754
J . Phys. Chem. 1988, 92, 754-759
Light-Harvesting Effect in Photoelectric Conversion with Dye MultiJayers on a Semiconductor Electrode’ Hiroyasu Sato,* Masahiro Kawasaki, Kazuo Kasatani, Yuji Higuchi, Terukazu Azuma, and Yasushi Nishiyama Chemistry Department of Resources, Faculty of Engineering, Mi’e University, Tsu 51 4, Japan (Received: March 9, 1987; In Final Form: September 9, 1987)
A light-harvesting effect in the sensitized photocurrent of dyecontaining multilayers on an Sb-doped SnOl optically transparent semiconductor electrode was studied. 2,8-Bis(dimethylamino)-l0-dodecylacridiniumbromide (BDA) was included as an antenna dye in the outer monolayer separated from the semiconductor surface with fatty acid monolayer(s), and dioctadecylthiacarbocyanine iodide (DTC) as a reaction center dye was incorporated in the monolayer in direct contact with the semiconductor. A remarkable enhancement of the photocurrent was observed as a result of efficient energy transfer from BDA to DTC.
Introduction The concept of solar energy conversion to electrical energy has been realized by the use of electrochemical photocells. Photocurrents at a stable semiconductor with a large band gap can be obtained with visible light by the use of organic dyes as sensitizers. The mechanism of such a photosensitization process has been studied extensively because of its importance to photoelectric conversion of solar energy and also to the photographic sensitization The use of dye-containing monolayers and multilayers formed by the Langmuir-Blodgett technique5 has a very attractive feature in this context, since dye molecules are in well-defined geometrical configurations to molecular size. Various monolayers and multilayers composed of fatty acids or fatty acid salts incorporating organic dyes have been prepared and their spectroscopic and electrical properties have been reported.6-12 Sensitized photocurrents of dye-containing monolayers on a semiconductor e l e c t r ~ d e ’and ~ . ~photographic ~ latent image formation with dye-containing multilayer on silver halide crystalsI5 have been studied. However, only the papers of Arden and Fromher~’~.’’ and Fujihira et a].]*have been published so far, to ~
~~~
the best of our knowledge, on photocurrents sensitized by a dye-containing multilayer on a semiconductor electrode. In these cases, the sensitizing dye and the electron-injecting dye were incorporated in adjacent monolayers. In the present paper, we studied sensitized photocurrents of dye-containing multilayers on a semiconductor electrode, with particular emphasis on enhancement of photocurrent by the use of the sensitizing dye, separated from the electron-injecting dye by more than two monolayers. The former and the latter dye correspond to the antenna and reaction-center dye in the chloroplasts, respectively. Multilayers of arachidic acid were deposited on an optically transparent Sb-doped SnOz electrode by means of the Langmuir-Blodgett t e c h n i q ~ e .Two ~ kinds of dyes were incorporated in multilayers: one was an antenna dye which was deposited in the “outer” layer separated from the semiconductor with fatty acid monolayer(s), and the other was the reaction-center dye which was incorporated in the “first” layer, in direct contact with the semiconductor. The antenna dye, excited by external illumination, transfers its excitation energy to the reaction-center dye, which in turn transfers an electron to the semiconductor. 2,8-Bis(dimethylamino)- 10-dodecylacridinium bromide (BDA) and dioctadecylthiacarbocyanine iodide (DTC) were used as the antenna and reaction center dye, respectively. The photocurrent quantum yields were studied for various types of multilayers. Experiments with only one dye, BDA, were made as blank tests to clarify the enhancement of photocurrent as the result of energy transfer.
(1) Briefly reported in: Higuchi, Y.; Kasatani, K.; Kawasaki, M.; Sato, H. Chem. Lett. 1986, 1651-1654. (2) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31-84. (3) Heller, A. Acc. Chem. Res. 1981, 14, 154-162. (4) Memming, R. Prog. Surf. Sci. 1984, 17, 7-74. (5) Blodgett, K. B. J. A m . Chem. SOC.1935, 57, 1007-1022. (6) Kuhn, H. Pure Appl. Chem. 1979,51,341-352; 1981.53, 2105-2122; J. Photochem. 1979, 10, 111-132 and references therein. (7) Whitten, D. G . Angew. Chem., Intl. Ed. Engl. 1979, 18, 440-450 and references therein. (8) Mann, B.; Kuhn, H . J. Appl. Phys. 1971, 42, 4398-4405. (9) Polymeropoulos, E. E. J. Appl. Phys. 1977, 48, 2404-2407. Polymeropoulos, E. E.; Sagiv, J. J . Chem. Phys. 1978, 69, 1836-1847. (10) Sugi, M.; Nembach, K.; Mobius, D.; Kuhn, H. Solid State Commun. 1974, 15, 1867-1870. Sugi, M.; Fukui, T.; Iizima, S. Appl. Phys. Lett. 1975, 27, 559-561. Sugi, M.; Nembach, K.; M6bius, D. Thin Solid Films 1975, 27, 205-216. Sugi, M.; Fukui, T.; Iizima, S. Mol. Cryst. Liq. Cryst. 1979, 50, 183-200. Sugi, M.; Iizima, S . Thin Solid Films 1980, 68, 199-204. ( 1 1) Mbbius, D. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 848-858. (12) Yamamoto, N.; Ohnishi, T.; Hatakeyama, M.; Tsubomura, H. Thin Solid Films 1980, 68, 191-198. (13) Memming, R. Faraday Discuss. Chem. SOC.1975, 58, 261-270. (14) Miyasaka, T.; Watanabe, T.; Fujishima, A,; Honda, K. J. Am. Chem. SOC.1978, 100, 6657-6665. Miyasaka, T.;Fujishima, A,; Honda, K. Bull. Chem. SOC.Jpn. 1981, 54, 957-961. (15) Steiger, R.; Heidiger, H.; Junod, P.; Kuhn, H.; Mbbius, D. Photogr. Sci. Eng. 1980, 24, 185-195. (16) Arden, W.; Fromherz, P. Ber. Bunsen-Ges. Phys. Chem. 1978, 82,
Experimental Section The dyes used are shown in Figure 1. The molecular structure of BDA is the same as acridine orange (AO), except for the presence of a long alkyl chain. BDA and DTC were purchased from Dojin Co. and the Japanese Research Institute for Photosensitizing Dyes, Co., respectively, and were used as received. As the substrate on which the multilayer was built in, an Sb-doped SnO, optically transparent electrode (Tokyo Eriko Co.) was used. The electrode was a 2 cm X 4 cm glass with one side coated with S n 0 2 . The charge-carrier density of the n-type material deter. mined from a Mott-Schottky plot was (3-8) X lOI9 ~ m - ~Prior to deposition of the multilayer, the electrode was washed with a chromic acid mixture and rinsed with doubly distilled water. Ohmic contact between the semiconductor and copper wire was made with indium. The multilayers were deposited by means of Langmuir-Blodgett technique. The arachidic acid monolayer including dye molecules was prepared on the water surface, and was transferred onto the substrate by raising it slowly out of the water trough. A constant
868-874. (17) Fromherz, P.; Arden, W. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 1045-1050; J. Am. Chem. Soc. 1980,102,621 1-6218. Arden, W.; Fromherz, P. J. Electrochem. SOC.1980, 127, 370-378.
(18) Fujihira, M.; Nishiyama, K.; Yamada, H. Thin Solid Films 1985, 132. 11.
0022-3654/88/2092-0754$01.50/00 1988 American Chemical Society
,
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 755
Photoelectric Conversion with Dye Multilayers
C12H25 1
[cH3.N"/cCH3 H3/
(a)
\ CH3
BDA
B;
io0
500 600 WAVELENGTH / n m
Figure 4. Absorption (i) and fluorescence (ii) spectra of BDA (2 X lo-' (b) DTC Figure 1. Structural formulas of (a) BDA and (b) DTC.
M) and absorption spectrum of DTC (2 X M) (iii) in chloform. The fluorescence spectrum has been corrected for the response of the photomultiplier. 71
400
500
600
700
0
WAVELENGTH / nm
Figure 5. Absorption (i) and fluorescence (ii) spectra of monolayer a D TC
c BOA
c-
FATTY ACID
Figure 2. Structures of multilayers. Multilayers a 4 were used for experiments containing only one dye (BDA) and e-h for experiments containing two dyes (BDA and DTC). DTC was deposited on the substrate directly.
Ar'
Laser
1
+
Filter
Mirror
containing BDA, and (iii) absorption spectrum of DTC-containing monolayer (DTC:arachidicacid = 1:lO). The fluorescence spectrum has been corrected for the response of the photomultiplier. trolyte was KCl (0.5 M). Thiourea (1 M) was added as a "supersensitizer"; Le., without this material, the observed photocurrent decreased drastically after each run of the measurement. Apparently, thiourea acts to reduce the oxidized dye.I6J7 A 150-W xenon arc lamp in combination with a Hitachi Model 650-10s spectrofluorometer was used as a light source for measurements of action spectra of the photocurrent. Measurements of the intrinsic current quantum efficiency were made with 476.5-nm light from a Spectra-Physics Model 165 Ar' laser for the monolayer and multilayers incorporating BDA or those incorporating both BDA and DTC; monochromatic light at several wavelengths between 540 and 570 nm from a Spectra-Physics Model 375 dye laser was used for the monolayers containing only DTC. In both cases, the exciting light was chopped at 15 Hz. Photocurrent was measured with an N F Electronic Instruments Model LI-570 lock-in amplifier equipped with a Model LI-75A preamplifier. The same measurement was repeated with 5-10 ~amp1es.I~Cyclic voltammograms of BDA were obtained with a home-made function generator, a potentiostat, and an X-Y recorder. All measurements were made at room temperature on aerated solutions, except for measurements of the temperature effect.
Results and Discussion Experiments with Two Dyes (BDA and DTC). Figure 4 shows absorption and fluorescence spectra of the dyes BDA and DTC in chloroform. Efficient energy transfer can be expected between these two dyes, in view of the high fluorescence yield of the donor (BDA) and the good overlap of its fluorescence band with the absorption band of the acceptor (DTC). We expected BDA in the outer monolayer to transfer its excitation energy to DTC in the first monolayer and DTC then transfer an electron to the semiconductor. Curves (i) and (ii) and Figure 5 show the absorption and fluorescence spectra of a BDA-containing monolayer (a in Figure 2), and curve (iii) in the same figure shows the absorption spectrum of a DTC-containing monolayer (DTC:arachidic acid = 1 :lo). We note that the positions of the absorption peaks are almost the (19) We did not add, e.g., CdC12 to bidistilled water in the transfer of monolayer onto the SnO, substrate, because the addition of CdCI2 did not cause any essential change to the results. No appreciable decrease of photocurrent was observed for repeated measurements when thiourea was used.
Sat0 et al.
756 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 -3
N
0 -
,-.
\
J z
-
l -
\
Q
m
n
0
-
v)
m
-
' I
'.
-5 +L v
1
0 0 J
-7
0
$ c
1
150
A
Figure 8. Logarithm of the intrinsic current quantum efficiency ( 9 ) at 476.5 nm vs the vertical distance between the layer of BDA chromophore
_I-
. ..-
W A V E LE NGTH/
50 100 DISTANCE I
i
and the semiconductor electrode. The symbols a-h correspond to those in Figure 2. No photocurrent could be detected for multilayer d.
nm
Figure 7. Photocurrent action spectrum of multilayer g containing BDA and DTC, normalized to the incident photon flux.
same as in chloroform solution. The absorption maximum and the shoulder in spectrum (i) found at 470 and 490 MI, respectively, must be due to the dimer and monomer of BDA, since (1) the absorption spectrum of BDA in H20/CH30H(99:l) showed that an increase of the concentration of BDA was accompanied by intensification of the 470-nm band at the expense of the 490-nm band; and (2) the absorption maxima of the dimer and monomer of A 0 in ether/ethanol(1:2) are reported to appear near 470 and 490 nm, respectively.20 We notice that a considerable portion of BDA molecules is dimerized in the monolayer. From a comparison of the absorption spectrum of Figure 5(i) with that reported by Zanker,20 the monomer:dimer molar ratio in this case can be estimated to be roughly 1:1. Generally speaking, there are two types of dimers of dye molecules, viz., with the two transition moments parallel (p) and antiparallel (a). The absorption band of the former appears on the shorter wavelength side of the monomer band; that of the latter type is expected to appear on the longer wavelength side, but it is not seen in many cases, since its intensity is very low. For convenience we call these two dimeric types p-dimer and a-dimer, respectively. The 470-nm band of BDA is apparently due to the p-dimer. We note in Figure 5 that the fluorescence band of BDA in the monolayer shows an abnormally large Stokes shift as compared to that in solution. The band in the monolayer is due partly to the a-dimer. We shall discuss this problem later. Figure 6 shows a typical absorption spectrum of a multilayer containing DTC and BDA (g in Figure 2).21 The appearance of this spectrum is very much like that of the BDA-containing monolayer (curve (i) in Figure 5). Figure 7 shows a photocurrent action spectrum of the multilayer (g in Figure 2) containing BDA and DTC; this spectrum is very similar to the absorption spectrum in Figure 6. We can therefore conclude that the photocurrent is due mainly to light absorption by BDA. In other words, the contribution of direct excitation of DTC to the photocurrent is very small and the photocurrent is due primarily to energy transfer from BDA to DTC. From Figures 6 and 7 it is evident that the (20) Zanker, V. Z . Phys. Chem. 1952,199,225-258; 1952,200,250-292. (21) The measurement of transmittance was made with near-normal in-
cidence. In general, one should take the reflection at the interface into account in the evaluation of the absorption of a monolayer on a high refractive index material (Orrit, M.; Mbbius, D.; Lehmann, U.; Mayer, H.J . Chem. Phys. 1986,85,4966-4979). The effect depends on the incidence angle. At normal incidence the reflectance at the air-glass or air-monolayer interface is small (ca. 4-5%), and hence the correction for the light reflected twice, four times, and so on is also small. We neglected this correction in the evaluation of absorbance.
Figure 9. Photocurrent action spectrum of monolayer a containing BDA, normalized to the incident photon flux.
monomer and dimer of BDA have almost the same efficiency of energy transfer to DTC. The efficiency of electron injection was determined in terms of the intrinsic current quantum efficiency ( q ) , which is defined for a single wavelength and is given as 9
=
ne/nph
(1)
where n, is the number of electrons which flow through the external circuit and nph is the number of photons absorbed by the dye. In this formula, ne was estimated from the photocurrent for excitation at 476.5 MI, and nph was estimated from the absorbance of BDA in the multilayer at the same wavelength. One should note that q is an effective quantity, corresponding to net electron transfer resulting from initial electron injection in competition with the reverse process when the latter occurs. Curve (ii) in Figure 8 shows the dependence of q on the vertical distance between the layer of BDA chromophore and the semiconductor electrode in the presence of DTC. Blank Experiments with One Dye (BDA). The photocurrent action spectrum shown in Figure 9 for the monolayer a is very similar to its absorption spectrum. This indicates that the monomer and p-dimer of BDA have almost the same efficiency of electron injection to the semiconductor electrode. In the following, BDA monomer plus BDA p-dimer are represented as BDA, except as specifically noted otherwise. For multilayers b and c, we observed a small photocurrent (ca. 7% and 0.04% of a, respectively). The dependence of the q values for these monolayermultilayer assemblies on the vertical distance between the layer of BDA chromophore and the semiconductor electrode is shown by curve (i) of Figure 8. Photocurrents measured between 20 and 50 O C showed little temperature dependence. Electron transfer across monolayers and multilayers has been studied for some time. Mann and Kuhn* studied the tunneling current across fatty acid salt monolayers. As for the multilayers, Sugi and co-workers'o studied electron transfer through 3-1 5 layers composed of Cd salts of fatty acids. The electric field used by these authors was usually small (C5 X lo4 V cm-I). Electron transfer was attributed to hopping with the intervention of interlayer "traps". In the case of a larger electric field (1 X lo6 V cm-I), electron transfer was attributed to hopping in the low-
The Journal of Physical Chemistry, Vol. 92, N o . 3, 1988 757
Photoelectric Conversion with Dye Multilayers temperature region (263-290 K) and to thermionic emission in the high-temperature region (290-333 K). The conductivity increased with temperature and this led these authors to their conclusion. Yamamoto et al.I2 attributed electron transfer across a multilayer to thermionic emission of electrons. In our case, little temperature dependence precludes the hopping or thermionic emission mechanism. However, decrease of the photocurrent with the dyesemiconductor distance is much smaller than that of the tunneling current observed by Mann and Kuhx8 Arden and Fromherz16 observed electron transfer across two tenside layers on an S n 0 2 semiconductor electrode with a transfer efficiency of 8-9% which they attributed to imperfections in the layers. Mobiusz2recently remarked that no fluorescence quenching was observed when the electron donor and acceptor were separated by two fatty acid monolayers. The photocurrent we observed for multilayers (b) and (c) may also be caused by an imperfection of the layers. In the following we shall treat the photocurrent observed for the multilayers containing only BDA as a blank, and investigate the effect of addition of DTC on the photocurrent. Comparison of curves (i) and (ii) in Figure 8 shows that, upon addition of DTC, q increases for a large distance, but decreases for a small distance. We assume that the total photocurrent efficiency can be given by eq 2, which takes into account both direct electron transfer from excited BDA to the semiconductor and electron transfer from DTC) DTC following the energy-transfer process (BDA
-
where d denotes the vertical distance between the layer containing the BDA chromophore and the semiconductor electrode,23qBDA(d) is the yield for direct electron transfer from the excited BDA to the semiconductor, &T(d) is the efficiency of energy transfer from excited BDA to DTC, and qDTc is the efficiency of electron transfer from DTC (in direct contact with the semiconductor electrode) to the semiconductor. In separate experiments on the DTCcontaining monolayer,24 we have found that both the monomer and p-dimer of DTC can inject electrons into an S n 0 2electrode with comparable e f f i c i e n ~ y . ~The ~ fluorescence band of BDA (monomer) at 530 nm has substantial overlap with the absorption bands of the monomer and p-dimer of DTC. Thus, we have to consider both DTC monomer and p-dimer as energy acceptors. In the following, the monomer plus p-dimer of DTC is denoted as DTC, except in cases where the species are specifically cited. The photocurrent caused by DTC excited directly by external illumination is neglected in eq 2, because direct excitation of DTC makes only a small contribution to the photocurrent as mentioned above (see Figure 7). The reduction potential of DTC (monomer) has been reported as -1 .OO V vs SCE,I5 and the potential of excited BDA (monomer) was estimed in this experiment as - 2 . 1 0 V vs SCE based on the potential of BDA in the ground state from cyclic voltammogram (0.43 V vs SCE) and excitation energy (2.53 eV). Hence electron transfer from excited BDA to DTC is possible on energy grounds. However, since the efficiency of electron transfer across more than two monolayers is expected to be negligibly small, as mentioned above, we assume that electron transfer from excited BDA to DTC can be ignored for multilayers f, g, and h. Addition of DTC increases q for multilayers g and h compared to the corresponding blanks, c and d. In these cases the blank photocurrent is negligibly small and the observed photocurrent is due essentially to electron transfer from DTC arising from energy transfer process (BDA DTC). Equation 2 then reduces to
-
(3) (22) Mobius, D., private communication. (23) The vertical distance between the BDA layer and the DTC layer is also d , since the chromophore of DTC resides in direct contact with the semiconductor electrode. (24) Azuma, T.;Nishiyama, Y.; Kawasaki, M.; Sato, H., to be published. (25) The participation of monomer and dimer dyes in sensitized photocurrent at a semiconductor electrode has been noted, for example, by MemmingI3 for 3,3'-distearyloxacarbocyanine perchlorate.
When we use the ratio of qDTC to qBDA (electron transfer efficiency from BDA at d = 0 to the semiconductor, Le., the q value obtained for monolayer a), eq 3 can be rewritten as q / q B D A = 4ET(d)(vDTC/qBDA) (4) Based on determinations of qDTC at 540-570 nm, we found that vDTC/qDTA = 0 . 1 1 . With this value and the data shown in Figure 8, we can calculate &T(d) for multilayers g and h to be 0.35 and 0.09, respectively. A rather complicated situation arises for multilayer f, becuase the observed photocurrent may include a contribution due to the imperfection of the layers. We estimated the latter to be at most the same as the photocurrent observed for the corresponding blank (Le., multilayer b), corrected for the presence of DTC which acts as a quencher for BDA in the same monolayer, and therefore we subtracted this contribution from the photocurrent observed for multilayer f. 4 E T values for multilayer f obtained with and without this correction were 0.80 and 0.88, respectively. In the following, we shall discuss the efficiency of energy transfer 4ET(d)for multilayers f, g, and h. Although the estimation of 4 E T for multilayer f has some uncertainty as mentioned above, those values for g and h are free from such an uncertainty. Drexhage et a1.26have given a formula for energy transfer in the case when the energy donor (D) is included in a monolayer and the energy acceptor (A) is situated in another monolayer at a vertical distance d from the former. (Their distance d has the same meaning as ours, since in our case the acceptor DTC is always incorporated in the first layer in direct contact with the semiconductor electrode.) According to these authors, the rate of energy transfer is given as
where u is the surface density of acceptor molecules (in the present case, 4 X A-Z, taking the surface areas for each arachidic acid molecule and each dye molecule to be 20 and 50 AZ,respectively). In eq 5, y is a constant that depends on the relative orientation of transition moments of D and A. The quantity C is related to the spectral overlap of the donor fluorescence and acceptor absorption as2' 9000 In 1 0 R C= 1 28iTsN~T~*t14 In this formula, N A is the Avogadro number, 7 D * is the natural fluorescence lifetime of the donor, and n is the refractive index of the medium. The quantity R is given from the spectral overlap of the fluorescence band of the donor and the absorption band of the acceptor as
wherefD(>) is the fluorescence spectrum (in cm-') normalized to the absorbed quanta and is the molar extinction coefficient of the acceptor (dm3 mol-' cm-I). The relation of C and the "critical transfer distance" (R,) in the three-dimensional Forster energy-transfer theory28 is given as
where K~ is a factor which depends on the relative orientation of fluorescence and absorption transition dipoles ( K ~= 2 / 3 for a three-dimensional statistical distribution of orientation), and q D is the fluorescence quantum yield of the donor in the absence of energy transfer: qD =
7D/TD*
(9)
According to the treatment of Drexhage et a1.,26the ratio of the (26) Drexhage, K. H.; Zwick, M. M.; Kuhn, H. Ber. Bunsen-Ges. Phys. Chem. 1963, 67, 62-67. (27) A factor of 9 instead of 9000 was used in the corresponding formula in ref 26; apparently, cA was given in cm3 mol-' cm-I. (28) Forster, Th. Ann. Phys. 1948, 2, 55-75; Z . Naturforsch., 1949, 4a, 321-327; Discuss. Faraday Sor. 1959, 27, 1-17.
Sato et al.
I58 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 107
d..70a dos80%
d..908 eL05C
,, d,.1008
I
0
d/A Figure 10. Simulation of the dependence of energy-transfer efficiency &T on d (the vertical distance between the layer of donor-chromophore and that of acceptor-chomophore). 0, experimental; - - -,calculated from eq 12, with do a s a n adjustable parameter.
fluorescence quantum yield with the acceptor (qF)to that without the acceptor (qFo)is
where do is the “critical distance” between the donor- and acceptor-including monolayer^.^^ MObius3O and Biicher et aL31 successfully appied this formula to their data on fluorescence quenching by energy transfer. When the two layers are separated at the distance do, the equation NWA(d0) = yaC/do4 = 1 / T D
(11)
holds, where T D is the fluorescence lifetime of the donor. The efficiency of energy transfer ~ $ ~ ~is (then d) = 1 - 1/[1 + (d0/d)41
(12)
As shown in Figure 10, we simulated the distance dependence of +ET(d) with this formula, taking do as an adjustable parameter, and obtained do = 90 f 5 A. The supersensitizing mechanism of thiourea for multilayers f, g, and h is not clear at present. Some of the thiourea molecules may be present in the vicinity of DTC due to imperfections in the layers and/or to diffusion through the layers. The fit to eq 12 indicates, however, that the systematic structure of the multilayer was, on the whole, maintained. Penetration of thiourea will be one of the factors limiting the photocurrent efficiency of the total system, however.32 Another factor that limits the photocurrent efficiency is the Occurrence of two-dimensional energy transfer from BDA (which contains both monomer and p-dimer, as mentioned above) to the a-dimer of BDA in the BDA-containing monolayer. In Figure 5 we noted the anomalously large Stokes shift of the fluorescence of BDA in the BDA-containing monolayer; this emission must be due to superposition of the monomer and a-dimer fluorescence. The BDA dimer may be fluorescent, because its homologue A 0 is noted for the peculiar property that its dimer is fluorescent with a lifetime longer than that of the m ~ n o m e r . ~Our ~ , ~study ~ on A 0 in micelles34indicates that the fluorescence band of the dimer (29) In eq 8 and 9, qD is the absolute value of fluorescence quantum yield, whereas $F and +Fo in eq IO are relative values. (30) Mobius, D. Z . Naturforsch. 1969, 24a, 251-253. (31) Biicher, H.; Elsner, 0. v.; Mobius, D.; Tillmann, P.; Wiegand J. Z . Phys. Chem. Frankfur? am Main 1969,65, 152-169. These authors pointed out that eq 10 provides a good approximation under the conditions that the average separation between the neighboring acceptor molecules in the acceptor-containing monolayer is much smaller than the vertical distance between the donor-containing and acceptor-containing monolayers. (32) One of the reviewers pointed out that our multilayers on SnO, electrode, with a hydrophobic surface facing the water, may reorganize on immersion in water or electrolyte solution. Although we cannot exclude such a possibility, it does not cause a serious change to our conclusion, Le., the very efficient energy transfer between BDA and DTC, since the reorganization of the top layer, if it occurred, makes the average distance of BDA and DTC somewhat larger, and hence the energy-transfer efficiency smaller than for the case without the reorganization. (33) Knof, J.; Theiss, F. J.; Weber, J. Z . Narurforsch. 1978, 33a,98-103. (34) Ban, T.; Kasatani, K.; Kawasaki, M.; Sato, H. Photochem. Photobiol 1983, 37, 131-139.
appears on the long-wavelength side of that of the monomer (A k 600 nm), apparently with a much smaller quantum yield. The a-dimer of BDA is a very poor donor species in energy transfer to DTC in the multilayer, because (1) the fluorescence band (A k 600 nm) of the a-dimer of BDA has only slight overlap with the absorption band of DTC, and (2) the fluorescence quantum yield of the a-dimer of BDA is much lower than that of the BDA monomer.35 Accordingly, two-dimensional energy transfer from BDA to the a-dimer of BDA will contribute little to the photocurrent and ends in dissipation of the exctiation energy. Hence, the presence of this dimer reduces the efficiency of energy transfer from BDA to DTC (reducing resulting in the unwanted overall reduction of the photocurrent quantum efficiency of the whole assembly. A large value of do (-90 f 5 A) was observed experimentally in spite of these limiting factors. BDA will reside in the monolayer with the long axis of its chromophore parallel to the monolayer plane. It has been reported that the transition of the redmost absorption of AO, which is essentially the chromophore of BDA, is polarized along the long axis of the m~lecule.~’DTC should reside in the monolayer with the long axis of its chromophore parallel to the monolayer plane. The transition dipole of cyanine dyes for the lowest energy excitation lies parallel to the long molecular a x i ~ . ~ ’ ,Therefore, ~* the transition dipoles of both BDA and DTC must be located horizontally in the plane of the monolayer. We have calculated the value of Q from the spectral overlap of the donor fluorescence and acceptor absorption; we used the absorption band of DTC in the monolayer (iii, in Figure 5) for the latter. However, we cannot use the fluorescence band of BDA in the monolayer (ii, in Figure 5) for the former, since it includes a contribution of thc a-dimer. Instead, the monomer fluorescence band in acetone solution (ii, in Figure 4) was used, mol-’ cm6. For the spatial and we obtained Q = 2.1 X distribution of transition dipoles mentioned above, y = 37r/16 (ref 26), and we obtain39do = 96 A, which is comparable with the experimental value (90 5 A). Thus, highly efficient energy transfer from BDA to DTC occurs in the mutilayers discussed here, which causes a large enhancement of the photocurrent, i.e., a light harvesting effect is observed in this system. When BDA is incorporated in the first layer with DTC (monolayer e), two-dimensional energy transfer from BDA to DTC occurs in the same monolayer. The photocurrent quantum yield is given, instead of eq 2, as
*
t = B B D A-( ~+ET) +
The electron-transfer efficiencies vBDA
(13) and t D T C are those for each ~ETBDTC
(35) It may sound strange that the a-dimer of BDA has a lower fluorescence quantum yield (q = T / T * ) compared to the monomer, although the fluorescence lifetime ( 7 ) of the a-dimer must be longer than that of the monomer as in the case of AO. This comes from the property of the a-dimer, Le., the very small magnitude of transition dipole owing to the antiparallel ordering of transition dipoles of two constituent monomers. This makes a-dimer’s Einstein probability of spontaneous emission much smaller and, therefore, a-dimer’s T* much longer compared to the corresponding quantities of the monomer. (36) This two-dimensional energy transfer occurs both in the presence and absence of DTC (Le., blank). Its effect on $F and 4 ; in eq 10 will cancel out, since the two-dimensional concentration of BDA (monomer plus p-dimer) and the a-dimer of BDA can be considered to be approximately the same in both cases. If the rate of this two-dimensional energy transfer is given by k,, we can formally set rD-] + k2 = T ~ ’ ’ and use the latter instead of rD-]. Thus, the effect of this unwanted two-dimensional energy transfer to the primary energy transfer (from BDA to DTC) can be treated formally as if some reduction of qD (or T ~ of) the BDA monomer occurred. This reduces the value of do,as is evident from eq 11. (37) Matsuoka, Y.; Yamaoka, K. Bull. Chem. Soc. J p n . 1979, 52, 3 163-3 170. (38) Salamon, Z . ; Skibinski, A,; Celnik, K. Z . Nururforsch. 1982, 37a, 1027- 1029. (39) In evaluation of do from 0 using eq 6, 9, and 11, the value of qD (fluorescence quantum yield of BDA monomer in a monolayer assembly) is necessary. This value is difficult to obtain experimentally, because of extensive dimerization of the dye in the monolayer. We have assumed qD to be close to unity, using the fluorescence quantum efficiency of 3,3’-diethylthiacarbocyanine iodide (an analogue of DTC) monomer in micelles (0.13, cf. Sato, H.; Kawasaki, M.; Kasatani, K.; Nakashima, N.; Yoshihara, K . Bull. Chem. SOC. Jpn. 1983, 56, 3588-3594) and the relation qBDA/qDTC = 0.1 1.
J . Phys. Chem. 1988, 92, 159-165
-
dye in the first monolayer. +ET is the two-dimensional energy transfer efficiency (BDA DTC) in the same monolayer. (Some contribution of electron transfer may be included.) Contributions from direct excitation of DTC can be neglected, since the magnitude of the additional term due to this effect is estimated to be roughly (I4) = 0.03?BDA The value of 7 decreases as a result of addition of DTC, because 9BDA > ?DTc. DTC thus acts as a quencher when it is incorporated in the same layer as BDA. Energy transfer from BDA to the a-dimer of BDA may occur to some extent. In this instance, however, this does not necessarily lead to reduction of photocurrent because the a-dimer of BDA may have some nonzero electroninjecting efficiency. Detailed results on monolayers containing BDA and DTC will be published elsewhere.24 (cDTC/cBDA)?DTC
i=
O.3(0*1?IBDA)
759
In conclusion, we find a remarkable enhancement in the photocurrent with the multilayer system with BDA as an antenna dye and DTC as a reaction-center dye when BDA is incorporated in the outer layer and DTC is in the first layer in direct contact with a semiconductor electrode. The energy-transfer process can be explained in terms of the Forster model applied to the multilayer system. Acknowledgment. We are grateful to Mr. Hirohisa Iwabayashi for his assistance in the experiments. This work was partially supported by a Grant-in-Aid for Scientific Research (60040004) of the Ministry of Education, Science, and Culture of Japan, and by the Asahi Glass Foundation for Industrial Technology. Registry No. BDA, 41 387-42-2; DTC, 26078-55-7; Sb, 7440-36-0; Sn02, 18282- 10-5; KCI, 7447-40-7; arachidic acid, 506-30-9; thiourea, 62-56-6.
Time-Resolved Fluorescence Depolarization of Rhodamine B and Octadecylrhodamine B in Triton X-100 Micelles and Aerosol OT Reversed Micelles Antonie J. W. G. Visser,* Kees Vos, Arie van Hoek,t and Jillert S. Santema Department of Biochemistry, Agricultural University, 6703 BC Wageningen, The Netherlands (Received: April 2, 1987: In Final Form: August 7, 1987)
Time-resolved fluorescence and fluorescence anisotropy experiments were conducted on rhodamine B and octadecylrhodamine B incorporated into aqueous Triton X-100micelles and in sodium bis(2-ethylhexyl) sulfosuccinate entrapped water or glycerol in a hydrocarbon solvent (heptane or dodecane). The time-resolved fluorescence behavior of the dye molecules in the micellar media was compared with that of the dye in homogeneous solution, from which a qualitative estimate of the polarity of the probe environment in the micelles could be inferred. The anisotropy decay of the fluorescent probes was analyzed with a biexponential decay model yielding correlation times characteristic for overall and internal micellar motion. The overall micellar rotation could be clearly distinguished from the faster internal motion in small water droplets and in glycerol droplets in heptane, for which there is good agreement between calculated and observed micellar rotation times. The hydrodynamic radii of glycerol droplets in dodecane medium are larger than the corresponding radii of droplets in heptane.
Introduction Reversed micelles have been the subject of intensive interdisciplinary research during the past decade. Reviews on reversed micelles pertained to chemistry and biology,' biochemistry and e n z y m o l ~ g y biote~hnology,~ ,~~~ and s p e c t r o ~ c o p y . ~Reversed ~~ micelles are usually considered as nanometer-scale, optically transparent water droplets, stabilized by a monolayer of surfactants in a bulk organic solvent. The most popular surfactant used thus far is sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or AOT), because no cosurfactant is required to form stable particles with a minimum interfacial tension between water and organic phase. A variety of techniques, summarized by Vos et al.,5 have proven that reversed micelles are dynamic entities, which exhibit internal motion and communicate with each other on microsecond time scale. It has been recognized previously that time-resolved fluorescence techniques can be employed to investigate the (sub)nanosecond dynamics of reversed micellesS6 In this investigation we describe the dynamic fluorescence properties of the versatile probe rhodamine B (RB) and the amphiphilic probe octadecylrhodamine B (ODRB) in AOT reversed micelles entrapped water or glycerol. Previously, ODRB has been utilized as a probe to monitor membrane fusion,' for distance determinations in biological aggregates,* and for the study of excitation transport in micelle^.^ Two organic solvents were used, namely, heptane and dodecane. The use of two different probe molecules enabled us to differentiate between two possible locations of the 'Permanent address: Department of Molecular Physics, Agricultural University, 6703 BC Wageningen, The Netherlands.
0022-365418812092-0759$01.SO10
probe. We also varied the parameter wo, which is defined as the molar ratio of surfactant and polar solvent (water or glycerol), in order to vary the size of the droplet. With photon correlation spectroscopy it was shown that the size of water-containing micelles is distinctly enhanced with increasing wO.lo The selection of different organic solvents is based on the different viscosities of heptane and dodecane. According to the Stokes-Einstein relation, variation of the viscosity would influence the rate of overall micellar rotation. In order to interpret the time-resolved fluorescence in an appropriate manner, control experiments of the probes in well-defined media (solvents and normal micelles in water) are also described. (1) (a) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Luisi, P. L.; Magid, L. J. CRC Crit. Rev. Biochem. 1986, 20, 409. (3) Martinek, K.; Levashov, A. V.; Klyachko, N.; Khmelnitski, Y. L.; Berezin, I. V. Eur. J . Biochem. 1986, 155, 453. (4) (a) Luisi, P. L. Angew. Chem., Int. Ed. Engl. 1985, 24, 439. (b) Luisi, P. L.; Laane, C. Trends Biotechnol. 1986, 4 , 153. (5) Vos, K.; Laane, C.; Visser, A. .I. W. G. Photochem. Photobiol. 1987, 45, 863. (6) (a) Eicke, H. F.; Zinsli, P. E. J. Colloid Interface Sci. 1978, 65, 131. (b) Zinsli, P. E. J . Phys. Chem. 1979,83, 3223. (c) Geladt, E.; De Schryver, F. C. In Reuerse Micelles; Luisi, P. L., Straub, B. E., Eds.; Plenum: New York, 1984. (d) Keh, E.; Valeur, B. J. Colloid Interface Sci. 1981, 79, 465. (e) Rodgers, M. A. J. J . Phys. Chem. 1981, 85, 3372. (f) Visser, A. J. W. G.; Santema, J. S.; Van Hoek, A. Photochem. Photobiol. 1984, 39, 11. (7) Hoekstra, D.; De Boer, T.; Klappe, K.; Wilschut, J. Biochemistry 1984, 23, 5675. (8) Holowka, D.; Baird, B. Biochemistry 1983, 22, 3475. (9) Ediger, M. D.; Domingue, R. P.; Fayer, M. D. J . Chem. Phys. 1984, 80, 1246. (10) Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979, 83, 480
0 1988 American Chemical Society
