Card pack aggregate

Well organized Langmuir-Blodgett (LB) films have been obtained from mixtures of a push-pull carotenoid and co-tricosenoic acid as shown in 7. These mixed films exhibit a very good cohesion, with an area of about 25 A2 per carotenoid molecule. They can easily be transferred onto solid substrates. Examination by UV-visible linear dichroism measurements confirms that the carotenoid chains are oriented perpendicularly to the surface of the substrate in card-packed aggregates, in which the polyenic chains interact via excitonic coupling, as indicated by the large hypsochromic shift of the tc-tc transition (20). 

The adsorption isotherms in Figure 11.3 are of interest for several reasons. First, it may seem surprising that an assemblage of kaolinite platelets should give a reversible isotherm. The adsorbent had a specific surface area of 17 m2g-1, which would appear to correspond to a platelet thickness of c. 50 nm. The particle rigidity and the house-of-cards packing have probably resulted in the formation of a macro-porous aggregate, which accounts for the appearance of the reversible Type II isotherm. 

Some reports on fluorescence occurring in, for instance, porous materials such as Nafion or aluminophosphates, " do not refer to azobenzene but to protonated azobenzene, which is classified as a pseudostilbene see Section 1.5). Emission from nonprotonated, isolated azobenzene-type molecules is still very rare. Aggregated systems, however, seem more prone to sho%v fluorescence emission. Shinomura and Kunitake have detected fluorescence bands with a maximum of near 600 nm in bilayer systems built from the monomers of 15. They have shown that the ability to emit is tied to the type of aggregation Head-to-tail aggregates emit relatively strongly, with quantum yields of up to < ) = 10" and lifetimes below 2 ns. Their prototype of card-packed dimers does not emit at all. This is expected because of the low transition probability at the lower band edge, which favors radiationless deactivation, probably via the Si state (see Figure 1.7). 

The spectroscopic behavior observed for confined Si(OEt)3-ImPV-F (blue-shifted absorption, decreased intensity, and red-shifted emission) could be attributed to an H-type aggregation such an aggregation may reasonably be expected from a card pack orientation of the distyrylbenzene imide chromophores in the silicate network. The formation of the silyl network supports a face-to-face organization of chromophores. 

There is then a sudden drop in the free energy as virtually all of the amphiphile molecules are incorporated into micelles, enabling their hydrophobic alkyl chains to be more or less completely shielded from the aqueous part of the phase. This leads to the familiar abrupt change in physical properties at the cmc. In chromonic systems there is also the aggregation of molecules in dilute solution before mesophase formation, but the pattern of association is different. The hydrophobic surfaces of the molecules cause them to aggregate in stacks like packs of cards. As these stacks grow, the fraction of the total hydrophobic surface area exposed to the aqueous part of the phase steadily falls, but there is no minimum free energy state, no cmc, and there is no structure directly comparable to the micelle [38]. 

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