Cristina Geiger

Supramolecular aggregates of photoreactive aromatics. Structure, photophysics and photochemistry of stilbene and azobenzene phospholipids

Abstract Stilbene and azobenzene-derivatized phospholipids have been synthesized as building blocks to study H-aggregate formation, structure and properties in both bilayer vesicles and LB films. The observed H-aggregates in bilayers display similar spectral characteristics to that in LB films and a strong induced circular dichroism spectrum. Studies of interconversion between H-aggregates and monomers or dimers have established relatively small, integral numbers for aggregate sizes. Based on these results as well as molecular cooling simulation, chiral cyclic pinwheel structures are proposed for these H-aggregates. Single crystal structure of a trans -stilbene derivative ( 4 ) has been obtained, and its remarkable similarity in structure to compressed monolayer obtained from molecular simulation and almost identical spectra to H-aggregate of stilbene support the proposed herringbone lattice for these aggregates. The very large arrays of aggregates may consist of a “mosaic” of small aggregates. The presence of aggregates as either stable or metastable entities has special consequences for microscopic as well as macroscopic properties of the medium in which they are generated. The aggregates of azobenzene in DPPC vesicles promote reagent release much more efficiently than the correspondent monomer and dimer. Trans—cis photoisomerization of azobenzene aggregates opens a convenient way to establish photoregulatable membrane and related materials.

SUPRAMOLECULAR STRUCTURES AND REACTIVITY IN BILAYERS AND LANGMUIR-BLODGETT ASSEMBLIES FORMED VIA SELF-ASSEMBLY OF POLYFUNCTIONAL AMPHIPHILES

Amphiphilic squaraine and trans-stilbene derivatives form highly organized aggregate assemblies when spread as mixed films at the air-water interface and transferred to rigid supports such as glass, quartz or optically transparent electrodes. The squaraines studied form both blue-shifted (“H”) aggregates and red-shifted (“J”) aggregates. Although both are evidently non-emissive, we have found moderate efficiencies for photocurrent generation when the “H” aggregates are incorporated into a photoelectrochemical experiment. The “H” aggregates function as photocatalysts in these experiments and a mechanism is proposed. The stilbene amphiphiles also form blue shifted “H” aggregates which are relatively strongly fluorescent. Our studies indicate that the “H” aggregate is a preferred structure favored by “apolar association” forces; interestingly we find that mixtures of different stilbene amphiphiles tend to self organize into zones of individual “H” aggregate.

Photoinduced electron transfer reactions oftrans-stilbene surfactants in Langmuir-Blodgett assemblies and phospholipid bilayers

The photochemistry and photophysics of a series of stilbene-functionalized fatty acids (SFA) have been examined in homogeneous solutions, supported Langmuir-Blodgett assemblies, and as guests in phospholipid bilayers and micellar dispersions. The spectroscopic characteristics of stilbene-functionalized derivatives of phosphatidyl choline (SFA-PC) have also been examined in organic solvents, aqueous dispersions and aqueous solution containing an excess of dipalmitoyl choline (DPPC), as well as in aqueous-methanol solutions containing γ-cyclodextrin. Langmuir-Blodgett assemblies of individual SFAs, as well as mixtures of various SFAs, exhibit spectroscopic properties (blue-shifted absorbance and red-shifted fluorescence relative to those observed in organic solvents) consistent with the formation of an “H” aggregate. The same effect is observed for SFA-PCs in aqueous dispersions and in aqueous-methanol solutions containing γ-cyclodextrin. The “H” aggregate is found to be the preferred geometric orientation of the stilbene chromophores which may correspond to an energy minimum for the systems investigated. Preliminary studies with SFA-PC in methanol-water solutions containing γ-cyclodextrin suggest that an association of only two stilbene chromophores is required to form the “H” aggregate.