Supramolecular Assemblies of Chromophores in LB Films and Related Media
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MRS BULLETIN/JUNE 1995
predictions developed by Kasha and Hochstrasser14"26 or by Czikkely, FSrsterling, and Kuhn. 17 The focus of the studies described here has been on aggregation occurring in LangmuirBlodgett (LB) films and related media, such as bilayer vesicles, which are characterized by an assembly of molecules in an interfacial situation where a polarnonpolar or water-hydrocarbon boundary should provide a strong organizing influence. In early cases where aggregates were encountered as prominent components of mixed LB films (even when relatively dilute mixtures were used), the phenomenon was usually dismissed as "microcrystallization," which was considered an unavoidable nuisance and not really due to fundamental intermolecular interactions. More recent studies have shown that aggregation in these media is really a significant molecular phenomenon that shows dependence both on the specific molecules and the topology of the film-forming surfactant. Although some previous investigations have been carried out with different results for various substrates, we have embarked on a study to correlate aggregation behavior for a number of
different chromophores incorporated into amphiphilic structures to obtain a general picture of the relative importance of different factors that can control aggregation phenomena.
Porphyrins The naturally occurring porphyrins and chlorophylls often exist as aggregated species as determined via spectral shifts and crystal structures. While both red-shifted and blue-shifted aggregates have been detected, little in the way of systematic study has been undertaken with the porphyrins or related compounds.22"2'' In several cases porphyrins and related compounds have been found to exhibit blue shifts in the prominent "Soret" band on aggregation, which is consistent with a face-to-face or "cardpack" association. Similar shifts are observed in cases where either neutral porphyrins or porphyrin 7r-cations form fairly tight dimers in what is believed to be a relatively strong TT-TT association.27"28 Porphyrin dimers and trimers, in which two or three p o r p h y r i n cores are strapped together by molecular bridges, also show blue-shifted Soret bands. Exciton theory has been applied to correlate the Soret shift with the interplanar separation and strength of the exciton interaction.29"32 However, the exciton interactions occurring for porphyrins are potentially complicated because the Soret band is composed of two transitions associated with the two transition dipoles that are in the porphyrin plane but perpendicular to one another.2223 While a simple card-pack array of porphyrins should give rise to a blue shift (H-aggregate interaction) for both transitions, other arrangements can lead to more complicated spectral shifts. This has been explicitly observed in two recent cases. For example, LB monolayers of 5,10,15,20-tetrakis[4-(l-octyloxy)phenyl]porphyrin are found to exhibit two Soret peaks compared to the monomer; one is shifted to higher energies and one to lower energies. This has bee
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