Capped Subporphyrins

Capped subporphyrins 12 – 16 with C 3 molecular symmetry were synthesized from 5,10,15‐tri(3‐aminophenyl)‐substituted subporphyrin 8 and tripodal trialdehydes 2 – 6 by Lindsey’s entropically favored macrocyclization. X‐ray diffraction analysis has revealed that the concave surface of the subporphyrin core is selectively capped with a 1,3,5‐substituted benzene moiety. Capped subporphyrins 15 and 16 , with a five‐atom arm and thus large inner cavities, exhibit solvent incorporation behavior in their crystal structures. On the other hand, subporphyrins 12 and 13 exhibit tight structures, in which the cap and subporphyrin core are found much closer with average interplanar separations of 3.56 and 3.15 Å, respectively. Variable‐temperature 1 H NMR measurements revealed that subporphyrins 12 , 13 , and 16 undergo spiral interconversions between P and M forms depending on the arm length and the electronic nature of the cap. Of these, subporphyrin 13 , with a 1,3,5‐tri(alkoxycarbonyl)benzene cap strapped by three‐atom arms, exhibits a considerably slow spiral interconversion with a large enthalpy change of Δ H ≠ =76.4 kJ mol −1 and a characteristic redshift of the Soret‐like band and enhancement of the Q(0,0) band. These properties are ascribed to considerable through‐space charge‐transfer interactions between the electron‐deficient cap and the subporphyrin core and the multiple CHπ interactions.