Polymer-bound chromophores

R. K. Guo and S. Tazuke, Microheterogeneity oflocal segment mobility in solid polymethacrylates as studied by a polymer-bound twisted intramolecular charge transfer chromophore, Macromolecules 22, 3286 (1989). 

The ICD of the dyes bound to saccharides through an ionic coupling or hydrophobic interaction may remain a conflicting problem. The side-chain chromophores covalently bound to saccharides permit CD bands in the far or near ultraviolet region. These side-chain chromophores can exhibit CD and thus provide more definitive information on the conformation of saccharide moieties. Thus, acetamide CD has been observed to reflect polymer secondary structure of glycosaminoglycans, which are the connective-tissue proteoglycans. 

UV absorption spectra, fluorescence emission spectra and photostabilization effect of 2-(2-hydroxy-4-acryloyloxyphenyl)-2/7-benzotriazole and of its polymer bound forms were studied in poly-CK-l,4-polybutadiene [337]. The following activity series was found copolymer with methyl methacrylate > homopolymer > monomer. It seems that chromophoric units incorporated into a macromolecule behave cooperatively (causing self-absorbance of the emitted radiation). 

Free and polymer-bound 2-benzyloxy-thioxanthone exhibit similar flash photolysis behaviour and the same photoreduction quantum yield in the presence of 2-(A, AT-diethylamino) ethanol. This clearly shows that the polymeric nature does not appear to affect photophysical properties of the thioxanthone moiety. The photoinitiated polymerization of MMA in benzene solution, using BOTX and poly(StX-co-St) in combination with 2-(MA -dieffiylamino) ethanol, indicates that the polymer-bound chromophore seems to operate in the same way and with similar efficiency as the free photoinitiator, at least in conditions of dilute chromophore concentration. 

In conclusion regarding the results obtained with polymer-bound azobenzene chromophores, establishing liquid crystalline phases that combine order and flexibility in the side chain region of the polymer seems to be the best way to obtain materials with a strong photoresponse. This is because a high density of chromophores can be combined with sufficient flexibility that the chromophores still can be photoisomerized in the dense packing of an LBK film. When a low density of chromophores is acceptable, however, the chromophores can be diluted along the polymer chain to reduce their interaction and secure sufficient free volume. 

Our discussion will now be specialized to triplet state processes for polymer bound chromophores. However in Table 1 is presented a representative sample of some work that has been done on the singlet state that illustrates some of the processes discussed above. 

Clearly this cannot be correct. It is also true that if the reaction is very ineffective, then there should be no (n) dependence since each reactant molecule will encounter the same number of polymer bound chromophores since the mole fraction of these chromophores in solution is held constant. An example of this behavior has been provided recently by Winnik and Maharaj (42) for n-alkanes (n from 6 to 36) undergoing a hydrogen abstraction reaction with triplet benzo-phenone. 

Figure 2 illustrates that similar information can be obtained from the pH-dependent fluorescence of a polymer-bound pyrene probe. The conformational transition of a copolymer of 2-ethylacrylic acid and 1-pyreneacrylic acid appears in the fluorescence experiment as a large increment in If around pH 6.2. Covalent binding of the chromophore has little effect on the position or the shape of the transition. 

Sassoon and Rabani [79] also prepared a two polymer system in which a chromophore was covalently bound to one polyelectrolyte and a donor or acceptor was electrostatically held by the other polyelectrolyte, and showed that its back ET underwent a similar retardation effect. They employed 26 as a photosensitizer, MV2+ as a mediator, and ferricyanide as an acceptor electrostatically bound to the added polycation (polybrene). 

As already shown, it is technically possible to incorporate additive functional groups within the structure of a polymer itself, thus dispensing with easily extractable small-molecular additives. However, the various attempts of incorporation of additive functionalities into the polymer chain, by copolymerisation or free radical initiated grafting, have not yet led to widespread practical use, mainly for economical reasons. Many macromolecular stabiliser-functionalised systems and reactive stabiliser-functionalised monomers have been described (cf. ref. [576]). Examples are bound-in chromophores, e.g. the benzotriazole moiety incorporated into polymers [577,578], but also copolymerisation with special monomers containing an inhibitor structural unit, leading to the incorporation of the antioxidant into the polymer chain. Copolymers of styrene and benzophenone-type UV stabilisers have been described [579]. Chemical combination of an antioxidant with the polymer leads to a high degree of resistance to (oil) extraction. 

Significantly, the bio-inorganic and polymer-containing PM nanocomposites showed no significant shift in the protein amide I and II vibration bands, or in the characteristic 567 nm optical absorption band of the retinal chromophore of BR, indicating that the structural and dynamical properties of the membrane-bound... 

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