Card pack

Figure 6.5. Exciton model for the coupling of dimers. When the transition dipoles are aligned (ft, W) in a card pack fashion only the transition to n is allowed thus a blue shift in the spectrum is expected.

FIGURE 8.4 Orientation factor for the card-pack and head-to-tail dimers. 

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. 

MoHiomor Card-packed Dimer Monoitref Head-to-tail Dimer FIGURE 1.7 Energy state scheme with exciton splitting (two states for dimers, bands for polymers). 

Some azobenzenes that are locked against rotation by bulky substituents in all four ortho positions may show fluorescence when frozen rigidly at 77 K 2,2 ,4,4 ,6,6 -hexaisopropyl 2,2 -difflethyl-4,4 6,6 -tetra-tert-butyl azobenzene belong to this series. Azobenzenophanes 7 to 13 do not emit, even at 77 K this is the expectation for card-packed dimers. 

Tsuda et al. found the same fluorescence at = 600 nm in a giant vesicle in a card-packed azobenzene arrangement. The noisy appearance of their fluorescence trace (if not due to the technique of confocal laser miGroscopy), however, suggests a very low emission intensity. Both Shinomura and Kunitake and Tsuda et al. report time-dependent orientation phenomena on Z-E isomerization in the supramolecular arrangement, which is reflected in the fluorescence intensity. So the former general statement that azobenzene-type azo molecules do not emit needs to be modified. 

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). 

CFC are produced via epitaxial deposition of carbon on the surface of metal catalyst. The filaments are shaped by graphite basal planes a filament can be imagined as either a pile of truncated cones put one onto another, each inside the next one, or, if flat, as a card pack [6,7]. Generally, a pile of cones with angle a made by the generatrix and the cones axes, varying from 0 to 90 , may be taken as a model of the CFC structure (Fig.2). The main structural parameters of the CFC s... 

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