Synthesis, characterization, and small hydrocarbon encapsulation of dicavitand-porphyrins

New host molecules "Dicavitand-Porphyrins" [H ₂C ₂P(syn,syn) (3), H ₂C ₂P(syn,anti) (4)] with small cavities on one or both sides of the porphyrin plane were synthesized from 5,10-bis(2,6-dihydroxyphenyl)-15,20-diphenylporphyrin and bis(chloromethyl) cavitand in 15 and 34% yield, respectively. Similarity of guest size selectivity of the host 4 in comparison with the reported "Cavitand-porphyrin" [H ₂CP(syn) (1)] suggests that these hosts have the same cavity size. The 1:1 and 1:2 association constants (K ₁₁ and K ₁₂) of the guest encapsulations into 3 were also obtained by ¹HNMR titration and nonlinear least square fittings. The guest size dependences of K ₁₁ and K ₁₂ values of 3 show that the initial cavity prefers larger guests such as ethane, while the second one does not. The induced-fit type very small structural changes (estimated within 1°) upon first guest encapsulation of the host 3 affects the guest encapsulation of the other cavity through the covalent linkages.