Crown-containing butadienyl dyes - 5. Structure and photoisomerization of a butadienyl dye containing a 15-crown-5 fragment

The structure of the E,E isomer of a butadienyl dye of the benzothiazole series containing a 15-crown-5 fragment was established by X-ray diffraction analysis. The stacking motif of the dye molecules in the crystal structure is, apparently, attributable to intermolecular pi-interactions between the large conjugated fragments. The possibility of [2+2] photocycloaddition occurring in the crystal is discussed. Reversible geometrical photoisomerization of the dye and its complex with Mg2+ in acetonitrile solution was studied by spectrophotometry and H-1 NMR spectroscopy. Photoirradiation of the complex of the E,E isomer with Mg2+ leads predominantly to isomerization of the C=C bond adjacent to the benzothiazole fragment. Regioselective E,E-->Z,E photoisomerization occurs via a singlet mechanism with a quantum yield of about 0.45. The quantum yield of reverse Z,E-->E,E photoisomerization is approximately 0.52. The structures of different geometrical isomers of the dye complex with Mg2+ were calculated by the quantum-chemical density functional theory (DFT).