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- W1988992558 abstract "The acidity constants ( pK a3 and pK a4 ) of a series of porphyrins and their corresponding [2]catenanes where the porphyrin is over-arched with a hydroquinol or naphthoquinol polyether strap and mechanically linked to a bipyridinium macrocycle cyclobis-paraquat(4,4'-biphenylene), are calculated from acid-base titration data. As the length of the strap decreases from tetraethylene through triethylene to diethylene, the pK a values decrease, and the separation between pK a3 and pK a4 increases. In the diethylene glycol strapped derivatives, only the monocation is accessible above pH = 0. The trend in values reflects the ease of protonation which becomes increasingly difficult as the coulombic repulsions between the protonated porphyrin and the tetracationic macrocycle becomes more severe, as a consequence of enforced proximity of the two charged centres. On protonation, any electronic interactions between the interlinked subunits is replaced by strong electrostatic repulsions. This is alleviated by a 90° rotation of the tetracation and a movement away from the porphyrin; such a reorientation is possible only in the longer strapped derivatives, and is prevented in the “tight” analogues." @default.
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- W1988992558 date "2003-10-01" @default.
- W1988992558 modified "2023-10-17" @default.
- W1988992558 title "The effects of catenation on the acidity constants of porphyrin [2]catenanes" @default.
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- W1988992558 doi "https://doi.org/10.1142/s1088424603000835" @default.
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