Steady-state and picosecond spectroscopy of Li+ or Ca2+ complexes with a crowned merocyanine. Reversible photorelease of cations

Reversible photorelease of Li+ and Ca2+ cations from complexes with a crowned derivative of the merocyanine dye DCM (4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4H- pyran) in acetonitrile is investigated by steady-state and time-resolved spectroscopy. The results obtained by fluorescence spectroscopy reveal the presence of two emitting species in solutions containing DCM-crown complexes. One species with a 2 ns lifetime is attributed to the free ligand resulting from cation photorelease, with a maximum formation yield of 86% for Li+ and 41% for Ca2+. The other one with a 400 ps slightly blue-shifted emission for both cations is suggested to be due to a loose complex. The mechanism of cation photorelease is studied by transient absorption and gain spectroscopy within 2 ns after excitation with a 0.7 ps pulse at 425 nm. In the 360-660 nm spectral range, the main changes observed take place within 30 ps. For longer time delays the time-resolved differential spectra indicate a dominant decay of 1.5-2 ns in all spectral regions. From the analysis of the early spectral changes, it is concluded that the dominant excited-State mechanism includes the disruption of the binding between the nitrogen atom of the crown and the cation in the locally excited state of the complex, followed by intraligand charge transfer at a slower rate for Ca2+ than for Li+. In spite of the absence of experimental evidence for cation diffusion to the bulk, a significant increase in concentration of free cations should be obtained by using light pulses of long duration.