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J. Phys. Chem. B 2006, 110, 17906-17911
Two-Photon Fluorescence Spectroscopy of Individually Trapped Pseudoisocyanine J-Aggregates in Aqueous Solution Yoshito Tanaka, Hiroyuki Yoshikawa,* and Hiroshi Masuhara* Department of Applied Physics and Handai Frontier Research Center, Osaka UniVersity, Suita, Osaka 565-0871, Japan ReceiVed: May 23, 2006; In Final Form: July 24, 2006
We have investigated a pseudoisocyanine dye aqueous solution including nanometer-sized J-aggregates by combining optical trapping and two-photon fluorescence spectroscopy. By focusing an intense near-infrared laser into an 8 × 10-3 M solution, the intense fluorescence from J-aggregates for a few to tens of seconds is observed intermittently, indicating that individual J-aggregates are trapped in and diffuse out from a focal spot. The peak position and full width at half-maximum of the J-band are different from each other. By measuring 171 J-aggregates, it was found that J-aggregates can be classified largely into two groups. The existence of two kinds of groups of J-aggregates could be attributed to the difference in the nucleation process, which is affected by the substrate. J-aggregates possessing a J-band of a narrower bandwidth in a shorter wavelength region are trapped for a longer period of time, indicating that highly ordered J-aggregates are trapped for a longer period of time because of their high polarizability.
Introduction Recently, self-assembly of small molecules into supramolecular aggregates by using noncovalent interactions has received much attention, because it is an elegant “bottom-up” method for fabricating nanostructured materials. These aggregates often show high functions, originating from their structures, which are quite different from those of isolated molecules or crystals. In this work we studied J-aggregates of 1,1′-diethyl-2,2′cyanine chloride in aqueous solution, commonly known as pseudoisocyanine (PIC). PIC is the most investigated cyanine dye forming J-aggregates, which have attracted considerable attention because of their high nonlinear optical properties, ultrafast optical response, and large molecular hyperpolarizability arising from the actions of the aligned molecules.1 Practical applications based on these characteristics such as nonlinear optical devices, photoelectric cells, and so on have recently been discussed.2-4 The formation of J-aggregates can be characterized by an intense narrow absorption band which is red-shifted with respect to the absorption spectrum of the monomer. Intermolecular interaction between their large transition moments causes the coherent delocalization of excitons over the J-aggregate, and the nonlinear optical property due to the delocalized exciton is expected to be enhanced. Furthermore, the spectral shift was explained by the formation of an excitonic state through the electronic coupling of the tightly packed dye molecules.5 However, with its propensity toward aggregation, PIC in solution can be found in many molecular arrangements such as a dimer,6 a crystal,7,8 and some kind of J-aggregate.9 In many cases, studies of J-aggregates have been conducted for an ensemble of many nanometer-sized J-aggregates, so that distributions in size, molecular arrangement, defects, and so on are not considered directly.10-12 In general, condensed-phase * To whom correspondence should be addressed. Phone: +81-6-68797839. Fax: +81-6-6879-7840. E-mail:
[email protected] (H.Y.),
[email protected] (H.M.).
optical spectra, measured for an ensemble, can be difficult to interpret due to inhomogeneous broadening. It is indispensable for various fields of nanoscience and nanotechnology to develop some methods for spectroscopically investigating the nanometersized objects one by one from an ensemble. Recently singlemolecule or single-particle spectroscopy has been done by dispersing individual molecules or particles on a substrate,13 but it is difficult to apply this method to molecular aggregates deposited in solution, because further aggregation proceeds during solvent evaporation after the solution is dropped on a glass substrate. In other words, molecular aggregates are in equilibrium with monomers in solution and cannot be taken from the solution to the substrate without any interaction with the latter. Therefore, other methods to investigate individual molecular aggregates formed in solution are desired. Considering this background, we propose optical trapping as the technique to take molecular aggregates one by one from an ensemble. Optical trapping by a focused laser beam has been developed into a noncontact and nondestructive manipulation technique for small particles in solution.14,15 A nanoparticle, which can be regarded as an electric dipole, experiences an attractive force in the center of the laser focus where the intensity of the electromagnetic field is at a maximum (i.e., the potential energy is at a minimum). In the present study, we introduced the optical trapping technique into the two-photon fluorescence spectroscopy of J-aggregates in solution. The two-photon excited fluorescence spectrum of an individual nanometer-sized J-aggregate could be obtained by holding it for a while in the laser focus by using the optical trapping technique. Experimental Section 1,1′-Diethyl-2,2′-cyanine chloride (Hayashibara Research Institute for Photosensitizing Dyes), whose chemical structure and molecular size are shown in Figure 1a, was used as received. PIC was dissolved in distilled water. The solution was stirred at room temperature for more than 24 h to fully dissolve the dye.
10.1021/jp063169t CCC: $33.50 © 2006 American Chemical Society Published on Web 08/18/2006
Fluorescence Spectroscopy of PIC J-Aggregates
Figure 1. (a) Molecular structure of PIC chloride. (b) A schematic diagram of the experimental setup. (c) Dependence of the fluorescence intensity on the NIR laser power for the PIC J-aggregates. The solid line indicates the fitting curve (Ifluor ∝ P2).
The experimental setup is described in Figure 1b. A 1064 nm near-infrared (NIR) laser beam from a continuous-wave (CW) Nd3+:YAG laser (Spectron Laser Systems, SL902T 1104) was used as an optical trapping light source. This laser beam was introduced to an inverted microscope (Carl ZEISS, Axiovert 200), and focused into a sample solution by an oil immersion objective lens (100× magnification, numerical aperture 1.30). The spot shape and size were almost a spheroid and ∼1.5 µm on the semimajor axis and ∼500 nm on the semiminor axis, respectively. J-aggregates are trapped in this volume and excited. A thin layer of the solution was prepared by putting it between two glasses or plastic (polyethylene) plates with a silicon rubber spacer of 1 mm thickness. As the laser intensity necessary for trapping nanometer-sized J-aggregates is so high (350 mW), the dyes absorb the NIR laser light via a two-photon excitation process, and then fluorescence from the focal spot can be concurrently detected with the same trapping laser beam. Figure 1c shows the laser power dependence of the fluorescence intensity of J-aggregates measured by focusing the NIR laser, demonstrating that this fluorescence actually occurs via a twophoton excitation process. Besides the NIR laser beam, a 532 nm visible laser beam from a diode-pumped solid-state laser (JDS Uniphase, 4611-020-1000, hereafter “green laser”) was also used as an excitation light source. The green laser beam was focused to a ∼300 nm spot size. Both focal points of green and NIR lasers were set at the same position, which is inside the solution and 200 µm from the surface of the coverslip. Particularly in the case of the NIR laser, this experimental condition allows us to measure the fluorescence of materials trapped in the laser focus without interference from the background fluorescence, which may come from the surroundings of the laser focus and molecules adsorbed on the coverslip. The
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17907
Figure 2. (a) Absorption and (b) fluorescence spectra (λex ) 532 nm) of PIC aqueous solutions at various concentrations: (solid black line) 0.015 mM, (fat dotted line) 6 mM, (solid gray line) 8 mM, (dotted line) 10 mM.
fluorescence spectrum was measured by a polychromator (Acton Research, Spectra Pro 300i) with a charge-coupled device (CCD) camera (Roper Scientific, PI-MAX1024HG18). Transmitted images were also observed by a high-sensitivity color CCD video camera (Flovel, HCC-600). Conventional absorption spectroscopy was performed with a spectrophotometer (Shimadzu, UV-3100PC). An optical cell with a 1 mm path length was used for the dilute sample, whose concentration was less than 1 mM. Concentrated samples (>1 mM) were measured by putting it between two glass plates, with an aluminum foil being used as a thin spacer (∼25 µm). Results and Discussion Figure 2a shows a set of absorption spectra of PIC aqueous solutions at various concentrations, which were measured with unpolarized light. The variation of the absorption spectrum with increasing concentration reflects the formation of aggregates. The most diluted one (C ) 0.015 mM) shows the well-known PIC monomer absorption spectrum, with vibronic maxima at 525 and 492 nm and a shoulder at shorter wavelength. With increasing concentration, e.g., 6 mM, a peak around 482 nm becomes distinct and the band at 525 nm slightly shifts to a long wavelength. These changes are ascribed to dimer formation.6,16 It is reported that the dimer spectrum is known to consist of a J-component on the longer wavelength side and an H-component on the shorter wavelength side of the original monomer absorption band. However, the J-component band of the dimer spectrum is not clear due to superposition on the monomer band at 525 nm. At dye concentrations exceeding
17908 J. Phys. Chem. B, Vol. 110, No. 36, 2006 8 mM, an intense and very narrow (∆ν ≈ 165 cm-1) absorption band appears at longer wavelength (572 nm) compared to the monomer band, demonstrating the formation of J-aggregates. The 8 mM PIC solution was still fluid at room temperature, but PIC solutions above 10 mM became very viscous and hardly flowed. Thus, the viscosity of the PIC solutions drastically increased within the narrow concentration range from 8 to 10 mM at room temperature. It has been reported that gelation arises when J-aggregates explosively grow and form an entanglement structure or permanent cross-linking points in a high-concentration solution.17 The bandwidth and peak wavelength of the J-band were not different between 8 and 10 mM, as shown in Figure 2a. In general, they are eventually attributed to the exciton delocalization length (or, in other words, coherent length of the exciton), rather than the total number of PIC molecules constituting the J-aggregates.18,19 This delocalization length is determined by the degree of order or molecular packing arrangement in J-aggregates.20 This means that, although the geometrical length of J-aggregates grew with increasing concentration, the delocalization length did not change. The fluorescence spectra (Figure 2b) also exhibit a spectral change similar to that of the absorption spectra following an increase of concentration. With increasing concentration from 0.015 mM, the fluorescence band shows a red-shift, which may be attributed to the dimer formation in the aqueous PIC solution. At concentrations higher than 8 mM, the J-aggregates show a narrow fluorescence band at 573 nm with an extremely small Stokes shift from the absorption peak. The fluorescence bandwidth and peak wavelength also did not change in the concentration range between 8 and 10 mM. We consider the physical size of J-aggregates dispersed in the 8 mM PIC-Cl solution. Although the 8 mM PIC solution shows clear J-bands in absorption and fluorescence spectra, transmission images observed via a 100× objective lens did not show visible aggregates. This means that the size of the J-aggregates is beyond the diffraction limit of the optical microscope, so that it is smaller than half of the light wavelength. Since the size of the PIC molecule is close to 1 nm, the J-aggregate is most likely to be on the order of nanometers in size. It should be noted that the size cannot be measured by electron and probe microscopy such as SEM, TEM, and AFM, because J-aggregates in solution exist in dynamic equilibrium with monomers, dimers, H-aggregates, other J-aggregates, and so on. For electron and probe microscopy, J-aggregates must be put on a substrate. However, once the solution is dropped on a substrate, the dynamic equilibrium changes according to condensation, and eventually the substrate is covered by a large amount of aggregates. In this substrate, we never identify J-aggregates dispersed in the original solution. Therefore, it is important to develop an experimental technique to investigate individual molecular aggregates in solution. By focusing the green laser in these PIC solutions, we measured 230 fluorescence spectra successively every 1 s as demonstrated in Figure 3. Consistent with Figure 2b, parts a and b of Figure 3 show that the 6 mM PIC solution has broad monomer and dimer emission and the 8 mM solution has a clear J-band at 573 ( 0.4 nm with a full width at half-maximum (fwhm) of ∼330 cm-1. The fluorescence spectral shape and intensity were constant during the measurement. Thus, both solutions were confirmed to be homogeneous, although the excitation green laser is tightly focused into the solution. Parts c and d of Figure 3 show fluorescence spectra measured with the NIR laser irradiation instead of the green laser. By focusing an intense NIR laser, fluorescence from J-aggregates
Tanaka et al.
Figure 3. Temporal variation of the fluorescence spectra of PIC aqueous solutions due to one-photon excitation with a green laser and two-photon excitation with a NIR laser: (a) 6 mM, (b) 8 mM, green laser irradiation; (c) 6 mM, (d) 8 mM, NIR laser irradiation. The 230 fluorescence spectra were measured with a 1 s exposure time per spectrum. The powers of the green laser and NIR laser were 25 nW and 350 mW, respectively.
can be detected, which is due to NIR two-photon absorption because of its extremely high intensity. Temporal variation of the fluorescence spectrum of the 8 mM PIC solution appears, suggesting inhomogeneity. Figure 4a shows one example of the temporal intensity profiles of the fluorescence spectra shown in Figure 3d. Since the NIR laser power is much stronger than the green laser power by a factor of 108, it is expected that the photon pressure would be exerted on the nanoaggregates, which have a higher refractive index than the surrounding medium. Furthermore, the dyes should absorb the NIR laser light via a two-photon excitation process, since fluorescence from the focal spot can be concurrently detected by using the same NIR laser. In general, the two-photon excited fluorescence intensity is proportional to the square of the excitation laser power; thus, the effective excitation region is restricted to the central portion of the focal spot, allowing only nanoaggregates trapped at the focal spot to be detected as demonstrated in ref 21. It was confirmed that the fluorescence spectrum of the PIC J-aggregates obtained by two-photon excitation is essentially identical to that by one-photon excitation in the former works on fluorescence spectroscopy of J-aggregates.22,23
Fluorescence Spectroscopy of PIC J-Aggregates
Figure 4. (a) Time profile of the fluorescence intensity. (b) Spectra of trapped PIC aggregates in 8 mM solution due to two-photon excitation with an NIR laser. The spectra labeled A, B, and C correspond to peaks in the time profile of fluorescence intensity. The inset shows the spectrum in the off state in the time profile.
As shown in Figure 3c, the fluorescence intensity of the 6 mM solution was very weak with no increment during NIR laser irradiation. On the other hand, time variation of the fluorescence spectra and intensity of the 8 mM solution indicated an on-off fashion, where the continuous intense fluorescence for a few to tens of seconds was observed intermittently, as shown in Figures 3d and 4a. This phenomenon can be explained as follows. Occasionally J-aggregates cross the NIR laser focus and emit fluorescence by two-photon absorption of the intense NIR laser beam. Since J-aggregates are of nanometer size as mentioned above, their diffusion time needed to cross the focal spot should be on the microsecond time scale at most. However, intense fluorescence continues for a few to a few tens of seconds as in Figure 4a. That is, J-aggregates stay in the focal spot for a long time, indicating that this intermittent intense fluorescence is due to optical trapping of J-aggregates. It should be noted that the increase and decrease in an on-off fashion of the fluorescence were done in one step. This strongly suggests that these fluorescences come from individual J-aggregates trapped in the laser focus, because fluorescence must gradually increase and decrease for optical trapping of multiple J-aggregates, as proved in our former study.24 In the case of a PIC solution exceeding 10 mM, the solution was hardly fluid, so that the fluorescence intensity gradually decreased during laser irradiation due to photobleaching (data not shown here). We have succeeded in trapping a single J-aggregate and measuring its fluorescence spectrum in the PIC solution at 8 mM. Figure 4b shows the fluorescence spectrum of each trapped J-aggregate observed in the series of spectral measurements of the 8 mM solution (Figure 3d). Three spectra, A, B, and C, correspond to peaks in the temporal profile of Figure 4a. The spectra are slightly different from one another, while
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17909
Figure 5. (a) A histogram where the number of trapped J-aggregates observed under the same conditions as for Figure 4 is plotted as a function of their peak wavelength. (b) A relation of the full width at half-maximum of the fluorescence band of trapped J-aggregates to their peak wavelength. The closed square indicates the datum obtained by green laser excitation. Gray solid lines indicate two approximate curves of the relation between fwhm and peak wavelength, one in the peak wavelength region shorter than 580 nm and another in the peak wavelength region longer than that.
all of them exhibit a narrow intense fluorescence band which characterizes J-aggregates. The difference in the peak position and fwhm of the fluorescence band must be attributed to a difference in the electronic structure of the individual J-aggregates. When J-aggregates are not trapped in the focal spot, i.e., in the off state in Figure 4a, the fluorescence spectrum exhibits a very weak broad dimer band, as shown in the inset of Figure 4b. We measured 171 fluorescence spectra of trapped Jaggregates in the 8 mM solution and analyzed their width and peak wavelength. The peak wavelength (λmax) distribution of the trapped J-aggregates is summarized as a histogram in Figure 5a. It seems to be composed of two distributions around 572 and 580 nm. This histogram indicates that mainly two kinds of J-aggregates exist in the 8 mM solution. This is supported by correlations between the fwhm and the peak wavelength of the J-aggregate fluorescence band, as shown in Figure 5b. This also clearly shows that trapped J-aggregates can be classified into two groups; one has a J-band in the wavelength region shorter than 580 nm and the other a J-band in the wavelength region longer than that. In each group, the J-band in the shorter wavelength region has a narrower bandwidth. Generally, a blue shift and narrowing of the J-bands are observed upon cooling the PIC solution with liquid nitrogen, which is interpreted as an increase in the exciton delocalization and a decrease in the disorder in the J-aggregates.25-28 Here, we abbreviate the former J-aggregates whose fluorescence peak is between 571 and 573 nm as Ja and those whose fluorescence peak is between
17910 J. Phys. Chem. B, Vol. 110, No. 36, 2006
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Figure 6. A histogram where the number of trapped J-aggregates is plotted as a function of their peak wavelength. Different from Figure 5a, a thin layer of the PIC solution was sandwiched between two plastic plates instead of glass plates.
580 and 583 nm as Jb. Figure 5 indicates that Ja-aggregates can take a higher degree of exciton delocalization, and/or a more closely stacked structure, than Jb-aggregates, since the fwhm of the Ja-band is narrower than that of the Jb-band. The fluorescence spectrum obtained by conventional spectroscopy is constructed by the spectral sum of many J-aggregates, whose peak wavelength and bandwidth are given in Figure 5. It is demonstrated that the optical trapping technique is useful for discriminating different kinds of J-aggregates. One possible explanation for the existence of two kinds of J-aggregates could be ascribed to the difference in nucleation. Previously, it was reported that J-aggregates in the water phase and in the vicinity of the soda lime glass have different fluorescence spectra with peaks at ∼570 and ∼580 nm, respectively.10 Such duplicity of the J-aggregate spectrum in aqueous solution was reported also by other research groups.11,12 Results of these former studies imply that J-aggregates deposited from the water-glass interface have a fluorescence peak around 580 nm, and some of those desorbed from a glass surface are occasionally trapped. To confirm this assumption, we have reproduced the optical trapping experiment by using a plastic plate instead of a glass one. Of course, the plastic does not contain exchangeable ions such as Na+ as in the glass, so that the inhomogeous nucleation could be eliminated on the plastic plate. Figure 6 shows a histogram showing the number of trapped J-aggregates as a function of the fluorescence peak wavelength. Jb-aggregates whose fluorescence peak is ∼580 nm obviously disappeared by using plastic plates. This strongly supports our assumption that Jb-aggregates are formed on the glass plates through inhomogeneous nucleation and dispersed in the solution. Apart from fluorescence spectroscopy of individual Jaggregates, optical trapping gives other interesting information. Parts a and b of Figure 7 show histograms in which trapped J-aggregates are classified by the trapping duration, τtrap, which is defined as the period during which the intense two-photon excited fluorescence is continuously detected as shown in Figure 4. Each histogram is mostly composed of one distribution, and the τtrap distribution of Ja-aggregates is clearly different from that of Jb-aggregates. Ja-aggregates are trapped for a longer period of time than Jb-aggregates, as the average τtrap values of Ja- and Jb-aggregates are 12.3 and 3.8 s, respectively. Here we consider the relation between τtrap and the optical trapping potential. In the present experiment, even if one aggregate is trapped in the focal spot, it escapes before the next one is trapped. Since the free Brownian motion of the aggregates is
Figure 7. Histograms where the numbers of trapped (a) Ja-aggregates with the peak between 571 and 573 nm and (b) Jb-aggregates with the peak between 580 and 583 nm are plotted as a function of their trapping duration, τtrap. τtrap is defined as the period during which the intense two-photon excited fluorescence is continuously detected.
biased by the small trapping potential, Utrap, the aggregates stay in the focal spot for a little longer time than in other places. Under such conditions, τtrap can be expressed by the following equation:29
( )
τtrap = τ0 exp -
Utrap kBT
(1)
where τ0 is the transit time of the aggregates in the laser focal spot without optical trapping potential, which is given by τ0 ) w02/(24D), where w0 is the beam waist and D is the diffusion coefficient. Here it should be noted that τtrap increases exponentially with the trapping potential depth, -Utrap. The trapping potential, Utrap, is described by using an electric field of light E and the polarizability of the aggregate, R, as follows:
1 R I Utrap = - RE 2 ) 2 nm0c
(2)
where nm is the refractive index of the surrounding medium, 0 and c are the dielectric constant and the velocity of light in a vacuum, respectively, and I is the laser intensity with a Gaussian beam profile. The trapping potential depth, -Utrap, increases linearly with increasing R. When the laser intensity, I, is fixed, the aggregate possessing larger polarizability is trapped for a longer period of time. Therefore, in the present experiment, it is explicit that R of Ja-aggregates is larger than that of Jb-aggregates. R is given by the following equation in classical electromagnetic theory:30
R ) 3nm0V
n2 - nm2 n2 + 2nm2
(3)
Fluorescence Spectroscopy of PIC J-Aggregates where V is the volume and n is the refractive index of the aggregate. This equation suggests that Ja-aggregates would have a higher refractive index assuming the same size for Ja- and Jb-aggregates or larger volume assuming the same refractive index. Of course, it is possible that they would have a higher refractive index and a larger volume. Since Ja-aggregates have highly delocalized excitons as compared to Jb-aggregates, which is deduced from the spectroscopic data, it is reasonable that Ja-aggregates have a higher refractive index than Jb-aggregates. Such a highly ordered structure may be energetically favored in aqueous solution, so that the volume of Ja-aggregates would be larger than that of Jb-aggregates. To estimate the volume of J-aggregates in this aqueous solution, dynamic light scattering (DLS) measurement was tried, but the signal could not be obtained. This suggests that the size of the J-aggregates is too small or the concentration is too low to be analyzed by DLS. Therefore, we cannot determine clearly the origin of differences in polarizability at the present stage. However, it is noteworthy that we could discuss the fluorescence spectral shape, duality of the structure, and polarizability of J-aggregates formed in aqueous solution by applying the optical trapping technique. Conclusion J-aggregates deposited in a PIC aqueous solution have been investigated by combining optical trapping and two-photon fluorescence spectroscopy. When an intense near-infrared laser is focused into the 8 mM PIC solution, the fluorescence, whose duration is between a few seconds and tens of seconds, was observed intermittently, indicating that individual J-aggregates were trapped in and diffused out from a focal spot. They showed the fluorescence bands characteristics of J-aggregates, and their peak wavelength and bandwidth showed two kinds of Jaggregates, which is also consistent with the optical trapping duration. We have demonstrated by a control experiment that the difference between the two types comes from the nucleation process. This work shows that the combination of optical trapping and two-photon fluorescence spectroscopy is very useful for the investigation of a solution including several nanometer-sized objects, which can be difficult to interpret by conventional spectroscopy due to inhomogeneous broadening. Particularly, the optical trapping duration, dependent on the polarizability of the trapped objects, supplies information regarding the volume or the refractive index of the trapped objects. In addition, optical trapping will develop into a selective assembly technique of the objects possessing a larger polarizability. Acknowledgment. This research was supported (financially) in part by a Grant-in-Aid for Scientific Research (S) (14103006) from the Japan Society for the Promotion of Science.
J. Phys. Chem. B, Vol. 110, No. 36, 2006 17911 References and Notes (1) Mishra, A.; Behera, R. K.; Mishra, B. K.; Behera, G. B. Chem. ReV. 2000, 100, 1973. (2) Meng, F.; Chen, K.; Tian, H.; Zuppiroli, L.; Nuesch, F. Appl. Phys. Lett. 2003, 82, 3788. (3) Tamaoki, N.; Keuren, E. V.; Matsuda, H.; Hasegawa, K.; Yamaoka, T. Appl. Phys. Lett. 1996, 69, 1188. (4) Dautel, O. J.; Wantz, G.; Almairac, R.; Flot, D.; Hirsch, L.; LerePorte, J.-P.; Parneix, J.-P.; Serein-Spirau, F.; Vignau, L.; Moreau, J. J. E. J. Am. Chem. Soc. 2006, 128, 4892. (5) Higgins, D. A.; Reid, P. J.; Barbara, P. F. J. Phys. Chem. 1996, 100, 1174. (6) Kopainsky, B.; Hallermeier, J. K.; Kaiser, W. Chem. Phys. Lett. 1981, 83, 498. (7) Vanden Bout, D. A.; Kerimo, J.; Higgins, D. A.; Barbara, P. F. J. Phys. Chem. 1996, 100, 11843. (8) von Berlepsch, H.; Moller, S.; Dahne, L. J. Phys. Chem. B 2001, 105, 5689. (9) Fidder, H.; Terpstra, J.; Wiersma, D. A. J. Chem. Phys. 1993, 94, 6895. (10) Yao, H.; Ikeda, H.; Kitamura, N. J. Phys. Chem. B 1998, 102, 7691. (11) Struganova, I.; Hazell, M.; Gaitor, J.; MacNally-Carr, D.; Zivanovic, S. J. Phys. Chem. A 2003, 107, 2650. (12) Yu, Z. X.; Lu, P. Y.; Alfano, R. R. Chem. Phys. 1983, 79, 289. (13) Single-Molecule Optical Detection, Imaging and Spectroscopy; Basche, T., Moerner, W. E., Orrit, M., Wild, U. P., Eds.; VCH: Weinheim, Germany, 1997. Single Organic Nanoparticles; Masuhara, H., Nakanishi, H., Sasaki, K., Eds.; Nanoscience and Technology; Springer: New York, 2003. (14) Sasaki, K.; Koshioka, M.; Misawa, H.; Kitamura, N.; Masuhara, H. Appl. Phys. Lett. 1993, 60, 807. (15) Grier, D. G. Nature (London) 2003, 424, 810. (16) Neumann, B. J. Phys. Chem. B 2001, 105, 8268. (17) von Berlepsch, H.; Bottcher, C.; Dahne, L. J. Phys. Chem. B 2000, 104, 8792. (18) Knoester, J. Chem. Phys. Lett. 1993, 203, 371. (19) Malyshev, V. J. Lumin. 1993, 55, 225. (20) Markov, R. V.; Plekhanov, A. I.; Shelkovnikov, V. V.; Knoester, J. Microelectron. Eng. 2003, 69, 528. (21) Florin, E. L.; Horber, J. K. H.; Stelzer, E. H. K. Appl. Phys. Lett. 1996, 69, 446. (22) Sanchez, E. J.; Novotny, L. Xie, X. S. Phys. ReV. Lett. 1999, 82, 4014. (23) Belfield, K. D.; Bondar, M. V.; Hernandez, F. E.; Przhonska, O. V.; Yao, S. Chem. Phys. 2006, 320, 118. (24) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Phys. ReV. E 2004, 70, 061410. (25) Renge, I.; Wild, U. P. J. Phys. Chem. A 1997, 101, 7977. (26) Shelkovnikov, V. V.; Markov, R. V.; Plekhanov, A. I.; Simanchuk, A. E.; Ivanova, Z. M. High Energy Chem. 2002, 36, 260. (27) Vacha, M.; Furuki, M.; Tani, T. J. Phys. Chem. B 1998, 102, 1916. (28) Muenter, A. A.; Brumbaugh, D. V.; Apolito, J.; Horn, L. A.; Spano, F. C.; Mukamel, S. J. Phys. Chem. B 1992, 96, 2783. (29) Svoboda, K.; Block, S. M. Opt. Lett. 1994, 19, 930. (30) Yoshikawa, H.; Matsui, T.; Masuhara, H. Phys. ReV. E 2004, 70, 061406.
