Copolymers of Methacrylates with Cholesteric Monomers

Groups, like 6-oxo-1,6-dihydropyrimidin-3-ium-4-oleates, are light-sensitive and undergo intramolecu lar photo-cyclization to bis-b-lactams when irradiated with UV light between 320 and 490 nm [189].

This was utilized to form copolymers of methacrylates with colesteric monomers [190]. An example of such copolymers and the rearrangement is shown below [190]:

It wasfoundthattherateofphoto-rearrangementisaffectedbythelengthofthealkylsidegroupinthe 5 position of the pyrimidinium oleate and by the embedding of a chromophore, as shown above [190]. In photosynthesis, antenna pigments harvest energy and transfer it into a reaction center for redox reactions [191]. Different antenna chromophores that surround the reaction center are arranged morphologically in an order of energy gradient [192–194]. Such an arrangement allows the energy to be sequentially transferred and efficiently funneled into reaction centers over small distances in the direction of decreasing band gaps. Considerable research has been carried out on means to develop the sequential multistep energy transfer systems. This was done not just to mimic the natural light harvesting process [192–194], but also for possible applications and use in optoelectronic and biological systems [195]. It is speculated that a conjugated polymeric backbone, with well-designed interruptions of conjugation by insulating spacers, might allow tuning the emission properties and by providing an alternate model [196].

Based on this concept, Krebs and coworkers [197] reported a synthesis of a light-harvesting material that consists of three structural domains. Two of them are conjugated homopolymers that are linked with a zinc porphyrin:

Sections A and B of the above block copolymer are illustrated above. The polymer has a constant ratio of the zinc porphyrin to the incorporated monomer units, regardless of the molecular weight. The ratio of zinc porphyrin to the polymer blocks can be varied in the material by varying the size of the blocks A. Studies of energy transfer from the polymer to the zinc porphyrin showed that there is actually very little energy transfer when the material is in solution. On the other hand, there is quantitative energy transfer in the solid state. Also, it was observed that the light-harvesting properties of the three-domain structures depend on the chain lengths of the conjugated polymers. Cheng and Luh reported that they are also trying to develop polymers that would mimic natural photosynthesis with synthetic polymers [198]. They point out that silylene moieties have been used extensively as insulating spacers [198]. In general, when the silylene spacer contains only one silicon atom, no conjugative interactions between the p systems and the silicon moiety are observed [199]. They believe, therefore, that introduction of an energy gradient with three well-designed chromophores into a silylene-spaced polymeric chain may lead to sequential energy transfer. To achieve this goal, they carried out preparations of regioregular silylene-spaced copolymers composed of energy gradients with three different chromophores. One of the polymers prepared in this way can be illustrated as follows:

Their synthesis utilized rhodium-catalyzed hydrosilylation of bis-vinylsilanes and bis-alkynes. The ratio of the three chromophores in the above polymer is 1:2:1, corresponding to D1, D2, and A chromophores, respectively. Cheng and Luh found that upon excitation of the donor chromophore D1, only emission from the acceptor A was observed [199].

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مواضيع ذات صلة

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams

Polymers with Pendant Azide Groups

Polymers with Pendant Abietate and Dibenzazepine Groups

Polymers with Pendant Maleimide Groups

Polymers with Pendant Furan Groups

Polymers with Functional Groups Similar to Cinnamates

Polymers with Functional Chalcone Groups