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J. Phys. Chem. B 1998, 102, 1896-1901
Chemical and Optical Mechanism of Microparticle Formation of Poly(N-vinylcarbazole) in N,N-Dimethylformamide by Photon Pressure of a Focused Near-Infrared Laser Beam Pawel Borowicz,† Jun-ichi Hotta, Keiji Sasaki,* and Hiroshi Masuhara* Department of Applied Physics, Osaka UniVersity Suita, Osaka 565, Japan ReceiVed: September 26, 1997; In Final Form: January 5, 1998
We focus our attention to the laser-controlled association process of poly(N-vinylcarbazole) by “pure” photon pressure effect. A few wt % N,N-dimethylformamide solution of the polymer was irradiated with a focused 1064 nm laser beam of sub wattage power, and a resultant condensation (microparticle formation) at a focal point of the microscope was initially probed by backscattering of He-Ne laser and later observed by transmission image of the microscope. To understand in detail the behavior of the polymer induced by the photon force, the relation between laser power and particle diameter, as well as repeated irradiation effect, were investigated. The laser-controlled association was successfully demonstrated even for short polymer chains of about 20 mean degrees of polymerization. Fluorescence spectra and their rise and decay curves were measured microspectroscopically for a formed microparticle. The results are different from those of the polymer in solution, suggesting the association structure is characteristic of photon pressure effect.
Introduction The photon pressure effect was first demonstrated and applied for trapping microparticles by Ashkin et al.1 This experimental technique, based on geometrical optics, was extended to threedimensional manipulation by us, which makes it possible to construct spatial pattern from micrometer-sized particles in solution.2 In the past few years, laser trapping was successfully applied to microparticle formation from polymer chains in aqueous solution.3 The behavior can be interpreted in the terms of wave optics, as polymers are much smaller than the laser wavelength. The first system is poly(N-isopropylacrylamide) (PNIPAM) in H2O which results in formation of a single microparticle with large (up to about 20 µm) diameter upon irradiation of the focused laser beam. The phenomena are rather complicated as photothermal heating due to near-infrared absorption at the trapping wavelength via an overtone band of O-H vibration involves thermally induced phase transition. Indeed, the size of condensed particle was reduced to about 1 µm in heavy water, as the photothermal effect does not have a major role in the microparticle formation in heavy water, due to the shift of overtone bands to the red. To demonstrate the laser trapping by “pure” photon pressure effect, we recently examined laser controlled microparticle formation for poly(N-vinylcarbazole) (hereafter abbreviated as PVCz) in organic solvents.4 The process of microparticle formation is dependent on solvent, concentrarion, laser power, and so on. Initially nothing is identified, then backscattering of He-Ne laser is observed, and the condensed polymer phase can be detected by optical transmission image. Association of polymer chains by “pure” photon pressure limits the size of condensed phase (called also “particle”, since under microscope it looks like microsphere) to the order of the focal spot dimension. Photothermal effect was not observed in organic solution of PVCz, since the lack of phase transition is established † Permanent address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.
for the polymer by systematic examination of the association process in the temperature range from 22 to 34 °C. To establish microparticle formation exclusively by photon pressure, we have studied here laser controlled association process of PVCz mainly in N,N-dimethylformide (DMF) in detail. The power of the trapping laser was extended to higher energy than in the former investigations,4 and the behavior of the sample upon sequence of irradiations was examined. To check the size limit of trapped polymer chains the samples with different molecular weights were under study. The fluorescence spectroscopy of PVCz in DMF was performed to obtain information on association structure of PVCz microparticle. Experimental Section Samples. The main sample of PVCz was received as a gift from Takasago International, Inc., as previously described.4,5 The polymers with smaller molecular weight were kindly given to us by Prof. A. Itaya (Kyoto Institute of Technology). PVCz(r) and PVCz(c) are prepared by radical and cationic polymerization, respectively. Both consist of iso- and syndiotactic sequences in ratio of 1:3 and 1:1, respectively.6 The degree of polymerization of PVCz(c) sample is about 20. In the case of PVCz(r) the precise value is unknown, but it is placed between 20-200. The degree of PVCz (Takasago International, Inc.) mainly used in the investigations is about 200 and its tacticity is considered to be similar to PVCz(r). Solvents. DMF (Wako, spectroscopic grade) and cyclohexanone (Nacalai Tesque, Inc., GR grade > 99%) (hereafter CHN) were checked in absorption and fluorescence before use with Shimadzu UV-vis-NIR (NIR ) near-infrared) 3100 spectrophotometer and Hitachi F4500 spectrofluorimeter, respectively. Experimental Setup. The setup demonstrating the association of the polymer chains and the experimental procedure were the same as described in our previous paper.4 Briefly, a continuous wave Nd3+: YAG (1064 nm) (Spectron, SI-903U) laser beam was introduced into a microscope (Nikon, Optiphoto 2) where it was focused to a micrometer-sized spot in the sample
S1089-5647(97)03121-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/24/1998
Microparticle Formation of Poly(N-vinylcarbazole)
J. Phys. Chem. B, Vol. 102, No. 11, 1998 1897
solution through microscope objective with high numerical aperture (NA ) 1.30). The He-Ne laser beam was introduced coaxially with near-infrared beam to detect particles smaller than the visible wavelength (hereafter we call them “invisible” particles). The solution cell was made of a slide glass with grooves and a cover glass. Since the setup used for fluorescence measurements was similar to that described in the literature,7 only the outline is given. The main part of the system was a confocal microscope (Zeiss UMSP-50, NA ) 1.25). As an excitation light, the third harmonic (λ ) 300 nm, repetition rate 0.4 MHz) of mode locked Ti: sapphire laser (3950-S2L, Spectra Physics) was introduced to the microscope together with the trapping beam. The steady-state fluorescence spectra were recorded with the intensified multichannel spectrophotometer (Hamamatsu Photonics, PMA10). For time-resolved measurements the system composed of monochromator (Zeiss 474345), microchannel-plate photomultiplier (Hamamatsu Photonics R2809U-07) and standard single-photon-counting electronics was used instead of an intensified multichannel spectrophotometer. Since the fluorescence intensity decreased very fast with the increase of the depth of focal spot, the measurements were performed at the distance of 5 µm (between the focal spot and a cover glass) to induce high fluorescence intensity. Results Microparticle Formation. The measurements were done in two DMF solutions with different concentration. When the solution with low concentration (3.5 wt %) was irradiated, a single particle reached a steady state in 40 min. Initially, particle formation is detected only by backscattering of He-Ne laser, then it increases the diameter to the value detected with microscope transmission image, and finally the steady state is achieved. In the case of high concentrated solution the necessary time of exposure was reduced to 20-25 min. Association is faster when the concentration is higher. In the case of 6.8 wt % the scattering can be observed at 3-5 min after introducing the trapping beam, and the steady state is achieved after 15-19 min. The main stages of formation process are similar to that reported in our former works.3,4 The examples of CCD images obtained for 6.8 wt % solution are shown in Figure 1. Since the visible particle is formed faster than in the case of 3.5 wt % solution, only the photograph at 300 s after introducing the laser beam was captured in scattering image. Frames at late stages present the association of polymer chains observed in transmission optical image. The development of the particle is reflected not only in its diameter but rather in the contrast of the recorded image. Changes of the particle size are not easy to detect due to the small dimension of the condensed polymer phase. Relation between Laser Power and Particle Diameter. As presented in Figure 2, an increase in the concentration of polymer chains results in the spread of power range where condensation of polymer chains is possible. It is valid not only for the low intensity limit where the threshold of formation is shifted from 80 mW down to 30 mW but also for the upper limit of the trapping laser intensity range where the association is detected. This value is shifted from 260 mW for concentration equal 3.5 wt % to above 310 mW for 6.8 wt %. In the latter case the value of the threshold was not reached, since further increase of the laser power may result in the damage of the trapping optics. The behavior that the trapped microparticle becomes unstable in the high power range is new and has never been observed for PNIPAM solutions. We examined it also in cyclohexanone. In this solvent microparticle was identified in
Figure 1. Microscope pictures of single particle formation of PVCz in DMF, concentrated 6.8 wt %: (a) 300 s, (b) 310 s, (c) 420 s, (d) 540 s, (e) 660 s, (f) 720 s, (g) 780 s, (h) 840 s, (i) 900 s, (j) 1020 s, and (k) 1100 s after introducing the near-infrared laser beam of 200 mW. Photograph (a) was obtained with backscattering of He-Ne laser, while photographs in (b)-(k) were obtained with the optical transmission image.
Figure 2. The diameter of the PVCz-particle as a function of nearinfrared laser power in the steady state: (a) concentration 3.5 wt % in DMF, (b) concentration 6.8 wt % in DMF. Full points and rhombuses indicate the particle is observed directly in transmission image, while open ones show the particle can be detected only with backscattering of He-Ne laser beam,
the range of 170-190 mW, while for the power above 200 mW and below 170 mW the particle was detected only with the scattering image. Also the diameter of the particle is slightly dependent on concentration. The relation between the condensation time, reaching the steady state, and the laser power is presented for 6.8 wt % solution in Figure 3. For 50 mW the time was about 30 min, while it decreased to 15 min with increasing laser power up to 120 mW. However, in the higher intensity range, the increase of laser power leads to slight increase of the condensation time. This is consistent with the above result that the microparticle diameter was again small for higher intensity.
1898 J. Phys. Chem. B, Vol. 102, No. 11, 1998
Figure 3. The time of steady-state achievement as a function of nearinfrared laser power for 6.8 wt % PVCz in DMF. Full rhombuses indicate the particle is observed with optical transmission image, while open one shows the particle can be detected only by backscattering of He-Ne laser beam.
Figure 4. The time of particle formation as a function of polymer concentration for PVCz(r) in DMF.
Relation Between Polymer Chain Length and Formation Process. The measurements of PVCz with different tacticity and different length were performed, although samples are limited because of difficulty of synthesis. During this investigations the laser power was fixed at about 200 mW. The images of PVCz(r) in DMF recorded by CCD camera are similar as in the case of the main PVCz sample described above. At the beginning of the formation process the particle can be observed only with backscattering of He-Ne laser, while in steady state the particle can be detected in optical transmission image. The diameter is similar to the particle formed by PVCz under the same irradiation condition of 200 mW. In Figure 4 the time after which steady state is reached is plotted as a function of concentration. It is shown that decreasing of the concentration of polymer chains results in prolongation of the time necessary to reach the steady state. However, the diameter of the particle is common even if the concentration changes over 1 order of magnitude. For PVCz(c) one can observe the process of particle formation, which means that even PVCz oligomers whose length is roughly estimated to be 10 nm can be trapped. The experiment is very difficult as it is strongly disturbed by trapping already precipitated pieces of polymer due to the lower solubility. Repeated Irradiation Effect. As the particle formation is due to laser irradiation, the microparticle disappears quickly after switching off the beam. If the concentration behavior comes back to its original homogeneous distribution, the same condensation should be reproduced upon the next irradiation. To confirm this simple expectation, the experiment changing the sequence of irradiations was performed for 3.5 and 6.8 wt % PVCz in DMF. The power of the trapping laser was fixed at 200 mW. The disappearance time of the particle is short in comparison with the time of the detected condensation. After a few seconds the solution becomes transparent and no “visible” object can be observed. The main change after the first exposition, in the case of high concentration, is the decrease of the formation time (after which the steady state is established) from about 17 to 3-5 min when the “dark” time between two
Borowicz et al.
Figure 5. The relation between formation time of the particle upon second irradiation and the length of the break between first and second irradiation (“dark” period), for 5.8 wt % PVCz in DMF.
irradiations is equal to 4 min. The abridgement of the break between two expositions, down to 1 min, provides no change in the time necessary to reach the steady state as shown in Figure 5. Also two or three cycles of irradiation with the same break between expositions do not change the time of particle formation. When the “dark” period decreases below 1 min, the time of particle formation significantly increases. After changing the place of irradiation the time of particle formation increases to its original value. Similar behavior, the abridgement of the condensation time after first irradiation, was also observed for low concentration case (3.5 wt %). Fluorescence Spectroscopy. The fluorescence spectroscopic measurement was done for PVCz in DMF of two concentrations: 3.5 and 5.8 wt %. In both cases steady-state spectra as well as their decay curves were recorded. In the latter case three wavelengths were chosen corresponding to the structure of PVCz fluorescence: 350, 380, and 460 nm. The steadystate fluorescence spectrum of this polymer at room temperature consists of two species. The first, high-energy part is emitted by so-called second or partial overlap excimer and its maximum is placed between 3706 and 380 nm.8 The low-energy part of the spectrum comes from the sandwich excimer, and its maximum is located between 4206 and 430 nm.8 Investigations provided for model compounds showed that the wavelength 350 nm corresponds to the energy of electronic transition in monomer carbazole chromophore.9 The time-resolved measurements in the picosecond time scale allowed us to detect the shoulder of monomer fluorescence of PVCz in the short wavelength region in solution,6,10 but it was not observed in PVCz films.6 The choice of 350 nm not only allowed to have a great contribution of the fluorescence from second excimer and large reduction of the fluorescence from the sandwich structure but also offers the possibility of the evidence of the monomer structure. The observation at 380 nm corresponds to the maximum of the partial overlap excimer and that at 460 nm reduces the fluorescence from the second excimer compared to the sandwich excimer fluorescence. Data were collected for the fresh solution (before introducing the trapping beam), after steady-state particle formation and after cutting the near-infrared laser beam (after complete dissolution of the particle). Some examples of fluorescence spectra and decay curves are presented in Figure 6. The decays measured at 350 and 380 nm could be analyzed with a two-exponential function. The measurement at both wavelengths gives similar result; for fresh solution two decay times are equal to about 3 and 18 ns, respectively. Namely, the contribution of the monomer component was not clearly detected, which is consistent with our previous results of solutions. After particle formation decay times decrease to about 1.5-2 and 15 ns, respectively. At 460 nm a single-exponential function gives a good fit. The decay time equal to about 23 ns in solution decreases to about 16 ns
Microparticle Formation of Poly(N-vinylcarbazole)
J. Phys. Chem. B, Vol. 102, No. 11, 1998 1899 Discussion
Figure 6. Fluorescence spectral data of 3.5 wt % PVCz in DMF; (a) steady-state fluorescence spectra (uncorrected) and (b), (c), and (d) fluorescence decays monitored at 350, 380, and 460 nm, respectively. Notations: crosses, fresh solution; closed circles, particle in steady state; and open rhombuses, solution after cutting off the trapping beam. Spectral behavior recorded for higher concentration (5.8 wt %) are almost identical.
TABLE 1: Fluorescence Decay Times and Amplitudes of PVCz in DMF for Solution and Microparticle Analyzed by a Two-Exponential Function solution
particle
wavelengths [nm]
relative amplitude
decay time
relative amplitude
decay time
350
0.798 ( 0.052 0.202 ( 0.052 0.776 ( 0.046 0.224 ( 0.046
2.68 ( 0.29 17.89 ( 0.48 2.80 ( 0.24 18.60 ( 0.48 22.97 ( 0.42
0.745 ( 0.071 0.255 ( 0.071 0.582 ( 0.063 0.418 ( 0.063
1.57 ( 0.27 14.95 ( 0.66 2.07 ( 0.07 14.99 ( 0.38460 16.31 ( 0.63
380 460
after particle formation. After cutting off the trapping laser the lifetimes were the same as measured for fresh solution. Obtained data are summarized in Table 1. To exclude the temperature elevation (due to possible local heating by focused 1064 nm beam) and electric field (due to high intensity of the focused laser light) effects due to nearinfrared laser beams additional measurements of the fresh solution were performed. In the first case the sample was warmed to about 40 °C, while investigations of the photon pressure effect were made at about 20 °C. No significant changes either in spectrum or in decay curves upon the higher temperature were observed. To check the electric field effect the spectrum and decay curves of the solution were measured just after introducing the trapping beam. In this case microparticle is still not formed, but the electric field upon PVCz dissolved in DMF could be exerted. However, no significant difference between irradiated and nonirradiated sample was be observed.
Chemical and Optical Equilibrium between the Focal Spot and the Bulk Solution. It is well-known3,4 that the photon pressure is a function of a few variables: laser power, intensity distribution in the laser beam, magnification, and numerical aperture of the microscope optics, refractive indices of trapped target molecular system and solvent, and the size of the trapped target. In the case of polymer chains, not only molecular weight (degree of polymerization) but also polymer conformation might influence the trapping force. Namely, before introducing the laser beam, free enthalpy is in principle constant in anywhere of the polymer solution and polymer chains diffuse randomly constituting micro-Brownian motion. Once the laser beam is introduced, polymer chain at the focal point feels optical potential, hence free energy at the point is lower than that of the surroundings, plural chains are gathered, and the association of the polymer chains leads to a decrease of entropy. As a results, the free enthalpy at the focal spot is lower compared to the bulk solution. It is worth noting that, during the polymer chains associate with each other, the exerted photon pressure is increased due to the increase of the size and/or refractive index of the condensed polymer phase. The photon pressure changes from time to time and converged, namely, after some time the system reaches the dynamic equilibrium. In this steady state, the polymer chains can move in and out of the particle, and not only the size of particle but also the chain packing does not change anymore. Temperature may affect the association process of polymers leading to chemical and optical equilibrium. It is reported that at least few kelvin is elevated in aqueous solutions because of photothermal effect due to absorption of a 1064 nm laser light by overtone of OH vibration of water.3 When we used poly(N-isopropylacrylamide) as a molecular target, temperature elevation results in breaking of hydrogen-bonded association of the polymer with water molecules, leading to aggregation. The exerted photon pressure becomes large if polymers are associated to a larger size, and accelarates further association. However, in the present PVCz in DMF such photothermal effect was not confirmed experimentally. Characteristic of Optical Potential for Microparticle Formation. In the steady state the dynamic equilibrium is established, and the photon pressure becomes time independent. Now we consider the size of trapped particle in terms of potential well generated by the near-infrared light. As shown in Figure 2, for the low power of the trapping beam, the diameter of the particle in steady state grows with the increase of the laser power. This should be reflected in the changes of the shape of potential well. When the polymer chains start to associate (after introducing the trapping beam) their packing should be dependent on the power of the trapping laser. In fact, for higher power, where the association provides highly condensed phase, trapped polymer chains form the microparticle with higher effective refractive index in comparison with the case of lower laser intensity. The structure with closer packing should modify the potential well deeper, which should be reflected in larger diameter of the particle in the steady state. This progress cannot be continued without limit. There are two possible reasons. One reason is of course optical condition of the experimental setup. Here the 1064 nm beam is focused into a 1 µm spot of diffraction limit, hence optical potential is deep around the focal spot. The peripheral area outside the spot has a high enthalpy. The microparticle could not grow up to a few µm which was confirmed in Figure 2. Second, the packing of polymer chains has a limit. It is considered that the
1900 J. Phys. Chem. B, Vol. 102, No. 11, 1998 chains are located so close that the density of the particle might become similar to that of the solid. In this case the increase of the laser power provides negligible changes in the chain packing and the effective refractive index of the condensed phase should be constant. Over such thresholds the diameter of the particle should not increase, even when the increase of the laser power provides to increase of the depth of potential well. This is well confirmed in the relation between the microparticle diameter and the laser power. For high energy range the decrease of the particle diameter with the increase of the laser power was observed. This behavior is extremely interesting but unfortunately not fully understood at the present stage of investigation. The most simple explanation may be due to local heating. For example, the overtone of H2O absorbs the 1064 nm light and causes elevation of a few kelvin.3 This possibility can be excluded since DMF as well as PVCz itself show negligible absorption at the wavelength of the trapping beam, as mentioned above. Another possibility is that the multiphoton absorption by PVCz heats the temperature but it has low probability, since at least three photons from trapping beam are necessary to reach the energy of polymer absorption band. Multiphoton excitation necessary for heating should not be induced. If we assume ionization of PVCz is brought about by multiphoton absorption, the cation of PVCz aggregate may accelerate dissolution. This is also improbable. If condensed polymer aggregates are too heavy, the disappearance is possible. But according to our experience of poly(N-isopropylacrylamide),3 a few tens micrometer size is usually necessary. In the present case only 2-3 µm is attained, hence this possibility can be also excluded. The dependence of diameter on laser power is reproduced also in the relation between condensation time and power of the near-infrared laser (Figure 3) where, at the beginning, the time necessary to achieve the steady state is decreasing with increasing the laser power. This is in principle quite reasonable, however, above 130 mW it reaches value about 17 min. The higher laser power of of course exerts the higher photon pressure upon accidentally trapped polymers and results in their gathering. As discussed above, the free entalpy of the focal spot should be lowered with the higher pressure, so that the equilibrium between the focal spot and the surroundings shifts to the former. Thus the steady-state is reached faster as the laser power is increased. Further increase of the laser intensity does not result in so significant change of the condensation time as it does in the low-power range. For high laser intensities even slight increase of the time necessary to achieve the steady state was suggested. The repeated irradiation results in the significant decrease of formation time of the particle. This phenomenon was observed both for 3.5 and 6.8 w% DMF solution. Although, the first particle condensation in fresh solution depends on the concentration, the time necessary to achieve steady state till the second irradiation was equal (within experimental accuracy) for concentrations of 3.5 and 6.8 wt %. Also the time necessary for relaxation of the solution to the “fresh” conditions seems to be very long at investigated concentration (5.8 wt %). This was confirmed by elongating the “dark” period to 40 min, which is longer than the time necessary to achieve the steady state (about 25 min). Photon pressure effect may also cause the changes of polymer chain conformations, which may change the effective size of trapped objects. The association of polymer chains may form condensed amorphous solid, and their time of dissolution and diffusion should be extended to a few hours. The time of relaxation necessary for the species to return to the standard
Borowicz et al. conformation in solution should be also extended to a few hours. If we move the focal spot in the different place of irradiation, the condensation time was the same to that of the fresh solution. This indicates that the local polymer concentration at the focal point is reserved for a few tens of minutes. Roughly speaking, it is consistent to the fact that dissolution of polymer microspheres in solution takes a few tens of minutes at room temperature. Minimum Size for Laser Trapping. It is interesting and important to note how small polymer can be trapped by a focused near-infrared laser beam. According to electromagnetic theory, Lorenz force responsible to the microparticle formation consists of gradient force and scattering force. The former attracts molecular targets to the central part of the beam, while the latter pushes the targets out of the focal spot. Usually the former is larger than the latter, and their sum is competitive with the Brownian motion. We have calculated the trapping force which is balanced with the Brownian motion at room temperature, assuming 1 W laser of 1064 nm. For about 10 nm this balance is achieved. Although the present system has a distribution of molecular weight, 20° of polymerization of PVCz(c) will give a size of 10 nm. Thus, the minimum molecular size which can be trapped by laser is demonstrated experimentally in the present work. Association Structure Analyzed by Fluorescence Spectroscopy. The steady-state fluorescence spectra of PVCz in solution and in film are composed of partial overlap and sandwich excimers.6, 8-11 By the detailed study, the decay of PVCz fluorescence in solution should be described by a fiveexponential function when measured at 350 or 380 nm or a four-exponential one for 460 nm. The rise times of sandwich excimer are 20 ps, 350 ps, and 1.6 ns,6 while the decay times published previously are 7 ns for the partial overlap excimer and 35 ns for the sandwich one.6 Dynamics in the film is also very complicated. After the present microparticle formation, the contribution of sandwich excimer decreases in comparison with partial overlap one, which suggests the former microparticle is not similar to the isolated polymer in solution in view of fluorescence spectroscopy. Although the temporal performance of the present setup is not high to obtain comparable data, the results are summarized in Table 1. The difference of time constant between the microparticle and the reference data of the solution is difficult to explain precisely at the present stage of study, since complicated scheme of PVCz8 may be perturbed by quenching by oxygen and singlet-singlet anihilation.11 However, it is clear that the fluorescence spectra and decay change upon the microparticle formation. The data measured for the particle present shorter decay time of the sandwich excimer than those observed for fresh solution. This novel result may be explained by the following two effects. The kinetic scheme of PVCz consists of radiative and nonradiative deactivation of excited monomer and both excimers as well as formation and dissociation processes of both excimers (including the interconversion between two type of excimers).8 It was also confirmed that the second excimer can be populated from the sandwich one via energy migration from iso- to syndiotactic sequence.10 Particle formation leads to very close packing of the polymer chains, which can result in acceleration of the population of the partial overlap excimer from the sandwich one and/or radiationless decay of the sandwich excimer. The other possibility is concerning with S1-S1 annihilation of sandwich excimer which can be detected as shortening of the decay time,11 as the excitation intensity is enough high under the microscope. It is clear that the S1-S1
Microparticle Formation of Poly(N-vinylcarbazole) annihilation is more efficient in the packed structure of the microparticle. Furthermore it is worth noting that the spectral change upon the microparticle formation is not similar to that of the film preparation. Now, it is concluded that PVCz in formed microparticle is not a simple assembly of PVCz chains in solution and in film, but a new association characteristic of photon pressure is probable. Acknowledgment. The present work was partly supported by NEDO, the proposal-based Advanced Industrial Technology Research and Development Program, Control of Molecular Assembling by Radiation Pressure of a Laser Beam, and by the Grant-in-Aid from Ministry of Education, Science, Sport and Culture (Grants 94170, 07241243, 08231247). One of the authors (Pawel Borowicz) gratefully acknowledges the New Energy and Industrial Technology Development Organization (NEDO) for financial support of his stay in Japan. PVCz was kindly provided by Prof. A. Itaya of Kyoto Institute of Technology and by Takasago International, Inc. References and Notes (1) Ashkin, A.; Dziedzic, J. M. Appl. Phys. Lett. 1971, 19, 283. Ashkin, A. Science 1980, 210, 1081. Ashkin, A.; Dziedzic, J. M.; Bjorkholm, J. E.; Chu, S. Opt. Lett. 1986, 11, 288. (2) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura N.; Masuhara, H. Jpn. J. Appl. Phys. 1991, 30, L907. Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura N.; Masuhara, H. Opt. Lett. 1991, 16, 1463. Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura N.; Masuhara, H. Appl. Phys. Lett. 1992, 60, 807. Sasaki, K.; Misawa, H. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B. V.: New York, 1994; pp 23-34.
J. Phys. Chem. B, Vol. 102, No. 11, 1998 1901 (3) Ishikawa, M.; Misawa, H.; Kitamura N.; Masuhara, H. Chem. Lett. 1993, 481. Kitamura, N.; Ishikawa, M.; Misawa H.; Fujisawa, R. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B. V.: New York, 1994; pp 79-92. Ishikawa, M.; Misawa, H.; Kitamura, N.; Fujisawa R.; Masuhara, H. Bull. Chem. Soc. Jpn. 1996, 69, 59. Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara, H.; Faes H.; De Schryver, F. Mol. Cryst. Liq. Cryst. 1996, 283, 165. Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara H.; Iwai, K. Langmuir 1997, 13, 414. Hofkens, J.; Hotta, J.; Sasaki, K.; Masuhara H.; Taniguchi, T.; Miyashita, T. J. Am. Chem. Soc. 1997, 119, 2741. (4) Borowicz, P.; Hotta, J.; Sasaki K.; Masuhara, H. J. Phys. Chem. B 1997, 101, 5900. (5) Watanabe, K.; Asahi, T.; Masuhara, H. J. Phys. Chem. 1996, 100, 18436. (6) Itaya, A.; Sakai, H.; Masuhara, H. Chem. Phys. Lett. 1987, 138, 231. Itaya, A.; Sakai, H.; Masuhara, H. Chem. Phys. Lett. 1988, 146, 570. (7) Sasaki, K.; Koshioka, M.; Masuhara, H. Appl. Spectrosc. 1991, 45 1041. Koshioka, M.; Misawa, H.; Sasaki, K.; Kitamura, N.; Masuhara, H. Chem. Lett. 1991, 469. Sasaki, K. Proc. SPIE-Opt. Eng. 1992, 117, 99 (High-Performance Optical Spectrometry). Masuhara, H. J. Photochem. Photobiol., A 1992, 62, 397. Masuhara, H.; Kitamura, N.; Misawa, H.; Sasaki, K.; Koshioka, M. J. Photochem. Photobiol. A: Chem. 1992, 65, 235. Sasaki, K.; Kashioka, M. Microchemistry: Spectroscopy and Chemistry in Small Domain; Masuhara, H., De Schryver, F. C., Kitamura, N., Tamai, N., Eds.; Elsevier Science B. V: New York, 1994; pp 185-196. Sasaki, K.; Kamada, K.; Masuhara, H. Jpn. J. Appl. Phys. 1994, 33, 1413. Masuhara, H.; Sasaki, K. Anal. Chim. Acta 1995, 299, 309. (8) Johnson, G. E. J. Chem. Phys. 1975, 62, 4697. (9) Itaya, A.; Okamoto, K-i.; Kusabayashi, S. Bull. Chem. Soc. Jpn. 1976, 49, 2082. Masuhara, H.; Tamai, N.; Mataga, N.; De Schryver, F. C.; Vandendriesche, J. J. Am. Chem. Soc. 1983, 105, 7256. (10) Sakai, H.; Itaya, A.; Masuhara, H.; Sasaki K.; Kawata, S. Chem. Phys. Lett. 1993, 208, 283. Sakai, H.; Itaya, A.; Masuhara, H.; Sasaki, K.; Kawata, S. Polymer 1996, 37, 31. (11) Masuhara, H.; Ohwada, S.; Mataga, N.; Itaya, A.; Okamoto, K-i.; Kusabayashi, S. J. Phys. Chem. 1980, 84, 2363.
