Evidence for Dual Pathway in Through-Space Singlet Energy Transfers in Flexible Cofacial Bisporphyrin Dyads

• Published in: Chemistry : a European journal 2009-01, Vol.15 (2), p.524-535
• Main Authors: Pognon, Grégory, Wytko, Jennifer A., Harvey, Pierre D., Weiss, Jean
• Format: Article
• Language: English
• Subjects: calixarenes; energy transfer; porphyrinoids; zinc
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200800299

Flexible “pacman” scaffolds built upon a calix[4]arene platform bearing a [18]crown‐6 ether and either two OH functions or two OPr groups at the lower rim have been used to generate donor–acceptor (D–A) dyads incorporating a zinc–porphyrin donor and a free‐base porphyrin acceptor. Through‐space singlet energy transfer (SET) in the D–A dyads was studied by time‐resolved fluorescence spectroscopy. Although the effects of conformational changes are well documented when the chromophores switch from a non‐cofacial to a cofacial arrangement, little is known about flexible pacman scaffolds in which the changes are limited to the distance between the chromophores. The known SET rates for reported, geometrically well‐defined, rigid pacman D–A dyads were used as calibration to estimate the D–A distances in the flexible pacman dyads. Due to the flexibility of the calix[4]arene spacer, the D–A dyads adopt a “closed” or “open” geometry that is tuned by intramolecular hydrogen bonds (OH⋅⋅⋅[18]crown‐6 ether) and by solvent interactions. Changes in the SET rates between the open and closed geometries were surprisingly less dramatic than expected, and are explained by a dual SET pathway that is specific to the calix[4]arene platform. Time‐resolved fluorescence studies support the hypothesis that, for the “open” conformer, the preferred through space SET pathway (i.e., at the shortest distance) is located within the calix[4]arene cavity through the cofacial phenyl groups. For the “closed” conformer, the preferred through space SET route is located between the zinc and free‐base porphyrins. The shortest pathway may not be a straight line for singlet energy transfer between a zinc porphyrin donor and a free‐base porphyrin, as through‐space communication within a calixarene spacer can be observed (see scheme).