Photochemical Reactions of Surfactant Molecules in Condensed Monolayer Assemblies-Environmental Control and Modification of Reactivity[**I By David G. Whitten["] An investigation of the effects of a controlled microenvironment on reactivity has been carried out by the synthesis and study in organized monolayers of surfactant molecules containing photochemically reactive chromophores. Reactions where substantial environmental effects have been observed include the cis-trans isomerization of olefins, ketone photoeliminations, ligand substitution processes, excimer and photodimer formation, and photoredox reactions. Both reactions occurring within the organized assemblies as well as interfacial phenomena have been investigated. Results obtained in these studies suggest possible extensions and applications for the development of new catalyst systems, biological models and controlled syntheses.

1. Introduction The phenomenon of monolayer film formation at an airwater interface has been known and studied for a long time. Such films are formed when molecules containing a hydrophilic group, for example a carboxyl group, and a sufficiently long hydrophobic chain are brought onto the surface of water; these molecules are referred to as surface active (or as surfactants). Techniques for the study and manipulation of such films were developed by Pockels"], Bl0dgett['.~1, and Langmuir13]and later extended by numerous other investigators. Langmuir found in 1917 that films spread on water could be transferred to solid supports[4]; later studies developed and refined the techniques for depositing several consecutive film layers. In recent years the techniques for the preparation and study of monolayers have been greatly elaborated and at the same time simplified, largely due to the eleThe possibilities for gant investigations of Kuhn et al. ['~~'"]. preparing assemblies of a known and controlled molecular architecture have been developed and used to investigate a wide range of phenomena ranging from electronic energy transfer and polymerization to protein-membrane interactions and photographic sensitization[']. A number of investigations have dealt with chemical reactions occurring within monolayer films or assemblies or occurring between a reactant contained in a monolayer and another present in an adjacent phase. Reactions investigated in some early studies include hydrolysis reactions[".'*', oxidat i o n ~ [ ' ~and ] , p o l y m e r i ~ a t i o n s In ~ ~ a~ ~number . of investigations the occurrence of specific reactions has indicated that considerable but frequently retarded penetration of monolayers by various reagents from adjacent bulk phases can occur["]. These observations as well as other evidence emphasize the intermediacy of the properties of compressed layers between those of liquids and solids. Thus the spacing indicated by surface pressure-area relationships in many condensed monolayers is regular and close to that occurring in ["I

Professor Dr. D. G. Whitten Department of Chemistry University of North Carolina Chapel Hill, North Carolina 27514 (USA)

[**I Photochemical Reactions in Organized Monolayer Assemblies, Part 12:Part 11: P. R. Worshum, D. W. Euker, D. G. Whitfen,J. Am. Chem. SOC.100, 7091 (1978).

440

0 Verluy Chemie, GmbH, 6940 Weinheim, 1979

highly ordered hydrocarbon crystals, yet the same monolayers can be penetrated by relatively large organic reagents such as a-naphtholl5,''I. Our interest in examining a variety of chemical processes and photoreactions in monolayers was stimulated by the attractive possibility of having reactive molecules present in a specific orientation and in a known and controlled microenvironment. The close resemblance of monolayer films and assemblies to biological membranes further suggested that phenomena occurring in monolayers might in some cases provide reasonable models for processes occurring within membranes or at membrane-solution interfaces. In our investigations of reactions occurring in monolayer films and assemblies and at monolayer interfaces, we have examined a variety of processes and in many cases we have been able to make comparisons between reactivity in solution, the solid state, monolayers and other organized media. Our results indicate that the microenvironment of a molecule after incorporation in a monolayer film or assembly can produce striking effects on reactivity that can be explained in terms of concentration and orientation effects, hydrophilic-hydrophobic relationships, restricted diffusion and packing phenomena. While most of the reactions examined to date have been carried out on an extremely small scale, the rather pronounced effects observed would justify the future development of practical applications.

2. Unimolecular Reactions in Monolayers 2.1. Photoisomerization of Olefins One of the most widely studied of all photoreactions is the cis-trans isomerization of olefins1'61. Although this reaction generally does not occur in crystalline solids it occurs readily in fluid solution. In most cases isomerization occurs via rapid relaxation of cis or trans excited states to a twisted intermediate which can decay to ground states of both isomers. In viscous solutions photoisomerization is sometimes retarded and in certain cases selective restriction of the isomerization in one of the two directions occurs[16.'71. For example, in the much investigated stilbenes, it has been observed that an increase in viscosity of the medium retards the trans to cis process selectively while allowing the reverse process to pro-

0570-0833/79 06 06-0440 $ 02.50/0

Anyew. Chem. Itit. Ed. Eiiql. 18. 440-450 11979)

ceedil7l.This viscosity effect has been ascribed to an increase in molecular volume along the path of the trans to cis process. Such an effect might be expected to play a major role in highly condensed monolayer systems, and for this reason we have examined the behavior of olefins (1)-(3), all of which can be incorporated into monolayer films and assemblies but in rather different environments[lx."1. n

All three olefins (as well as their non-surfactant analogs) undergo facile cis-trans photoisomerization in both directions in solution. For (1) and (2) selective wavelength irradiation of solutions enables preparation of monolayer films and assemblies rich in one or the other isomer. For (3) only the photobehavior of the trans isomer has been investigated'l-ll. The photoisomerization behavior of (I) in films and assemblies has been the most extensively investigated["]. The thioindigo chromophore absorbs strongly in the visible region and the spectra of the two isomers in assemblies are only slightly altered from those in solution. From measurements of the absorption spectrum with polarized light, in with structurally analogy to results obtained by Kuhn et. al.L51 related cyanine dyes, we conclude that the chromophore of (1) lies at or near the hydrophilic interface parallel to the surface in films and assemblies. On irradiation of films or assemblies containing cis-(I) a rapid and irreversible conversion to trans-(l) is observed. In contrast films or assemblies containing trans-(I) were found to be photostable and no conversion to the cis isomer or other photoreaction was detected even on relatively long term irradiation. A study of the pressure-area isotherms of spread films of cis- and trans-(I) on the water surface indicates that a greater cross sectional area per molecule is required for the cis isomer and that a significant reduction in pressure occurs when films of cis-(I) are irradiated and undergo conversion into trans-(I). Thus there appears to be a very reasonable parallel between the behavior of (1) in the films and assemblies and the viscosity effects previously observed with olefins such as the stilbehavior has been observed for the stilbab e n e ~ ~ "A~ similar . zole derivative (2) in films and assemblies. This positively charged analog of stilbene evidently also resides at a site at or near the hydrophilic interface. Irradiation of cis-(2) in monolayer assemblies results in rapid and irreversible formation of the trans isomer. The trans isomer does not form cis(2) on irradiation in the assemblies; however, in this case the trans isomer undergoes photoreaction to form other products (cf. Section 3.2)[".2"1.That a restriction to increased molecular volume caused by tight packing plays the major role in inhibiting the trans to cis process in ( I ) and (2) rather than simple orientation effects is indicated by the finding that (2) undergoes efficient isomerization in both directions (trans s cis, cis + trans) as its chief photoreaction upon incorporation into cationic micelles. A~~grC u h. m i . In/. Ed. Etqjl. 18. 440-450 ( 1 979)

The surfactant styrene (3) also readily forms monolayer films and assemblies12"; however, for this compound the olefinic chromophore lies at the end of a hydrocarbon chain in a highly hydrophobic environment. The behavior of (3) in films and assemblies closely resembles that of (2). Irradiation of trans-(3) leads rapidly and cleanly to a photoproduct, but no isomerization to the cis-isomer A pressure-area study of trans-(3) suggests that highly compressed films and assemblies are formed which can not easily expand to accomodate the bulkier cis isomer. The similar behavior of the three rather different olefinic systems described here would indicate that partial inhibition of isomerization could be a fairly general phenomenon in highly condensed assemblies owing to the effective high pressures caused by close packing of the paraffin chains and hydrophilic (head) groups.

2.2. Photoreactions of Ketones in Monolayers We have recently investigated another intramolecular photoreaction in monolayers which should be extremely sensitive to the environment: the Norrish type I1 photoelimination of ketones [eq. (a)]. This reaction, which occurs with moderate quantum efficiency, is the dominant photoreaction of most aliphatic and aromatic ketones possessing abstractable y-hydrogens in ~ o l u t i o n [ ~The ~.~ reaction ~ ~ . proceeds via H

y-hydrogen abstraction in a cyclic six-membered transition state followed by cleavage of the diradical to form an en01 of the methyl ketone and an olefin. The reaction has been examined in m i c e l l e ~ ' ~and ~ ' in several polymer systems[25~261. Studies on polymers have shown that the rates of reaction and efficiencies are relatively high in polyethylene and vinyl ketone copolymers above the glass transition point1261.At low temperatures below the glass transition point and in some crystalline solids very little reaction via the type I1 process is observed because the rigidity of the medium makes it difficult to achieve the proper geometry for reaction to occur[z61. In our investigations of the type I1 process [eq. (a)] in monolayers we used the 0x0 acid (4), which was expected to form films and assemblies with the ketone in a somewhat ordered hydrophobic region. It was found that (4) does in fact form good films and assemblies in 1 : l mixtures with arachidic acid[271.In compressed films formed from these mix-

tures the area per molecule of (4) is 20 A2; this corresponds closely to the cross sectional area of a paraffin chain. Irradiation of (4) in benzene solution gives the expected methyl ketone (5) as the chief volatile product with a quantum effciency (+) of 0.2. In contrast irradiation of (4) in assemblies leads to disappearance of the ketone chromophore (as meas-

441

ured by spectral changes in the ultraviolet or infrared) with a quantum efficiency of 0.06[271.However, the methyl ketone (5) is only a minor product formed in low yield. Thus, the quantum yield for the type I1 elimination ranges from 0.0001-0.03 (depending on assembly architecture), down considerably from the value in solution. The low quantum efficiency for the type I1 elimination in monolayer assemblies can be attributed to a perpendicular (zig-zag) arrangement of the paraffin chains which cannot be easily distorted to allow the cyclic transition state necessary for y-hydrogen abstraction to be attained. The contrast between the extremely low quantum yield for the reaction of (4) in the type I1 process [eq. (a)] and the moderate quantum yield for the disappearance of (4) suggests that other photochemical reactions occur. One such process which might be anticipated is the photoelimination of type I (a-cleavage) [eq. (b)]. This would be expected to result in chain scission of (4), leading to either or both p-methylbenzaldehyde and toluene. However, analysis of the volatile products obtained on photolysis of (4) in the assemblies

(41

/

\

(b)

reveals that neither product is formed in detectable yield. The lack of type I cleavage products [eq. (b)] is perhaps surprising but also reasonable if the monolayer environment prevents diffusion of free radicals formed by a-cleavage such that only recombination occurs. The disappearance of (4) must therefore be accounted for by other reactions; a reasonable possibility is intermolecular hydrogen abstraction from a paraffin chain in close proximity to the carbonyl group. Such a process could lead to net photoreduction of the keto group or to a cross-linking reaction. We are currently examining the non-volatile products in order to determine precisely the major reaction path. We are also examining reactions of the 0x0 acid (4) as a structural probe in other organized media and in modified monolayer assemblies. In anionic sodium dodecyl sulfate (SDS) micelles we find that (4) undergoes Norrish type I1 photoelimination to give the methyl ketone (5) as the only volatile product. The quantum yield for the process in SDS micelles is 0.8, four times the value obtained in hydrocarbon solution. This value is comparable to those obtained for nonsurfactant ketones solubilized in cationic micelle~'*~'; in these studies it was inferred that the increase in quantum efficiency in micelles was consistent with location of the ketone chromophore in a relatively polar environment, since the Norrish I1 photoelimination in solution increases markedly in alcohols or other polar solvent^[**^^^^. A reasonable inference from the present results is that for (4) in SDS micelles the polymethylene chain is coiled back on itself so that both ends (carbonyl and carboxy group) may lie near the micellar surface. The high value obtained for the photoelimination of (4) in SDS micelles = 0.8) emphasizes the contrast in order and rigidity or microviscosity between monolayers and 442

micelles. Thus, even though both media are formed by selforganization of amphipathic molecules, the one structure closely resembles a crystalline solid in many of its properties while the other is essentially solution-like.

2.3. Ligand Exchange Processes in Metalloporphyrin Complexes Previous indications that the microenvironment provided by deposited monolayer assemblies is intermediate between a crystalline solid and a viscous solution suggested that ligand exchange processes would be a particularly attractive class of reactions to study in these media. Both thermal and photochemical ligand exchange reactions have been observed for a wide variety of transition metal complexes; while the former may involve both association and dissociation processes, photochemical reactions almost invariably involve ejection of a ligand to generate an intermediary, highly reactive coordinatively unsaturated metal center. In solution such intermediates are usually very short-lived; the reactive complex is normally consumed rapidly by reaction with ligating solvent or solute molecules or by recapture of the ejected fragment. We hoped that ligand photoejection in organized monolayer assemblies might permit the isolation and selective reaction of the reactive complexes thus formed. Our investigations in this area have centered around ligand exchange processes occurring with metalloporphyrin complexes. The photoejection of bound CO from iron(rr) porphyrin complexes such as carboxyhemoglobin and carboxymyoglobin as well as simpler synthetic complexes has been observed in numerous investigations128.2yl; more recent work has shown that photoejection processes involving other ligands as well as other metals can occur in porphyrin c ~ m p l e x e s ~ ~ " - ' ~ ~ . We have investigated the photoejection of CO from ruthenium(i1) and osmium(i1) porphyrin complexes in solutionr3"'. In these investigations the overall reaction observed was replacement of the departing CO by a new ligand. The CO-free metalloporphyrin which is formed in the photoejection cannot be isolated in solution even though it can be detected by flash photolysis. In our studies with surfactant metalloporphyrins in monolayer assemblies we found that the carbonylruthenium(I1) complex of the dioctadecyl ester of mesoporphyrin IX (porph) forms good monolayer assemblies in which the porphyrin is photochemically The complex can be introduced into assemblies as either the aquo-CO or pyridine-CO complex; both complexes exhibit similar photochemical behavior. Irradiation of dry monolayer assemblies containing these complexes, L(porph)RuCO (where L is pyridine or water), in vacuo leads to isolation of the CO-free complex. This complex, L(porph)Ru, is extremely reactive on exposure to air, nitrogen, oxygen, pyridine or CO, but it is fairly stable in a highly evacuated cell. The reactions we have observed to date are summarized in Scheme ( c ) ' ~ ~The ] . absence of ligating molecules in the dry

and evacuated monolayer environment permits isolation of the otherwise unobtainable L(porph)Ru and thus its exploitation for the directed synthesis of novel nitrogen and oxygen complexes'341. It is interesting to note that, subsequent to these investigations, solution studies on the photolysis of similar carbonylruthenium(i1) porphyrin complexes in weakly ligating solvents such as acetonitrile have led to the production of labile oxygen complexes having spectra similar to those of the oxygen complexes L(porph)RuO, isolated in the mono layer^[^^]. Here a labile L,(porph)Ru species i s evidenced as intermediate in the formation of the oxygen complex. We have also examined ligand exchange processes with surfactant iron porphyrins in monolayer as ~ emb l i esr~~' As-. semblies are generally prepared by introducing the air-stable iron(ii1j complex into the monolayer film and reducing the complex after it has been incorporated into a multilayer assembly. We have found that the iron(rrr) porphyrins in monolayer assemblies can be reduced to the corresponding iron(ii) complex by two rather different procedures. Exposure of dry, evacuated assemblies to the reducing ligand piperidine leads to quantitative reduction to the iron(rrj complex. With the surfactant iron(1i) porphyrin (6), R=cholesqOOR

tanyl [cf. (18b)],we find that exposure to oxygen or carbon monoxide in the monolayer assemblies leads to the formation of substituted iron(1i) complexes and not to the corresponding iron(ii1) complex. Both the oxygen- as well as the carbonyl-complex regenerate the unsubstituted iron(ii) complex on evacuation of the assembly[361.We have also found that reduction of iron(m) porphyrins can occur through addition of other ligand-reducing agent systems. Addition of NO to evacuated assemblies results in formation of an NO complex having a characteristic iron(1r) NO spectrum. This complex can be decomposed by addition of methanol to give the iron(i1) complex [eq. (d)][36]. +

(porph)Fel'NO'Cle +CH,OH

+

(porph)Fe"+ CH,ONO

+ HCl

(d)

Reactions of iron(ii) complexes prepared in assemblies by this method are currently under investigation.

3. Bimolecular Reactions in Monolayer Films and Assemblies 3.1. Dimer Formation and Aggregation Investigations with surfactant dyes and other compounds with strongly absorbing chromophores have indicated that A n y w ( I i c ~ r n lilt Ed Enql 18.440-450 ( 1 9 7 9 )

considerable aggregation occurs in monolayer films and assemblies" ']. This can frequently be observed both in pure and in mixed monolayers and can involve both inter- and intra-layer interactions. The aggregation can most easily be established by comparison of the absorption and emission spectra of the molecules under investigation in dilute so h tion and in the condensed monolayer assemblies. Aggregation can result as a consequence of several factors, depending upon the system under investigation and the techniques used to form the monolayer In many cases studies of isotherms obtained from separate and mixed monolayers indicate that the latter are not homogeneous but consist of doWhile in certain mains or patches of separate monolayers~3x~3y1. cases separate phases may result from differences in solubility of the components in the spreading solvent, in other systems the occurrence of separate phases may be due to nonmixing or nonideality in the spread filmf3']. Such factors are evidently much more severe in organized media than in conventional solutions; it has recently been shown, for example, that in micellesformed frommixtures offluorocarbon and hydrocarbon surfactants there is a nearly complete segregation into distinct phases of the individual component^[^"^. Even where the components of mixed monolayers are completely miscible in the spread film, in cases where both components are present in anywhere near equimolar amounts the effective local concentrations obtained in the films can be quite high. Thus, the films and assemblies frequently provide a means for obtaining concentrations much higher than in solution, such that dimer formation or aggregation is conceivable even if there is only relatively weak attraction between the molecules. The tight packing of components in condensed monolayers may also lead to a n interlocking or association of groups which might not associate in solution, particularly if facilitated by the spatial arrangements in the component surfactants. Examples where such factors have been demonstrated as being important include the packing of surfactant cyanine dyes into Scheibe and the interlocking functional units consisting of an azobenzene dye oriented perpendicular to a cyanine dye which have recently been prepared by Polymeropoulos, Mobius and Kuhn14'].

Our work in this area has demonstrated the importance of dimerization and aggregation for several different surfactant molecules in monolayer films and multilayer assemblies. Some of our first investigations involved a study of monolayers formed from various porphyrins and their metal complexes. In solution studies we were able to show that porphyrins and their zinc complexes form dimers at low temperatures in moderately dilute M) hydrocarbon solution~'~*]. When the same porphyrin chromophore is incorporated into a surfactant molecule we find in several cases that the resulting monolayer assembly formed by transfer of several film layers from water gives a temperature-independent absorption spectrum nearly identical to that of the dimer obtained in solution at low temperature^[^*]. Such dimers, which are formed by contact of the chromophores of neighboring layers can easily be distinguished from the dimers formed within a layer in which dye layers are separated by intercalated fatty acid layers. Hence, for several surfactant porphyrins the dimerization. i. e. association of two chromophores, is a purely intralayer phenomenon such that identical spectra are obtained both from multilayers in contact and 443

those in which “inert” arachidate “spacer layers” separate the layers containing porphyrin. However with certain porphyrins, most notably those of the “picket fence” type such as (7), the dimers obtained in multilayers are exclusively or predominantly interlayer as determined by the “spacer layer” technique‘431.The fact that (7) and some closely related “picket fence” porphyrins form interlayer dimers indicates that the porphyrin must lie at the hydrophilic interface in a rather well-defined site. We are currently attempting to take advantage of this property to construct assemblies of controlled architecture in which two different metal complexes are in close proximity with each another.

The tendency of porphyrins to associate in monolayer films and assemblies led to interesting but troublesome results in our attempts to study iron porphyrins in assemblies[44’.Since iron(rr) porphyrins are not air-stable in solution, it was anticipated that assemblies should be constructed from the corresponding iron(m) complexes which could then be reduced by various reagents. As outlined in Section 2.3, for certain complexes this procedure has proven feasible. However, for several surface-active iron porphyrins it was found that spreading of films from the chloroiron(II1) porphyrin led to immediate and irreversible formation of the corresponding (*-ox0 compounds [eq.(e)][441.This reaction which can be viewed as a simple three-step substitution process [eqs (Q-(h)], occurs in solution, but only at high pH.. . and then rather ~ l o w l y [ ~ ~In - ~contrast, ’~. we found that in the monolayer films the reaction is complete and irreversible even where the subphase is strongly acidic (down to pH=2)[441. The results can be fairly easily interpreted by 2(porph)FeC1+20Hw + (porph)FeOFe(porph)+H,O+2CI0 (porph)FeCl+ OH“ + (porph)FeOH + C1“ (porph)FeOH+ OHe + (porph)FeO” + H 2 0 (porph)FeOe + (porph)FeCI + (porph)FeOFe(porph) + C1”

(e)

(0 (g)

(h)

assuming that reaction (h) is highly favored in the dimers (or aggregates) formed in the spread film while steps leading to decomposition of the p-0x0 dimer should be rendered slower and more difficult. The net effect of the microenvironment provided by the monolayer film in this case is to produce a rather pronounced alteration of both rate processes and equilibria.

3.2. Excimer and Photodimer Formation

The trans isomers of olefins (2) and (3) can be incorporated into monolayer films and assemblies in which the usual solution photoreaction, trans to cis isomerization, does not occur. For the stilbazole derivative (2) it was found that the absorp444

tion spectrum in the assemblies is nearly identical to that obtained for dilute acetonitrile solutions of (2)[201(see Section 2.1). Consequently, there is little indication that complex formation in the ground state is important. Compound (2) fluoresces strongly both in solution and in monolayer assemblies: however, the fluorescence in the assemblies is shifted about 50 nm to longer wavelength as compared to acetonitrile solutions. Although (2) does not undergo trans-cis isomerization in assemblies upon irradiation, it undergoes a rapid photobleaching which was shown to be due to dimer format i ~ n [ ” . ~A~ similar ]. red-shifted fluorescence and rapid photobleaching was also observed for crystals of (21, X=p-chlorobenzenesulfonate or p-bromobenzenesulfonate. For these crystals it was found that the dimers were of the cyclobutanetype (8),formed from parallel addition of two molecules of (2)[j9’. The formation of the photodimers in crystalline (3) and in monolayers appears to be closely related-though there are some notable differences. In all cases the occurrence of dimerization is accompanied by a red-shift in the fluorescence. Consequently, it can be inferred that the redshifted fluorescence is due to an excimer, which is probably an intermediate in the formation of (8) [eqs. (i)-(l)].

H

While excimer formation is frequently observed in solution as a diffusion process, excimer formation in crystals and monolayer assemblies of (2) must involve pre-formed units in which relatively little molecular motion occurs following excitation. Interestingly, no excimer fluorescence has been detected for (2) or similar stilbazole quaternary salts in solution, although photodimerization to give a number of cyclobutane dimers of different stereochemistry than (8) has been ob~erved[~(’.~’l. As pointed out above, excimer fluorescence and photodimer formation are coupled in crystals of (2); while both processes are observed for some anions, other anions yield crystals that are photostable and show only monomer fluorescence. For monolayers of (2) the photobehavior is independent of the anion used (this may simply reflect rapid exchange with the anions in the aqueous phase); however, a strong dependence upon assembly architecture has been observed. Thus, when (2) is diluted with increasing amounts of an inert surfactant such as tripalmitine or arachidic acid, a reduction in both the rate and extent of the photoreaction is observed‘’’! This is, of course, reasonable in view of the need for “pre-formed” dimer sites to afford the possibility of reaction. Furthermore, it was found that both the rate and extent

of photoreaction are reduced when assemblies are constructed with the layers containing (2) separated by inert “spacer layers” of tripalmitine or arachidate[”]. The two observations indicate that for (2) in monolayers the dimerizable sites can be formed both within single layers and between adjacent layers. Although it has not yet been verified, it may be anticipated that interlayer dimerization would produce a dimer having a different stereochemistry from that of (8). The fact that excimer fluorescence and photodimer formation for (2) are observed in solid crystals and monolayers but not in concentrated solutions suggests that the tight packing in the former phases may play a major role in this phenomenon. Schmidt et al. have made extensive investigations of solidstate photoaddition and photodimerization reactions in which it has been shown that properly aligned monomer units of one must be within a distance of ca. 4.2 this is the typical interchain spacing determined for ordered hydrocarbons to permit reactions[501.When photoactive crystals of (2) (X =p-chlorobenzenesulfonate) are heated it is found that reversible loss of excimer fluorescence occurs at temperatures considerably below the melting point[201.One possible explanation for this would be a “melting” of the hydrocarbon chains with a corresponding release of the chromophores from close proximity to one another. It is frequently observed that phospholipids and similar substances undergo phase transitions below their melting points; the hydrocarbon chains become more disordered and liquid-like with an increase in the interchain separation[”’. Self-organization alone of hydrophilic and hydrophobic regions in an organized assembly is not sufficient for excimer formation in the olefin (2). This is indicated by the fact that no photodimerization or excimer formation is observed when (2) is incorporated into cationic cetyltrimethylammonium bromide (CTAB) micelles[201.Appreciable concentrations of (2) in micellar CTAB can be obtained, indicating that there are several molecules/micelle as well as (2): CTAB ratios corresponding to the (2): surfactant ratios in monolayer assemblies where both excimer fluorescence and photodimerization are observed. For these micelles, however, only monomer fluorescence and relatively efficient trans-icis photoisomerization of (2) are We have recently investigated photochemical and thermal reactions of the surfactant styrene derivatives (3) and (9) in different media including the solid state, monolayer films, deposited monolayer assemblies, micelles and solution[*’].

absorption spectrum of monolayers of (3) is altered considerably as compared to the solution spectrum (cf. Fig. 1); thus in this case-in contrast to the surfactant (2)-there is evidently

A

200

19)

Like the 0x0 acids (4) these compounds possess a potentially reactive chromophore in what would be anticipated to be a highly hydrophobic site in organized media. Both (3) and (9) form good films which can be easily manipulated and transferred to yield deposited monolayer assemblies. The styrene (3) exhibits especially favorable properties, forming good films from the pure compound which yield compressed monolayers very similar to those of saturated fatty acids such as arachidate. The area per molecule (3) in a monomolecular film is 19 A’ for a surface pressure of 20 dyn/cm[211.The UV Angew

[nml

< hcm. In[. Ed. Engl.

18, 440-450 (1979)

350

Fig. 1. Comparison of absorption spectra of styrene (3, in hexane solution (and in quartz supported multilayers (---).

250

300

A [nml C-(CH2),,COOH

300

A

200

’‘

250

Fig. 2. Spectra of styrene (3) in quart;. wpported multilayers. min (---I, 15 min (....) and 60 min (.- 1 irradiation.

(--),

)

350 and after 5

considerable interaction between the chromophores, even in the ground state. On irradiation of assemblies containing (3) at wavelengths between 2 5 6 3 0 0 nm there is a rapid bleaching of the styrene absorption (cf. Fig. 2). Good isosbestic points are obtained throughout the reaction, suggesting that only a single reaction is taking place. A similar bleaching occurs when films of (3) are irradiated at an air-water interface. The bleaching is due to formation of a photodimer similar to that obtained from (2) in crystals and monolayer assemblies. The product could be isolated by preparative scale irradiation of large slides containing several styrene multilayers or by recovery of material from several films spread and irra-

445

diated at an air-water interface. The product was found to consist of a single product having NMR, I R and mass spectral data in accord with those expected for a cyclobutane structure as in (10) or ( f l ) . CH3

CH3

QQ

CH3

the quenching-energy conversion processes [eqs. (m) and (o)] are followed by energy-wasting back reactions which regenerate ground states of the starting species [eqs. (n) and (p)]. The obvious potential utility of reactions of this type for energy conversion and storage has led to several investigations in which means have been sought to capture or utilize the high energy redox products or otherwise avert the back reactions"x 631. Our own work in this area has included extensive investigations in solution as well as in monolayer assemblies. MCn@'+Ox + MC(fl+I)@+OXred MC(n+1)s+ Oxrcd+ MC"@+ Ox MCna'+Red MC(n-Ij'3 +Red"* MC(L I I* + Red"" --t MC"" + Red Me?*- excited metal complex. Ox = oxidant, OX"^ = reduced

(m) (n)

~

Irradiation of (3) in solution, in SDS micelles, and in crystals also leads to the formation of dimeric products which are evidently cyclobutane derivativesI2''. However, preliminary indications are that products having different stereochemistry are formed in the solid state and in SDS micelles. The reactions in solution and micelles also appear to produce more than one product. Although we have not yet obtained conclusive evidence, it appears that the product formed from (3) in the monolayers is the trans-syn-transdimer (10) resulting from the parallel alignment of the chromophore in the monolayer[2'1. The fact that only a single product is formed in relatively high quantum yield (4 =0.14) affords reasonable confirmation of the extended regular structure of the hydrocarbon chain expected for the compressed monolayer. The results obtained in this case seem quite analogous to the "topological control" observed in the solid-state photochemical dimerization and addition reactions investigated by Schmidt, Cohen et a1.[4'.5'.521. Although the scale for the reactions in monolayer films or assemblies is obviously somewhat limited, the results obtained with (3) suggest a number of exciting possibilities for obtaining regio- and stereospecific control of reactions by the construction and use of especially tailored surfactants. A particular advantage illustrated by the reactions of both (2) and (3) is that hydrophobic-hydrophilic interactions and the close packing of the hydrocarbon chains dictate the structure of the films and assemblies in a predictable fashion. Thus, it appears that reactions in monolayers offer many of the same possibilities obtainable in the solid state by "crystal engineering" with the possible advantage that monolayer structure may be somewhat easier to predict and control.

3.3. Photoinduced Electron Transfer Reactions in Monolayer Assemblies Photoinduced electron transfer reactions of transition metal complexes, dyes and other substances having relatively low excitation energies have been the subject of extensive investigation during recent years. It has been found that excited states of metal complexes, for example, can be quenched by electron donors or electron acceptors in redox processes that can involve efficient conversion of light energy into chemical energy in the form of energy-rich products. Both the initial photoreactions and subsequent "dark" processes have been well-characterized for several systems'53 "I. In most cases, 446

(0)

(P) oxi-

dant, Red = reductant, Red"" =oxidized reductant.

One of the most widely used metal complexes in studies of electron transfer reactions has been tris(2,2'-bipyridyl)ruthenium(rI), (Ru(bpy): ' ) . This complex is strongly luminescent in fluid solution and reasonably photostable; hence, the quenching of its luminescence by electron donors and acceptors has proven to be a convenient means for studying these reactions. Our interest in studying electron transfer quenching processes in monolayer assemblies led us to prepare the hydrophobic surfactant complex (12). This complex

was found to form monolayer films which could be transferred to solid supports[h41.We found that dry assemblies containing (12) exhibited intense luminescence similar to that of Ru(bpy): in solution. Since its luminescence is strongly quenched by electron acceptors such as the dication paraquat (13a), we decided to investigate quenching of the luminescence of (12) in monolayer assemblies with the surfactant counterpart of paraquat, (13b). Assemblies were constructed +

with (12) and (13b) in the same layer and with (12) and (13b) in adjacent layers having hydrophilic-hydrophilic contact. In both cases significant quenching of the luminescence of (12) was observed[h51.Separation of layers containing (13b) from those containing (12) by two or more layers of cadmium arachidate generally resulted in no quenching of the luminescence-as might be expected for an electron transfer process requiring near-contact of donor and acceptor. In cases where there was quenching of the luminescence of (12) no permanent bleaching was observed, at least on short term irradiat i ~ n @ This ~ ] . suggests that oxidative electron transfer in the assemblies is followed in this case by rapid reverse electron transfer [cf. eq. (n)] analogous to the solution behavior. SubAiiqrw. Chem. lnr. Ed. Enql. 18. 440-450 11979)

sequent to our investigations more extensive studies on electron transfer quenching in monolayers have been reported["]. In these investigations it has been found that quenching of the luminescence of several cyanine dyes occurs over statistical spacings of up to R = 75 A in contrast to the average quenching distance between (12) and (13b) of 10 A. These very interesting results indicate a correlation between the distance and the ionization energy of the excited state of the electron donor[66]. One case where electron transfer quenching of an excited state leads to formation of permanent products, albeit rather inefficiently, is the photoreduction of metalloporphyrins by various reducing agents. It has been found, for example, in solution that a number of metal porphyrin complexes (14) including those of tin(Iv), germanium(Iv), zinc(n), platinum(rr) and palladium(I1) can be photoreduced sequentially to chlorin (dihydroporphyrin) (15) and tetrahydroporphyrin (16) products [eq. (q)] with reducing agents (Red) such as stannous chloride, ascorbic acid and tertiary a m i n e ~ ' ~ ' - ~ ~ ] .

The reduction of palladium(I1) porphyrins with tertiary amines has been extensively investigated; evidently the initial step involves electron transfer from the amine to the excited p ~ r p h y r i n [ ~ 'Depending ]. upon the medium, subsequent steps may involve proton transfers, secondary electron transfer steps and free radicaI processes. In an effort to determine whether such a sequence of reactions can occur in monolayer assemblies we have examined reactions of the palladium(1r) complexes of surfactant porphyrins (6) and (7) with the sur-

H37c'8;N0

( I 7)

H37C18

factant tertiary amine (14)["]. In very recent experiments we have found that both dry assemblies and assemblies immersed in water undergo partial conversion as outlined in eq. (9) when the amine (17) and palladium porphyrin are present in a 1:l ratio in the same layer with an excess of arachidate host. We have also observed similar reactivity for assemblies in which (17) and palladium porphyrin are present in adjacent layers with hydrophilic-hydrophilic contactl7'1. In both cases only partial conversion has been observed. Fairly rapid conversion occurs on initial irradiation, resulting even in formation of some tetrahydroporphyrin (16), but prolonged irradiation results in little additional conversion. Evidently here, as with the dimerization phenomena observed with (2). there are some sites capable of reaction while others are non-reactive; once the "reactive sites" are exhausted no further reaction can occur. We are currently exploring different combinations of porphyrin reductant and host surfactant to define what constitutes a reactive or an unreactive site. Anyew. Chein. I n t . Ed. Engl. 18.440-450 ( 1 9 7 9 )

4. Interfacial Phenomena: Monolayer Solution and Monolayer-Gas Reactions Recently, considerable interest has centered around interfacial processes in which an immobilized reactant or catalyst interacts with a reagent present in a contacting solution or vapor phase. The relative ease of constructing monolayer assemblies on various supports would suggest that this might be an area where monolayers could offer considerable utility both for mechanistic studies as well as practical applications. Our initial interest in this area centered around the possibility of carrying out heterogeneous light-induced electron transfer processes involving an immobilized light absorbing complex and potential oxidants or reductants in solution. The first system planned for investigation involved monolayer assemblies containing the ruthenium complex (12) and aqueous solutions of various electron acceptors. Initial experiments with assemblies containing an outer hydrophobic layer of (12) revealed that the strong luminescence observed from the dry assemblies was almost completely quenched upon immersion of these assemblies in water[641.The absorption spectrum of (12) in the assemblies was little altered by this process and we found that gentle heating in a vacuum restored the luminescence to nearly its original intensity[64.651. In contrast we found that the luminescence of (12) in organic solvents was virtually unaffected by addition of considerable amounts of water, aqueous acid or aqueous base. The observation that water quenched the luminescence of monolayer assembly-bound (12) suggested the possibility that a photoreaction involving (12) and water could be occurring. Initial experiments on the irradiation of monolayer assemblies of the surfactant ruthenium complex (12) in the presence of water led to the production of hydrogen in an apparent light-induced cleavage of water by the monolayerbound complex[641.Although slides containing monolayers formed from the initial preparation of (12) repeatedly gave sustained gas evolution upon irradiation, irradiation of assemblies formed from subsequent preparations of the complex under identical conditions has not produced comparable It is now clear that light-induced water cleavage does not occur with pure (12) in monolayer assemblies; the underlying reason behind the initial results remains as yet an unsolved mystery. The original preparation of (12) was found to contain several impurities including other surfactant ruthenium(I1) complexes and it seems likely that the net process of water cleavage in the layers may require at least two different complexes. Attempts to prepare useful systems with the complex (12) in assemblies have been somewhat disappointing because of the extreme sensitivity of its film properties to impurities-especially anions-and to its rapid degradation on contact with ~ a t e r ' ~ ~ Although , ~ ~ ] . different samples of carefully purified complex have been thoroughly investigated in several different laboratories, there still remain significant discrepancies regarding its photobehavior in monolayer films and a s s e m b l i e ~ [ ~ ~ ~ ~ ~ ~ ~ ~ ] . The instability of (12) in monolayers in the presence of water led us to focus our studies on interfacial electron transfer processes in monolayers on other metal complexes; however, our work with (12) initiated numerous other studies on structurally related hydrophobic ruthenium(I1) complexes in s o l ~ t i o n [ ~ ~We .~~ have . ~ ~found l . that the photochemical reac447

tivity of these complexes having the general structure (18) is significantly different from that of the parent complex Ru(bipy): . This renders them potentially useful in energy storage applications and at the same time provides some insight into how the original preparation may have functioned. We have prepared and studied a series of complexes (18) in which the parent complex Ru(bpy):+ is surrounded by hydrophobic groups ranging in size from isopropylcarbonyl in (18a) to cholestanyloxycarbonyl in (18b)‘73J. All of these +

complexes are insoluble in water but can be readily studied in a variety of organic solvents. The presence of the electronwithdrawing carboxyester groups on the bipyridine ligands shifts the redox potentials of the complexes to more anodic values as compared to that of Ru(bipy): . Figure 3 compares the redox potentials in the ground state and in the excited state. An interesting consequence of the shifted redox poten+

Fig. 3. Formal redox relationships for ground and excited states of ruthenium(r~) complexes: left: Ru(hipy),, right: hydrophobic complexes (18). Energy in volts referred to SCE.

tials for the hydrophobic complexes is that while both Ru(bipy): and the various hydrophobic complexes should be stable towards self-quenching electron transfer processes Icf. eq. (r)], a quenching process with one of the hydrophobic complexes (18) as an electron acceptor and Ru(bipy): as a donor should be energetically favored [cf. eq. (s)]. Although we have not yet obtained evidence for the reaction according to eq. (s), we have found evidence that efficient light induced electron transfer processes involving the complex (18) can result in irreversible reactions for both oxidative[741and reductive q ~ e n c h i n g ‘ ~ ~Although , ~ ~ . ~ ~it ~is. beyond the scope of this paper to report in detail on reactions of hydrophobic complexes (18) in solution, it is worth noting that the hydrophobic groups produce significant effects on the course of excited state electron transfer processes and subsequent events. Thus both excited state quenching and reverse electron transfer processes involving (18a) and (18b) are slowed down considerably in several cases as compared to those in the case of Ru(bipy): ’ . The bulky groups evidently retard approach +

+

448

by several reagents-especially cationic species-such that while both efficient but slower oxidative [cf. eq. (m)] and reductive [cf. eq. (o)] quenching processes occur, the backtransfer of electrons is rendered sufficiently slow to permit other processes to ~ o m p e t e [ ~ Thus ~ . ~ ~considerable ]. energy can be stored in the form of one or both of the high energy products produced in eqs. (m) and (0). Returning to interfacial processes occurring in monolayer assemblies we have recently examined reactions of monolayer-bound porphyrins with potential reducing agents present in a contacting aqueous solution[7r1.As with the results reported in Section 3.3, most of our studies in this area have involved palladium(I1) complexes of the surfactant porphyrins (6) and (7) (see Section 3.3). We have found that irradiation of multilayer assemblies containing the palladium porphyrins in contact with aqueous solutions of N,N-dimethylaniline or triethylamine can result in reduction of the porphyrin as outlined in eq. (9). The reaction has been examined using assemblies having different architecture. It has been found that a photoreduction takes place both in multilayer assemblies of porphyrins (6) or (7) as well as in systems containing only a single outer hydrophilic or hydrophobic layer; however, reaction is considerably more rapid when assemblies containing only 1-4 outer monolayers of palladiumporphyrin are irradiatedl7’I. The fact that both porphyrins react at comparable rates is interesting since porphyrin (6) is believed to lie in a relatively more hydrophobic environment than (7), which lies at the hydrophilic interface. It seems reasonable that in this case the moderately polar amines can penetrate both hydrophobic and hydrophilic portions of the monolayer assembly. Such penetration most likely involves a temporary “dissolving” of microscopic portions of the assembly; in agreement with this it is found that addition of excess amine to the contacting aqueous solution can result in removal of the mutlilayer from the support. The occurrence of considerable reaction in multilayers without removal of porphyrin would indicate that penetration through several layers can occur. In contrast to the lack of discrimination between reactivity of porphyrins at hydrophilic 11s hydrophobic sites in the case of amine-mediated photoreduction, there is a pronounced site-dependence upon the interfacial incorporation of metal ions from aqueous solutions contacted with monolayerbound surfactant porphyrins. Thus, while all the surfactant porphyrins examined readily incorporate Cu2 in solution, we find the microenvironment provided by the monolayer produces major effects in both spread films and multilayer assemblies[431.Films or assemblies containing porphyrins such as (7), which evidently reside in a highly hydrophilic site, are rapidly metalated by aqueous Cu*+. In contrast (6) and other porphyrins which are located in more hydrophobic regions are totally inert towards Cu2+ incorporation in films and assemblies. The different rates of Cu’+ incorporation in (7) can also be correlated with the relative hydrophilicity of the site[43’.We have found that monolayer-incorporated porphyrin (7) incorporates several other divalent metal ions from aqueous solution, but the relative ease of metal ion incorporation differs substantially from that observed in homogeneous solution[431.Thus while the rate of incorporation of metal ion for a number of porphyrins in solution decreases along the series Cu” > Zn2+ > Co” > Ni2+ % Mg2+[751, +

Anqew. Chem. Int. Ed. Engl. 18.440-4S0 (1979)

we find that (6) incorporates Mg2+, Co2+,Ni2+ and Mn2+ from aqueous solutions but not Zn2+ or Fe3+. The failure of zinc and iron to metalate in contrast with the facile incorporation of magnesium is especially striking. We are currently studying interfacial metalation phenomena with other more hydrophobic metalating reagents, but it is clear from the foregoing results that the microenvironment provided by the monolayer assembly can produce striking effects on reactivity, particularly when there is limited or selective penetration of one of the reagents from the solution. In other studies of reactions involving surfactant porphyrins at a n assembly-air or assembly-water interface we have likewise observed differences from the behavior of the same reagents in solution. One example where a substantial difference has been observed is the photooxidation of the protoporphyrin derivatives (19)1761. In solution the major oxidation product obtained from irradiation of protoporphyrin IX esters in the presence of oxygen is the hydroxyaldehyde (20) (and its isomer where the other vinyl group has H

hv

02

In contrast, irradiation of protoporphyrin esters in micelles, monolayer films or dry monolayer assemblies leads to production of the diformyl derivative (21) as the major product[”]. The ratio of the products (21)/(20) increases on going from micelles to spread films to condensed monolayer assemblies. Although formation of both (20) and (21) can be rationalized in terms of well-established singlet oxygen addition paths, it is clear that the path leading to (20) sould be favored in the absence of special effects. Although it is not possible to specify the precise cause of the change in product distribution for this reaction it appears once again that the microenvironment plays a major role in the organized media, either in controlling access of the incoming oxygen or in favoring or disfavoring formation of a reaction intermediate.

5. Summary The results summarized in this review of reactions occurring in organized monolayer assemblies indicate that the particular microenvironment provided by the assemblies can often produce major changes i n behavior compared to in solution and even other organized media. In view of recent interests in processes occurring in organized media it is partiAiigrw. Chrm. I n t . E d . Engl. 18.440-450 11979)

cularly instructive to compare and contrast reactivity in monolayers with that occurring in micelles, bilayers, lipid vesicles and microemulsions. While the above examples include some cases where similar effects on reactivity are obtained in different media, there are more often striking differences between monolayers and the other media which can largely be attributed to differences in packing and the degree of molecular organization. Thus in many cases the particular environment produced by a monolayer assembly affords the opportunity for a specific and unique reaction. The examples discussed in this article include some reactive systems where the role of the microenvironment can be quite clearly defined as well as others where precise structure-reactivity relationships remain to be determined. The number of reactions investigated thus far represent only a beginning and it is clear that as more reactions are studied the role of the microenvironment will become sharply defined. One of the most important future uses of monolayer assemblies will probably be the construction of increasingly more complex functional units in which reactions are mediated through cooperative interaction of the individual components. In this way it should be possible to build model systems of increasing complexity which hopefully will approach the structure and reactivity of biological systems. A number of simple systems such as mixed metal complexes of specific structure appear capable of realization as extensions of some of the present experiments. Certainly an interesting area for future study will be interfacial reactions involving interaction of such functional units with mobile substrates. The relative ease of construction of monolayer assemblies of different molecular architecture suggests that extended studies of interfacial reactions should be useful for the design and development of catalysts. Whether catalyst systems based on monolayers can be developed for practical purposes remains to be seen; certainly the fragility of the layers and the small amounts of material contained in the assemblies provide formidable problems. The rather striking examples of topological control obtained in the photodimerization reactions suggests that considerable control and prediction of regio- and stereochemistry can be obtained in assemblies, in analogy to the “crystal engineering” proposed for solid state processes. In fact, in many ways the monolayer systems should be perhaps simpler to design and easier to construct than crystal-based systems. The major limitation on synthetic processes employing monolayer films and assemblies is the extremely limited scale of reaction possible with present techniques and equipment. I am indebted io my co-workers, present and past, at ihe University of North Carolina, who carried out most of the experiments described in this article and who provided valuable insights into iheir interpretation. I am also indebted to Professor H . Kuhn and Dr. D. Mobius at the Max Planck Institut fur biophysikalische Chemie in Gottingen for valuable discussion and advice in several aspects of this work and to the Alexandervon-Humboldt-Stiftung for a Senior U. S. Scientist Award (1975) which permitted extended collaboration with the Gottingen group. Received: May th, 1978 [ A 273 IE] German version: Angew. Chem. Y l . 412 (1979)

449

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Angew. Chem. I n t . Ed. Engl. 18.440-450 ( I 979)