Evidence for dual pathway in through-space singlet energy transfers in flexible cofacial bisporphyrin dyads.

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.