Thermodynamics and Kinetics of Guest-Induced Switching between “Basket Handle” Porphyrin Isomers

The synthesis and switching properties of two “basket handle” porphyrin isomers is described. The cis-oriented meso-phenyl groups of these porphyrins are linked at their ortho-positons via benzocrown-ether-based spacers, which as a result of slow atropisomerization are located either on the same side of the porphyrin plane (cis), or on opposite sides (trans). In solution, the cis-linked isomer slowly isomerizes in the direction of the thermodynamically more stable trans-isomer. In the presence of viologen (N,N'-dialkyl-4,4'-bipyridinium) derivatives, which have different affinities for the two isomers, the isomerization equilibrium could be significantly influenced. In addition, the presence of these guests was found to enhance the rate of the switching process, which was suggested to be caused by favorable interactions between the positively charged guest and the crown ethers of the receptor, stabilizing the transition state energies of the isomerization reaction between the two isomers.