Guest binding and new self-assembly of bisporphyrins

In organic medium, bisprophyrins 1-6 connected by aromatic linkers self-assemble via subtle forces such as van der Waals, pi-pi stacking, and CH/pi to form supramolecular dimers. The structures of bisporphyrin dimer 1 center dot 1 were discussed using our chemical shift simulation, revealing that 1 center dot 1 mainly adopts the self-complementary structure A. ESI mass experiments of the bisporphyrins showed that 1-4 form only the dimers; however, trimers as well as the dimers of 5 and 6 were observed in the gas phase. Thus, the assemblies of bisporphyrin 5 and 6 should adopt structure B, which still has a binding site to which another bisporphyrin can fit to form oligomeric structures. The dimerization constant of bisporphyrin I is dependent on the solvent polarity: the values decrease in the order of toluene > chloroform > 20% methanol -chloroform. The thermodynamic studies of the dimerization processes revealed that desolvation as well as pi-pi stacking interactions play a key role in the formation of the self-complementary dimers. The binding studies of bisporphyrin 1 with a variety of electron deficient aromatic guests 9-17 were carried out in chloroform. Soret and Q-bands of I showed the characteristic changes with the addition of guests 9-13 and 15, and large upfield shifts of their protons were observed in their complexation studies with H-1 NMR spectroscopy. These results suggested that the electron deficient aromatic guests bound within the cleft of bisporphyrin 1 via charge transfer as well as pi-pi stacking interactions between the guests and the porphyrin rings. The dimerization constant of 1 center dot 1 is much smaller than the association constant of 1 center dot 9, suggesting that the dissociation of dimer 1 center dot 1 can be regulated by binding of 9 within the cleft. The addition of 9 into the solution of 1 center dot 1 resulted in the quick dissociation of the dimer and the formation of 1 center dot 9.