Reversible bulk photorelease of strontium ion from a crown ether-linked merocyanine

The use of a crown-ether-linked merocyanine, DCM-crown, for producing photoinduced, long-lived and reversible concentration jumps of metal cations is reexamined. In this new investigation, the excited-state behavior of DCM-crown complexed with a strontium ion in acetonitrile is found to exhibit the same trends as those previously reported with calcium and lithium ions. However, some new features provide evidence for cation photorelease to the bulk. Subpicosecond transient absorption experiments confirm the initial fast photodisruption of the interaction between the ion and the crown, and the formation of a loose complex after intramolecular charge transfer within the chromophore. Two additional steps are observed. Firstly, a continuous red shift of the gain spectrum is seen on the subnanosecond scale. It is assigned to the movement of the Sr2+ cation away from the chromophore, partly to the bulk of the solvent and partly towards the formation of an ultraloose complex with oxygen atoms of the crown. Secondly, a free-ligand-like absorption remains after the complete decay of the excited state. This band, which signals a total photorelease of the Sr2+ ion, disappears with a characteristic time of about 110 ns, attributed to the recomplexation of the crown in the ground state of the dye.