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J. Phys. Chem. B 1998, 102, 10098-10111
FEATURE ARTICLE Self-Assembly of Aromatic-Functionalized Amphiphiles: The Role and Consequences of Aromatic-Aromatic Noncovalent Interactions in Building Supramolecular Aggregates and Novel Assemblies David G. Whitten,* Liaohai Chen, H. Cristina Geiger, Jerry Perlstein, and Xuedong Song Chemical Science and Technology DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Center for Photoinduced Charge Transfer, Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627 ReceiVed: June 3, 1998; In Final Form: September 9, 1998
This feature article presents an overview of a study of several different aromatic-functionalized amphiphiless fatty acid and phospholipid derivatives. These amphiphiles form organized assemblies when the fatty acids are spread as monolayers at the air-water interface or when the phospholipids are dispersed in aqueous solutions. For a wide range of aromatic chromophoresstrans-stilbene derivatives and a series of “vinylogues” (1,4-diphenyl-1,3-butadiene and 1,6-diphenyl-1,3,5-hexatriene), diphenylacetylenes, and azobenzenes such as phenyl, biphenyl, and terphenyl derivatives and modified stilbenes (styryl thiophenes and styryl naphthalenes)sassembly formation is accompanied by formation of aggregates of the aromatic groups. Results of experimental studies and simulations indicate that in many cases the aromatics form a small, stable “unit aggregate” characterized by strong “noncovalent” edge-to-face interactions among adjacent aromatics. Although the unit aggregates exhibit characteristic spectral shifts and strong induced circular dichroism indicating a chiral “pinwheel” aggregate structure, they may be packed together in pure films or dispersions to form an extended glide or herringbone structure. Although the “pinwheel” unit aggregate and the extended glide structure is favored for the majority of aromatics studied, for certain aromatics (styrenes, styrylthiophenes, and R-styrylnaphthalenes) a translation layer, characterized by face-to-face noncovalent interactions, is preferred. The glide or herringbone aggregates are readily distinguished from the translation aggregates by different spectral signatures and different photochemical and photophysical behavior. Factors controlling the type of aggregate and hence extended structure formed from different aromatic functionalized aromatics include shape and steric factors and strength of the competing noncovalent edge-face and face-face interactions.
Introduction The process of self-assembly of amphiphiles such as fatty acids or phospholipids in films at the air-water interface or in aqueous dispersions, respectively, has been attributed in large part to “hydrophobic effects” in which a major part of the driving force for forming the “organized assembly” is an entropically favored release of interfacial water.1-7 In numerous investigations of mixed monolayers or vesicles containing aromatic chromophores or dyes incorporated into the assemblies either as coamphiphiles or as nonamphiphilic solutes, it has been found that aggregation of the aromatic or dye chromophore is a commonly encountered phenomenon.8-10 In several cases formation of a Langmuir-Blodgett multilayer assembly has proven to be an especially convenient way to prepare aggregates not easily attainable otherwise.11-17 It has often been concluded that aggregate formation may occur because of the combination of a very high “local” concentration within the film with the geometric constraints that occur when the film is compressed into a highly ordered array. Depending upon the specific chromophore and substitution pattern when the chromophore is incorporated into an amphiphile structure, both absorption
spectrally red-shifted (“J”) or blue-shifted (“H”) aggregates may be obtained.18-20 In several recent examples it has been shown to be possible to specifically design and synthesize chromophore-functionalized amphiphiles to achieve a particular type of aggregate in both LB films and vesicles.19-21 Several years ago we began a study of the self-assembly processes that occur with functionalized amphiphiles incorporating rodlike aromatic chromophores such as trans-stilbene, diphenylacetylene, or trans-azobenzene into the hydrophobic portion of a fatty acid or phospholipid. Our first studies with trans-stilbene fatty acid derivatives (SFA’s) indicated that these amphiphiles exhibited film-forming properties very similar to those for saturated fatty acids of comparable overall dimensions.22 Not surprisingly, we observed that the assemblies from pure SFA’s showed strongly blue-shifted absorption and redshifted fluorescence characteristic of an “H” aggregate, which would be anticipated if the molecules had their transition dipoles (along the long axis for trans-stilbene) in a “card pack” array.23,24 What was at least initially surprising was the finding that the aggregates for the SFA’s were very resistant to dilution upon addition of saturated fatty acids and the indication that no “mixed” aggregates could be detected when several isomeric
10.1021/jp9824656 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/13/1998
Feature Article
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10099
CHART 1: Structures of trans-Stilbene Fatty Acid and Phospholipds and Related Compounds
SFA’s were spread from a mixed solution.25-28 In an extended study of the aggregation occurring for the trans-stilbene chromophore in films at the air-water interface, LB multilayers, and aqueous dispersions of the corresponding trans-stilbene phospholipids (SPL’s), we have been able to determine that the key species in controlling aggregation phenomena is a relatively stable “supramolecular” unit aggregate.29-31 The simplest unit aggregate for the trans-stilbene chromophore is a tetramer characterized by strong edge-face interactions among the π systems of adjacent molecules.30,31 In subsequent investigations we have found that similar aggregation behavior occurs for a variety of aromatic-functionalized amphiphiles ranging from very simple benzene-derivatized fatty acids to much more complicated dye structures. The results of our investigations as well as others now suggest that strong stabilizing noncovalent interactions can provide an important organizing force that can control both the structures and properties of a number of different types of assemblies. In this feature article we discuss the formation of these aggregates, the structures generated by their self-assembly at meso- and macroscale, and the changes in molecular properties and reactivity that occur upon aggregate formation and attempts to correlate molecular structure of the amphiphile with the type of aggregate that is formed. Although we and others have observed formation of similar aggregates for a wide variety of chromophores and functionalized amphiphiles, we will restrict
our discussion in this paper to a series of relatively simple amphiphiles having an aromatic chromophore replacing several carbons in the linear hydrocarbon chain of a fatty acid or phospholipid. To simplify the paper, we will discuss aggregation by focusing on the different chromophores. trans-Stilbene, Diphenylbutadienes, Tolans, and Diphenylhexatrienes Some of the trans-stilbene amphiphiles studied are shown in Chart 1. As indicated in the Introduction, formation of a pure film of a SFA or aqueous dispersion of a SPL results in changes in absorption and fluorescence associated with a characteristic “H” aggregate (Figure 1). The spectral shifts are ascribed to an excitonic splitting associated with a “card pack” array of transition dipole moments as developed in the approaches of Kasha and Hochstrasser32 and Czikkely, Foersterling and Kuhn.33,34 Although the aggregates formed in films at the airwater interface are rather resistant to dilution, aqueous dispersions of pure SPLs may be diluted by treatment with a large excess of dispersions of aqueous saturated phospholipids such as dimyrystoyl phosphatidyl choline (DMPC) or dipalmitoylphosphatidyl choline (DPPC) at temperatures above the phase-transition temperatures of both the saturated phospholipid and the SPL.29-31 The dilution is characterized by several spectral isosbestic points, suggesting a direct conversion of the
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Figure 1. Absorption (a-c) and fluorescence (a′-c′) spectra of 4 in CH2Cl2, DMPC, and water and induced circular dichroism (ICD) (inset) of 4 in water (θ, molecular ellipticity of 1 × 104 deg cm2/dmol): (-, a′) fluorescence in CH2Cl2; (‚‚‚, b′) fluorescence of 4/DMPC (1:10) in water; (- - -, c′) fluorescence of 4 in water; (‚‚‚, a) absorption of 4 in CH2Cl2; (‚‚‚, b) absorption of 4/DMPC (1:10) in water; (- - -, c) absorption of 4 in water.
TABLE 1: Aggregation Parameters for trans-Stilbene and trans-Styrylthiophene Phospholipids in Aqueous DMPC compound
aggregation no.a
chromophore/aggregation
K (temp)b
k (min-1)c (temp)
∆H (kcal mol-1)
∆S (cal K-1 mol-1)
4 24 25
3 2 5.96
6 4 12
3 × 10-3 (30 °C) 1.8 × 10-2 (35 °C) 1.2 × 10-7 (55 °C)
2 × 10-2 (30 °C) 2.1 × 10-2 (35 °C) 1 × 10-3 (50 °C)
30 19.5
90 55
k
Aggn (DMPC) h n dimer (DMPC)
(2)
K ) [dimer]nDMPC/[agg]DMPC
(3)
K
a Determined from Benesi-Hidebrand treatment (refs 31 and 73). b K for eq 2, given by eq 3 below, in units dependent on n. c Pseudo-first-order rate constants for decomposition of aggregate in eq 2.
aggregate to dimer (or monomer, depending upon whether a SPL with one or two trans-stilbenes was examined) without evolution through several spectrally distinct aggregated species. The kinetics of the “dilution” are first order in SPL and zero order in saturated phospholipid, suggesting that extrusion of SPL molecule from the SPL assembly is rate-determining.31 Through studies of the “equilibrium” associated with the dilution it is possible to measure aggregation numbers and even thermodynamic parameters. Values for the various aggregation parameters for 4 and some related amphiphiles are shown in Table 1; aggregation numbers are typically small integral values, and the ensembles are characterized by rather strong binding energies. About the same time we were examining aggregation of the SFAs and SPLs in assemblies, we examined host-guest complexes of the same amphiphiles with R, β, and γ cyclodextrins.31 As expected, R and β cyclodextrin can accommodate only a single trans-stilbene chromophore, and consequently, only monomer absorption and fluorescence are observed. Models indicate that γ cyclodextrin should be capable of accommodating
two trans-stilbene chromophores in a face-to-face “dimer”, and in fact, both the SFAs and SPLs form complexes that evidently have a pair of stilbenes. The absorption and fluorescence of the “dimer” entrapped in γ-cyclodextrin are broadened somewhat and slightly blue-shifted compared to monomer but are quite different from those associated with the aggregate.31 Similarly, when an SPL containing two trans-stilbenes is dispersed in excess saturated phospholipid as described above, absorption and fluorescence very similar to those obtained with γ cyclodextrin are observed. For all three cyclodextrin complexes of the stilbenes, circular dichroism spectra in the regions where only the stilbene absorbs show a relatively strong induced circular dichroism signal for the achiral stilbene chromophore. This is a reasonable finding, since the chiral cyclodextrin host is close to the stilbene in the host-guest complex. In contrast, when the chiral SPLs are dispersed in water in the presence of excess saturated phospholipid such that only “dimer” or monomer is present, there is no (or extremely weak) ICD signal obtained corresponding to the stilbene absorption. The lack of an ICD signal can be attributed to the fact that the trans-stilbene
Feature Article
Figure 2. Schematic of different arrays of aromatic-functionalized amphiphiles (obtained from monolayer simulations of aromatic derivatized fatty acids). Upper part shows the translation layer structure that is similar to lowest energy array for styrene 21. Middle part shows the glide or herringbone array that is similar to lowest energy array for several stilbene and azobenzene amphiphiles. Lower part shows the chiral “pinwheel” unit aggregate from the extended glide layer.
chromophore is relatively far from a chiral center compared to the cyclodextrin complexes. However, when pure SPL is dispersed in water or when SPL-saturated phospholipid mixtures are examined under conditions where aggregate predominates, a strong biphasic ICD signal is detected (see Figure 1). The type of ICD spectrum observed is characteristic of an “excitonic” state and is most reasonably attributed to a diastereomeric interaction between the chiral center in the phospholipid and a chiral aggregate. To gain some insights into possible structures for the aggregates of the stilbenes, we performed simulations on the structures of extended two-dimensional arrays of simplified SFA’s such as 1. The Monte Carlo cooling simulations35 suggested that several of the lowest energy structures for these ensembles of 1 are glide or herringbone arrays that are characterized by nearest-neighbor edge-to-face interactions. Figure 2 shows a schematic of these arrays and some possible structures for aggregates formed from the SPLs and related amphiphiles. From the simulated structures for the arrays of 1 it is possible to estimate several “measurables” such as tilt angle and area per molecule in the corresponding LB film. It is also possible to calculate excitonic splittings using the extended dipole-extended dipole treatment of Czikkely, Foersterling, and Kuhn.33,34 Monte Carlo simulations for arrays such as those shown in Figure 2 indicate remarkable agreement between several of the measurable and predicted properties. From the experimental results and simulations discussed above we were able to propose a tetramer “pinwheel” as the basic structure of the aggregate. Although this may be extended into a mosaic or large array in films of pure SFA or in aqueous dispersions of SPLs, our results suggest that the tetramer has a nearly limiting spectral shift for both absorption and fluorescence and is stabilized by the edge-to-face interactions between each nearest neighbor. Although the SPLs do not readily form crystals suitable for X-ray diffraction, we found that the surrogate amphiphiles 7-10
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10101 do form crystals whose structures can easily be determined.36 Interestingly, we found that all four of these compounds crystallize in structures that resemble LB multilayers with the chromophores arranged in layers having head-to-head and tailto-tail contact between layers. The arrangement of 7-10 in a single layer in each crystal is remarkably close to the simulated glide layers (Figure 2) “predicted” for monolayers of 1. The edge-to-face relationships are prominent, and an analysis of the energetics of the crystal packing indicates that these are likely the most important forces controlling the crystallization. Hydrogen bonding between adjacent layers seems to be less important in these crystals, since the ester 10 shows virtually the same structure as its corresponding carboxylic acid, 7. It is interesting to review the finding of predominant edgeto-face interactions for the stilbene amphiphiles with results from other systems in which π-π interactions are important. Although it is clear that molecular geometry (shape and steric effects) is the key determinant in many cases, where both faceto-face (parallel stacking) and edge-to-face (T-stacking) are possible, much evidence suggests the latter may be more favorable.37-44 Theoretical calculations on the interaction of two benzene rings suggest there is an energy minimum when the two rings are in a tilted “T” arrangement;43,45 experimental results verify that the most stable configuration for the benzene “dimer” is a “T” configuration.46 There is growing evidence for several noncharge-transfer aromatics that edge-to-face structures are favored in simple dimers or larger aggregated arrays and that these interactions may be structure-determining for several systems.47-50 Directly measured values for a single edge-to-face interaction, depending on the aromatic system, are on the order of 1.5 kJ mol-1 from measurements using a molecular torsion balance or a double-mutant approach.51 The edge-to-face interaction between a naphthyl and a phenyl ring is reported to be in the range 1-2.5 kcal mol-1.52 The proposed “unit aggregate” or pinwheel maximizes the number of edgeto-face interactions and thus should be especially favored on both electronic and steric (packing) grounds. In our initial experiments with films of SFAs at the airwater interface we supposed that the initially spread film would consist largely of monomeric stilbene, prior to compression, and that as compression occurred, there would be an onset of aggregation as the molecules were forced into the “card pack” arrangement. To the contrary, direct observation of the film by reflectance spectroscopy indicates that for the SFAs the aggregate is the only detectable species even for the most dilute films prior to compression.31 As the SFA is compressed, the aggregate spectrum persists, effectively unchanged up to the collapse point. This suggests that aggregate formation occurs prior to compression and emphasizes that the formation plays an important role in the self-assembly process and not vice versa. Similar results have been observed in our studies of amphiphilic squaraine dyes, which form “H” type aggregates, while for some cyanine dyes and other squaraines, which form “J” aggregates, we observe that aggregate formation occurs only as a tightly compressed film is formed while the aggregate disappears as the film is decompressed.19,21 Although we initially anticipated that the various SPLs would form closed bilayer vesicles similar to those formed from DMPC or DPPC, we found several indications that self-assembly of the functionalized lipids may result in quite different structures. Light scattering studies of several SPLs suggested particle sizes in the range 200-500 nm, more than 10-fold higher than for the saturated phospholipids.31 In agreement with the light scattering studies, we found that aqueous dispersions of the pure
10102 J. Phys. Chem. B, Vol. 102, No. 50, 1998 SPLs could not be filtered through 100 nm pore polycarbonate membranes. A cryotransmission electron microscopy examination of an aqueous dispersion of 4 showed a mixture of structures primarily consisting of large open tubules and smaller openended tubules.31 We assume these structures consist of bilayers of 4 and that the failure to form small unilamellar vesicles may be attributed to resistance of the aggregated stilbenes to adopting the tight curvature required for vesicle formation. Our initial interest in the SFAs was their potential utility as photophysical probes of the environment provided by different microheterogeneous media such as micelles, microemulsions, and vesicles. We anticipated that the competition between fluorescence and photoisomerization might provide a sensitive indication of the order and/or microviscosity of the local microenvironment. In fact, very large differences are observed when very dilute samples of SFAs such as 2 are incorporated into different assemblies under conditions where the stilbene is monomeric. As shown in Figure 1, the fluorescence observed when the stilbene is aggregated shows a large red shift compared to monomer and may be structured or unstructured, depending upon the specific SFA or SPL as well as the composition of the sample. As detailed elsewhere,31,53 we have interpreted the fluorescence observed from the aggregate to arise predominantly from two species: the excited aggregate itself, characterized by a structured fluorescence with a lifetime of several nanoseconds; an “excimer” with an unstructured spectrum having a much longer (15-25 ns) lifetime. In samples such as 5 where the “aggregate” fluorescence predominates, we observe little or no photoreaction. In samples such as 6 where the excimer is the dominant fluorescing species there is a very inefficient photobleaching that occurs on prolonged irradiation. In neither case do we observe changes consistent with photoisomerization. The lack of prominent photoreactivity in arrays with the edgeface aggregated structures shown in Figure 2 is not surprising. The ordered structures clearly should inhibit photoisomerization. In addition, the edge-to-face relationship in the glide or herringbone “lattice” should inhibit photodimerization, the other reaction observed for highly concentrated solutions of stilbene derivatives and certain crystals in which a “topologically controlled” reaction may occur.54-56 The spectral changes occurring in the inefficient photobleaching, which is observed where “excimer” emission predominates, are consistent with photodimerization, although we have not been able to verify that a dimer is formed. In other cases where stilbene derivatives give excimers, photodimerization has been found to frequently accompany the formation of the excimer.57 For the aqueous dispersions of SPLs the tendency to see purely “aggregate” fluorescence is strongest when the stilbene chromophore is at the end of a moderately long chain and well-separated from the phospholipid headgroup, as for 5.31,53 In this situation, the aggregation number is generally large and the aggregate is expected to be very stable with few defects and minimum perturbations arising from mismatches between packing of the headgroup and stilbene chromophore. In contrast, “excimer” fluorescence is predominant in cases where the stilbene chromophore is in the middle of the chain or close to the headgroup. We would expect that “defects” in the aggregate structure may be more frequent in these cases or that the excited aggregate might easily collapse to the lower energy “excimer” excited state. Although they have not been studied as extensively, the “vinylogues” of trans-stilbene, trans,trans-1,4-diphenyl-1,3butadiene (DPB), and trans,trans,trans-1,6-diphenyl-1,3,5hexatriene (DPH) have also been incorporated into fatty acid
Whitten et al. CHART 2: Azobenzene Phospholipids
structures (11 and 12, respectively) to provide amphiphiles that form well-behaved films at the air-water interface and that can be transferred layer by layer to form LB multilayers.25,58 For example, we find that both 11 and 12 form LB films exhibiting spectra and spectral shifts from the monomer in both absorption and fluorescence that are similar to those observed for the SFAs.58 It seems quite reasonable that similar aggregates are formed from these chromophores; preliminary indications are that these aggregates may not be as stable, since it appears much easier to “dilute” both 11 and12 to monomer by spreading mixtures with saturated fatty acids such as arachidic acid.58 In addition to the simple stilbene, DPB and DPH amphiphiles, we have also found that polar stilbene and tolan (diphenylacetylenes) fatty acid derivatives exhibit aggregation in LB films.18,59 Here again, the aggregates are characterized by spectral blue shifts in absorption and a red-shifted relatively long-lived fluorescence. Azobenzene Derivatives Given the structural similarity of trans-azobenzene to transstilbene, it is not surprising that incorporation of the transazobenzene chromophore into the hydrophobic portion of an amphiphile should lead to molecules exhibiting similar aggregation behavior. Most of our studies of azobenzene amphiphiles have focused on the several phospholipids whose structures are shown in Chart 2. Since much of this work has recently been published,60 we will only review some of the key findings that contribute to the overall picture of the role of aromatic-aromatic interactions in self-assembly. Both Monte Carlo simulations
Feature Article and experimental findings indicate that the extended azobenzene amphiphiles should pack with glide or herringbone arrays that are similar to those described above for the trans-stilbene amphiphiles.60,61 The azobenzene phospholipids (APL’s) with oxygen as a linking atom (14-16) are considerably more soluble in water than the stilbene phospholipids discussed above; consequently, it is possible to carry out microcalorimetry with their aqueous dispersions and to readily observe the phase transitions that occur. Each of the three APL’s should have similar chain lengths comparable to that of DMPC, which has a phase-transition temperature, Tc, of 23 °C.62 Compound 14, which has the azobenzene chromophore farthest from the headgroup, shows a very sharp phase transition at 74.5 °C with an enthalpy for the main transition of ∆H ) 11.1 kcal/mol.60 For 15, which has the azobenzene chromophore much nearer the phospholipid headgroup, the phase transition occurs at 40.1 °C and the enthalpy for the transition is lower. For the mixed APL 16, the main transition occurs at a still lower temperature (35.2 °C) and the enthalpy is much lower. The trends apparent in the results obtained from the microcalorimetry studies also emerge in the aggregation numbers and measured sizes for aqueous dispersions of the APL’s. Thus, 14 shows a much higher aggregation number (42 azobenzenes/aggregate) than the other APL’s shown in Chart 2 (all close to 3 azobenzenes/ aggregate), while the sizes estimated by light scattering or microfiltration are much larger for the aqueous dispersions of 14 than for the other three APL’s.60,61 The differences between the different APL’s are most easily attributed to “mismatches” between the different self-assembling elements in the aromaticderivatized phospholipid. Thus, in 15 and 16, where the azobenzene chromophore is close to the phospholipid headgroup, formation of the azobenzene aggregate may be impeded by association of the phospholipid headgroups as a bilayer is formed. However, for 14 the relatively large number of intervening carbon (and oxygen) atoms may provide enough flexibility to allow both the azobenzene units and the phospholipid headgroups to associate without serious mutual perturbations. These differences may also manifest themselves in the different structures formed by self-assembly in the aqueous dispersions. Thus, 14 forms structures detected by cryotransmission electron microscopy (cryo-TEM), which appear to be large (>100 nm in length) plates, while 15 forms long tubules.60 Although the structures formed from pure dispersions of the individual APL’s in water are not small closed bilayer vesicles, mixtures of the APL’s with DMPC or DPPC do form small unilamellar vesicles that are capable of entrapping dyes or reagents such as carboxyfluorescein (CF). What renders the APL’s interesting compared to the SPL’s is their photoreactivity in aqueous dispersions. Thus, although the trans-stilbene phospholipids are unreactive toward photoisomerization in aqueous dispersions, the trans-azobenzene phospholipids are readily and reversibly photoisomerized upon irradiation presumably because of a different photoisomerization mechanism for the latter. Several changes occurring concurrently with the trans-cis photoisomerization suggest that the cis-azobenzene phospholipids may have a much lower tendency to aggregate or self-assemble. The absorption spectra of aqueous dispersions of the cis-APL’s are very broad and featureless but differ little from those obtained for the corresponding organic solutions of the cis-APL’s or the corresponding cis-azobenzene fatty acids, where it seems certain only monomer is present. Additionally, although the aqueous dispersions of the pure trans-APL’s will not pass through 100 nm polycarbonate membranes, the corresponding cis-APL dispersions pass
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10103 completely through the same membranes.61 The strong excitonic ICD spectra described above for aqueous dispersions of the trans-SPL’s are also observed for the trans-APL’s. However, upon irradiation and photoisomerization, the ICD spectra associated with the trans-APL aggregates disappear. Irradiation of the cis-APL dispersions results in formation of aggregated trans-APL in each case and for 13 and 14 regeneration of an ICD spectrum identical to that for the original dispersion in water prior to the photoisomerization. Finally, when aqueous dispersions of 14 are examined by cryo-TEM before and immediately following photoisomerization to near-complete conversion to cis, it is observed that the complex and large structures associated with the trans disappear completely and that no new structures can be detected.60 When samples that have been partially reconverted to trans are examined, a mixture of structures can be observed by cryo-TEM ranging from the “plates” associated with pure trans-14 to structures that appear to be spherical and to resemble the vesicles observed for DPPC or mixtures of 14 with DPPC. Given the shape of the cis-azobenzene chromophore, it is not surprising that amphiphiles with a cis-azobenzene at the end or in the middle of the hydrocarbon chain may show a drastically reduced tendency to self-assemble. As indicated above, pure aqueous dispersions of trans-APL’s such as 13-16 form relatively large structures and not simple closed bilayer vesicles. However, mixtures containing up to 10 mol % of the trans-APL’s in excess DPPC do form closed bilayer vesicles that are relatively monodisperse and of size very similar to those of the pure saturated phospholipid.60,61 These “mixed vesicles” are capable of entrapping water-soluble reagents such as carboxyfluorescein; the vesicles containing entrapped CF, where [CF] > 100 mmol, exhibit little CF fluorescence because of self-quenching.63-65 When CF fluorescence is monitored, it is found that these vesicles are stable in the dark for several days at temperatures below Tc.60 When the mole ratio of APL (or the corresponding azobenzene fatty acid) to DPPC is varied, it is possible to construct vesicles that contain the azobenzene as predominantly monomer, dimer, or aggregate. In each case irradiation of the trans results in photoisomerization to cis. However, the consequence of photoisomerization as far as the properties of the vesicle are concerned is strongly dependent upon the aggregation state of the azobenzene. When the azobenzene is present as monomer or dimer, irradiation to produce nearly complete conversion from trans to cis results in very little release of entrapped CF, as monitored by the level of CF fluorescence. However, when aggregated trans-APL is photoisomerized, a rapid increase in CF fluorescence is observed. Irradiation of aggregated APL to produce near-complete conversion to cis results in essentially a total release of the entrapped CF. Examination of these solutions before and after irradiation by cryo-TEM reveals that the vesicle structures are almost completely destroyed following the photoisomerization. Evidently, when the azobenzene is monomeric or dimeric, the photoisomerization may be followed by extrusion of the cis-azobenzene amphiphile without major disruption of the vesicle, and consequently, only a slight release of entrapped CF occurs. In contrast, when the aggregated transazobenzene is photoisomerized, there must be a much more serious disruption of the vesicle structure, perhaps extrusion of an entire region containing the extended aggregate followed by collapse of the vesicle structure. Similar results are observed for all four APL’s shown in Chart 2; thus, release of entrapped CF can be triggered by the photoisomerization regardless of whether the azobenzene is in the middle of the chain as in 13,
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CHART 3: Phenyl, Biphenyl, Terphenyl, and Styrene Surfactants
15, or 16 or at the hydrophobic end of the chain as in 14. Interestingly, consistent with its higher tendency to aggregate and the larger aggregate size, a much lower mole percentage (less than 2%) of 14 is required in mixed vesicles with DPPC to render them photolabile. Amphiphiles Containing Simple Aromatics: Phenyl, Biphenyl, Terphenyl We have recently synthesized and examined a series of amphiphiles containing simple aromaticssphenyl, biphenyl, and terphenylsinserted into the hydrocarbon chain.66 Chart 3 shows several of the compounds in this series. In several respects the behavior of these amphiphiles in aqueous dispersions and LB films is similar to the trans-stilbene and azobenzene amphiphiles discussed above. For example, the biphenyl phospholipid 18a shows absorption and fluorescence activation spectra in water or LB films that are slightly blue-shifted compared to those of organic solutions in acetonitrile (Figure 3). As with the stilbene assemblies, the fluorescence is considerably red-shifted and structured, suggesting that emission occurs to a bound ground state. Although similar spectral shifts were observed for biphenyls tethered to chains of six carbons plus one oxygen and eight carbons plus one oxygen, there were no spectral shifts in either absorption or emission when shorter-chain compounds such as 19 or the corresponding acid were dispersed in water.66 Evidently, the shorter-chain compounds do not self-assemble into any structure resulting in chromophore aggregation. Smaller shifts in fluorescence (there is almost no shift in absorption) are observed for the phenyl phospholipid 17, while the terphenyl amphiphiles show somewhat larger shifts in both absorption and
emission. For the biphenyl and terphenyl phospholipids, dilution with an excess of saturated phospholipid such as DPPC results in spectral shifts compared to a species showing absorption and fluorescence only slightly different from those attributed to monomer (organic solutions or short-chain biphenyl amphiphiles in water). Unfortunately, there is considerable overlap between the spectra of pure aromatic phospholipid and the DPPC diluted mixture so that it is not possible to determine the aggregation number by the Benesi-Hildebrand approach. Nonetheless, several other properties of the aqueous dispersions suggest that aggregates formed from these amphiphiles are similar in structure to those formed with the stilbene amphiphiles and related compounds. For example, as shown in Figure 3, 18a shows a strong biphasic ICD spectrum. No ICD is observed for the samples of 18a diluted with DPPC or for acetonitrile solutions. The lack of an ICD for the DPPC diluted aqueous solutions of 18a is consistent with the findings for the SPL’s and suggests that diastereomeric interactions between chiral centers in the phospholipid and a chiral aggregate are responsible. Interestingly, Monte Carlo simulations on a twodimensional gas-phase cluster of the corresponding fatty acid (18b) yield a glide or herringbone structure (Figure 4) very similar to that observed for the stilbene crystals or simulated for the stilbene amphiphiles discussed earlier.67 Here again, edge-to-face interactions among the nearest-neighbor aromatics appear to be the most prominent noncovalent interactions. Although the experimental evidence described above strongly suggests that similar aggregation occurs for the biphenyl and terphenyl phospholipids (and perhaps even for the phenyl phospholipid 17) to that observed with the stilbene and
Feature Article
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10105
Figure 3. Emission and excitation spectra for 18a in water and acetonitrile and ICD spectrum (inset) of 18a in water. The 255 and 435 nm spectra are excitation and emission, respectively, in water. The 273 and 322 nm are excitation and emission, respectively, in acetonitrile.
azobenzene phospholipids, the mesoscopic structures formed from the biphenyl and terphenyl phospholipids are quite different. An attempt to estimate the size of the assemblies formed from the pure biphenyl and terphenyl phospholipids was made using dynamic light scattering. The light scattering from aqueous dispersions of the pure phospholipids 18a, 19, and 20 indicated diameters (assuming spherical shapes) estimated in the range 116-140 nm. A cryo-TEM study of 18a and 19 showed that in both cases the aqueous dispersions of the pure phospholipids are spherical and similar in diameter to those estimated by the light scattering. The strongest indication that 18a forms closed bilayer vesicles comes from experiments in which it was found that CF can be entrapped within vesicles formed from pure 18a or a 1:10 mixture of 18a/DPPC. The vesicles formed from 18a gradually released entrapped CF (50% “escape after 80 h), while the mixed vesicles showed comparable stability to pure DPPC vesicles.
exclusively the trans-syn-trans photodimer 23 (eq 1).68
Styrene Fatty Acids and Phospholipids The styrene fatty acid (21) and the corresponding phospholipid (22) appear on first inspection to have structures very similar to those of stilbene, azobenzene, and other aromaticderivatized amphiphiles discussed above. In fact, when 21 is incorporated into LB films or 22 into aqueous dispersions, the absorption spectrum of the aromatic undergoes the blue shift characteristically associated with the “H” aggregation as described above.68 However, for both 21 and 22 the blue shift is much smaller than that seen with the stilbene, azobenzene, or diphenylacetylene aggregates and the two styrene derivatives, strongly fluorescent in organic solutions, are nonfluorescent in the assemblies. Upon irradiation quartz-supported LB multilayers of 21 undergo a very clean photoreaction to produce
Similarly, irradiation of aqueous dispersions of 22 followed by a hydrolytic workup leads to the same photodimer. In each case the production of a single photodimer upon irradiation recalls the topological control observed in solid-state reactions where irradiation of crystals of molecules such as cinnamic acid derivatives produces a single dimer (or no dimer) depending upon the arrangement of molecules in the crystal.54-56,69,70 Typically, photodimerization in the crystal is observed only when the molecules are properly aligned and within a “magic distance” of 3.5-4.2 Å.54,55 The observation of relatively efficient photodimerization for styrenes 21 and 22 in their
10106 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Whitten et al.
Figure 4. Overhead view of simulated layer array (global minimum) of 18b in monolayer. The alternate rows are printed in green and red to emphasize the glide array.
respective assemblies would not be anticipated from a unit aggregate having a “pinwheel” structure similar to that proposed for the stilbenes or azobenzenes. In an extended glide or herringbone structure, the nearest neighbors should not be able to photodimerize and a “collapse” of the structure to permit formation of a dimer should not lead to a dimer of the indicated stereochemistry. Thus, the observed results suggest that the selfassembly of 21 and 22 must produce an aggregate of different structure than those described above. Monte Carlo simulations on assemblies of 21 suggest that the most stable structure in the monolayer should be a simple translation aggregate.68 Energy minimization on a layer of 21 indicates that the global minimum is a translation layer with an area per molecule of 20.7 Å2, which is in good agreement with the measured value of 21 Å2. Similarly, calculation of the exciton shift for the aggregate corresponding to this structure by the extended dipole-extended dipole approximation of Czikkely, Foersterling, and Kuhn33,34 gives a predicted λmax for the aggregate of 241 nm in reasonable agreement with the
measured value of 244 nm. For this structure the separation between nearest neighbors is 4.10 Å, which is within the “magic distance” required for photodimerization in the crystal. The contrast between the behavior of the styrenes and the other aromatic-functionalized amphiphiles described above is at first surprising; however, in several cases the simulations of “layer” structures reveal that both glide and translation layer structures may be of similar energies. Since these are gas-phase simulations that do not provide any estimation of the role of the solvent, it may be anticipated that subtle changes in molecular structure may influence the actual structure occurring upon self-assembly and that in some cases coexistence or even fluctuations between different aggregate structures may occur. Styrylthiophenes A particularly interesting case that we have recently studied involves the styrylthiophene surfactants 24-27 (structures shown in Chart 4).71 The trans-styrylthiophene chromophore
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CHART 4: Styrylthiophene and Styrylnaphthalene Fatty Acids and Phospholipids
is structurally quite similar to trans-stilbene and might be expected to exhibit similar chemical and photophysical behavior. In fact, simple trans-styrylthiophene has absorption and fluorescence behavior very similar to that of trans-stilbene in solution. It has been found to undergo photoisomerization in solution but not photodimerization.72 Crystals of trans-styrylthiophene are photostable, as expected from the crystal structure that shows unfavorable packing for topologically controlled dimerization. Some substituted trans-styrylthiophene derivatives have been found to undergo photodimerization both in the solid state and in solution.69 Both phospholipids 24 and 25 form clear aqueous dispersions with blue shifts in absorption and red shifts in fluorescence typical of those observed for the stilbene amphiphiles and related compounds in the same medium. Similarly, the longer chain fatty acid 27 forms LB mono- and multilayers exhibiting almost identical spectral shifts. The phospholipids 24 and 25 undergo similar disaggregation compared to the corresponding SPL’s by addition of excess DMPC, and a study of the disaggregation equilibrium indicates aggregation numbers (by the Benesi-Hildebrand analysis73) of 2.02 and 5.96 for 24 and 25, respectively (4 and 12 styrylthiophene chromophores per aggregate). The aggregates of 24 are very similar in stability compared to those of the corresponding SPL having the same number of methylenes in the side chains. Both the enthalpy (ca. 5 kcal mol-1 chromophore-1) and entropy (ca. 15 cal mol-1 K-1 chromophore-1) as well as the rate constants for dissaggregation (ca. 2 × 10-2 min-1 at 30 °C) are very similar for 24 and 4 (Table 1) even though the aggregation numbers are slightly different. Other evidence suggesting similar aggregation behavior for the phospholipids 24 and 25 comes from very similar ICD
spectra for the monomer, dimer, and aggregates in aqueous dispersion compared to the corresponding stilbene and azobenzene derivatives. Thus, we find strong biphasic (excitonic) ICD in the regions of the aggregate absorption with a crossover point coinciding with the absorption maximum. For the monomer or dimer dispersions we see little, if any, evidence of induced circular dichroism spectra. The aqueous dispersions of 24 and 25 also show evidence of structures larger than those of simple spherical bilayer vesicles. Thus, although DPPC shows an approximate diameter of 9.3 nm, the dispersions of 24 and 25 have particle sizes estimated by light scattering or membrane filtration of 150 and 100 nm, respectively. The simulated packing of 26 and 27 into monolayers using the Monte Carlo cooling techniques applied earlier for the stilbenes, azobenzenes, and styrenes suggests that there may be some differences between the styrylthiophenes and stilbenes.71 Thus, for 27 we find that the lowest energy simulated structures are all translation layers, similar to the simulations for the styrene amphiphiles described above. This is in sharp contrast to the simulations for comparable chain-length stilbene fatty acids, which generally show glide structures as the lowest energy two-dimensional layer. Several of these low-energy translation structures yield predicted areas per molecule and exciton shifts close to those actually measured for both monolayers of 26 and 27 as well as for the aqueous dispersions of the corresponding phospholipids. The shorter-chain styrylthiophene fatty acid 26 shows a mixture of glide and translation layer structures among the minimum energy configurations; for 26 the predicted values for surface area per molecule and exciton shift and splitting appear reasonable for both glide and translation structures.
10108 J. Phys. Chem. B, Vol. 102, No. 50, 1998 The main area where there are sharp differences between the behavior of the aggregated styrylthiophene amphiphiles and the corresponding stilbene amphiphile assemblies is in photoreactivity.71 Thus, as mentioned above, although the stilbene assemblies show very little photochemical reactivity, the styrylthiophenes in either aqueous dispersions or LB films exhibit photodimerization. Both of the styrylthiophene phospholipids 24 and 25 undergo photobleaching upon irradiation; concurrent with the photobleaching is a disappearance of the ICD spectrum attributed to the “chiral” aggregate. The product from irradiation of 24 was recovered and analyzed by NMR and mass spectrometry and found to be a single photodimer. The NMR was most consistent with a trans-syn-trans dimer structure analogous to that produced from the styrene amphiphiles 21 and 22. The formation of only a single photodimer suggests, once again, a “topological” control in which the product formed is determined by the arrangement of monomer molecules in the assemblies. This result would be consistent with a translation layer structure but incompatible with a glide layer arrangement. If we examine the translation layer structures predicted as global or local energy minima for 26 or 27, we find that only one of these structures (the structure of energy rank 9 for 26) has the combination of double bonds within the “magic distance” limit of 4.2 Å and reasonable agreement between measured and predicted area and spectral parameters. This suggests that perhaps the restrictions for topological control in “soft assemblies” such as aqueous bilayers may not be quite as rigid as for crystals or LB films. As indicated above, styryl thiophene phospholipids 24 and 25 apparently form larger structures that may not be closed bilayer vesicles similar to their stilbene and azobenzene counterparts. However, analogously to the APL’s, we find that mixtures of DPPC and 25 form vesicles that can entrap reagents such as CF for periods of more than 1 week in the dark. From the results, which seem at first glance truly remarkable, we find that incorporation of only a small percentage of 25 (
