Intracomplex electron transfer in a hydrogen-bonded calixarene-porphyrin conjugate: tweezers for a quinone

The synthesis and characterization of a new supramolecular assembly 1, wherein photoinduced electron transfer through non-covalent interactions may be probed, is reported. Ensemble I is based on supramolecular contacts between the phenolic hydroxyl groups of a calix[4]arene-substituted Zn(Il) metalloporphyrin photodonor la and the carbonyl groups of a benzoquinone acceptor 6. Ensemble I is formed with a K-a of 70 +/- 10 dm(3) mol(-1) in CDCl3 as judged by H-1 NMR spectroscopic analysis. Upon irradiation of the porphyrin subunit of I at 400 nm, a photoinduced intramolecular electron transfer from the Zn(fl) metalloporphyrin to the benzoquinone occurs with a rate constant of 3.3 x 10(10) s(-1). Two phenolic hydroxyl groups of the calix[4]arene serve not only as tweezers to capture the benzoquinone by two-point hydrogen bonding fixation, but also as useful building blocks in the construction of non-covalent donor-acceptor electron transfer model systems. (C) 2001 Elsevier Science B.V. All rights reserved.