J. Phys. Chem. 1983, 87, 3759-3769
3759
Energy Transfer between Rhodamine 6G and Pinacyanol Enhanced with Sodium Dodecyl Sulfate in the Premicellar Region. Formation of Dye-Rich Induced Micelles‘ Hlroyasu Sato,
Masahlro Kawasakl, and Kazuo Kasatani
Chemistry Department of Resources, Faculty of Engineering, Mi‘e University, Tsu, 514 Japan (Received: September 29, 1982; In Final Form: March 8, 1983)
The energy transfer (ET) between rhodamine 6G (Rh-6G)and pinacyanol, both being cationic dyes, was studied in the presence of sodium dodecyl sulfate (SDS), an anionic detergent, as a function of the concentration of SDS ([SDS]). When we monitored the energy-transferefficiencyby pinacyanol monomer fluorescence,it increased with decreasing [SDS] and showed a distinct peak. The peak appeared below the critical micelle concentration (cmc),i.e., in the premicellar region when the dye concentration was low, although it shifted to a higher [SDS] with the increase in the dye concentration. On the basis of the [SDSI-dependent change in the absorption spectra of pinacyanol, it was concluded that the observed peak of the energy-transfer efficiency in the premicellar region is due to the association of both dyes in dye-rich induced micelles, which reduces the average distance between the donor (rhodamine 6G) and the acceptor (pinacyanol). The locally concentrated structure is supported also by picosecond fluorescence decay measurements.
Introduction The efficiency of energy transfer (ET) between dyes is enhanced by the presence of an appropriate detergent. This effect has been the subject of recent intense reEarlier studies were made on the donor-acceptor dye systems thionine-methylene blue2 and coumarin 1-rhodamine 6G (Rh-6G).3 An effective E T was observed at the concentrations of donor and acceptor dyes as low as 10-5-10-6M.2*3These dye-detergent systems can be considered as model membrane systems of chlorop l a s t ~ ,which ~ * ~ are peculiar in their very high efficiency as light-harvesting systems.1° While these studies were limited to the detergent concentrations above the critical micelle concentration (cmc),ll the measurement of the E T efficiency was made both above and below the cmc in the study of the Rh-GG-3,3’-diethylthiacarbocyanineiodide (DTC) system with sodium dodecyl sulfate (SDS) as a dete~-gent.~l~ In this case, the maximum efficiency was observed at the SDS concentration ([SDS]) a little below the cmc (i.e., in the premicellar r e g i ~ n ) The . ~ enhanced energy transfer in the premicellar region was attributed to the formation of dye-rich induced micelles. A similar enhancement of the E T efficiency in the premicellar region was recently reported by Mandal and Demas’ for tris(2,2’-bipyridine)ruthenium(II)-cresyl violet, etc. They attributed the effect to the formation of submicellar particles. The presence of submicellar particles (dye-rich
induced micelles) in the premicellar region for some dyedetergent systems has been reported by studies of absorption and luminescence spectra, luminescence decay or quenching, and electric The enhancement of the electron transfer in the premicellar region was found, and attributed to submicellar aggregates (dye-rich induced micelle^).'^^^^ However, the participation of dye-rich induced micelles in the enhancement of E T has not been directly proved. In the present paper is given the direct proof for such a mechanism, using Rh-6G and pinacyanol as the donor and the acceptor, respectively. This system is especially suited to such a study, because (1)pinacyanol is known to form dye-rich induced micelles with SDS12and because (2) the [SDSI-dependent formation of dye-rich induced micelles can be monitored with the [SDSI-dependent change in the absorption spectra of pinacyano1.l2v2l The effect of SDS, an anionic detergent, is discussed in relation to that of potassium poly(viny1 sulfate) (PVSK), an anionic polyelectrolyte, that of polyethylene glycol mono [p-(1,1,3,3-tetramethylbutyl)phenyl] ether (PGME), a nonionic detergent, and that of hexadecyltrimethylammonium bromide (HTAB), a cationic detergent.
(1)Reported briefly at the 10th International Conference on Photochemistry, Crete, Greece, 1981. A brief abstract is given in H. Sato, M. Kawasaki, and K. Kasatani, J . Photochem., 17,243 (1981). (2)G. S. Singhal, E. Rabinowitch, J. Hevesi, and V. Srinivasan, Photochem. Photobiol., 11, 531 (1970). (3)G. A.Kenney-Wallace, J. H. Flint, and S. C. Wallace, Chem. Phys. Lett., 32,71 (1975). (4)Y. Usui and A. Gotou, Photochem. Photobiol., 29, 165 (1979). (5)Y. Kusumoto and H. Sato, Chem. Phys. Lett., 68,13 (1979). (6)H. Sato, Y. Kusumoto, N. Nakashima, and K. Yoshihara, Chem. Phys. Lett., 71,326 (1980). (7)K. Mandal and J. N. Demas, Chem. Phys. Lett., 84,410 (1981). (8) T.Matsuo, Y. As0 and K. Kano, Ber. Bunsenges. Phys. Chem.. 84, 146 (1980). (9)P. K. Koelin, D. J. Miller, J. Steinwandel. and M. Hauser. J. Phvs. Chem., 85,2365 (1981). (IO) R. K. Clayton in “The Chlorophylls”, L. P. Vernon and G . R. Seely, Eds., Academic Press, New York, 1966,pp 620-3. (11)It is quoted in ref 2 that A. K. Ghosh found the evidence of enhanced energy transfer below the cmc. However, his data are not included in that article.
(12)P. Mukerjee and K. J. Mysels, J.Am. Chem. Soc., 77,2937 (1955). (13)M. Koizumi and N. Mataga, Nippon Kagaku Zasshi, 73,814,879 (1952);N.Mataga and M. Koizumi, ibid., 75,269,273(1954);M. Koizumi and N. Mataga, Bull. Chem. SOC.Jpn., 26,115 (1953);N.Mataga and M. Koizumi, ibid., 27, 197 (1954). (14)E. Lehoczki and J. Heveki, Dokl. Akad. Nauk SSSR, 206,1158 (1972),and references therein. (15)S. S. Atik and L. A. Singer, J. Am. Chem. Soc., 101,6759 (1979). (16)H.Sato, M. Kawasaki, K. Kasatani, Y. Kusumoto, N. Nakashima and K. Yoshihara, Chem. Lett., 1529 (1980). (17)J. H. Baxendale and M. A. J. Rodgers, Chem. Phys. Lett., 72,424 (1980);J. H. Baxendale and M. A. J. Rodgers, J . Phys. Chem., 86,4906 (1982). (18)K. Y. Law, Photochem. Photobiol., 33, 799 (1981). (19)M.A. J. Rodgers and J. C. Becker, J. Phys. Chem., 84, 2762 (1980). ’ (20) H. Sato, M. Kawasaki, K. Kasatani, and T . Ban, Chem. Lett., 1139 (1982). (21) Pinacyanol was used in the “spectral change method” of determining the cmc: W. D. Harkins, “The Physical Chemistry of Surface Films”, Reinhold, New York, 1952.
Experimental Section Rh-6G and pinacyanol (Eastman Kodak) were spectroscopically pure and were used without further purification.
0022-3654/83/2087-3759$01.50/00 1983 American Chemical Society
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SDS (protein research grade of Nakarai Chemicals) was recrystallized from ethanol. PGME (Nakarai, liquid scintillation grade), HTAB (Nakarai, guaranteed reagent), and PVSK (Wako, n = 1500 up, 90.0% esterification) were used as received. Water was distilled twice. In the measurements of absorption and fluorescence spectra, stock solutions of the dye were mixed with those of SDS. Because the fluorescence intensity changed with time for some solutions containing pinacyanol and SDS, the solutions were kept in the dark until the intensity change was undetectable (ca. 2 h for the samples with [SDS] 2 6-8 mM, longer time for those with [SDS] S 6-8 mM) before the measurement. For PGME, HTAB, or PVSK, the stock solutions of each reagent were mixed with those of the dyes. Absorption spectra were obtained by a Cary-14 spectrophotometer. Fluorescence spectra were measured by a Hitachi 650-10s fluorescence spectrophotometer with a Hamamatsu R 928 photomultiplier. All fluorescence intensities were corrected for the attenuation of the exciting and emitting light beams in the sample solution. Briefly, the intensity of exciting light was obtained as a function of penetration depth by using Beer's law. The amount of fluorescence absorbed by the solution for a fluorescing volume element at a given distance from the window was also obtained by Beer's law. The correction was made by integrating over all distances. The corrections were also made for the photomultiplier response function. All spectral measurements were made at room temperature. Because purging with nitrogen did not cause any appreciable change, the measurements were made for aerated solutions. In the measurement of ET, the stock solutions of Rh-6G and those of pinacyanol were mixed, and the resulting solution was mixed with the stock solutions of SDS. The concentration ratio of Rh-6G and pinacyanol was adjusted to be ca. 1:lO. The sample solution contained in a 1-cm Pyrex cell was excited with 480-nm light. This excitation wavelength was selected to excite Rh-6G and to minimize the direct excitation of pinacyanol. A correction for a small amount of fluorescence intensity of pinacyanol excited directly was made by measuring the fluorescence of the pinacyanol-SDS solution as a blank. The blank solution was prepared at the same time as the sample solution in such a way that the only difference was the presence and absence of Rh-6G. Their fluorescence measurements were made successively, each sample after the corresponding blank. In the fluorescence decay measurements, the dye solution in a quartz spectrofluorometer cell w8s pumped with the third harmonic of an Nd3+:YAGlaser. Fluorescence decay was measured with a Hamamatsu C 979 streak camera. Fluorescence of Rh-6G a t 550 nm was selected by a Nihon-Shinkukogaku interference filter. The time scale of the streak camera was fully calibrated. All measurements were made at room temperature for aerated solutions. The results of three shots on the same sample were averaged. The cmc of SDS was determined conductometrically, first for SDS alone and then for (SDS plus both dyes) mixtures. The electrodes of the conductivity cell were platinum plates without platinum black. The temperature of the cell was kept a t 25.0 f 0.1 "C. The bridges used were a Yanaco MY-8 conductivity outfit or a Yokogawa Hewlett-Packard 4255A universal bridge. The cmc of PGME was obtained at 25.0 f 0.1 "C by stalagmometry. Results and Discussion 1. Cmc Of sDsSolutions* The cmc determined for the SDS solution in the absence of the dye was 7.4 f 0.1 mM,
I
I
I
7
8
I
9
10
I I
11
Flgure 1. Equivalent conductivity of sodium dodecyl sulfate: (0)in water, (0)with Rh-6G (1.4 X 10-5 M) and pinacyanol (1.6 X 104 M), not corrected for the CI- present (cf. footnote 23).
in good agreement with the generally accepted published value (8.1 mM).22 In the presence of both dyes (Rh-6G, 1.4 X M; pinacyanol, 1.6 X lov4M), the equivalent conductivity vs. [SDS]1/2plot showed a deviation from linearity for [SDS] 2 6.1 mM as shown in Figure lF3 This deviation is quite similar to that found by Mukerjee and Mysels12for the pinacyanol-SDS system. If we estimate the cmc of this system by the point of intersection of two lines extrapolating from above and below the cmc, we obtain the value of 7.0 f 0.1 mM. The presence of dyes caused only a little shift in the cmc. 2. Spectra. The absorption and fluorescence spectra of pinacyanol-SDS and Rh-6GSDS solutions are shown in Figure 2 in comparison with those of aqueous solutions. ( A ) Pinacyanol in Aqueous and SDS Solutions. For dilute aqueous solutions of pinacyanol (e.g., 1.1X 10" and 3.6 X M), the observed absorption band at 600 nm and that at 548 nm are apparently correlated to monomeric and dimeric species, respectively, since the latter increased in intensity in comparison with the former by an increase in the dye concentration. For a larger concentration of pinacyanol (e.g., 1.6 X M), a shoulder appeared at ca. 520 nm. This is due to the H aggregate of the dye.24 Absorption spectra of pinacyanol-SDS solutions showed a very striking [SDSI-dependent change, which was essentially the same as Mukerjee and Mysels12reported. In the higher [SDS] region (hereafter called region A), the solution was blue. Two absorption bands were observed ( a band at 610 nm and p band at 565 nm). A fluorescence band appeared at 656 nm. In the lower [SDS] region (hereafter called region B), the solution was red. Both the LY band and the p band disappeared. An absorption band (y band) appeared at 480 nm instead of them. This band can be assigned, after Mukerjee and Mysels,12 to the (22) J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, 1975. (23) The equivalent conductivity is not corrected for the Cl- ion present in the dye-SDS solution. The upward shift of the straight line in the left side (the Onsager slope) is due to this effect. (24) (a) W. West and S. Pearce, J.Phys. Chem., 69, 1894 (1965); (b) W. West, S. P. Lovell, and W. Cooper, Photogr. Sci. Eng., 14, 52 (1970). Although the y band has an appearance similar to the band of the H2 aggregate (a higher aggregate than the H aggregate) found in alcoholic solvents a t low temuerature in ref 24b. it showed a different time-dependent behavior: tke y band was stable or changed into the H aggregate with a bathochromic shift, while the H2 aggregate changed into the H* (higher than H2) aggregate with a hypsochromic shift.
The Journal of Physical Chemistty, Vol. 87, No. 19, 1983
Formation of Dye-Rich Induced Micelles
"
400
500 600 700 WAVELENGTH/nm
800
SI
I
"
400
500
3761
600
Wavelengt h/nm Flgure 3. Absorption spectra: (i-iv) pinacyanol (1.2X M) plus M), within 3 min, 30 min, 1 h, and 1 1 h after mixing, SDS (2.0X respectively; (v) pinacyanol (1.2X lo-' M) in water.
200
500 600 700 WAVELENGTHhm
800
io0
500 600 700 WAVELENGTH/nm
800
Flgure 2. Absorption and fluorescence spectra: (-) Rh4G (1.1 X M), (---) pinacyanol (1.1 X M); (a) [SDS] = 0, (b) [SDS] = 10.0 X M, (c) [SDS] = 5.0 X M.
dye-detergent complex ("salt") (D+S-),, where D+and Sare the dye cation and dodecyl sulfate anion, respectively. In this complex (salt), dye cations are aggregated with S-. An emission band (Amu 790 nm) was observed. For small concentrations of pinacyanol (e.g., 1.1 X 10" and 3.6 X 10" M), the y band was found to be stable. For a larger concentration (e.g., 1.6 X M), however, its intensity decreased with time, and another band appeared gradually at 519 nm, which has the characteristic bandshape of the H aggregate reported by West and PearceZ4(Figure 3). The H-aggregate formation was assisted by the presence of SDS in this case. In the transient region between regions A and B, the a and p bands increased and the y band decreased in
-
intensity with [SDS]. The 656-nm fluorescence was observed in this region. The boundary of this region with region A can be determined either by the disappearance of the y band or by the leveling off of the P-band intensity. When we determined the boundary in the latter way, it was found in the premicellar region when the dye concentrations were low. However, it shifted to a higher [SDS] with the concentration of pinacyanol (e.g., 6.4 mM for 1.1 X M dye, 7.2 mM for 3.6 X M dye). The spectral change reported here was found to be reversible, in the sence that the spectra in region A or B could be obtained repeatedly by increasing or decreasing the [SDS] of the solutions. ( B )Rh-6G i n Aqueous and SDS Solutions. The absorption bands at 530 and 500 nm of aqueous solution of Rh-6G are those of monomer and dimer, respectively. With [SDS] far below the cmc, the intensity of the 500-nm band increased at the expense of that of the monomer band. The quenching of fluorescence occurred in this region. These findings agree well with those of Koizumi and Mataga.13 They attributed these to the formation of dimer or higher aggregates of dye molecules by the interaction with SDS. With [SDS] above the cmc, the absorption band was found at 535 nm with a shoulder at 500 nm. The fluorescence band was found at 564 nm. The monomer-type appearance of the absorption band and the recovery in the fluorescence intensity indicated the deaggregation of the dye. ( C ) Fluorescence Intensity of Rh-6G and Pinacyanol in SDS Solutions. The change of the fluorescence intensity (peak height) with [SDS] (normalized at large [SDS]) is shown in Figure 4. The fluorescence intensity was essentially constant for large [SDS] and decreased with decreasing [SDS] below a certain limiting [SDS]. The limiting value shifted to a larger [SDS] with the concentration of the dye. One may note that the limiting [SDS] of pinacyanol is larger than that of Rh-6G of the same concentration. In Figure 5, the absorbance of the 610-nm band (monomer band) of pinacyanol (denoted as is shown in comparison with the intensity of the fluorescence band at 656 nm (IF). (Both quantities are normalized at large [SDS].) The absorbance Ae10 took a constant value for large [SDS] and decreased with decreasing [SDS] below a certain limiting [SDS]. The decrease of fluorescence intensity started at the larger [SDS] than that of absorbance. In other words, IF/A610was constant for large [SDS] but showed a marked decrease in the marginal [SDS] region. These findings clearly show for pinacyanol (a) that only the monomer is fluorescent and (b) that the
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The Journal of Physical Chemistry, Vol. 87, No. 19, 1983
5t
r
f
:
i
300
LL
, O S
10
15 EOSI/ mM
I
20
10
t
500 1
1
10 15 l S D S l / mM
800
Figure 6. Absorption and fluorescence spectra of Rh-6G-PVSK and pinacyanol-PVSK systems ([PVSK] = 1.0 X g/mL): (-) Rh-6G (1.0 X M), (---) pinacyanol (1.0 X lo-' M).
Flgure 4. Fluorescence intensity as a function of SDS concentration: (a) Rh-6G (Amx = 564 nm, excited at 480 nm), (b) pinacyanol A(, = 656 nm, excited at 550 nm). Dye concentration: (0)1.2 X lo-' and (0)3.9 X M. Intensities are normalized at large [SDS].
O' 5
500 600 700 Wavelength (nm)
400
20
Figue 5. Fluorescenceintensity (IF)and absorbance at the monomer peak (A e,o) of pinacyanol (1.1 X M) as a function of SDS concentration. Both values are normalized at large [SDS] .
monomer fluorescence is quenched by the dimer which appears in the marginal [SDS] region. The same can be mentioned about Rh-6G, and we can conclude that the decrease of IFwith decreasing [SDS] as shown in Figure 4 is caused by the two factors, a and b, mentioned above. (D) Rh-6G and PinacyanollPVSK and IPGME. Absorption and fluorescence spectra of these dyes in PVSK and PGME solutions are shown in Figures 6 and 7, reM)-PVSK, the intensity spectively. For Rh-6G (1.0 X ratio of the 500-nm (dimer) band to the 530-nm (monomer) band was larger than in aqueous solution for a small [PVSK] (e.g., 1.0 X g/mL). Fluorescence quenching was observed in this case. The intensity ratio approached that of aqueous solution for a larger [PVSK] (e.g., 1.0 x g/mL), with the recovery of the fluorescence intensity. Such a finding is in accordance with that of Koizumi and Mataga13 and shows the dye aggregate formation for the small [PVSK]. The intensity ratio was smaller than in aqueous solution for Rh-6G (1.0 X M)-PGME (20.2 w t YO),showing the deaggregation effect of PGME on the dye. (The cmc of PGME was determined to be 1.3 X w t YO at 25.0 f 0.1 "C by stalagmometry.) For the pinacyanol-PVSK system with a small [PVSK] (e.g., 1.0 x g/mL), a red solution (XZA = 480,550 nm; AEax = 780 nm) was obtained as shown in Figure 6. The spectra are similar to those of SDS solutions in region B. A blue solution (A& = 570,610 nm; XEax = 660 nm) was obtained for the pinacyanol-PGME system ([PGME] 2 0.2 wt %), as shown in Figure 7. The spectra are similar to those of SDS solutions in region A.
400
600
800
"
Wavelengt h/nm
Figure 7. Absorptlon and fluorescence spectra of Rh-6G-PGME and pinacyanol-PGME systems ([PGME] = 0.8 wt %): (-) Rh-6G (1.0 X lo-' M), (---) pinacyanol (1.0 X M).
( E )Effect of SDS on the Aggregation State of the Dye. The [SDSI-dependent spectral behaviors of the dye-SDS systems and the comparison of them with those of the dye-PVSK and -PGME systems reveal two different effects of SDS on the dye, aggregation and deaggregation. In region B the effect of SDS is similar to that of PVSK, suggesting the formation of the essentially 1:l complex (DW),. Dye molecules are aggregated with S- in this complex. When the dye concentration is high enough, the complex changed into the H aggregate with time. The H-aggregate formation is assisted by the presence of SDS in this case. A similar spectral behavior was found in the dye-PVSK systems. Thus, it is clear that the interaction of the dye cation with the OSO, group is responsible for the action of SDS causing aggregation of the dyes. In region A, the effect like that of PGME, i.e., the deaggregation of the dye by association with micelles, is apparent. Thus, the micellization of the dyes, which includes the interaction of dye cations with the hydrophobic micellar interior, is shown to be the main cause of the deaggregation of the dye. SDS exerts two opposite effects on the aggregation state of the dye, one appearing in region B and the other in region A. 3. Energy-Transfer Efficiency. ( A )Energy-Transfer Efficiency i n SDS Solutions. In the following discussion of ET, we measure the efficiency of ET to the pinacyanol monomer (eET)by monitoring the 656-nm fluorescence. The other species which quench the fluorescence of the donor in competition with the pinacyanol monomer are considered as quenchers. The efficiency of E T to these quenchers (e,) is not monitored. So, in the estimation of ET efficiency as a function of [SDS], we studied only the
Formation of Dye-Rich Induced Mlcelles
The Journal of Physical Chemlstty, Vol. 87, No. 19, 1983 3763
ceptor monomer) on the peak height baskz5 eET. When the pinacyanol monomer is the only acceptor (case I), Zl= aN(1- em)qDand Z2 - 4 = b N e d k Factors a and b are used because the fluorescence intensities are expressed in arbitrary units. We assume a = b and we have NeETqA
N(1 - eET)qD
= -I 2 - 4
11
(1)
or ET =
II/qD
+ (12 - 1 3 ) / q A
(2)
When there is quencher(s) besides the pinacyanol monomer (case 11),eq 1 and 2 have to be modified. eq is the ET efficiency to the quencher(s), and we have
CSDSl/ mM i
(I2 - I 3 ) / q A
NeETqA N(1 - ET - e,)qD
i
BET
=
e, =
-- 12 - 13
(I2 - 4 ) / q A
Il/qD +
(I2
- I 3 ) / q A + nq
n, I l / q D + (I2 - I 3 ) / q A
(3)
I1
+ nq
(4) (5)
where n, corresponds to the number of donor monomers which transferred the excitation energy to the quencher(s). eEL. The other way of obtaining the ET efficiency is with the “energy loss”, the fractional loss of donor fluorescence: 26*27 EL = (Io - Ii)/Io i
5
10 C SDS1/
i
15 mM
20
Figure 8. Observed fluorescence intensities: (0)I , , (0)I , - I 3 (multiplied as shown), ( 0 )lOI,,(B)1011, - I&. The concentrations of R h 4 G and pinacyanol are as follows: (I) 1.1 X 10“ and 1.1 X M, and (ii) 3.6 X 10“ and 3.6 X M. The ratio ( I , 13)/1,is also given. For T, A,, and A,, see text.
-
region where the 656-nm fluorescence of pinacyanol was observed, i.e., region A and the transient region. We measured the fluorescence spectra of mixed solutions of Rh-6G and pinacyanol at fixed dye concentrations in the presence of SDS. The concentrations of Rh-6G and pinacyanol were as follows: (i) 1.1 X lo4 and 1.1 X M, and (ii) 3.6 X lo4 and 3.6 X lob M. Energy transfer was revealed by the quenching of the donor fluorescence and by the appearance of the sensitized fluorescence of the acceptor. The fluorescence intensities (peak heights)%of the donor and acceptor observed in the mixed solution are denoted as Il and 12,respectively. The fluorescence intensity of the blank solution (acceptor-SDS) is denoted as 13. The observed fluorescence intensities Zland I2- I3 are shown with their ratio as a function of [SDS] in Figure 8. The E T efficiency can be obtained in either of the following two ways, em and eEb We put N = number of the excited donor monomer, and q D (or qA) = relative fluorescence efficiency of the donor monomer (orthe ac(25) The fluorescence intensities are expressed by peak heights (Pa) instead of the integrated intensities on the wavenumber scale (Ss). (The ratio S / I for each dye was found to be essentially constant through the (SDS] region studied.) Then q A or is the quantum yield divided by a factor to convert I into S. Because the S / I value of the acceptor was 1.4 times larger than them of the donor, q A / q D is (1/1.4) timea the relative quantum yield.
(6)
where Io is the fluorescence intensity of the donor (Rh6G)-SDS solution which has the same concentration of Rh-6G as the Rh-6Gpinacyanol-SDS solution in question. When one uses the relation Io = aNqD,it is easy to show that em = em for case I and eEL= em + e, for case 11. In the treatment given above it is implicitly assumed that one donor molecule is deexcited by every ET (including the quenching) process. In other words, the E T is assumed to be solely nonradiative. ( B ) [SDS] Far Above the Cmc. Intermicellar Energy Tramfer. As shown in Figure 8, both Zl and I2 - I3 increased with [SDS]. They showed the tendency to level off at large [SDS]. At [SDS] 20 mM, both Il and I2I3 were nearly independent of [SDS]. In these cases, both the donor and acceptor dyes are present essentially as monomers associated with micelles, as revealed by their absorption spectra. (In the present study we can ignore the dye dissolved in the aqueous phase, since its quantity can be estimated to be very small in comparison with the micellized dye, judging from the absorption spectra.) The energy transfer occurs between Rh-6G monomer and pinacyanol monomer (case I). It must be governed essentially by the intermicellar process. Both q D and q A can be assumed to be constant. The ratio qA/qD was experimentally determined as nearly equal to 1/310.28 When we put this value into eq 2 together with the observed values of Il and Z2- I,, we obtain a very large ET efficiency
-
(26) A. Mehreteab and G . Strauss, Photochem. Photobiol., 28, 369 (1978). (27) Y.Aso, K. Kano, and T. Matsuo, Biochim. Biophys. Acta, 599, 403 (1980). (28) The relative fluorescence quantum yield of the pinacyanol-SDS system with respect to the Rh-GGSDS system was measured at [SDS] = 20 mM for the dye concentrations i and ii to be 1/220. The excitation wavelength was 480 nm. Then qa/qD = (1/1.4)(1/220) = 1/310 (see ref 25).
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The Journal of Physical Chemistry, Vol. 87, No. 19, 1983
-
2 - -0.21
l
0
" 5
10
15
IO
,
I
20
3p
l
40
60
50
20
CSDS l/m M Figure 9, Energy loss (ea). [Rh-60] and [plnacyanol] are the Same as in Figure 8. (BET). However, the following consideration shows that such a high efficiency is not explicable if the E T is solely due to the nonradiative (the F o r ~ t e r - t y p emechanism ~~) for the present case. The efficiency of the intermicellar nonradiative E T between micelles occupied by only one dye molecule can be estimated. This process is different from the E T between dye molecules in the homogeneous solution in two respects, i.e., (1) the wavelength shift and intensity change in the absorption and fluorescence spectra, and (2) the change in the translational and rotational diffusion rate30 which leads to a different value in the orientation factor.31 The critical transfer distance (R,) calculated for [SDS] = 20 mM from the spectral overlap was 65 A, which gives the critical concentration Co = 1.2 X M. (These values are almost the same as those in the homogeneous solution: Ro = 64 A and Co = 1.2 X M in water.) We can calculate the nonradiative E T efficiency due to the Forster mechanism by the formulasz9
edcd =
exp(s2)[1- 4(dl
(7)
I
O;
,OOlEiO
'
500 '
I
'
' 1000 I '
Figure 10. (a) In [I (f)/IF(f)] plotted as a functlon of Ia(t): [Rh-60] = 1.0 X 103 M, [plnacyanol] = 1.0 X lo-' M, [SDS] = 20 mM. I#): [Rh-BG] = 1.0 X lo-' M, [SDS] = 20 mM. The range of the scatter in the data is shown by vertical bars. The slope of the straight llne was calculated with R , = 65 A. (b) I#) and I a ( t ) . [Rh-60] and [plnacyanol]: the same as above, [SDS] = 6.4 mM. Note the difference of the tlme scale for 0 It I180 ps and t > 180 ps. The range of the scatter In data Is shown by vertical bars. Curves I-lv are calculated by eq 9. [A],/Co are 4, 5, 6, and 7 for i-lv, respectively. Curves v and VI are calculated by eq 13 and 14. n = 1 and 2 for v and VI, respectlvely.
of acceptor molecules, when the homogeneous distribution of acceptors can be assumed: 6 m 3 2 9 3 3
where [A] and Co are the acceptor concentration used and the critical concentration of ET, respectively. For the dye concentrations i and ii, we obtain edcd = 0.016 and 0.051 M. using Ro = 65 A and Co = 1.2 X The ET efficiency by the nonradiatiue process can be obtained by the energy loss (eEL)of eq 6. (Recall that Il is corrected for the reabsorption of fluorescence in the solution. So, it should, in principle, be free from the quenching due to the radiative ET (the trivial process).) Its values are shown as a function of [SDS] in Figure 9. The values at [SDS] = 20 mM were 0.05 and 0.09 for the dye concentrations i and ii, respectively. They are of the same order as the calculated values. The difference may be due to the presence of a small amount of dye aggregates even in this [SDS] region. It may, however, come from the error in the correction for the reabsorption. The fluorescence decays of Rh-6G in the absence (IF(t)) and in the presence ( Z e ( t ) ) of pinacyanol were studied at [SDS] = 20 mM. The concentrations of the dyes were those of i. The result is shown in Figure loa. It is known that the following holds for the time-dependent fluorescence decay of the donor in the presence of a large number (29)Th. Forster, Discuss. Faraday SOC.,27, 7 (1959). (30)H.E. Leasing and A. von Jena, Chem. Phys.,41, 395 (1979). is 2/3 and 0.475for (31)The average value of an orientation factor (2) the cases that the rate of rotation ( T ; ~ ) is much larger and much smaller than that of ET, respectively. In the calculation of Ro we assumed K* = in the aqueous solution, since the reported value of T~ (160ps, H. J. Eichler, U. Klein, and D. Langhans, Chem. Phys. Lett., 67,21(1979))is very small compared to the fluorescence lifetime of Rh-6G (4ns). We assumed K~ = 0.475in micelles, since values of Beveral nanoseconds were reported as T ? of several dyes including Rh-6G in micelles (ref 30).
In [ r ~ ( t ) / I ~ ( t=) ]In [IQ(O)/IF(O)]- ~ ( [ A ] / C J T D - ' / ~ ~ ~ / ~ (11) where 7 D is the fluorescence lifetime of the donor in the absence of ET. These equations can be applied to the present case, because (1) the ET is of an intermicellar nature and (2) micelles are distributed homogeneously in the solution. As shown in Figure loa, the difference in the fluorescence intensity caused by the presence of 1.0 X 10" M of pinacyanol w a ~ very small and was hidden within the experimental error. No indication of any larger E T efficiency than that expected from eELwas found. These findings show that the E T efficiency due to the nonradiative process is very small in comparison to the observed BET, which has a very large value. We must conclude, therefore, that the major part of the large observed value of I2 - I3 comes from the radiatiue E T in which the acceptor absorbs the fluorescence of the donor. Equation 1 is to be modified as
--- 13 - N[eET" + (1 - eE$)eETrlqA 12
I1
--
N(1 - eETn)(l- eET')qD
eET'qA (12) (1 - eETn)(l- eETr)qD (1 - eET')qD where e&' and eETr are the E T efficiency due to the nonradiative and radiative ET, respectively. The latter eETnqA
+
(32)J. B. Birks, J. Phys. B,Ser. 2, 1, 946 (1968). (33)G.Porter and C. J. Tredwell, Chem. Phys.Lett., 56, 278 (1978).
Formation of Dye-Rich Induced Micelles
The Journal of Physical Chemistry, Vol. 87,No. 19, 1983
I
i
\
I
,43 0.03 0.02 a 0.01
'?J
Rd
t
-0.01 -0.02t
Rfn
450 500 550 600 650 700 WAVELENGTH/ nm
Figure 11. (a) Absorption spectra of Rh-BO-pinacyanol-SDS (l), of plnacyanol-SDS (2), and of Rh-BG-SDS (3). (b) The difference in absorbance (AA = 1 (2 -I-3)). R,, R,, P@ and P, Indicate the positions of peaks of Rh-6G and pinacyanol (seetext). [Rh-601 = 1.1 X 10" M, [plnacyanol] = 1.1 X M, [SDS] = 6.8 mM. (c, inset) AA at 480 nm vs. [SDS]. [Rh-BG] and [pinacyanol] are same as above.
-
depends on the absorbance of the acceptor at the wavelength of the donor fluorescence (AA). It is independent of [SDS] as far as AA is so. When the radiative E T is the dominating part in the total ET (i.e., emn
