10702
J . Phys. Chem. 1993,97, 10702-10707
Photophysical Processes on a Latex Surface: Electronic Energy Transfer from Rhodamine Dyes to Malachite Green K. Nakashima,+ J. Duhamel, and M. A. Winnik’ Department of Chemistry and Erindale College, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada MSS 1 A1 Received: January 28, 1993; In Final Form: July 28, 1993’
W e bound pairs of donor-acceptor dyes to the surface of polystyrene latex particles and studied the characteristics of the surface as a reaction field for electronic energy transfer. The donors are rhodamine B (RB) and rhodamine 6G (R6G), and the acceptor is malachite green (MG). Steady-state fluorescence measurements revealed that the dyes are highly concentrated on the latex surface; thus, energy transfer occurs up to 100 times more effectively on the surface than in an aqueous solution. Picosecond fluorescence measurements of donor decay in the presence of the acceptor provided us with the fractal dimensions of the systems: 1.75 from RB-MG and 1.82 from R6G-MG. The values suggest that the latex surface is flat and smooth on a molecular scale and that the distribution of the dyes is slightly inhomogeneous. The kinetics of exchange of the dyes between the latex particles were also studied. The rate constants obtained are 6.4 min-l for RB and 2.2 min-1 for R6G. The results seem to be useful for characterizing the latex surface as a reaction field for photophysical processes.
TABLE I: Emulsion Polymerization Recipe and Conditions
Introduction Various microheterogenous systems (MHS) have been employed as a “reaction field” for photophysical and photochemical processes over the past 2 decades.’ The systems so far examined cover a wide range of materials such as detergent micelles, vesicles, Langmuir-Blodget films, liquid-crystalline media, semiconductors, cyclodextrins, porous glasses, silica, and polyelctrolytes.’ More recently, polymer micelles have become a target of such studies.2 The study of photoprocesses in M H S seems to be generated from several motivations: (i) M H S provide a different type of reaction field than homogeneous systems, so that phenomena in MHS stimulate the interests of theorists as well as experimentalists; (ii) some types of M H S are useful materials for controlling photoreactions for synthesis applications; (iii) photoprocesses are powerful tools for characterizing MHS; and (iv) MHS can be applied todeveloping new materials with required functionality (e.g., photochromic materials). The surfaces of latex particles offer the possibility of providing a new type of reaction field for photoprocesses. The surface of polystyrene (PS) latex, for example, consists of hydrophobic domains embedded dispersely with negatively charged sulfate groups. Thus the surface hasdiscrete adsorption sites for cationic species and continuous adsorption domains for hydrophobic species. As the charge density and particle size of the latex (ranging from 50 nm to 5 ~ L Mcan ) be controlled through synthesis, we can design a reaction field with various charge densities, domain sizes, and surface curvatures. Despite these unique features of a latex surface, there are few examples which employ the latex surface as a reaction field for photoprocesses.3 In this work we studied direct nonradiative electronic energy transfer (DET) from rhodamine B (RB) or rhodamine 6G (R6G) to malachite green (MG) on a PS latex surface. From steadystate fluorescence measurements, we observed that DET occurs much more effectively on the surface compared to that in homogeneous aqueous solution. By analyzing the decay curves ofthedonorsin the presenceoftheacceptor, weobtain information about the morphology of the latex surface, as well as about the distribution of the dyes on the surface. The kinetics of exchange of the dyes between latex particles was also studied. The results On leave from Laboratory of Chemistry, Collegeof Liberal Arts, University of Saga, 1 Honjo, Saga 842, Japan. @Abstractpublished in Advance ACS Abstracts. September 15, 1993.
0022-365419312097- 10702$04.00/0
recipe water styrene SDS KZSZOB
100 mL 10 mL
condition 70 OC 6h
0.1oog 0.050 g
obtained in this study will be useful for characterizing the latex surface as a reaction field for photophysical processes.
Experimental Section RB (laser grade, Aldrich), R6G (laser grade, Aldrich), and MG (Aldrich) were used as received. Styrene monomer was vacuum distilled immediately before use. The PS latex was synthesized by standard emulsion polymerization in the presence of sodium dodecyl sulfate (SDS). The recipe and conditions of the polymerization are given in Table I. The latex was purified by successive ultrafiltration and serum replacement. The mean diameter determined by dynamic light scattering is 155 nm. Stock solutions of the donors and the acceptor were prepared in distilled and ultrafiltered water, to give a concentration of 1.0 X mol L-l. Aliquots of the PS dispersion and the stock solution of the donor were mixed in PS dispersion and the stock solution of the donor were mixed in a IO-mL volumetric flask, followed by ultrasonic agitation during 5 min. Then each amount of the stock solution of the acceptor was added to a flask. After the flask was filled with water to give a total volume of 10 mL, it was sonicated for 5 min.‘ By this procedure the donor molecules were first adsorbed onto the latex followed by the acceptor. Similar results were obtained if acceptor was adsorbed first followed by the donor. Absorption spectra were recorded on a Jasco Ubest-50 Spectrophotometer equipped with an integrating sphere Model TIS-417 for turbid latex samples. Steady-state fluorescence measurements were carried out with a Spex Fluorolog 2 spectrofluorometer and with a Hitachi F-4000 spectrofluorometer. Fluorescence and excitation spectra were corrected by a conventional rhodamine B method and by the use of a standard tungsten lamp with a known color temperature, respectively. A sample cell with a 2-mm optical path length was used in the front-face configuration to minimize reabsorption of fluorescence 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10703
Photophysical Processes on a Latex Surface
I
I
I
io4 10' IO' IO1
U
AI nm
T i m e (ns)
w
Ili
500
550
600
650
U
700
Alnm Figure 1. Fluorescence spectra of RB (- -) and R6G (-a) and absorption spectra of MG (-) in (a) aqueous solution and (b) PS latex dispersion. [MG] = 2 X 10-6 mol L-1, [RBI = [R6G] = 1 X 10-6 mol L-I, and [PSI = 1.88 g L-1. Excitation wavelengths for RB and R6G are 525 and 470
-
nm, respectively. Fluorescence spectra are normalized at the maximum intensity. since the effect is significant for RB and R6G due to the small Stokes shifts. Fluorescence decay curves were obtained by a time-correlated single-photon-counting apparatus equipped with a synchronously pumped, cavity-dumped dye laser (Coherent, Model 701-3) as an excitation light source and a microchannel-plate photomultiplier (Hamamatsu R 1564U-01) as a d e t e c t ~ r .The ~ laser dye employed for exciting the donors was either R6G or R575. The fluorescence from the donors was detected in the front-face configuration. To minimize the interference of reflectedexcitation light on the fluorescence detection, the angle of incident light to the surface of the cell was adjusted to 1.05 rad. In addition, sharp-cutoff glass filters were set on the emission side of the sample chamber. A Glan-Taylor prism polarizer was also put on theemissionside a t the magic angle (0.956 rad) to the polarized plane of excitation light so as to remove the effect of photoselection excitation on large molecules.6 The excitation pulse profile for deconvolution calculation was observed using an aqueous dispersion of silica powder (Ludox) as scattering material. The pulse width (fwhm) of the instrumental response function of the scattered laser light was 1 10 ps. The decay curves were analyzed by using an iterative nonlinear least-squares deconvolution program.
Results and Discussion Change of Spectroscopic Parameters of Dyes on Adsorption onto PS latex. Fluorescence spectra of RB and R6G and absorption spectra of MG in aqueous solution and in a PS latex dispersion are shown in Figure 1. Red-shifts of about 4 and 7 nm in the fluorescence of RB and R6G, respectively, are observed on going from the aqueous phase to the latex surface. A larger red-shift (1 5 nm) was observed for the absorption spectrum of MG upon adsorption onto the latex. It is notable that the absorption of M G is enhanced upon adsorption onto the latex. At present we do not know the origin of the enhancement. One possibility is that the molecule may bind to the surface in a more
4.3
.rl
ffl - 4 . 3 (u
n
11
22
T i m e (ns) Figure 2. Fluorescence decay curve of RB in (a) aqueous solution and (b) PS latex dispersion. [RBI = 1 X 10-6 mol L-I, [PSI = 1.88 g L-'. Excitation light from R6G dye laser is tuned to 567 nm. Fluorescence emission is collected at 600 nm.
ci
TABLE II: Fluorescence Lifetimes of RB and R6C lifetime/ns medium
RB
R6G
H20 PS latex
1.76 3.56
3.86 4.21
planar conformation than in solutions. This might perturb the electronic structure of M G so that the transition moment of SI SOabsorption could be increased. Figure 2 shows fluorescence decay curves of RB in aqueous solution and in the latex dispersion. At a glance we notice that the lifetime of RB drastically increases on going from aqueous solution (Figure 2a) to the latex dispersion (Figure 2b). Both of the curves are well-fitted by a single-exponential function. Generally, RB shows a different lifetime in an adsorbed state than in aqueous solution because the internal rotation of the diethylamino group, the main path of intramolecular radiationless decay of this m ~ l e c u l eis, ~suppressed upon adsorption. Therefore, the single-exponential decay observed for the latex dispersion strongly indicates that almost all of RB molecules are adsorbed ontothelatexand that thecontribution fromspeciesin theaqueous phase is negligible. For R6G in the latex dispersion, we also obtained a single-exponential decay curve. The fluorescence lifetimes of these donor dyes in aqueous solution and in the latex dispersions are summarized in Table 11. The critical energy-transfer distance, Ro, is an important parameter for characterizing the nonradiative energy-transfer process. It is defined as the distance at which the rate of energy transfer is equal to that of intramolecular deactivation processes. For the dipole-dipole coupling mechanism, Ro is related to an overlap of donor fluorescence with acceptor absorption as follows:
-
where f#Jd is the quantum yield of the donor fluorescence in the
Nakashima et al.
10704 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
TABLE 111: Critical Energy-Transfer Distance for RB-MG and R6C-MG Rnlnm
medium
RB-MG
R6G-MG
(a) K Z = 2/j' H2O PS latex
6.10 6.57 (b) K~ = 0.475' 5.76 6.21
6.30 5.91
H20 5.95 PS latex 5.58 K is an anisotropic factor for a dipole-dipole interaction. See text. absence of the acceptor; NA,Avogadro's number; n, the refractive index of the medium; Fd(u), the fluorescence intensity of the donor at frequency Y , with the total intensity normalized to unity; e&), the molecular extinction coefficient of the acceptor at frequency Y . ~2 is a factor describing the relative orientation of the transition moments of the donor and acceptor: K~ = 2 / 3 for rapidly rotating dipoles; ~2 = 0.475 for random pairs of immobile dipoles.*-9 From the overlap integral data in Figure 1, we tried to estimate Ro values for two representative cases of donoracceptor orientation: Le., for K~ = 2/3 and 0.475. The values of the parameters employed in the calculation are n = 1.33 for water and r$d = 0.30 for RBIo and 0.59 for R6G" in aqueous solution. As for r$d in the latex, we evaluated them by6J1
I P S I l g L-'
- 0.6'-
P, [I
0.4-
>
# \
I
$
/
a, :,
0.2 -
/
L"B
(3) where 7 is the fluorescence lifetime of a donor and td(Y) is the molecular extinction coefficient of a donor at frequency Y . The subscripts L and W denote latex and water, respectively.I2 The values of Ro,thus obtained, are listed in Table 111. Effect of PS Latex on Dimer Formation of RB and R6C. RB and R6G are well-known to form nonfluorescent dimers.I3 It is interesting to examine if the dimer formation is accelerated or suppressed upon adsorption onto the latex surface. Thus we examined the dependence of the fluorescence intensity of the two dyeson PS particleconcentration. The results areshownin Figure 3. As we can see, the dependence is quite similar for RB and R6G. At low PS concentration (point A in each figure), most dye molecules exist in the aqueous phase. Here the fluorescence intensity is nearly equal to that in aqueous solution. As the concentration of PS is increased (point B), the fraction of the adsorbed dye molecules becomes dominant. Under these conditions, the dyes undergo extensive dimer formation on the latex due to their high local concentration, and their fluorescence intensity is diminished. If we continue to increase the PS microsphere concentration, the dyes redistribute on the latex, so that dimerization is suppressed (point C). A similar behavior of R6G on dihexadecyl phosphate vesicles was reported by Tamai et al.14 Kinetics of Exchange of Dyes between PS Latex Particles. In our initial experiments, we sonicated the mixture of dyes and latexdispersion because we were concerned that the equilibration process would be slow. We later realized that it would be straightforward to measure these equilibration times by adding increments of additional latex to preequilibrated samples and observing the time profile of the increase or decrease in fluorescence. Curves obtained in this way are shown in Figure 4. These experiments allow us to conclude, even in the absence of a kinetic model, that the equilibration time is on the order of a few minutes at most. Under our experimental conditions, we can neglect dyes in the aqueous phase. Thus the increase in fluorescence intensity can be ascribed to the increase in monomeric dyes on the latex surface.
0
1
2
3
4
3
4
t I min
I
I
0
1
2
5
tlmin
Figure 4. Rise curves of fluorescenceof (a) RB and (b) R6G after sudden increase of PS latex concentration. [RBI = [R6G] = 1 X 1W mol L-I = const. PS latex concentrationsare 0.056 g L-1 (the point B) and 1.88 g L-l (the point C) in Figure 4a; 0.188 g L-' (the point B) and 1.88 g L-I (the point C) in Figure 4b.
In principle, there are a number of mechanisms for the transfer of dyes from precoated to uncoated latex particles. In practice, one needs to consider only two possible pathways for redistribution of dyes on the latex:
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10705
Photophysical Processes on a Latex Surface
-
kl'
ki'
+ (1
+()+(1
k3'
+ D + ( 1+ ( )
(D) + (D) (5) In this scheme D and ( ) stand for a donor dye and a latex particle, respectively. Hence (Dz), for example, means a dimer of donor dye adsorbed on a latex. If we assume that the desorption of dyes is the rate-determining step, we can express processes 4 and 5 by a single kinetic equation: (D2)
D,
+
D
+
d[(D,)lldt = -k[(D,)I (6) where [(D,)] denotes the concentration of (D2) and k is the dissociation rate constant. The solution of eq 6 is [(D2)1 = [(Dz)lo exp(-kt) (7) where [(D2)]0 is the initial concentration of (D2). Thus the concentration of monomeric dye on the latex, [(D)], is expressed by
-
2 3.0
*-
I
v
[ ( W l = 2[(D,)lO(l - exp(-kt)) (8) As the fluorescence from monomer dyes in aqueous phase is negligible, the fluorescence intensity of the system, If, is proportional to [(D)], and we obtain Zf(t) = If"( 1 - exp(-kt))
C
2.0-
0
10
20
30
1
3
tlS
(9)
I{ is the fluorescence intensity at the time when the system reaches the equilibrium. Equation 9 leads to the linear relation between ln(Z{ - If) and t : ln(Z: - If) = -kt
-
Figure 5. Plots of In(1f - If)against for the notations of If and I f .
1:
(a) RB and (b) R6G. See text
+ const
(10) In Figure 5 we plot ln(If - If) against t . The linearity between ln(Z{-Zf) and tis fairly good for both dyes: correlation coefficients a r e 4 9 9 7 for RB a n d 4 9 9 5 for R6G. Thissupports thevalidity of our analysis. The rate constants obtained are 0.106 s-I (6.36 min-I) for RB and 0.0371 s-l (2.23 min-I) for R6G. The partition coefficient ( K ) of a dye between the latex and aqueous phases is given by the equation
= kadsorp/
(1 1) where kahrpand kdgorpare the rate constants of adsorption and desorption of dyes onto the latex, respectively. We can estimate values of K by assuming that the adsorption is diffusion controlled and that kdgorp is equal to the rate constant in eq 6. In this way we obtained the values of 5.6 X 10l2 mol-l L for RB and 1.6 X 1013 mol-1 L for R6G.15 Energy Transfer on the Latex Surface. The high fluorescence quantum yields of RB (& = 0.30)'O and R6G (r#q = 0.59)l*and lowquantumyieldof MG (q5f N 10-4-10-2)16enablesus toobserve fluorescence of the donors without interference from acceptor fluorescence. Figure 6 presents typical fluorescence spectra of R6G in the presence of MG. As we expected, we do not see the overlap of M G fluorescence on that of R6G. This is an advantage of the present D-A combination. If we compare parts a and b of Figures 6, we note that the quenching through energy transfer occurs about 10, times more effectively in the latex dispersion than in aqueous solution. As regards the RB-MG pair, we also observed 30- to 50-fold enhancement of DET on the latex. Generally, enhancement of DET in M H S can be ascribed to several factors: (i) an increase of local concentration of dyes, (ii) an increase of overlap between donor fluorescence and acceptor absorption, and (iii) preferable orientation of donor with respect to acceptor. In the present cases, the factor (i) seems to be dominant because the Ro values, which depend on (ii) and (iii), for RB-MG and R6G-MG pairs are not so changed on going from water to the latex particle surface. kdesorp
0 Alnm
Alnm Figure 6. Fluorescence spectra of R6G ( 1 X 1O-' mol L-I) in the presence of MG in (a) aqueous solution and (b) PS latex dispersion. [PSI = 1.88 g L-I. Concentrations of MG are (0) 0, (1) 1.0 X 10-4 mol L-I, (2) 2.0 X 10-4 mol L-l, (3) 3.0 X 10-4 mol L-l in Figure 6a; (0), 0, (1) 1.0 X 1 V mol L-I, (2) 2.0 X 1 V mol L-I, (3) 3.0 X 1V mol L-I in Figure 6b. R6G is excited at 470 nm.
Kinetics of Energy Transfer on the PS Surface. Because we use the front-face configuration for detection of emission, and turbidity of the latex sample prevents excitation light from penetrating deeply into the sample, the inner filter effect of M G
10706 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
X
-6
-4
=-
-2
0 0
2 4 6 8 1 0 I M G l l l O - ’ m o l L‘’
0
1 2 3 4 5 [ M G l l l O ~ ’ m o lL-’
4F‘p Nakashima et al.
P3
Figure 7. Perrin plot (0) and Stern-Volmer plot (X) for (a) RB-MG and (b) R6G-MG pairs in PS latex dispersion. [RBI = [R6G] = 1 X mol L-I. [PSI = 1.88 g L-I.
1
0 0
2
4
6
0 1 0
0 2 4 6 8 1 0 L” [MG1/lO-’mol L-‘ Figure 9. Plot of P and d for (a) RB-MG and (b) R6G-MG pairs. [RBI = [R6G] = 2 X mol L-I. [PSI= 1.88 g L-I. P M O is number density of MG on PS latex surface, which is calculatedfrom weight concentration (1.88 g L-I), the diameter (1 55 nm) and the density (1.05 g cm-)) of the latex particle under the assumption that the latex is a sphere and its surface is smooth.
c’
i
cn
c c CI
x 1 o4
c’
lo3
:
lo2
=
10’
c, i i
0
( M GI I1 O-’mol
x
m
0
2
0
-.
22
11
i m e (ns) Figure 8. Fluorescence decay curve of (a) RB and (b) R6G in PS latex dispersion in the presence of MG. [RBI = [R6G] = 2 X lk7mol L-I. [MG] = 6 X lo-’ mol L-I and [PSI = 1.88 g L-I in both figures. RB and R6G are excited at 556 nm, and their fluorescences are detected at 600 nm. seems to be negligible on the data in Figure 6b. Therefore, when we made Perrin plots and Stern-Volmer plots for the data, we chose not to make a correction for any possible inner field effect. The results are shown in Figure 7. Good linearity is obtained for the Perrin plots, whereas the Stern-Volmer plots are curved, for both RB-MG and R6G-MG. This demonstrates that a static quenching mechanism operates for the RB-MG and R6G-MG pairs on the latex surface.17 Fluorescence decay curves of RB and R6G in the presence of M G are presented in Figure 8. The curvature at early times indicates energy transfer. According to Klafter and Blumen,l* the intensity decay I D ( t ) of donor fluorescence under DET is described by the expression 1
I D ( t ) = Bl exp[-(t/?D)
- P(f/?D)‘”]
(12)
where
P = A,(d/d)r(i - L I / S ) ( R , / U ) ~ (13) Here T D is the lifetime of donor without quencher, s is the order of the multiple interaction (s = 6 for a dipole-dipole mechanism). A , is the fraction of the sites occupied by the acceptor ( A I
