Photochemistry in Organized Media J a n o s H. Fendler Department of Chemistry and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, N Y 13676 Most of our textbooks and essentially all of our teaching is restricted to homogeneous media. Real life chemistry and real photochemistry occur at interfaces, surfaces, and in multinhase heteroeeneous svstems. Heterogeneity allows the organization of reactants A d products in ~omp&ments of given microenvironments and facilitates transport of substrates to and from different locations. The key concept is organization. C o m ~ l e xbiochemical processes are all based on molecular light is transformedto chemica~kner~y which is then utilized for carbon dioxide reduction. The process is described by the "2-scheme" (Fig. 1).Briefly, light is absorbed by two pigment systems: photosystem I, PS I (PIOO),and photosystem 11, P S I1 (P680). These two systems operate in series; two photons are absorbed for every electron liberated from water. Lightinduced charge separation in P S I1 leads to the formation of a strong oxidant, Z+ (E, = +0.8V), and weak reductant, Q(E, 0.OV). Although the reduction potential of Z+ is sufficient for water oxidation, evolution of molecular oxygen demands the accumulation of four vositive charges. - Electron flows from Q-, via pool plastoquinones and other carriers, to a weak oxidant. (En = +0.4V). generated along with a strong reductant, X (E,= -0.6V) & P S I. This eiectron flow is coupled to phosphorylation which converts adenosine dlphosphate, ADP, and inorganic phosphate to adenosine triohosohate. ATP. With the aid of ATP, X- reduces carbon hiox:de tocarbohydrate (I). The precise, yet incompletely understood, arrangements in the thylakoid membrane are responsible for theefficient energy deposition and transmission, for prevention of charge recombination, and for creating a proton gradient essential for photophosphorylation (2). Advantages of molecular organization are increasingly recognized in photochemical studies ( 3 , 4 ) .Sensitizers, photoreactants, transition states and products can be concentrated and localized. Molecular organization can lead to altered ionization, oxidation, and reduction potentials as well as to different quantum efficiencies, photophysical and photochemical pathways, reactivities, and stereochemistries. Furthermore, separation of photoproducts, even isotopes ( 5 ) , can be favorably effected by compartmentalization.
--
Organization is best accomplished by imitating the supreme molecular organizer: the biological membrane (Fig. 2). The term "membrane mimetic chemistry" has been coined to describe a new discipline: the practical exploitation of membrane-mediated processes in relatively simple chemical systems (6). No attempt is made in membrane mimetic chemistry to reproduce all aspects of natural membranes. Micelles, microemulsions, monolayers, bilayers, vesicles, polyions, polymers, colloidal semiconductors, and even clay particles are considered to function as organized media and thus mimic aspects of membranes. Following a brief description of the most commonly used organized media, emphasis will be placed on selected recent photophysical and photochemical exploitations of these systems. Comparative Description of the Different Organized Media Oreanized media can he convenientlv divided into those assembled from surfactants, those acting as hosts (cyclodextrins, crown ethers, for example), colloidal and subcolloidal particles (Ti02, Pt, etc.), and macromolecules (polymers, polyions, etc.), The present discussion will be limited to surfactant aggregates. Figure 3 is a schematic representation of the different structures formed from surfactants. Surfactants are, of course, amphiphatic molecules, having distinct hydrophobic (water-hating) and hydrophilic (water-loving) regions (7-11). Hexadecyltrimethylammonium bromide, CTAB, CH3(CH2)15N+(CH3)~Br-;sodium dodecyl sulfate, SDS, CH3(CH2)laSOhNa+;sodium bis(2-ethy1hexyl)snlfosuccinate, Aerosol-OT, [CH3(CH2)3CH(C2H5)0COCH2][CH3(CH2)3CH(CaH5)OCO]CHSOm; dioctadecyldimethylammonium chloride, DODAC, [CH3(CH2)17]2N+[CH3I2C1-; and dihexadecyl phosphate, DHP, [CHs(CH2)150]2P0;. are typical surfactants. As seen, depending on their headgroups, surfactants can be cationic, anionic, zwitterionic, or uncharged. The hydrophobic chain can be a different length, contain one or more unsaturated double bonds, and/or consist of two or more chains. Importantly, surfactants can be functionalized (6, 7). Aggregation behavior
NAOW
Figure 1. Zigzag (2)scheme of the vectorial pathways of electrons and protons in the thylakoid membrane.
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Journal of Chemical Education
Figure 2. The fluid mosaic model of the biological membrane.
of surfactauts depends upon the nature of the solvents used and on the method of preparation (6). Opposing forces of repulsion between the polar headgroups and of association between the hydrocarbon chains of the surfictants are responsible for the formation of micelles in water. Micelle formation is dynamic. At a concentration known as the critical micelle concentration (cmc). 50-100 individual surfactants cooperatively assemble to a more-orless spherical aggregate: the micelle. Diameters of micelles are typically.40-60 A. Increasing the surfactant concentration leads to the formation of rodlike micelles. The longer the hydrocarbon tail, the lower the cmc, hence the lower the concentration a t which rodlike micelles begin to form. With further increase of surfactant concentration, especially in the presence of an alcohol (the cosurfactant), oil-in-water (olw) &croemulsions and liquid crystals may form. Certain surfactants with the right hydrophohic-lypophilic balance (Aerosol-OT,for example) associate to form inverted or reversed micelles in apolar solvents (6,12-14). Formation of reversed micelles requires at leasttraces of water. Reversed micelles can he considered, therefore, to he surfactant-entrapped water pools in the sea of hydrocarbon solvent. Increasing the concentration of entrapped water, i.e., the size of water 0001s. a t a given surfactant concentration. results in the formation of larger aggregates. If the water concentration is further increased water-in-oil (w/o) microemulsions herin to appear. Snreadine" an oreanic solution of a surfactant on water re" suits in monolayer formation a t the air-water interface (6, 15-19). Techniques have been developed for transferring the monolayer onto a solid support and for building up organized multilayer assemblies in controlled topological arrangements (17-19). Photochemistry in these systems have been described (19). Brushing an organic solution of a surfactant across a pinhole
Within a few minutes the film thins and the reflected lieht
sweliing of dried bhospholiplds or surfactants results in the formation of onion-like multicom~artmentvesicles. Sonication above the phase transition temperature yields single compartment hilayer vesicles whose diameters are in the stable for weeks, even months. Importantly, vesicles are capable of soluhilizing a large number of substrates per aggregate. They also have the largest number of solubilization sites. Hydrophobic molecules can he distributed among the hydrocarbon hilayers of the vesicles. Polar molecules may move about relatively freely in vesicle-entrapped water pools, particularly if they are electrostatically repelled from the inner surface. Small charged ions can he electrostatically attached
Phase transition is an imoortant ~ r o. ~ e.rof t vmonolavers. BLMs, and vesicles. ~ e p e n d i n gon the surface area-presk& isotherm. monolavers mav be in a easeous, fluid. or solid state. Thermotropic phase transitions o r ~ ~ ~ s 'vksic~es a n d involve changes in the arrangements of lipids without altering the gross structural features of the bilayers. Below the phase transition temperature, the surfactant c~nstitnentsof BLMs and vesicles are in highly ordered "solid" states, with their alkyl chains in all-trans conformations. Above the phase transition temperatures, lipids become fluid as the result of gauche rotations and kink formation. Micelles do not usually have temperature-induced phase transitions. There are additional motions of surfactants within the BLMs and vesicles. Surfactants may undergo segmental and rotational motions, lateral diffusion, and flip-flop. Artificial Photosynthesis in Organized Media The table provides a somewhat oversimplified comparison of natural and artificial photosynthesis (6, 21-23). Such Comparison of Natural and Artificial Photosynthesis
monoloyer
spherical micelle
rod-like micelle
Natural
Function
reversed mlcelle
' W/O
I
,
Photosynthesis
-
Carbohydrate Production: H 2 0 CO? ICH201 + 0 2
+
Organization of
PhotoCKemislry: Captured visible light is transformedto chemical energy Chemistry: Stored energy is used in dark reactions In thyiakoid membrane
ComDonents Light harvesting
Antenna chlorophyiis (-400).
-
m~croemuls~on o/w m~croemulsion
Artificial Photosynthesis in Organized Media
Redox events are used water splining: H20 H, + %02
-
in organized media
Sensitizers: ruthenium, porphyrin derivatives.acridine dyes Electron transporl Complex, yet inwmpietely Simple relay compounds: understood chain of events methylviologen, r'Z-scheme") europium cation Charge separation Through potentials developed Through potential3 in thylakoid membrane developed in organized proteins (?)
media Water splitting
multlcompartment vesicle Figure 3. Organized structures formed frwn surfactants
catalyst Conversion efficiency
Mn-containing enzyme or complex 10%
Pt,
hydrogenase for H,,
RuOz for O2 Up to 6 %
Volume 60 Number 10 October 1983
873
multifaceted functions as energy conversion, water oxidation, broad set of &ndit&ns. Not unexpectedly, this requires complex molecules performing complex chemistries: Conversely, artificial photosynthesis contents itself with 6ne reaction in controlled laboratory conditions: water splitting. Mother Nature need not be, therefore, slavishly reproduced. The simplest artificial system is likely to perform best. Equation (1) illustrates the simplest potentially cyclic photosensitized water-splitting apparatus consisting of only sensitizers (S),electron relays (R), and catalysts:
acetate, EDTA, as the sacrificial electron donor, tris-(2,2'bipyridine)ruthenium cation, Ru2+,as the sensitizer, hexylviologen, H V + , as the relay, and benzylnicotinamide, BNA+, as the electron acceptor. ThG sensitizer and electron donor were placed in the surfactant-entrapped water pools while the acceptor was localized a t the interface.
water pool
Development of viable artificial photosynthesis requires (1 l ~i -the ., - -use ~ of sensitizers and relavs with suitahle s ~ e c t r aand redox properties as well as with chemical and photochemical stabilities, (2) the efficient photoelectron transfer from the sensitizer, (3) the prevention of hack electron transfer which would reform R and S without water splitting, (4) the employment of catalysts which facilitate water reduction and oxidation (andlor the design of relays and sensitizers capable of simultaneously transferring two and four electrons, respectively), and (5) the separation, in space or in time, of the ultimate photosynthetic products-oxygen and hydrogen. In the natural system thebe requirements are well met by organizing the components of the photosynthetic apparatus in the thylakoid membrane. Artificial membranes-organized medl8-have to perform functions similar to the thylakoid membrane. Initial applications of organized assemblies were limited to the demonstration of principles relating to compartmentalization and charge separation. Simple aqueous micelles were used most frequently. Energy transfer from micellar sodium-dodecyl-sulfate (SDS) 2soluhilized naphthalene (N) to terbium chloride illustrates ;he beneficial advantages of compartmentalization (24).The most efficient energy transfer is observed when less than one N molecule is localized in each micelle, but there are Iage numbers of terbium cations hound to the negative surface of SDS [eqn. (4)). In the absence of the micellar cage, there is no energy transfer. Naphthalene triplet-triplet annihilation predominates (eqn. (3)). ~~~~
~
~~
~
In water I n micelle
a", /me
0: micelle
micelle
*
~bl*
3N* has no partner in the micellar cage, and it can only be deactivated by energy transfer to Tb3+ present in close proximity a t high local concentrations. Important advances have been made using reversed rnicelies and w/o microemulsions in charge separation (25,26). Sacrificial oxidative and reductive half cells have been constructed in dodecylammonium propionate-reversed micelles in toluene. The oxidative half cell consisted of ethylenediamine tetra874
Journal of Chemical Education
orgame phase
Charge separation is accomplished by ejecting B N A to the organic phase (eqn. (5)).The reduction half cell consisted of the sacrificial electron donor, thiophenol, the sensitizer, Ru2+ or water-soluble zinc porphyrin, and the electron acceptor, methylviologen, MV2+ (eqn. (6)). The sensitizer and electron acceptor were located in the surfactant-entrapped water pools while the electron donor was distributed in the water-oil interface. Charge separation is, once again, accomplished by the teversed micelle. The reduced nroduct. MV.+ remains in the -~ aqueous solution, while the oxidized product, diphenyldisulfide. into the organic uhase (25.26').Couuline ~, is eiected " . . .. the oxidative and reductive half cells together remains, of course, to be accomplished. ~~~~~~
~
water
interface
toluene
Surfactant vesicles have also been utilized to mediate artificial photosynthesis (6,27,28).Efficient electron transfer had been demonstrated from a sacrificial donor, EDTA, to an acceptor, M V + , via a sensitizer, tris-(2,2-hipyridine)ruthenium chloride, Ru(bpy)jt. Ru(hpy)ii was attached to the outer surfaces and MV2+ was placed on the inner surface of negatively charged dihexadecyl phasphate (DHP) vesicles while EDTA was distrihuted in the bulk aqueous solution. Initially, electron transfer was considered to occur across the vesicle bilayers. Subsequently, electron transfer was shown to occur on the same outer surface of DHP vesicles, following photosensitized leakage of MV" (29). These results as well as the relatively poor long-term stability of vesicles demand alternative approaches. We have chosen polymerized surfactant and surfactant vesicles as media for artificial photosynthesis (30-34). Others have concentrated their efforts on colloidal semiconductors (23). Either of these, or indeed a judicious combination of both approaches, will undoubtedly provide the much needed breakthrough in this fascinating and highly relevant area of research. Photochemical Reactivity Control in Oiganized Media Reactivity control is accomplished by placing reactants in the different compartments provided by the organized media. Photoexcitation of a simple molecule of paphthalene, caged in a micelle covered by Th3+,has already been shown to result in energy transfer (eqn. (4)) in contrast to nonproductive triplet-triplet quenching (eqn. (3)) occurring in homogeneous solution (24). Similar principles have been employed in altering photochemical pathways. Photodecarbonylations have been investigated most extensively. Photodecarbonylation
of dissymmetrical dibenzylketones in homogeneous solution results in statistical product formation (35):
I1
2
P~CH~CCH~A~ 0
I1
ACB
A~ PhCHzCHzPh + P ~ C H ~ C Ht ~ArCHzCHzAr
AA
AB
BB
25%
50%
25%
(7)
Product distribution is dramatically altered in the presence of micellar hexadecyltrimethylammoninm chloride, CTACI. Increasing the surfactant-to-ketone ratios results in a sigmoidal increase of the cross products, AB, at the expense of coupled products AA and BB. When there is less than one reactant per micelle, AB becomes the exclusive photoproduct. Under this condition the photolytically generated A. and .B readily react with each other prior to their escape from the micellar cage. Conversely, in homogeneous solution there is nothing to prevent the radicals from reacting with each other in a statistical manner (35). More recently, wlo microemulsions and surfactant vesicles have been employed as media for the in situ photochemical generation of colloidal gold (36) and platinum (37) particles. The rationale of this approach is that the aggregation of reduced metal particles is necessarily limited by the concentration of the parent metal halide per aggregate as well as by the size of the water pools in microemulsions and vesicles. The smaller and the more uniform the colloidal metal particles, the more efficient and selective. of course. their catalvtic activity. Photoreduction of HA~CI:entrappeb in the wa&r pools of pentaetbyleneglycol dodecylether microemulsions in hexane resulted in the more efficient formation of smaller and more uniform oarticles than in water (Fie. 4 (36)).Under the particles th;ough the exchange of the contents of neighboring microemulsions (eqn. (8)):
In this case colloidal gold formation is relatively slow and the size is limited by the size of the microemulsions (36). Since the contents of polymerized microemulsions or vesicles cannot interchange, these aystems provide a better control in the formation of colloidal catalysts. Colloidal growth in polymerized microemulaions or vesicles is limited, therefore, by
Figure 4. Electron micrograph of colloidal gold formation in water (top) and in microem~l~l~n~ (bottom).
the number of monomeric ions placed into the water pool prior to colloid formation (37). Utilizing organized media for promoting photoinitiated chiral recognition has been a long-cherished problem (6,7). Maximum stereospecificities are to be expected for systems which provide relatively rigid and specific binding sites for a given substrate and its reactive transition state. Cavities of functionalized, optically active cyclodextrins and crown ethers are expected to provide more rigid and hence energetically more favorable binding sites for chiral discriminat:on than those available in micelles. Indeed, host-guest systems have been used as "resolving machines" for racemic amino acids (38). Relatively little work bas been carried out on photoinitiated chiral recognitions in organized medias (6). Photophysical techniques provide opportunities, however, for recognizing subtle effects. For example, no differences in fluorescence lifetimes (7)and time-dependent anisotropies (TR) have been found between P-cyclodextrin-entrapped (-1- and (+I-a-(1-nanbthvl)ethvl amine (NEA) in water (391. Dia' . iteremirrlt I I M rinlinatiun IX.VIIIIIC oh,en.nl)lr in llqueous IjOcc I)MSO. where r m i ~ h ximmatim 11, tawll NF.A and DMSO as wellas between DMSO and P-cyclodextrin facilitates the required rigid orientation. Differences between chiral P-cyclodextrin-entrapped (-)-NEA and (+I-NEA in 60% DMSO manifested in different excitation and emission snectra and rvalues for the diastereomers (39). Organized assemblies have also been used for enriching mag'etic isotopes (5, 40, 41 j. The underlying principle 6 based on the radical pair model of chemically induced dynamic nuclear polarization, CIDNP (42-45). According to this theory, the chemical reactivity of radical pairs is expected to depend on the hyperfine interactions of
