Langmuir 1996, 12, 269-275
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Energy Transfer between Pyrene and Proflavine Solubilized in Polymer/Surfactant Complexes Katumitu Hayakawa,* Takafumi Nakano, and Iwao Satake Department of Chemistry, Faculty of Science, Kagoshima University, Korimoto-1, Kagoshima, Japan 890
Jan C. T. Kwak Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received June 20, 1994. In Final Form: August 30, 1995X Energy transfer from excited pyrene (donor) to proflavine (acceptor) was measured in sodium dextran sulfate/dodecyltrimethylammonium bromide (DxS/DTAB), DxS/dodecyltrimethylammonium chloride (DxS/ DTAC), poly(vinyl sulfate)/DTAB (PVS/DTAB), and poly(styrenesulfonate)/DTAB (PSS/DTAB) mixed solutions by monitoring both the quenching of pyrene emission and the enhancement of proflavine emission as a function of acceptor concentration at 0.1, 0.2, 0.3, 0.4, and 0.5 mmol dm-3 surfactant. A remarkably high enhancement of energy transfer was found in DxS/DTAB, DxS/DTAC, and PVS/DTAB complexes, but the energy transfer efficiency was low in PSS/DTAB complexes. The enhanced energy transfer is attributed to the presence of polymer/surfactant (P/S) aggregates which solubilize the donor and acceptor in monomeric form. The high energy transfer efficiency in the DxS and PVS systems is due to the larger aggregate sizes in these systems, related to the higher degree of cooperativity observed in the surfactant binding isotherm. Smaller aggregate sizes in the PSS/DTAB complex lead to a lower energy transfer efficiency. ET efficiences are relatively independent of the surfactant counterion, bromide or chloride, indicating the complete exclusion of the counterion from the surface of the P/S aggregate.
Introduction The composition, the structure, and the dynamics of organized amphiphile systems such as micelles, vesicles, Langmuir-Blodget films, and microemulsions have been the subject of extensive studies recently. The photophysics and photochemistry of these systems have been investigated by various photoluminescence techniques. Aggregates of polyions with surfactants of opposite charge form a special class of organized amphiphiles. In these systems, the surfactant ions bind to the polyion by means of cooperative hydrophobic interactions between the bound surfactant ions, to form hydrophobic surfactant clusters similar to micelles in the polyion domain.1-7 Clustering of the bound surfactant ions can be demonstrated by means of dynamic photophysical techniques using hydrophobic probe molecules.6,8,9 It can also be proven indirectly from binding isotherms, solubilization of water-insoluble dyes,10 spectroscopy of the solubilizates,11 and induction of ordered conformations of ionic polypeptides.12,13 For fluorescence probe molecules we can therefore expect an enhancement * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (2) Hayakawa, K.; Kwak, J. C. T. In Cationic surfactants: Physical chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; Vol. 37; p 189. (3) Satake, I.; Hayakawa, K.; Komaki, M.; Maeda, T. Bull. Chem. Soc. Jpn. 1984, 57, 2995. (4) Shirahama, K.; Tashiro, M. Bull. Chem. Soc. Jpn. 1984, 57, 377. (5) Zana, R.; Binana-Linbele, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992, 96, 5461. (6) Almgren, M.; Hansson, P.; Mukhtar, E.; van Stam, J. Langmuir 1992, 8, 2405. (7) Maltesh, C.; Somasundaran, P. Colloids Surf. 1992, 69, 167. (8) Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J. Am. Chem. Soc. 1982, 104, 5673. (9) Chu, D.-Y.; Thomas, J. K. Macromolecules 1984, 17, 2142. (10) Hayakawa, K.; Fukutome, T.; Satake, I. Langmuir 1990, 6, 1495. (11) Hayakawa, K.; Ohta, J.; Maeda, T.; Satake, I.; Kwak, J. C. T. Langmuir 1987, 3, 377. (12) Satake, I.; Hayakawa, K. Chem. Lett. 1990, 1051. (13) Hayakawa, K.; Fujita, M.; Yokoi, S.; Satake, I. J. Bioact. Compat. Polym. 1991, 6, 36.
0743-7463/96/2412-0269$12.00/0
of energy transfer between probes solubilized in the polymer-surfactant complexes. This was indeed observed between proflavine and rhodamine 6G solubilized in the complex of sodium dextran sulfate with dodecyltrimethylammonium ions.14 Since the cluster size depends on the cooperativity of the surfactant binding process,15 the efficiency of the energy transfer may be expected to depend on the type of surfactant binding. In the present study, energy transfer from excited pyrene as a donor to proflavine (3,6diaminoacridine hydrochloride) as an acceptor was examined in polymer-surfactant systems, i.e., dodecyltrimethylammonium (DTA+) with dextran sulfate (DxS), poly(vinyl sulfate) (PVS), and poly(styrenesulfonate) (PSS). These three polymers exhibit different cooperative binding processes for the DTA+ ion. We use the pyrene and proflavine pair as the donor-acceptor pair because the photophysical properties are well established. Experimental Section Materials. Pyrene from Wako Pure Chemicals was recrystallized from ethanol, chromatographed on an alumina column eluted with cyclohexane, and again recrystallized twice from ethanol. The pyrene solid was suspended in a mixed solution of 0.50 mmol dm-3 polymer and 0.20 mmol dm-3 surfactant by stirring overnight. The resulting supernatant was used as stock solution. The absorbance at 338 nm was used to estimate the pyrene concentration. A molar absorbance of 42 300 mol-1 dm3 cm-1 for pyrene at 338 nm was used, determined from the dependence of the absorbance on pyrene concentration in 0.1 mol dm-3 DTAB micellar solutions. A fresh stock solution was prepared before the preparation of each series of test solutions. The absorbance was kept constant (less than 0.06 at 338 nm which corresponds to 1.4 µmol dm-3 pyrene) in a series of test solutions. Proflavine (3,6-diaminoacridine hydrochloride, Aldrich, purity > 95%) was purified by recrystallization from ethanol. The concentration was calculated from the absorbance of a diluted solution of the stock aqueous solution using a molar absorbance (14) Hayakawa, K.; Ohyama, T.; Maeda, T.; Satake, I.; Kwak, J. C. T. Langmuir 1988, 4, 481. (15) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263.
© 1996 American Chemical Society
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of 41 000 at 510 nm.16 Again a fresh stock solution of proflavine was used for the preparation of each test solution. Dodecyltrimethylammonium bromide and chloride (TCI, GR and EP grade, respectively) were purified by repeated recrystallization from acetone including a small amount of ethanol. Surfactant solutions and all other mixed solutions were prepared by weight. Solutions of sodium dextran sulfate (Nacalai Tesque), sodium poly(styrenesulfonate) (Polyscience), and potassium poly(vinyl sulfate) (Wako Pure Chemicals) in 0.5 mol dm-3 NaCl were dialyzed against doubly distilled water until no chloride ion could be detected by silver nitrate addition to the dialysate. The ionic concentrations of the polyelectrolyte solutions were determined by colloid titration with PVS. Quinine sulfate and 2-naphthol (Wako Pure Chemicals, GR) were used for the correction of the instrument sensitivity of the spectrofluorometer. An aqueous solution of quinine sulfate was mixed with dilute ammonia solution. The dried precipitate was dissolved in 0.05 mol dm-3 sulfuric acid, and the concentration was adjusted to 5.0 × 10-5 mol dm-3. 2-naphthol was purified by recrystallization from benzene followed by sublimation. A solution of 2.0 × 10-5 mol dm-3 was prepared at pH 4.67 in 0.02 mol dm-3 acetate buffer. Measurements. Stock solutions of polymer, surfactant, and proflavine and the solution of pyrene solubilized in the polymer/ DTAB mixture were mixed into a given amount of water under stirring. The absorbance of pyrene was kept constant below 0.06; the concentration of polymer was kept constant at 0.50 mmol dm-3 equivalent ionic concentration. The concentration of the surfactant was varied from 0.10 to 0.40 mmol dm-3 for the DxS and PVS systems and from 0.10 to 0.50 mmol dm-3 for the PSS system. The concentration of proflavine was varied from 0 to 2.0 µmol dm-3 for DxS and PVS systems and from 0 to 10 µmol dm-3 for the PSS system. Absorption and emission spectra were recorded at room temperature using a Hitachi 228 spectrophotometer and a Hitachi 650-10S spectrofluorometer, respectively. Some spectra were also measured using a Cary 219 spectrophotometer and a PerkinElmer MPF 66 spectrofluorometer with automatic intensity correction. The wavelength of the excitation light was set at 338 nm in the measurements of the emission spectrum. The sensitivity of the Hitachi 650-10S spectrofluorometer was corrected by a comparison to the observed emission spectrum of the standard sample solution of quinine sulfate and 2-naphthol to the reference spectrum.17,18 The correction factors were 1.92 at 510 nm and 1.15 at 374 nm. The quantum yield of proflavine was determined in excess polymer/surfactant solution by a comparison of the emission spectrum to the observed emission spectrum of the standard sample solution of chlorophyll a in a micellar solution (quantum yield ) 0.0095) to the reference spectrum at the same excitation wavelength and an equal absorbance. Since the precise determination of the absorbance of pyrene was disturbed due to the microheterogeneity of the polymer/surfactant mixed solution, the quantum yield was determined as 0.51 for the micellar solution in dodecyltrimethylammonium chloride by a comparison to the observed emission spectrum of pyrene in a micellar solution of sodium dodecyl sulfate (SDS), where the quantum yield was reported as 0.53.19 The lifetime of pyrene fluorescence emission was determined with a System 3000 single photon counting system (Photochemical Research Associates, London, Ontario, Canada) using deconvolution and a standard four-parameter fitting procedure. The quantity of bound surfactant was determined potentiometrically by using a surfactant ion selective electrode responding to DTA+ ion.1
Results and Discussion Absorption and Fluorescence Spectra. The absorption and fluorescence spectra of proflavine in aqueous (16) Vitagliano, V. In Aggregation processes in solution; Wyn-Jones, E., Gormally, J., Eds.; Elsevier: Amsterdam, 1983; p 271. (17) Lippert, E.; Na¨gele, W.; Seibold-Blankenstein, I.; Staiger, U.; Voss, W. Z. Anal. Chem. 1959, 17, 1. (18) Kokubun, H. In Kiso Gijutsu 3. Hikari 1; Ito, M., Ed.; Maruzen: Tokyo, 1976; Vol. 4; p 505. (19) Kano, K.; Kawazumi, H.; Ogawa, T. J. Phys. Chem. 1981, 85, 2998.
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Figure 1. Absorption spectra of a mixture of pyrene and proflavine in DxS/DTAB mixed solution. DxS, 0.5 mmol dm-3; DTAB, 0.3 mmol dm-3; pyrene, 1.4 µmol dm-3; and proflavine, (0) 0.0, (1) 0.2, (2) 0.4, (3) 0.6, (4) 0.8, (5) 1.0, (6) 1.2, (7) 1.5, (8) 2.0 µmol dm-3.
solution exhibit spectral band maxima at 445 and 510 nm, respectively.20 The maximum in the absorption spectrum shifts to 470 nm in a mixture of DxS and DTAB.11 The 0-0 vibronic transition of pyrene is observed at 338 nm in the absorption spectrum and at 374 nm in the fluorescence spectrum. No shift of the maximum in the absorption spectrum was observed in the DxS/DTAB mixed solutions. The absorption spectrum of a mixture of pyrene and proflavine is simply additive for the two components. Figure 1 shows an example of the absorption spectra of a mixture of pyrene and proflavine in DxS/ DTAB mixed solutions, where the pyrene concentration is kept constant and the concentration of proflavine varies from 0 to 2.0 µmol dm-3. The increase in baseline at shorter wavelengths may be due to increased scattering from the microheterogeneous solution. The absorption of proflavine at 470 nm increases with increasing proflavine concentration, while the pyrene absorbance remains constant. The absorption maximum of proflavine is observed at 470 nm and is independent of the concentrations of DTAB and proflavine in the DxS and PVS systems. Since this band is observed at 445 nm in aqueous solution, the shift to longer wavelength indicates the solubilization of proflavine in the polymer/surfactant complexes.11 In the case of the PVS/DTAB system, a broad band with a shoulder at 450 nm is observed at 0.1 mmol dm-3 DTAB, showing the coexistence of an aggregate form of proflavine in the PVS polyion domain due to cooperative binding. The appearance of the 470 nm band indicates that proflavine is completely solubilized in the monomeric form (but still partly aggregated in the 5:1 PVS/DTAB complex) in these polyion/DTAB complexes.11 In the PSS/DTAB system, a broad maximum is also observed. In this system, upon increasing proflavine concentration, the wavelength at maximum absorption varies from 457 to 453 nm at 0.1 mmol dm-3 DTAB, from 457 to 444 nm at 0.3 mmol dm-3 DTAB, and from 450 to 444 nm at 0.5 mmol dm-3 DTAB. These observations have been ascribed to the specific interaction of proflavine with PSS,21 and they are not due to a release of proflavine from the polyion domain. The emission spectra of the mixed solutions, with selective excitation at 338 nm (the pyrene absorption maximum), are given in Figure 2. No emission of proflavine was observed without pyrene (a in Figure 2), and in the absence of proflavine (0), the typical fluorescence spectrum of pyrene was observed. In the presence of proflavine, the proflavine emission band at 510 nm appears and its intensity increases as the proflavine content increases, while the emission band by pyrene decreases (0-4). This figure clearly indicates the energy transfer (20) Haugen, G. R.; Melhuish, W. H. Trans. Faraday Soc. 1964, 60, 386. (21) Hayakawa, K.; Satake, I.; Kwak, J. C. T. Colloid Polym. Sci. 1994, 272, 876.
Energy Transfer between Pyrene and Proflavine
Langmuir, Vol. 12, No. 2, 1996 271
Figure 2. Fluorescence spectra of a mixture of pyrene and proflavine in DxS/DTAB mixed solution. DxS, 0.5 mmol dm-3; DTAB, 0.3 mmol dm-3; pyrene, 1.4 µmol dm-3; and proflavine, (0) 0.0, (1) 0.4, (2) 0.8, (3) 1.2, (4) 2.0 µmol dm-3.
from the excited pyrene to proflavine, leading to emission from the excited proflavine as shown in the following equations:
Py + hν f Py* f Py + hνPy
(1)
Py* + PF f Py + PF*
(2)
PF* f PF + hνPF
(3)
where Py and PF stand for pyrene and proflavine, respectively, hν represents the irradiation light, hνPy and hνPF stand for the emitted photon from pyrene and proflavine, respectively, and the asterisks stand for the corresponding excited molecules. At 0.1 mmol dm-3 DTAB, a weak emission of the pyrene excimer was observed in the absence of proflavine, but it disappeared in the presence of proflavine for the PVS/DTAB system. Energy Transfer. In calculating intensity enhancement of the proflavine emission at 510 nm (IA), the intensity values were corrected by use of the emission spectra of standard samples (quinine sulfate and 2-naphthol) and normalized relative to the intensity at 374 nm in the pyrene emission spectrum obtained in the mixed solution of 0.5 mmol dm-3 polymer and 0.1 mmol dm-3 DTAB. The dependence of IA on the concentration of proflavine is given in Figure 3 for the DxS/DTAB, DxS/ DTAC, PVS/DTAB, and PSS/DTAB mixed systems. At 0.1 mmol dm-3 DTAB or DTAC in parts A and B of Figure 3, at first the emission of proflavine quickly increases at low proflavine concentration, followed by a decrease at higher concentration. On the other hand, at 0.4 mmol dm-3 DTAB, the enhancement of the proflavine emission gradually increases and tends to reach a maximum. A similar behavior is observed in the enhancement of the proflavine emission at 510 nm in the PVS/DTAB mixed solutions (Figure 3C). The surfactant concentration corresponding to the maximum of emission enhancement clearly shifts to higher concentrations of proflavine with increasing DTAB concentration. These observations are ascribed to aggregation at the higher proflavine concentration due to the cooperative binding of proflavine.22 The maximum enhancement is larger in the PVS/DTAB (22) Schwarz, G.; Klose, S.; Balthasar, W. Eur. J. Biochem. 1970, 12, 454.
Figure 3. Enhancement of proflavine emission as a function of proflavine concentration (A) in DxS/DTAB, (B) in DxS/DTAC, (C) in PVS/DTAB, and (D) in PSS/DTAB. Surfactant concentration: (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.4, (5) 0.5 mmol dm-3.
mixed system than in the DxS/DTAB or DxS/DTAC mixed systems. The similarity in the spectra with bromide and chloride as surfactant counterions indicates that bromide, the more effective quencher, has no effect on the energy transfer between pyrene and proflavine. This demonstrates that energy transfer occurs not in the aqueous bulk solution but in the anionic polymer/surfactant complexes and, in turn, that both probes are solubilized in the polymer/surfactant complexes. A similar dependence of the enhancement of acceptor emission on the concentration of acceptor and DTAB was also found for the energy transfer of excited proflavine to rhodamine 6G in the DxS/DTAB mixed solutions.14 In PSS/DTAB mixed solutions (Figure 3D), a weak enhancement of proflavine emission is observed and its dependence on the proflavine
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concentration is different from what is observed in the other mixed solutions. In this system, the emission intensity is almost independent of the surfactant concentration. This observation may relate to the aggregation process of both bound surfactants and dyes, as will be discussed later. The quenching efficiency of pyrene emission ID showed corresponding behavior due to the energy transfer to proflavine (data are not shown). Strong quenching was observed in 0.1 mmol dm-3 DTAB already at low proflavine concentration, while a more gradual increase in quenching was observed at the higher DTAB concentration. In the PSS/DTAB system, relatively strong quenching was also observed. These results indicate that energy transfer occurs between pyrene and proflavine bound to the polyion for two reasons. The donor and acceptor concentrations are too low for energy transfer to occur in bulk solution, and the absorption spectra clearly indicate solubilization of proflavine in the polymer/surfactant complexes (Figure 1). Although the vibronic peak intensity ratio I1/I3 is relatively high (1.3 for DxS/DTAB and 1.2 for PSS/DTAB), indicating a relatively polar environment compared to a SDS micelle, fluorescence lifetime measurements clearly show pyrene to be solubilized in the polymer-surfactant aggregates. In DxS/DTAB we find lifetimes of 280 ns in mixtures with a 5:1 DxS/DTAB ratio, 320 ns (5:2 DxS/ DTAB), 330 ns (5:3 DxS/DTAB), and 330 ns (5:4 DxS/ DTAB). In PSS/DTAB we find 305 ns (5:1 PSS/DTAB), 350 ns (5:3 PSS/DTAB), and 370 ns (5:5 PSS/DTAB). In deaerated DxS or PSS solution in the absence of DTAB the pyrene has the same solubility as in water, and the measured lifetime is 200 ns for both polymers. In 0.03 M DTAB the lifetime is 150 ns (showing the effect of the bromide ions on the micellar surface); in deaerated 0.06 M SDS we determine a lifetime of 400 ns. The longer lifetimes in the polymer-surfactant solutions clearly indicate the solubilization of pyrene in the polymer/ surfactant complexes, with exclusion of the bromide ions. The slightly higher I1/I3 ratios therefore can be interpreted as showing a slightly more polar environment in the polymer-surfactant aggregate compared to the micellar interior, i.e., a more polar environment in polyion/DTAB complexes than in pure surfactant micelles (the ratio is 1.2 in DTAB micelles). These observations are in accordance with the results of Almgren et al.6 Reaction 2 is, therefore, considered not due to a diffusion mechanism, under Stern-Volmer conditions, but due to a Fo¨rster type energy transfer, where effective energy transfer occurs through dipole-dipole interaction between monomeric donor-acceptor pairs with separation distances within a critical transfer distance as given by the Fo¨rster equation. The molar ratio of probes to ionic sites on the polymer is 1/360 for pyrene (about 1.4 µmol dm-3) and ranges from 0 to 1/250 for proflavine in the DxS and PVS systems and from 0 to 1/50 for proflavine in the PSS system. Assuming both probes to be completely bound to the polyions and a random distribution of the probes along the linear polyions, all of which have an average linear charge separation of about 0.26 nm, the average distance between donor and acceptor will be, at the shortest, about 40 nm in the DxS and PVS systems and about 13 nm in the PSS systems. This estimate would predict a more effective energy transfer in the PSS system compared to the DxS and PVS systems. To the contrary, we observe a more effective energy transfer in the DxS and PVS systems than in the PSS systems. In the following we will consider that effective energy transfer occurs only between pyrene and proflavine solubilized in the polymer/surfactant complexes. If the
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distribution of both pyrene (donor) and proflavine (acceptor) meets the condition of a Poisson distribution among surfactant clusters on polymer, the following equations are derived for the emission intensity of donor (ID) and acceptor (IA):23
ID ) IaΦDe
IA ) IaΦAe
〈m〉m
-〈m〉
∑ m)0 m!
P〈m〉
-〈m〉
1
(4)
1 + Pm
〈m〉m-1
1
∑ m)1 (m - 1)! 1 + Pm
(5)
where Ia stands for the radiation intensity absorbed by pyrene, m for the number of proflavine molecules in a surfactant cluster which includes excited pyrene, and 〈m〉 for the average number of proflavine molecules solubilized in a surfactant cluster. ΦD and ΦA are the corresponding fluorescence yields, and P is the efficiency of energy transfer from D*, defined by eq 6
P)
kET kf,D + kn,D
(6)
where kET is the rate constant for reaction 2, and kf,D and kn,D are the radiative and nonradiative decay constants of deactivation of the excited donor, respectively. Quenching of excited donor emission may occur due to excimer formation of the donor as well as to energy transfer to the acceptor. As noted before, a small excimer emission band of pyrene is observed at 480 nm when the surfactant to polymer ratio is small. For this reason we calculate the energy transfer efficiency to proflavine from the emission intensity of acceptor. From eqs 4 and 5, the following expression is easily derived.
IA Φ A [A] ) ID ΦD [cluster]
(7)
where [cluster] stands for the concentration of surfactant clusters defined as [S]/NS, where NS is the average number of surfactant ions in a cluster. [A] stands for the acceptor (proflavine) concentration. Therefore, the slope of a plot of IA/ID against [proflavine] is a function of the concentration of surfactant clusters. Plots of IA/ID versus [proflavine] corresponding to eq 7 are given in Figure 4, and the inverse slope corresponds to [cluster]ΦD/ΦA. The quantum yield of proflavine, ΦA, was determined as 0.69 in the 5:4 DxS/ DTAB and PVS/DTAB complexes and 0.67 in the 5:5 PSS/ DTAB complexes. These values are comparable to the value determined in a tetradecyltrimethylammonium chloride micellar solution. If we use 0.51 for the quantum yield of pyrene, ΦD, determined in a TTAC micellar solution, the ratio of ΦA/ΦD is 1.35 for the DxS/DTAB and PVS/DTAB mixed systems and 1.3 for the PSS/DTAB mixed system. We, therefore, estimate the concentration of surfactant clusters to be 0.5-2.0 µmol dm-3 for the DxS and PVS systems and 12-20 µmol dm-3 for the PSS system (Table 1). These values are too small to have a Poisson distribution of pyrene (1.4 µmol dm-3) and proflavine (up to 10 µmol dm-3) in surfactant clusters on polyion. Nevertheless, if we calculate the aggregation numbers from these values and from the concentration of bound surfactant determined by potentiometry using surfactant ion selective electrode (given in Table 1), the aggregation numbers of the surfactant cluster is estimated as 190(23) Atik, S. S.; Nam, M.; Singer, L. A. Chem. Phys. Lett. 1979, 67, 75.
Energy Transfer between Pyrene and Proflavine
Langmuir, Vol. 12, No. 2, 1996 273 Table 1. Concentrations of Surfactant, Cluster, and Bound Surfactant polymer/ surfactant
[surfactant], mmol dm-3
[cluster], µmol dm-3
[bound surfactant], µmol dm-3
DxS/DTAB
0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 0.1 0.2 0.3 0.4 0.1 0.3 0.5
0.46 0.86 1.4 1.8 0.49 0.86 1.5 2.0 0.42 0.76 1.2 1.2 (22) 12 20
92 190 290 380 95 190 280 370 41 140 230 260 100 280 370
DxS/DTAC
PVS/DTAB
PSS/DTAB
lated here indeed suggest that intercluster energy transfer may occur. Therefore, we use our data to try to estimate the distance between pyrene and proflavine. Energy Transfer Distance. Fo¨rster investigated the radiationless energy transfer through the dipole-dipole interaction between a donor (D) and an acceptor (A) and derived eq 8 for the critical transfer distance (R0) for energy transfer with a yield of 50%27-29
R06 )
9000K2ΦD ln 10 5 4
128π n L
∫
fD(ν) A(ν) ν4
dν
(8)
where K2 is an orientation factor, usually taken as 2/3 for a random distribution, n is the refractive index of the medium, L is Avogadro’s constant, and fD(ν) and A(ν) are the spectral distributions of emission of D and of absorption of A, respectively, on a wavenumber scale. Since the dipole-dipole interaction is proportional to R-6, the energy transfer efficiency is given by eq 9
(
eET )
)
IA/ΦA R-6 ) -6 IA/ΦA + ID/ΦD R + R0-6
(9)
where IA and ID are the emission intensities of A and D, respectively, and R is the mean distance between D and A. Therefore, we calculate the effective distance for the energy transfer Reff in the polyion/surfactant complexes by eq 10
Reff ) Figure 4. Intensity ratio of proflavine emission to pyrene emission as a function of proflavine concentration. Notation as in Figure 3.
230 for the DxS systems, 190-220 for the PVS system, and 20-23 for the PSS system. In this calculation, the data at 0.1 mol dm-3 surfactant were not adopted due to the observation of excimer emission. These values may be compared to reported aggregation numbers of 7-10 for PSS/DTAB complexes,24 105 ( 10 (which was not corrected for the concentrations of unaggregated surfactant) for the complex of decyltrimethylammonium ions with poly(methacrylic acid),25 65 for polyacrylate/DTA+ cation at the very dilute aqueous solution, and 80, 70, and 150 for polyacrylate/DTAB, DTAC, and CTAC in the two-phase region of the phase diagram.26 The larger values calcu(24) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 6274. (25) Chu, D.-Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (26) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115.
(
)
1 - eET eET
1/6
R0
(10)
Although we are not in a position to calculate the critical transfer distance because we do not have values of the absolute quantum yield of pyrene in polyelectrolyte/ surfactant complexes, we may infer an analogy to the situation in micellar solutions. Kano and co-workers estimated a critical distance (R0) of 3.8 nm for the energy transfer from pyrene to proflavine in a micellar solution of SDS.19 We can use this value and a conventional calculation of eET from IA/(ID + IA) to obtain a rough estimate of Reff in the polyion/surfactant complexes. Figure 5 gives such calculated values of Reff as a function of added proflavine concentration. The effective distance between pyrene and proflavine is 3-4 nm for proflavine concentrations above 1 µmol dm-3 in the DxS and PVS (27) Fo¨rster, T. Z. Naturforsch. A 1949, 4, 321. (28) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7. (29) Fo¨rster, T. In Modern Quantum Chemistry; Sinanoglu, O., Ed.; Academic Press: New York, 1965; p 93.
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Figure 5. Effective transfer distance as a function of proflavine concentration. Notation as in Figure 3.
systems. At a low surfactant concentration, Reff tends to increase at 2 µmol dm-3 proflavine due to quenching of IA by self-aggregation of proflavine on the polyion. The large value of Reff at 0.4 mmol dm-3 surfactant indicates that the bound surfactant ions are effective dispersants for the bound proflavine and pyrene molecules. In the PSS/ DTAB system, a long effective distance is calculated, reflecting the low efficiency of energy transfer. The PSS/ DTAB complex is considered to have lower solubilizing power due to a smaller degree of cooperative binding of DTAB. This may lead to aggregation of proflavine on PSS, causing self-quenching of the emission of proflavine and accordingly a low efficiency of energy transfer. We introduce the critical transfer concentration defined at eET ) 0.76 by eq 11,
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Figure 6. Effective local concentration of proflavine as a function of proflavine concentration. Notation as in Figure 3.
C0 )
(
)
0.765 nm 3000 ) R0 2π3/2LR03
3
(11)
eq 12 can be derived for eET
eET ) xπ Q exp(Q2)[1 - φ(Q)]
(12)
Q ) [A]eff/C0
(13)
where
φ(Q) )
∫0Qexp(-x2) dx
2 xπ
(14)
The acceptor concentration [A]eff equals the bulk concentration in a homogeneous solution, but it is a local
Energy Transfer between Pyrene and Proflavine
concentration effective for the energy transfer in the present heterogeneous system. When R0 ) 3.8 nm, C0 is calculated as 8.2 × 10-3 mol dm-3. From eq 12, we determine Q and [A]eff for each value of eET. [A]eff is plotted in Figure 6. A striking finding is that the local concentration effective for energy transfer at high surfactant concentration increases linearly as the proflavine concentration increases, indicating a homogeneous distribution of proflavine in the polyion/surfactant complexes. On the other hand, at low surfactant concentration, a steep rise is observed at low proflavine concentration, followed by saturation or even a maximum in the effective transfer concentration, [proflavine]eff. The steep rise indicates solubilization of proflavine close to pyrene in the polyion/ surfactant complexes. The saturation effect suggests that when too many proflavine molecules are present there is interference of proflavine emission presumably due to the formation of nonfluorescent aggregates of proflavine. Once again, in PSS/DTAB mixtures the critical transfer concentration is low; as explained before this is presumed to be due to the smaller cluster size in this system, leading
Langmuir, Vol. 12, No. 2, 1996 275
to poor dispersion of the proflavine and self-quenching of the acceptor molecules. Conclusions Effective energy transfer was observed between pyrene and proflavine solubilized in DxS/DTAB, DxS/DTAC, and PVS/DTAB mixed solutions. The energy transfer was less effective in the PSS/DTAB mixed solutions. These observations relate strongly to the binding mode of surfactant to polyions. Highly cooperative binding systems induce large surfactant aggregates and consequently are good dispersants for the probes, causing the observed effective energy transfer. The PSS/DTAB system exhibits lower cooperativity with smaller aggregates, leading to a low energy transfer efficiency. A treatment based on stationary quenching yields nonrealistic estimates of aggregation numbers of surfactant clusters in the polyion domain, which may be due to intercluster energy transfer between probes solubilized in different surfactant clusters bound to a polyion. LA940494B
