J. Phys. Chem. B 2002, 106, 803-808
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Photoinduced Electron Transfer from Pyrenes to Alkyl Viologens on the Surface of Polystyrene Latex Particles: Effects of Polarities of the Donors and Charge Densities of the Particles Yong-Kuan Gong and Kenichi Nakashima* Department of Chemistry, Faculty of Science and Engineering, Saga UniVersity, 1 Honjo-machi, Saga 840-8502, Japan ReceiVed: May 22, 2001; In Final Form: NoVember 17, 2001
Photoinduced electron transfers (PET) from pyrene (Py), 1-pyrenemethanol (PyM), and 1-Pyrenebutyltrimethylammonium bromide (PBTAB) to alkyl viologens, such as methyl viologen (MV2+) and Octyl viologen (OV2+), in polystyrene latex (PSL) aqueous dispersions have been studied by steady-state fluorescence, timeresolved fluorescence, and adsorption/desorption measurements. From steady state fluorescence measurements, it is found that the PET is remarkably enhanced on going from the aqueous homogeneous solution to the latex dispersion. The PET efficiency increases with increasing polarity of the electron donors in the PSL dispersions; PBTAB shows the highest efficiency and Py shows the lowest one. The low PET efficiency of Py in the PSL dispersion is attributed to penetration of Py, as revealed by the I1/I3 ratio in the vibronic fine structure of Py fluorescence spectrum. The PET efficiency of PBTAB in PSL dispersion increases with the increase in the charge density of the particle, but decreases with the increase in alkyl-chain length of the quencher. The alkyl viologen with longer alkyl-chain weakens the PET efficiency by desorbing the donors from the latex surface and/or preventing the donors from effective approach with higher steric hindrance effect.
1. Introduction Polystyrene latex (PSL) particles are solid nano- or microspheres with functional groups attached on the surfaces.1 Latex particles have received increasing attention in latex coatings,2,3 biomedical fields,4-11 and drug delivery.12 They are also used as model systems in colloid research because of their welldefined physicochemical characteristics.13-15 Recently, we used latex particles as micro-substrates for photoreactions and found that the efficiencies of the reactions were dramatically enhanced.16-21 It is well-known that the fluorescence of pyrene (Py), anthracene (An), and their derivatives is quenched by alkyl viologens through a photoinduced electron transfer (PET) mechanism.21-23 However, the efficiency of PET was surprisingly different between An and its cationic derivative,21 although both cationic and neutral probes were supposed to be adsorbed on the surface of the latex particles. Another phenomenon we observed is that the efficiency of PET from Py to methyl viologen (MV2+) decreases as the loading time (storage time after mixed with PSL particles) increases.21 The decrease in efficiency suggests that small channels or holes exist on the particle surface, through which Py and An molecules can go into the inner part of the particle and are protected from quenching. In this paper, we report studies of PET from Py, 1-pyrenemethanol (PyM) and 1-pyrenebutyltrimethylammonium bromide (PBTAB) to methyl viologen (MV2+), octyl viologen (OV2+) and octadecyl viologen (ODV2+) on the surface of PSL particles in aqueous dispersions. Several factors such as polarity of * To whom correspondence should be addressed. Fax: +81-952-288548. E-mail:
[email protected] electron donors, chain length of the alkyl viologens, charge density of PSL particles, and loading time of the probes, on the efficiency of PET have been investigated and discussed in details. 2. Experimental Section PyM and PBTAB (Molecular Probes, Inc.), OV2+ and ODV2+ (Tokyo Kasei Kogyo Co., Ltd.) were used as received. MV2+ from Katayama Chemical Co. Ltd., were guaranteed grade, and used without further purification. Py was purified by vacuum sublimation. Structural formulas of these compounds are shown in Figure 1. Distilled water was purified with a Millipore Milli Q purification system. PSL samples (L711 and CG17) with sulfate groups on the particle surfaces were employed in this research. The negatively charged surface groups are originated from the initiator (K2S2O8). L711 was synthesized by a standard emulsion polymerization method.18 CG17 was synthesized according to the recipe for the latex CG17 in the work by Charreyre et al.24 The purification and characterization methods of PSL were reported elsewhere.15,25 The characteristics of the latexes used are summarized in Table 1. Stock solutions of PBTAB, MV2+, and OV2+ in water were prepared to give the concentrations of 100 µM, 20 mM, and 20 mM, respectively. The solubility of ODV2+ is too low (less than 1 µM) to be used in this experiment. The methanol solutions of Py and PyM were used as their stock solutions, and the solvent was completely removed by N2 purging before adding any other stock solutions.21 PSL dispersion was introduced (1.0 g/L) after the donor and acceptor stocks were well mixed in a 10-mL volumetric flask, followed by sonication for 5 min. The samples were stored in the dark for 20 min before the
10.1021/jp0119532 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/29/2001
804 J. Phys. Chem. B, Vol. 106, No. 4, 2002
Gong and Nakashima
Figure 1. Structural formulas of electron donors and acceptors.
TABLE 1: Characteristics of the Latex Particles latex L711
CG17
212 216 1.02
94 96 1.02
0.30 27.0
1.26 60.8
a
DLS diameter Dn (nm) Dw (nm) polydispersity charge density σ (10-7 mol/m2) specific surface (m2/g)b
a D and D are the number and weight average diameter, respecn w tively. b The number average diameter is used in the calculation.
spectroscopic measurements unless otherwise mentioned. Airsaturated samples were used in all fluorescence experiments. To compare the quenching or PET efficiency of Py derivatives, the same concentration of the electron donors (1.0 µM) was introduced in all the quenching experiments, except the penetration and adsorption isotherm measurements. In the penetration measurements, the total concentration of Py was reduced to 0.4 µM to exclude any aggregate of the donor before adsorption. Adsorption isotherms of the donors and acceptors onto the latex particles were measured by ultra-centrifugation.21,25 Portions of the latex dispersion containing known amounts of the probe were centrifuged with Beckman Avanti-30 ultracentrifuge for 60 min at 2.60 × 104 rpm (5.70 × 104 g) in the case of L711 or a Hitachi 55P-72 ultracentrifuge for 30 min at 4.00 × 104 rpm (1.05 × 105 g) in the case of CG17. Concentration of the donors in the supernatant after centrifugation was determined by fluorescence spectroscopy. Concentrations of the quenchers were determined by UV-vis absorption spectroscopy. The amount of the donors or quenchers adsorbed was calculated from the difference between the total concentration in the dispersion and equilibrium concentration in the serum. Fluorescence spectra were recorded on a Hitachi F-4000 spectrofluorometer. All the samples were excited at 334 nm in a four-wall transparent quartz cell with an 1 cm optical path length. The sample cell was adjusted to the front-face configuration when the turbid latex sample was measured. The spectra were corrected by the use of a standard tungsten lamp with a known color temperature. As Py and its derivatives usually show 5 peaks in their fluorescence spectra, their maximum λem are
Figure 2. Stern-Volmer plots of Py, PyM and PBTAB fluorescence as quenched by MV2+ in CG17 aqueous dispersions (a), and in aqueous solutions (b).
slightly different from each other and also change with the micro-environment of the probes, steady-state fluorescence intensity is expressed in terms of peak height of a maximum band after the background was deducted, unless otherwise stated. Absorption spectra were measured with a Jasco Ubest-50 spectrophotometer. Fluorescence lifetime measurements were carried out with a Horiba NAES 1100 time-resolved spectrofluorometer which employs a time-correlated single photon counting technique. 3. Results and Discussion 3.1 Effect of the Polarities of the Electron Donors on the PET. We observed fluorescence spectra of Py, PyM, and PBTAB in the absence and presence of MV2+. The effects of MV2+ on PBTAB fluorescence spectra in an aqueous solution and in the PSL dispersion are very similar to those on the spectra of 3-(9-anthracene)-propyltrimethylammonium bromide (APTAB) in the same condition,21 (data not shown); PBTAB fluorescence was markedly quenched by MV2+ on going from an aqueous homogeneous solution to the PSL (CG17) dispersion (Figure 2a and 2b). To reach the same degree of quenching on PSL surface, MV2+ concentration needed in aqueous solution is thousand times higher than that in the PSL dispersion. Quenching of the PyM fluorescence by MV2+ was also much enhanced by the existence of PSL particles. The PET quenching efficiency of PyM is lower than that of PBTAB, but is higher than that of Py in the PSL dispersion. These are clearly shown by the Stern-Volmer plots in Figure 2a. The PET efficiency seems to depend on the polarity of the donors. The ionic donor (PBTAB) has the highest efficiency and the nonpolar one (Py) has the lowest efficiency in the PSL dispersion. The PET efficiencies and polarity effect in aqueous solutions are much different from those in the PSL dispersions. As seen
PET from Pyrenes to Alkyl Viologens
Figure 3. Adsorption isotherms of Py and PM and PBTAB onto CG17 particles in aqueous dispersions at room temperature.
from Figure 2b, PBTAB has the lowest PET efficiency. The electrostatic repulsion between cationic PBTAB and bicationic MV2+ prevents them from approaching each other. Therefore, the PET efficiency is the lowest. In contrast, Py and PyM have relatively high efficiency because they can approach MV2+ without electrostatic repulsion. The adsorption isotherm measurements elucidated that all the three donors were efficiently adsorbed onto the particles in the low concentration range. Figure 3 represents the examples of the adsorption isotherms on CG17. When the total concentration of the donors is lower than 2 µM, more than 95% of the donors are adsorbed onto the latex surfaces. Therefore, the quenching in the PSL dispersion is dominated by the quenching on the particle surface. The large differences among the Stern-Volmer plots of the donors in the PSL dispersions suggest that their behaviors on the PSL particles are different. The cationic Py derivative and viologen can be adsorbed effectively onto the sulfate groups on the PSL surface by electrostatic attraction. Because PBTAB and MV2+ have positive charges, there is more adsorption of both compounds on the negatively charged surface, which means a shorter average distance between PBTAB and MV2+, although repulsion exists. This is the reason PET between the two cationic species is very efficient in the PSL dispersion. On the other hand, it might be difficult to understand the relatively low efficiency of Py-MV2+ and PyM-MV2+ pairs on PSL surface. If both of the donors were located only on the surface, MV2+ could approach Py and PyM more easily than PBTAB, and should have higher PET efficiency because the electrostatic repulsion force is absent. However, the efficiency decreases with the decrease in the polarity of the donor’s. There are two hypotheses to explain the low PET efficiency. One is rearrangement of the donor molecules at the surface of the latex. In fact, the latex surface is not smooth, with the presence of small anchored styrene oligomers, formed during latex synthesis at high conversion. Due to its high hydrophobicity, Py could diffuse/penetrate under some of these oligomers. Then, Py could be surrounded by PS chains and protected from quenching because the adsorbed MV2+ would remain at the water/PS interface. The other hypothesis is penetration of the Py molecule into the hydrophobic sphere through channels or holes.21,26 Because the holes are very small and hydrophobic, only nonpolar species could penetrate/diffuse into the solid sphere very slowly. Both of the hypotheses suggest that Py diffuse under the latex surface and be surrounded by PS chains. However, the deepness of Py penetration and the time of
J. Phys. Chem. B, Vol. 106, No. 4, 2002 805
Figure 4. Loading time effect on the quenching of PyM fluorescence by MV2+ in L711 aqueous dispersions at room temperature.
Figure 5. Loading time effect on the quenching of Py fluorescence by MV2+ in L711 aqueous dispersions at room temperature.
equilibrium must be different between the two mechanisms. As discussed in the next subsection, both mechanisms seem to be responsible for the low PET efficiencies in Py and PyM cases. 3.2 Effect of Loading Time on the PET. As discussed above, the adsorbed fluorescence probes may diffuse into the inner part of the sphere depending on their polarity. If this is true, more Py and PyM molecules will be located inside the particles and the PET efficiency will decrease as the loading time increases. Figure 4 shows an example of loading time effect on the PET efficiency of PyM in latex L711 dispersion.27 The obvious decrease of PET efficiency with the loading time indicates that PyM penetrated slowly to the inner part of the PSL particle. Loading time effect on the PET efficiency is more significant in the Py-MV2+ pair than in the PyM-MV2+ pair. The PET process was completely suppressed with long loading time as shown in Figure 5. Moreover, it is surprising that the SternVolmer plot for the longest loading (140 h) sample has a slightly negative slope. One of the possible reasons is that the presence of the electron acceptor (quencher) finally acts as a driver to force more Py molecules go to more inner part of the particle. As Py molecules penetrate deeply into the PSL particles, the increase in fluorescence quantum yield seems to surpass the quenching effect by MV2+. It is known that fluorescence quantum yield increase as a probe moves from fluid to solid environments. To confirm the penetration of Py into the PSL particle, the microenvironment of Py molecule was measured by the I1/I3 ratio of the Py fluorescence spectrum.28-30 In aqueous solution, the I1/I3 ratio of Py is about 1.84. After loaded on PSL, the
806 J. Phys. Chem. B, Vol. 106, No. 4, 2002
Figure 6. Loading time effect on the I1/I3 ratios of Py, PyM, and PBTAB in L711 aqueous dispersions at room temperature.
ratio decreases to 1.22 in 2-minutes as shown in Figure 6. Then it decreases gradually to 1.08 in 5-days at room temperature. It is certain that the value of 1.08 28,29 represent the inner environment of PSL sphere because this value is very similar to the ratio in benzene (1.05) and far from that in water (1.84). It should also be noted that the I1/I3 ratio reaches a constant value after 30 h or more. Because there is no way to reduce the I1/I3 ratio to such a low value and to take such a long equilibration time except that Py penetrates into the particle, we conclude that small holes exist in the particles. Time-resolved experiment shows that the lifetime of Py increases remarkably with the loading time in PSL dispersions.26 The increase of lifetime with loading time is another evidence of the penetration. When the PSL was dried to form film and annealed at 150∼180 °C (>Tg) for 20 min, the I1/I3 ratio did not decrease below 1.2 as the loading time increased.26 This fact not only supports our hypothesis, but also suggests a way to modify the microstructure of such materials. In conclusion, the rearrangement of Py at the latex surface probably is the dominant mechanism in the beginning several minutes of the loading, as suggested from the rapid decrease of the I1/I3 ratio. The following penetration of Py into the PSL particle seems to a slow but significant mechanism to suppress the quenching of the cationic quenchers in the PSL dispersion. This effect increases with the decreasing polarity of the donor and also decreases with the loading. 3.3 Effect of Surface Charge Density on the PET. To investigate the effect of surface charge density on the PET efficiency, two kinds of PSL particles were used. The surface charge density of L711 is 0.30 × 10-7 mol/m2, and CG17 is 1.26 × 10-7 mol/m2. Illustrated in Figure 7 are the SternVolmer plots of PBTAB-MV2+ pair in the two PSL dispersions. It is clear from Figure 7 that the fluorescence quenching (i.e., PET) efficiency depends strongly on the surface charge density. High PET efficiency is reached in highly charged PSL dispersion. In this case, both PBTAB and MV2+ are adsorbed on the surface (in the electric double layer) mainly due to electrostatic attraction,15 and one cationic PBTAB can be surrounded by several MV2+. The adsorption isotherms (shown later) indicate that the cationic species can be effectively adsorbed until the negative charges of the particles are neutralized.21,25 Up to the maximum adsorption point, the quenching efficiency increases steadily as the increase of MV2+. After the adsorption reaches its maximum, the increase of quenching efficiency becomes very slow.
Gong and Nakashima
Figure 7. Effect of the charge density of PSL particle surfaces on the quenching of PBTAB fluorescence. The charge density of L711 is 0.30 × 10-7 mol/m2 and CG17 is 1.26 × 10-7 mol/m2.
Figure 8. Stern-Volmer plots of PBTAB fluorescence quenched by MV2+ (b) and OV2+ (9) in CG17 aqueous dispersions.
In the case of low charge density, the adsorption sites on the surface can be completely occupied by small amount of the counterions. PBTAB has priority to occupy the adsorption sites because its hydrophobic interaction with PSL surface is much stronger than that of MV2+ and the adsorbed PBTAB cannot be replaced by MV2+. When the amount of PBTAB is fixed in a dispersion, the low charge density of the particle yields low adsorbed fraction of MV2+. Therefore, the PET efficiency decreases with the decrease of the charge density. This charge density effect is similar to the effect of occupation ratio of APTAB.21 3.4 Effect of Chain Length of Alkyl Viologen on the PET. Two quenchers with different alkyl-chain length were used to investigate the alkyl-chain effect on the quenching of PBTAB fluorescence in PSL aqueous dispersions. Figure 8 shows the Stern-Volmer plots of PBTAB fluorescence quenched by MV2+ and OV2+ in dispersions of CG17. The considerable difference between the two plots implies that the long alkyl-chains in the OV2+ molecule cause the decrease in the quenching efficiency. To investigate the mechanism of the alkyl-chain effect, the adsorption isotherms of the quenchers (Figure 9) onto CG17 particles were determined together with the isotherms for the desorption of PBTAB from the particles which was brought about by the quenchers (Figure 10). MV2+ and OV2+ show almost the same adsorption isotherms. However, their interactions with the particle surface depend on the lengths of alkylchains; the longer the alkyl-chain is, the stronger the hydrophobic interaction will be. The consequent results are evident
PET from Pyrenes to Alkyl Viologens
J. Phys. Chem. B, Vol. 106, No. 4, 2002 807 desorption of PBTAB. Both of these effects decrease the PET efficiency on PSL particle surfaces. 4. Conclusions
Figure 9. Adsorption isotherms of MV2+ and OV2+ onto CG17 particles in aqueous dispersions at room temperature.
The photoinduced electron transfers from PBTAB, PyM, and Py to alkyl viologens are remarkably enhanced on going from aqueous solutions to PSL dispersions. The PET efficiency is directly related to the polarity of the electron donors. The ionic donor, PBTAB, shows the highest efficiency of PET in the PSL dispersions. High charge density of the particle also favors the PET process. The efficiency of PET can be restrained with several days of loading, especially for the probes with low polarity. This suggests that the probes with low polarity penetrate slowly into the PS particles, escaping the quenching by (ionic) alkyl viologen. Longer alkyl-chains of alkyl viologens can also weaken the quenching efficiency by either desorbing the donors from the latex surface or preventing effective approach of the donor with higher steric hindrance effect. Acknowledgment. The authors thank Professor Fumio Kato (Saga University) for the use of Hitachi 55p-72 ultracentrifuge. One of the authors (Y-K. Gong) acknowledges a financial support by the Sasakawa Scientific Research Grant from the Japan Science Society. The present work is partly defrayed by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan (No. 12640559). References and Notes
Figure 10. Isotherms for the desorption of PBTAB from CG17 particles induced by competitive adsorption of MV2+ (b) and OV2+ (9). The symbol RV2+ denotes MV2+ or OV2+. [PS] ) 1.00 g/L; [PBTAB] ) 1.00 µM.
in Figure 10. PBTAB is considerably desorbed by OV2+ from the surface of CG17 particles, but is not desorbed by MV2+. The desorbed PBTAB goes to the aqueous phase where the quenching is negligible in the very low concentration range (as shown in Figure 2b). Therefore, the desorption of PBTAB from CG17 particle surface is one of the reasons for the low efficiency of the quenching by OV2+. Another reason is unraveled after investigating the two SternVolmer plots (Figure 8) and the desorption isotherms (Figure 10) in the low concentration range. When the concentration of the quenchers is very low (e3 µM), the adsorbed concentrations of MV2+ and OV2+ are the same, and neither of them desorbs PBTAB because there are a lot of vacant adsorption sites on the surface as suggested from Figure 10. However, the quenching efficiency by OV2+ is much lower than that by MV2+. This suggests another effect of the long alkyl-chain besides the increase of the adsorption interaction. It is known that effective approach or collision between electron donor and acceptor is essential for PET. The effective approach between PBTAB and OV2+ will be hindered significantly by the octyl chains located at both ends of the viologen moiety. Therefore, we can ascribe the second reason to steric hindrance effect of the alkyl-chain. In summary, the alkyl-chain has two effects. Long alkyl-chains of the alkyl viologen increase the steric hindrance and the
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808 J. Phys. Chem. B, Vol. 106, No. 4, 2002 (23) Hsiao, J.-S.; Webber, S. E. J. Phys. Chem. 1993, 97, 8296. (24) Charreyre, M.-T.; Zhang, P.; Winnik, M. A.; Pichot, C.; Graillat, C. J. Colloid Interface Sci. 1995, 170, 374. (25) Gong, Y.-K.; Nakashima, K.; Xu, R. Langmuir 2000, 16, 8546. (26) Gong, Y.-K.; Nakashima, K. Chem. Commun. 2001, 1772. (27) We carried out the experiments on loading time effect using both latexes CG17 and L711, and obtained similar preliminary results. However,
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