J. Phys. Chem. B 2004, 108, 10887-10892
10887
Enhanced Fluorescent Emission of Organic Nanoparticles of an Intramolecular Proton Transfer Compound and Spontaneous Formation of One-Dimensional Nanostructures Shayu Li,† Liming He,† Fei Xiong,† Yi Li,*,‡ and Guoqiang Yang*,† Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, P. R. China ReceiVed: March 18, 2004; In Final Form: April 18, 2004
Nanoparticles of intramolecular proton transfer molecule N,N′-bis(salicylidene)-p-phenylenediamine have been prepared with the reprecipitation method in water. The evolving processes have been monitored by means of UV-vis absorption, fluorescence emission, transient emission spectroscopy, and scanning electron microscopy. The fluorescent intensity is increased beyond 60 times in the nanoparticles compared to that in solution. With the increase of aging time, nanoparticles convert spontaneously from spherical to rodlike and finally to beltlike aggregates. A mechanism for the enhanced emission is proposed on the basis of flattened molecules, and a possible model for change of the nanoparticles’ shape is also proposed on the basis of theoretical calculations.
Introduction There is considerable interest regarding the possible favorable effects on optoelectronic devices by luminescent nanoparticles applied. Over the past decade, many kinds of luminescent nanoparticles have been prepared and investigated for various potential applications including light-emitting devices,1 photovoltaic cells,1c,2 tagging applications,3 biological nanosensors,4 etc. In these applications, numerous attempts to generate inorganic semiconductor and metal nanoparticles have been discovered, but there are only a few and latter-day approaches to organic nanostructures.5 Compared to inorganic nanoparticles, organic nanoparticles have their own advantages and applicable potentials because they allow much more variety and flexibility in materials synthesis. The methods for preparation of organic nanoparticles included emulsification evaporation, emulsification diffusion, and solvent displacement methods.6 Reprecipitation, one kind of solvent displacement method, becomes a popular technique7 for the preparation of the nanoparticles due to its easy and versatile operation. Some studies changed conditions, including temperature, precipitation time, adding template, and adding stabilizer to acquire organic nanoparticles with different structures, various shapes, and diverse sizes and investigated their unique or more applicable properties as compared to monomer or bulk materials.5 It is well-known that properties of many nanoscaled aggregates (quantum dots, rods, tubes and wires) have unique size dependence and are substantially different from those bulk materials composed of the same atoms or molecules.6 Regarding luminescent organic nanoparticles, recently several articles reported that they show different and size dependent emission and absorption.5,7 So far, it is not very clear yet on the formation process for organic nanoparticles, especially in the initial stage. Attracted by the well applicable possibility of the luminescent organic nanoparticles on optoelectronic devices and to qualitatively probe the evolving process of the nanoparticles, we * To whom correspondence should be addressed. E-mail: G.Y.,
[email protected]; Y.L.,
[email protected]. † Institute of Chemistry. ‡ Technical Institute of Physics and Chemistry.
chose fluorescent organic material N,N′-bis(salicylidene)-pphenylenediamine (p-BSP) as our object compound. p-BSP is one of salicylideneaniline (SA) type and a symmetric aromatic Schiff base molecule from its structural formula. But in reality, environments that surround molecules influence its symmetry intensively. Such as in a polar solvent, it is asymmetrical.8 In the crystal state, the p-BSP were reported and studied for its thermochromic properties.9 In solution (liquid and solid), its photochromic properties were widely studied.10 The molecular mechanisms of the thermo- and photochromism of p-BSP are essentially uniform, on the basis of intramolecular proton transfer processes.11 p-BSP as a model compound of intramolecular proton transfer have also been deeply studied by femtosecond time-resolved laser spectroscopy,11,12 soft X-ray spectroscopy,13 and theory computation,14 etc. It is interesting that p-BSP itself is low fluorescent in dilute solution, but its emission in crystal is extremely strong. As known, these are different from most organic chromophores that show higher luminescence efficiency in solution and decreased efficiency in the solid state due to their intermolecular interactions that induce the nonradiative deactivation process.7b,18 In analogy to other organic and inorganic nanoparticles, we expected that nanoparticles of p-BSP would have special photophysics properties that are not obtainable by the monomer in solution or by the bulk type of the molecules. Reprecipitation is rapidly injecting microamounts of the solution, a dye in a good solvent, into macroamounts of poor solvent. In this process, sudden changes of environment for organic molecules induce precipitation. For p-BSP, its photophysical properties vary a lot when the molecular conformation is transferred, induced by the environmental changing.11 It may help us to probe the evolving process of p-BSP nanoparticles. In this paper, we report the preparation of p-BSP nanoparticles, the photophysical behavior of these nanoparticles, and their evolving process. It is found that p-BSP nanoparticles have no photochromic properties and their emission intensity is much stronger than that in dilute solution with equal concentration. Additionally, we observe the evolving process of the nanoparticles that are growing spontaneously from a sphere with a
10.1021/jp0488012 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004
10888 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Li et al.
diameter of about 10 nm to a 1D belt with about 2000 nm long and 100 nm wide. Experimental Section Materials. The crystals of p-BSP were prepared according to the methods described in ref 15. Methanol and tetrahydrofuran (THF) were obtained from DIKMA (HPLC grade) and used without further treatment. Particles Synthesis. A 100 µL solution of p-BSP in THF (1.0 × 10-3 mol/L) was rapidly injected into 10 mL of water (18 MΩ) with vigorous stirring at 302 K in a thermostated cell (this process is temperature-sensitive), then stirring for 1 min for complete solvent mixing, and standing for ripeness. SEM Samples. A drop of the nanoparticle solution with variable evolving time was placed onto a silicon plate and allowed to evaporate the solvent. This process for every SEM sample was restricted in 1 min. Then the sample was covered with platinum by a vacuum coat. Particle Characterization. The UV-visible absorption spectra of p-BSP nanoparticle dispersions in water and p-BSPmethanol solution were recorded using a Hitachi U-3010 spectrophotometer. The steady state emission fluorescence spectra were measured with an excitation wavelength at 383 nm by a Hitachi F-4500 spectrometer. Fluorescence quantum yields (Φf) were determined with the excitation wavelength at 420 nm, using a standard of 0.95 for fluorescein.16 Timeresolved fluorescence measurements were carried out using a time-correlated single-photon counting (TCSPC) spectrometer from Edinburgh Instruments Co. A pulsed diode laser (500 ps fwhm and 2.5 MHz frequency) was used to excite the sample. Fluorescence was collected into a monochromator with an MCPPMT (E3059-500, Hamamatsu) for detection and the maximum count was 5000. The spectrum measurements of every sample were limited in 2 min. The sizes and shapes of the nanoparticles were observed by means of cold field emission scanning electron microscopes (FESEM, Hitachi S-4300). Computational Methods. The structures used for calculations were obtained from crystal data in ref 15. Single point energy of one and two molecules were calculated using the BLYP/6-31G method in the Gaussian 98 suite of programs.17 Results Scanning Electron Microscopy (SEM) Data. Figure 1 shows the SEM images of particles deposited onto silica wafers from the solution at different aging time. When the aging time is zero, we cannot get an image of the aggregate at the nanometer scale. Considering the size of platinum particles is about 2-3 nm, it may suggest that the size of congeries for p-BSP molecules at 0 min is less than 2 nm. When sample is ripened about 15 min, small spherical particles (D ) 10-15 nm) have been observed. Particles evolve to bigger with diameters 50 nm when the aging time is 30 min. The image of nanoparticles with 60 min aging shows some rodlike structures with lengths up to 150-200 nm and diameters 50 nm and some spherical particles in analogy to the sample at 30 min. These nanorod and nanosphere all become blocklike structures with different sizes when the aging time reaches 180 min. The nanoparticles with the longest aging time (6 h) in our prepared samples become 1D aggregates (L ) 500-2000 nm, W ) 100200 nm). It is noticed that dispersion of small spherical particles is narrow, whereas that of large size beltlike structures is reverse. In addition, we found that 1D particles undergo further aggregation and finally precipitate from solution after a few days and the solution is colorless.
Figure 1. SEM images of p-BSP nanoparticles at different aging times: (a) 15 min; (b) 30 min; (c) 60 min; (d) 180 min; (e) 6 h.
UV-Vis Spectroscopy Data. p-BSP is an orange crystal at room temperature. It is soluble in conventional organic solvents such as methanol, acetonitrile, toluene, ethyl acetate, tetrahydrofuran, etc., and in these solvents, absorption and fluorescence spectra are analogous or identical. Its dilute methanol solution was prepared as a reference, and this solution was yellow under natural light. Nanoparticle solutions at various aging times were also yellow but deeper than the methanol solution. Figure 2 shows the UV-vis absorption spectra of the p-BSP dilute solution in methanol (1.0 × 10-5 mol/L) and the p-BSP nanoparticles dispersion in water (molecular concentration 1.0 × 10-5 mol/L) at various aging times. In methanol, p-BSP is molecularly dissolved at this concentration. It has a single absorption peak with maximum at 370 nm which was assigned to the π-π* transition. The absorption of p-BSP nanoparticles is red-shifted from that in the dilute solution. They are characterized as one maximum that is at 383 nm and three shoulders are at 365, 402, and 426 nm, respectively. These multipeaks could also be observed in the excitation spectrum (see Figure 3b), so they would be assigned as the absorption bands of J-aggregates (see the discussion later). Meanwhile, with the increase of aging time, the baseline of the absorption at longer wavelength gets higher, implying that the scattering from the formation of larger particles could also be detected at the longer aging times. Before 180 min and with the aging time increasing, changes of absorption intensity for two local maxima at shorter wavelengths are minimal. However, at the long wavelength band, two shoulders become much stronger and their shapes are from shoulder to peak. This process is accompanied by a slight red shift. After 180 min, changes of the absorption spectra become much more significant. The absorption intensity of the sample that is ripened for 6 h decreased from that of earlier intensities but the absorption tail becomes stronger. At the moment, the solution exhibited buff turbidity under natural light due to the light scattering of the nanoparticles. Emission Spectra. The fluorescence spectra of p-BSP nanoparticles in water and methanol dilute solutions are shown in Figure 3a. p-BSP in methanol shows one peak with the maximum at 535 nm and a flat and very weak roof from 480 to 425 nm. Different from that of the methanol solution, the fluorescence emission of nanoparticles has only one peak with the maximum at 546 nm and is aging-time dependent. p-BSP shows a large change of fluorescence intensity from disperse molecules in methanol to the nanoparticles in water. The relative quantum yield (Φf) increases from 0.00111 in dilute solution to 0.060 in nanoparticle solution. The maximum of Φf for p-BSP nanoparticles is more than 60 times higher than that of the dilute methanol solution. With increasing aging time, intensities of the emission decrease monotonically but the peak positions are consistent. The Φf decreased in turn from 0.060 to 0.055, 0.051, 0.048, 0.040, and 0.025. Figure 3b contains the excitation spectra of nanoparticles with various aging times. The shapes of most excitation spectra are very analogous to their absorption spectra
Fluorescent Emission of Organic Nanoparticles
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10889
Figure 2. UV-vis absorption spectra of p-BSP nanoparticles (1.0 × 10-5 mol/L in molecule) in water at different aging times (left) and p-BSP in methanol solution (1.0 × 10-5 mol/L) (right).
Figure 3. (a) (left) Fluorescence emission spectra of p-BSP nanoparticles (1.0 × 10-5 mol/L in molecule) in water at different aging times and p-BSP in methanol solution (1.0 × 10-5 mol/L, multiplied by a factor of 30). (b) (right) Fluorescence excitation spectra of p-BSP nanoparticles (1.0 × 10-5 mol/L in molecule) in water at different aging times.
TABLE 1: Radiative and Total Nonradiative Rate Data aging time τ (ns) Φf kr (107 s-1) knr (107 s-1)
0 min 2.10 ( 0.05 0.060 2.86 44.8
15 min 2.03 ( 0.05 0.055 2.70 46.3
30 min 1.98 ( 0.05 0.051 2.57 47.9
except for the longest ripen time one. These may indicate that fluorescent species in nanoparticles of various aging times are the same. Additionally, no significant changes in shapes of nanoparticle emission spectra are observed when the excitation wavelength changes from 300 to 500 nm. Fluorescence Decay Measurement. The fluorescence decay curves of nanoparticles obtained at various aging times are shown in Table 1. Every decay datum was collected at 546 nm in 2 min. It is obvious that the fluorescence decay process of nanoparticles is more rapid with increasing aging time. After stirring is stopped, when the aging time is zero, the emission of the nanoparticles has a monoexponential decay with a lifetime (τ) of 2.10 ( 0.05 ns. When nanoparticles are ripened to 180 min, the lifetime changes to 1.55 ( 0.05 ns. After 6 h, the lifetime decreases to 0.95 ( 0.05 ns. The fluorescence lifetime of p-BSP in methanol solution is about 10 ps,11 which is too short to be measured with our instrument. It implies that the formation of the nanoparticles restricts the rotation and vibration of the groups in the molecules, so that longer emission lifetimes have been detected. According to equations18 τ-1 ) kr + knr and kr ) Φf/τ, the radiative rate constant kr and total nonradiative rate constant knr of nanoparticles with various aging times were
60 min 1.83 ( 0.05 0.045 2.62 52.1
180 min 1.55 ( 0.05 0.040 2.56 61.6
6h 0.95 ( 0.05 0.025 2.63 102
calculated and also listed in Table 1. With increasing aging time, kr has just a slight change. The factor that induces fluorescent decreasing is mainly ascribed to the increase of knr. Discussion Enhanced Emission Considerations. Enhanced emission in p-BSP nanoparticles is an interesting phenomenon and may have some potential applications. In recent years, a few cases of enhanced emission in the solid state of specific organic molecules have been reported.19 Most of these organic molecules are stilbene derivatives or have a stilbene group. The reasons for these phenomena were interpreted in terms of the intra- and intermolecular effects exerted by fluorophore aggregation.7b,19,20 Intramolecular effects on fluorescence enhancement are simply explained by a supposition that the twisted conformations and the rotation of chromophores in solution tend to suppress the radiation process, whereas planar ones of chromophores induced in the solid state activate the radiation process.21 We also know that the increase of rigidity in a molecule can lead to the decrease of vibration in a molecule, in addition to probably decreased internal conversion of excited molecules, and may increase
10890 J. Phys. Chem. B, Vol. 108, No. 30, 2004
Li et al.
SCHEME 1: Photoinduced Isomerization of p-BSP
Φf. Intermolecular effects in π-conjugated chromophores are correlated with aggregation structures such as H- and J-dimers, H- and J- aggregates, etc. UV-vis absorption spectroscopy was the most common method to obtain qualitative and/or quantitative data for the aggregation. For H-structure where molecules are aligned parallel to each other, the higher electronic state carries all oscillator strength whereas transitions from the ground state to a lower coupled excited state of molecules are forbidden. So the absorption of H-structure is blue-shifted. In this case, internal conversion from a higher electronic state to a lower ones is much faster than emission, so the emission of the H-structure is effectively quenched. However, for the J-structure those molecules arrange into a slanted stack, and the transition to the lower couple excited state of molecules is allowed. As a result, the absorption is red-shifted and the emission is stronger than that of the monomer.22,23 In our case, both intra- and intermolecular effects may affect the enhanced emission of p-BSP nanoparticles. In the good solvents such as THF and methanol, the p-BSP molecule is nonplanar and may undergo some conformational changes. The two halves of the molecule are much less coupled due to the free rotation around the single bond. As mentioned above, p-BSP in solution exhibits photochromism due to excited state intramolecular proton transfer. Most studies about SA and its derivatives, p-BSP among them, have a general conclusion that the excitation of molecules only localize on one salicylidene subunit, which is the site of proton transfer reactivity.11 Once a p-BSP molecule is excited, one salicylidene subunit will undergo the single proton transfer and lead the p-BSP molecule to the excited monoketo tautomer. This tautomer can experience torsion and transform to a photochromic transient. The process of transfer and transform are shown in Scheme 1. The main emission of the p-BSP solution and solid is from the excited state of the monoketo tautomer.9,11 When THF and water are mixed, p-BSP molecules aggregate together due to their poor solubility in water. Rotations around a single bond are restricted. It is hard to form the photochromic species. These would indicate that photochromism disappears in nanoparticles. In addition, the p-BSP in crystal is a planar symmetric molecule,9a,15 so the intermolecular aggregation would induce planarization and rigidity in water. Molecular mechanics force field computations with MM+ parametrization
in the HyperChem 7.0 package24 confirm this presumption. The optimized geometry of a single p-BSP molecule with 392 surrounding water molecules shows two phenyl rings of salicylidene are perpendicular to a third phenyl ring in the molecule. When the second p-BSP molecule is added to the system, if the distance of the two sample molecules is smaller than a variant that is closely correlated to the intermolecular torsion angle, the two molecules may influence each other and result in each being more planar than a single molecule in water. Figure 4 shows the conformation(s) of single and double p-BSP molecule(s) in water. As mentioned above, absorptions at 426 nm in nanoparticle solution increase synchronously with particle evolution. Because the peak could also be observed in the excitation spectroscopy, it should be assigned as the sum of the absorption band of J-aggregates structure and scattering from the formation of larger particles. In general, J-aggregation absorption in dyes is redshifted and intensely narrow. But that of p-BSP nanoparticles is broad and weak. According to ref 7b, these results may come from a less optimal way of J-aggregation in nanoparticles. We can observe actually in Figures 1 and 2 that molecular arranging is more ordered with increasing aging time and, at the same time, absorption of the J-aggregation band is more apparent. We noticed that emission of nanoparticles decreased with increasing aging time. Intermolecular charge transfer induced self-quenching is considered a possible reason of it.18 The azomethine groups and the benzene rings may show considerable intermolecular charge transfer interactions. In general, intermolecular charge transfer will induce fluorescence quenching. Therefore, the enhanced emission of p-BSP is attributed to the combined effects of molecular planarization, restricted molecules motions, and J-aggregate formation. Considering the maximum absorption of nanoparticles gives only one bathochromic shift from 370 to 382 nm, the latter two may be the main factors. When the aggregate forms, the interactions between the molecules limit the group rotation of the molecules, and the intramolecular hydrogen bonding conformation is favorable. When the molecules are excited by the irradiation, the intramolecular proton transfer occurs easily and the emitting ketone form is the main excited species. Furthermore, the restriction of the group rotation in the aggregate also limits the isomerization of the excited species to a nonemitting conforma-
Fluorescent Emission of Organic Nanoparticles
J. Phys. Chem. B, Vol. 108, No. 30, 2004 10891
Figure 4. (a) Conformation of a single p-BSP molecule in water: Conformations of two p-BSP molecules with different inclination of two longaxis; (b) 45°; (c) 90°; (d) 0°.
tion. Thus, the enhanced fluorescent emission is observed efficiently. When the crystal forms at the longer aging time, the intermolecular charge transfer between the molecules is increased. It enhances the nonradiation relaxation of the excited energy. A relatively smaller emission than at the beginning is then detected for the sample with the longer aging time. Evolving Process. Concerning the reprecipitation method that was used in our experiments, it is generally accepted to introduce a concentrated organic solution in an aqueous medium resulting in the formation of fine droplets.25 In our preparing process, the THF solution has just been introduced in water. The good solvent is replaced by water and disperses in the bulk water. This step is no more than a few seconds.6 In our experimental condition, the stirring state lasted 1 min. So it is hard to monitor this very beginning process. After this step, a completed water shell surrounds the p-BSP molecule. In this case, the p-BSP molecule exposed in the poor solvent tends to aggregate together. This process leads to the formation of amorphous particles that have a range of size.26 Aggregation is energetically favorable, whereas the introduction of an interface is unfavorable, so there is a balance between the two processes. In general, for the reprecipitation system in which the organic molecule is poorly soluble in water, an equilibrium spherical shape will be formed to minimize the interfacial energies. Actually, from the SEM observation in our experiments, p-BSP nanoparticles are spherical up to the aging time of 15 and 30 min, respectively. It may assume that particles in the shorter aging time are spherical too. As we know, the amorphous aggregations are growing; then according to the type of organic molecules, some may form crystals and others remain amorphous. According to the observation shown in Figure 1, beginning from the 60th minute, p-BSP nanoparticles have a tendency of anisotropic growth. It indicates that aggregations of sample are growing to crystallization. It is obvious that once sizes of nanoparticles exceed about 50 nm, the growth rates in different directions are no longer equal. Finally, all spherical aggregates convert
SCHEME 2: Interaction between Azomethine Group and Benzene Ring
to beltlike microcrystals whose long/wide ratio is about 10. The reason for the dimensional-like growth is not very clear, but it is affirmed that interactions between the azomethine group in one molecule and the benzene ring in the other molecule can produce much effect.27 In our calculations, the interaction between the two neighboring molecules with the J-aggregate is as high as 39.0 kJ/mol (shown in Scheme 2). This large interaction should prefer to form the crystal with one-dimensional growth. In summary, the evolving process of p-BSP particles is a competitive process between single-direction aggregation and spherical aggregation (interface energy minimum). It can be described as below: The molecules disperse in the THF-rich environment first; then they are surrounded by water molecules and aggregate to small amorphous particles. Furthermore, the molecules aggregate to large amorphous structures with some germs of crystallization and finally form the microcrystals. The SEM images obtained confirm majority phenomena of this process. It gives the beginning step of crystal growth for the organic compound in a poorly soluble solvent. Conclusion Nanoparticles of an intramolecular proton transfer molecule, N,N′-bis(salicylidene)-p-phenylenediamine, have been prepared by reprecipitation and characterized by SEM, absorption, and fluorescence spectra. The nanoparticles exhibit an enhanced
10892 J. Phys. Chem. B, Vol. 108, No. 30, 2004 emission compared to the same concentration dilute solution in a good solvent. The enhanced emission is considered to be the result of the variation of intra- and intermolecular interactions. THe aggregation process of particles has been studied by means of SEM and UV-vis spectroscopy. It is observed clearly that the aggregations grow from a spherical amorphous state to the 1D microcrystal form. Such well-defined organic 1D particles may be used as building blocks for nanoscale electronic devices for which the proton motion is essentially correlated to the electron conduction and beamed array molecules contribute to electron conduction in one direction. On the other hand, it can acquire large amounts of consistent nanobelts by improving the experimental conditions. Acknowledgment. This work was supported by the Major State Basic Research Development Program of China (Grant No. G2000078100) and National Natural Science Foundation of China. References and Notes (1) (a) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837-5842. (b) Bozano, L.; Tuttle, S. E.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1998, 73, 3911-3913. (c) Godovsky, D. Y. AdV. Polym. Sci. 2000, 153, 163-205. (2) Gratzel, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 314-321. (3) (a) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 2013-2016. (b) Chan, W. C. W.; Maxwell, D. J.; Gao, X. H.; Bailey, R. E.; Han, M. Y.; Nie, S. M. Curr. Opin. Biotechnol. 2002, 13, 40-46. (4) (a) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52. (b) Bergey, E. J.; Levy, L.; Wang, X. P.; Krebs, L. J.; Lal, M.; Kim, K. S.; Pakatchi, S.; Liebow, C.; Prasad, P. N. Biomed. MicroDeVices 2002, 4, 293-299. (5) (a) Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Ito, O.; Nakanishi, H. Chem. Lett. 1997, 1181-1182. (b) Taylor, J. R.; Fang, M. M.; Nie, S. M. Anal. Chem. 2000, 72, 19791986. (c) Fu, H. B.; Yao, J. N. J. Am. Chem. Soc. 2001, 123, 1434-1439. (6) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4331-4361. (7) (a) Gong, X. C.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, C. M. J. Am. Chem. Soc. 2002, 124, 14290-14291. (b) An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410-14415. (8) Kownacki, K.; Kaczmarek, L.; Grabowska, A. Chem. Phys. Lett. 1993, 210, 373-379. (9) (a) Inabe, T.; Hoshino, N.; Mitani, T.; Maruyama, Y. Bull. Soc. Chem. Jpn. 1989, 62, 2245-2251. (b) Sekikawa, T.; Kobayashi, T.; Inabe, T. J. Phys. Chem. B 1997, 101, 10645-10652.
Li et al. (10) Alarcon, S. H.; Olivieri, A. C.; Nordon, A.; Harris, R. K. J. Chem. Soc., Perkin Trans. 2 1996, 11, 2293-2296. (11) Ziolek, M.; Kubicki, J.; Maciejewski, A.; Naskrecki, R.; Grabowska, A. Chem. Phys. Lett. 2003, 369, 80-89. (12) Kownacki, K.; Mordzinski, A.; Wilbrandt, R.; Grabowska, A. Chem. Phys. Lett. 1994, 227, 270-276. (13) Ito, E.; Oji, H.; Araki, T.; Oichi, K.; Ishii, H.; Ouchi, Y.; Ohta, T.; Kosugi, N.; Maruyama, Y.; Naito, T.; Inabe, T.; Seki, K. J. Am. Chem. Soc. 1997, 119, 6336-6344. (14) (a) Zhang, Y.; Lu, Z. H. Mater. Chem. Phys. 1999, 57, 253-259. (b) Zgierski, M. Z. J. Chem. Phys. 2001, 115, 8351-8358. (15) Hoshino, N.; Inabe, T.; Mitani, T.; Maruyama, Y. Bull. Chem. Soc. Jpn. 1988, 61, 4207-4214. (16) Lakowiez, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic Plenum: New York, 1999. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian, Inc: Pittsburgh, PA, 1998. (18) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings Publishing Company, Inc.: Menlo Park, CA, 1978. (19) (a) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F. Macromolecules 2000, 33, 652-654. (b) Deans, R.; Kim, J.; Machacek, M. R.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 85658566. (c) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Chem. Commun. 2001, 1740-1741. (20) Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15, 5676-5680. (21) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.; Hanack, M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902-1907. (22) Akins, D. L.; Macklin, J. W. J. Phys. Chem. 1989, 93, 59996007. (23) Chowdhury, A.; Wachsmann-Hogiu, S.; Bangal, P. R.; Raheem, I.; Peteanu, L. A. J. Phys. Chem. B 2001, 105, 12196-12201. (24) HyperCube Inc., Copyright 2002. (25) Nalwa, H. S.; Kasai, H.; Okada, S.; Oikawa, H.; Matsuda, H.; Kakuta, A.; Mukoh, A.; Nakanishi, H. AdV. Mater. 1993, 5, 758-760. (26) Bertorelle, F.; Lavabre, D.; Fery-Forgues, S. J. Am. Chem. Soc. 2003, 125, 6244-6253. (27) Bregman, J.; Leiserowitz, L.; Osaki, K. J. Chem. Soc. 1964, 20862100.
