STEADY-STATE AND PICOSECOND SPECTROSCOPY OF LI- REVERSIBLE PHOTORELEASE OF CATIONS( OR CA2+ COMPLEXES WITH A CROWNED MEROCYANINE )

Reversible photorelease of Li+ and Ca2+ cations from complexes with a crowned derivative of the merocyanine dye DCM ethylene)-2-methyl-6-(p- (dimethylamino)styryl)-4H- pyran) in acetonitrile is investigated by s teady-state and time-resolved spectroscopy. The results obtained by fl uorescence spectroscopy reveal the presence of two emitting species in solutions containing DCM-crown complexes. One species with a 2 ns lif etime is attributed to the free ligand resulting from cation photorele ase, with a maximum formation yield of 86% for Li+ and 41% for Ca2+. T he other one with a 400 ps slightly blue-shifted emission for both cat ions is suggested to be due to a loose complex. The mechanism of catio n photorelease is studied by transient absorption and gain spectroscop y within 2 ns after excitation with a 0.7 ps pulse at 425 nm. In the 3 60-660 nm spectral range, the main changes observed take place within 30 ps. For longer time delays the time-resolved differential spectra i ndicate 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 domi nant excited-State mechanism includes the disruption of the binding be tween the nitrogen atom of the crown and the cation in the locally exc ited state of the complex, followed by intraligand charge transfer at a slower rate for Ca2+ than for Li+. In spite of the absence of experi mental evidence for cation diffusion to the bulk, a significant increa se in concentration of free cations should be obtained by using light pulses of long duration.