DEPENDENCE OF METAL-ION COMPLEXATION AND INTERMOLECULAR AGGREGATION ON PHOTOINDUCED GEOMETRIC ISOMERISM IN A CROWN-ETHER STYRYL DYE

Photoisomerization and complexation with alkaline-earth metal cations of a 2-[2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopentaoxacyclopentadecin-16-yl)ethenyl]-3-(3-sulfopropyl)benzothiazolium betaine has been studied by steady-state and time-resolved electronic spectroscopy. The dye is highly sensitive to complexed metal cations, showing characteristic hypsochromic shifts of the long wavelength absorption and fluorescence bands, as well as a decrease in the fluorescence quantum yield upon complexation. The stabilities of the complexes are drastically changed by photoisomerization, with enhanced stability of the cis ligand complex being attributed to the intramolecular interaction of the complexed cation with a tethered sulfonate anion. This intramolecular capping also induces a strong hypsochromic shift of up to 70 nm from the absorption maximum of the uncapped trans ligand complex. Unlike this capped cis complex, the uncapped trans complex is partially aggregated, even at low concentration, as a result of the association of the sulfonate anion of one dye with the "crowned' cation of a second molecule. At high metal ion concentrations, the aggregates dissociate as the sulfonate group becomes associated with a free metal cation. Similar interactions are also responsible for the relaxation of the anion-capped cis complex to its "uncapped" form at high cation concentrations.