A ‘‘reverse interrupter’’: the novel molecular design of a fluorescent photochromic DTE-based bipyridinew

Fluorescent switching systems containing photochromic molecules have attracted significant attention on account of their potential uses as optical memory media and photoswitchable devices. Most systems offering the photomodulation of luminescence involve a photochrome covalently linked to an organic or metal–organic fluorophore, wherein the luminescence is regulated by an intramolecular energy transfer from the excited fluorophore to the photochromic unit. In contrast, little attention has been devoted to luminescent photoswitches based on donor–bridge–acceptor (D–p–A) derivatives, in which the photochromic unit is a part of the p-conjugated bridge. We have previously shown that 2,20-bipyridines featuring p-donor-conjugated substituents are good building blocks for the construction of metal complexes with very large non-linear optical responses and interesting luminescent properties, which are governed by strong intra-ligand charge transfer (ILCT) transitions. In the course of our continuing exploration of such systems and the search for the photomodulation of these properties, we have recently incorporated a dithienylethene (DTE) unit into a donor-substituted styrylbipyridine ligand, allowing the preparation of the first example of metalcontaining photochromic bipyridine ligands displaying an efficient switching of the NLO responses (Scheme 1(a)). However, these free bipyridine ligands, in which the donor (D) and acceptor (A) groups are located at each end of the DTE unit, are not emissive under ambient conditions, neither in the open nor closed forms. Thus, as an alternative way to obtain a fluorescent photoswitch, we sought to design a push–pull chromophore featuring D and A moieties located at the 2and 5-positions of the same thiophene ring of the DTE unit (Scheme 1(b)), rather than them being located on two different thiophene rings. In this so-called ‘‘reverse interrupter’’, p-conjugation between A and D is efficient only when the DTE unit is in its open form (‘‘ON’’ state), whereas in the closed ring isomer (‘‘OFF’’ state), the formation of a tetrahedral sp center at the C carbon disconnects the D and A parts of the molecule. In these systems, it was anticipated that the rupture of the intramolecular charge transfer accompanying the photoisomerization process would allow modulation of the emission properties of the chromophore. As a first approach, we wish to disclose our preliminary results in the synthesis, and photochromic and photophysical studies of reverse interrupter 1, containing dimethylaminophenyl and vinylbipyridine moieties as the D and A end groups, respectively (Scheme 2). We show that the photochromic reaction can be triggered upon excitation with UV and visible light. In addition, this process, which can itself be switched on or off according to the polarity of the solvent, allows an efficient modulation of the fluorescence. Substituted 4,40-vinyl-2,20-bipyridine derivatives are accessible via a Wadsworth–Emmons reaction of bipyridine-bis (phosphonate) 7 and the appropriate aldehyde in the presence of a base. We have used this approach to prepare compound 1 (Scheme 3). The two distinct thienyl fragments, 3 and 4, were first prepared independently. The p-dimethylaminophenyl