Photoactive chromophores

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Meisel etal. [18-20] were the first to investigate how the addition of a polyelectrolyte affects photoinduced ET reactions. They found that charge separation was enhanced as a result of the retardation of the back ET when poly(vinyl sulfate) was added to an aqueous reaction system consisting of tris(2,2 -bipyridine)ruthenium(II) chloride (cationic photoactive chromophore) and neutral electron acceptors [21]. More recently, Sassoon and Rabani [22] observed that the addition of polybrene (a polycation) had a significant effect on separating the photoinduced ET products in an aqueous solution containing cir-dicyano-bis(2,2 -bipyridine)ruthenium(II) (photoactive donor) and potassium hexacyano-ferrate(III) (acceptor). These findings are ascribable to the electrostatic potential of the added polyelectrolytes. 

In contrast, if photoactive chromophores are covalently tethered to the polyelectrolyte main chain, the chromophore reactivity in the photoinduced ET is much more greatly changed because the reaction sites are totally constrained to the polyelectrolyte molecular surface. 

The microphase structure of amphiphilic polyelectrolytes in aqueous solution provides photoinduced ET with an interesting microenvironment, where a photoactive chromophore and a donor or acceptor can be held apart at different locations. Photoinduced ET in such separated donor-acceptor systems allows an efficient charge separation to be achieved. 

Although the electrostatic potential on the surface of the polyelectrolyte effectively prevents the diffusional back electron transfer, it is unable to retard the very fast charge recombination of a geminate ion pair formed in the primary process within the photochemical cage. Compartmentalization of a photoactive chromophore in the microphase structure of the amphiphilic polyelectrolyte provides a separated donor-acceptor system, in which the charge recombination is effectively suppressed. Thus, with a compartmentalized system, it is possible to achieve efficient charge separation. 

Nanoscale Morphological Change of PS-b-P4VP Block Copolymer Films Induced by Site-Selective Doping of a Photoactive Chromophore... 

Machida, S., Nakata, H., Yamada, K. and Itaya, A. (2007) Morphological change of a diblock copolymer film induced by selective doping of a photoactive chromophore./, Polym. Sci. B, Polym. Phys. Ed., 45, 368-375. 

Another type of negative tone Novolak resin was obtained by direct incorporation of the photoactive chromophore into the polymer chains as shown... 

A problem of the experimental measurement of local polarity in the vicinity of donor and acceptor centers incorporated into a protein (bovine serum albumin, BSA) was solved with the use of the dual fluorescence-nitroxide probe (Bystryak et al., 1986 Rubtsova et al., 1993 Fogel et al., 1994 Likhtenshtein, 1993, 1996 Likhtenshtein et al., 2001). In such a hybrid molecule, the photoactive chromophore fragment in the excited singlet state can... 

Block Copolymers with Photoactive Chromophores and Non-photoactive Mesogenic Side-Groups... 

As an alternative to holographic experiments, the photoinduced birefringence can also be measured by the response of the material to polarized light. Its value depends on the concentration of the photoactive chromophores, which decreases with increasing amount of the homopolymer. Normalizing the results to the azobenzene content yields very similar results for each series of blends in which the same block copolymer is used (see Fig. 37). This also confirms that the photoresponse is independent of the homopolymer content. 

This synthetic protein fixates heme and zinc protoporphyrin IX very well in water < 10 M). The heme s iron(II) is a mixture of high spin and low spin, but no oxygen fixation is possible in water. Such a relatively simple designed protein may, however, fixate various redox- and photoactive chromophores and may be combined with anionic redox systems at its open end. Fast electron transfer may be achieved. The redox potential of the Fe /Fe° pair drops by 90 mV if the peptide environment of the heme changes from hydrophilic to hydrophobic. Hydrophobic-ity increases the binding constants of the peptides to the heme iron. They scale with more negative reduction potentials (Huffmann et al., 1998). 

As a further example of difficulties in identifying the photoactive chromophore in a commercial polymer, poly(propylene) can be cited. Here some workers, using conventional methods, have produced data that favor hydroperoxides as the initiating species (123,130,131), whereas using luminescence measurements a good correlation has been found between onset of photooxldatlon and formation of luminescent ag-unsaturated carbonyl compounds in this... 

Chart 8.3 Substituted phenyl propanols that constitute the structural units of lignins. Table 8.1 Photoactive chromophores (pigments) of photoreceptor proteins [9, 20-25]. 

Hydrophobic associations in random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and some methacrylamides and methacrylates substituted with bulky hydrophobes are described with a focus on preferential intrapolymer self-association which leads to the formation of single-macromolecular assemblies (i.e., unimolecular micelles). Structural parameters that critically determine the type of the macromolecular association (i.e., intra- vs. interpolymer associations) are discussed, which include the type of hydrophobes, their content in a polymer, sequence distribution of electrolyte and hydrophobic monomer units, and the type of spacer bonding. Functionalization of single-macromolecular assemblies with some photoactive chromophores is also presented. 

This chapter will discuss hydrophobic associations in random copolymers of AMPS and some hydrophobic methacrylamide and methacrylate comonomers with a focus on the intra- versus interpolymer self-association in connection witih the type of hydrophobes, their content in the polymers, and spacer bonding. A particular emphasis will be placed on intrapolymer association of hydrophobes which leads to single-molecular self-assemblies. Functionalization of the single-macromolecular assemblies with some photoactive chromophores will also be presented briefly. 

Trans-to-cis isomerisation of photoactive chromophores usually occurs through a standard one-bond-flip mechanism in the gas phase. In contrast, spatial constraints... 

VI. UNIMER MICELLES FUNCTIONALIZED WITH PHOTOACTIVE CHROMOPHORES... 

Over the last decade, there has been increasing interest in photochemistry in constraining space. If photoactive chromophores are compartmentalized in the hydrophobic microdomains of the unimer micelles, the chromophores will be completely isolated from one another in highly constraining nonpolar microenvironments and protected from the aqueous phase. Owing to these unusual microenvironmental effects, such chromophore-functionalized unimer micelles, unlike the conventional molecular assembly systems, induce a large modification of the photophysical and photochemical behavior of the compartmentalized chromophores. 

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