Fulgides as switches for intramolecular energy transfer

Switching of intramolecular energy transfer is a challenging subject [ l-3 1. The problem is: is it possible to use a photochromic molecule F as central part of a supermolecule D-F-A to allow or to hinder energy transfer from D (donor) to A (acceptor) by switching F with light of the appropriate wavelength from one of its photochromic configurations into the other one and back? Promising candidates for usable photochromic molecules are the fulgides. It is known that molecules of this class can be transformed by light in a reversible way from a configuration E (see fig. 1) into a configuration C [4-61. Irradiation with UV light causes an intramolecular ring closure reaction (colouring), visible light causes a ring opening reaction (bleaching) [ 7 1. The kinetics of such processes and the stability of educts and products during many cycles have been studied on selected fulgides [ g-101. In order to follow this process step by step, we also investigated the molecules 3 and 4 which are the parts D-F and F-A, respectively, of 5, D-F-A [ 1 l-131. Our first attempts to use the ring opening-ring closure reaction of F (1 in scheme 1) to interrupt or to close the conjugation in a polyene-like part of molecules D-F or F-A were not successful [ 11,12,14]. The photoreaction during colouring produced sevFE Fc