J-type aggregates

Purrello R, Monsu Scolaro L, Bellacchio E et al (1998) Chiral H- and J-type aggregates of m so-tetrakis(4-sulfonatophenyl)porphine on a-helical polyglutamic acid induced by cationic porphyrins. Inorg Chem 37 3647-3648... 

Lauceri R, Gurrieri S, Bellacchio E et al (2000) J-type aggregates of the anionic mero-tetrakis (4-sulfonatophenyl)porphine induced by hindered cationic porphyrins. Supramol Chem 12 193-202... 

Fig. 18 H-type and J-type aggregation possibility of the azobenzenic groups...

The aggregation behavior of the squaraine dyes 26 and 27 (Structure 8) in water in the presence and absence of polyvinylpyrrolidone has been studied [66]. In aqueous solutions, these dyes show two absorption bands with maxima around 595 and 645 nm, which have been attributed to those of the aggregated and monomeric forms, respectively. The aggregated forms were identified as dimers from concentration dependent changes in the absorption spectra. The blue shift in absorption of the aggregated forms indicate the formation of H-type aggregates [82,83], imlike in the case of the bis(hydroxyphenyl)squaraine derivatives, where J-type aggregates were obtained [76]. 

The structures of representative examples of cyanines with the potential to form helicogenic and/or helicophilic J-type aggregates are shown in Fig. 2.7. The syntheses of some of these cyanines were described previously [45]. These... 

Fig. 2.7. Chemical structures of cyanine dyes that may form J-type aggregates...

Snpra-molecnlar bio-heUx with cyanine J-type aggregates (red ovals) formed. Light tnrned on ... 

The J-type aggregates (side-by-side) were formed for transitions polarized parallel to the long axis of the aggregate, while H-type (face-to-face) for transitions pwlarized perpendicular to it (Fig. 14). 

Some H- or J-type aggregates of porphyrins play a role as light harvesting assemblies to gather and transfer energy to the assembled devices, and to obtain a higher incident photon-to-photocurrent generation efficiency ( Kamat et al.,2000 Sudeep et al., 2002). 

It can be seen from UV-visible characterization of the species that the tetraaryl porphyrinic chromophores 125(M) tend to form J-type aggregates (red-shifted absorptions). In these assemblies, the use of equimolar mixtures of 125 (Zn) and 125(H2) led to a very efficient (81%) ET process, via a postulated and reasonable Forster mechanism, with a rate constant of A et = 3.1 X 10 s-l. When iron(III) porphyrins 125(Fe) are used, free base protoporphyrin 124 bearing alkyphosphocholine side arms can be incorporated in the fluid part of the vesicles. In the presence of a sacrificial electron donor on the outside of the vesicle, a vectorial eT process occurs from the excited free base porphyrins to the iron(III) species, leading to photoreduction of the iron(III) to iron(II). Previous work had already shown that this could also be achieved between zinc porphyrins and iron porphyrins in vesicles ... 

Supramolecular systems with H-aggregates do not possess fluorescence [2], however they can be recommended for PDT of first type. Supramolecular systems of monomeric metal phthalocyanines possess fluorescence and are effective in the diagnosis as fluorescent labels. J-type aggregates are also fluoresce, but their absorption band is bathochromic shifted. This property increases the possibility of using of aggregated supramolecular systems in the diagnosis of several diseases. 

The structure of porphyrins in SAMs has been widely studied because of the strong and characteristic UV/vis absorption features of porphyrin chromophores (see Sect. 5.1) [78,142,144]. Porphyrins may form two different types of aggregation a face-to-face porphyrin 7r-aggregation (sandwich-type H-aggregate) and a side-by-side porphyrin tt-aggregation (J-aggregate) (Fig. 16). The J-type aggregation is often foimd with cyanine dyes [145,146]. 

Just briefly mentioned should be quinone dyes 28 [201], cyanine SAMs 29 where the cyanine dye forms highly ordered 2D J-aggregates on a cysteamine SAM with a 7 X V3 superlattice [ 145,146] and a perylene bisimide dye 30, which shows J-type aggregation as demonstrated by the bathochromic shift of the fluorescence spectra [202] (Fig. 33). 

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