Convenient Scaffold for Forming Heteroporphyrin Arrays in Aqueous Media

A new methodology for preparing heteroporphyrin arrays in aqueous solution has been presented. The present method is based on the extremely strong ability of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMe-beta-CD) to include 5,10,15,20-tetrakis(p-substituted-phenyl)porphyrins (Por) affording trans-type 1:2 complexes of the porphyrins and TMe-beta-CD. Two different Por-per-O-methylated beta-CD (per-Me-beta-CD) conjugates were synthesized. Conjugate 2 was prepared by an S(N)2 reaction of 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin and per-O-methylated beta-cyclodextrin having one primary OTs group. Four per-Me-beta-CD moieties are attached to the meso positions of 2. Conjugate 3, synthesized from 5-(p-hydroxyphenyl)-10,15,20-tris(3,5-dicarboxyphenyl)porphyrin and monotosylated per-O-methylated beta-cyclodextrin, has one per-Me-beta-CD moiety at the periphery of the porphyrin. Conjugate 2 yields a stable 1:4 complex with the zinc complex of 5-phenyl-10,15,20-tris(3,5-dicarboxyphenyl)porphyrin (8) in the dissociated form. In this system, the energy transfer from photoexcited Zn-8 to free base 2 occurs with 85% efficiency. Conjugate 3 forms a very stable 1:1 complex with Zn-8 (K = (7.0 +/- 0.3) x 10(5) dm(3) mol(-1)) with an energy transfer efficiency (93%) larger than that obtained in the case of 2. The structure of the 3-Zn-8 complex, which can account for the efficient energy transfer, was deduced from (1)H NMR spectroscopy. Intramolecular fluorescence quenching of 2 and 3 by Fe(III)-8 also occurred through an electron-transfer process as the main quenching mechanism. The present method is a very simple and convenient means to construct various heteroporphyrin arrays in aqueous solution.