J. Phys. Chem. 1985,89, 2730-2740
2730
FEATURE ARTICLE Photochemical Solar Energy Conversion. An Assessment of Scienttfic Accomplishments Janos H.Fendler Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676 (Received: January 7, 1985)
Research on photochemical solar energy conversion has grown exponentially. Basic sciences have benefited most from this research. The tenet of the present Feature Article is that solar energy research has appreciably contributed to the ongoing renaissance of colloid chemistry and to the development of ever more sophisticated models for light-sensitizeddistancecontrolled electron transfers. The need for compartmentalizingcomponentsof the photosynthetic apparatussensitizers: electron donors, acceptors, and relays; and catalysts-has focussed attention to microheterogeneous systems. Micelles, monolayers, organized multilayers, vesicles, polymerized vesicles, and dispersed colloidal semiconductors have been used as molecular organizers in artificial photosynthesis. Properties and potentials of these systems have been summarized. Emphasis has been placed on insights gained through photochemical investigations. Electron-transfer studies in covalently linked porphyrin-quinone, carotenoid polyene-porphyrin-quinone model compounds, and proteins have been summarized.
Introduction The perceived energy crisis in the early seventies has prompted worldwide research into solar energy conversion. This research has been directed toward obtaining storable, pollution-free electrical and chemical energy from sunshine at an economically acceptable price. Government and private agencies have been created to provide support for the scientific efforts. Inspired by Mother Nature’s billion year old photosynthesis, scientists have focussed their attention to photovoltaic, photoelectrochemical, photochemical, and photobiological solar energy conversions. Their approach has been truly multidisciplinary. It involved solid-state physics, theoretical chemistry, photophysics, photochemistry, photobiology, synthetic chemistry, polymer chemistry, colloid chemistry, electrochemistry, biophysics, and cell biology. All in all, scientists have risen valiently to the challenge of recreating photosynthesis in their laboratories. A challenge indeed it has been. Formidable obstacles have to be overcome prior to developing viable systems. Even the conceptually simplest photochemical water-splitting system (eq 1) presents many difficulties. hv
\ t
Sensitizers ( S ) and relays (R) have to be found which are chemically and photochemically stable. They have to be capable of operating over the range of solar spectrum and have to have appropriate redox properties. The forward photoelectron transfer (S* + R S+ + R- in eq 1) has to be efficient and the backreaction (S+ R- S + R ) needs to be obviated. Suitable catalysts have to be found for facilitating water reduction (a two-electron process) and oxidation (a four-electron process). Finally, hydrogen and oxygen have to be separated either in space or in time. Needless to say that the unrealistically simplistic water splitting scheme, indicated by eq 1, has not been experimentally realized. Considerable progress has been made, however, toward the optimization of water photoreduction in sacrificial half cells (eq 2). Since half cells are noncyclic, electron donors (D) have been used to produce hydrogen. Research in artificial photosynthesis has grown exponentially in the past decade. Initially it appeared to capture the popular
-+ -
support needed for proclaiming solar energy conversion to be a national goal. But the oil-glut nipped the euphoria in the bud and terminated any serious industrial commitment. Nevertheless, much has been accomplished, as documented in primary research publications and summarized in numerous review articlesl-10 and book^.^^-'^ There is no need for an additional review popularizing solar energy research. It would only pollute the scientific literature. There is room, however, for the critical assessment of the benefits the basic sciences have gained from the solar energy program. The purpose of the present article is to highlight some of the scientific spinoffs from these researches. Attention will be focussed on the renaissance of colloid chemistry and on the development (1) Porter, G.; Archer, M. D. Interdiscip. Sc;. Reu. 1976, 1 , 119-143. (2) Tollin, G. J . Phys. Chem. 1976, 80, 2274-2277. (3) Almgren, M. Photochem. Photobiol. 1978, 2 7 , 6 0 3 6 0 9 . (4) Porter, G. Proc. R . Soc. London, Ser. A 1978,362, 281-303. (5) Calvin, M. Acc. Chem. Res. 1978, 11, 369-374. (6) Zamaraeu, K. I.; Parmon, V. N. R w s . Chem. Reu. 1980,49,695-717. (7) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83-90. (8) Fendler, J. H. J. Phys. Chem. 1980,84, 1485-1491. (9) Gratzel, M. Acc. Chem. Res. 1981, 14, 376-384. (10) Kiwi,J.; Kalyanasundaran, K.; Gratzel, M. In ‘Visible Light I n d u d Cleavage of Water into Hydrogen and Oxygen in Colloidal Microheterogenmus Systems”, Springer-Verlag, Heidelberg, 1981. Structure Bonding (Berlin) 1982, 49, 37-125. (1 1) Claesson, S.;Engstrom, M. “Solar Energy-Photochemical Conversion and Storage”. National Swedish Board for Energy Source Development, Stockholm, 1977. (12) Barber, J. ‘Photosynthesis in Relation to Model Systems”; Elsevier: New York, 1979. (13) Gerischer, H., Katz, J. J. ”Light Induced Separation in Biology and Chemistry”; Verlag Chemie: New York, 1979. M. “Energy Resources through Photochemistry and (14) ?tzel, Catalysis ; Academic Press: New York, 1983.
0022-3654185 12089-2730%01.SO10 0 1985 American Chemical Societv
Feature Article of ever more sophisticated models for light-sensitized distancecontrolled electron transfers. No attempts will be made to be exhaustive. Inevitably, the selection of topics will reflect somewhat our own interest. Research in homogeneous solutions will not be covered. The importance of recently discovered metal complexes having unusual oxidation states15which are capable of mediating water oxidation16 and reduction” should be recognized. Apart from their intrinsic interests these complexes may well be employed in photochemical solar energy conversion in colloidal, semiconductor-based, or other heterogeneous systems. Orgaoizational Imperative in Solar Energy Conversion. Renaissance of Colloid Chemistry Compartmentalization is an essential feature of natural photosynthesis. Components of the photosynthetic apparatus are dynamically arranged in the thylakoid membrane. This arrangement is responsible for light energy harvesting, charge separation, vectorial electron transfer, water oxidation, and carbon fi~ation.’~J~ The need for compartmentalization in artificial photosynthesis has been recognized for some time.14 Organization of sensitizers, electron donors and acceptors, relays, and catalysts in microheterogeneous systems has led to altered ionization, oxidation, and reduction potentials, photophysical and photochemical pathways, and quantum efficiencies. Most importantly, compartmentalization concentrates and localizes components of the artificial photosynthetic unit and facilitates vectorial charge separations. Micelles, microemulsions, monolayers, bilayers, organized multilayers, polymers, vesicles, polymerized vesicles, and colloidal semiconductors-collectively referred to as membrane mimetic agents”*l-have been used in artificial photosynthesis as microheterogeneous systems. Important aspects of solar energy conversion have been demonstrated in membrane mimetic systems. Progress, however, demanded the development of improved and new molecular organizers. This, in turn, necessitated a careful reassessment of the physical chemical properties of micelles, microemulsions, monolayers, bilayers, organized multilayers, vesicles, and colloidal semiconductors. The renewed attention, infusion of different approaches, and availability of modern instrumentation all have led to the ongoing renaissance of colloid chemistry. The tenet of this article is that solar energy research has appreciably contributed to the revitalization of colloid science. Emphasis will be placed on areas which benefited most from recent studies. I . Micelles. Aqueous micelles are spherical aggregates (40-80 A in diameter) which are dynamically formed from amphiphatic molecules above a characteristicconcentration (the critical micelle concentration, the cmc) in water.19*22-24 Depending on the chemical structure of their hydrophilic head groups, surfactants can be neutral or negatively or positively charged. The alkyl chains of the hydrophobic parts typically contain between 6 and 20 carbon atoms. Micelles serve as eminently suitable media for demonstrating the advantages of compartmentalization and charge separation. Energy transfer from micellar sodium dodecyl sulfate (SDS) solubilized naphthalene (N) to terbium chloride illustrates the beneficial advantages of c~mpartmentalization.~~ The most (15) Sutin, N. Progr. Inorg. Chem. 1983, 30,441-498. (16) Brunswig, B. S.;Chou, M. H.; Creutz, C.; Ghosh, P.; Sutin, N. J . Am. Chem. Soc. 1983, 105,4832-4833. (17) Krishman, C. V.; Creutz, C.; Mahajan, D.; Schwarz, H. A,; Sutin, N. Isr. J . Chem. 1982, 22, 98-106. (18) Clayton, R. K. ‘Photosynthesis: Physical Mechanisms and Chemical Patterns”; Cambridge University Press: London, 1980. (19) Fendler, J. H. “Membrane Mmetic Chemistry, Characterizationsand
Applications of Micelles, Microemulsions, Monolayers, Bilayers, Vesicles, Host-Guest Systems and Polyions”; Wiley: New York, 1982. (20) Fendler, J. H. Chem. Eng. News,1984 (Jan 2) 62, 25-38. (21) Fendler, J. H. Science 1984, 223, 888-894. Effect”, Wiley-Interscience: New (22) Tanford. C. ‘The Hydrophobic . York, 1980; 2nd ed. (23) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1-86. (24) Lindman, B.; Wennerstrom, H. Top. Current Chem. 1980,87, 1-81.
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2731 efficient energy transfer is observed when less than one N molecule is localized in each micelle, but there are large numbers of terbium cations bound to the negative surface of SDS (eq 5). In the (3)
In water 3N*
+ 3N* -. quenching
In micelle 3N*
+ Tb3+
-
(4)
IN + Tb3+*
absence of the micellar cage, there is no energy transfer. Naphthalene triplet-triplet annihilation predominates (eq 4). 3N* has no partner in the micellar cage, and it can only be deactivated by energy transfer to Tb3+present in close proximity at high local concentrations. The advantage of micelle-mediated charge separations can also be readily shown. An example is provided by photoinitiated electron ejection in SDS micelle solubilized phenothiazine.26 Negative charges on the micelle surface decrease the probability of electron reentry and, hence, electron recombination with SDS localized phenothiazine cation radical (formed in the photoionization). The lifetime of phenothiazine cation in micellar SDS is considerably longer than that in water. This, in turn, facilitates electron transfer to appropriate donors or acceptors.26 The examples cited may give the impression of micelles as miniturized charged cages. Indeed, this was starting point for the statistical treatments developed for the characterization of substrate-micelle interactions in terms of Poisson’s d i s t r i b ~ t i o n ? ~ , ~ Micelles are, however, dynamic entities. They rapidly break up and re-form by two known processes?+31 The first process occurs on the microsecond time scale and is due to the release and subsequent reincorporation of a single surfactant from and back to the micelle. The second process occurs on the millisecond time scale and is ascribed to the dissolution of the micelle and to the subsequent reassociation of the monomers. Credit should be given to photochemists who recognized the need for considering the lifetimes and volumes of reactants relative to those of the micelle~.~~-~~*~~ Both intermicellar and intramicellar reactions have been realized experimentally and treated theoretically. Much fundamental insight has been gained from fluorescence q ~ e n c h i n g . ~ ~ - ~ O (25) Escabi-Perez,J.; Nome, F.; Fendler, J. H. J . Am. Chem. SOC.1977, 99, 7749-7754. (26) Alkaitis, S.A.; Beck, G.; Gratzel, M. J. Am. Chem. Soc. 1975, 97, 5723-5729. (27) Infelta, P. P.;Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 1906. ( 2 8 ) Infelta, P. P.; Gratzel, M. J . Chem. Phys. 1979, 70, 179-186. (29) Aniansson, E. A. G.; Wall, S. N. J . Phys. Chem. 1974, 78, 1024-1 030. (30) Aniansson, E. A. G. J . Phys. Chem. 1978,82, 2805-2808. (31) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.;
Keilman, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976,80, 905-922. (32) Muller, N. In ‘Solution Chemistry of Surfactants”;Mittal, K. L., Ed.; Plenum Press: New York, 1979; pp 279-295. (33) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133-138. (34) T w o , N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980,19, 675-696. (35) Quina, F. H.; Toscano, V. G. J . Phys. Chem. 1977,81, 1750-1754. (36) Atik, S.;Singer, L. A. J . Am. Chem. Soc. 1978, 100, 3234-3235. (37) Infelta, P. P. Chem. Phys. Lerr. 1979, 61, 88-91. (38) Atik, S. S.;Kwan, C. L.; Singer, L. A. J. Am. Chem. Soc. 1979,101, 5696-5702. (39) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978, 100, 5951-5952.
2732 The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 Equation 6 describes the general scheme for treating fluorescence
quenching in micelles,@where P* is the fluorescing probe, Q (the quencher) may be in the aqueous, Qaq,or in the micellar phase, P Q. The circles symbolize the micelles, M, without implying any preferred sites for P, P*, P* Qq, and so on; T~ is the fluorescence lifetime in the absence of quenchers; k+ and k- are rate constants for entry into and exit from the micelles; and k, is the intramicellar quenching rate constant. If conditions are chosen carefully, fluorescence quenching affords a means for the determination of the mean micellar aggregation number, Both P and Q must be localized exclusively in the micelles. The concentration of P and the excitation light intensity must be kept as low as possible. P* must be rapidly and completely quenched by Q (static quenching, manifested in a single exponential decay of fluorescence intensity, which is independent of [Q]). Under these conditions, emission occurs only from that fraction of P* containing micelles that are free of Q. If the distribution of quenchers obeys Poisson's statistics, the measured luminescence intensity of P* in the absence (lo)and in the presence ( I ) of Q is given by fi.3793"3
[ / l o = e-IQI/[Ml
afford values for K, k+, k-, and n. Solubilizate binding constants (Kvalues) and entry (k,) and exit (k-) rate constants have been determined in micellar SDS for benzene, naphthalene, anthracene, and pyrene to be in the order of 103-106 M-I, 1Olo M-' s-l, and 104-106 s-1,34,44 A more general stochastic master equation has been derived recently for the kinetic analysis of intramicellar or intravesicular excited-state p r o c e s ~ e s . ~Equations ~' have been formulated for a number of reversible and irreversible photoinitiated diffusioncontrolled reactions. The equation derived for second-order reversible processes hu
A-A* A*
+B
kl3
G C
k-I3
(12)
+D
(13)
will be shown here.& At time t = 0, A and B are assumed to be independently distributed among micelles of equal volume ( u ) according to Poisson's statistics. At all subsequent times, the species A, B, C, and D are constrained to remain within the host micelle; selecting one micelle from the ensemble, the probability that it has n A's, m B's, p C's, and q D's at time t is denoted P(n,m,p,q,t).At the initial time t = 0 and assuming only reactant species are present so that p = q = 0, the initial probability distribution is
(7)
where the micelle concentration, [MI, is related to the total detergent (surfactant) concentration, [DET], and to the concentration of free monomers, [S,], by
Combination of eq 7 and 8 leads to
[Qln In (zo/o= [DET] - [SI]
Fendler
(9)
which allows the determination of fi and [SI]. Since either [Q] or [DET] can be varied, the method is not limited to measurements of fi close to the cmc value of the surfactant. It is also useful for determining the dependency of [SI] on [DET], hence it provides a means for testing the different models used for micellization. If a quencher is chosen such that it partitions between the micellar and bulk aqueous phases, then steady-state emission still occurs from the fraction of micellized P* that is free of Q and for steady-state illumination
where the first and second terms are contributions to intensity quenching from dynamic and static quenching, respectively. Equation 10 reduces to eq 7 when Q is completely in the micellar phase (KIM] >> 1 ) . The lifetime of emission, T , is however, reduced by dynamic diffusional quenching by the water-solubilized quenchers, Qas.
where XA and AB are the average occupation numbers of species A and B per micelles. The evolution of the system away from this initial state is formulated by considering terms which reflect the population/depopulation of the state (n,m,p,q). Combining the contributions from the birth and death processes for each of the forward and reverse reactions yields the stochastic master equation describing the time evolution of the state (n,m,p,q) dP(n,m,p,q;t)/dt = ( k 1 3 / u ) ( n+ 1 ) X ( m + l)P(n+l,m+l,p-1,q-1;t) + ( k - 1 3 / v ) @+ l ) ( q + l)P(n-l,m-l,p+l ,q+ 1 ; t ) - ( 1 / ~ ) ( k , 3 n m+ k-13pq)P(n,m,p,q;t)( 1 5 ) Solution of 15 (and similar master equations developed for more complex cases)45leads to several important conclusions. First, the kinetics of compartmentalized reactions are characterized by apparent pseudo-first-order behavior. Secondly, the apparent equilibrium constant for reversible reactions occurring in micelles, Q, is different from that which governs the same reaction in homogeneous solution (9.Thirdly, stochastic equations describe well available experimental data. Fourthly, and perhaps most importantly, micellar systems provide an experimental approach for the investigation of the effects of confined geometries and confined volumes on r e a c t i v i t i e ~ . ~ ~ Ultrafast electron and proton transfers occurring on micellar surface^^'^^ can be rationalized by taking into consideration the reductions of the diffusion lengths of the reaction partners and the reduction of the dimensionality of the processes i n v o l ~ e d . ~ ~ ~ ~ ~ , ~ (44) Almgren, M.; Grieser, F.; Thomas, J. K. J . Am. Chem. SOC.1979, 101, 279-291.
Luminescence lifetimes are found to increase with increasing concentrations of micelles. Steady-state and time-resolved data
(45) Hatlee, M. D.; Kozak, J. J . Chem. Phys. 1980, 72, 4358-4367. (46) Hatlee, M. D.; Kozak, J. J . Chem. Phys. 1981, 74, 1098-1140. (47) Hatlee, M. D.;Kozak, J. J . Chem. Phys. 1981, 74, 5627-5635. (48) It is worth remembering that the radius of a typical micelle is 15-20 A, and that of a reacting molecule is 3-7 A. (49) Frank, A. J.; Gratzcl, M.; Kozak, J. J . Am. Chem. Soc. 1976, 98, 3317-3321 (50) Henglein, A,; Proake, Th. Ber. Bunrenges, Phys. Chcm. 1978, 82, 471-476. (51) Escabi-Perez, J. R.; Fendler, J. H. J . Am. Chem. SOC.1978, 100, 2234-2236. (52) Hatlee, M. D.; Kozak, J. J.; Rothenberger, G.; Infelta, P. P.; Gratzel, M. J . Phys. Chem. 1980, 84, 1508-1519. I
(40) Yekta, A.; Aikawa, M.; Turro, N. J. Ch" Phys. Lctt. 1979, 63, 543-548. (41) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62-67. (42) Kratohvil, J. P. J . Colloid Interface Sci. 1980, 75, 271-275. (43) Almgren, M.; Lofroth, J. E. J . Colloid Interface Sci., in press.
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2733
Feature Article Reduction of a three-dimensional volume to a two-dimensional surface in which molecules react by diffusion has been recognized to contribute to the efficiency of enzyme The reaction Brp
+Brl
-
Br3-
+ Br-
(16)
in the presence of micellar CTAB led to the identification of two kinetic processes-a two-dimensional surface diffusion and a three-dimensional intermicellar reaction.49 Rate constants of kM = 2.1 X 106 s-I and kM = 1.46 X lo9 M-’ s-’ have been determined for these processes. The ratio of these rate constants (kzd/2k3) has been substantiated by calculating the ratios of steps a random walker needs to take in a three-dimensional, ( n ) 3 rvs. a two-dimensional, (n)2rspace. Using Montroll’s lattice statistics57 and estimating the concentration of traps (2.0 X traps/total number of lattice sites on the CTAB surface and 4.44 X traps/total number of sites in the micelle) led to ( n ) 3 3.4 X 10“ and ( n ) 2 82. Taking the time for the reactions of two B r r radicals in two- or three-dimensional space to be ( n ) i = ( n ) i t i (where ti is the time required for a diffusional jump from one lattice point to the next and the subscript i is the dimensionality) and assuming the time and distance between jumps to be the same in two- and three-dimensional space yields ( 7 ) 3 / (r)2 = ( n ) 3 /( n)3 and hence
-
-
Substitution of [Br2-.] = 2.69 X M and ( n ) 3= 3.4 X lo4 an& ( n ) 2= 82 into eq 17 leads to k2d/2K3d= 7.7 X which is in good agreement with the experimentally obtained ratio of two- to three-dimensional rate constants, 1.5 X 10-3.49 Reduction of dimensionality has been treated more exactly in terms of a Markovien theory of reaction e f f i c i e n ~ y . ~Group ~.~~ theoretical arguments have been used to facilitate the determination of the average number of steps required (n) for a diffusing coreactant A to react with stationary target molecule B. Changes in n as a function of dimensionality reductions have been calculated for systems having symmetrical and tubular geometries. These calculations provide evidence for the complexity of the interplay among dimensionality, geometry, and reactivity. Elucidation of micellar structures has presented an awesome challenge to physical and theoretical chemists.m2 The classical micelle was pictured as an oil droplet with a polar Coat.6M The validity of this model was seriously questioned by Menger who summarized conflicting experimental data and interpretations concerning microviscosities and solubilization sites.6s His space-filling models have allowed for considerable water pene(53) Austumian, R. D.; Schelly, Z . A. J . Am. Chem. SOC.1984, 106, 304-308. (54) Adam, G.; Delbruch, M. In “Structural Chemistry and Molecular Biology”, Rich, A., Davidson, N., Eds.; Freeman: San Francisco, 1968; pp 198-203. (55) Rjchter, P. H.; Eigen, M. Biophys. Chem. 1974, 2, 255-263. (56) Eigen, M. In “Quantum Statistical Mechanics in the Natural ~~
Sciences”, Kursunoglu, B.; Mintz, S. L.; Windmeyer, S. M., Eds.; Plenum Press: New York, 1974. (57) Montroll, E. W. J. Math. Phys. 1969, 10, 753-765. (58) Musho, M. K.; Kozak, J. J. J . Chem. Phys. 1984, 80, 159-169. (59) Lee, P.; Kozak, J. J. J. Chem. Phys. 1984.80, 705-713. (60) Fromherz, P. Chem. Phys. Lett. 1981, 77, 460-466. Fromherz, P. Ber. Bunsenges. Phys. Chem. 1981, 85, 891-899. Fromherz, P. In *Surfactants in Solution”, Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; pp 321-336. (61) Gruen, W. R.;delacey, E. H. B. In “Surfactants in Solution”, Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; pp 279-306. (62) Dill, K. A,; Flory, P. J. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 676-680. Dill, K. A. J . Phys. Chem. 1982, 86, 1498-1500. Dill, K. A,; Cantor, R.S. Macromolecules 1984.17, 380-384. Cantor, R. J.; Dill, K.A. Macromolecules 1984, 17, 384-388. Dill, K. A. In “Surfactants in Solution”, Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; pp 307-320. (63) Hartley, G. S.Trans. Faraday Soc. 1935,31,31-50. Hartley, G. S. Q. Reu. Chem. Soc. 1948, 2, 152-183. (64) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978.82, 1620-1627. (65) Menger, F. M. Acc. Chem. Res. 1979, 12, 111-117.
tration. The extent of water penetration has, in fact, been a major bone of contention. At one time or other, water was believed to penetrate the micelle not a t all,67or indeed to any intermediate le11gth.6~3~~Considering the exposure of the surfactant hydrocarbons to water at the micellar interface is more appropriate than discussing water penetration. This important but subtle difference has been verified recently by small angle neutron scattering (SANS) e~periments.~“-~~ The modem micelle is pictured to contain a virtually waterless core but to have substantial hydrocarbon-water contact at its interface.’O A corollary of this model is the long advocated viewI6 that substrates do not deeply penetrate into the micelles but are dynamically solubilized at the rugged wet micellar surface. Where do we go from here? Theoreticians will continue to devote their attention to micelles and micellesubstrate interactions. Synthetic chemists will continue to make functionalized micelle-forming surfactants. The most promising utilization of micelles will remain to be based on their ability to act as molecular cages. Their ability to amplify magnetic effects of photochemical processes is but73v74one potential utilization of micellar cages. There are many other, some not yet recognized, potentials for exploiting the organizational abilities of aqueous micelles. 2. Monolayers and Organized Multilayers. Monolayers (monomolecular layers) are formed by spreading naturally occurring lipids or synthetic surfactants, dissolved in a volatile solvent, over water in a Langmuir t r o ~ g h . ~ ~The - ’ ~ polar headgroups of the amphiphiles are in contact with water, the subphase, while their hydrocarbon tails protrude above it. Surface pressuresurface area curves, surface potential, and surface viscosity determinations are used for monolayer chara~terization.~~ Interest in energy and electron transfers in organized molecular a s s e m b l i e ~ ~has * ~ contributed ~q~~ to the renaissance of monolayers. Improved techniques have been introduced which allowed the reproducible transfer of monolayers to solid surfaces.77 Glass slides, stainless steel, or platinum plates have been used as solid support. Transfer of the monolayer has been accomplished by dipping the glass slide or metal plate through the monolayer covered liquid. Repeating the process results in the buildup of multilayers. The method of dipping determines the point of surfactant attachment. Thus, it is possible to form multilayers by sequential (plate-tail-heat-tail-heat etc., or plate-headtail-heat-tail etc., or plate-head-head-tail-tail etc.) or random attachments. Multilayers can be separated by depositing a thin film of polyvinyl alcohol (PVA) on the outer monolayer. Once the PVA film dries it is removed, along with the monolayer attached to it, from the plate.77 Monolayers are the most suitable systems for investigating two-dimensional transformationss0 and r e a ~ t i v i t i e s . ~Unlike ~ (66) Svens, B.; Rosenholm, B. J. Colloid Interface Sci. 1973,44,495-504. (67) Stigter, D. J. Phys. Chem. 1974, 78, 2480-2485. (68) Muller, N.; Birkhahn, R. H. J . Phys. Chem. 1967, 71, 957-962. (69) Podo, F.; Ray, A.; Nemethy, G. J . Am. Chem. SOC.1973, 95, 6164-6174. (70) Dill, K. A.; Koppel, D. E.; Cantor, R. S.;Dill, J. D.; Bendedouch, D.; Chen, S.H. Nature (London) 1984, 309,42-45. (71) Bendedouch, D.; Chen, S.H.; Koehler, W. C. J . Phys. Chem. 1983, 87, 153-159. (72) Cabane, B. In “Surfactants in Solution”, Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; pp 373-404. (73) Turro, N. J.; Kreutler, B. Acc. Chem. Res. 1980,13, 369-377. Turro, N. J. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 609-621. (74) Scaiano, J. C.; Abuin, E. B.; Stewart, L. C. J . Am. Chem. SOC.1982, 104, 5673-5679. (75) Gaines, G. L., Jr. “Insoluble Monolayers at Liquid-Gas Interfaces”; Interscience: New York, 1966. Gaines, G. L.,Jr. In “MTP International Review of Science”, Vol. 7, Kerker, M., Ed.; Butterworth: London, 1972; pp 1-24. (76) Gershfeld, N. L. Annu. Rev. Phys. Chem. 1976, 27, 349-368. (77) Kuhn, H.; Mobius, D. Angew. Chem., In?. Ed. Engl. 1971, 10, 620-637. Kuhn, H.; Mobius, D.; Bucher, H. In “Physical Methods for
Chemistry”, Weissberger, A.; Rossiter, B. H., Eds.; Wiley-Interscience: New York, 1972; Vol. 1, Part 111B, pp 577-701. (78) Kuhn, H. J . Photochem. 1979, 10, 111-132. Kuhn, H. In “Light Induced Charge Separation in Biology and Chemistry”, Gerischer, H., Katz, J. J., Eds.; Verlag-Chemic: New York, 1979; pp 151-169. (79) Mobius, D. Acc. Chem. Res. 1981, 14, 63-68.
2734
The Journal of Physical Chemistry, Vol. 89,No. 13, 1985
micelles, they are highly organized. In principle, energy and electron transfers can be examined within a monolayer or across several layers by localizing appropriate concentrations of donor and acceptor molecules a t appropriate positions. An example is provided by the intramonolayer quenching of the N,N'-dioctadecyloxacyanine (1) by N,N'-dioctadecylthiacyamine (2)
C18H37
Fendler c hemlca I acllvrtlon
I
CI-st-
TT
*SI-o-S-
-CI
I
I
732%
C18H37
I a + ; & C H * ; n
(c'o4-)
I
I
C18H37
C18H37
2
Significantly, extremely efficient energy transfer was observed. One molecule of 2 was sufficient to quench 50% of the fluorescence intensity of 10000 molecules of Photosensitized electron transfer has been examined from a donor (2, for example) to an acceptor (3, for example), separately
3
localized in different monolayer^.^^,^' The distance between the donor and acceptor molecules was varied by an interlaying fatty acid monolayer whose chain length could be altered from C- 14 to C-22. Determination of the donor fluorescence intensities in the absence (lo)and in the presence of the acceptor (l) led to the rate constants for the electron transfer, kET
where 7 is the fluorescence lifetime of the donor in the absence of acceptors. kETvalues were found to decrease exponentially with increasing distance separating the donors from the acceptors indicating electron t ~ n n e l i n g . ' ~ * ~ ~ Vectorial electron transfer has also been demonstrated in organized multilayers placed between two semitransparent metal junction^.^^*^^ Illumination, in the presence of electron donors, acceptors, photocatalysts, and/or conducting wires, has resulted in observable photocurrent. Preparation of multilayers demands scrupulous cleanliness. Short- and long-term stability is a serious problem with organized multilayers. On standing, multilayers are known to rearrange. Solvents, even in trace amounts, dissolve in multilayers and introduce i m p u r i t i e ~ . ~ ~ - ~ ~ A promising new development is the preparation of organized multilayers by adsorbing bifunctional surfactants onto a polar solid. Building up the multilayer is accomplished by sequential coupling to other surfactants (eq 19).87 (80) Patterson, L. K.; MacCarty, J. E.; Kozak, J. J. Chem. Phys. Lett. 1982,89, 435-437. Tembe, B. L.;MacCarty, J. E.; Kozak, J. J. J . Phys. Chem. 1984,87,452-453. (81) Mobius, D. Ber. Bunrenges. Phys. Chem. 1978,82,848-858. (82)Janzen, A. F.; Bolton, J. R. J. Am. Chem. Soc. 1979,101,6342-6348. Janzen, A. F.; Bolton, J. R.; Stillman, M. J. J. Am. Chem. SOC.1979,101, 6337-6341. (83)Polymeropoulos, E. E.; Sagiv, J. J. Chem. Phys. 1978,69,1836-1847. Polymeropoulos, E. E.; Mobius, D.; Kuhn, H. J. Chem. Phys. 1978, 68, 3918-3931. Polymeropoulos,E. E.; Mobius, D.; Kuhn, H. Thin Solid Films 1980.68, 173-190. (84) Richard, M. A.; Dutch, J.; Whitesides, G. M. J . Am. Chem. SOC. 1978,100, 6613-6625. (85)Mercer-Smith, J. A.; Whitten, D. G. J . Am. Chem. Soc. 1979,101, 6620-6625. (86) Windreich, S.; Silberberg, A. J . Colloid Interface Sci. 1980, 77, 427-434.
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