with that of laser-induced bubble formation, which was a sequential process after shockwave emission, cavitation bubble formation, and collapse. Sufficient supersaturation for crystallization decreased with an increase in the pulse energy. These results suggest that crystallization is triggered in the processes resulting in bubble formation. Furthermore, crystallization was enhanced at the surface of the bubble. The crystallization mechanism was completely different from that reported previously based on photochemical reactions or molecular alignment due to a strong optical field. Introduction Light-induced crystallization in supersaturated solutions of organic and biological molecules has been a very attractive topic in the fields of crystal science and engineering. Recently, we have introduced an intense focused femtosecond laser irradiation to crystallization studies as a trigger and succeeded in crystallizing not only organic molecules such as urea1,2 and 4-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST)3 but also hen egg-white lysozyme (HEWL),4 membrane protein,5 and so on.6,7 Now, our interest is directed at clarifying the mechanism of femtosecond laser-induced crystallization. Previously, light-induced crystallization by means of nanosecond pulsed laser and continuous light has been investigated on relatively small molecules. Garetz et al. found the crystallization of urea and glycine by 1064-nm nanosecond pulsed laser irradiation8-12 and concluded that an optical field due to the laser light induces optical Kerr alignment of molecules within urea and glycine clusters, leading to the nucleation. On the other hand, Okutsu et al. demonstrated light-induced crystallization and their morphology control due to photochemical reactions,13,14 where dianthracene and benzopinacol were respectively produced from anthracene and benzophenone with 355nm nanosecond pulsed laser irradiation. Furthermore, they proposed a crystallization method for proteins in which intermediate radicals produced by ultraviolet light are applied as a nucleus for crystallization.15 In addition to these crystallization mechanisms, the femtosecond laser provides further possibilities. When an intense femtosecond laser is focused into the supersaturated solution, it creates femtosecond-induced nonlinear phenomena such as shockwaves, cavitation bubbles, and jet flow formations, which * To whom correspondence should be addressed. Tel: +81-06-68797837. Fax: +81-06-6879-7840. E-mail:
[email protected] (Y.H.),
[email protected] (H.M.). † Graduate School of Frontier Biosciences, Osaka University. ‡ Department of Applied Physics and Center for Advanced Science and Innovation, Osaka University. # CREST JST, Honcho.
are considered to be due to optical breakdown16,17 and photomechanical ablation.18,19 If shockwaves, bubbles, and jet flow initiate the nucleation, it will trigger crystallization. To reveal the possibility, it is indispensable to elucidate the relationship between the dynamics of these nonlinear phenomena and the primary process of the crystallization; however, it has not been confirmed sufficiently because the crystallization speed in the previously studied solution was too slow to examine directly the relations. For example, HEWL crystals appeared a few days after the laser irradiation. In this work, we have succeeded in inducing crystallization of anthracene immediately after single-shot femtosecond laser irradiation at the vicinity of the laser focal point. The growth speed of anthracene crystal was much faster than that of compounds used in our previous studies.1-7 Here for the first time it is shown that anthracene crystallization is related to a femtosecond laser-induced bubble formation. Such characteristics of the femtosecond laser-induced crystallization are evaluated as functions of the pulse energy and supersaturation and compared with light-induced crystallization based on the optical field effect and chemical reactions. Experimental Section Anthracene (Nakarai Tesque, 99%, GC) and cyclohexane (Wako, Spectrochemical Anal.) were used without further purification. After a solution of anthracene suspended in cyclohexane was heated to 60 °C and stirred for 2 h, the supernatant solution with a concentration of 0.03 M was put into a 10 mm rectangular optical cell and set on an inverted microscope (Olympus, IX71). The solution temperature in the optical cell was cooled to room temperature (25 °C) for 20 min on the microscope. Its temporal profile of the solution temperature is shown in Figure 1, which was measured by putting a thermocouple into the solution. Since anthracene was not crystallized during the cooling process, the supersaturation of the solution became higher with time. 800-nm femtosecond laser pulses with a duration of 120 fs were generated by a regeneratively amplified femtosecond Ti: sapphire laser system (Spectra Physics, Hurricane), led into the microscope, and focused by an objective lens (10×, NA. 0.25) as shown in Figure 1. The laser focal point was adjusted a few millimeters from the bottom
10.1021/cg060631q CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007
886 Crystal Growth & Design, Vol. 7, No. 5, 2007
Nakamura et al.
Figure 1. Experimental setup for femtosecond laser-induced crystallization. The inset indicates the temporal profile of the temperature of the sample solution on the microscope. of the optical cell. The single-shot laser pulse was picked up from the pulse train with a repetition rate of 125 Hz by a mechanical shutter. The pulse energy was ranged from 0 to 20.6 µJ/pulse by adjusting the angle between a half wavelength plate and a polarizer. While temperature decreased from 60 °C to room temperature, the single pulse was intermittently shot into the solution with a time interval of 10 s. During the interval time, the appearance of crystals and its growth at the surrounding of the laser focal point were checked by a CCD camera (Ikegami, ICD-878) attached to the microscope. The laser focal point was slightly shifted shot-by-shot to exclude the effects of the former irradiation. On the other hand, the initial process of the crystallization was also monitored using a high-speed CMOS camera (Photoron, FASTCAM-MAX) as transmission images, whose gate time was 0.67 ms in the case of 1500 frame/s (fps) and 20 µs in the case of 50 000 fps. The chemical purity of the crystals generated by the laser irradiation was analyzed by means of high-performance liquid chromatography (HPLC) (Shimadz, LC-10Avp). To obtain enough crystals for the analysis, 6 × 104 shots of femtosecond laser pulses, whose energy and repetition rate were 10 µJ/pulse and 1 kHz, were irradiated to the sample solution at room temperature. These crystals were collected, washed with ethanol (Wako, 99.5%, for HPLC), and dissolved in ethanol. The analysis was performed by using a reversed-phase column (Shimadz, shim-pack, VP-ODS). As a reference, the original anthracene was also analyzed and confirmed with the same procedure.
Results and Discussion Figure 2 shows a primary crystallization process monitored by the high-speed CMOS camera with a frame rate of 1500 fps, where a single-shot femtosecond laser pulse with a pulse energy of 12.7 µJ/pulse was focused into the solution at 25 °C. Here, bubbles were observed at the vicinity of the laser focal point at 0.67 ms after the irradiation. At 300 ms, some crystalline objects appeared near the laser focal point as shown in Figure 2b, and the number and size of the objects increased with time (Figure 2c). At 1.3 s, the polyhedral shape of each object was clearly observed (Figure 2d). The objects were attributed to anthracene crystals from the results of the HPLC analysis, in
Figure 2. Microscopic images of the crystallization induced by singleshot femtosecond laser irradiation at 12.7 µJ/pulse, which were obtained using a high-speed CMOS camera. The solution temperature was 25 °C. The delay time after the laser irradiation is given in the images.
which the obtained retention profile was in good agreement with that of anthracene. Namely, photoproducts were not detected in our crystals. The appearance and shape of the generated anthracene crystals depended on the laser pulse energy. The representative examples monitored by the CCD camera were summarized in Figure 3. In energy regions of 3.1-16.0 µJ/pulse, all crystals had a shape of polyhedron; a representative result is shown in Figure 3a.
Anthracene Crystallization Induced by Laser Irradiation
Crystal Growth & Design, Vol. 7, No. 5, 2007 887
Figure 4. The pulse energy dependence of the critical temperature (a), below which anthracene crystallization was always induced by single-shot laser irradiation (see text), and supersaturation for the crystallization for each pulse energy (b). Inset of (b) represents the solubility of anthracene in cyclohexane for each critical temperature, which was experimentally estimated.
in the temperature. The supersaturation (σ) for each critical temperature was calculated by the following equation.
σ) Figure 3. Microscopic images of generated anthracene crystals at a pulse energy of 6.7 µJ/pulse (a) and a growth process of a bending film-like crystal created at the surface of a large laser-induced bubble at a pulse energy of 16.5 µJ/pulse (b-d). Crystals of (a) and (b-d) were observed at 30 and 32 °C, respectively. Right side illustrations represent generated crystals in left side photographs.
When the pulse energy was above 16.0 µJ/pulse, the initial shape of some crystals was sometimes (less than 10%) like a bending film as shown in Figure 3b, although most crystals (more than 10%) were polyhedron. The bending film soon grew to be the polyhedral crystal in the time scale of a few seconds (Figure 3b-d). When the single-shot pulses with a pulse energy of 12.7 µJ/ pulse were intermittently shot into the solution during the cooling process, crystals appeared only at a solution temperature lower than 36 °C. Alternatively, when the pulse energy was 20.6 µJ/pulse, crystallization occurred only below 38 °C. The pulse energy dependence of such critical temperatures was summarized in Figure 4 a. The critical temperature was increased with the pulse energy. Here, we must note that crystallization was only observed immediately after the laser irradiation. For example, when the laser pulse at 12.7 µJ/pulse was shot above the critical temperature and then kept at room temperature for a few hours, no crystallization appeared. Since anthracene was not spontaneously crystallized during the cooling process, the supersaturation of the solution became higher with a decrease
C - Ce Ce
where C is concentration at 60 °C (0.03 M), and Ce is solubility at each critical temperature. Ce was estimated from an exponential fitting curve for experimentally measured solubility (inset of Figure 4b). The supersaturation was dramatically increased with decreasing temperature as shown in Figure 4b. Therefore, the temperature dependence can be interpreted as follows: the crystallization is only induced when the supersaturation is achieved to a certain degree, which depends on the pulse energy. On the other hand, at the pulse energy less than 3.1 µJ/pulse, no crystallization was observed even when the temperature reached room temperature and then was kept at that temperature for a few hours. Since the threshold energy of the laser-induced bubble formation was 3.1 µJ/pulse, it is considered that the bubble formation or its precursor process triggers the crystallization. The high-speed CMOS camera with a frame rate of 50 000 fps monitored the dynamics of the bubble formation. A representative result at 12.7 µJ/pulse at 25 °C is shown in Figure 5. At 40 µs after the single-shot irradiation, a large bubble with a diameter of apparently 240 µm was created at the laser focal point. The size of the bubble increased with the pulse energy. The large bubble asymmetrically collapsed in a time scale of less than 100 µs. After the collapse of the initially large bubble (Figure 5d-f), a few small bubbles with a diameter of a few tens of micrometers were observed. These small bubbles remained for a few seconds in the solution. The bubbles shown in Figure 2a correspond to the latter bubbles. The size of the
888 Crystal Growth & Design, Vol. 7, No. 5, 2007
Figure 5. (a-f) Microscopic images of the bubble formation process induced by single-shot femtosecond laser irradiation at an energy of 12.7 µJ/pulse. The solution temperature was 25 °C. An arrow represents the laser focal point. The delay time after the single-shot irradiation is given above the images. Here the bubbles are out of focus, as the bubbling occurs three-dimensionally and not always perfectly on the focal plane.
remaining bubbles also increased with the pulse energy, while their number strongly fluctuated shot-by-shot. Only when the pulse energy was above 16.5 µJ/pulse and the single large bubble with a diameter of a few hundred micrometers remained, a crystal with a shape like a bending film (Figure 3b-d) appeared. Both initial and remaining bubbles also were formed in pure cyclohexane, so that the bubble formation should be dominated by multiphoton absorption of cyclohexane. On the basis of published studies on the pulsed laser ablation and subsequent bubble formation in pure water,17-19 the mechanism of bubble formation can be explained as follows. Energy from the femtosecond laser pulse is efficiently absorbed at the laser focal point through a multiphoton absorption process. Then, excited and ionized states of solute and solvent molecules are densely generated. The energetic molecular motions induced by their nonradiative relaxation will lead to an ultrafast temperature rise. It initiates rapid thermal expansion at the laser focal point and creates shockwave emission and subsequent void formation called cavitation.16 The large bubble in Figure 5b should be attributed to the cavitation bubble. The latter bubbles are probably due to boiling of cyclohexane and can be called a gas bubble, which isinitiated by local heating due to adiabatic compression in the process of the collapse of the cavitation bubble. Additionally, jet flow generated by an asymmetry collapse of the cavitation bubble creates an asymmetrical convection around the laser focal point. This convection would split the produced gas bubble into a few bubbles. Thus, the number of the gas bubbles would strongly fluctuate shot-byshot. On the other hand, when the cavitation bubbles collapsed symmetrically, a large gas bubble with a diameter of a few hundred micrometers was probably produced near the laser focal point, and its surface would accelerate growth of the bending film-like crystal. Now, we consider our crystallization in comparison with the previously reported mechanism. When a near-infrared nanosecond pulsed laser with a repetition rate of 10 Hz was continuously shot in a supersaturated solution of urea and glycine, the molecules were aligned by the optical field and
Nakamura et al.
were crystallized in a few seconds and in about 30 min,8,10 respectively. Also, in the case due to photochemical reactions,13,14 dianthracene and benzopinacol were continuously produced from anthracene and benzophenone by a few hundred shots of UV nanosecond laser pulses and crystallized in the laser-irradiated region. On the other hand, laser-induced crystallization presented here was purely initiated by a single-shot femtosecond laser irradiation. Here, the optical field created by the femtosecond laser pulse only affects the solution for 120 fs, which is too short to produce nuclei by the optical field. Even if dianthracene molecules are produced by the single-shot laser irradiation and aggregates as nuclei, under the present experimental condition, the nuclei should disperse from the laser focal point within 40 µs, and the dispersed nuclei should be dissociated. On the other hand, dianthracene molecules as a photoproduct are produced through a photodimerization process, so that photochemical nucleation should not depend on the threshold energy of the cavitation bubble formation; however, the threshold energy of the presented crystallization was in good agreement with that of the cavitation bubble formation. Therefore, it is concluded that the present femtosecond laser-induced crystallization is not attributed to optical field or chemical reactions but to several nonlinear phenomena resulting in the bubble formation. In the bubble formation processes, we can consider some possible processes to trigger crystallization. For example, the explosive expansion and collapse of the cavitation bubble generate transient pressure (MPa order).20 Also the shockwave emission creates the transient pressure with a magnitude reaching the GPa order.21 Such pressure fluctuations will probably be a perturbation to induce the crystallization. The size of the cavitation bubble and the magnitude of the shockwave were increased with the pulse energy. If the probability for nucleation is enhanced by these increases, the supersaturation required for the crystallization will decrease with the pulse energy, which is shown and summarized in Figure 4b. As a further effect, the surface of the latter bubbles with lifetime of a few seconds would allow adsorption of anthracene molecules, nucleation, and several kinds of association, which will accelerate nucleation and crystal growth. In fact, we have confirmed that crystallization of HEWL is promoted at the interface between its solution and a laser-induced bubble.22 The crystal shaped like a bending film shown in Figure 3b may reflect at least partly this process. Conclusion In this paper, we have succeeded in inducing anthracene crystallization immediately after the single-shot femtosecond laser irradiation. The crystallization was observed above the threshold energy of the laser-induced bubble formation. Sufficient supersaturation for crystallization decreased with an increase in the pulse energy. Furthermore, crystallization was enhanced at the surface of the bubble having lifetimes of a few seconds. In a previous study, we have also confirmed a crystallization of HEWL at the surface of the laser-induced bubbles.23 These results suggest that bubbles created by an intense focused femtosecond laser irradiation act as a preferential field for the crystallization. Such a crystallization mechanism will be completely different from that based on optical field8-12 and photochemical reactions at the laser focal point.13-15 Further investigation of femtosecond laser-induced crystallization will provide new possibilities for the fields of crystallography and crystal engineering.
Anthracene Crystallization Induced by Laser Irradiation
Acknowledgment. This work was partly supported by the Grant-in-Aid for the Bio-Medical Cluster Project in Saito (Northern Part of Osaka Prefecture) from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and by the Grant-in-Aid Project on Scientific Research (KAKENHI) on Priority Are on “Molecular Nano Dynamics” from the MEXT. References (1) Yoshikawa, H. Y.; Hosokawa, Y.; Masuhara, H. Cryst. Growth Des. 2006, 6, 302. (2) Yoshikawa, H. Y.; Hosokawa, Y.; Masuhara, H. Jpn. J. Appl. Phys. 2006, 45, L23. (3) Hosokawa, Y.; Adachi, H.; Yoshimura, M.; Mori, Y.; Sasaki, T.; Masuhara, H. Cryst. Growth Des. 2005, 5, 861. (4) Adachi, H.; Takano, K.: Hosokawa, Y.; Inoue, T.; Mori, Y.; Matsumura, H.; Yoshimura, M.; Tsunaka, Y.; Morikawa, M.; Kanaya, S.; Masuhara, H.; Kai, Y.; Sasaki, T. Jpn. J. Appl. Phys. 2003, 42, L798. (5) Kitano, H.; Adachi, H.; Sato, A.; Murakami, A.; Matsumura, H.; Takano, K.; Inoue, T.; Mori, Y.; Doi, M.; Sasaki, T. Jpn. J. Appl. Phys. 2004, 43, L1271. (6) Numata, T.; Ikeuchi, Y.; Fukai, S.; Adachi, H.; Matsumura, K.; Takano, K.; Murakami, S.; Inoue, T.; Mori, Y.; Sasaki, T.; Suzuki, T.; Nureki, O. Acta Crystallogr. 2006, F62, 368. (7) Tsukazaki, T.; Mori, H.; Fukai, S.; Numata, T.; Perederina, A.; Adachi, H.; Matsumura, H.; Takano, K.; Murakami, S.; Inoue, T.; Mori, Y.; Sasaki, T.; Vassylyev, D. G.; Nureki, O.; Ito, K. Acta Crystallogr. 2006, F62, 376. (8) Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young, R. G.; Myerson, A. S. Phys. ReV. Lett. 1996, 77, 3475.
Crystal Growth & Design, Vol. 7, No. 5, 2007 889 (9) Zaccaro, J.; Matic, J.; Myerson, A. S.; Garetz, B. A. Cryst. Growth Des. 2001, 1, 5. (10) Garetz, B. A.; Matic, J.; Myerson, A. S. Phys. ReV. Lett. 2002, 89, 175501. (11) Matic, J.; Sun, X.; Garetz, B. A.; Myerson, A. S. Cryst. Growth Des. 2005, 5, 1565. (12) Aber, J. E.; Arnold, S.; Garetz, B. A.; Myerson, A. S. Phys. ReV. Lett. 2005, 94, 145505. (13) Okutsu, T.; Nakamura, K.; Haneda, H.; Hiratsuka, H. Cryst. Growth Des. 2004, 4, 113. (14) Okutsu, T.; Isomura, K.; Kakinuma, N.; Horiuchi, H.; Unno, M.; Matsumoto, H.; Hiratsuka, H. Cryst. Growth Des. 2005, 5, 461. (15) Okutsu, T.; Furuta, K.; Terao, M.; Hiratsuka, H.; Yamato, A.; Ferte, N.; Veesler, S. Cryst. Growth Des. 2005, 5, 1393. (16) Vogel, A.; Venugopalan, V. Chem. ReV. 2003, 103, 577. (17) Vogel, A.; Noack, J.; Huettman, G.; Paltauf, G. Appl. Phys. B 2005, 81, 1015. (18) Hosokawa, Y.; Yashiro, M.; Asahi, T.; Masuhara, H. J. Photochem. Photobiol. A Chem. 2001, 142, 197. (19) Paltauf, G.; Dyer, P. E. Chem. ReV. 2003, 103, 487. (20) Franz, M.; Koenz, F.; Pratisto, H.; Weber, H. P.; Silenok, A. S.; Konov, V. I. J. Appl. Phys. 1998, 84, 5905. (21) Noack, J.; Hammer, D.; Noojin, G.; Rockwell, B.; Vogel, A. J. Appl. Phys. 1998, 83, 7488. (22) Davey, R.; Garside, J. From Molecules to Crystallizers; Oxford University Press: Oxford, UK, 2000; p 20. (23) Nakamura, K.; Sora, Y.; Yoshikawa, Y. H.; Hosokawa, Y.; Mori, Y.; Sasaki, T.; Masuhara, H. J. Appl. Sur. Sci., manuscript submitted.
CG060631Q
